Cr-spinel, an excellent micro-container for retaining primitive melts – implications for a hydrous...

Cr-spinel, an excellent micro-container forretaining primitive melts ^ implications for a hydrous plume

origin for komatiites

Kenji Shimizu a;*, Tsuyoshi Komiya a, Kei Hirose a, Nobumichi Shimizu b,Shigenori Maruyama a

a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japanb Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Received 24 October 2000; received in revised form 1 March 2001; accepted 18 April 2001

Abstract

Ultramafic melt inclusions were discovered in Cr-spinels of 2.7 Ga Al-undepleted komatiites from the BelingweGreenstone Belt, Zimbabwe. The inclusions consist of glass and sub-micrometer-size quench crystals of olivine andclinopyroxene. Homogenized melt inclusions are highly magnesian, ranging from 12.5 to 19.5 wt% in MgO content, andare also close to host komatiitic magma in other compositions. This fact indicates entrapment of melt into host spinelduring the early stages of crystallization. The water content of two melt inclusions was determined using an ion probe;the high magnesian melt inclusion, 17.5 wt% in MgO, contains 1.1 wt% H2O, whereas the moderately magnesian meltinclusion, 11.8 wt% in MgO, has 1.7 wt% H2O. This evidence suggests that the primary komatiite melt contained 0.8^0.9 wt% H2O and 23.4^25.0 wt% MgO. The water content is about five times greater than previous estimates from meltinclusions in olivine [McDonough and Danyushevsky, EOS Trans. AGU 76 (1995) S266]. In addition, even the highH2O content preserved in melt inclusions within Cr-spinel may represent the minimum estimates of the parentalcomposition, because part of the water should be dehydrated from parental magma during crystallization in the magmachamber. If the komatiite melt was formed by a high degree of partial melting of a peridotite, the source mantle shouldcontain considerable amounts of water (V0.5 wt%). However, recent melting experiments of hydrous peridotiteindicate that the addition of 0.5 wt% H2O to mantle peridotite would not significantly decrease komatiite liquidustemperature [Asahara et al., Geophys. Res. Lett. 25 (1998) 2201^2204]. Petrological study of melt inclusions togetherwith experimental data suggest that Belingwe komatiites were formed from a hydrous plume at hightemperatures. ß 2001 Elsevier Science B.V. All rights reserved.

Keywords: komatiite; chrome spinel; inclusions; Belingwe greenstone belt ; plumes; high pressure; transition zones

1. Introduction

Komatiites are the most magnesian extrusiverocks on the Earth, and are restricted to Archeangreenstone belts except for the Cretaceous ko-matiite on Gorgona Island. It has been widely

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 3 5 9 - 4

* Corresponding author. Tel. : +81-3-5734-2618;Fax: +81-3-5734-3558; E-mail: [email protected]

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Earth and Planetary Science Letters 189 (2001) 177^188

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believed that komatiites were produced from drymantle plumes under ultra-high temperature andpressure conditions [3,4] ; hence, Archean mantleplumes were about 300^400³C hotter thanpresent-day plumes [5]. Komatiites are thusthought to be a good temperature indicator ofthe deep mantle, and the best samples to constrainthe secular change in mantle and plume temper-atures. However, high temperature may not be theunique factor for the origin of komatiites. A num-ber of recent studies have emphasized the impor-tant role of water in the genesis of komatiites.Recent ultra-high pressure experiments haveshown that komatiitic melts can be produced bypartial melting of hydrous mantle even at normaltemperature [2,6,7] (for example, addition of1 wt% H2O to peridotite decreases the tempera-ture of komatiite formation by V100³C [2]). Evi-dence of wet komatiite has been supported by theoccurrence of igneous hornblende in the 2.7 Gakomatiites of the Abitibi greenstone belt, Canada[8], and from the compositions of clinopyroxeneswith spinifex texture from Al-depleted komatiitein the Barberton greenstone belt, South Africa [9].However, Arndt et al. [10] argued that most ko-matiites are substantially dry, based on the lack ofdegassing textures and structures, and the chem-ical and isotopic compositions of most komatiites.However, in order to evaluate the role of water inkomatiite magma genesis, precise in situ measure-ments of the water contents of primary komatiitesare required.

Melt inclusions in the liquidus phase of komati-ite such as olivine or Cr-spinel may preserve theprimary H2O content before the post-extrusivedegassing. Melt inclusions in olivine have alreadybeen found in the 2.7 Ga komatiite of the Be-lingwe Greenstone Belt, Zimbabwe [11]. Fouriertransform infrared (FTIR) analyses of these meltinclusions indicated that the primary komatiitemagma (MgO = 25 wt%) contains only 0.19 wt%H2O [1]. However, it is more di¤cult to estimatethe primary water content from a melt inclusionwithin olivine, because most melt inclusions with-in olivine are relatively less magnesian, and al-ready evolved. Melt inclusions in Cr-spinel haverecently been found in various rock types in-cluding boninite [12], mid-oceanic ridge basalt

(MORB) [13] and dunite [14], and have providedimportant constraints on petrogenesis and tecton-ic setting. In this paper, we present the petrologyand H2O content of highly magnesian melt inclu-sions in Cr-spinel ¢rst discovered from the Al-un-depleted komatiites of the Belingwe GreenstoneBelt.

2. Analytical methods

Major element concentrations of whole rockswere analyzed by a Simultix 3550 X-ray £uores-cence spectrometer (Rigaku) at the Tokyo Insti-tute of Technology. Details of the analyticalmethod were reported by Goto and Tatsumi[15]. Mineral and melt inclusion compositionswere determined by an electron probe microana-lyzer, a JEOL JXA-8800A, at the Tokyo Instituteof Technology. Phenocrysts and melt inclusionswere analyzed at 15 kV accelerating voltage and12 nA beam current with 10^30 s counting time.To analyze melt inclusions in Cr-spinel and oli-vine, the beam was defocused up to 10 Wm diam-eter to cover both glass and ¢ne-grained quenchcrystal. As melt inclusions are not homogeneousglass (Fig. 1a), separated Cr-spinels were annealedwith an atmospheric furnace and with an inter-nally heated pressure vessel at 1, 2 and 3 kbar,and at 1200, 1250, 1300, 1325, 1350, 1375 and1400³C for 10 min, with oxygen fugacity at theQFM bu¡er. The minimum experimental temper-ature for homogenizing the melt inclusions was1325³C at all pressures (Fig. 1b). Reheated meltinclusions in Cr-spinel were analyzed with a defo-cused beam up to 8 Wm to ¢t the size of meltinclusions.

H2O contents of melt inclusions were deter-mined using a Cameca IMS 3f ion microprobeat the Woods Hole Oceanographic Institution.Basaltic glasses with various H2O contents (0.15,0.60, 1.41 and 1.71 wt%) were used as standardsamples to establish the calibration curve (H/Si vs.H2O [wt%]). Sample preparation and analyses ofthe hydrogen content in melt inclusions were per-formed as follows: (1) the standard and unknownsamples were left in a vacuum for 48 h; (2) abeam of negatively charged oxygen ions with a

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K. Shimizu et al. / Earth and Planetary Science Letters 189 (2001) 177^188178

current of ca. 2.5 nA was rastered over an area150U150 Wm for 15 min; (3) pre-sputtering with aspot size less than 10 Wm in diameter was done for3 min; (4) the intensity data on 1H and 30Si weretaken at a 350 V o¡set to calculate the H/Si ratio.Spot-to-spot reproducibility was V3 to 5%.

3. Petrological description of sample and meltinclusion

The Cr-spinels containing melt inclusions de-scribed here were sampled from unusually freshkomatiites in a cumulate zone of a thin komatiite£ow near SASKMAR [11,16]. The komatiite sam-ples contain olivine and Cr-spinel as phenocrystsand groundmass clinopyroxene with spinifex tex-ture. Secondary serpentine and magnetite occuralong cracks and rims of olivine; chlorite ispresent in the matrix. The associated basalts arealso weakly metamorphosed to form chlorite, cal-cite, quartz, and Ca-zeolites, characterizing zeo-lite-facies metamorphism. Fine-grained euhedraland cruciform Cr-spinels are scattered over bothspinifex and cumulate zones in the £ows andrange from 10 to 150 Wm in size (mean size:V50 Wm). Cr-spinel constitutes less than V0.5vol% of the rock even in the cumulate zone.Some Cr-spinels are included in the cores ofhigh-magnesian olivine, indicating that they crys-tallized at an early stage. Most Cr-spinel crystalsshow a remarkable decrease in Mg# [ = Mg/(Mg+Fe2�)], but a constant ratio in Cr# [ = Cr/(Cr+Al)] from core to rim. The compositions ofCr-spinels range from 0.71^0.78 in the cores to0.72^0.78 in the rims in Cr#, 0.48^0.78 to 0.35^0.74 in Mg# and 0.040^0.088 to 0.055^0.114 inFe3�/(Cr+Al+Fe3�). The FeO to Fe2O3 ratios inCr-spinel were calculated from the stoichiometryconstraints. Most Cr-spinel cores preserve theirigneous compositions, but some are re-equilibrat-ed with the residual melt at solidus temperature ofkomatiite magma as low as V1000³C [17]. Noneof them have been modi¢ed by the late-stage, low-grade metamorphism.

The orthocumulate rock (BW130) in the cumu-late zone of a £ow was selected from over 450samples from the Belingwe Greenstone Belt for

mineral separation of Cr-spinels and olivines, be-cause it contains many large Cr-spinels, comparedwith samples in spinifex zones and other cumulaterocks, in addition to its lack of alteration. Cr-spinel was separated by a magnetic separationsystem, heavy solution and hydrogen £uoride so-lution. The separates were mounted in epoxy andpolished for detailed petrographic and composi-tional examinations. Although melt inclusionsare distributed randomly in their host minerals,most are concentrated near the core of the hostCr-spinels. Most of them have a spherical shape,and have a grain size less than 3 Wm across, exceptfor rare occurrences of large melt inclusions, V15Wm across. They contain residual glass andquench crystals of olivine and pyroxene (Fig.1a). Laser Raman spectroscopic analyses of thesemelt inclusions indicated the presence of olivineand clinopyroxene, but did not reveal any hy-drous minerals, such as chlorite or serpentine.On the other hand, some melt inclusions pene-trated by cracks running into the matrix su¡eredchloritization or serpentinization in the originallyentrapped glass portions. Such altered melt inclu-sions were excluded in the present study.

The compositions of unheated melt inclusionsdisplay signi¢cant variation and are not consistentwith the compositional trends of the whole-rockchemistry, probably due to the variable extent ofin situ crystallization of quench olivine and pyrox-ene within the melt inclusions (Fig. 2). In addi-tion, overgrowth of the host Cr-spinels by theconsumption of the entrapped melt and the Fe^Mg exchange between the entrapped melt and Cr-spinels also have an in£uence on the compositionof the melt inclusions. Assuming an Fe3�/gFe ra-tio of the komatiite from 0.07 (MORB [20]) to0.13 (island arc basalt [21]), the Fe2�/Mg partitioncoe¤cient between melt inclusions and hostspinels [KFe2�=Mg

D �sp=liq�] was calculated to rangefrom 0.96 to 3.25. The calculated partition coef-¢cients are highly variable and some of them areinconsistent with the results of melting experi-ments of komatiite at 1 atm, 0.98^1.52 in KD

[22]. The inconsistency indicates signi¢cant Feloss from the entrapped melt, due to Fe^Mg ex-change between the melt inclusions and host Cr-spinels. The Cr2O3 contents of all the melt inclu-

EPSL 5869 19-6-01

K. Shimizu et al. / Earth and Planetary Science Letters 189 (2001) 177^188 179

sion within Cr-spinel are over 1 wt% (Table 1).These concentrations are signi¢cantly higher thanthe Cr2O3 contents of the whole rocks (0.2^0.3wt%). The excess content also indicates redistri-bution of Cr2O3 between melts and host spinel tosome extent.

Although the MgO content in homogenized

melt inclusions ranges from 12.5 to 19.5 wt%with a mean value of 17.4 wt% for the 18 heatedmelt inclusions and these are slightly less magne-sian than the whole rock composition, they havecompositions close to the whole rock composi-tion of the komatiites except for Cr2O3 (Fig. 2,Table 1). KFe2�=Mg

D �sp=liq� between the heated

Fig. 1. Back-scattered image of Cr-spinel containing melt inclusions (scale bar: 10 Wm). (a) Unheated melt inclusions consist ofglass and sub-micrometer-sized olivine and pyroxenes. (b) Heated melt inclusions at 1 atm, 1325³C for 10 min, showing thehomogeneity and the presence of condensed bubbles which may have formed during quenching.

EPSL 5869 19-6-01


Tab

le1

Com

posi

tion

ofre

pres

enta

tive

mel

tin

clus

ions

and

thei

rho

stsp

inel

Hos

tko

mat

iite

BW

130

Hea

ted

incl

usio

ns(1

atm

)H

eate

din

clus

ions

(1kb

ar)

Ave

rage

of18

heat

edin

clus

ions

Unh

eate

din

clus

ion

Est

imat

epr

imar

yko

mat

iitea

Raw

data

Cor

r.da

ta

SiO

244

.05

50.5

949

.19

49.5

547

.44

50.3

750

.37

49.8

648

.56

50.2

948

.04

50.2

548

.28

49.4

31.

1252

.71

50.7

653

.39

48.8

2T

iO2

0.31

0.33

0.40

0.40

0.79

0.38

0.39

0.40

0.49

0.30

0.43

0.40

0.40

0.44

0.11

0.08

0.25

0.28

0.37

Al 2

O3

5.71

9.41

9.24

8.86

9.65

9.47

8.52

8.92

9.34

9.38

8.74

9.13

8.92

9.07

0.49

20.7

216

.72

10.0

67.

65F

eO*

10.6

510

.71

10.3

910

.99

11.8

210

.79

11.0

610

.62

11.1

210

.78

11.6

29.

9811

.93

11.1

41.

134.

347.

146.

8210

.68

MnO

0.18

0.23

0.19

0.16

0.22

0.20

0.20

0.16

0.24

0.17

0.19

0.18

0.16

0.20

0.03

0.05

0.14

0.15

0.19

MgO

27.4

317

.51

18.6

518

.04

16.1

917

.73

16.7

618

.51

18.3

217

.68

17.8

817

.10

17.3

517

.41

1.01

10.4

511

.58

17.5

623

.44

CaO

6.27

8.95

8.12

8.19

8.42

8.28

7.31

7.79

8.78

8.53

9.54

8.92

8.24

8.11

0.75

5.99

8.57

8.16

6.84

Na 2

O0.

550.

890.

710.

680.

700.

730.

740.

670.

730.

680.

430.

750.

700.

700.

053.

662.

431.

200.

59K

2O

0.02

0.04

0.03

0.02

0.06

0.00

0.02

0.04

0.02

0.06

0.01

0.06

0.04

0.03

0.02

0.21

0.13

0.10

0.02

Cr 2

O3

^1.

401.

471.

231.

861.

771.

441.

261.

991.

621.

551.

381.

751.

570.

240.

921.

151.

20^

H2O

2.6

1.7

1.1

0.9

Tot

al95

.17

100.

0598

.38

98.1

297

.14

99.7

296

.82

98.2

399

.58

99.4

798

.42

98.1

597

.76

98.0

90.

8910

1.73

100.

5810

0.02

99.5

0M

g#74

.46

77.6

876

.07

72.6

376

.10

74.5

977

.16

76.1

574

.51

73.2

876

.84

73.7

975

.13

2.78

79.6

4H

ost

spin

els

Mg#

67.7

672

.68

71.2

969

.14

74.2

268

.14

70.0

573

.65

69.8

169

.81

71.6

064

.14

69.9

64.

3571

.71

69.8

9C

r#77

.46

77.0

675

.98

76.0

274

.17

73.8

976

.15

73.0

775

.15

75.1

575

.20

77.6

875

.49

1.98

77.7

075

.17

Fe3�

/(C

r+A

l+F

e3�)

0.05

10.

061

0.07

40.

068

0.06

80.

067

0.06

10.

080

0.05

20.

052

0.07

70.

074

0.06

90.

010

0.04

40.

052

KD

(sp-

mel

t)1.

541.

311.

281.

181.

111.

371.

441.

141.

401.

321.

321.

571.

842.

15

Mel

tin

clus

ions

wer

ere

heat

edat

1325

³Cfo

r10

min

.F

eO*

:to

tal

iron

asF

eO.

Fe2�

ofm

elt

incl

usio

nsas

0.93

Fe t

otal

.M

g#:

Mg/

(Mg+

Fe2�

)100

.C

r#:

Cr/

(Cr+

Al)

100.

KD

(sp-

mel

t):

(Fe2�

/Mg)

sp/(

Fe2�

/Mg)

mel

t.C

orr.

data

:co

rrec

tion

data

for

post

-ent

rapm

ent

crys

talli

zati

onof

clin

opyr

oxen

e.It

alic

¢gur

es:

1cof

aver

age

com

posi

tion

.aF

e2�of

mel

tin

clus

ions

as0.

93F

e tot

alan

d(F

e/M

g)ol

ivin

e/(F

e2�/M

g)m

elt=

0.3

wer

eas

sum

ed.

EPSL 5869 19-6-01


MgO (wt%)

SiO2 (wt%)

MgO (wt%)

Al 2O3 (wt%)

MgO (wt%)

FeOt (wt%)

MgO (wt%)

CaO (wt%)

MgO (wt%)

Na2O (wt%)

MgO (wt%)

TiO2 (wt%)

40

45

50

55

60

65

70

0 5 10 15 20 25 30

0

0.5

1

0 5 10 15 20 25 30

0

5

10

15

0 5 10 15 20 25 305

10

15

20

0 5 10 15 20 25 30

5

10

15

20

0 5 10 15 20 25 30

0

1

2

3

4

0 5 10 15 20 25 30

whole rockmelt inclusion in olivine(unheated)melt inclusion in Cr-spinel(unheated)melt inclusion in Cr-spinel (heated)

Fig. 2. MgO variation diagrams of major elements for whole rocks, unheated melt inclusions within olivine and unheated andheated (1325³C at 1 atm and 1 kbar for 10 min) melt inclusions within Cr-spinel. The dashed line area indicates whole rock com-positional variation of komatiite from the Belingwe Greenstone Belt from the literature [11,16,18,19]. The ¢lled star representsthe primary komatiite derived from melt inclusions within Cr-spinel. The dispersion in compositions of unheated melt inclusionsin Cr-spinel is due to crystallization of quench crystals and host mineral^melt interaction after entrapment. The compositions ofmany melt inclusions annealed at moderate temperatures are consistent with the compositional trend of the bulk composition ofkomatiites, indicating that melt inclusions within Cr-spinel retain their original composition. On the other hand, compositions ofmelt inclusions within olivine are di¡erent from the host komatiite.

EPSL 5869 19-6-01


melt inclusions and the host Cr-spinels is 0.93^1.64, suggesting that the annealed melt inclusionsare in equilibrium with the host Cr-spinel. Nochemical di¡erence was observed among themelt inclusions annealed at various experimentalpressures.

4. Water content in komatiite

Two unheated fresh melt inclusions, V15 Wmin size, without cracks were selected to analyzetheir water content by SIMS. One melt inclusioncomprises both residual glass (ca. 10 Wm across)and quench crystals of clinopyroxene (ca. 2 Wmwidth); the analyzed spot on a glassy part yielded2.6 wt% H2O. The entrapped melt has 11.6 wt%in MgO, and 1.7 wt% H2O with a correction forpost-entrapment crystallization of clinopyroxenewithin the inclusion on the basis of the back-scat-tered electron image. The other melt inclusionyielded 1.1 wt% H2O, where sub-micrometerquench crystals of clinopyroxene are distributeduniformly over the residual melt (Fig. 3, Table 1).

We have calculated the crystallization sequence

of a komatiite magma and the compositionalchange of residual melts as shown in Fig. 3. Thevariations of whole-rock compositions for the Be-lingwe komatiite and komatiitic basalt are shownin Fig. 2; these plots, especially CaO against MgOcontent, suggest that olivine (+spinel) was thedominant phase during early crystallization untilthe MgO content of the residual melt decreased toV15 wt%, then clinopyroxene participated in thefractionation of the komatiite magma. Therefore,the composition of parental magma was calcu-lated from the addition of equilibrium olivine tothe unheated melt inclusion, 17.5 wt% MgO, with1.1 wt% H2O to attain its equilibrium with themost magnesian olivine (Fo = 93.5). The Fe2�/Mg ratio of the water-bearing melt inclusionused for this calculation was assumed to be theaverage ratio of the heated melt inclusions, be-cause the Fe loss of unheated melt inclusionscould be signi¢cant. Assuming that the Fe2�^Mgpartition coe¤cient between olivine and melt is0.3^0.33 [23], the water content of the primarykomatiitic magma (MgO 23.4^25.0 wt%) is esti-mated to be 0.8^0.9 wt% H2O (Fig. 3). The calcu-lated water content of the less magnesian meltinclusion with 11.8 wt% MgO is slightly higherthan that of the high magnesian melt inclusionwith 17.5 wt% MgO, because of crystal fraction-ation of V14% olivine and V22 wt% clinopy-roxene from the melt with 17.5 wt% MgO.

5. Discussion

5.1. Cr-spinel, a superior water container

This study shows that melt inclusions withinCr-spinel preserve four to ¢ve times more H2O(0.8^0.9 wt%) than inclusions in olivine (0.19wt% [1]). The di¡erence is caused by the di¡erentdegree of fractional crystallization where olivineand Cr-spinel precipitated, and by the di¡erentpermeability of water within Cr-spinel and oli-vine. Mackwell and Kohlstedt [24] suggestedthat hydrogen in olivine may di¡use through de-fects during Mg^Fe2� re-equilibration. If hydro-gen escaped through the host minerals by thisprocess during cooling, the water content of a

Fig. 3. The calculated path of fractional crystallization ofmelt inclusions in terms of H2O against MgO by weight per-cent (open circles: analyzed melt inclusions). The addition ofabout 20% olivine to a magnesian melt inclusion with 17.5wt% MgO would produce a primary komatiite composition,which is in equilibrium with the most refractory olivine(Fo = 93.5). The calculation indicates that a primary komati-ite with 23.4^25.0 wt% MgO contains 0.8^0.9 wt% H2O. Theless magnesian melt inclusion has a higher H2O content be-cause of still more fractional crystallization of V14% of oli-vine and V22% of clinopyroxene.

EPSL 5869 19-6-01


melt inclusion in Cr-spinel would be smaller thanthat in such an inclusion in olivine, because Mg^Fe2� di¡usion in Cr-spinel is faster than in olivine[25]. Fig. 4 is a plane-polarized light view of thecumulate zone in a Belingwe komatiitic £ow; itshows dislocations within olivine heated at 900³C,1 atm for 1 h, following the technique of Kohl-stedt et al. [26]. Dislocations are visualized by O2

di¡usion into the defect tubes of olivine and itsreaction with olivine to form magnetite and or-thopyroxene [26]. Irrespective of the existence ofmelt inclusions, dislocations were observed inmost olivine grains. Dislocation densities ofmost olivines are V105 cm32, indicating thatthe dislocations formed by plastic deformationat high temperatures in the presence of a melt[27]. Defect tubes in olivine are large enough todi¡use O2 through olivine. Therefore, H2, OH, O2

and H2O in melt inclusions can easily escape fromhost olivine during magma emplacement. In addi-tion, the movement of those molecular com-pounds would not lead to a signi¢cant increaseof oxygen fugacity in a melt inclusion in olivine,in contrast to the escape of hydrogen from hy-drous magma. On the other hand, no dislocation

within Cr-spinel exists even in ultra-thin sectionsof V10 Wm. The lack of dislocation within Cr-spinel is because the small size, density and ri-gidity of Cr-spinel allow it to sustain the stressof high-temperature plastic deformation.

Another advantage of Cr-spinel as a superiorH2O container compared with olivine is that oli-vine can easily be replaced by serpentine evenduring low-temperature alteration [28]. Even inthe unusually fresh komatiites in the BelingweGreenstone Belt, rims of all olivine grains are re-placed by serpentine. H2O in melt inclusions mayreact with the host olivine to form serpentine,hence decreasing the water concentration. Onthe other hand, no Cr-spinels from the Belingwekomatiite were replaced by hydro-oxide such asgoethite.

In general, small melt inclusions have the pos-sibility to increase incompatible elements by theboundary layer e¡ect. In particular, the size ofmelt inclusions within Cr-spinel in this study isso small (V15 Wm) that the boundary layer e¡ectmay be relevant. However, the SIMS analyses ofV20 Wm melt inclusions in olivine as small asthose in Cr-spinels from the Belingwe komatiite

Fig. 4. Microphotograph of dislocations in olivine with a melt inclusion (scale bar: 50 Wm). The characteristic texture of high-temperature deformation suggests the high possibility of escape of H2O from melt inclusions within olivine through these defecttubes.

EPSL 5869 19-6-01


indicate that concentrations of the highly incom-patible trace elements (e.g. rare earth elements), atleast, were not increased by the boundary layere¡ect [29]. Based on these results, we assumethat the boundary layer e¡ect is insigni¢cant forhighly incompatible trace elements and even forwater contents in melt inclusions within Cr-spinels.

5.2. The origin of 2.7 Ga komatiite from a hydrousplume

The petrogenesis of komatiitic magma is stillcontroversial [2^7]. The degree of melting of ko-matiite is one of the keys to elucidate the mecha-nism of formation. Low abundances of incompat-ible trace elements in Belingwe komatiites [19]suggest that they were produced by a high degreeof partial melting of mantle, whether the sourcemantle was depleted or primordial. Assuming thatthe degree of melting was 50 wt% based on TiO2

and heavy rare earth elements contents, and thatH2O behaved completely as an incompatible ele-ment, the source mantle of komatiitic magmawith 0.9 wt% H2O would contain 0.45 wt%H2O. Although partial melting of H2O-saturatedperidotite at low pressure would not yield koma-tiitic melt, but high-Mg andesite [30], the additionof 0.5 wt% H2O to the mantle peridotite lowersthe solidus temperature by 200³C at low pressure,e.g. at 1.0 GPa [31]. In addition, Parman andGrove [32] showed that hydrous (5 wt% H2O)Al-depleted komatiite is multiply saturated onits liquidus with olivine and orthopyroxene at2.2 GPa and 1430³C, and suggested that the Bar-berton komatiites in South Africa were producedat a depth of 66 km in the presence of V5 wt%H2O.

The tectonic setting of komatiitic magma is stillunder dispute. Nisbet et al. [5] proposed that Be-lingwe komatiite was formed beneath a thick con-tinental craton. Grove et al. [33] suggested thatthe Barberton komatiite was a subduction-relatedmagma because of the high content of H2O con-tent. However, high H2O concentration does notnecessarily result from subduction-related mag-matism. In addition, H2O may not have been sup-plied to the mantle wedge at 2.2 GPa because an

early Archean subduction zone geotherm wasmuch hotter than at present, and because com-plete melting and dehydration of a subductingslab occurred at pressures of V1 GPa [34]. There-fore, in our model, we assume that the komatiitewas extruded as an oceanic plateau [29,35].Young volcanic rocks contain H2O at the level

Fig. 5. Schematic diagram illustrating generation of wet ko-matiitic melt and the related basalt from a hydrous plume atTp = 1700³C and dry komatiite from an anhydrous plume atTp = 1800³C [5] in an intra-oceanic plate setting. Their P^Tpaths are respectively shown as a solid line and a dotted linewith arrows. Numbers in parentheses represent the maximumwater content by weight percent in major minerals constitut-ing peridotitic mantle [39,40]. The wet solidus of peridotitewith V0.5 wt% H2O at depths v410 km may be quite simi-lar to the dry solidus, because much water can be ¢xed inmajor constituent minerals. However, a hydrous plume mayrelease H2O-rich £uid, when it rises up through the L^Kphase transition. In this case, the solidus temperature drasti-cally decreases, because free H2O-rich £uid is formed in themantle peridotite. As a result, the komatiitic melt may beformed at ultra-high pressure. As the plume ascends to ashallower level, the sheath of the plume is cooler because ofadiabatic upwelling. However, the temperatures may intersectthe dry solidus at relatively low pressure, because the de-crease of dry solidus of mantle peridotite is larger than thatof the temperature of the adiabatically upwelling plume. Asa result, much basaltic magma is produced.

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V0.2 wt% for MORB [36] and V0.4 wt% forOIB [37], whereas this study shows that the 2.7Ga komatiite contained 0.8^0.9 wt% H2O. Thehigh H2O concentration would have played animportant role in the petrogenesis of the Archeankomatiite, such as causing a decrease of solidustemperature and initiation of partial melting athigh pressure. In Fig. 5, we schematically demon-strate the origin of the 2.7 Ga Al-undepleted ko-matiite from hydrous mantle plume. Recent high-pressure melting experiments suggest that komati-ite magma can be formed from a hydrous mantleperidotite with 1 wt% H2O at a lower temperatureof V100³C than from a dry source mantle [2].L- and Q-spinels in the mantle transition zone(410^660 km) can contain as much as 3.3 and2.8 wt% H2O, respectively [38,39], but K-olivinecan contain only 0.12 wt% H2O [40]. This lineof evidence predicts that the solidus of a hydrousmantle with V0.5 wt% H2O is similar to a drysolidus at high pressure, where L- and Q-spinel arestable, whereas it is signi¢cantly lower than thedry solidus in the K-olivine stability ¢eld. Thewet solidus in the K-olivine stability ¢eld was cal-culated by Kawamoto and Holloway [7] from themelting experiments of H2O-saturated KLB-1.Moreover, in the case that a hydrous peridotiticplume with V0.5 wt% H2O rose up from s 410km deep mantle, free H2O-rich £uid would beexpected to be released from the hydrous mantle,when it reached the K-olivine stability ¢eld. Ko-matiitic melt is continuously formed from a hy-drous plume until the melt is extracted from itssource, because the solidus of the ascendingplume falls drastically in the presence of freeH2O-rich £uid. The melt separation may occurat depths 6V200 km [2,6,7], because the Al-un-depleted komatiite in the Belingwe GreenstoneBelt shows no signature of residual garnet [19].For the isentropic melting, the P^T path of anascending plume mantle shifts extensively towardslower temperatures compared to an adiabaticpath without melting. In addition, the solidus ofthe ascending mantle increases drastically becauseall the free water is also extracted from the sourcemantle. As a result, the melting ceases and theP^T path follows the dry adiabat. The cool anddry sheath of the plume may intersect the dry

solidus again at shallower depth to form the over-lying basaltic £ows, as shown in Fig. 5.

Our systematic petrological studies of melt in-clusions in Cr-spinel suggest that Cr-spinel is abetter H2O container than olivine and that meltinclusions in Cr-spinel preserve four to ¢ve timesmore H2O than melt inclusions in olivine. How-ever, even the H2O content preserved in melt in-clusions in Cr-spinel may represent the minimumestimate, because the komatiitic melt may alreadyhave released part of its volatiles before the en-trapment of melt within Cr-spinel. The absolutewater content of primary komatiites remains un-clear, but our study indicates that water even at aminimum has played an important role in komati-ite genesis.

Acknowledgements

We are indebted to J.G. Liou and B.F. Windleyfor critical reading of the manuscript and for cor-recting the English. We thank E. Takahashi, T.Inoue, N.T. Arndt, C. Herzberg, T.L. Grove, andC.D. Parkinson for constructive discussions andcareful reviews. This work was supported by theJapanese Society for the Promotion of Science forJapan Junior Scientists and partly supportedby grants from the Ministry of Education, Sci-ence, and Culture of Japan (No. 09041099 and07238105).[BW]

References

[1] W.F. McDonough, L.V. Danyushevsky, Water and sulfurcontents of melt inclusions from Archean komatiites, EOSTrans. AGU 76 (1995) S266.

[2] Y. Asahara, E. Ohtani, A. Suzuki, Melting relations ofhydrous and dry mantle compositions and the genesis ofkomatiites, Geophys. Res. Lett. 25 (1998) 2201^2204.

[3] E. Takahashi, C.M. Scarfe, Melting of peridotite to 14GPa and the genesis of komatiites, Nature 315 (1985)566^568.

[4] C. Herzberg, J. Zhang, Melting experiments on anhydrousperidotite KLB-1: compositions of magmas in the uppermantle and transition zone, J. Geophys. Res. 101 (1996)8271^8295.

[5] E.G. Nisbet, M.J. Cheadle, N.T. Arndt, M.J. Bickle, Con-straining the potential temperature of the Archaean man-

EPSL 5869 19-6-01


tle a review of the evidence from komatiites, Lithos 30(1993) 291^307.

[6] T. Inoue, H. Sawamoto, High pressure melting of pyroliteunder hydrous condition and its geophysical implication,in: Y. Syono, M.H. Manghnani (Eds.), High-PressureResearch: Application to Earth and Planetary Sciences,Terra, Tokyo and AGU, Washington, DC, 1992, pp. 323^331.

[7] T. Kawamoto, J.R. Holloway, Melting temperature andpartial melt chemistry of H2O-saturated mantle peridotiteto 11 gigapascals, Science 276 (1997) 240^243.

[8] W.E. Stone, E. Deloule, M.S. Larson, C.M. Lesher, Evi-dence for hydrous high-MgO melts in the Precambrian,Geology 25 (1997) 143^146.

[9] S.W. Parman, J.C. Dann, T.L. Grove, M.J. de Wit, Em-placement conditions of komatiite magmas from the 3.49Ga Komati Formation, Barberton Greenstone Belt, SouthAfrica, Earth Planet. Sci. Lett. 150 (1997) 303^323.

[10] N. Arndt, C. Ginibre, C. Chauvel, F. Albare©de, M. Chea-dle, C. Herzberg, G. Jenner, Y. Lahaye, Were komatiiteswet?, Geology 26 (1998) 739^742.

[11] E.G. Nisbet, N.T. Arndt, M.J. Bickle, W.E. Cameron, C.Chauvel, M. Cheadle, E. Hegner, T.K. Kyser, A. Martin,R. Renner, E. Roedder, Uniquely fresh 2.7 Ga komatiitesfrom the Belingwe greenstone belt, Zimbabwe, Geology15 (1987) 1147^1150.

[12] A.V. Sobolev, V.S. Kamenetsky, A.J. Crawford, Very hotH2O-bearing Phanerozoic ultrama¢c melt inclusions inphenocrysts from Papua-New Guinea Boninite, EOSTrans. AGU 75 (1995) S270.

[13] V. Kamenetsky, Methodology for the study of melt inclu-sions in Cr-spinel, and implications for parental melts ofMORB from FAMOUS area, Earth Planet. Sci. Lett. 142(1996) 479^486.

[14] P. Schiano, R. Clocchiatti, J.P. Lorand, D. Massare, E.Deloule, M. Chaussidon, Primitive basaltic melts includedin podiform chromites from the Oman ophiolite, EarthPlanet. Sci. Lett. 146 (1997) 489^497.

[15] A. Goto, Y. Tatsumi, Quantitative analysis of rock sam-ples by an X-ray £uorescence spectrometer (I), Rigaku J.11 (1994) 40^59.

[16] E.G. Nisbet, M.J. Bickle, A. Martin, The ma¢c and ultra-ma¢c lavas of the Belingwe greenstone belt, Rhodesia,J. Petrol. 18 (1977) 521^566.

[17] S.J. Barnes, Chromite in komatiites. 1. Magmatic controlson crystallization and composition, J. Petrol. 39 (1998)1689^1720.

[18] M.J. Bickle, A. Martin, E.G. Nisbet, Basaltic and perido-titic komatiites and stromatolites above a basal uncon-formity in the Belingwe greenstone belt, Rhodesia, EarthPlanet. Sci. Lett. 27 (1975) 155^162.

[19] M.J. Bickle, N.T. Arndt, E.G. Nisbet, J.L. Orpen, A.Martin, R.R. Keays, R. Renner, Geochemistry of theigneous rocks of the Belingwe Greenstone Belt: alteration,contamination and petrogenesis, in: M.J. Bickle, E.G.Nisbet (Eds.), The Geology of the Belingwe GreenstoneBelt, Zimbabwe: A Study of Archean Continental Crust,

Geol. Soc. Zimbabwe Spec. Pub. 2, Balkema, Rotterdam,1993, pp. 175^209.

[20] D.M. Christie, I.S.E. Carmichael, C.H. Langmuir, Oxida-tion states of mid-ocean ridge basalt glasses, Earth Planet.Sci. Lett. 79 (1986) 397^411.

[21] C. Lecuyer, Y. Ricard, Long-term £uxes and budget offerric iron: implication for the redox states of the Earth'smantle and atmosphere, Earth Planet. Sci. Lett. 165(1999) 197^211.

[22] P. Thy, Low-pressure experimental constraints on theevolution of komatiites, J. Petrol. 36 (1995) 1529^1548.

[23] P. Ulmer, The dependence of the Fe2�-Mg cation parti-tioning between olivine and basaltic liquid on pressure,temperature and composition: an experimental studyon 30 kbars, Contrib. Mineral. Petrol. 101 (1989) 261^273.

[24] S.J. Mackwell, D.L. Kohlstedt, Di¡usion of hydrogen inolivine: implications for water in mantle, J. Geophys. Res.95 (1990) 5079^5088.

[25] P.A.H. Scowen, P.L. Roeder, R.T. Helz, Reequilibrationof chromite within Kilauea Iki lava lake, Hawaii, Contrib.Mineral. Petrol. 107 (1991) 8^20.

[26] D.L. Kohlstedt, C. Goetze, W.B. Durham, J.B. vanderSande, New technique for decorating dislocations in oli-vine, Science 191 (1976) 1045^1046.

[27] R. Takeguchi, E. Takahashi, Primary magma composi-tion of Koolau volcano, Oahu, Hawaii, Earth Planet.Sci. Lett., submitted.

[28] B.A. Cressy, Electron microscopy of serpentinite textures,Can. Mineral. 17 (1979) 741^756.

[29] W.F. McDonough, T.R. Ireland, Intraplate origin of ko-matiites inferred from trace elements in glass inclusions,Nature 365 (1993) 432^434.

[30] I. Kushiro, E¡ect of water on the composition of magmasformed at high pressures, J. Petrol. 13 (1972) 311^334.

[31] K. Hirose, Melting experiments on lherzolite KLB-1under hydrous conditions and generation of high-magne-sian andesitic melts, Geology 25 (1997) 42^44.

[32] S.W. Parman, T.L. Grove, High pressure water undersa-turated liquidus phase relations of komatiite from theBarberton Mountainland, South Africa, EOS TransAGU 78 (1998) S374.

[33] T.L. Grove, S.W. Parman, J.C. Dann, Melt generationconditions for hydrous Barberton komatiite magmas:subduction zone versus deep mantle plumes, EOS TransAGU 80 (1999) S370.

[34] T. Komiya, S. Maruyama, S. Nohda, T. Masuda, M.Hayashi, S. Okamoto, Plate tectonics at 3.8^3.7 Ga; Fieldevidence from the Isua accretionary complex, southernWest Greenland, J. Geol. 107 (1999) 515^554.

[35] T.M. Kusky, W.S.F. Kidd, Remnants of an Archean oce-anic plateau, Belingwe greenstone belt, Zimbabwe, Geol-ogy 20 (1992) 43^46.

[36] A.V. Sobolev, M. Chaussidon, H2O concentrations in pri-mary melts from supra-subduction zones and mid-oceanridges: implications for H2O storage and recycling in themantle, Earth Planet. Sci. Lett. 137 (1996) 45^55.

EPSL 5869 19-6-01


[37] D.A. Clague, W.S. Weber, J.E. Dixon, Picritic glassesfrom Hawaii, Nature 353 (1991) 553^556.

[38] T. Inoue, H. Yurimoto, Y. Kudoh, Hydrous modi¢edspinel, Mg1:75SiH0:5O4 : a new water reservoir in lower-most upper mantle, Geophys. Res. Lett. 22 (1995) 117^120.

[39] H. Yusa, T. Inoue, Y. Ohishi, Isothermal compressibilityof hydrous ringwoodite and its relation to the mantlediscontinuities, Geophys. Res. Lett. 27 (2000) 413^416.

[40] D.L. Kohlstedt, H. Keppler, D.C. Rubie, Solubility ofwater in the K, L and Q phases of (Mg, Fe)2SiO4, Contrib.Mineral. Petrol. 123 (1996) 345^357.

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