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37. CRYSTAL MORPHOLOGIES AND PYROXENE COMPOSITIONS IN BONINITES ANDTHOLEIITIC BASALTS FROM DEEP SEA DRILLING PROJECT HOLES 458 AND 459B

IN THE MARIANA FORE-ARC REGION1

James H. Natland, Deep Sea Drilling Project, Scripps Institution of Oceanography, La Jolla, California

ABSTRACT

Not all boninites are glassy lavas. Those of Hole 458 in the Mariana fore-arc region are submarine pillow lavas andmore massive flows in which glass occurs only in quenched margins. Pillow and flow interiors have abundant Plagio-clase spherulites, microlites, or even larger crystals but can be recognized as boninites by (1) occurrence of bronzite, (2)presence of augite-bronzite microphenocryst intergrowths, and (3) reversal of the usual basaltic groundmass crystalliza-tion sequence of plagioclase-augite to augite-plagioclase. The latter is accentuated by sharply contrasting augite andPlagioclase crystal morphologies near pillow margins, a consequence of rapid cooling rates. This crystallization se-quence appears to be a consequence of boninites having higher SiO2 and Mg/Mg + Fe than basalts but lowerCaO/Al2O3.

Microprobe data are used to illustrate the effects of rapid cooling on the compositions of pyroxene and micropheno-crysts in a glassy boninite sample and to estimate temperatures of crystallization of coexisting bronzite and augite. Arange from 1320°C to 1200°C is calculated with an average of 125O°C. This is higher by 120°-230° than the knownrange for western Pacific arc tholeiites and by over 300° than for calc-alkalic andesites.

Boninites of Hole 458 lack olivine and clinoenstatite but are otherwise chemically and petrographically similar toboninites that have these minerals. In order to distinguish the two types, the Hole 458 lavas are here termed boninitesand the others are termed olivine boninites.

Arc tholeiite pillow lavas from Holes 458 and 459B are briefly described and their textures compared to frac-tionated, moderately iron-enriched, abyssal tholeiites. Massive tholeiite flows contain striking quartz-alkali feldsparmicrographic intergrowths with coarsely spherulitic textures resulting from in situ magmatic differentiation. Such inter-growths are rare in massive abyssal tholeiites cored by DSDP and probably occur here because arc tholeiites have highernormative quartz at comparable degrees of iron enrichment—a result of higher oxygen fugacities and earlier separationof titanomagnetite—than abyssal tholeiites.

INTRODUCTIONThis chapter describes crystal morphologies and tex-

tures of lavas recovered from Holes 458 and 459B of theMariana fore-arc region during Deep Sea Drilling Proj-ect Leg 60 (Fig. 1). The majority of lavas in Hole 458 arechemically and petrographically distinct members ofwhat has been termed the boninite series by Bloomer etal. (1979) and Meijer et al. (this volume). The lavas wereextruded as pillows and massive flows onto the seafloorand consequently have a considerable range in crystal-Unity, with great differences in the sizes and shapes ofcrystals from sample to sample. The recovery of an ex-tensive suite of these lavas from Hole 458 allows them tobe described in a more comprehensive manner than haspreviously been attempted; the principal objective is torelate glassy samples to more coarsely crystallinesamples from pillow interiors and massive flows. Alsooccurring in both holes are tholeiitic basalts with thechemical characteristics of island arc tholeiites (Wood etal., this volume). These also were extruded as pillowsand massive flows and, although more altered than theboninite series lavas, can be described in much the sameway. The descriptions may be of interest to those whowish to compare these lavas with subaerial equivalentsor abyssal tholeiites.

Initial Reports of the Deep Sea Drilling Project, Volume 60.

BACKGROUNDThe recovery of boninite series lavas from Hole 458

was one of the most significant results of Leg 60. At thetime of the drilling, petrologists were just begining to beaware of the role of these rocks in island arc magmatism(e.g., Kuroda and Shiraki, 1975). Boninites had beennamed and described as early as the late 19th century(Kikuchi, 1888, 1890; Petersen, 1891) but remained littlemore than a curiosity among rock types for almost 90years. Dallwitz et al. (1966) described clinoenstatite-bearing lavas from the Cape Vogel area of Papua, butmainly to emphasize their unusual mineralogy. Theselavas are now recognized to be boninites (Cameron etal., 1980). Although Dietrich et al. (1978) described bo-ninites from dredge hauls in the Mariana Trench, theyidentified the suite as originally ocean crust because theboninites occurred in an "ophiolitic" assemblage of pil-low basalts, dolerites, gabbros, and ultramafic rocks.Only a combined study of the Bonin Islands (Kurodaand Shiraki, 1975; Shiraki and Kuroda, 1975, 1977;Kuroda et al., 1978; Shiraki et al., 1977, 1978) and Leg60 drilling have prompted the general realization thatthese are island arc lavas (e.g., Meijer, 1980; Cameronet al., 1979, 1980; Sharaskin et al., 1980).

The evidence that the lavas from Hole 458 are islandarc lavas is that arc-derived turbidites began draping theentire fore-arc region, including the pillow lavas ofHoles 458 and 459B, in the late Eocene-early Oligocene

681

J. H. NATLAND

130°

Ridges (all types)

140°

Trenches with related subduction

150°

Proposed spreading centers(dashed where no well-defined rift)

17°45'

17°30'

146°30'E

B

146°45 147 147°15' 147° 30' 147°45'

Figure 1. A. Location of Sites 458 and 459 (open circles) in the Mariana fore-arc region. Other Leg 60 sites are closed circles. Leg 59 sites are cir-cled dots. Site 290 (Leg 31) is shown by a star. The box encloses area of B, bathymetry of the fore-arc and trench region near Sites 458 and 459,from Hussong and Fryer (this volume). Contour interval is 200 meters.

682

CRYSTAL MORPHOLOGIES

(see reports for Sites 458 and 459, this volume). More-over, basalts associated with the boninite series lavas inHole 458 and all the lavas of Hole 459 have island arctholeiite compositions (Wood et al., Meijer et al.; Shara-skin; Bougault et al.; Hickey and Frey, all this volume).Additionally, island arc tholeiites and a boninite-typerock were recovered at Sites 460 and 461 in the MarianaTrench in talus beneath Eocene sediments (see Sites 460and 461 reports; Meijer et al.; Sharaskin, all this vol-ume). The fore-arc basement is thus of island arc com-position, as old as the oldest lavas on Guam and Saipan(Schmidt, 1957; Stark, 1963), and probably crops out infault exposures in the trench wall. Cameron et al. (1980)have described boninite occurrences in the Troodos, aswell as other ophiolites, and postulate that ophiolitescontaining boninites originate in tensional environmentseither at the onset of subduction or at a time of suddenchange in the direction of plate motion. Natland andTarney (this volume) suggest that the top of basement inHoles 458 and 459B is the potential upper part of an insitu ophiolite succession, one which originated not asocean crust but in an island arc setting.

Apart from these tectonic considerations, boninitesand related rocks have compositions, mineralogies, andtextural features of no little theoretical interest. Chemi-cally, they typically have high SiO2 (52-58%) coupledwith high MgO (9% or more), as well as several hundredparts per million apiece of Cr and Ni, but low TiO2(0.2-0.5%) and Zr (20-40 ppm). Glassy boninites usu-ally have a fair amount ( — 5%) of dissolved water (e.g.,Cameron et al., 1979; Meijer, 1980). These features sug-gest that the rocks are derived by partial melting of ahydrous peridotite, as indicated by experimental data(O'Hara, 1965; Kushiro, 1969, 1972a, 1974; Mysen etal., 1974; Green, 1976). Of great interest has been theoccurrence in some boninites of clinoenstatite (Dallwitzet al., 1966; Dietrich et al., 1978; Komatsu, 1980). Priorto the descriptions of Dallwitz et al. (1966), terrestrialexamples of lavas demonstrating the inversion of proto-to clinoenstatite were unknown, even though clinoensta-tite had been produced experimentally by stressing en-statite (Turner et al., 1960) and phase relationships hadbeen determined establishing that clinoenstatite couldform metastably by cooling protoenstatite through theprotoenstatite ^ orthoenstatite inversion (Boyd andSchairer, 1964; Brown, 1968, 1972; Nakamura, 1971;Mori, 1978). As a consequence of the latter, there hasbeen a general recognition that the occurrence of clino-enstatite in these siliceous glassy rocks reflects not onlytheir unusual compositions but also the high coolingrates they had undergone.

One difficulty in identifying boninites and relatedrocks stems from the original definition of the term.Petersen (1891) named boninites on the basis of boththeir characteristic mineral assemblages and their un-usual composition. Johannsen (1937) defined boniniteas glass-rich, nearly feldspar-free olivine-bronzite ande-site with phenocrysts of olivine, bronzite, and augite.Recently, Cameron et al. (1979) redefined boninite as "ahighly magnesian but relatively siliceous glassy rockcontaining one or more varieties of pyroxene, some or

all of which have a morphology characteristic of rapidgrowth, accessory magnesiochromite, and, commonly,minor amounts of olivine. Laths of amphibole or pla-gioclase microlites are rare" (p. 550).

Despite these definitions, it is still difficult to identifythe Hole 458 lavas as boninites. The chief differencesbetween these lavas and the material in the definitionsare that the glassy Hole 458 lavas lack olivine and clino-enstatite—although that does not seem to be crucial.Magnesiochromite is also very rare, whereas it is quiteabundant in olivine-bearing boninites. The most impor-tant difference is that few of the Hole 458 lavas areglassy. The majority contain abundant spherulitic tomicrolitic Plagioclase, and many have intersertal andeven subophitic textures in which Plagioclase is a domi-nant mineral.

The explanation for the lack of olivine and clinoen-statite seems to be that the Hole 458 lavas are somewhatfractionated. Kushiro (this volume) determined that twosamples from Hole 458 he examined experimentallycould not have melted in equilibrium with a hydrousperidotite mantle, unlike boninites previously investi-gated; instead, they had a more magnesian boniniteparent which experienced fractionation of olivine andenstatite. Meijer (1980), noting the lower magnesiancomposition of the Hole 458 lavas compared withtypical boninites but recognizing their obvious generalcompositional similarities, proposed the existence of aboninite series, analogous in certain respects to the is-land-arc-tholeiite and calc-alkalic magma series of is-land arcs (this had been proposed earlier by Bloomer etal., 1979). He defined boninite series on normative crite-ria rather than on mineralogy.

But even broadening the definition of boninites inthis way to include the Hole 458 lavas does not confrontthe problem of contrasting conditions of crystallizationof these lavas, whether they are rigorously boninites ornot. It is unusual in the history of petrography for thedefinition of a mafic rock to be based on the charac-teristics of glassy, rapidly quenched lavas. Pillow basaltsare commonly glassy only in their outermost few centi-meters, and these lavas are no different. It is to be ex-pected then that the vast majority of pillowed boninites(or high-MgO bronzite andesites) will be far more crys-talline in the interior than in the narrow exterior glassyrind. But by the petrographic definitions just cited, eventhe interiors of small pillows could not be termed bonin-ites because they usually lack glass. Should the samelava composition erupt on land, there would be little orno glassy outer margin. Obviously, any proper petro-graphic identification of subaerial boninite must bebased on descriptions of lavas cooled more slowly thanpillow margins.

In this chapter, then, a principal objective will beto outline the crystallization of the Hole 458 boniniteseries lavas through the entire range of cooling rates andundercooling represented by pillow rims, pillow interi-ors, and massive flows. From this, reliable criteria foridentifying boninite-series lavas among coarser grainedrocks can be stated. Certain features in the glassy lavasdo not occur in the more coarsely crystalline rocks.

683

J. H.NATLAND

Electron microprobe analyses of minerals will be used toshow that these features are the consequence of rapidquenching at pillow margins. Pyroxene compositionswill be used to estimate temperatures of crystallizationof the lavas. This done, the general problem of boniniteseries classification will be considered. Finally, the pet-rographic features of associated tholeiitic basalts will bedescribed. These have some geochemical features, suchas iron enrichment, that are reminiscent of abyssal tho-leiites, to which they will be compared.

This chapter is the third in a series of mine devoted tocrystal morphologies of submarine lavas recovered bythe Deep Sea Drilling Project and is the first to deal withisland arc lavas. The basic concept of the previous twopapers (Natland, 1978, 1980) will be followed here. Mypremise is that undercooling is most extreme at coolingunit boundaries (where crystallization is suppressed tosuch an extent that glass forms) and least in the interiorsand that one may therefore trace modifications in crys-tal morphologies and increasing crystallinity literallyfrom the outside of cooling units into the interior as anapproximate function of decreasing undercooling. Thefoundations for such an approach were laid by Kirkpat-rick (1978), following experimental verification that awide range of crystal morphologies of Plagioclase (Lof-gren, 1974) and olivines (Donaldson, 1976) can be re-lated systematically to undercooling. The boninite serieslavas of Hole 458, however, are striking in that pyrox-ene assumes much of the role that Plagioclase does inbasalts in the development of rock textures, and somefeatures of pyroxene microphenocrysts have no experi-mental or natural analog among olivines or plagio-clases. These will be pointed out as they are discussed.Apart from these unusual occurrences, I will use theterminology of Lofgren (1971, 1974) and Donaldson(1976) for crystal morphologies and spherulitic forms ingeneral.

BASEMENT LITHOLOGIES AND COMPOSITIONSIN HOLES 458 AND 459B

Lithologic representations of basement, identifica-tion of the principal chemical types and subtypes basedon data of Wood et al. (this volume), and summarymagnetic stratigraphies for Holes 458 and 459B areshown in Figure 2. The two basic chemical types in bothholes are designated A, for boninite series lavas, and B,for basalts. Subtypes are identified by numerical sub-scripts. Average chemical analyses and CIPW norms arelisted in Table 1. There are two intervals of boniniteseries lavas in Hole 458, comprising Lithologic Units 1,2, 3, and part of 4. The abrupt change in magnetic incli-nations between Lithologic Units 3 and 4 was probablythe result of a reversal in the Earth's magnetic fieldbetween the times these two units were extruded (Bleil,this volume; Natland and Tarney, this volume). Withinthe boninite series lavas, Lithologic Units 1 and 3 aremainly pillows; Lithologic Unit 2 is a pair of fairly mas-sive flows. Lithologic Units 4 and 5 are pillows and thinflows.

The boninite series lavas of Lithologic Unit 4 differcompositionally from those of Units 1-3 (Table 1), hav-

ing more TiO2 and Zr at a given abundance of MgO(Wood et al., this volume; Bougault et al., this volume).Within Chemical Type A] (Lithologic Units 1-3) there issome chemical variation, but TiO2 abundances are solow (less than 0.4%) that this normally sensitive dis-criminant is useless for defining chemical units. Mostother major oxides are either poorer discriminants orare highly mobile during alteration, which is locally ex-tensive. Most of the lavas have MgO contents between5% and 7%. Those intervals with MgO greater than 8%in reasonably fresh samples are indicated by brackets tothe right side of the lithologic column (Fig. 2). Samplesfrom the upper flow of Lithologic Unit 2 have unusuallyhigh SiO2 (57-59%) coupled with low MgO (4.5-5.5%)leading to high normative quartz (8-10%; see Table 1).It seems likely therefore that Chemical Type A j consistsof several chemical subtypes, but their boundaries havebeen obscured by alteration (see Site 458 report, thisvolume).

Hole 459B consists of two major basaltic subtypes,Bj and B2, which differ in that Bj has higher MgO con-tents and lower Fe2θJ contents than B2. Still finer com-positional subtypes can be defined on the basis of smallchemical differences (see Table 2 and Site 459 Report,this volume). Subtype B1 A is actually compositionallyvery similar to Subtype A2 of Hole 458 but consists offlows rather than glassy pillows and lacks orthopyrox-ene; hence it was not identified as belonging to the bo-ninite series. Alteration in Hole 459B is more extensivethan in Hole 458 and could very well have destroyed anyorthopyroxene phenocrysts present.

The various B2 subtypes in Figure 2 show variable en-richment in Fe2θJ and TiO2. This apparently influencedthe formation of abundant titanomagnetite in the lavas,producing high intensities of magnetization (Bleil, thisvolume). The two most iron-enriched lavas are B2 ofHole 458 and B2A of Hole 459B. Both have over 13.5%Fe2θJ (Table 1; see discussion in Natland and Tarney,this volume).

Petrography of Hole 458 Boninite Series Lavas

Because of the extensive alteration, the outer, glassi-est parts of boninite series pillows were not recovered inHole 458. Those samples which appear to have beenclosest to the outer edges of pillows occur in LithologicUnit 3 (Chemical Type Aj) and are fairly altered. Theycontain abundant clinopyroxene spherulites that usu-ally consist of small crystals surrounded by dense over-growths of tiny fibers in pale brown glass (Fig. 3A).Many of the small crystals have fibers attached mainlyfrom their ends. In rocks that crystallized at slightlylower cooling rates (farther into pillows), these fibrousattachments are shorter and less pronounced (Fig. 3B).In still other glassy rocks, acicular rather than spheru-litic clinopyroxene microlites are well developed (Fig. 3,C-F). These occur mainly near the top of LithologicUnit 1 (Type A ). In these rocks, there are also brownspherulites (Fig. 3, C and D) that contain sworls ofneedlelike Plagioclase and abundant vesicles. These arethe only places where vesicles occur in the glassy sam-ples. Within about 1 cm of the outermost portions of

684


Hole 458

Hole 459B

35'

14°

33°

21 =

37=

- 1 2 °

-12°

- 3 C

- 1 0 °

4700

4800

Dl j) flj

2 c .E

N < J £ R+12=

-12 C

- 12 C

-24°

+11°

+7"

High Mg BronziteAndesites

(Chemical Types A)

Pillows

Thin to massive

flows or sills

Inferred contact

. — _ _ ^ — N Possible contact

Sediments

\\ II, muddy

vitric tuff

claystone

Figure 2. Basement lithologic, chemical, and magnetic stratigraphy, Sites 458 and 459.

MagneticInclination

IM

inomalpolarity

reverse

polarity

no oriented

samples

Zones of IntenseFracturing

vesicle trains

the rock, where these isolated Plagioclase spherulitesfirst occur, they become so abundant that they coalesceand the entire zone of the pillow is highly vesicular. Mei-jer et al. (this volume) speculate that segregation of dis-solved volatiles into vesicles locally reduced the watercontent of the glass, allowing Plagioclase to crystallize.However, apart from the vesicles, these Plagioclasespherulites occur in precisely the same position and havethe same form as in weakly vesicular abyssal tholeiites

(e.g., Kirkpatrick, 1978; Natland, 1978, 1980). Thiscould mean that it was the crystallization of the Plagio-clase in the spherulites that forced the segregation ofvesicles, not the other way around.

The clinopyroxene microlites in these rocks have ter-minations which generally spread out into two or moredendritic or skeletal arms, a feature typical of crystalsthat grew at high rates of cooling (Fig. 3E). Other, lessabundant clinopyroxenes have several curved radiating

685

J. H. NATLAND

Table 1. Average of chemical types and subtypes of boninites and island arc tholeiites, DSDP Holes 458 and 459B.a

S i O 2

Tiθ2

F e 2 O 3

T

MnOMgOCaO

N a 2 OK 2 OP 2 O 5

Total

Mg/Mg + Fe

NiCrSrZr

YNb

QOr

AbAnNe

WoDi En

FsM . . EnH y Fs

01 I0

FaMtII

Ap

Ave.Type A ]Boninite

l ( 2 1 ) b

55.00.32

15.29.310.126.879.592.600.980.03

100.02

0.594

7 6212

105

3332

4.315.78

21.9826.90

0.00S.544.833.34

12.398.58O.fX)0.001.720.610.07

Sample458-29-2, 37 cm

Boninite2

52.00.36

14.410.230.129.718.702.630.880.01

99.01

0.653

76

216

103

30

2

3

0.005.19

22.2424.87

0.007.594.652.51

15.198.19

3.101.841.900.690,02

Ave. ofBronzite

Andesite FlowCores 32-35

3(6)b

58.50.31

14.29.120.115.629.792.480.960.04

101.13

0.550

61

185

103

32

8

5

10.865.67

20.9724.77

0.009.815.214.298.897.320.000.001.690.590.09

Hole 458

Sample458-40-2, 90 cm

Boninite4

52.80.35

14.910.240.098.977.982.471.030.01

98.86

0.634

97

290

104

38

67

0.826.08

20.8826.51

0.005.423.241.90

19.1511.220.000.001.900.670.02

TypeB]

Basalt5b

53.21.13

15.59.660.063.736.125.701.520.20

96.82

0.433

Ave.Type A 2

Boninite6(6)b

53.90.52

14.310.540.136.218.332.650.660.03

97.27

0.558

Ave.Type B 2

Basalt7(6)t>

52.01.10

14.013.630.116.696.553.310.750.09

98.23

0.493

Trace Elements (ppm)

1111

169

101

317

0.008.97

44.2412.212.157.023.193.770.000.004.305.611.792.150.46

74

167

11049

15

4

29

33

139

62

24

5

CIPW Norms 0

6.253.90

22.4125.16

0.006.653.492.96

12.1010.260.000.001.950.990.07

0.914.43

27.9921.12

O.fX)4.492.222.18

14.5214.220.00O.fX)2.532.100.21

Ave.Type B 1 A

Basalt8<3) b

54.30.69

14.69.950.145.78

10.822.870.480.07

99.70

0.535

56153

11937

154

4.492.83

24.2725.52

0.0011.54

6.095.108.457.07O.fX)0.001.841.310.16

Ave.Type B I B

Basalt9(2)b

51.2

0.8314.310.670.149.648.473.310.620.09

99.27

0.641

6062

125

5329

6

0.003.66

27.9922.32

0.007.964.892.618.474.527.534.441.981.580.21

B2ABasalt

10

52.61.21

13.913.630.166.076.063.730.890.06

98.35

0.462

24

13

125

68

2^

6

0.525.25

31.5418.550.004.632.222.35

13.0713.860.00O.fX)2.532.300.14

Hole 459B

Ave.Type B 2 ß

Basaltl l ( 3 ) b

56.•7

0.9713.111.890.155.097.253.180.980.08

99.39

0.459

2315

1085323

2

8.565.78

26.8918.570.007.033.293.669.55

10.62O.fX)0.002.201.850.19

Ave.Type B2c

Basalt12(2)b

57.70.74

13.310.820.125.52^.633.151.290.08

100.35

0.502

2923

13049193

8.067.61

26.6318.330.007.923.963.789.909.450.000.002.011.410.19

Ave.Type B 2 D

Basalt13(2)b

55.70.91

14.311.010.115.408.253.710.910.10

100.40

0.493

2638

1355926

4

3.545.37

3 1 . 3 7

19.670.008.594.264.159.299.06O.CX)

O.fX)2.041.730.23

Ave.Type B 2 E

Basalt14(4)b

55.11.13

13.911.910.115.197.404.021.020.12

99.90

0.463

1416

1346932

5

2.196.02

33.9916.86

O.fX)

7.953.774.079.269.990.00O.fX)

2.202.150.28

a Compiled from data of Wood et al . (this volume),b Number of analyses in parentheses if more than one .c Calculated assuming Fe2 + / ( F e 2 + + Fe3 + ) = 0.86.

Table 2. Summary of phenocryst and groundmass crystal morphologies in Hole 458 boninite pillows.

Pillow Zones

groundmass

1

glass

2

augitespherulites +glass

— Toward Pillow Interiors —

3

skeletal augitemicrolites + isolatedPlagioclase spherulites

4

augite microlites +coalescedPlagioclase spherulites

augfanpla;

5

ite microlites +and bowtie

^ioclase spherulites

6

augite microlites +Plagioclase microlites

phenocrysts euhedral bronzite (no dendrites)augite with dendritesblocky augitecommon bronzite-augite intergrowths

(1) bowtie(2) bronzite with augite overgrowths

euhedral bronzite (some with dendrites)granular augite (rare dendrites)blocky augite rarerare bronzite-augite intergrowths, none

with overgrowths

axial projections (described as ''coxcomb" morphologyby Meijer et al., this volume) that are a form of spheru-lite. One of these is shown lower center in Figure 3E.

The most completely crystallized rocks of LithologicUnit 1 contain clinopyroxene microlites in a matrix offine but distinct needlelike Plagioclase crystals, arrangedin fans or sheafs. In Lithologic Unit 3, some inter-granular rocks with abundant Plagioclase are inter-bedded with the glassier clinopyroxene-rich rocks, butthe degree of alteration makes it uncertain that theseoriginally had closely similar compositions. No com-plete flows or pillows with textures ranging from spher-ulitic to intergranular were found among the LithologicUnit 3 rocks.

The acicular clinopyroxenes and spherulitic plagio-clases are the only minerals that appear to have formedduring the final stages of pillow extrusion. All otherminerals in the rocks are microphenocrysts that grewprior to pillow extrusion, forming distinctive dendritic

projections along their major crystallographic axes (Fig.4A). Orthopyroxenes, which have bronzite composi-tions, tend to be euhedral in glassy and spheruliticsamples or complexly intergrown with augite. They haveno dendritic or other overgrowths in quenched glassysamples but can have dendritic projections in samplescooled somewhat more slowly (Fig. 4B).

Microphenocrysts of both augite and bronzite inLithologic Units 1 and 3 range from simple euhedralcrystals to complexly intergrown multicrystal aggre-gates. The latter are less abundant in Lithologic Unit 3than in Unit 1 and in the few thin sections prepared onboard the Glomar Challenger are also altered, or partlyaltered, to clays. Only two thin sections were preparedon board ship from the boninite series lavas of Litho-logic Unit 4 (Chemical Type A2). These were intenselyaltered, had only rare microphenocrysts, and consistedprimarily of clinopyroxene microlites surrounded byspherulitic fans of Plagioclase. The remaining discus-

686


0.1 mm 1.0 mm

1.0 mm 1.0 mm

0.1 mm 0.1 mm

Figure 3. Crystal morphologies is glassy boninites, Hole 458. A. Sample 458-40-1, 33-35 cm. Plane light.Sperulitic clinopyroxene in altered glassy matrix of Lithologic Unit 3 boninite pillow rim. B. Sample458-28-1, 64-67 cm. Plane light. Clinopyroxene microlites, many with fibrous sperulitic projections(dark) in glassy matrix (light), Lithologic Unit 1 boninite. C. Sample 458-28-1, 142-146 cm. Plane light.Clinopyroxene microlites in glassy Lithologic Unit 1 boninite pillow rim. Subparallel arrangement ofmicrolites may have been the result of the glass having been stretched plastically during pillow formation.Three vesicular Plagioclase spherulites are also shown. D. Sample same as C in plane light but at reducedmagnification to show augite microphenocrysts and larger vesicular Plagioclase spherulites. E. Sample458-29-1, 1-3 cm. Crossed nichols. Detail of clinopyroxene microlites, showing development of central001 grooves leading to bifurcation of the central crystal into two arms. A branched spherulite is at bot-tom. F. Sample 458-30-1, 101-104 cm. Crossed nichols. Center is an irregular augite microphenocrystwith dendritic projections. At lower right are two intergrown clinopyroxene microlites.

sion of microphenocrysts therefore applies to LithologicUnit 1 (Type A,) only.

Clinopyroxene microphenocrysts can have irregularextinction and blocklike morphology (Fig. 4C) pro-duced in part by what may have been a type of hetero-geneous nucleation (attachment of crystals along othercrystal faces, sometimes but not always in optical con-tinuity). A typical growth feature is to have blocklikecrystals attached to each other endwise, resulting inskeletal composite crystals (Fig. 4D). This type ofgrowth is reminiscent of the growth of certain types of

Plagioclase microphenocrysts in abyssal tholeiites (com-pare to Natland, 1978, fig. 8, A and D).

Most bronzites occur as single crystals. The one inFigure 4E has a twinned termination (left side). The thinbright lines that show up when the rest of the crystal isat extinction appear to be clinopyroxene exsolutionlamellae. Other bronzite crystals occur in bow tie inter-growths with clinopyroxene (Fig. 5, A and B). In glassysamples they are quite commonly irregular in shape withthin rims of clinopyroxene (Fig. 5C). Multicrystal inter-growths of bronzite and clinopyroxene show blocky

687

J. H. NATLAND

1.0 mm

gE•sg

<rf* •%

t - r

«

an•B **"

0.1 mm 0.1 mm

r •-

I L *

S v

0.1 mm 0.1 mm

Figure 4. Pyroxene phenocrysts and spherulitic groundmass in Hole 458 boninites. A. Sample 458-29-1, 1-3cm. Crossed nichols. Augite microphenocryst with dendritic spines growing from both ends. B. Sample458-28-1, 9-13 cm. Crossed nichols. Typical fine fibrous spherulitic Plagioclase growing betweenclinopyroxene microlites in one of the more crystalline boninites of Lithologic Unit 1. C. Sample458-30-1, 71-73 cm. Crossed nichols. "Blocky" augite microphenocryst in Lithologic Unit 1 boninite.Growth was by lateral attachment, causing parallel crystals, but upper-left crystal appears to have grownpartly by filling in of space between dendritic spines, such as those of Figure 4A. D. Sample 458-30-1,71-73 cm. Crossed nichols. Another blocky augite glomerocryst. Blocky additions at right end havetrapped groundmass material within the glomerocryst. E. Sample 458-28-1, 64-67 cm. Plane light. Bron-zite microphenocryst. Note "serrated" crystal termination on left side of crystal. F. Same sample as Ebut with nichols crossed. Thin clinopyroxene lamellae extend the length of the crystal up to the"serrated" crystal termination at the left end, where they stop.

growth morphology (Fig. 5, D and E), but in some ofthem blocky growth has given way (at the crystal ex-tremities) to development of thin dendritic clinopyrox-ene arms enclosing bronzite along 001 crystal faces (Fig.6, A and B). Pyroxene compositional variations in thistype of growth will be discussed later.

Finally, some clinopyroxene phenocrysts have ex-tremely irregular zoning (Fig. 6, C and D). This patternof growth appears to be a combination of blocky growthand dendritic extension, perhaps reflecting a range ofundercoolings.

In summary, the sequence of mineral crystallizationin these rocks is (1) bronzite, (2) clinopyroxene, (3) pla-gioclase, and (4) Fe-Ti oxides. Olivine and clinoensta-tite do not occur, nor do Fe-Ti oxides in glassy samples.Scattered Fe-Ti oxides occur, however, in the coarser-grained rocks of Lithologic Units 2 and 3. Meijer et al.(this volume) report that chrome spinel occurs as raretiny crystals surrounded by augite microphenocrysts.

Table 2 summarizes crystal morphologies in Litho-logic Unit 1 boninite series lavas. The six divisions ofcrystal morphologies shown are inferred to represent


0.1 mm 0.1 mm

"•"• > l - i t

1.0 mm 1.0 mm

Figure 5. Two-pyroxene intergrowths in Hole 458 boninites. A. Sample 458-28-1, 9-13 cm. Crossed nichols.Bow tie intergrowth of bronzite (longer crystal) and twinned augite. Note spherulitic Plagioclase inmatrix. B. Sample 458-28-1, 142-146 cm. Crossed nichols. Intergrowth of bronzite (wide crystal) andelongate augite, in hyalopilitic matrix with no Plagioclase. Crystals are smaller than in 5A and have lessregular terminations. C. Sample 458-29-1, 1-3 cm. Crossed nichols. Irregular-shaped bronzite rimmedwith augite. Note dendritic projections on skeletal bronzite, lower left. D. Sample 458-28-1, 142-146 cm.Crossed nichols. Intergrown clinopyroxene microphenocrysts with typical separations into two dendritic"arms." Microphenocryst at lower left has a bronzite core (darker gray at this orientation relative to ex-tinction) marginally "zoned" with narrow clinopyroxene borders which project into the glass beyond thebronzite. E. Sample 458-28-1, 142-146 cm. Plane light. Complexly intergrown pyroxene micropheno-crysts in a hyalopilitic matrix. F. Sample 458-28-1, 142-146 cm. Same as 5E, except that nichols arecrossed. The "blocky" growth pattern of the larger augitic portion of the intergrowth is revealed. Thetwo more slender arms also contain intergrown bronzite, shown in Figure 6A.

progressive distance from pillow rims to pillow inte-riors, hence decreasing undercooling, of rocks that areessentially identical chemically. The zones correspondapproximately with the pillow zones of Kirkpatrick(1978). The ranges in chemistry within Lithologic Units1 and 3 probably would not affect these relations otherthan perhaps to modify abundances of micropheno-crysts. For example, it is probably that the more mag-nesian/res7i lavas of Chemical Type A in Core 29 (Fig.2) have more bronzite phenocrysts than other lavas ofthis general chemical type. Table 2 illustrates that apart

from increasing crystallinity of the groundmass andabundance of groundmass Plagioclase, decreased under-cooling modifies microphenocryst relationships consid-erably. The abundance of intergrowths is diminished,and there is a clear tendency for individual phenocrystsrather than clumps to form. Dendritic overgrowths donot occur on clinopyroxene but can on bronzite. Theserelationships favor the interpretation that the micro-phenocrysts themselves grew at conditions of height-ened undercooling rather than near equilibrium in aslowly cooled magma chamber. Probably they formed

689

J. H. NATLAND

fi Λ «,.... ••

0.1 mm 1.0 mm

1.0 mm 1.0 mm

1.0 mm 1.0 mm

Figure 6. Two-pyroxene intergrowths in glassy boninites and groundmass textures in massive bronziteandesites. A. Sample 458-28-1, 142-146 cm. Crossed nichols. Detail of the lower arm of the intergrowthshown in Figure 5, E and F. Outer portions of " a r m " (augite) appear to combine a "blocky" growthmorphology (nearer the intersection with the larger cross-arm) and separation into two quasi-dendriticbranches, separated by bronzite. B and C. Sample 458-28-1, 9-13 cm. Crossed nichols. Two views of asingle complexly intergrown glomerocryst of augite and bronzite, at slightly different angles. The extinctinterior of the glomerocryst in B is bronzite, the rest augite. Turning the stage to C puts other augiticportions of the glomerocryst into extinction but brings bronzite and some augite out of extinction. D.Sample 458-33-1, 9-13 cm. Crossed nichols. Subophitic bronzite andesite with elongate euhedra of augiteand some of Plagioclase in a matrix consisting mainly of irregular anhedral Plagioclase. E and F. Sample458-31-1, 2-7 cm. In plane light and with nichols crossed, respectively. Spherulitic interstitial Plagioclasesurrounding Plagioclase euhedra in groundmass of bronzite andesite. The radiating pattern is accen-tuated by inclusions of a needlelike mineral (quartz?) with higher relief.

as the lavas were extruded and continued to grow asthey flowed, prior to budding of pillows. It is also prob-able, therefore, the composition of the fresh lavas isclose to that of the liquid—without accumulations ofphenocrysts.

The coarse-grained massive flows of Lithologic Unit2 share some of the petrographic features of the pillows.Primarily, the crystallization sequence is the same—bronzite, clinopyroxene, Plagioclase, and Fe-Ti oxides.However, apart from euhedral orthopyroxenes, it is dif-ficult to identify phenocrysts that are discernibly larger

than the augites and feldspars surrounding them. Thesemassive rocks, however, have a more fractionated com-position than most of the pillows (Table 1), hence theircrystallization cannot strictly be linked to the sequenceof decreasing undercooling of Table 2. One importantfeature of these massive flows is that they evidentlyunderwent localized in situ magmatic differentiation.The principal evidence for this is the occurrence ofcoarse patches of spherulitic Plagioclase enclosing morecalcic euhedral Plagioclase laths (Fig. 6, E and F). Thelaths themselves have spherulitic terminations, and the

690


patchy feldspar surrounding them is intergrown with aclear mineral that may be quartz, arrayed in a crudelyradial pattern in Figure 6, E and F. In some coarse-grained samples, there are fairly large crystals of freequartz, irregular in shape, associated with similar patchyfeldspar, and opaque minerals. This is in accord with thehigh SiO2 (58-59%) and normative quartz (8-10%) of themassive rocks (Table 1).

MINERALOGY OF THE HOLE 458BONINITE SERIES LAVAS

To elucidate the effects of rapid cooling on the bo-ninite series pillow lavas of Hole 458, electron micro-probe analyses of orthopyroxenes and clinopyroxeneswere obtained from a glassy sample having particularlyprominent two-pyroxene intergrowths. The data were ob-tained on an ARL 9-channel electron microprobe at theSmithsonian Institution and are listed in Table 3.

In all cases, the orthopyroxenes are bronzites2 andthe clinopyroxenes augites. The normalized pyroxeneend-members, calculated from structural formulae inTable 3, are plotted on a pyroxene quadrilateral (Fig.7A). Several distinctions are made among the plottedaugites according to their occurrence as individual crys-tals (microphenocrysts or microlites) or as parts of two-pyroxene intergrowths. It is readily apparent that mostof the bronzites are very nearly identical in composition,regardless of their occurrence as individual crystals or incrystal aggregates. The two analyses that fall outside thegrouping have somewhat higher structural Wo and Fs,which may reflect partial involvement of the probebeam with clinopyroxene and glass, respectively.

In contrast, there is a clear compositional differencebetween augite crystals, and augite borders on bronzite,the latter being both more calcic and iron-rich. The bor-ders are also richer in tetrahedrally coordinated alumi-num (Alz) and Ti, as shown on Figure 8.

I interpret these data to mean that the augite bordersowe their compositions to growth from glass enriched inCa, Fe, Al, and Ti caused by the prior growth of thebronzite cores. This could have been a disequilibriumeffect caused by reduced diffusion rates of polymerscontaining these elements in the glass occasioned byheightened undercooling and increased melt viscosityduring pillow extrusion. Probably the growth of thecrystals and their borders was nearly simultaneous. Insome cases, the augite borders extend somewhat beyondthe ends of the bronzite cores (Figs. 4D and 5A), a mor-phology comparable to the dendritic projections onaugite microphenocrysts (Fig. 4A).

For comparison, the data of Sharaskin (this volume)and Meijer et al. (this volume) are plotted on Figure 7B.Their samples include several from the massive flows ofLithologic Unit 2 (Cores 31 and 33). Augites reported bySharaskin (this volume) from these intervals form sub-ophitic intergrowths with Plagioclase and are normallyzoned. The sense of that zoning, as indicated on Figure

The bronzites have more than 5% Wo in their structural formulae, hence they resemblemagnesian pigeonites. However, they are too magnesian, strictly speaking, to be so classified,so I refer to them here as bronzites (see Fig. 7C).

7B, is toward less calcic, more iron-enriched composi-tions than the augite borders on bronzites of Figure 7A.This is presumably a consequence of reaction with suc-cessively fractionated interstitial melt compositions dur-ing in situ magmatic differentiation. The orthopyrox-enes reported by Sharaskin (this volume) and Meijer etal. (this volume) are also bronzites, very similar to thoseof Table 3, although in both reports they were fromsamples of Chemical Type A2 (Lithologic Unit 4).

The type of crystal intergrowth discussed here hasbeen noticed previously in boninites from other local-ities. Dallwitz et al. (1966) described it as "marginalzoning" in samples from Cape Vogel, and presented mi-croprobe data for augite mantling clinoenstatite. Cam-eron et al. (1979) figured a boninite from the Troodosophiolite, with augite microlites identical to those de-scribed here, and an orthopyroxene microphenocryst"jacketed" by augite. This type of crystal intergrowththerefore seems to be a characteristic feature of glassyboninitic lavas. On the basis of the data presented here,augite borders on Mg-Fe pyroxene evidently occur be-cause of changes in the properties of melts near growingMg-Fe pyroxenes at the marked undercoolings sustainedby the margins of extruding pillows.

Other types of marginal zoning have been describedfor more mafic boninites, including rims of clinoensta-tite on olivine, bronzite on olivine, bronzite on clinoen-statite (Komatsu,1980), clinoenstatite on bronzite, andaugite on clinoenstatite (Dallwitz et al., 1966; Kuroda etal., 1978; Komatsu, 1980). Some of these, such as feath-ery terminations of clinopyroxene on clinoenstatite mi-crolites described by Dallwitz et al. (1966), clearly alsooriginated in response to high undercooling. Komatsu(1980), however, inferred a reaction relationship be-tween clinoenstatite and olivine. In some of the sampleshe examined and in samples described by Kuroda et al.(1978), the bordering compositions are more magnesianthan the core minerals (i.e., clinoenstatite rims on bronz-ite). Komatsu suggested this effect was caused by an in-crease of magma temperatures upon extrusion becauseof an oxidation reaction (e.g., Smith, 1969). Clearly,pyroxene interrelationships in boninites are complex.The data and arguments presented here point to the im-portance of kinetic factors during rapid cooling as thecause for at least some of these features.

Bougault et al. (this volume) presented microprobedata for pyroxenes in a remarkable specimen from Core44, Section 1 (Chemical Type A2), plotted here as Fig-ure 7C. In this specimen, there is a wider range of bothortho- and clinopyroxene compositions, including theoccurrence of strikingly iron-enriched ferroaugite andferrohedenbergite phenocrysts. Bronzite phenocrystsare similar to those of Table 3, but orthopyroxene mi-crolites are more calcic, with some either forming rimson, or being rimmed by, iron-rich clinopyroxene.

These iron-rich clinopyroxene phenocrysts may beunique among lavas. They are known elsewhere onlyfrom granophyric members of ferrogabbros (i.e., theSkaergaard intrusion; Wager and Brown, 1967) and inquartz syenites, alkali granites, and shonkinites, wherethey are commonly associated with fayalite (Deer et al.,

691

Table 3. Electron microprobe analyses of pyroxenes in boninite Sample 458-28-1, 142-146 cm.

SiO2

A12O3

FeOMgOCaONa2OK 2 OTiO2

SiAlAlFeMgCaNaKTi

49

49.078.48

16.7822.11

3.370.130.050.21

100.20

l 7 9 3 l 2 000.207r•W

0.1540.5131.2040.1320.0090.0020.006

2.024

W o 7.1 E n 65.1 F s 27.7

54

53.381.698.98

27.526.140.070.010.09

97.88

1.940l2 ( X )

o.oeor 0 0

0.0120.2731.4910.2390.0050.0010.003

2.024

Won.9En74 4Fsi3 g

55

54.461.549.04

29.163.240.050.010.04

97.91

1 9 6 5 l 2 00o.θ35r•w

0.0310.2731.5690.1250.0040.0010.001

2.004

W0 6.4En79. 8Fs 1 3.9

57

52.662.93

10.0628.78

3.220.100.010.16

97.91

1.9080.0920.0330.3051.5540.1250.0070.0010.004

2.00

2.029

Wo6.3En7 8.3Fs15.4

62

54.891.199.80

30.022.670.050.010.07

98.64

Orthopyroxenes

Analysis Numbera

63

54.222.949.46

29.662.970.050.010.10

99.25

66

54.621.71

10.6428.93

2.960.050.010.09

98.95

Structural Formulae Computed on the Bas s of 6(O)

'•962hnnn0.038/0.0120.2931.6000.1020.0040.0010.002

2.014

Wo5.iEn8o.2Fsi4.7

O ^ T I 2 ' 0 0 0

0.0460.2811.5680.1130.0030.0010.003

2.015

Wo 5 . 6 En 7 8 .3Fs 1 5 .4

1.9550.0450.0270.3191.5430.1140.0040.0010.002

2.000

2.010

w°5.8En78.1Fsi6.1

67

54.811.60

10.8729.312.690.050.010.08

99.36

1.9540.0460.0210.3241.5580.1030.0040.0010.002

2.000

2.012

w °5.2 E n7 8 .5Fsi6.3

68

53.992.51

10.7829.11

2.660.050.010.13

99.18

o9m)2xm

0.0350.3221.5500.1020.0040.0010.004

2.018

Wo 5 . 2 En 7 8 . 5 Fsi 6 .3

69

54.981.59

10.9929.142.840.050.010.08

99.62

1.9560.0440.0230.3271.5460.1080.0040.0010.002

2.000

2.011

Wθ5.5En 7 8. 0Fsi 6.5

72

54.541.57

10.8828.992.800.050.010.05

98.83

1.9560.0440.0220.3261.5500.1080.0040.0010.001

2.000

2.012

w °5.5 E n 78.1 F s 16.5

73

54.602.94

10.0029.12

2.700.050.010.08

99.44

0.0590.2971.5390.1030.0030.0010.002

2.004

Wo5,3Eπ79.4Fsi5.3

a Orthopyroxenes are microphenocrysts or parts of two-pyroxene intergrowths. 49 and 54 may show effects of partial overlap of the probe beam with glass and clinopyroxene,° Clinopyroxene Analyses 50, 58, 70, 71, and 74 are of rims of bronzite. All others are cores to larger microphrenocrysts or grains in two-pyroxene intergrowths.

spectively.

Table 3. (Continued).

Clinopyroxenes

Analysis Numberb

58

53.872.336.95

19.4517.420.050.010.37

100.45

1.9480.0520.0470.2101.0480.6750.0040.0010.010

w°34.9Er•54.2Fs10.9

49.807.536.46

16.1215.31

1.470.070.76

97.52

1.85210.14810.1820.2010.8930.610 2.0160.1060.003

2.000

52.212.667.84

17.6118.040.140.000.14

98.64

1.9380.0620.0540.2440.9750.7180.0100.0000.0050.021)

W035.8En52.4Fs1 i.g W037.1E1151.3FS12.6

53.002.007.64

20.1515.120.140.020.09

98.18

1.9580.0420.0450.2361.1090.5990.0100.0010.003

52.212.196.79

18.7917.700.150.040.10

97.97

5.609.36

15.8617.960.160.010.34

102.05

52.982.106.68

19.3217.870.110.010.11

99.18

Structural Formulae Computed on the Basis of 6(O)

1.9420.0580.0380.21:11.0420.7060.0110.0020.003

2.000 1.8640.1360.1090.2900.8760.7130.0120.0010.010

2.000

2.011

1.9440.0560.0350.2051.0570.7030.0080.0010.003

53.462.186.89

19.2317.470.150.080.10

99.56

1.9530.0470.0460.2111.0470.6840.0110.0040.003

46.829.49

12.5911.2118.490.170.040.55

99.36

1.7720.2280.1960-.3990.6330.7500.0130.0020.016

48.317.63

11.9813.4317.980.220.010.38

99.94

48.206.12

10.3115.0318.370.290.020.31

98.65

1.809 I Mil0.191 - "0.1460.3750.7500.7220.0160.0010.011

2.021

1.82210.17810.0950.3260.8470.7440.0210.0010.009

Wθ35.8En53.8Fs1 0.4 W042.1E1135.5FS22.4 w°39.lEn40.6FS20.3 w°38.8E n44.2Fsi7.0

Sample 458-28-1142-146 cm

× augites+ high-AI augite crystal©high-AI augite rims on broi

augite J

Bon mple,£. Hole 458, Bougault et al.* this volume). Core 44 Type A 2

• phenocrysts©core!

rim I r n ' c r o P r i e n o c r y s t s

+ microlites

Other Hole 458boninites

Clinopyroxenes

x459β} M e i i e r e t a l

+ 459B This study

1) field of augitephenocrysts, boniniteSample458-28-1. 142-146 cm.this study.

2) field of Hole 458boninite augites fromMeijer et al. andSharaskin, both in this

^volume.

Figure 7. Pyroxene quadrilaterals comparing compositions of bronzites and augites in Sample 458-28-1, 142-146 cm (A), with pyroxenes reported in Meijer et al. (this volume) and Sharaskin(this volume) (B), and in Bougault et al. (this volume) (C). Also plotted on A are pyroxenes from boninites of Chichi-jima, Bonin Islands (Kuroda et al., 1978). Compositional boundariesfor calcic pyroxenes are shown on C. D is augites from tholeiitic basalts, both this study and Meijer et al. (this volume). Fields on D are those of augites from A (field 1) and B (field 2). o

%rO?o-oXO

OOm

J. H. NATLAND

U.U24

0.022

0.020

0.018

0.016

0.014

Ti 0.012

0.010

0.008

0.006

0.004

0.002

rλ /s•\/ \ \

\ \\ / \

\ / \

\ / 458 \

\ / Boninite \\ /\ / augite borders o459B \ / .\ / o n b r o n z i t e

i \ /I × l /I ® 7 / /

/458 1 /

/

o /

\® /

\

458

1 J>Z/ boninite augitephenocrysts

i i

Boninites Hole 458• augite phenocrysts

and microphenocrystsO augite borders to

bronzite

This study only

Arc Tholeiites0 458θ459B 1 A

®459B9

Meijer et al.

(this volume)x 459B2

This study

0.10 0.20 0.30

Al

Figure 8. Ti versus Al z from pyroxene structural formulae. Data are from Tables 3 and 5, Meijer et al.(this volume) and Sharaskin (this volume).

1963, p. 68). They do not occur in calc-alkalic ortholeiitic lava suites of Japan (Kuno, 1968) or the Boninand Mariana islands (Shiraki et al., 1978). The occur-rences in coarse-grained rocks are generally thought toreflect circumstances of very low oxygen fugacity (Os-borne, 1959, 1962, 1979). Such conditions would not beexpected in magmas as hydrous as boninites generallyseem to be. Dowty et al. (1974) report similar iron-richclinopyroxenes in lunar pyroxene-phyric basalts, butthey are not phenocrysts and, since they occur in thegroundmass with metallic iron, they clearly formed atlow oxygen fugacities.

Glass compositions in the sample analyzed by Bou-gault et al. (column A of Table 4) compare generally toglass compositions obtained in other samples from Hole

458 (columns B-D), including glass from Sample 458-28-1, 142-146 cm, for which pyroxene analyses aregiven in Table 3. All of these glasses have andesitic com-positions comparable to a boninite series hyperstheneandesite from the Bonin Islands (column F) reported byKuroda et al. (1978). The pyroxene mineral relation-ships of Figure 7C differ so greatly from those shown inA and B that some unusual process has obviously oc-curred. With no data other than the mineral composi-tions for evidence, it would seem that magma hybridiza-tion between boninite and an iron-rich tholeiite grano-phyre has occurred, with incomplete mixing of two meltcompositions providing two sharply contrasting do-mains of pyroxene crystallization within the same rock.If this interpretation is correct, it is also evidence for the

694


Table 4. Electron microprobe analyses of glasses in boninite pillowfragments, DSDP Hole 458, compared with silicic glass and lavasfrom the Bonin Islands.

SiO2

A12O3

FeO*MgOCaONa2OK2OTiO2

P 2 O 5

Mg/Mg + Fe

QcOrAbAn

(WoDWEn

iFsIT /En

^ I F SMt11Ap

A

62.1517.595.451.566.721.07a

0.490.260.05

95.34

0.338

33.943.202.899.05

32.990.000.000.003.877.651.130.500.12

B

56.8713.867.676.629.801.800.400.29—

97.31

0.606

c

58.6415.388.024.568.741.840.350.520.17

98.34

0.503

CIPW Norms

13.360.002.36

15.2228.54

8.364.803.190.000.001.580.550.00

18.580.002.07

15.5632.653.991.981.930.000.001.650.990.39

D

61.7218.864.260.645.242.530.560.330.11

94.26

0.211b

30.324.833.31

21.3925.260.000.000.001.595.790.880.630.25

E

59.3416.636.191.506.343.600.560.08—

94.33

0.302

17.130.003.31

30.4427.55

1.620.491.203.237.871.270.150.00

F

62.7314.996.491.695.992.390.890.260.02

95.45

0.317

26.340.005.25

20.2127.530.850.280.603.928.611.330.500.05

G

65.7912.545.621.564.543.090.900.310.04

94.39

0.331

29.820.005.31

26.1317.68

1.910.651.313.226.521.160.590.09

Note: A, Sample 458-28-1, 142-146 cm, glass. B, Sample 458-39-1, 23-26 cm,glass (Meijer et al., this volume). C, Sample 458-43-2, 34-37 cm, glass (Meijeret al., this volume). D, Sample 458-44-1, 53-56 cm, glass (Bougault et al., thisvolume). E, Glass in boninite pillow, Tsuri-hama, Chichi-jima, Bonin Islands(Kuroda et al., 1980). F, Hypersthene andesite, near Omura Pier, Chichi-jima, Bonin Islands (Kuroda et al., 1980). G, Perlitic dacite, SE of Miyano-hama, Chichi-jima, Bonin Islands (Kuroda et al., 1978).

a Low Na2O result of volatilization. New analysis by J. W. Hawkins hasNa2O = 2.29<%.

b Computed assuming Fe2 + /Fe2 + + Fe3 + = 0.86.

simultaneous availability of boninite and arc tholeiitemagmas in the fore-arc region.

GEOTHERMOMETRY OF BONINITESBecause the phase relationships of coexisting or-

thopyroxenes and clinopyroxenes are reasonably wellknown, it is possible to estimate the temperature ofcrystallization of these minerals if both occur in thesame rock. Here, the equation of Wood and Banno(1973) is used, as modified by Ishii (1980). The alter-native modification of the Wood-Banno equation pro-posed by Wells (1977) produces almost identical results,to within a few degrees.

In a quenched pillow margin, because of rapid cool-ing rate, one might expect significant departures fromequilibrium crystallization and hence in mineral compo-sitions. We have seen, for example, that the composi-tions of augites in two-pyroxene intergrowths in a glassysample are affected by the prior growth of bronzite.Such effects would make estimates of crystallization tem-peratures unreliable, based on mineral compositions. Inthis particular sample, choice of the wrong augite couldresult in temperature estimates low by over 100°C. Aglassy rock may also have crystallized through a par-ticular temperature range forming phenocrysts beforequenching, which would produce a range of mineralcompositions accurately reflected in a calculated rangein the temperatures of crystallization. Here, to avoid the

effects of kinetics of crystal growth, augite borders onbronzites have been excluded from the calculations.Also excluded are any grains which, on the basis of probeanalyses, seem spuriously contaminated with glass oranother mineral phase. The remaining mineral analysesare of larger microphenocrysts which are presumed toapproximate crystallization nearer to equilibrium condi-tions. Ranges of compositions within these remainingaugites and bronzites are assumed to reflect the combinedeffects of analytical precision and small degrees of crys-tallization fractionation.

Using the ratio Mg/Mg + Fe computed from thestructural formulae of Table 3 as an index of fractiona-tion, calculations were made for bronzite-augite pairshaving both the highest and lowest values of this ratioamong the analyzed pyroxenes (analysis pairs 55-53 and69-52, respectively, of Table 3). These give temperaturesof 1319°C and 1198°C, representing the possible max-imum range of crystallization temperatures for the phe-nocrysts analyzed in the sample. Using the averagebronzite and augite compositions gives a temperature of1251°C.

One can compare these estimates of crystallizationtemperature with those of Ishii (1980) from lavas drilledduring Leg 59 on the Palau-Kyushu Ridge (Site 448) andthe West Mariana Ridge (Site 451), both older, riftedfragments of the Mariana arc. Ishii (1980) based his esti-mates on a variety of pyroxene geothermometers. On aplot of the ratio Mg/Mg + Fe in orthopyroxenes (XFe)versus crystallization temperature (Fig. 9), Ishii showedthat clear differences exist in calculated temperatures ofcrystallization of pyroxenes in lavas of the two sites,with samples from the Palau-Kyushu Ridge havinghigher temperatures. He described the samples fromSite 448 as "high-temperature tholeiites" and thosefrom Site 451 as "low-temperature tholeiites" (twosamples) and a calc-alkalic andesite (lowest temperaturesample plotted on Fig. 9). It is evident from the figurethat the boninite sample from Site 458 has much highertemperatures of crystallization than either tholeiitic ba-salts or calc-alkalic andesites from other portions of theMariana arc, or Recent Japanese lavas from Hakonevolcano. Temperatures of crystallization estimated frompyroxene compositions of other boninite-type lavas arealso high. Based on data in Sharaskin et al. (1980), thetemperature of crystallization of pyroxenes in a "mari-anite" sample dredged from the Mariana trench is1128°C. Using pyroxene analyses in Kuroda et al.(1978) for a boninite from Chichi-jima in the BoninIslands (plotted on Fig. 7A), I calculated an upper tem-perature of crystallization of 1136°C. Crawford (1980)calculated a range of 1300°C to 1150°C for 15 pairsof crystals in a cumulate clinoenstatite-bearing olivinepyroxenite sill in Victoria, Australia, using the Wells(1977) modification of Wood and Banno's equation.This particular boninitic material appears to be derivedfrom an extremely refractory source, since Ti, Ca, Al,and Na are extremely low, lower than in any of the otherexamples cited here. The similar range of crystallizationtemperatures of the Site 458 lavas and the Australian sillresult from nearly identical compositions of both pyrox-

695

J. H.NATLAND

high

1300

1200

® average

• low

boπiniteChichi-jimi,Bonin islands

boninite Sample458-28-1, 142-146 cm

Pigeonite eutectoid reaction line (Ishii and Takeda, 1974)

% 1100

1000

Hakone calk-alkalicandesite

A Fe(opx)

Figure 9. Temperatures of crystallization of coexisting orthopyroxene and clinopyroxene in western Pacific island arc lavas drilled during Legs 59and 60, plotted against X F e ( o p x ) ( = Fe/Mg + Fe in the orthopyroxene). Diagram and Leg 59 data from Ishii (1980). Fields are for Site 448 (Palau-Kyushu Ridge) and Site 451 (West Mariana Ridge) drilled during Leg 59. Boninites plotted are Sample 458-28-1, 142-146 cm of this study (high,low, and average temperatures computed from data of Table 3) and Sample Bl, boninite center of pillow, Tsuri-hama, Chichi-jima, BoninIslands (Kuroda et al., 1978, tables 2 and 4). The difference in temperature of crystallization results from differences in augite compositions (Fig.7A). Both boninites plot at temperatures as high as or higher than arc tholeiites and calc-alkalic andesites from Sites 448 and 451, as well as fromHakone volcano, Japan.

enes in the two sets of samples. The lower computedtemperatures for the marianite and the Chichi-jimasample result primarily from their having more iron-richaugites (Fig. 7A); coexisting bronzites have nearly iden-tical compositions in all the suites. The result of this isthat the seemingly more fractionated olivine-free bonin-itic lavas of Hole 485 have high-temperature pyroxenessimilar in composition to those in some extremely re-fractory boninites carrying olivine and clinoenstatite(the Australian sill). But other lavas which are mineral -ogically less fractionated, the olivine- and clinoenstatite-bearing marianites of Sharaskin et al. (1980), havelower-temperature bronzite-augite pairs than the Hole

458 boninites. Evidently, the occurrence of neitherclinoenstatite nor olivine has much relationship to thetemperatures of crystallization of augite and bronzite inthese lavas.

The cause of this seemingly contradictory state of af-fairs is not obvious. The calculations for the marianiteand Chichi-jima sample were based on limited publishedmicroprobe data (the most magnesian of only two orthree bronzite-augite pairs apiece were used). A morethorough data set might have produced more consis-tency among the calculated temperatures for the variousboninites. Probably, however, the compositional dif-ferences among the augites are genuine and result from

696


inherent compositional differences in the host magmaswhich affect both the shape of the pyroxene solvus inthese magmas and the extent of the resulting pyroxenemiscibility gap. A fundamentally higher Mg/Mg + Feratio for a given Ca abundance in the Hole 458 lavas,for example, might have been involved. This would bedifficult to establish because of the effects of alteration.Alternatively, the abundance of minor components suchas Na2O, TiO2, or volatiles might be responsible. Ishii(1980), for example, has concluded that elevated /?(H2O)among calc-alkalic lavas (such as those of Hakone vol-cano on Fig. 9) produces lower calculated temperaturesof pyroxene crystallization at a given value of XFe (theratio Mg/Mg + Fe in orthopyroxenes) than among arctholeiites. This result, too, is a consequence of moreiron-rich clinopyroxenes coexisting with orthopyroxenesof a given XFe than in the arc tholeiites. On this reason-ing we could speculate that the Hole 458 and Australiansill samples were less hydrous than the marianites orChichi-jima boninites.

These possible complications aside, boninitic lavas ingeneral appear to have been high-temperature melts,with the lowest calculated temperatures of pyroxenephenocryst crystallization still as high as the maximumcalculated by Ishii (1980) for magnesian arc tholeiiteswith olivine and bronzite. This is consistent with therefractory compositions of the mantle sources for bo-ninites inferred from their overall compositions andwith the occurrence of extremely magnesian olivine(Fo92_94) and chrome-rich magnesiochromite in someof them (e.g., Cameron et al., 1979, 1980; Crawford,1980). This, coupled with low Na2O, TiO2, CaO, anddepleted rare earth element abundances (Wood et al.,this volume; Hickey and Frey, this volume), indicatesmantle sources that have already undergone previousepisodes of melting. Crawford (1980) invoked the two-stage melting hypothesis of Duncan and Green (1978) toexplain the boninite-arc tholeiite association such asthat found at Site 458. Rather than anhydrous melting,however—which produces contemporaneous MgO-richand MgO-poor lavas such as komatiites and tholeiites—it is melting that occurred under hydrous conditionswhich produces the boninites. This has the importantpetrological effect of allowing silica-saturated residualmelts to be derived from olivine normative parents (thatis, allowing the olivine-orthopyroxene reaction relation-ship, which is actually observed in olivine-bearing bo-ninites, to occur; e.g., Crawford, 1980). It also allowsmelting to occur at geologically reasonable tempera-tures. Crawford (1980) felt that the peridotite meltingexperiments under hydrous conditions of Mysen andKushiro (1977) produced reasonable analogs to bo-ninites at temperatures of 1500-1600 °C and pressuresof 20 kbar. The experiments were performed on a garnetlherzolite with 1.9% H2O in the charges. The range oftemperatures at which melting occurred was also con-sistent with the rather high temperatures of crystalliza-tion of pyroxenes calculated by Crawford to have oc-curred upon injection of his sill.

At odds with this, however, are experimental resultson the melting behavior of basalts and andesites at ele-vated p(H2O). Kuroda et al. (1978) cite the experiments

of Yoder and Tilley (1962) on basalts and of Eggler(1972) on andesites to argue that boninite melting tem-peratures need only be 1000-1100°C at/?(H2O) of 5-10kbar. Similarly, the liquidus temperatures of a magne-sian andesite (Kushiro and Sato, 1978) and a boninitefrom Hole 458 (Kushiro, this volume) drop to values aslow as these at similar water pressures. The latter ex-periments were done with 12-16% H2O in the capsulesto insure conditions of water saturation at elevatedpressures. Between 8 and 14.5 kbar, clinopyroxene inthe Hole 458 boninite was the experimentally producedliquidus phase at temperatures of only between 1025 °Cand 1075°C. The upper value is less by nearly 55° thanthe lowest value I have calculated for any boninite and isless by 175° than the average value calculated for thepyroxenes of the Hole 458 sample listed in Table 3. Ifthe calculated temperatures of crystallization are cor-rect, they cast into serious doubt the validity of the con-ditions of melting simulated by the experiments. Itseems unavoidable to conclude that melting of boninitesis not accomplished under water-saturated conditions atelevated pressure. However much water may have facil-itated melting, it did not suppress it by the nearly 500°indicated by the difference between melting tempera-tures of "damp" garnet lherzolite (Mysen and Kushiro,1977) and those suggested by water-saturated experi-ments done on the actual lavas. Clearly the precise con-ditions of boninite melting, and the phase relationshipsthat existed during that melting, are poorly constrainedexperimentally. Accurate statements about geothermalgradients, other than that they were probably high, arenot yet possible.

CLASSIFICATION OF THE BONINITE SERIES

Classifications of island arc magmas have been basedeither on mineralogy or chemistry. As examples of theformer, the pigeonitic and hypersthenic suites of Kuno(1968) come to mind. Their approximate chemical equi-valents are the island arc tholeiite series of Jakes andGill (1970) and the calc-alkalic series of Peacock (1931)and many subsequent workers. As discussed earlier, theboninitic lavas of Hole 458 are not petrographicallyequivalent to many boninites previously described. Forthis reason, Meijer (1980) and Meijer et al. (this volume)have proposed a chemical classification that includes ina boninite series both the Hole 458 rocks and previouslydescribed boninites. Because the Hole 458 samples lackolivine and clinoenstatite, however, and because theyare more evolved than lavas which carry these minerals,the Leg 60 shipboard party elected to call these rocks"high-MgO bronzite andesites" in the site chapters andin other chapters of this volume. This term denotes boththe chemical and petrographic features of the Hole 458lavas.

However, it is obvious that the fundamental miner-alogic and petrographic features of the Hole 458 lavasand other boninites are nearly identical. Moreover, thecompositions of bronzites and augites in the rocks de-scribed from Hole 458 are also nearly identical to thosein other boninites, notwithstanding the lack of olivineand clinoenstatite. Some of them also have MgO con-tents approaching those of boninites from other local-

697

J. H. NATLAND

ities. Others, though, have MgO contents so low (lessthan 4.5%) that they should not be grouped with rocksdescribed as "high MgO."

I propose that the earlier described examples of bo-ninite series lavas which contain olivine and clinoensta-tite be termed olivine boninites and that the Hole 458lavas be called boninites to emphasize that they lackolivine. The distinction would be analogous to the termsolivine tholeiite and tholeiite for the Hawaiian petro-graphic province (Macdonald and Katsura, 1962, 1964).Bronzite-bearing lavas with a high modal proportion ofPlagioclase and quartzose segregations in coarse-grainedsamples could then be described as bronzite andesiteswithout concern for the abundance of MgO. At Hole458, by this terminology, lavas range from boninites tobronzite andesites, with the boninites being the com-paratively MgO-rich lavas of Cores 29, 39, and 40 (Fig.2) and the bronzite andesites the massive lavas of Cores32-34. Intermediate lavas could be termed boniniticandesites, in a manner analogous to basaltic andesites.These distinctions have the advantage of utilizing a sim-ple mineralogical modifier to express real differences inchemical composition but leaving no doubt as to themagma type. They also avoid potential confusion withhigh-MgO andesites not related to boninites, such asthose described from the Aleutian arc (Kay, 1978) andJapan (Kushiro and Sato, 1978).

Other members of the boninite magma series de-scribed elsewhere include, at the mafic end, marianite,which is basically an olivine boninite rich in clinoensta-tite (Sharaskin et al., 1980), and at the silicic end, bronz-ite or hypersthene dacite (Kuroda et al., 1978). Amongknown examples of all these lava types, however, geo-chemical relationships are complex, implying a diversityof mantle source compositions and melting conditionsin the mantle (e.g., Hickey and Frey, this volume; Mei-jer, 1980). Probably no complete cogenetic boninite frac-tionation series has yet been sampled.

RECOGNITION OF NONGLASSY BONINITEFrom the preceding discussion, criteria for the identi-

fication of boninites in thin sections which are free ofglass can be summarized. The most important criteriaare (1) the occurrence of bronzite ± clinoenstatite micro-phenocrysts ± olivine; (2) the occurrence of augite-bronz-ite microphenocryst intergrowths and, perhaps addi-tionally, intergrowths involving bronzite, clinoenstatite,and olivine; and (3) the occurrence of tabular or skeletalaugite microlites, surrounded by later-grown, and usu-ally finer-grained, Plagioclase. Of these criteria, thethird is probably the most universal, since, as we haveseen, olivine and clinoenstatite phenocrysts do not occurin all boninites, bronzite microphenocrysts may be rare,and bronzite-augite intergrowths are not nearly so com-mon in coarse-grained as in fine-grained and glassy bo-ninites. It is this third criterion which clearly distin-guishes boninites from both basalts and typical calc-al-kalic andesites, since groundmass Plagioclase crystal-lizes first in both these rock types. The effect of highcooling rates near pillow margins amplifies the texturalcontrast between basalts and boninites. Whereas in most

abyssal tholeiites, particularly those with olivine, the ef-fect of extreme undercooling is first to suppress alto-gether clinopyroxene crystallization next to glassy rims,in boninites it is Plagioclase crystallization that is firstsuppressed. In both rock types, as undercooling in-creases toward pillow rims, crystals change morphologyfrom granular and euhedral to skeletal and finally tospherulitic forms. Minerals that form late in an equili-brium crystallization series progress through this se-quence before minerals that form early. In basalts, thisprogression occurs to clinopyroxene before it does toPlagioclase; in boninites, it occurs to Plagioclase beforeit does to clinopyroxene.

This contrast can be shown diagrammatically onschematic plots of temperature versus cooling rate. Forthe case of a Hole 458 boninite (Fig. 10A), the curvesshow the relative temperatures at which minerals crys-tallize at different cooling rates. The left side of the dia-gram corresponds to equilibrium temperatures of crys-tallization. Following each curve to lower temperatureand higher cooling rates, there are systematic changes incrystal morphology. At the left, along each curve,crystals are granular or euhedral, toward the midpointof each curve they are skeletal, and toward the bottomthey are spherulitic or, in some cases, dendritic. At anygiven cooling rate, corresponding to a particular dis-tance from the edge of a pillow or other cooling unit, aline traced upward from the bottom of the diagramintersects the mineral curves at different points; hencethe crystals assume different morphologies. Put anotherway, at any given cooling rate, the undercooling for,say, clinopyroxene, given by the difference between itsequilibrium temperature of crystallization (the left sideof the diagram) and its actual temperature of crystalliza-tion, is less than for Plagioclase. At cooling rates corre-sponding to spherulitic Plagioclase, then, clinopyroxeneforms skeletal or dendritic microlites, and magnetitedoes not crystallize at all. At cooling rates correspond-ing to spherulitic clinopyroxene, neither magnetite norPlagioclase forms.

For comparison, a similar plot for a Mid-AtlanticRidge aphyric olivine tholeiite is shown on Figure 10B,from Kirkpatrick (1978). The same general variations inmorphologies apply, but the contrasts with boninitesare that the Plagioclase and clinopyroxene curves areswitched, and olivine occurs instead of orthopyroxene.The various crystallization zones of Table *2 are sum-marized on Figure 10A. The corresponding basalt crys-tallization zones of Kirkpatrick (1978) are summarizedon Figure 10B. Together, Figure 10A and Table 2 con-stitute a general petrographic description for pillowedboninites. Somewhat different relationships would pre-vail for olivine boninites or marianites. For bronziteandesites, the clinopyroxene and Plagioclase curves ofFigure 10A would be closer together.

COMPOSITIONAL AND KINETIC CONTROLSON BONINITE CRYSTALLIZATION

Although many workers have discussed the mecha-nism by which boninitic melts can be derived from themantle, little attention has been given to how those melts

698


Massive Flow Interiors Pillow Interiors- Pillow Rims

Cooling Rate

o S.E£ ° o

^ c9" -

.2 -c

_° α a) 'σi o 01

, σ> × o

rà"? Q.S ra P ε E l

E O•Q. 9-

Eö× .-

o a ~ o.

Pillow Interiors Pillow Rims

Cooling RatePillow Zones

Figure 10. A. Schematic diagram showing deviations from the sequence of equilibriumcrystallization temperatures (left side of diagram) for minerals in Hole 458 boniniteas cooling rate is increased (to the right). The pillow zones of Table 2 are summarizedat the bottom of the diagram. The generalized changes in crystal morphologies forPlagioclase are shown next to that curve. Similar variations occur with the otherminerals, and in the same order, although not necessarily along precisely the sameportions of their curves. B. A similar diagram for a Mid-Atlantic Ridge olivinetholeiite from DSDP Site 396, after Kirkpatrick (1978). His pillow zones are in-dicated on the diagram.

699

J. H. NATLAND

produce the peculiar mineralogies and textures of bo-ninites. It seems obvious that crystallization of orthopy-roxene rather than olivine results from combining highSiO2 with high MgO and Mg/Mg + Fe (e.g., Dallwitz etal., 1966; Dallwitz, 1968; Crawford, 1980). It is not soobvious why clinopyroxene crystallizes ahead of Plagio-clase. The explanation appears to be related to high SiO2/A12O3 and low CaO/Al2O3 in boninitic magmas. Thelatter ratio averages 0.63 in Chemical Type A! boniniticlavas of Hole 458 and 0.59 in Type A2 boninites (cal-culated from data in Wood et al., this volume). Cam-eron et al. (1979) cite CaO/Al2O3 in some boninites aslow as 0.55. Similarly, the marianites of Sharaskin et al.(1980) have CaO/Al2O3 ranging between 0.40 and 0.63,with an average of 0.55. These compare with ratios be-tween 0.75 and 0.88 in typical basalts and in fact moreclosely resemble values in andesites than in basalts. Theresult of this is a propensity for boninitic magmas tocrystallize comparatively anorthite-poor (and thus lower-temperature) plagioclases, in spite of having very mag-nesian and high-temperature pyroxenes. Bougault et al.(this volume) report plagioclases in Hole 458 boniniteswith SiO2 between 50% and 54% and ranging in compo-sition between An77 to An63. These are similar to Plagio-clase compositions in fractionated basalts with lowerMg/Mg + Fe than boninites.

A fair approximation to phase relations in the Hole458 boninites is the system diopside-albite-anorthite(Bowen, 1915), and it can be used to illustrate the con-trast between basalts and boninites shown on Figure 10.Boninite compositions would correspond to those onthe diopside side of the cotectic boundary separating thediopside and Plagioclase fields (Fig. 11). Basalts wouldcorrespond to compositions on the Plagioclase side. Any

NaAISi- 20 40 60Weight (%)

80 CaAI2Si20g

Figure 11. Projection of the liquidus surface of the system Di-An-Abat 1 atm (Bowen, 1915; Kushiro, 1972b). The contrast betweenboninite and basalt crystallization sequences (Fig. 10) is approxi-mated by the difference in crystallization sequences between com-positions plotting on opposite sides of the cotectic boundary,boninite in the diopside field, basalts in the Plagioclase field.

liquid with less SiO2 and higher CaO/Al2O3 than bo-ninites would correspond to compositions closer to theanorthite corner of the diagram—that is, away from thediopside field—and would more closely resemble ba-salts. Compositions analogous to boninites in this sys-tem first crystallize diopside. Fractionation shifts resid-ual liquid compositions away from the diopside apexuntil the plagioclase-diopside cotectic is reached, whenceboth minerals crystallize together. With basalts, Plagio-clase crystallizes first, until residual liquids reach thesame cotectic boundary, and both Plagioclase and diop-side crystallize together.

High volatile contents, particularly the concentrationof H2O in the melt, might be thought to hinder the crys-tallization of Plagioclase, inasmuch as the liquidus curvein the system Ab-An is known to drop in temperature atelevated /?(H2O) (Yoder, 1969; Johannes, 1978). Meijeret al. (this volume) alluded to this possibility in consid-eration of the crystallization of Plagioclase spherulitesin Hole 458 boninite glasses. For such an effect to be ap-preciable, however, drops in water pressures of severalkilobars would be required. Yoder (1969) documented ashift of the cotectic boundary shown on Figure 11 to-ward the anorthite corner at water pressures of 5-10kbar. However, we are dealing with minerals whichformed in lavas extruded under no more than 3000-4000meters of water, and probably less. Judging from Yo-der^ data, this might change the sequence of mineralcrystallization if the original melt composition were veryclose to the cotectic boundary to begin with. In my view,the compositional differences between boninite andbasalt must be much more important than the concen-trations of volatiles in explaining observed mineralogi-cal and textural differences between the two. Basalts ofthe Mariana Trough such as those cored at Site 454 dur-ing Leg 60, for example, are quite vesicular. Basaltsdredged from the center of the Trough have mainly wa-ter in vesicles trapped in glass (Garcia et al., 1979).These basalts were all erupted in water depths of about3500 meters, yet all have typical basalt textures and min-eralogies. None have experienced a reversal in the pla-gioclase-clinopyroxene crystallization sequence.

There are two terrestrial lava types with textures simi-lar to boninites of which I am aware. Cameron et al.(1979, 1980) have pointed out textural and composi-tional similarities between boninites and Archean ba-saltic komatiites, although they stress that the latterwere generated under nearly anhydrous conditions. Likeboninites, the basaltic komatiites they describe haveaugite microlites and two-pyroxene intergrowths, but noPlagioclase, in glassy samples. But they are much moreolivine-rich and have CaO/Al2O3 ratios of about 0.8,similar to values in basalts. The appearance of clino-pyroxene before Plagioclase in these basalts must berelated only to their high MgO and low SiO2 rather thanlow CaO/Al2O3. On Figure 11, they would be approxi-mated by compositions in the diopside field but clos-er to the diopside-anorthite side of the diagram thanboninites.

The other terrestrial lava type with some texturessimilar to boninites is submarine examples of the nephe-

700


Unite series of oceanic volcanoes, namely basanites andolivine nephelinites. Pillows of these compositions havebeen dredged from a number of places, including theLine Islands seamount chain (Natland, 1976). Thesecarry (altered) olivine phenocrysts and abundant titan-augite microlites, but no Plagioclase, in glassy samples.In these lavas, suppression of Plagioclase crystallizationstems from low abundances of both SiO2 and A12O3.The lavas are highly undersaturated with respect toSiO2, partly also because of high alkali contents, hencecannot crystallize orthopyroxene. Although the systemDi-Ab-An is a rather poor analog for such rocks, onFigure 11 they would be approximated by compositionsalso in the diopside field but closer to the diopside-al-bite side than boninites.

One other close textural analog to boninites is thegroup of lunar quartz-normative, pyroxene-phyric ba-salts mentioned earlier and described by Dowty et al.(1974). In those rocks, phenocrysts are pigeonite (ratherthan bronzite) and augite. Augite microlites are abun-dant, and Plagioclase is absent in glassy samples. Incoarser-grained samples, Plagioclase crystallizes wellafter the appearance of both pyroxenes, and its nuclea-tion drastically modifies all subsequent pyroxenes tomuch less calcic compositions. These basalts experi-enced extreme cooling rates upon extrusion onto thesurface of the lunar maria, cooling rates matched natu-rally by few circumstances on the Earth's surface otherthan submarine pillow eruption. Like the boninites,these lunar basalts have augite overgrowths on low-Capyroxenes (pigeonite), which tend to have skeletal mor-phologies in glassy samples. Coarser-grained samplesof these lunar basalts even contain ferroaugite and sub-calcic ferroaugite, some of which have compositionssimilar to pyroxenes in the boninite studied by Bou-gault et al. (this volume), shown on Figure 7C. Thelavas are considerably richer in iron and TiO2 than bo-ninites, however, and contain virtually no water. As aresult, even the initial pyroxenes to form were compara-tively iron-rich magnesian pigeonites, not bronzites, andground-mass iron and titanium enrichments were ex-treme, leading to formation of interstitial metallic ironand ilmenite.

In an effort to model the crystallization of these lunarpyroxene-phyric basalts, Dowty (1980) applied numeri-cal techniques to natural composition and the syntheticcomposition Di6OAn30Ab10 that plots in the diopsidefield of Figure 11. His results bear on one aspect of thetextures in the Hole 458 boninites: the high modal pro-portion of clinopyroxene in glassy samples. Dowty con-sidered two cases: crystallization under conditions ofconstant decrease of temperature (1 deg/hr) and condi-tions of constant loss of heat (0.001 cal/s). Different ef-fects were observed in the two cases, but in both the liq-uid composition overshot the cotectic so that diopsidecontinued to crystallize even after the liquid was well in-to the Plagioclase field. In effect, the duration of theperiod of diopside crystallization was extended, andthat of Plagioclase diminished, by rapid cooling rates.Plagioclase began to crystallize only when the criticalundercooling necessary for its nucleation was reached.

This type of phenomenon would explain, for example,the increase in Al contents of clinopyroxenes in Hole458 boninites (Figs. 7A and 8) and the siliceous, essen-tially dacitic composition of the glass from the samesample (Table 4, column 1). The modeling also sug-gested that the initial stages of crystallization should bedominated by homogeneous or quasi-homogeneous nu-cleation, which would produce mainly individual crys-tals with a limited range in size (i.e., the clinopyroxenemicrolites of Fig. 4). In olivine tholeiites, however, somany Plagioclase crystals have formed by the time clino-pyroxene crystallizes that nucleation of pyroxene is pri-marily heterogeneous, producing rows of parallel andclosely spaced dendrites with faceted crystal termina-tions, all attached at their other ends to earlier-formedcrystals (see Natland, 1980, fig. 5B).

A final question concerns the development of por-phyritic texture in the Hole 458 boninites. Until re-cently, conventional wisdom held that porphyritic tex-ture was a consequence of two or more stages of cool-ing. But there is now fairly considerable evidence thatporphyritic texture can be produced by a single-stagecooling event (i.e., cooling at constant rates; see sum-mary by Lofgren, 1980). The critical requirement is thatthe magma not be multiply saturated—that is, that theliquidus phase be modally dominant and crystallize byitself before introduction of any other modally signifi-cant phase, as, for example, augite before Plagioclase inboninites. When the second phase finally begins to crys-tallize, so many nuclei form that the sizes of new crys-tals of the first phase decrease abruptly, producing arock with two sizes of that mineral. The composition ofthat mineral may also change abruptly when a secondphase nucleates (Dowty et al., 1974).

In glassy Hole 485 boninites, however, there are twosizes of clinopyroxene crystals, but there is no Plagio-clase. The larger crystals include blocky augite glomero-crysts, isolated augite phenocrysts, and augite-bronz-ite intergrowths. These are surrounded by myriads ofsmaller augite microlites set in glass. The glomerocrystsand intergrowths suggest types of heterogeneous nuclea-tion, whereas the microlites probably formed homo-geneously. Their period of growth coincided with thatof the growth of dendritic projections on the pheno-crysts. At least two stages of cooling are thus implied.

In fact, there were probably more than two stages ofcooling, which may have influenced phenocryst growth.The lavas themselves had to undergo a series of abruptincreases in cooling rate, corresponding to periods ofascent into the shallow crust, eruption, large-scale flowof lava, and budding of pillows. Because pillows formfrom each other, a given packet of lava may havesquirted through several pillows before finally freezing,undergoing at each pillow a change in cooling rate. Thespecific cooling histories of different packets of lavaduring these later stages of eruption will thus vary. Thatthe phenocrysts in the Hole 458 boninites formed duringthese later stages of eruption is evident from their vari-ety in any given sample, the different forms they takefrom sample to sample, and their apparent absence inthe coarsest-grained boninites. Put another way, there is

701

J. H. N ATL AND

no population of phenocrysts common to all samplesthat appears to be left over from any large holding reser-voir or magma chamber. The simplest form of the two-stage cooling hypothesis has phenocrysts forming inmagma chambers, and in the groundmass upon erup-tion. That did not occur here.

In summary, the crystallization sequence of boninitesis determined primarily by their chemistry, but their tex-tures, modes, crystal morphologies, and mineral com-positions are dictated in large measure by kinetic fac-tors. There are several other lava types whose crystal-lization at high cooling rates is more or less analogous tothat of boninites. Laboratory experiments and modelingpromise to improve our understanding of the crystal-lization of all these rocks.

CRYSTAL MORPHOLOGIES IN BASALTSOF HOLES 458 AND 459B

Basalts of arc tholeiite composition interbedded withboninites in Hole 458 are for the most part intenselyaltered and fractured. Glassy and spherulitic zones wereespecially vulnerable to alteration, hence were either notrecovered during coring or were totally reconstituted toclays. Available thin sections made on board ship are ofaltered hyalopilitic samples, with small acicular Plagio-clase microlites set in groundmasses either completelytransformed to clays (Fig. 12A) or retaining traces ofspherulitic clinopyroxene and opaque minerals (Fig.12B).

In the basalts of Hole 459B as well, alteration is in-tense, and the outermost glassy and spherulitic zoneswere not recovered. Two types of basalt were cored,coarse-grained massive rocks of Lithologic Units 1 and3 (Fig. 2) and fine-grained pillows in Lithologic Units 2and 4. Some finer-grained spherulitic rocks occur at thetop of Lithologic Unit 1 and may represent the upperchilled margin of this 29-meter-thick flow. Despite therebeing two major and several minor chemical subdivi-sions among the basalts, petrographic distinctions arenot easy to make, owing to the combined effects ofscarcity of phenocrysts, similarity of groundmass tex-tures among the different lithologic units, and altera-tion. The two Bj basalt subtypes (Fig. 2) are relativelyricher in MgO and poorer in iron (as Fe2O3) than the B2subtypes (Wood et al., this volume), but this appears tobe reflected mainly in a higher proportion of clino-pyroxene, and somewhat less titanomagnetite, in therocks. All the basalts appear to be saturated in bothclinopyroxene and Plagioclase, hence would be repre-sented by cotectic crystallization on Figure 11.

The finest-grained basalts in Hole 459B are hyalopi-litic, with acicular microlites of both Plagioclase andclinopyroxene in an altered mesostasis (Fig. 12C). Thepyroxenes are pale brown in color, and both miner-als are usually surrounded by fuzzy spherulitic over-growths. The Plagioclase microlites flare into spheruliticterminations at either end.

Other somewhat more crystalline samples are quitevesicular (Fig. 12D), with scattered Plagioclase micro-lites separating into two arms at the ends of manycrystals. These are surrounded by finer-grained spheru-

litic sheafs of clinopyroxene. Closer inspection of thesereveals them to be bundles of individual pyroxene fiberstypically splaying from larger elongate crystals whichmight be termed "core fibers" (Fig. 12, E and F). Cubicand octahedral titanomagnetite grains cluster aroundthe edges of these bundles.

Examples of the coarsest-grained pillow basalts areshown in Figure 12, G and H. These are weakly vesicu-lar, suggesting that basalts with abundant vesicles areconfined to narrow zones close to pillow margins. Vesi-cles that occur in the coarser-grained samples, thoughrare, are often large and concentrated into trains (Fig.12H). Plagioclases in the coarser-grained portions ofpillows are elongate crystals, many of them separatingat their ends into two dendritic arms, a consequence ofheightened undercooling (e.g., Kirkpatrick, 1978). InFigure 12G these are surrounded by bundles and clustersof spherulitic clinopyroxene. Overall, these rocks can bedescribed as having mesh textures, with abundant rela-tively short crystals rather than fewer longer crystals;hence they appear to have had many centers of nuclea-tion. This indicates cooling from somewhat below ratherthan somewhat above liquidus temperatures (Walker etal., 1979; Lofgren, 1980).

An interesting feature of hyalopilitic samples is thatseveral contain segregation vesicles lined with dark red-dish brown devitrified glass and containing elongateopaque minerals (Fig. 13A and B). The glass is made upof fine spherulitic material of optically indeterminantorigin. The crystallization of titanomagnetite beforePlagioclase and clinopyroxene in these vesicles is a re-versal of the crystallization sequence elsewhere in therocks, and probably indicates elevated oxygen fugacitiesin the vesicles.

Phenocrysts are rare in these basalts. Two examplesare shown in Figure 13, C and D. The first, in a rathercoarse-grained sample, is a cluster of intergrown eu-hedral augites. The second is an irregular isolated augitethat may be a xenocryst or a type of skeletal crystal; ithas a narrow reddish outer zone. Tabular Plagioclasemicrophenocrysts also occur but are very rare.

The massive flows of Cores 66-68 in Hole 459B havecoarsely subophitic or intergranular textures in their in-teriors. Alteration is not as pervasive in these basalts asin the pillows and typically consists of clay minerals(smectites and celadonite) lining cavities and replacingPlagioclase and augite (Natland and Mahoney, this vol-ume). In situ magmatic differentiation has been locallyextensive in these massive rocks, producing micrographicintergrowths of quartz and two feldspars (Fig. 13, E andF) and locally coarse patches of quartz (Fig. 13, G andH). Some of the micrographic intergrowths have a dis-tinctively spherulitic texture, with the intergrown feld-spar splaying from the ends of larger Plagioclase eu-hedra (Fig. 13F). One can speculate that these late-stagetextures resulted from increased viscosity of the silicicresidual liquids, reducing diffusion rates in the melt rela-tive to crystal growth, hence promoting growth of spher-lites. More conventional interpretations of micrograph-ic intergrowths are (1) that they represent eutectic crys-tallization of quartz and alkali feldspar (Vogt, 1921) and

702


0.1 mm

0.1 mm 1.0 mm

0.1 mm 0.1 mm

0.1 mm 1.0 mm

Figure 12. Petrographic features of tholeiitic basalts, Holes 458 and 459B. A. Sample 458-46-1, 78-80 cm.Plagioclase microlites in an altered, formerly glassy mesostasis. Plane light. B. Same sample as A, also plane-polarized light. Titanomagnetites are fairly abundant at lower right. C. Sample 459B-63-1, 24-27 cm. Plagio-clase microlites with spherulitic overgrowths in an altered glassy mesostasis. Plane light. D. Sample459B-60-1, 44-47 cm. Plane light. Chemical Subtype B1A. Microlites of Plagioclase and spherulitic clinopy-roxene in a vesicular basalt. E and F. Same sample as D, both in plane light. Examples of branching spheru-litic clinopyroxene, speckled with titanomagnetite. G. Sample 459B-71-1, 31-33 cm. Plane light. Plagioclasemicrolites and clinopyroxene spherulites in a Type B2 basalt. Dark spots are clumps of titanomagnetite withoutlines obscured by alteration. H. Same sample as G, plane light, showing portion of a train of vesicles.

703

J. H. NATLAND

1.0 mm 0.1 mm

0.1 mm 0.1 mm

0.1 mm 0.1 mm

0.1 mm 0.1 mm

Figure 13. Segregation vesicles, phenocrysts, and quartz-feldspar intergrowths in coarse-grained tholeiiticbasalts, Hole 459B. A. Sample 459B-10-1, 69-71 cm, plane light. A segregation vesicle in hyalopiliticbasalt with Plagioclase microlites. B. Detail of A, showing abundant elongate skeletal titanomagnetite insegregation vesicle, surrounded by very fine spherulitic material. C. Sample 459B-60-2, 96-99. Crossednichols. Intergrowth of several euhedral augite phenocrysts, in a finer-grained subophitic matrix. D.Sample 459B-63-1, 81-83 cm. Crossed nichols. Irregular single augite microphenocryst in hyalopiliticbasalt. E-H. Sample 459B-66-3, 3-4 cm. Crossed nichols. E and F show patchy intergrowths of a morecalcic Plagioclase (generally darker and twinned) and a less calcic Plagioclase (lighter), with F particularlyshowing spherulitic morphology extending from the large upright Plagioclase at lower center. G and Hshow patchy quartz (low relief, darker at this orientation) intergrown with Plagioclase.

704


(2) that they are produced at elevated /?(H2O) (Luth etal., 1964). In these rocks, however, the feldspar inter-grown with quartz is Plagioclase (see the following)—that is, it is not potassic, hence eutectic compositionswere not reached. Moreover, the experiments of Luthet al. {loc. cit.) relate to crystallization at 2-10 kbar/?(H2O) in the granite system, not to late-stage crystal-lization of basalt flows. Here, the experiments of Fenn(1977a, 1977b), who produced alkali feldspars and al-bite with coarsely spherulitic morphologies from under-cooled silicic melts, seem most relevant, given the ob-served textures.

COMPARISON OF ARC ANDABYSSAL THOLEIITES

In several respects, these rocks resemble East PacificRise and Galapagos Rift ferrobasalts (Natland, 1980)more closely than they do olivine and Plagioclase tholei-ites of either the East Pacific Rise or the Mid-AtlanticRidge (Natland, 1978, 1980). The critical points of simi-larity to the former and contrasts with the latter are (1)lack of olivine phenocrysts; (2) occurrence of clinopy-roxene phenocrysts and, more rarely, Plagioclase pheno-crysts; (3) development of acicular and even equant cli-nopyroxene crystals in the same spherulitic rocks areacicular and skeletal plagioclases; and (4) abundance ofopaque minerals. Regarding the opaque minerals, theintensity of magnetization of some of the finer-grainedHole 459B basalts is the highest ever found in DSDPmaterials (Bleil, this volume), fully five times greaterthan the highest intensity recorded among pillow basaltswith >15% iron as Fe2O3 and 1.9% TiO2 at the Ga-lapagos Rift (Petersen and Roggenthen, 1980). How-ever, these are not as iron-enriched (see Table 1), in-dicating that elevated oxygen fugacity may have acted topromote crystallization of titanomagnetite. The Hole459B arc tholeiites are also more vesicular than easternPacific ferrobasalts, a feature perhaps consistent withelevated oxygen fugacity. The early crystallization of

titanomagnetite may also have prevented an extreme ofiron enrichment in interstitial glass. This probably pre-vented the extreme darkening of spherulitic zones soprevalent among eastern Pacific ferrobasalts (Natland,1980) and promoted the development of quartz-norma-tive residual melts that produced so much quartz inthe coarser-grained flow interiors (e.g., Osborne, 1958,1962, 1979). With 56-59% SiO2, samples from the mas-sive flows of Cores 66-68 have higher SiO2 and norma-tive quartz (8-10%) than comparably iron-enriched (11-12% as Fe2O3) abyssal tholeiites.

PYROXENE AND FELDSPAR COMPOSITIONS

Two clinopyroxene and three Plagioclase composi-tions were obtained from a single sample from Hole459B during an electron microprobe reconnaisance ofclay mineral compositions (described in Natland andMahoney, this volume). The analyses are listed in Table5. The sample was from one of the coarse-grainedmassive flows with quartz-feldspar micrographic inter-growths. The analyzed pyroxenes are the cores of twolarge augite crystals and resemble augite compositionsobtained by Meijer et al. (this volume), as shown onFigure 7D. The Plagioclase analyses are of the core andrim of a large euhedral crystal and of a nearby smallfeldspar grain intergrown with quartz. The large crystalis zoned from calcic labradorite to andesine, and thesmaller crystal is calcic oligoclase, containing slightlymore structural Or than the larger crystal (Table 5).More alkalic feldspar such as anorthoclase may moregenerally be intergrown with quartz elsewhere in therock but has not been analyzed with a microprobe.

The arc tholeiite augites listed in Table 5, as well asthose of Meijer et al. (this volume), have greater struc-tural Wo than the boninite augites of Table 3, as shownon Figure 7D. However, some augites in Hole 458 bo-ninites analyzed by other workers in this volume are ascalcic as the arc tholeiite augites, as indicated by thelarger of the two fields superposed on Figure 7D. A

Table 5. Electron microprobe analyses of pyroxenes and plagioclases, Sample 459B-66-3, 3-4 cm (#1).

SiO2

A12O3

FeOMnOMgOCaONa 2 OK2OTiO 2

SiAlAlTiFeMnMgCaNaK

Clinopyroxenes

49.312.45

14.400.31

12.6918.120.160.030.48

97.95

48.292.37

14.320.33

12.8717.240.250.020.44

96.13

Number of Ions on Basis of 6(O)

L 9 1 5 1 2 0000.085 / Z W W

0.0230.0140.4680.0100.7350.7540.0120.002

2 022

w038.6E n37.5F s23.9

1-911) 2 0 0 0

0.089 J 2 0 0 0

0.021'0.0130.4740.0110.7590.7310.0190.001

W θ 3 7 . 2 E n 3 8 . 7 F s 2 4 . i

50.9330.640.810.010.13

13.493.360.030.01

99.41

Plagioclases

56.8927.47

0.850.030.059.465.540.040.06

100.34

65.0924.42

0.550.000.005.337.830.190.05

103.46

Number of Ions on Basis of 32(0)

9.330

6.6180.0010.1240.0010.0362.6481.1940.007

-15 950-15.950

A πiπt.uiu

An 69Ab 3 1Or 0.002

10.182 Λ

5.797 J0.0080.1270.0050.0131.8141.9230.009

15.978

3.900

An 4 8 . 4 Ab 5 1 .3θro.3

11.1191

4.919^0.0060.0770.0000.0000.9762.5940.041

16.037

3.696

An27.0Ab7i.8On.2

705

J. H. NATLAND

greater contrast can be seen for most pyroxenes onFigure 8, which plots tetrahedrally coordinated Al ver-sus Ti. Boninitic augite phenocrysts have very low Ti,whereas most of the arc tholeiite clinopyroxenes haveconsiderably higher Ti at comparable Alz. The twoaugites with least Ti from Hole 459B are from Core 60,Chemical Subtype B1A, which earlier was noted as hav-ing a nearly boninite composition. Among the boniniteaugites, only those that mantle bronzite have elevatedTi, but they also have elevated Alz and therefore do notoverlap the arc-tholeiite augite compositions.

CONCLUSIONS

The two primary aims of this chapter have been (1) toestablish a petrographic basis for identifying boninites,whether or not they are glassy, and (2) to outline thevarious ways in which kinetic factors influence modes,textures, crystal morphologies, and mineral composi-tions in these pillow lavas. The treatment has been mostlyqualitative, and the mineralogical data so far are lim-ited. Nevertheless, several important conclusions can bedrawn.

Boninites assuredly have unusual compositions whichmust be seen as having a major role in determining theirmineralogy. However, because the Hole 458 boninitesare pillow lavas, rocks with various grain sizes and crys-tallinity had to be examined before this could becomefully apparent. Analogies to laboratory experiments re-producing crystal morphologies and rock textures wererequired to understand the significance of phenocrystsand the lack of Plagioclase in glassy boninites. Sur-prising textural and mineralogical analogs to boninitesexist in nature, particularly the lunar pyroxene-phyricbasalts, whose textures have been studied extensively.However, rigorous understanding of boninite crystal-lization will be realized only when laboratory experi-ments are carried out on boninite compositions. It willbe necessary to conduct these with varying proportionsof water, particularly to determine the effect of water onthe crystallization temperatures of pyroxenes. In view ofthe significance of boninites to magma generation in is-land arcs, the need for further experimentation shouldbe evident.

ACKNOWLEDGMENTS

I am grateful to the crew of marine technicians on board GlomarChallenger during Leg 60 for skillful preparation of samples and thinsections. Bill Melson made the time available for me to use the Smith-sonian Institution^ electron microprobe, and I am grateful to him andto Eugene Jarosewich for assistance in operating the equipment. Sher-man Bloomer and Jim Kirkpatrick provided helpful discussion andcomments on the manuscript.

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