Chemical Geology, 113 ( 1994 ) 191-204 191 Elsevier Science B.V., Amsterdam

[EW]

Characterization of spinel peridotites by olivine-spinel compositional relationships: Review and interpretation

Shoji Arai Department of Earth Sciences, Kanazawa University, Kanazawa, Ishikawa 920-11, Japan

(Received April 27, 1992; revised and accepted August 3, 1993)

ABSTRACT

A comprehensive review on igneous petrological characteristics of mantle-derived spinel peridotites was made on the basis of their olivine-spinel compositional relationships. The spinel peridotites (harzburgites and lherzolites), of both massif and xenolithic derivations, plot in a narrow band, the olivine-spinel mantle array, in terms of Fo of olivine and C # (Cr/(Cr + A1) atomic ratio) of chromian spinel. The Cr # of spinel grows rapidly within the olivine-spinel mantle array with a slight increase of Fo. Their Cpx/(Opx + Cpx ) volume ratio decreases towards the high-Fo, high-Cr # end; it is 0.1 for Cr # = 0.5-0.6. Peridotite suites from each tectonic setting lie in a particular part of the olivine-spinel mantle array.

The olivine-spinel mantle array possibly consists of integrated Fo-Cr# residual trends formed at various conditions (Ptou~ and Pn2o ). Cr# of residual spinel coexisting with olivine of a particular Fo increases with a decrease of Ptot~J and/ or with an increase of Pmo on partial melting. Origins of peridotite suites can be constrained to some extent in terms of the Fo-Cr# residual trends, for example, some of the Japan-arc mantle peridotites and fore-arc peridotites are low-pres- sure and/or hydrous restites, and subcontinental and oceanic hot-spot peridotites are high-pressure and/or anhydrous restites. Fertile alpine-type lherzolites with Cr # ~ 0.1 are of subcontinental origin. Other alpine lherzolites are most fre- quently of sub-arc origin, sometimes of sub-ocean floor origin, and rarely of sub-continental origin. Most of the alpine- type harzburgites are of fore-arc origin.

1. Introduction Fujii and Scarfe, 1985). Dick and Bullen (1984) are successful in characterizing abyssal

Spinel peridotites are interpreted to be man- and alpine-type spinel peridotites by using spi- tle restites (e.g., Dick and BuUen, 1984; Arai, nel chemistry, especially by its Cr# [ =Cr/ 1987) formed in relatively low-pressure re- (Cr+A1) atomic ratio]. Later, Bonatti and gions of the upper mantle (e.g., Kushiro and Michael (1989) summarized petrological Yoder, 1966; Green and Ringwood, 1967). characteristics of spinel peridotites from con- Refractory low-Ca,A1 peridotites, however, can tinental rifts, ocean basins and fore-arc regions have modal chromian spinel both in the pla- in terms of various parameters including Fo of gioclase lherzolite and in the garnet lherzolite olivine and Cr# of spinel. They concluded that stability fields (e.g., MacGregor, 1970). The there is a variation in the degree of depletion spinel-bearing peridotites are the most corn- (or partial fusion) of mantle peridotites in re- mon of all mantle-derived rocks that we can sponse to the difference of tectonic setting. In obtain on the Earth's surface both as xenoliths this paper I intend to systematically compile in volcanic rocks and as alpine-type peridotite igneous petrological characteristics of mantle- masses (e.g., Nixon, 1987; Arai, 1990). Many derived spinel peridotites in terms of combi- important magmas could be formed or re- nation of Cr# of spinel and Fo content of co- leased from the mantle restites within the spi- existing olivine. To use simple Fo-Cr # rela- nel-peridotitefield (e.g.,Tatsumietal. , 1983; tionships for the compilation is of great

0009-2541/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDL" 0009-2541 (93 ) E0196-Z

192 s. ARA]

advantage; F o - C r # data for peridotites are burgite (or lherzolite) xenoliths in African easily available both from literature and from kimberlites (e.g., Boyd and Nixon, 1975; Her- natural rocks in the laboratory. Reid and vig et al., 1980) are omit ted from the data Woods (1978) also discussed the origin of source for the olivine-spinel mantle array as mantle peridotite xenoliths by means of a sim- ment ioned below (Fig. 1 ). The Cpx/ ilar plot, Fo of olivine vs. C r / ( C r + A1 + Fe 3 + ) (Opx + Cpx) volume ratio gradually decreases atomic ratio, towards the refractory (high-Fo, high-Cr# )

end of the olivine-spinel mantle array; the ra- 2. Olivine-spinel mantle array tio is 0.1 at C r # =0.5-0.6. In this article

"harzburgite" and "lherzolite" are used in a 2.1. Def in i t ion different way from the standard IUGS nomen-

clature; the boundary between them is set on

The olivine-spinel mantle array was pro- the C p x / ( O p x + C p x ) ratio=0.1 instead of the posed by Arai ( 1987, 1990) as a mantle peri- Cpx mode of 5%. The degree of refractoriness dotite restite t rend formed in the spinel lher- and other petrological characters ofperidotites zolite field in terms of Fo content of olivine depend rather on the ratio than on simple cli- and Cr # of spinel (Fig. 1 ). Xenolithic and al- nopyroxene volume (e.g., Arai, 1984; Dick and pine-type peridotites as a whole show essen- Fisher, 1984). tially the same trend on the F o - C r # plane, al- Proportion and chemistry of minerals in though frequency of the rock type (or C r # of peridotites are variable even within a speci- spinel) is different between the two (Fig. 1 ). men (cf., Dick et al., 1984). The F o - C r # re- Note that garnet-free spinel (chromite) harz- lationships are, therefore, dependent on which

grains are analyzed. For the purpose of this ~o study the core compositions of large grains may

Xe,o/,,h A,p,,e-,ype be favorable. Fo content of olivine in perido- o~ tites is essentially unchanged during subsoli-

ooo ~ dus stages except for local variations near cli- o +~ ° ° nopyroxene and chromian spinel. It could be

o °~,. significantly changed by metasomatism as ~'° ".ai~o ment ioned below. Cr # of spinel may be kept 0 5 i. °

• 4 almost constant during subsolidus stages be- .~'1 . . # ~ °

:'.:.~ "":.'.'. cause Cr-A1 partitioning between spinel and "'~.'~.~r" "'~" orthopyroxene, one of main phases which

• "" could contain appreciable amounts of A1 and " "~..... Cr, is invariable at subsolidus temperatures

~5 . . . . 9~ . . . . 0'5 9's . . . . 9'0 . . . . A' (Fig. 2). Cr # of spinel rims, however, could Fo olivine Fo olivine be changed during subsolidus deformation

Fig. 1. Relationships between the Fo content of olivine stages (Ozawa, 1989). The volume of spinel and the Cr/(Cr+A1) atomic ratio ( = C r # ) of spinel in may increase with temperature decrease as mantle-derived spinel peridotites (Arai, 1987). Xenol- Tschermak's components are extracted from iths are mainly from alkali basalts in continental rifts and oceanic hot spots. Chromite peridotite xenoliths in kim- pyroxenes to form modal spinel (Green and berlite (e.g., Hervig et al., 1980) are omitted as men- Ringwood, 1967). The Fo-Cr# relationships tioned below in the text. Open circles= harzburgite; closed in mantle-derived peridotites, which are m o r e circles = lherzolite. Note that the peridotites from the two or less recrystallized under subsolidus condi- derivations make a roughly common trend ( = olivine- spinel mantle array). See text for further details. For data tions, are essentially inherited from the ig- sources see Arai (1990). neous-stage ones (also see Ozawa, 1986 ). The

C H A R A C T E R I Z A T I O N O F S P I N E L P E R I D O T I T E S B Y O L I V I N E - S P I N E L C O M P O S I T I O N A L R E L A T I O N S H I P S 19 3

• Noyamedakef&200°C) (~. subsol idus fo rmat ion of A I - r i c h phase

• Kurose (900~I,000°C) b . metasomaf i sm • CIM (800~900°c)

1.0 ~ C. f rac t iona l crysta l l izat ion

....... ,,to.o \ ? . j . - " ,~-~.-':--"¢~ ~ Greet Dyke

2 05 o~3 ,,--~ --

/ " I1~ ,,i,~." Ryozen volcano, NE Japan

• J25o .~ deques ' Greenfl9eo) 0 5 • • • • • • c " ' " " - " J , % - ~ % , % - ' 2 - " - -.~C 'F, ",A # / ' o Pyrolite

0 ~ I i

0.] O.2 0 . 3 ~ C I i i , , i I , , i , I , i , , I i i ,

Opx Cr / (C r÷A I ) 95 90 85 80

Fo o l i v i n e

Fig. 2. Cr-A1 partitioning between spinel and orthopyrox- ene in peridotitic assemblages with different equilibrium Fig. 3. Fo-Cr # relationships of cumulates and modified conditions. Noyamadake & Kurose=xenoliths from the peridotites. Data for Great Dyke, Zimbabwe, cumulates Southwest Japan arc (Table 1; Arai and Hirai, 1983; Hirai, and Ryozen volcanics, northeast Japan (Yoshida et al., 1986); CIM=low-temperature alpine peridotites from the 1985 ) are after Wilson (1982) and S. Arai (unpublished Circum-Izu Massif region, central Japan (Arai, 1991); data, 1990), respectively. Trends a and b are after N. Tak- Pyrolite & Tinaquillo=residues of anhydrous melting ex- ahashi (1988) and K. Goto and Arai (1987), respec- periments at 10 and 15 kbar (italic numbers= tively. OSMA=olivine-spinel mantle array. temperature ) by Jaques and Green (1980). Note that the partition coefficient does not depend on temperature, lationships of the Horoman peridotites, north-

ern Japan. Subsolidus formation of other alu- scattering o f F o - C r # in peridotites (Fig. 1 ) i s

minous phases, especially plagioclase, makes partly real and partly due to some analytical the remaining spinel Cr-enriched by selective

problems, including analytical uncertainties and inadequate choice of analyzed grains. We consumption of Al-spinel components (N. should notice how carefully olivine and spinel Takahashi, 1988 ). The subsolidus formation of were analyzed when we use data from the plagioclase or possibly garnet may shift the literature, spinel peridotites of f the olivine-spinel mantle

array in h igh-Cr# direction at constant Fo

2.2. Peridotites plotting off the olivine-spinel (Fig. 3 ). mantle array If peridotites are of cumulate origin they plot

within or in low-Fo area off the olivine-spinel

The spinel peridotites metasomat ized by mantle array (Fig. 3). Coexisting olivine-spi- melts of fluids enriched with incompatible ele- nel pairs in ultramafic cumulates or in Mg-rich ments are sometimes shifted off the ol ivine- volcanics always plot in this way (Fig. 3 ).

spinel mant le array in low-Fo directions (Irv- ing, 1980; K. Goto and Arai, 1987) (Fig. 3 ). 3. Upper-mantle spinel peridotites from known Arai and Takahashi (1989 ) demonstra te that tectonic settings formation of secondary phlogopite and amphi- bole, for which alkali-rich aqueous fluids were Spinel peridotites from a particular tectonic responsible, did not alter original F o - C r # re- setting lie, on average, in their particular area

194 s. ARAI

within the Fo-Cr # plane (Fig. 4). In the dia- ray (Fig. 4A). Note that the oceanic perido- grams apparent metasomafized peridotites, rites ever documented are mostly from frac- that is, peridotites which contain secondary A1- ture zones of slow-spreading oceanic ridge rich mineral (s) such as pargasite and phlogo- systems. It may not be guaranteed, therefore, pite, are excluded, that they represent the upper mantle of ordi-

nary ocean floors sufficiently remote from the 3.1. Ocean-floorperidotites fracture zones, especially those of oceans with

a fast-spreading ridge system such as the Pa-

The ocean-floor peridotites from the Atlan- cific Ocean. tic and Indian Oceans are extensively de- scribed by Hamlyn and Bonatti (1980), Dick 3.2. Back-arc basin peridotites

and Bullen (1984), Dick et al. (1984), Mi- chael and Bonatti (1985), and Dick (1989). Data on back-arc basin peridotites are scarce. Cr # of their spinels varies from 0.1 to 0.6 (Fig. Ishii (1987) reported a peridotite xenolith in 4A); relatively fertile lherzolites with an alkali basalt dredged in the Sea of Japan Cr# <0.3 are dominant (Dick and Bullen, (38 ° 12.2'N, 132°34.7'E). It contains olivine 1984). The ocean-floor peridotites sometimes of Fo9o.5 and spinel of Cr# =0.46 (Ishii, contain primary plagioclase (Dick and Bullen, 1987 ). Ninomiya and Arai ( 1992 ) described 1984; Michael and Bonatti, 1985; Dick, 1989). a harzburgite xenolith in a recent calc-alkaline They roughly make a trend in a medium-Fo andesite from the Oshima-Oshima volcano in part (at a particular Cr # ) of the low-Cr# half the Sea of Japan, a back-arc basin, which is (Cr # < 0.6 ) of the olivine-spinel mantle or- representative of the back-arc basin mantle and

has olivine of Fo91 and chromian spinel of Cr # = 0.5. The Circum-Izu Massifperidotites,

'°[ . [A] . [B] , [C] central Japan, may represent the Miocene up- -'~ 1 \ ~ \Ocean Floor \ \ \ c- t ~_ ~, ~ \ . , Fore-Arc 4\ \\lOceon~CHot Spot p e r m a n t l e of the Shikoku Basin, a n d b a c k - a r c

~_ l ,~ , ', ', , basin of the Izu-Bonin arc system (Arai, f ~ .~ "'" , ,~ 1991 ). They have olivine of FO9o_92 and spinel

" . I . - I ~1 I o5 : . , , of Cr# 0.4-0.6. Those data, although in-

<~ [ ', "~ '~ ~ ' complete, suggest that the back-arc basin peri- \ , ,." • , dotites may be refractory lherzolite and Cpx-

' " ' ,~'". bearing harzburgite, with Cr # = 0.4-0.6. • . . i \

9'5 9o 85 95' 9'0 ~5 ~5 . . . . ~o . . . . ~'5

Vo o,v~ne 3.3. Fore-arc peridotites , o [ " , [D ] - . \ [E ] . I F ]

1 \ \ ' J a p a n A r c s \ \ \C on t i ne n t ~ ' . , . '~\ \ \ A f r i c a n Fore-arc peridotites are available from con- "Q- | ~ ' \\ "'" '/ ";4 * ' Croton

• , , : ' , tinent-ward walls or trench-slope breaks of ~' f ~'~' " "" '~' "" ~", "'~'~i " ~ some trenches such as Tonga and Mariana o5 '~.~,.~... '.~',,,...~,~.., ,,, ~ which are non-accreting convergent plate

", ,~:~, , ~ boundaries (Bloomer and Hawkins, 1983; + ~ ~ "?, ~ ~ . . I I

[ ~, X ~ . ~ . , , \ Bloomer and Fisher, 1987). In contrast to the - " ocean-floor peridotites (Fig. 4A) the fore-arc

95 9o ~5 95 9o 85 ~5 . . . . 9'o . . . . & peridotites lie in the high-Cr# half of the O1- Fo olivine

Fig. 4. Fo-Cr# relationships of spinel peridotites f rom ivine-spinel mantle array (fig. 4B); they are known settings. For the data sources see text and Arai mostly harzburgites (Fig. 1 ). The available (1990). spinel chem!stry (Bloomer and Hawkins, 1983;

CHARACTERIZATION OF SPINEL PERIDOTITES BY OLIVINE-SPINEL COMPOSITIONAL RELATIONSHIPS 19 5

Bloomer and Fisher, 1987 ) suggests that the Japan arcs had been set up. The xenolith local- fore-arc peridotites may cover a slightly wider ities are concentrated on the Japan Sea side of range, 0 . 4 < C r # <0.8, of the mantle array. A1- the arcs or within the Sea of Japan (e.g., E. though we need more systematic data, they Takahashi, 1978; Aoki, 1987). It is possible, seem to lie in a medium- to low-Fo area of the therefore, that they are representative not of high-Cr # part of the mantle array (Fig. 4B). typical island-arc mantle but of some transi-

tional mantle from the island-arc proper to 3.4. Oceanic hot-spot peridotites back-arc setting.

The range of Cr # of spinel in the Japan arc Deep parts of the oceanic lithosphere may peridotites (Fig. 4D) is almost the same as that

be represented, at least partly, by peridotite in the ocean-floor peridotites (Fig. 4A); they xenoliths in highly alkaline magmas in some are mostly lherzolites and rarely harzburgites oceanic hot spots such as Hawaii (A. Goto and (Fig. 1 ). Compared to the data of Dick and Yokoyama, 1988; Sen, 1987, 1988) andTahiti Bullen (1984) the Japan arc peridotites are (Tracy, 1980). They are dominated by rela- slightly more refractory than the ocean-floor tively fertile lherzolites with Cr# < 0.3 (Fig. peridotites on average. 4C). Note that the oceanic hot-spot perido- The Japan arc peridotites are similar in the tires have generally lower Cr # of spinel at a olivine-spinel mantle array to the ocean-floor given Fo value of olivine than the peridotites peridotites but are more diffuse towards low obtained from the ocean floors (Fig. 4AC). It Fo (Fig. 4AD). is possible that the majority of them occupy a higher-Fo, lower-Cr# part of the olivine-spi- 3.6. Subcontinentalperidotites nel mantle array relative to the ocean-floor ones (Fig. 4AC). The oceanic hot-spot peridotites Subcontinental peridotites are available from resemble a fertile group of subcontinental per- xenoliths in alkaline basalts which usually erupt idotites described below (Fig. 4E). on continental rift zones (e.g., Nixon, 1987).

Alkali basalts from Loihi Seamount, Ha- Fertile lherzolites with Cr# <0.2 (especially waii, contain dunite and harzburgite xenoliths around 0.1 ) surpass other kinds of peridotites which have Mg-rich (Fo89.5_92.6) olivine and in the subcontinental upper mantle (Fig. 4E). Cr-rich (Cr # , > 0.5 up to 0.75 ) spinel (Cla- It is noteworthy that the majority of subconti- gue, 1988 ). As Clague ( 1988 ) concluded they nental lherzolites lie in a relatively high-Fo area represent the uppermost part of the mantle, of the low-Cr # part of the olivine-spinel man- possibly residual peridotites for the Pacific tie array (Fig. 4E; cf. Fig. 4D). mid-ocean ridge basalts (MORB). Some of the Loihi harzburgites which have high-Cr# 3.7. Subcratonicspinelharzburgites (>0 .7) , high-TiO2 ( ~ 1 wt%) spinel could be

genetically related to shield-building tholeiite Xenoliths in kimberlites, mainly from Afri- activities of Hawaii Island. can cratons, are frequently garnet-free spinel

harzburgites and lherzolites. They contain 3.5. Japan islandarcs high-Fo olivine (up to Fo95 ) (e.g., Boyd and

Nixon, 1975; Hervig et al., 1980), and they On the Japan arcs almost all mantle xeno- show a high-Fo trend on the Fo-Cr# plane,

lith-bearing magmas, which are younger than quite different from the ordinary olivine-spi- 11 Ma (Uto et al., 1986), erupted after the nel mantle array (Fig. 4F). The extremely de- main stage of Sea of Japan opening ( 14-16 Ma; pleted spinel harzburgites and lherzolites be- Otofuji et al., 1985), that is, after the present neath the African cratons were formed not in

19 6 S. ARAI

the spinel-lherzolite stability field but in the ~o "0phlolitie . . . . Root-zone"

garnet-lherzolite stability field as restites or cumulates related to some Mg-rich magmas o such as picrites and/or komatiites (e.g., E. _= Takahashi, 1990). ~ o°oo

o O~o .

4. Alpine-type peridotites o5 °"~° ..4 o

. ° Z " " °

On-land alpine-type peridotites have a wide • '-" :. ... range of C r # from 0.08 to 0.95 (Fig. 1 ). They • .'" " are evenly distributed within the olivine-spi- ~ ~" ".-; nel mantle array (Fig. 1 ), in contrast to the "~...... xenolithic peridotites, in which relatively fer- , . . . . . , . . . . . . . . . . . . . tile lherzolites are predominant (Fig. 1 ). A1- 95 90 95 90 85 ,Co olivine Fo olivine pine peridotites are usually divided into two categories, harzburgite (ophiolitic) and lher- Fig. 5. F o - C r # relationships in alpine-type peridotites

from "ophiolit ic" (or harzburgite subtype) masses and zolite (root-zone) subtypes (den Tex, 1969; from "root-zone" (or lherzolite subtype) masses. The Jackson and Thayer, 1972 ). The former often distinction of the two types of alpine masses are after den form the basal peridotite member of the "rex ( 1 9 6 9 ) a n d J a c k s o n a n d T h a y e r (1972). Open cir- ophiolite suite, usually associated with subor- des= harzburgite; closed circles=lherzolite.

dinate amounts of dunite, lherzolite and Cr# <0.5 (Fig. 5). In terms o f F o - C r # rela-

chromitite. The latter make lherzolite-domi- nant masses usually free from the so-called tionship the root-zone peridotites are similar ophiolitic igneous sequence. Nicolas and Jack- to either the subcontinental, the suboceanic or son (1972) interpreted that the harzburgite- the sub-arc peridotites (Fig. 4). The fertile

lherzolites with Cr# of ~ 0.1 from some A1- subtype peridotites in the Mediterranean area are of suboceanic origin and the lherzolite-sub- pine masses (e.g., Balmuccia and Baldissero; type ones are of subcontinental origin. Note Ernst, 1978) are identical in mineralogy to the that this classification is too simple and is not subcontinental xenolithic spinel peridotites.

always definitely applied to individual peri- dotite masses (Spray, 1982). Exotic blocks of 5. Discussion fertile lherzolite are sometimes included by harzburgite-dominant tectonites or vice versa 5.1. Olivine-spinel mantle array as Fo-Cr # (Ozawa, 1988; N. Takahashi, 1991 ). residual trends

The Cr# of the ophiolitic peridotites is > 0.2 (Fig. 5). Peridotites near the harzburgite- The olivine-spinel mantle array was imi- lherzolite boundary (Cr# ~0.5) are predom- tared in the laboratory by Jaques and Green inant. Ophiolitic harzburgites resemble fore-arc ( 1980 ). They conducted melting experiments peridotites in terms of F o - C r # relationship at anhydrous conditions (up to 15 kbar) on a (Fig. 4B). Ophiolitic lherzolites resemble some pyrolite and a Tinaquillo lherzolite minus 40% of the ocean-floor ones (Fig. 4A). It is note- of olivine, successfully leaving residual phases worthy, however, that the lherzolites with ofolivine, pyroxenes and spinel (Fig. 6A).The Cr # < 0.3, the most common type in the sub- residual mineralogy of their experiments (Ja- oceanic mantle, are rare in the ophiolitic peri- ques and Green, 1980; A.L. Jaques, pers. com- dotites (Fig. 5 ). mun., 1988 ) indicates that the F o - C r # resid-

The root-zone peridotites are lherzolites with ual trend has a sigmoid form from the lherzolite

CHARACTERIZATION OF SPINEL PERIDOTITES BY OLIVINE-SPINEL COMPOSITIONAL RELATIONSHIPS 197

(fertile) end to the dunite (refractory) end ~o °®4 \ (A} (Fig. 6A ). Note that the residue of Jaques and "¢ \ ~ dunite

\ I 3o%\\ O horzburgife Green (1980) grow Fo-rich peridotites more 1~(~5k b \ • lherzolite rapidly than the natural ones (Fig. 6A), which I \, "ri ~ 2°°/° 4O'~o ~/I~'~'// is due to rapid olivine consumption because the

starting material is an olivine-subtracted (by / ,~ '/ 40%) natural lherzolite.

05 / ~ I The experimental results of Jaques and l~Skb 1 Green (1980) also demonstrated a possibility ~ / of pressure dependence of the residual Fo-Cr #

"~ / lOkb I i \ trend (Fig. 6A). Cr# of spinel equilibrated

~- ~ L \ with lower-pressure melts is higher than that \ \ \ equilibrated with higher-pressure melt for a

\ \ given Fo value (or a given degree of partial fu- , , , . . . . J . . . . J sion) (Fig. 6B). This may be due to change in

95 90 85 Cr-AI partition coefficient between spinel and ,Co olivine melt, which is severely dependent on melt chemistry (e.g., Allan et al., 1988), especially on SiO2 contents (cf. Dick and Bullen, 1984).

Dry (a) Cr may be more preferentially partitioned into spinel when melts have higher SiO: contents,

.~ that is, higher degree of Si-O polymerization. " The SiO2 content of partial melts in mantle ,o

~ \ ~ \ peridotites increases with a decrease of total \ pressure (e.g., Jaques and Green, 1980; E. o Takahashi and Kushiro, 1983) or with an in-

x e \ / crease OfPH20 (Kushiro, 1969). Hydrous Fo- Cr # residual trends are expected to be shifted

~ to the high-Cr # direction relative to the an- ~ hydrous ones. We cannot, however, discrimi-

nate low-pressure anhydrous restites from hy- pr~=ord~o, drous restites in terms ofFo-Cr # relationship

alone. Fo Data from natural peridotites may support

the possibility of pressure dependence of the Fig. 6. Fo -Cr# residual trends and the olivine-spinel mantle array. Fo-Cr # residual trend: A. Fo-Cr# residual trends determined by Jaques and ( 1 ) The relationship between the ocean- Green (1980) and a possible form of the entire olivine- f loor peridotites (Fig. 4A ) a n d the oceanic hot- spinel mantle array. The trends of Jaques and Green spot peridotites (Fig. 4C), which are t he shal - (1980) are slightly deviated from the natural mantle ar- ray because their starting material is a modified (40% ol- lower and deeper parts, respectively, of the ivine subtracted) one from a Tinaquillo peridotite. De- oceanic lithosphere, is consistent with the gree of partial fusion (% of melt not in peridotite minus interpretation mentioned above. The latter olivine but in peridotite) is shown by thick broken lines have spinel with lower Cr # than the former at after Jaques and Green (1980). B. A schematic diagram to show a possible pressure de- a given Fo value of olivine (or have higher Fo pendence of the Fo -Cr# residual trend at anhydrous a t a given Cr# ) (Fig. 4). conditions (pl= plagioclase; sp = spinel; ga = garnet). ( 2 ) The Fo-Cr # trend of peridotite x e n o -

198 s. ARAI

liths from Kurose, southwestern Japan, is on 900-1000°C) (Arai andHirai, 1983) (Fig. 2). average slightly shifted in the low-Fo direction The geobarometer of Kohler and Brey (1990) compared to those ofperidotite xenoliths from yields different pressures, 5 and 17 kbar for Noyamadake, southwestern Japan (Fig. 7; Ta- Kurose and Noyamadake peridotites, respec- ble 1 ). As mentioned above, the Noyamadake tively. The Ca contents in olivine were care- peridotites have a higher equilibrium temper- fully determined by ion probe (Table 1 ). Their ature than the Kurose ones (~1200°C vs. Fo-Cr# trends are qualitatively consistent

with the results of Jaques and Green (1980) ~ ° (A) (Fig. 6A). The Noyamadake basalt has higher ~ Noyamadake(1,200 C) \ o L \ contents of normative nepheline (9.1 wt%) / ~ ~ \ and olivine (17 wt%) than the Kurose ones,

I Kurose (9OONt,OOO *C) which has 6.0 and 13 wt% respectively (Hirai, ~ \ ~ / 1986), indicating a deeper origin for the Noy-

amadake basalt than the Kurose one (e.g., Ku- o5 shiro, 1969). Na content of clinopyroxene in

~c , peridotites is also concordant with the differ- ence in the depth of derivation (cf. Korn-

-~ probst et al., 1981 ). It may depend both on the 5 degree and the depth of partial fusion (e.g.,

Mysen and Boettcher, 1975) if the starting ~ \ material is the same. Combined with the Cr #

\ of coexisting spinel as a guide to the degree of o , . . . , . . , , partial fusion, the Na of clinopyroxene is

95 90 s~ clearly higher in the Noyamadake lherzolites Fo olivine

than in the Kurose ones (Fig. 7; Table 1 ). It is concluded, therefore, that the Noyamadake

N%O cp, (B) Fo-Cr# trend was made at higher pressure

o Noyomodoke than the Kurose one. 2 " Kurose A subsolidus formation of spinel peridotites

from garnet peridotites may be possible o° o (Smith, 1977). Garnet peridotites are, how-

ever, usually suffered from partial fusion upon o oO o ° decompression. Garnet may be transformed

o 2 into pyroxene-spinel aggregate with melt ex- : o • traction (Nicolas et al., 1987; N. Takahashi and

, , , Arai, 1989). In such cases and if the degree of o~ o2 o3 o4 o5 0.6 o,7 08 partial fusion is sufficiently high, spinel could

Cr/(Cr+AI,) spinel be in equilibrium with olivine and melt. Fig. 7. Mineral chemical characteristics of two xenolith suites (Kurose and Noyamadake) from the Southwest Ja- pan arc (Arai and Hirai, 1983; Hirai, 1986). 5.2. Origin ofdunites A. Fo-Cr # trends (squares= Noyamadake peridotites; circles=Kuroseperidotites; open symbols=lherzolite; If the olivine-spinel mantle array is the filled symbols= harzburgite), mantle restite trend, the dunite restites, if any, B. Relationships between Na20 wt% ofclinopyroxene and should lie in a region more refractory than the Cr # of spinel. The Noyamadake xenoliths are derived from a greater depth than the Kurose ones as discussed in harzburgites on the mantle array (Fig. 7A; the text. OSMA =olivine-spinel mantle array. Mysen and Kushiro, 1977; Dick and Fisher,

B1

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200 s. ARAI

1984)• This is not the case, however, for natu- and off the mantle array (Fig. 8A). They rarely ral dunites (Fig. 8A). The dunites, both of plot in the high-Fo region offthe mantle array xenolithic and alpine derivations, plot within (Fig. 8A). Their Fo-Cr # relationships in re-

spect to the olivine-spinel mantle array (Fig. ( A ) Duni te 8A) are almost the same as those of volcanic

1 0 • Xenolith rocks (pairs of phenocrystic olivine-spinel) : . . ~ , ~ o • Alpine-type relative to the mantle array (Fig. 8B). This

\ "" .. '.~ definitely shows that the dunites are mostly not _ • ...... .i::~:i,~:~.....:.._.. ~'. '~ mantle restites but could be cumulates from

~\ i relatively primitive magmas. However, Dun- .!j~. ~:(t~i ~ gun and Ave Lallemant ( 1 9 7 7 ) a n d Quick . , • o .~

" o'oo'" (1981) reported replacive dunites, and fur: o

°" "°.~ thermore Nicolas and Prinzhofer (1983 ) con- / cluded that the transition-zone dunites of

., o.. '~ ophiolites are mostly of replacive origin. If the dunites are of replacive origin, that is, if they i , . . • \

~ " \ are produced from mantle peridotites by selec- ~\~ \ tive consumption of pyroxenes (e.g., Fisk,

1986), their constituent minerals (especially . . . . . . . . . . . olivine) are expected to be rather similar in 9 5 O0 8 5

Fo olivine chemistry to equivalents in mantle peridotites. This may result from that the composition (e.g., Mg# ) of dunite should be buffered by large amounts of mantle minerals (especially

([3) Volconic Rocks olivine) (e.g., Kelemen, 1990), especially when the melt/mantle peridotite ratio is low. I O . .

~ - Dunites plotted within the olivine-spinel • mantle array are, therefore, possibly of repla-

\ ~ "" \ \ cive origin• Preliminary analyses for Troodos \ . \ . \. : (Cyprus) and Oman peridotites indicate that

~3 + \~. ":'. l'~'" ~ """ -:' . •. olivine in the transition-zone dunites is almost / • C :'." similar in chemistry to that in mantle harzbur-

gites, being consistent with their replacive ori- o~ ~ " "' (- "'~'" "'% "" gin. Chromian spinel in the dunites is, how- I " ' " ~ . " • : : " . . : "

I " : ~Ii ~ "." . . . ~';. ever, more refractory than that in harzburgites.

~.'.,(-" . ' : : ' . In the Troodos ophiolite a transition-zone • ..~..- / \~'" "'" dunite has olivine of FO9L5 and spinel of ~ Cr# =0.686 whereas the surrounding harz- \

o, burgite has olivine of FO9o.9 and spinel of , . . . . . . . . . . . . ~ Cr# = 0.533. In the Oman ophiolite olivine in 95 9o 8s 8o transition-zone dunites is slightly less magne-

,Co o l i v i n e

sian (FO9o.7 and FO9t.2 ) than that in harzbur- Fig. 8. Fo -Cr# relationships in dunites (A) and in vol- gite (Fo91 5), whereas spinel in the former is canic rocks (B). Note that almost all points are within or at lower Fo contents than the olivine-spinel mantle array more refractory (Cr # = 0.581, 0.761 ) than in (OSMA). the latter (Cr # = 0.461 ).

CHARACTERIZATION OF SPINEL PERIDOTITES BY OLIVINE-SPINEL COMPOSITIONAL RELATIONSHIPS 201

5.3. Evaluation of the origins of spinel 6. Conclusions peridotites in terms of the olivine-spinel mantle array (1) Upper-mantle spinel peridotites (lher-

zolites and harzburgites ) have a limited range of Fo-Cr # , which is termed the "olivine-spi-

There is a contrast between the Japan arc nel mantle array", on the Fo-Cr# plane. mantle peridotites and the continental ones in Harzburgites and lherzolites occupy the high- the Fo-Cr # relationship: Fo of olivine at given Cr # ( > 0.5 ) and low-Cr # ( < 0.5 ) halves, re- Cr# 's is lower in the former than in the latter spectively, of the olivine-spinel mantle array. especially in the low-Cr # region (Fig. 4). The (2) The olivine-spinel mantle array con- former may be higher-pressure restites than the sists of integrated Fo-Cr # residual trends latter, which also could be restites formed un- formed at various conditions. Restites formed der hydrous conditions (Fig. 6). Low-pressure by low-pressure melting have higher Cr # of and/or hydrous conditions are more easily spinel at a given degree of partial fusion than available within the wedge mantle than within those formed by high-pressure melting. the subcontinental mantle. The fore-arc peri- (3) Upper-mantle peridotites from various dotites may be high-Cr # equivalents to the is- tectonic settings occupy respectively distinc- land-arc peridotites (Fig. 4). tive regions within the olivine-spinel mantle

The averaged Fo-Cr # trend for the alpine- array, showing different mantle melting con- type peridotites, except for the fertile ones with ditions. The Japan arc mantle peridotites, low Cr # ( ~ 0.1 ), is slightly Fo poorer at a which have a relatively high Cr # of spinel at a given Cr # than that for the xenolithic perido- given Fo of olivine, may be restites formed by tires (Fig. 1 ). This may mean that the former low pressure and/or hydrous melting. The fore- have shallower origins than the latter, most of arc peridotites are high-Cr # analogues of the which represent the subcontinental or the deep Japan arc peridotites. In contrast, the perido- suboceanic mantle (Fig. 6). The fertile alpine tires from continental rifts and oceanic hot peridotites which have spinel with Cr # ~ 0.1 spots, which have a relatively low Cr # of spi- and Na-rich ( > 1 wt% of Na20) clinopyrox- nel at a given Fo of olivine, may be restites ene are of subcontinental origin. Other alpine formed by high-pressure melting. The subcra- peridotites are mostly either of sub-arc origin tonic spinel harzburgites, which have high Fo or of suboceanic origin, and rarely of subcon- of olivine and intermediate to high Cr # of spi- tinental origin (Figs. 1, 4 and 5 ). The ophiol- nel, may be high-pressure restites formed in the itic harzburgites (Fig. 5) are most probably garnet lherzolite stability field. derived from fore-arc mantle (Figs. 4 and 5 ). (4) Dunites are either of cumulus or of re-

Peridotites from St. Paul's Rocks, South At- placive origin in most cases. lantic (Roden et al., 1984) and from Zabargad ( 5 ) Alpine-type peridotites with low-Cr # Island, Red Sea (Bonatti et al., 1986 ), are very ( ~ 0.1 ) spinel and high-Na clinopyroxene are similar to the subcontinental peridotites in of subcontinental origin. Other alpine lherzo- terms of mineral chemistry; their Fo-Cr # re- lites are mostly either of sub-arc origin or of lationship (F088.7-90.4, C r# 0.09-0.15 ) is typi- suboceanic origin. Ophiolitic harzburgites are cal of that in the subcontinental peridotites o f f ore-arc origin. (Fig. 4). They are not representative of the ocean-floor mantle but are fragments of conti- Acknowledgements nental (=pre-oceanic rift) mantle as con- cluded by Bonatti and Michael (1989) and The author wishes to acknowledge K. Ozawa, Bonatti (1990). E. Takahashi, H. Hirai and N. Takahashi for

202 s. ARAI

discussions and encouragements. Comments a nonaccreting plate boundary. J. Geol., 95: 469-495. b y M.K. R o d e n , D. S m i t h a n d E.B. W a t s o n Bloomer, S.H. and Hawkins, J.A., 1983. Gabbroicandul-

tramafic rocks from the Mariana Trench: an island arc were very constructive and helpful to improve ophiolite. In: D.E. Hayes (Editor), The Tectonic and the manuscript. H. Yurimato kindly assisted Geologic Evolution of Southeast Asian Seas and Is- the S IMS analysis . K. N a k a m u r a k ind ly d ra f t ed lands, Part 2. Am. Geophys. Union, Geophys. Mon- the f igures u sed in th is pape r . T h i s s t udy was ogr., 27: 274-317.

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CHARACTERIZATION OF SPINEL PERIDOTITES BY OLIVINE-SPINEL COMPOSITIONAL RELATIONSHIPS 203

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