Dunite Formation Processes in Highly Depleted Peridotite: Case

JOURNAL OF PETROLOGY VOLUME 43 NUMBER 3 PAGES 423–448 2002

Dunite Formation Processes in HighlyDepleted Peridotite: Case Study of theIwanaidake Peridotite, Hokkaido, Japan

K. KUBO∗DEPARTMENT OF EARTH AND PLANETARY SCIENCES, TOKYO INSTITUTE OF TECHNOLOGY, 2-12-1 OOKAYAMA,

MEGURO-KU, TOKYO 152-8551, JAPAN

RECEIVED JULY 25, 2000; REVISED TYPESCRIPT ACCEPTED SEPTEMBER 5, 2001

Dunite formation processes in highly depleted peridotites are discussed mantle, it is useful to investigate this type of lithology.based upon a detailed study of the Iwanaidake peridotite, Hokkaido, The Mg# [= 100 × Mg/(Mg + Fe2+)] of olivine andJapan, which consists mainly of harzburgite with a small amount Cr# [= 100× Cr/(Cr+ Al)] of spinel have been usedof dunite. In the harzburgites, the Mg# [= 100 × Mg/(Mg previously as measures of the degree of chemical depletion+ Fe2+)] of olivine ranges from 91·5 to 92·5, and the Cr# [= as a result of partial melting and melt extraction in the100 × Cr/(Cr + Al)] of spinel from 30 to 70; in the dunites, mantle (e.g. Dick & Bullen, 1984; Arai, 1994). A highthe Mg# of olivine ranges from 92·5 to 94 and the Cr# of spinel Mg# of olivine and high Cr# of spinel indicate a highfrom 60 to 85, respectively. The NiO wt % of olivine in harzburgites degree of melting, as is suggested by peridotite meltingranges from 0·38 to 0·44, and in dunites from 0·35 to 0·37. experiments (Mysen & Kushiro, 1977; Jaques & Green,The Mg# and Cr# are higher and NiO wt % is lower in the 1980). Highly depleted peridotites (with high Mg# ofdunites than in the harzburgites surrounding the dunites. The Mg# olivine and Cr# of spinel) are found in a number ofand Cr# exhibit normal depletion trends expected from simple tectonic settings, such as subduction zones (e.g. ser-partial melting, whereas the NiO wt % shows an abnormal trend. pentinite seamounts at fore-arcs, Ishii et al., 1992; xeno-On the basis of mass balance calculations, dunites are considered liths in Japan, Kamchatka, and the Luzon–Taiwan arcs,to be derived from the harzburgites by a process involving incongruent Arai et al., 1998), cratonic regions (e.g. xenoliths inmelting of orthopyroxene (orthopyroxene→ olivine+ Si-rich melt).

kimberlites; Boyd & Nixon, 1975; Hervig et al., 1980), orHydrous conditions were necessary to lower the solidus, and thus

in the mantle section of ophiolites (e.g. Jaques & Chappell,melting of harzburgite was probably triggered by the introduction of

1980). In general, peridotite massifs are more suitablehydrous silicate melt. The dunite in this massif may have formedthan peridotite xenoliths for investigation of lithologicalin the mantle wedge above a subduction zone.spatial relationships in the upper mantle.

The Iwanaidake peridotite, central Hokkaido, Japan,is composed of harzburgite with intercalated dunite (Niida& Kato, 1978). Peridotite massifs are exposed alongKEY WORDS: depleted peridotite; hydrous melt; incongruent melting;

residual dunite; Iwanaidake peridotite the Kamuikotan belt intermittently and these massifs,including the Iwanaidake, are highly depleted in basalticcomponents (Kato & Nakagawa, 1986). Unlike the otherperidotite massifs in the Kamuikotan belt, however, the

INTRODUCTION Iwanaidake peridotite is free from extensive ser-pentinization. Thus, the Iwanaidake peridotite is highlyA highly depleted peridotite, such as dunite, is one ofsuitable for a case study of the petrogenesis of highlythe terminal products of partial melting in the upper

mantle. Therefore, to trace the evolution of the upper depleted peridotite massifs. In the Iwanaidake, dunite is

∗Corresponding author. Telephone: +81-3-5734-2338. Fax: +81-3-5734-3538. E-mail: [email protected] Oxford University Press 2002

JOURNAL OF PETROLOGY VOLUME 43 NUMBER 3 MARCH 2002

more depleted than harzburgite, based on Mg# of olivine 20 km; it consists of high- and low-pressure metamorphicrocks with a subordinate amount of ultramafic rocksand Cr# of spinel.

Recently, dunite formation processes have been ex- (Ishizuka et al., 1983).Some of the ultramafic rocks within the Kamuikotantensively debated. Dunite is an important constituent of

both ultramafic massifs and xenolith suites. Some dunite belt are considered to be sections of ophiolite complexes,as lithologies include pelagic sediments overlying thebodies show evidence of magma–wall rock reaction in

the upper mantle and the formation of dunite would basal ultramafic rocks (e.g. Horokanai ophiolite, Ishizukaet al., 1983). However, most of the ultramafic rocks arehave changed the composition of both the melt and

the host peridotite (e.g. Quick, 1981; Kelemen, 1990; not associated with an ophiolite sequence but ratheroccur as isolated peridotite massifs. Spatial variationsKelemen et al., 1995; Allan & Dick, 1996; Arai & Mat-

sukage, 1996; Dick & Natland, 1996). For these reasons, in mineral assemblage and chemical composition areobserved within the belt (Kato & Nakagawa, 1986). In thedunite formation processes are important to under-

standing magma genesis and upper-mantle evolution. northern part, the Al concentrations in orthopyroxene,clinopyroxene, and spinel are low, and the Mg# of olivineThree origins of dunite have been proposed: (1) residual

dunite; (2) cumulative dunite; (3) replacive dunite. Re- is high, suggesting that the peridotites here are a morerefractory residue than those in the southern part (Katosidual dunite is formed after extensive partial melting of

peridotite. Cumulative dunite is formed by fractionation & Nakagawa, 1986). According to Niida & Kato (1978),the Sarugawa ultramafic massif is divided into westernof olivine from a mafic melt. Replacive dunite is a product

of the reaction between a pyroxene-bearing host rock and eastern units (Fig. 1). The western unit peridotite issurrounded by the Sarugawa formation, composedand an olivine-saturated magma, which dissolves ortho-

pyroxene in the host peridotite and sometimes crystallizes mainly of mafic lavas and pyroclastic rocks with slate,limestone, chert, and sandstone. The Sarugawa formationolivine.

The main purpose of this paper is to propose a quan- corresponds to the Sorachi group, which is in the up-permost part of the Hidaka supergroup (Niida & Kato,titative model of dunite formation based on a detailed

petrologic study of the Iwanaidake peridotite. The re- 1978). The eastern unit peridotite is located within theunclassified Hidaka supergroup, which is considered tolationship between dunite and harzburgite has been

investigated by detailed petrological observations and be older than the Sarugawa formation, and which iscomposed mainly of slate and pelitic schist with sandstone,chemical analysis of mineral compositions. In this massif,

dunite is more depleted than harzburgite, although harz- chert, and pyroclastic rocks. Both units of the Sarugawaultramafic massif are in fault contact with the surroundingburgite itself is already depleted. Using this compositional

relationship between dunite and harzburgite, it is clear rocks, and the two units are also separated by faults.The Iwanaidake peridotite occurs within the Sarugawathat the dunite is a residue after a partial melting of the

harzburgite. On the basis of this interpretation, a model ultramafic massif and crops out around the top of Mt.Iwanaidake, where the rocks are free from severe ser-is proposed that involves the injection of a hydrous

melt, causing partial melting of the host harzburgite and pentinization. It comprises harzburgite with a smallamount of dunite, minor websterite, orthopyroxenite andresulting in formation of residual dunite. To verify this

model, mass balance calculations using the scheme of chromitite. In this study, the study area is treated as partof the Iwanaidake peridotite massif and is divided intoOzawa (1997) were carried out to estimate the change

of composition and modal abundance of the main min- sub-areas A, B, C, and D based on the modal abundanceof clinopyroxene in harzburgite: A, 1–3 vol. %; B, <1erals. In addition, results of peridotite melting ex-

periments under dry conditions (e.g. Mysen & Kushiro, vol. %; C, gradually changing from >1 vol. % in thesouthern part to almost 0 vol. % in the northern part;1977) and under hydrous conditions (Green, 1973) are

used to constrain the P–T and H2O conditions. These D, almost 0 vol. % (Fig. 2).constraints suggest that dunite formation occurred in ahydrous environment such as the mantle wedge above asubduction zone.

PETROGRAPHYIn this study, the boundary between dunite and harz-burgite is defined by 2 vol. % of orthopyroxene. This is

GEOLOGICAL SETTING different from the IUGS classification (‘dunite’ is definedas peridotite with 90–100 vol. % of olivine), but is moreThe Iwanaidake peridotite is part of the Sarugawa ultra-

mafic massif, which was emplaced in the southern part useful for describing detailed petrologic features in theIwanaidake peridotite. As will be seen in the followingof the Kamuikotan belt in the central axial part of

Hokkaido, Japan. The Kamuikotan belt extends from section, dunite and harzburgite defined by this criterioncan be clearly distinguished by chemical composition.north to south and has a length of 320 km and a width of

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KUBO DUNITE FORMATION PROCESSES

Fig. 1. Outcrop map of peridotite massifs within the Kamuikotan belt, showing the location of the Sarugawa ultramafic massif. Shaded areasare peridotite massifs exposed along the Kamuikotan belt. (b) Detailed geological map of the study area within the Sarugawa massif [modifiedafter Niida & Kato (1978)]. The Iwanaidake peridotite is located around the top of Mt. Iwanaidake and belongs to the western unit peridotiteof the Sarugawa ultramafic massif. The studied area is shown by a rectangle. Both the western and eastern peridotite units of the Sarugawaultramafic massif are composed of harzburgite with small amounts of dunite and minor websterite, orthopyroxenite and chromitite.

Harzburgite spinel. In most harzburgites, the modal abundance ofolivine is 70–90 vol. %, orthopyroxene 10–30 vol. %Harzburgite is the most abundant rock type in theand spinel >1 vol. %, and clinopyroxene is rare. TheIwanaidake peridotite and consists of olivine, ortho-

pyroxene and a lesser amount of clinopyroxene and modal abundance of these minerals varies even within a

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Fig. 2. Sample location map within the studied area. Circles show sampling points and numbers show sample numbers. HD2, HD3 and HD7are sampling points for sections studied in detail. The study area is subdivided into four areas, A, B, C and D, by abundance of clinopyroxenein harzburgite (A, 1–3 vol. %; B, <1 vol. %; C, gradually changes from >1 vol. % at the southern part to almost 0 vol. % at the northernpart; D, almost 0 vol. %). Thick continuous line indicates the road; thin continuous line indicates the contour line in meters.

single hand specimen (Fig. 3a). Serpentine, brucite, talc, Spinel is generally anhedral with the exception of thoseeuhedral crystals that are surrounded completely byopaque minerals and a small amount of tremolite are

also observed. olivine. The grain size of spinel ranges from 0·1 to 1 mm,and sometimes is larger than a few millimeters. SpinelOlivine is usually anhedral, and ranges in grain size

from very fine (<10 �m in diameter) to very coarse sometimes forms in a vermicular intergrowth with ortho-pyroxene, and rarely occurs as lamellae in orthopyroxene.(larger than a few centimeters). Orthopyroxene is usually

anhedral and sometimes porphyroclastic, mostly with a It is resistant to alteration, and usually is not altered toopaque minerals, even close to serpentine veins.grain size of 0·5–5 mm and sometimes >10 mm. Ortho-

pyroxene crystals are usually deformed, often contain Sometimes a weak foliation and lineation, formedby orthopyroxene alignment, exists and rarely strongexsolution lamellae of clinopyroxene and rarely contain

spinel lamellae. Although orthopyroxene is usually more foliations and lineations are developed. However, thedirection of these structures varies even within a singleresistant to serpentinization than olivine, some crystals are

partly replaced by talc. Replacement of orthopyroxene by outcrop, and therefore, the regional structure of theIwanaidake peridotite is difficult to discern. Sometimestalc is preferentially observed in areas C and D, and this

alteration is more intensive and selective than ser- dunite exists in parallel bands to the foliation or lineationof orthopyroxene (see Fig. 3a). Harzburgite shows de-pentinization of olivine. Clinopyroxene is absent or small

in amount if present, usually occurring at the rim of formation textures, such as porphyroclastic texture (some-times mylonitic), kink banding in olivine, and distortedorthopyroxene and rarely as anhedral isolated grains.

Clinopyroxene exsolution lamellae in orthopyroxene are grains of orthopyroxene and clinopyroxene. Por-phyroclasts are olivine and orthopyroxene, whereas neo-common. The grain size of clinopyroxene crystals at the

rim of orthopyroxene ranges from <10 �m to 0·3 mm, blasts are olivine alone, indicating that olivine deformsmore easily than the other minerals. Porphyroclasts areand the isolated grains are >0·5 mm in size. The oc-

currence and abundance of clinopyroxene varies spatially. sometimes as large as a few centimeters in size, whereas

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developed in orthopyroxene-poor domains (>1 cm2 orlarger), where grain size is a few tens of micrometers to100 �m in diameter.

DuniteThe grain size of olivine is usually larger than that inharzburgite and reaches a few centimeters in diameter.The modal abundance of spinel is 1–2 vol. %. The spinelgrains in dunite are usually euhedral, and are generally0·1–1 mm in diameter although sometimes 2–3 mm.Dunite occasionally contains clinopyroxene, with or with-out a small amount of orthopyroxene, and the grain sizesof both pyroxenes are a few millimeters. Rare arrays ofcoarse-grained clinopyroxene are found in a dunite of2 m thickness in area C. Spinel lineation exists commonlyin dunite but not in harzburgite. Porphyroclastic texturein dunite consists of porphyroclasts and neoblasts ofolivine.

The boundary between dunite and harzburgite is im-portant in considering the formation of dunite. Bothgradual boundaries (orthopyroxene fades out and thewidth of the transition zone ranges from a few to a fewtens of centimeters) and sharp boundaries (orthopyroxenedisappears immediately) exist in this massif. The shapeof the dunite bodies is also important. Two types arerecognized: lenticular-shaped dunites (Fig. 3b) and ir-regular-shaped dunites (Fig. 3c). The former is morecommon in this massif, corresponding to the bandedtexture of dunite and harzburgite described by Niida &Kato (1978). The thickness of lenticular-shaped dunitesranges from a few centimeters to a few meters. Lenticular-shaped dunite sometimes occurs in concordance with theorthopyroxene foliation or lineation of the surroundingharzburgite. The irregular-shaped dunite is uncommonin the Iwanaidake massif and occurs only in areas C andD. The structural relationship between irregular-shapeddunite bodies and the foliation or lineation of surroundingharzburgite is not clear.

Fig. 3. Photographs of typical harzburgite and dunite. ha, harzburgite;du, dunite; broken line shows the boundary between dunite and ROCK SAMPLES AND ANALYTICALharzburgite. (a) Typical harzburgite with a thin dunite band (about

METHODS3 cm wide) from area B. Dark spots are grains of orthopyroxene andlight parts are olivine grains. The modal abundance of orthopyroxene Minerals in 38 samples of harzburgite and dunite wereis heterogeneous in this limited area. The upper boundary between

analyzed following microscopic observation. Theseharzburgite and dunite is sharp, whereas the lower boundary is gradual.samples were collected from sites away from lithological(b) Dunite with a lenticular shape, with relatively sharp boundary, from

area C. About 5 cm in thickness and about 1 m in length. Left side of boundaries between dunite and harzburgite. Samplingthe dunite becomes thinner. (c) Dunite with an irregular shape, with a was conducted to cover the entire massif of the Iwanai-relatively sharp boundary with the surrounding harzburgite from area

dake peridotite, and major sampling sites are shownD. The shape of dunite is irregular in three dimensions.in Fig. 2. To eliminate the effects of subsolidus re-equilibration and alteration, only the compositions of the

mylonitic olivine grains are less than a few micrometers. cores of each mineral are discussed below. These samplesshow a compositional trend in the Iwanaidake peridotiteIn harzburgite, an equigranular texture of olivine is

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referred to subsequently as the Iwanaidake General as Mg# increases), which is opposite to the establishedTrend. general compositional trends in mantle peridotite olivines

To examine the relationship between dunite and harz- [i.e. mantle olivine array (MOA) by Takahashi, 1986a;burgite, three harzburgite–dunite sections were in- Fig. 4b]. The compositional relationships of the Iwanai-tensively sampled: HD2 (relatively thick lenticular-shaped dake peridotite in Fig. 4a and b are called the Iwanaidakedunite with broad contact zones), HD3 (thin lenticular- General Trend. The data for HD2 (representing the localshaped dunite) and HD7 (irregular-shaped dunite). Fur- trend of HD2) harzburgite plot in a gap within thether description of these three sections is given in the Iwanaidake General Trend.next section. The relationships between the modal abundance of

The modal abundance of minerals was measured by orthopyroxene and the chemical composition of olivinethe point counting method. Mineral compositions were and spinel are shown in Fig. 5. The Mg# of olivine andanalyzed by electron probe micro-analysis (EPMA), using Cr# of spinel in the dunite are systematically higher,the JEOL JCMA-733Mk II and JXA-8900 systems at and NiO wt % of olivine in dunite is systematically lower,the Department of Earth and Planetary Science of the than in normal harzburgite (with modal abundance ofUniversity of Tokyo. Olivine was analyzed at 25 kV, 5·0 orthopyroxene >10 vol. %). Also in this figure, data for× 10−8 A and for 200 s duration with the JCMA-733Mk HD2 fall in the gap of the Iwanaidake General Trend.II and corrected with the ZAF method. Compositions of Details of HD2 and the relationship between the Iwanai-orthopyroxene, clinopyroxene and spinel were analyzed dake General Trend and the local trend of HD2 areat 15 kV, 1·2 × 10−8 A and 10 s duration using the discussed later.JCMA-733Mk II with the correction by the Bence andAlbee algorithm and the JXA-8900 with the ZAF oxidemethod. Average values of chemical compositions within

Local trends (HD2, HD3 and HD7)a single thin section (about five points) and their varianceThree harzburgite–dunite sections that represent differ-shown by their standard deviation (1�) are given in Tableent dunite lithologies are examined: HD2 (Fig. 6), HD31. Representative compositions of olivine are given in(Fig. 7) and HD7 (Fig. 8).Table 2, and those of orthopyroxene, clinopyroxene and

HD2 is a harzburgite with a relatively thick lenticular-spinel are given in Tables 3, 4 and 5, respectively. Theshaped dunite and broad contact zones between, whichstandard deviation (1�) of the measured Mg# of olivinewas taken from area D (Fig. 6a). HD2 contains a duniteis>0·01 and that of NiO wt % in olivine>0·001 basedof 77·5 cm thickness and adjacent harzburgite. Twenty-on repeated analysis of an internal standard, the Santwo samples were collected from HD2 over a 150 cmCarlos olivine (Arizona).distance. In the left-hand harzburgite, when observedwith the naked eye, the abundance of orthopyroxenegrains seems to decrease gradually toward the dunite. InCHEMICAL COMPOSITIONALthin section, however, the modal proportion of ortho-

VARIATIONS pyroxene is difficult to determine because of severeIwanaidake General Trend alteration to talc (Fig. 6f ). In the right-hand harzburgite,

the boundary is relatively sharp. The Mg# of olivine inIn the Iwanaidake peridotite, the Mg# of olivine rangesthe left-hand harzburgite increases gradually from thefrom 91·5 to 92·5 in harzburgite, and from 92·5 to 94·0left end (92·2) to the dunite contact (92·8) and is highestin dunite (Fig. 4a and b). Data from HD2 are plottedin the center of the dunite (93·3, Fig. 6b). NiO wt % offor comparison. The Cr# of spinel in harzburgite rangesolivine shows the opposite trend: it decreases from thefrom 28·7 to 70·0 and that in dunite ranges from 66·7left end of the harzburgite (0·41 wt %) to the duniteto 84·4. The compositional relationship between Mg#contact (0·38 wt %) and is lowest at the center of duniteof olivine and Cr# of spinel is shown in Fig. 4a, in which(0·35 wt %, Fig. 6c). The Cr# of spinel increases graduallya positive correlation is seen for harzburgite and dunite.from the left end (59·1) to the dunite contact (72·8) andIn Fig. 4a, the olivine–spinel mantle array (OSMA) byis highest in the central position of the dunite (78·0, Fig.Arai (1994) is also shown, which represents a residual6d). The Cr# of spinel shows a trend similar to that ofmantle peridotite trend in the spinel lherzolite field.the Mg# of olivine. Fe3+/(Cr + Al + Fe3+) is very lowOlivine and spinel in the Iwanaidake peridotite plot(<0·05) in both dunite and harzburgite (Fig. 6e).within the OSMA. The Mg# of olivine is basically

HD3 is a harzburgite with very thin lenticular-shapedconstant within one thin section both in dunite anddunite of 3·2 cm thickness, which was taken from areaharzburgite. The NiO content of olivine in harzburgiteD (Fig. 7a). In a small drill core sample, 2·4 cm inis 0·38–0·43 wt % and that in dunite is 0·35–0·38 wt %.diameter, the modal abundance of orthopyroxeneThe Mg# and NiO wt % of olivine in dunite and

harzburgite show a negative correlation (NiO decreases changes abruptly at the boundaries between dunite and

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Table 1: Modal abundance of minerals and average compositions

No. Place Type Abundance (vol. %) Average Standard deviation

ol opx cpx sp ol Mg# NiO sp Cr# Fe3+ ol Mg# NiO sp Cr# Fe3+

(wt %) (wt %)

Iwanaidake General Trend

1 A ha 70·33 26·65 2·04 0·98 91·46 0·40 47·14 0·010 — — 0·91 0·002

2 A ha 78·13 20·06 0·27 1·53 91·83 0·38 55·18 0·056 0·08 0·01 2·14 0·068

3 A ha 69·58 27·05 1·61 1·77 91·53 0·38 28·71 0·010 0·06 0·01 2·65 0·002

4 B du 99·33 0·00 0·00 0·67 93·00 0·36 79·51 0·031 0·12 0·01 0·23 0·008

5 B du 99·44 0·00 0·00 0·56 92·56 0·35 81·56 0·005 0·08 0·00 0·36 0·005

6 B du 98·53 0·00 0·00 1·47 92·47 0·37 68·28 0·040 0·11 0·01 0·26 0·006

7 B ha 75·77 22·06 0·59 1·58 91·78 0·38 41·48 0·004 0·10 0·02 2·68 0·000

8 B ha 78·13 19·13 0·53 0·76 91·55 0·40 48·23 0·008 0·18 0·01 0·83 0·003

9 B ha 80·80 18·40 0·16 0·64 91·59 0·39 56·18 0·002 0·11 0·00 0·65 0·004

10 B ha 86·77 12·50 0·33 0·40 91·62 0·39 46·41 0·000 0·13 0·00 0·67 0·000

11 B ha 73·43 25·63 0·31 0·63 91·58 0·42 46·82 0·001 0·16 0·02 — —

12 B ha 57·34 19·15 0·11 23·39 91·91 0·39 53·78 0·000 0·12 0·01 0·02 0·000

13 C du 98·98 0·00 0·00 1·02 92·46 0·37 74·50 0·015 0·43 0·02 0·83 0·005

14 C du 98·97 0·00 0·00 1·03 94·03 0·37 84·35 0·023 0·26 0·01 0·59 0·006

15 C du 98·79 0·00 0·00 1·21 93·05 0·37 75·17 0·019 0·22 0·00 0·35 0·002

16 C du 94·20 0·00 0·00 0·92 92·50 0·37 67·02 0·018 0·34 0·04 1·41 0·003

17 C du 99·48 0·00 0·00 0·52 92·98 0·37 73·83 0·028 0·07 0·00 1·70 0·048

18 C du 92·37 0·00 0·00 7·63 93·37 0·38 76·13 0·011 0·47 0·01 0·39 0·005

19 C du 99·83 0·00 0·00 0·17 92·61 0·38 73·94 0·018 0·46 0·01 0·58 0·002

20 C ha 85·68 13·97 0·00 0·35 91·72 0·38 51·99 0·005 0·09 0·00 0·49 0·009

21 C ha 79·22 19·55 0·22 1·01 91·74 0·41 54·08 0·004 0·17 0·01 — —

22 C ha 74·67 24·64 0·12 0·58 92·19 0·40 57·93 0·007 0·54 0·01 0·23 0·002

23 C ha 79·99 17·02 0·48 2·51 91·68 0·39 46·21 0·042 0·45 0·02 1·77 0·046

24 C ha 72·79 26·56 0·00 0·65 91·78 0·41 56·51 0·006 0·11 0·01 1·80 0·004

25 C ha 87·84 11·83 0·00 0·33 91·83 0·39 68·55 0·013 0·07 0·01 0·95 0·005

26 C ha 75·72 22·78 0·50 1·00 92·05 0·44 59·55 0·003 0·65 0·02 2·48 0·003

27 C ha 72·25 26·14 0·19 1·41 92·85 0·39 58·45 0·001 0·25 0·03 0·33 0·001

28 C ha 83·15 15·73 0·00 1·12 91·82 0·39 45·72 0·004 0·12 0·00 4·68 0·003

29 C ha 85·87 13·93 0·00 0·20 91·53 0·40 47·45 0·000 0·13 0·02 3·62 0·001

30 C ha 70·04 29·09 0·50 0·37 91·70 0·42 54·30 0·006 0·06 0·01 — —

31 D du 99·21 0·00 0·00 0·79 92·99 0·36 78·02 0·007 0·12 0·00 0·87 0·001

32 D du 99·46 0·00 0·00 0·54 94·06 0·38 83·98 0·012 0·07 0·00 0·82 0·006

33 D du 99·26 0·00 0·00 0·74 93·10 0·36 79·07 0·023 0·12 0·01 1·56 0·011

34 D du 99·52 0·00 0·00 0·48 93·08 0·36 75·45 0·017 0·08 0·00 2·75 0·012

35 D du 98·22 0·00 0·00 1·78 93·25 0·36 77·16 0·051 0·14 0·02 0·92 0·085

36 D ha 79·28 19·62 0·00 1·09 91·45 0·40 57·60 0·006 0·13 0·01 0·38 0·003

37 D ha 86·14 13·24 0·00 0·63 92·09 0·40 64·75 0·011 0·34 0·00 0·33 0·003

38 D ha 79·67 19·88 0·00 0·46 92·48 0·38 70·53 0·005 0·07 0·01 1·52 0·005

harzburgite (Fig. 7a and f ). The grain size of minerals surrounding harzburgite. Two core samples have bothdunite and harzburgite parts and they were examinedin HD3 dunite ranges from a few micrometers to a few

millimeters, which is not very different from that of the separately. Therefore, two samples of dunite and four

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Table 1: continued

Distance Type Abundance (vol. %) Average Standard deviation

(cm)


(wt %) (wt %)

HD2

1 ha 87·85 11·53 0·00 0·67 92·19 0·41 58·94 0·004 0·32 0·01 1·41 0·003

5 ha 94·48 4·91 0·00 0·62 91·90 0·40 59·31 0·005 0·37 0·01 2·30 0·006

10 ha 88·23 11·40 0·00 0·37 92·33 0·41 60·08 0·001 0·28 0·01 1·11 0·002

15 ha 96·64 2·58 0·00 0·78 92·62 0·42 61·38 0·000 0·62 0·03 0·66 0·000

20 ha 83·84 15·49 0·00 0·67 92·44 0·40 60·99 0·000 0·31 0·01 1·41 0·001

25 ha 86·89 12·70 0·00 0·41 92·13 0·40 61·10 0·011 0·16 0·01 1·87 0·011

30 ha 90·17 8·96 0·00 0·87 92·43 0·40 63·27 0·009 0·21 0·01 0·75 0·001

35 ha 80·45 18·77 0·00 0·78 92·74 0·41 64·19 0·008 0·47 0·02 0·91 0·002

40 ha 95·00 4·59 0·00 0·41 92·42 0·39 64·16 0·009 0·14 0·01 1·99 0·003

45 ha 96·62 2·97 0·00 0·41 92·66 0·39 67·28 0·023 0·51 0·02 2·85 0·011

50 ha 88·99 10·46 0·00 0·55 92·57 0·40 68·74 0·017 0·11 0·01 3·02 0·035

55 ha 91·82 6·93 0·00 1·25 92·63 0·39 69·99 0·007 0·82 0·02 1·68 0·006

60 ha 90·52 8·25 0·00 1·23 92·58 0·38 70·88 0·003 0·68 0·05 0·86 0·004

65 ha 97·29 2·11 0·00 0·60 92·81 0·38 73·08 0·006 0·24 0·01 1·06 0·006

70 du 98·22 0·00 0·00 1·78 92·81 0·37 75·45 0·014 0·33 0·01 0·40 0·003

90 du 99·04 0·21 0·00 0·74 93·23 0·38 75·96 0·014 0·41 0·01 0·58 0·004

100 du 99·52 0·00 0·00 0·48 92·87 0·36 76·15 0·010 0·18 0·03 1·11 0·004

110 du 99·59 0·00 0·00 0·41 93·28 0·35 75·63 0·016 0·65 0·04 0·43 0·006

120 du 99·20 0·00 0·00 0·68 93·28 0·37 76·23 0·013 0·37 0·02 1·20 0·007

130 du 99·36 0·00 0·00 0·64 93·14 0·36 74·80 0·009 0·55 0·02 1·86 0·005

140 du 98·60 0·35 0·00 1·05 93·09 0·38 75·54 0·012 0·30 0·00 1·10 0·004

150 ha 88·46 9·81 0·12 1·60 92·84 0·39 72·11 0·010 0·55 0·01 0·61 0·011

HD3

0·0 ha 88·47 10·94 0·00 0·58 91·81 0·40 60·96 0·007 0·17 0·02 0·58 0·009

1·1 du 98·45 0·00 0·00 1·55 91·97 0·39 61·59 0·006 0·25 0·01 2·87 0·005

3·3 du 98·11 0·00 0·00 1·89 92·12 0·39 63·61 0·007 0·28 0·02 0·72 0·004

4·4 ha 79·37 19·53 0·00 1·10 91·87 0·40 60·14 0·008 0·18 0·02 5·15 0·009

6·5 ha 83·61 15·14 0·00 1·25 92·02 0·40 61·18 0·006 0·19 0·02 0·51 0·006

9·5 ha 82·92 16·48 0·10 0·50 91·83 0·39 60·25 0·005 0·42 0·01 1·24 0·006

samples of harzburgite were taken from HD3. The modal the dunite than in the harzburgite, although the differ-ences are very small. Fe3+/(Cr + Al + Fe3+) is veryabundance of orthopyroxene in the harzburgite ranges

from 11·0 to 19·7 (Fig. 7f ). Mineral compositions are low (<0·05) in both dunite and harzburgite (Fig. 7e).HD7 is a harzburgite with an irregular-shaped dunite,homogeneous throughout the dunite and harzburgite. In

the harzburgite, the Mg# of olivine ranges from 91·8 to which was taken from area D (Fig. 8a). HD7 is dividedinto a dunite–harzburgite alternation zone and a host92·0, NiO wt % of olivine ranges from 0·39 to 0·40, and

Cr# of spinel ranges from 60·1 to 61·2, whereas these harzburgite zone. Twenty-seven samples were taken froma section of 130 cm length and three samples (one fromvalues range from 92·0 to 92·1, from 0·38 to 0·39 and

from 61·6 to 63·6, respectively, in the dunite (Fig. 7b, c the left-most side and two from the right-most side) areclassified as host harzburgite. In the dunite–harzburgiteand d, respectively). The Mg# of olivine and Cr# of

spinel are higher and NiO wt % of olivine is lower in alternation zone, dunite and harzburgite alternate three

430


Distance Type Abundance (vol. %) Average Standard deviation

(cm)


(wt %) (wt %)

HD7

0·0 ha 93·94 5·32 0·00 0·74 92·51 0·37 70·26 0·011 0·09 0·00 0·26 0·000

20·0 ha 79·98 19·48 0·00 0·54 92·42 0·37 70·14 0·009 0·10 0·00 1·03 0·004

25·0 ha 88·24 11·24 0·00 0·52 92·55 0·37 72·00 0·009 0·08 0·01 0·55 0·003

28·5 ha 83·52 15·34 0·00 1·14 92·55 0·37 70·85 0·011 0·22 0·01 0·59 0·002

31·5 du 99·47 0·18 0·00 0·35 92·48 0·37 72·29 0·014 0·08 0·00 1·05 0·002

35·0 ha 96·19 2·92 0·00 0·89 92·53 0·36 73·37 0·020 0·07 0·00 0·75 0·006

39·5 du 98·42 0·80 0·00 0·78 92·47 0·36 73·77 0·018 0·07 0·00 0·45 0·004

43·5 du 97·74 1·86 0·13 0·27 92·51 0·36 75·97 0·015 0·11 0·00 0·26 0·002

47·5 ha 94·96 4·03 0·00 1·01 92·47 0·37 74·64 0·015 0·11 0·01 0·37 0·002

50·0 ha 96·66 2·60 0·00 0·74 92·54 0·37 74·69 0·011 0·14 0·01 1·66 0·003

55·0 ha 86·26 13·44 0·00 0·29 92·40 0·37 77·29 0·008 0·09 0·01 0·48 0·003

58·5 ha 95·56 2·62 0·00 1·81 92·48 0·36 77·49 0·020 0·14 0·00 1·01 0·012

61·0 ha 95·68 3·87 0·00 0·45 92·54 0·36 75·43 0·012 0·11 0·00 0·97 0·008

65·0 du 99·87 0·00 0·00 0·13 92·51 0·35 77·85 0·026 0·10 0·01 1·40 0·009

70·0 du 98·98 0·00 0·00 1·02 92·52 0·35 75·36 0·020 0·09 0·00 1·84 0·005

74·0 du 99·34 0·00 0·00 0·66 92·53 0·35 65·08 0·023 0·16 0·02 3·57 0·011

78·0 du 99·58 0·29 0·00 0·13 92·46 0·35 68·67 0·019 0·08 0·02 1·81 0·006

82·0 ha 96·10 2·72 0·00 1·19 92·53 0·36 74·19 0·007 0·08 0·01 1·01 0·003

84·0 ha 88·39 10·93 0·00 0·69 92·48 0·37 78·19 0·008 0·06 0·01 1·69 0·007

88·0 ha 89·72 9·97 0·00 0·31 92·49 0·38 78·17 0·012 0·09 0·01 0·26 0·003

91·0 du 97·75 0·56 0·00 1·69 92·45 0·37 75·13 0·019 0·08 0·00 1·28 0·005

94·0 du 98·66 0·00 0·00 1·34 92·51 0·37 77·69 0·030 0·09 0·00 0·70 0·004

98·0 du 98·13 1·47 0·00 0·40 92·47 0·37 75·64 0·026 0·22 0·00 0·64 0·001

100·0 ha 87·31 12·55 0·00 0·13 92·51 0·37 76·83 0·024 0·10 0·01 0·40 0·003

104·0 ha 87·08 12·12 0·00 0·80 92·44 0·38 76·32 0·020 0·14 0·01 1·03 0·002

118·0 ha 82·77 16·64 0·00 0·44 92·28 0·39 74·27 0·017 0·10 0·01 0·81 0·002

129·0 ha 85·77 14·06 0·00 0·18 92·17 0·38 69·74 0·019 0·11 0·01 2·85 0·003

times in a narrow band (80 cm in thickness). Harzburgite this dunite has the lowest values (NiO wt % of olivine:0·35; Cr# of spinel: 65·1). Spinel in the other dunitesin the alternation zone is orthopyroxene poor, the modal

abundance being >5 vol. % and at most 13·5 vol. % does not have low Cr# of spinel, but shows slightly lowerNiO wt %. In the host harzburgite, the Mg# of olivine(Fig. 8f ). In the harzburgite of the alternation zone, the

Mg# of olivine is nearly constant (92·4–92·5), whereas ranges from 92·2 to 92·5, which is not very differentfrom that in the alternation zone. The Cr# of spinelthe NiO wt % of olivine and Cr# of spinel show small

variations (Fig. 8b, c and d). The NiO wt % of olivine ranges from 69·7 to 70·3, which is slightly lower thanthat in the harzburgite in the alternation zone. Likewise,and Cr# of spinel range from 0·35 to 0·38 and from

68·7 to 78·2, respectively. In the alternation zone, the the NiO wt % of olivine ranges from 0·37 to 0·39, whichis slightly higher than in the harzburgite in the alternationMg# of olivine in the dunite does not differ from that

in the harzburgite. The thickest central dunite in HD7 zone. Fe3+/(Cr + Al + Fe3+) is very low (<0·05) inboth dunite and harzburgite; however, the ratio seemshas a lower NiO wt % of olivine and Cr# of spinel than

the adjacent harzburgite, and in particular, the center of relatively higher in dunite than harzburgite (Fig. 8e).

431


Table 2: Representative compositions of olivine

No. SiO2 MnO NiO FeO MgO Total NiO∗ No. SiO2 MnO NiO FeO MgO Total NiO∗

Iwanaidake General Trend 45 40·46 0·12 0·387 7·39 51·64 100·00 0·3871 39·99 0·12 0·395 8·49 50·99 99·99 0·395 50 40·65 0·11 0·398 7·46 51·41 100·03 0·3982 39·76 0·12 0·390 8·11 51·76 100·14 0·389 55 40·35 0·11 0·388 7·29 51·83 99·97 0·3883 39·86 0·12 0·379 8·51 51·22 100·10 0·379 60 40·51 0·11 0·385 7·10 52·02 100·12 0·3854 40·26 0·10 0·359 6·93 52·22 99·87 0·359 65 40·36 0·10 0·382 7·10 52·42 100·36 0·3815 40·04 0·11 0·349 7·34 52·01 99·86 0·350 70 40·38 0·11 0·376 7·02 52·01 99·90 0·3766 40·13 0·11 0·371 7·62 51·85 100·08 0·371 90 40·94 0·10 0·385 6·33 52·29 100·05 0·3857 39·95 0·13 0·378 8·18 51·58 100·21 0·377 100 40·57 0·11 0·370 6·97 52·75 100·77 0·3678 40·52 0·12 0·397 8·45 50·55 100·03 0·397 110 40·39 0·12 0·365 6·95 52·43 100·25 0·3649 39·93 0·12 0·387 8·49 51·09 100·02 0·387 120 40·26 0·11 0·374 6·99 52·74 100·47 0·37210 40·07 0·11 0·391 8·33 51·17 100·07 0·391 130 40·95 0·11 0·371 7·30 51·58 100·31 0·37011 40·03 0·12 0·420 8·50 51·08 100·15 0·419 140 40·39 0·11 0·384 6·95 52·36 100·19 0·38312 40·19 0·11 0·389 7·97 51·37 100·03 0·389 150 40·64 0·12 0·381 7·28 51·80 100·21 0·38013 39·64 0·12 0·362 7·27 51·39 98·76 0·367

HD314 40·05 0·09 0·372 6·07 53·35 99·93 0·372

0·0 40·29 0·13 0·398 8·01 50·83 99·66 0·39915 40·04 0·11 0·372 6·94 51·79 99·25 0·375

1·1 40·26 0·12 0·391 8·06 50·88 99·71 0·39216 40·16 0·11 0·360 7·25 52·04 99·92 0·360

3·3 40·02 0·13 0·386 8·05 51·19 99·78 0·38717 40·20 0·11 0·368 7·06 51·98 99·72 0·369

4·4 40·04 0·13 0·398 8·06 51·15 99·78 0·39918 40·33 0·10 0·388 6·11 52·70 99·63 0·389

6·5 40·28 0·12 0·394 7·98 50·82 99·59 0·39619 39·65 0·11 0·377 7·27 52·09 99·50 0·379

9·5 39·96 0·12 0·388 8·03 51·42 99·92 0·38820 39·55 0·12 0·379 8·32 51·77 100·14 0·378

HD721 40·05 0·12 0·414 8·37 51·22 100·17 0·413

0·0 39·69 0·11 0·368 7·65 51·96 99·77 0·36922 40·11 0·11 0·398 7·88 51·67 100·17 0·397

20·0 40·50 0·11 0·374 7·41 51·65 100·04 0·37423 39·93 0·12 0·398 8·37 50·81 99·63 0·399

25·0 40·19 0·11 0·370 7·52 52·04 100·23 0·36924 39·94 0·12 0·406 8·20 50·87 99·54 0·408

28·5 40·13 0·11 0·372 7·50 52·21 100·32 0·37125 39·75 0·12 0·391 8·20 51·25 99·72 0·392

31·5 40·65 0·11 0·366 7·43 51·47 100·02 0·36626 39·83 0·12 0·437 8·29 51·09 99·77 0·43835·0 40·65 0·11 0·361 7·45 51·52 100·10 0·36127 40·03 0·11 0·370 6·92 52·18 99·60 0·37139·5 40·48 0·12 0·362 7·63 51·52 100·11 0·36228 39·86 0·13 0·392 8·17 51·08 99·62 0·39343·5 40·79 0·11 0·361 7·56 52·38 101·21 0·35729 39·81 0·12 0·402 8·28 51·06 99·67 0·40347·5 40·72 0·11 0·370 7·43 51·73 100·36 0·36930 39·82 0·12 0·419 8·42 51·54 100·31 0·41850·0 40·29 0·11 0·368 7·48 52·26 100·50 0·36631 40·03 0·10 0·359 7·02 52·49 100·00 0·35955·0 40·36 0·12 0·366 7·52 51·75 100·10 0·36632 40·22 0·09 0·383 5·98 52·87 99·55 0·38558·5 40·41 0·11 0·364 7·40 52·24 100·53 0·36233 40·51 0·10 0·358 6·90 52·51 100·37 0·35761·0 40·28 0·12 0·364 7·44 51·81 100·01 0·36434 40·21 0·11 0·361 6·83 52·10 99·61 0·36265·0 40·52 0·12 0·354 7·56 51·73 100·29 0·35335 40·08 0·11 0·364 6·72 52·29 99·56 0·36670·0 40·47 0·11 0·350 7·52 51·79 100·24 0·34936 39·52 0·12 0·404 8·56 50·94 99·54 0·40674·0 40·16 0·17 0·352 7·03 52·30 100·02 0·35237 40·24 0·11 0·404 7·33 51·88 99·96 0·40478·0 40·43 0·11 0·362 7·44 51·82 100·17 0·36138 39·90 0·11 0·374 7·39 51·72 99·50 0·376

82·0 40·63 0·12 0·363 7·56 51·96 100·64 0·361HD2

84·0 40·63 0·12 0·374 7·52 51·81 100·45 0·3721 40·08 0·12 0·404 8·07 51·28 99·96 0·404

88·0 40·39 0·11 0·379 7·53 51·96 100·36 0·3785 40·28 0·12 0·406 8·03 51·10 99·94 0·406

91·0 40·16 0·12 0·376 7·68 52·19 100·53 0·37410 40·22 0·12 0·413 7·78 51·07 99·60 0·415

94·0 40·42 0·12 0·370 7·50 51·96 100·36 0·36915 40·38 0·11 0·432 6·66 51·94 99·52 0·434

98·0 39·75 0·13 0·367 8·11 51·70 100·06 0·36720 40·06 0·12 0·399 7·84 51·54 99·96 0·399

100·0 40·27 0·11 0·377 7·41 52·47 100·63 0·37525 40·34 0·11 0·402 7·64 51·52 100·01 0·402

104·0 40·54 0·11 0·382 7·56 51·95 100·53 0·38030 40·36 0·13 0·401 7·95 51·16 100·01 0·401

118·0 40·35 0·12 0·395 7·72 52·32 100·90 0·39135 40·58 0·12 0·412 7·54 51·64 100·29 0·411

129·0 40·10 0·13 0·383 7·97 51·82 100·39 0·38140 40·30 0·12 0·390 7·74 51·54 100·09 0·390

NiO∗ = (NiO wt %/total) × 100.

432


Table 3: Representative compositions of orthopyroxene

No. SiO2 Al2O3 TiO2 FeO MnO MgO CaO Na2O K2O Cr2O3 V2O3 NiO Total


1 56·47 2·14 0·04 4·96 0·16 34·81 0·51 0·01 0·00 0·68 0·00 0·08 99·87

2 56·62 1·82 0·00 5·55 0·13 34·73 0·81 0·03 0·01 0·50 0·00 0·04 100·24

3 54·77 3·85 0·00 5·83 0·14 33·75 0·81 0·02 0·01 0·95 0·01 0·11 100·24

7 55·34 2·19 0·05 5·86 0·19 34·45 0·51 0·02 0·00 0·56 0·00 0·07 99·22

8 56·02 2·12 0·05 5·83 0·17 34·71 0·71 0·00 0·00 0·46 0·00 0·12 100·19

9 56·90 2·01 0·01 5·31 0·12 34·75 0·67 0·00 0·00 0·69 0·00 0·11 100·57

10 56·40 1·80 0·03 5·49 0·15 34·88 0·56 0·02 0·00 0·39 0·02 0·08 99·82

11 55·55 2·48 0·04 4·95 0·13 31·80 4·16 0·01 0·03 0·78 0·02 0·11 100·06

12 56·18 2·09 0·00 5·43 0·14 33·90 2·19 0·01 0·01 0·85 0·00 0·11 100·91

20 56·11 2·03 0·00 5·59 0·16 34·88 0·50 0·02 0·03 0·59 0·02 0·12 100·04

21 56·20 2·18 0·00 5·15 0·16 34·54 0·52 0·00 0·00 0·67 0·03 0·10 99·55

22 56·32 1·73 0·01 5·34 0·13 35·23 0·52 0·02 0·02 0·60 0·04 0·07 100·04

23 55·48 2·26 0·04 5·70 0·18 34·96 0·53 0·03 0·02 0·74 0·00 0·06 99·99

24 56·64 1·54 0·02 5·55 0·18 34·67 0·56 0·01 0·00 0·61 0·00 0·14 99·91

25 57·11 1·13 0·01 5·50 0·12 35·22 0·45 0·02 0·00 0·50 0·02 0·11 100·19

26 56·34 1·45 0·00 5·39 0·14 33·65 2·20 0·01 0·02 0·60 0·02 0·10 99·91

27 56·35 1·92 0·03 4·96 0·13 35·15 0·36 0·00 0·01 0·69 0·01 0·13 99·74

28 57·19 1·40 0·01 5·48 0·17 35·32 0·39 0·01 0·00 0·27 0·01 0·11 100·36

29 57·50 0·83 0·02 5·78 0·18 35·19 0·32 0·00 0·01 0·13 0·07 0·06 100·09

30 56·22 1·96 0·06 5·71 0·13 35·12 0·61 0·02 0·02 0·76 0·00 0·19 100·78

36 55·76 1·70 0·03 5·10 0·13 33·99 1·85 0·02 0·02 0·63 0·04 0·10 99·37

37 56·82 1·48 0·02 5·60 0·10 35·21 0·70 0·00 0·02 0·58 0·01 0·10 100·64

38 57·29 0·70 0·01 5·11 0·11 35·53 0·54 0·01 0·01 0·31 0·00 0·08 99·70

Distance SiO2 Al2O3 TiO2 FeO MnO MgO CaO Na2O K2O Cr2O3 V2O3 NiO Total

(cm)

HD2

1 56·41 1·63 0·02 5·33 0·13 34·74 0·91 0·00 0·00 0·57 — 0·09 99·82

5 56·59 1·58 0·00 5·24 0·13 34·99 0·77 0·00 0·00 0·66 — 0·09 100·04

10 56·39 1·63 0·00 5·10 0·14 34·95 0·48 0·00 0·00 0·63 — 0·09 99·41

15 56·25 1·61 0·01 5·02 0·15 34·94 0·57 0·00 0·00 0·65 — 0·11 99·31

20 56·45 1·53 0·01 5·08 0·13 34·77 0·94 0·00 0·00 0·60 — 0·07 99·58

25 56·44 1·63 0·00 5·04 0·12 35·29 0·29 0·00 0·00 0·68 — 0·07 99·55

30 56·62 1·57 0·00 5·12 0·11 35·20 0·53 0·00 0·00 0·68 — 0·06 99·89

35 57·26 1·22 0·01 5·11 0·13 35·66 0·31 0·00 0·00 0·45 — 0·07 100·21

40 57·08 1·31 0·02 5·06 0·00 34·07 1·17 0·01 0·00 0·54 — 0·10 99·36

45 56·05 1·25 0·04 4·71 0·00 33·22 1·69 0·02 0·01 0·54 — 0·07 97·59

50 57·69 1·11 0·00 4·93 0·00 34·37 0·79 0·04 0·00 0·45 — 0·09 99·46

55 57·14 1·04 0·00 4·68 0·00 34·98 0·86 0·03 0·00 0·51 — 0·06 99·29

60 57·30 0·82 0·00 4·92 0·13 36·04 0·35 0·00 0·00 0·36 — 0·10 100·02

65 57·42 0·83 0·00 4·81 0·12 35·69 0·48 0·00 0·00 0·39 — 0·13 99·87

150 58·69 1·03 0·01 4·64 0·10 34·37 0·70 0·00 0·00 0·51 — 0·09 100·14

433


Table 3: continued


HD3

0·0 58·03 1·38 0·01 5·27 0·13 34·55 0·36 0·00 0·00 0·50 0·02 0·08 100·34

4·4 57·50 1·54 0·03 4·93 0·10 33·40 1·83 0·00 0·00 0·64 0·03 0·09 100·09

6·5 57·93 1·55 0·02 5·21 0·13 34·09 0·79 0·00 0·00 0·55 0·00 0·09 100·35

9·5 57·55 1·54 0·01 5·05 0·15 34·27 0·75 0·00 0·01 0·60 0·02 0·10 100·05

HD7

0·0 57·95 1·06 0·02 4·85 0·10 34·45 0·50 0·00 — 0·49 — 0·09 99·51

20·0 58·43 1·01 0·04 4·98 0·10 34·70 0·44 0·04 — 0·47 — 0·08 100·29

25·0 58·43 1·03 0·03 4·87 0·14 34·63 0·52 0·00 — 0·54 — 0·09 100·29

28·5 57·90 0·92 0·00 4·90 0·16 34·27 0·57 0·00 — 0·47 — 0·06 99·24

31·5 57·73 0·91 0·00 4·92 0·10 34·04 0·58 0·00 — 0·42 — 0·05 98·77

35·0 55·41 0·71 0·01 4·75 0·00 33·42 1·15 0·03 — 0·41 — 0·15 96·04

43·5 58·08 0·82 0·02 5·24 0·10 34·68 0·26 0·02 — 0·41 — 0·03 99·65

47·5 58·58 0·74 0·00 4·91 0·14 34·93 0·32 0·00 — 0·36 — 0·09 100·06

50·0 58·42 0·74 0·00 4·92 0·14 34·76 0·40 0·02 — 0·37 — 0·05 99·82

55·0 58·64 0·60 0·00 5·16 0·15 34·68 0·39 0·01 — 0·34 — 0·05 100·03

58·5 58·25 0·60 0·04 5·11 0·00 35·10 0·50 0·00 — 0·26 — 0·07 99·92

61·0 58·69 0·69 0·02 4·98 0·15 34·79 0·55 0·02 — 0·38 — 0·07 100·33

78·0 59·96 0·85 0·02 4·84 0·12 33·97 0·25 0·03 — 0·43 — 0·08 100·55

82·0 58·39 0·89 0·00 5·04 0·13 34·20 1·06 0·01 — 0·49 — 0·00 100·22

84·0 58·16 0·56 0·00 5·05 0·13 35·10 0·35 0·01 — 0·30 — 0·06 99·71

88·0 58·33 0·61 0·02 4·98 0·16 34·65 0·37 0·01 — 0·33 — 0·05 99·51

91·0 56·78 0·77 0·01 5·30 0·14 35·32 1·07 0·00 — 0·39 — 0·08 99·86

98·0 57·86 0·61 0·00 5·22 0·13 35·81 0·55 0·00 — 0·30 — 0·06 100·55

100·0 57·87 0·72 0·02 5·14 0·13 35·75 0·56 0·01 — 0·38 — 0·05 100·63

104·0 57·85 0·67 0·00 5·20 0·12 35·88 0·49 0·03 — 0·39 — 0·05 100·68

118·0 57·65 0·97 0·01 5·39 0·12 35·50 0·57 0·00 — 0·51 — 0·04 100·76

129·0 57·75 0·77 0·00 5·22 0·13 35·31 0·61 0·00 — 0·39 — 0·04 100·21

undergoing intense deformation, dunite bodies of anySIZE EFFECTS ON THE CHEMICALshape could be changed into a lenticular shape by flat-

COMPOSITION OF DUNITE BODIES tening and stretching (Nicolas, 1984). Therefore, the thinTo discuss primary magmatic stages recorded in peri- lenticular-shaped dunite (e.g. HD3, Fig. 7) may havedotites, it is necessary to eliminate the effect of subsequent been generated by deformation of a thicker body (e.g.subsolidus processes. In this section, deformation and HD2, Fig. 6), and the irregular-shaped dunite (e.g. HD7,diffusion in the subsolidus stage is discussed. Fig. 8) may preserve the original undeformed structure

of the dunite.

Formation of lenticular-shaped dunitebodies

Diffusion effectsAs discussed above, a variety of sizes and lithologies ofTo evaluate the effects of diffusion at the dunite–dunite bodies are present in the Iwanaidake peridotite.harzburgite contact, interdiffusion of Mg–Fe in olivineLenticular-shaped dunite bodies may have formed fromin the vicinity of a planar contact between dunite anddunite bodies with more complex original shapes. Thisharzburgite is modeled. A one-dimensional diffusioninterpretation is supported by the concordant structuremodel is considered with the concentration profile beingof the lenticular-shaped dunite bodies with the foliation

or lineation of the ambient harzburgite. In a peridotite initially step-like: Mg# of olivine is 91 in the harzburgite

434


Table 4: Representative compositions of clinopyroxene



1 53·18 1·97 0·04 1·84 0·11 17·91 24·29 0·13 0·02 0·68 0·05 0·04 100·25

2 53·87 2·29 0·00 1·86 0·08 17·30 23·87 0·30 0·01 1·04 0·02 0·01 100·64

3 51·66 4·36 0·13 1·94 0·10 16·41 23·61 0·22 0·00 1·39 0·00 0·04 99·85

7 54·27 1·76 0·14 1·69 0·09 17·59 24·45 0·19 0·01 0·43 0·01 0·05 100·69

8 53·89 1·67 0·07 1·69 0·05 17·67 24·64 0·08 0·01 0·69 0·00 0·04 100·51

9 53·86 2·30 0·05 1·63 0·07 17·38 24·11 0·26 0·01 1·07 0·07 0·07 100·87

10 53·17 1·78 0·03 1·74 0·06 18·79 23·54 0·11 0·01 0·74 0·02 0·06 100·04

11 53·45 2·41 0·03 1·74 0·09 17·17 24·76 0·15 0·01 0·96 0·00 0·05 100·82

12 55·04 0·98 0·02 1·46 0·08 18·31 24·97 0·07 0·02 0·47 0·05 0·04 101·50

17 54·57 0·75 0·05 1·05 0·01 17·49 25·51 0·19 0·00 0·57 0·01 0·03 100·24

19 55·08 0·12 0·01 1·10 0·04 18·11 25·74 0·04 0·03 0·11 0·01 0·00 100·39

22 53·66 1·89 0·05 1·87 0·08 17·88 23·49 0·25 0·02 0·90 0·02 0·05 100·13

24 53·56 2·04 0·02 1·46 0·04 17·26 23·84 0·24 0·00 1·11 0·03 0·03 99·62

25 55·93 1·15 0·00 2·98 0·11 27·88 11·77 0·06 0·01 0·68 0·02 0·12 100·70

26 53·98 1·54 0·04 1·34 0·10 17·54 24·70 0·11 0·00 0·85 0·04 0·09 100·33

27 53·79 1·73 0·06 1·74 0·09 17·70 23·37 0·22 0·00 0·86 0·01 0·02 99·58

28 55·01 1·14 0·02 1·67 0·06 17·71 24·62 0·06 0·00 0·27 0·04 0·03 100·63

29 53·56 2·22 0·09 1·78 0·10 17·20 23·91 0·21 0·01 1·07 0·03 0·06 100·23

30 52·90 1·94 0·05 1·83 0·08 17·71 23·89 0·10 0·02 0·90 0·04 0·08 99·54

36 53·15 1·61 0·02 1·58 0·07 17·53 24·25 0·08 0·01 0·73 0·03 0·00 99·05

and 93 in the dunite plate (sandwiched between harz- (estimated from the experimental melting of pyroliteunder water-saturated conditions; Green, 1973). Rangesburgite). According to Crank (1956), the Mg# of olivineof Mg–Fe interdiffusion coefficients have been reported(CMg2SiO4) can be expressed as a function of place (x) andfor olivine (Buening & Buseck, 1973; Misener, 1974;time (t):Chakraborty, 1997). According to Chakraborty (1997),the diffusion coefficient at 1100°C is >10−17 m2/s. InCMg2SiO4

C 0Mg2SiO4

=12 �erf �a− x

2�Dt�+ erf �a+ x

2�Dt�� this case the Mg# of olivine in the center of the 2 cmdunite decreases and homogenizes with the surrounding

where D is the interdiffusion coefficient of Mg and Fe2+ harzburgite after 10 My. In contrast, that in the 200 cmin the peridotite (assuming Dharzburgite = Ddunite), C 0

Mg2SiO4 is dunite remains unchanged even after 1 Gy. The diffusionMg# of olivine in dunite at t = 0 (C 0

Mg2SiO4 = 93), a is coefficients of Buening & Buseck (1973) and Misenerthe half-thickness of the dunite plate and t is time since (1974) at 1100°C are both >10−15 m2/s. In this case,dunite formation. As the Mg# values of olivine and the Mg# of olivine in the center of the 2 cm dunite isorthopyroxene are similar in the Iwanaidake peridotite, homogenized after 0·1 My and that in the 200 cm dunitewe assume that the Mg# of olivine in the harzburgite after 1 Gy. It is difficult to estimate the duration of thewill not be lowered by Mg–Fe exchange between olivine annealing time because of very large uncertainty ofand orthopyroxene. diffusion coefficients and temperatures involved in the

The change in the Mg# of olivine by Mg–Fe inter- Iwanaidake. It is suggested that it takes a geologicallydiffusion is examined for two dunite bodies of different significant time for a thick dunite body to be homogenizedthicknesses: 2 cm and 200 cm, respectively. A comparison with the surrounding harzburgite. A thick dunite bodybetween the two shows that the time taken for the dunite may have retained its original Mg# in the center whereasof 200 cm thickness to homogenize with the surrounding the Mg# in the center of a thinner dunite can easilyharzburgite is 10 000 times longer than for the dunite of change, even by solid-state diffusion, during the geological2 cm thickness. As an example, a temperature of 1100°C history of the Iwanaidake peridotite.is considered, which corresponds to the lowest tem- Several workers have reported Ni diffusion coefficients

in olivine (Morioka, 1981; Nagasawa & Morioka, 1996;perature at which dunite can be formed at 1 GPa

435


Table 5: Representative compositions of spinel

No. SiO2 Al2O3 TiO2 FeO MnO MgO CaO Na2O K2O Cr2O3 V2O3 NiO Total Fe3+ Al Cr


1 0·05 29·25 0·07 16·35 0·22 13·56 0·00 0·02 0·01 40·12 0·20 0·10 99·95 0·008 0·517 0·475

2 0·50 25·48 0·01 16·25 0·24 13·64 0·02 0·02 0·00 44·00 0·18 0·10 100·44 0·008 0·460 0·532

3 0·02 41·69 0·09 13·75 0·19 16·56 0·03 0·01 0·01 27·39 0·15 0·17 100·04 0·008 0·688 0·303

4 0·04 10·29 0·09 19·84 0·34 10·08 0·01 0·01 0·00 58·96 0·23 0·03 99·92 0·026 0·201 0·773

5 0·02 9·27 0·07 19·22 0·34 9·24 0·01 0·01 0·00 61·78 0·15 0·07 100·17 0·002 0·183 0·816

6 0·00 16·30 0·11 20·44 0·30 10·80 0·01 0·01 0·02 51·59 0·22 0·09 99·90 0·036 0·309 0·656

7 0·16 35·25 0·13 14·96 0·19 15·19 0·00 0·00 0·00 34·44 0·20 0·10 100·62 0·004 0·602 0·394

8 0·04 28·74 0·08 16·56 0·29 13·50 0·01 0·03 0·00 40·88 0·19 0·07 100·38 0·009 0·507 0·484

9 0·03 24·72 0·02 16·30 0·24 12·69 0·01 0·00 0·00 46·58 0·19 0·09 100·87 0·000 0·442 0·558

10 0·01 30·47 0·10 14·45 0·18 14·33 0·00 0·01 0·00 40·10 0·15 0·11 99·91 0·000 0·531 0·469

11 0·05 30·72 0·05 14·92 0·21 14·51 0·01 0·02 0·01 40·33 0·14 0·08 101·04 0·001 0·531 0·468

12 0·02 26·08 0·01 14·85 0·25 13·56 0·01 0·00 0·00 45·20 0·13 0·14 100·26 0·000 0·462 0·538

13 0·01 12·72 0·09 19·15 0·34 10·32 0·01 0·02 0·01 56·19 0·30 0·05 99·21 0·020 0·247 0·733

14 0·18 7·61 0·03 17·31 0·29 11·07 0·02 0·05 0·02 62·37 0·07 0·05 99·07 0·019 0·151 0·830

15 0·07 12·36 0·10 17·89 0·29 10·82 0·02 0·00 0·01 56·09 0·18 0·07 97·90 0·018 0·243 0·739

16 0·01 16·04 0·06 20·55 0·42 9·08 0·01 0·00 0·00 50·84 0·19 0·07 97·25 0·016 0·315 0·669

17 0·03 13·02 0·09 17·36 0·33 11·00 0·01 0·00 0·01 57·16 0·26 0·05 99·31 0·008 0·251 0·741

18 0·03 11·95 0·06 16·24 0·31 11·81 0·00 0·00 0·01 59·14 0·16 0·06 99·77 0·010 0·229 0·761

19 0·07 13·56 0·10 17·26 0·33 11·60 0·01 0·00 0·02 56·18 0·23 0·04 99·40 0·016 0·260 0·723

20 0·00 27·46 0·01 16·09 0·27 13·15 0·01 0·03 0·02 43·79 0·16 0·05 101·02 0·000 0·483 0·517

21 0·01 25·68 0·04 16·26 0·21 13·15 0·00 0·01 0·01 45·10 0·23 0·11 100·80 0·004 0·457 0·539

22 0·01 23·15 0·09 16·01 0·27 12·99 0·03 0·09 0·04 47·07 0·13 0·11 99·98 0·007 0·420 0·573

23 1·32 27·01 0·05 24·75 0·21 13·99 0·04 0·05 0·02 33·17 0·20 0·11 100·92 0·104 0·492 0·405

24 0·00 23·86 0·02 15·70 0·27 12·48 0·01 0·00 0·01 45·91 0·16 0·08 98·50 0·000 0·437 0·563

25 0·08 16·20 0·09 19·16 0·27 10·44 0·00 0·00 0·00 51·90 0·31 0·06 98·51 0·016 0·312 0·671

26 0·04 21·54 0·03 17·46 0·28 11·93 0·02 0·00 0·00 48·52 0·24 0·05 100·12 0·007 0·396 0·598

27 0·03 22·96 0·10 15·87 0·27 12·89 0·01 0·00 0·00 47·70 0·12 0·09 100·03 0·002 0·417 0·581

28 0·12 29·44 0·06 15·35 0·27 14·13 0·00 0·00 0·01 39·79 0·17 0·07 99·41 0·008 0·521 0·472

29 0·01 28·90 0·09 16·03 0·14 13·68 0·01 0·00 0·00 41·99 0·18 0·08 101·11 0·002 0·506 0·493

30 0·15 24·80 0·09 17·50 0·24 12·19 0·00 0·02 0·02 43·93 0·22 0·03 99·19 0·006 0·454 0·540

31 0·00 11·52 0·13 18·70 0·31 9·99 0·00 0·01 0·03 58·86 0·22 0·07 99·82 0·006 0·225 0·769

32 0·03 8·79 0·05 17·64 0·28 11·15 0·01 0·00 0·00 61·80 0·16 0·01 99·93 0·020 0·171 0·808

33 0·00 10·92 0·06 19·95 0·35 10·09 0·00 0·00 0·00 57·59 0·25 0·02 99·23 0·030 0·214 0·756

34 0·11 12·49 0·06 23·63 0·30 8·67 0·00 0·00 0·01 54·20 0·15 0·07 99·69 0·043 0·245 0·712

35 0·03 12·08 0·12 16·60 0·30 11·50 0·00 0·00 0·01 59·00 0·16 0·04 99·83 0·006 0·232 0·761

36 0·05 22·93 0·03 17·49 0·26 11·94 0·00 0·03 0·02 47·00 0·23 0·03 100·00 0·004 0·419 0·577

37 0·10 18·55 0·05 17·74 0·30 11·52 0·00 0·00 0·01 51·33 0·23 0·04 99·87 0·008 0·347 0·644

38 0·01 16·38 0·09 17·82 0·27 10·94 0·00 0·01 0·00 53·63 0·27 0·02 99·44 0·004 0·312 0·684

Ito et al., 1999). These values are slightly smaller than the PROCESS OF DUNITE FORMATIONMg–Fe interdiffusion coefficients in olivine. Therefore,

Similarity of HD2 with the IwanaidakeNiO wt % of olivine in dunite would change sim-General Trendultaneously with Mg# of olivine, or more slowly. It mayThe local trend of HD2 (with a relatively thick dunite)be difficult to change the Cr# of spinel in dunite byshows a wide range of variation in mineral compositions,Al–Cr interdiffusion because the spinel fraction is very

small and spinel grains are isolated from each other. suggesting that the core of this dunite may preserve the

436


Distance SiO2 Al2O3 TiO2 FeO MnO MgO CaO Na2O K2O Cr2O3 V2O3 NiO Total Fe3+ Al Cr

(cm)

HD2

1 0·02 22·96 0·00 18·54 0·29 11·61 0·01 0·00 0·00 47·48 — 0·01 100·92 0·007 0·416 0·577

5 0·04 22·63 0·02 17·78 0·29 11·95 0·00 0·00 0·00 48·28 — 0·05 101·05 0·004 0·410 0·586

10 0·05 21·52 0·00 23·43 0·44 7·15 0·01 0·00 0·00 47·46 — 0·04 100·10 0·000 0·403 0·597

15 0·05 21·22 0·02 17·35 0·32 10·84 0·00 0·00 0·00 49·25 — 0·02 99·07 0·000 0·391 0·609

20 0·03 21·31 0·00 16·52 0·25 11·22 0·00 0·01 0·00 50·12 — 0·00 99·46 0·000 0·388 0·612

25 0·14 20·68 0·01 17·72 0·33 11·81 0·00 0·00 0·00 49·23 — 0·05 99·96 0·008 0·382 0·610

30 0·03 19·53 0·05 18·54 0·34 11·13 0·02 0·00 0·00 50·58 — 0·04 100·25 0·008 0·362 0·629

35 0·03 19·68 0·04 15·81 0·29 12·90 0·01 0·00 0·00 51·06 — 0·04 99·85 0·008 0·362 0·630

40 0·05 18·92 0·04 17·33 0·33 11·84 0·03 0·02 0·00 51·21 — 0·06 99·82 0·010 0·352 0·638

45 0·00 17·69 0·05 17·08 0·29 12·30 0·02 0·00 0·01 52·52 — 0·10 100·06 0·018 0·328 0·654

50 0·11 16·92 0·00 18·31 0·31 11·26 0·03 0·01 0·00 53·21 — 0·00 100·15 0·012 0·318 0·670

55 0·02 16·47 0·05 18·18 0·40 11·05 0·00 0·01 0·01 54·46 — 0·05 100·68 0·007 0·309 0·684

60 0·02 15·73 0·03 16·88 0·33 11·41 0·03 0·00 0·01 55·54 — 0·05 100·01 0·001 0·297 0·702

65 0·05 14·52 0·02 21·11 0·41 8·83 0·03 0·00 0·00 55·07 — 0·07 100·11 0·008 0·280 0·712

70 0·04 12·47 0·08 20·72 0·40 9·09 0·01 0·00 0·00 56·98 — 0·06 99·86 0·013 0·243 0·744

90 0·07 12·57 0·03 19·58 0·38 9·93 0·01 0·00 0·00 57·38 — 0·03 99·97 0·014 0·243 0·744

100 0·07 12·67 0·00 18·25 0·32 10·68 0·00 0·01 0·00 58·13 — 0·02 100·16 0·010 0·243 0·747

110 0·05 12·73 0·02 20·21 0·42 9·24 0·00 0·03 0·00 56·69 — 0·01 99·39 0·011 0·248 0·741

120 0·06 11·92 0·01 20·42 0·37 9·46 0·01 0·00 0·00 58·40 — 0·04 100·69 0·014 0·230 0·756

130 0·08 13·87 0·04 16·11 0·33 11·97 0·01 0·00 0·00 57·57 — 0·04 100·03 0·005 0·263 0·732

140 0·07 12·82 0·03 20·95 0·41 9·33 0·01 0·00 0·00 57·56 — 0·00 101·20 0·014 0·246 0·740

150 0·07 14·76 0·01 17·54 0·33 11·23 0·01 0·00 0·00 56·49 — 0·06 100·49 0·006 0·279 0·715

HD3

0·0 0·10 21·43 0·04 16·59 0·28 11·96 0·00 0·00 0·01 49·51 0·18 0·05 100·15 0·000 0·392 0·608

1·1 0·08 20·95 0·00 17·48 0·31 12·26 0·01 0·00 0·01 49·15 0·22 0·04 100·50 0·013 0·384 0·603

3·3 0·08 20·06 0·00 17·76 0·26 11·66 0·02 0·01 0·02 49·89 0·16 0·04 99·95 0·008 0·372 0·620

4·4 0·08 20·91 0·00 16·97 0·29 12·07 0·01 0·00 0·00 50·15 0·15 0·05 100·67 0·001 0·383 0·616

6·5 0·20 20·28 0·00 18·47 0·32 11·80 0·02 0·00 0·01 48·21 0·18 0·07 99·55 0·021 0·378 0·602

9·5 0·09 21·57 0·00 18·30 0·30 11·72 0·02 0·03 0·00 47·46 0·23 0·05 99·75 0·015 0·398 0·587

original mineral compositions. Within the massif as a is seen with Mg# of olivine and Cr# of spinel, andpositive correlation is seen with NiO wt % of olivine.whole, a correlation is evident between the thickness of

the dunite layers and chemical composition [Figs 6 and However, considering that the modal abundance oforthopyroxene in HD2 covers the gap between dunite7, and Tamura et al. (1999)].

Figure 4 shows the relationship between the Mg# of and harzburgite in the General Trend, HD2 seems torepresent the intermediate way to make the Generalolivine, Cr# of spinel and NiO wt % of olivine in the

harzburgite and dunite, compared with HD2. Figure 5 Trend. Therefore, it can be considered that dunite inthe General Trend was produced by the same formationshows the relationship between the modal abundance ofprocess as HD2-type dunite.orthopyroxene and mineral compositions for the same

samples. The HD2 harzburgite appears to bridge thegap between dunite and harzburgite in the IwanaidakeGeneral Trend. No apparent relationship is evident

Incongruent melting of orthopyroxenebetween the modal abundance of orthopyroxene andchemical compositions in the Iwanaidake General Trend; First, Mg# of olivine and Cr# of spinel are examined

using experimental data on peridotite melting (e.g. Mysenhowever, in the local trend of HD2, a negative correlation

437


Table 5: continued

Distance SiO2 Al2O3 TiO2 FeO MnO MgO CaO Na2O K2O Cr2O3 V2O3 NiO Total Fe3+ Al Cr

(cm)

HD7

0·0 0·05 15·38 0·08 18·64 0·31 10·65 0·00 0·00 — 54·20 — 0·07 99·38 0·011 0·294 0·695

20·0 0·05 15·63 0·07 16·47 0·30 11·81 0·02 0·02 — 55·11 — 0·05 99·52 0·005 0·296 0·699

25·0 0·18 14·45 0·05 17·32 0·32 11·50 0·01 0·00 — 55·52 — 0·05 99·40 0·011 0·276 0·712

28·5 0·14 15·38 0·07 18·32 0·32 11·00 0·01 0·00 — 54·48 — 0·00 99·71 0·011 0·293 0·696

31·5 0·08 14·82 0·07 18·49 0·34 10·83 0·01 0·00 — 54·92 — 0·03 99·58 0·013 0·283 0·704

35·0 0·02 13·29 0·08 20·00 0·37 10·11 0·02 0·00 — 55·23 — 0·02 99·14 0·025 0·257 0·718

39·5 0·10 13·05 0·09 18·92 0·36 10·43 0·01 0·00 — 56·13 — 0·03 99·13 0·016 0·253 0·731

43·5 0·05 12·38 0·07 18·87 0·34 10·31 0·01 0·01 — 57·60 — 0·00 99·64 0·012 0·240 0·748

47·5 0·02 13·14 0·05 19·93 0·33 9·82 0·00 0·00 — 56·04 — 0·02 99·35 0·017 0·255 0·728

50·0 0·04 14·01 0·06 19·27 0·37 10·22 0·00 0·02 — 55·07 — 0·02 99·08 0·016 0·270 0·713

55·0 0·03 11·45 0·07 18·86 0·34 10·04 0·02 0·00 — 58·16 — 0·03 99·00 0·012 0·224 0·763

58·5 0·51 11·74 0·08 18·15 0·36 10·83 0·01 0·06 — 58·22 — 0·02 99·97 0·006 0·230 0·764

61·0 0·04 12·86 0·06 18·61 0·35 10·52 0·01 0·00 — 57·07 — 0·05 99·58 0·014 0·248 0·738

65·0 0·05 11·55 0·10 19·78 0·36 10·47 0·00 0·00 — 56·46 — 0·05 98·82 0·034 0·226 0·740

70·0 0·09 12·32 0·10 18·39 0·32 10·75 0·00 0·02 — 56·69 — 0·05 98·73 0·018 0·240 0·741

74·0 0·22 18·13 0·11 17·51 0·37 11·69 0·00 0·01 — 50·90 — 0·03 98·97 0·010 0·343 0·646

78·0 0·00 16·22 0·09 19·04 0·34 10·71 0·00 0·01 — 52·74 — 0·01 99·17 0·017 0·309 0·674

82·0 0·08 13·43 0·06 18·59 0·36 10·14 0·00 0·00 — 56·60 — 0·03 99·29 0·005 0·260 0·735

84·0 0·02 11·22 0·07 19·43 0·38 10·05 0·02 0·00 — 58·61 — 0·02 99·82 0·018 0·218 0·764

88·0 0·06 11·07 0·06 18·43 0·31 10·38 0·00 0·02 — 59·21 — 0·03 99·58 0·011 0·216 0·773

91·0 0·04 13·09 0·07 17·63 0·32 11·29 0·01 0·00 — 56·77 — 0·03 99·24 0·016 0·252 0·732

94·0 0·03 10·93 0·03 20·56 0·38 10·14 0·01 0·00 — 57·98 — 0·03 100·10 0·035 0·212 0·753

98·0 0·05 12·61 0·02 19·88 0·38 10·35 0·01 0·00 — 57·05 — 0·01 100·35 0·025 0·242 0·734

100·0 0·04 11·97 0·01 19·62 0·32 10·37 0·00 0·00 — 57·91 — 0·01 100·25 0·023 0·230 0·747

104·0 0·02 12·20 0·03 19·38 0·39 10·18 0·01 0·00 — 57·70 — 0·02 99·93 0·018 0·235 0·747

118·0 0·04 13·20 0·03 19·24 0·37 10·34 0·00 0·00 — 56·79 — 0·09 100·10 0·016 0·253 0·731

129·0 0·03 16·62 0·03 19·44 0·32 10·78 0·00 0·00 — 52·83 — 0·00 100·06 0·019 0·313 0·668

& Kushiro, 1977; Jaques & Green, 1980). According to (e.g. Kelemen, 1990), in which reaction between pyr-oxene-bearing host rock and olivine-saturated magmathe experiments, as the degree of melting is increased,

the rock type changes from lherzolite to harzburgite and produces dunite, can explain the Mg#–NiO wt % neg-ative correlation, this model seems not to explain ad-finally to dunite, and the Mg# of olivine and Cr# of

spinel in the residue increase gradually (Mysen & Kushiro, equately the Mg#–Cr# positive correlationsimultaneously. To solve this problem, an incongruent1977; Jaques & Green, 1980). Because dunite has a

higher Mg# of olivine and Cr# of spinel than harzburgite melting model of orthopyroxene is proposed here; thatis, the reaction orthopyroxene → olivine + SiO2-richin the local trend of HD2 (and also in the Iwanaidake

General Trend), it is suggested that dunite may have melt. The phase diagram in the simple system MgSiO3–H2O (Kushiro et al., 1968) shows that orthopyroxenebeen derived from harzburgite by melting. However, the

lower NiO wt % of olivine in dunite than in harzburgite melts incongruently to liquid and olivine at pressuresless than >0·5 GPa under dry conditions and thatconflicts with this simple melting model. Melting of

harzburgite should increase the NiO wt % of olivine in incongruent melting occurs at 3 GPa under water-sat-urated conditions. If the mass of olivine increases as aresidues, because the partition coefficient between olivine

and melt is usually larger than unity (Kinzler et al., 1990; result of the above reaction, the concentration of NiOin olivine should decrease because of the dilution effect.Beattie et al., 1991). Although the replacive dunite model

438


(1997). To consider the melting processes of HD2, themost fertile harzburgite (the left-most side with the lowestCr# of spinel) in HD2 was used as the starting com-position and cation ratio for this calculation (Table 6).Because clinopyroxene is rare in HD2, only olivine,orthopyroxene and spinel are used. The relationshipsbetween minerals and melt are described by one net-transfer reaction. Six elements (Mg, Fe, Ca, Al, Cr andNi) are used for this calculation and the cation ratio ofSi is given by subtracting the others from unity. Theexchange distribution coefficients for Mg–Fe (MF), Al–Cr(AC) and Mg–Ca (MC) between melt and mineral (�)are expressed as K�

(MF,AC,MC) and given as

K OlMF=

C OlFe

C OlMg

·C lq

Mg

C lqFe= 0·275, K Opx

MF =C Opx

Fe

C OpxMg

·C lq

Mg

C lqFe= 0·27,

K SpMF=

C SpFe

C SpMg

·C lq

Mg

C lqFe= 2·38

K SpAC=

C SpAl

C SpCr

·C lq

Cr

C lqAl=0·007, K Opx(M1)

AC =C Opx(M1)

Al

C Opx(M1)Cr

·C lq

Cr

C lqAl=0.01

K OpxMC =

C OpxCa

C OpxMg

·(C Opx

Mg + C OpxFe )

(1/4− C OpxCa )

·C lq

Mg

C lqCa= 0·03.

The values of exchange partition coefficients are modi-fied to satisfy the chemical compositional relationshipsin the starting sample of HD2. For Ni, a simple Nernst-type partition coefficient was used:

D Ol/lqNi =

C OlNi

C lqNi= 10.

Aluminum content in orthopyroxene is also given as

K OpxAl =

C SpMF

C OlMF

·(C lq

FeKOlMF+ C lq

Mg)(C lq

FeKSpMF+ C lq

Mg)·

(C SpAl )2

C Opx(M1)Al

= 5.

Fig. 4. Chemical compositional relationships for the Iwanaidake Gen-Mass balance equations are formulated for each mineral:eral Trend and the local trend of HD2. (a) Mg# of olivine vs Cr# of

spinel. The olivine–spinel mantle array (OSMA; Arai, 1994) is shaded.The range of Cr# of spinel in abyssal peridotite (Dick & Bullen, 1984) C Ol,0

Mg xOl,0+ C Opx,0Mg xOpx,0+ C Sp,0

Mg xSp,0= (1− f )is also shown. It should be noted that the local trend of HD2 resemblesthe General Trend and shows a positive correlation similar to OSMA. {C Ol

Mg xOl+ C OpxMg xOpx+ C Sp

Mg xSp}+ f C lqMg(b) Mg# vs NiO wt % of olivine; 100% normalized values of NiO wt %

were used. The mantle olivine compositional array (MOA, Takahashi,1986a) is shown as the shaded area. It should be noted that the local

C Ol,0Fe xOl,0+ C Opx,0

Fe xOpx,0+ C Sp,0Fe xSp,0= (1− f )trend of HD2 resembles the General Trend; however, they are different

from the MOA. The harzburgite samples plot within the MOA;{C Ol

Fe xOl+ C OpxFe xOpx+ C Sp

Fe xSp}+ f C lqFehowever, the dunites are unusually low in NiO wt % for a given Mg#.

C Opx(M1),0Al xOpx,0+ C Opx(T1),0

Al xOpx,0+ C Sp,0Al xSp,0=Calculation of melting mode

To verify this hypothesis of orthopyroxene incongruent (1− f ){C Opx(M1)Al xOpx+ C Opx(T)

Al xOpx+ C SpAlx

Sp}+ f C lqAlmelting, model calculations of mineral compositional

change using a variety of melting modes were carriedout using the calculation scheme developed by Ozawa C Opx,0

Ca xOpx,0= (1− f )(C OpxCa xOpx)+ f C lq

Ca

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Fig. 5. Relationship between the modal abundance of orthopyroxene and mineral chemical compositions (average values) for the IwanaidakeGeneral Trend and the local trend of HD2. (a) Relationship between Mg# of olivine and the modal abundance of orthopyroxene. In bothtrends, the Mg# of olivine in dunite (modal abundance of orthopyroxene <2 vol. %) is higher than in harzburgite. (b) NiO wt % of olivine. Inboth trends, NiO wt % of olivine in dunite is lower than in harzburgite. (c) Cr# of spinel. In both trends, Cr# of spinel in dunite is higher thanin harzburgite. (d) Fe3+/(Cr + Al + Fe3+) of spinel. In both trends this ratio is very low (<0·05) in both dunite and harzburgite.

C Opx,0Cr xOpx,0+ C Sp,0

Cr xSp,0= (1− f ) a very silica-rich melt. Case (2) corresponds to the inter-mediate case where orthopyroxene melts almost con-

{C OpxCr xOpx+ C SpxSp

Cr}+ f C lqCr gruently. By using these three chemical reactions, both

batch melting and fractional melting (in this calculation,C Ol,0

Ni xOl,0= (1 – f )C OlNix

Ol+ f C lqNi. 1% incremental melting is used as a proxy for fractional

melting) were examined. The calculations were continuedBy defining the degree of melting f, the modal abundanceuntil orthopyroxene disappeared (i.e. until dunite wasof minerals and melt, the compositions of minerals andformed).melt are defined.

The results of the calculations are shown in Fig. 9.Here, three melting models have been computed:Case (3) with 1·497 orthopyroxene + 0·003 spinel →(1) 0·497 orthopyroxene+ 0·5 olivine+ 0·003 spinel1 melt + 0·5 olivine by batch melting (Fig. 9a and b)→ 1 melt;reproduces the trend where the Cr# of spinel increases(2) 0·997 orthopyroxene + 0·003 spinel → 1 melt;and the NiO wt % of olivine decreases, as the Mg# of(3) 1·497 orthopyroxene + 0·003 spinel → 1 melt +olivine increases. This trend is similar to the HD2 local0·5 olivine.trend (and hence, the Iwanaidake General Trend). In thisCase (1) corresponds to eutectic melting of ortho-chemical reaction, orthopyroxene melts incongruently topyroxene and olivine in nearly equal amount. Case (3)olivine and melt, and the Mg# of olivine and Cr# ofcorresponds to the case of extreme incongruent melting;

one-third of orthopyroxene reacts to form olivine, yielding spinel increase and NiO wt % of olivine decreases, as

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Fig. 6. HD2: relationships between distance and mineral chemical composition. Average values and error bars, which represent the 1� ofmeasured points in one thin section, are plotted. (a) Photograph of HD2. The dunite is>70 cm thick. The left side of the harzburgite graduallychanges into dunite. Total width of the section is >150 cm. (b) Mg# of olivine as a function of distance. (c) NiO wt % of olivine. (d) Cr# ofspinel. (e) Fe3+/(Cr + Al + Fe3+) of spinel. (f ) The modal abundance of orthopyroxene. Clear compositional zoning across the dunite layerexists in Mg#, NiO wt % of olivine and Cr# of spinel.

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Fig. 7. HD3: relationships between distance and mineral chemical composition. Average values and the 1� in one core sample are plotted.Where core samples have both dunite and harzburgite, they are examined separately. (a) Photograph of HD3. Section is >10 cm wide. Thethin dunite has a sharp boundary with the surrounding harzburgite. (b)–(f ) as in Fig. 6. It should be noted that there is no appreciable zoningacross the dunite layer.

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Fig. 8. HD7: relationships between distance and mineral chemical composition. (a) Photograph of HD7. Section is >130 cm wide. Threedunite zones are seen in this section. (b)–(f ) as in Fig. 6.

the degree of melting increases. Although the parameters between the new melt and the surrounding harzburgite,the agreement of the calculated trends with those ob-used in the calculation have some uncertainty and we

have to consider the possibilities of re-equilibrium served in HD2 (and the Iwanaidake peridotite) supports

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melting and dunite formation in the Iwanaidake peri-Table 6: Starting compositions of minerals fordotite may have been stimulated by the presence of

calculation water. Dunite bodies in the Iwanaidake peridotite arescattered in the host harzburgite, suggesting that the

wt % cation ratio host harzburgite melted only locally. This pattern ofoccurrence can be explained by injection of hydrous meltinto the host harzburgite.ol opx sp ol opx sp

The existence of water in the Iwanaidake peridotitehas been demonstrated by studies of fluid inclusions inSiO2 40·33 56·71 — Si 0·33 0·49 0·00olivine (Arai & Hirai, 1985; Hirai & Arai, 1987). CrystalAl2O3 — 1·66 22·47 Al 0·00 0·02 0·27phases in the fluid inclusions are brucite and serpentine,FeO 7·78 5·23 17·28 Fe 0·05 0·04 0·15a combination implying that the fluid comprised mainlyMgO 51·49 34·81 12·15 Mg 0·62 0·45 0·19H2O. Of the range of possible environments in the upperCaO — 0·96 — Ca 0·00 0·01 0·00mantle in which water might be present, the mantleCr2O3 — 0·62 48·10 Cr 0·00 0·00 0·39wedge above a subduction zone seems the most likelyNiO 0·41 — — Ni 0·00 0·00 0·00tectonic setting. The existence of water in the mantle

Total 100·00 100·00 100·00 Total 1·00 1·00 1·00wedge above subduction zones is substantiated by studiesof mantle peridotite xenoliths entrained in arc magmas.

Modal abundance of mineralsHydrous minerals, including amphibole, were reported

ol opx spin peridotite xenoliths from the Ichinomegata crater, NEJapan (Takahashi, 1986b). They were derived from the

wt % 87·70 11·40 0·90 mantle wedge beneath the NE Japan arc.vol. % 87·40 11·50 0·70 Both relatively high temperatures (>1100°C) and

water-excess conditions are necessary to melt harzburgiteThe starting composition is the most fertile harzburgite (the to produce dunite as described above. Many models forleft-most side with the lowest Cr# of spinel) in HD2.

the temperature distribution and mantle lithologies insubduction zones have been suggested (Tatsumi, 1995;Green & Falloon, 1998; Iwamori, 1998), and it is es-

the hypothesis that dunite formed by batch partial melting timated that the main dehydration of the slab finishesof harzburgite with the incongruent melting of ortho- within the fore-arc region and the released water ispyroxene. trapped by the mantle peridotite above the slab. This

suggests that the supply of large quantities of water intomantle peridotite occurs mainly at relatively shallowdepth near the trench. These models also suggest that the

Effect of water in dunite formation mantle wedge is warmed by upwelling of asthenosphericThe supply of hydrous melt into the host harzburgite is mantle material and that the high-temperature fieldthe most likely triggering mechanism for formation of >1100°C extends towards the trench side of the volcanicthe dunite bodies in the Iwanaidake peridotite. This idea front. Therefore, a part of the fore-arc may satisfy bothis based on the following reasons. Because harzburgite the high-temperature and water-excess conditions. Thein the Iwanaidake has been itself depleted, judging from Iwanaidake peridotite may have undergone injection ofits high Mg# of olivine and Cr# of spinel, it would be hydrous melts in the fore-arc, as a result of which smalldifficult to form even more depleted dunite by simple patches of dunite were formed. Tamura et al. (1999)partial melting of this refractory harzburgite under also proposed a similar hydrous melting process for theanhydrous conditions. However, it has been dem- Takadomari, Nukabira and Iwanaidake massifs in theonstrated that the solidus of peridotite is decreased, Kamuikotan belt.the temperature of the orthopyroxene-out reaction isdecreased, and the degree of melting is increased by theaddition of the water. Green (1973) showed that in a

The model of dunite formation in thepyrolite bulk composition at 1 GPa, orthopyroxene meltsIwanaidake peridotiteout at >1400°C under dry conditions and >1100°CFigure 10a illustrates the proposed model of duniteunder water-saturated conditions. In addition, the field

of orthopyroxene incongruent melting expands up to 3 formation in the Iwanaidake peridotite. Let us supposethat the hydrous melt was in equilibrium with olivine ofGPa (Kushiro et al., 1968) under hydrous conditions,

whereas it occurs only at lower pressures <0·5 GPa under Mg# >91, NiO wt % >0·40 wt % and spinel of Cr#>60 and was undersaturated with respect to SiO2. Thisdry conditions. Therefore, orthopyroxene-incongruent

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Fig. 9. Results of mass balance calculations for peridotite partial melting involving three representative melting reactions. Variations in thedegree of melting are shown by thin continuous lines. (See Table 6 for starting material.) (a) Mg# of olivine vs Cr# of spinel; batch melting. (b)Mg# of olivine vs NiO wt % of olivine; batch melting. (c) Mg# of olivine vs Cr# of spinel; fractional melting (in this calculation, 1% incrementalmelting is used as a proxy). (d) Mg# of olivine vs NiO wt % of olivine; fractional melting.

composition corresponds to the melt in equilibrium with addition of the H2O-rich melt. Both the effects of thesilica-poor melt and the presence of H2O would causethe surrounding harzburgite (with olivine of Mg# >91,

NiO wt % >0·40 wt % and spinel of Cr# >60) at the orthopyroxene in the harzburgite to melt in-congruently and to produce olivine. The incongruentgreater depth. When the hydrous melt upwells into the

shallower harzburgite, the melt would no longer be in melting causes the melt to become more silica rich. Inthis case, the NiO wt % of olivine would decrease asequilibrium with the surrounding harzburgite, and the

solidus of the harzburgite would be lowered by the the mass of olivine increased, and as the melt fraction

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Fig. 10. The model for the formation of HD2-type dunites in the Iwanaidake peridotite. (a) We suppose that the hydrous melt is in equilibriumwith olivine of Mg# >91, NiO wt % >0·40 wt % and spinel of Cr# >60. The hydrous melt upwells to a shallower position, so the melt canno longer be in equilibrium with surrounding harzburgite and the solidus of the harzburgite is lowered by the addition of H2O. Orthopyroxenein the harzburgite melts incongruently, and the olivine NiO wt % decreases as the olivine mass increases. At the same time as melt fractionincreases, the Mg# of olivine and Cr# of spinel increase. The new melt is more siliceous than the initial melt. (b) Deformation occurs after theHD2-type dunite was formed. The dunite bands became thinner by deformation and thin lenticular-shaped dunites are formed.

increased by incongruent melting, Mg# of olivine and formation of the Iwanaidake General Trend. Second,deformation occurred in the Iwanaidake peridotite andCr# of spinel would increase.

The relationships between the shapes of the dunite the observed dunites were stretched or flattened, formingthe lenticular shapes. Thinner dunites (e.g. HD3) maybodies and the mineral compositional trends may rep-

resent the history of melt supply and deformation in the have been formed from thicker dunites by this de-formation process. Finally, after deformation, irregular-Iwanaidake peridotite. Figure 10b illustrates the de-

formation history. First, HD2-type dunites were formed shaped dunites such as HD7 were formed. Because HD7has a different trend of mineral compositions from the(as illustrated in Fig. 10a), which corresponds to the

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other lenticular-shaped dunites, as shown in Fig. 8, an- The melting mode is estimated to be approximately 1·5other dunite formation process, which was different from orthopyroxene → 0·5 olivine + 1 melt. A supply ofthat of the Iwanaidake General Trend (e.g. precipitation water would have been necessary to lower the solidus ofof olivine from Si-poor melt), may have occurred during the harzburgite and the incongruent melting temperaturethe final stage of the dunite formation processes in the of orthopyroxene. The mantle wedge above a subductionIwanaidake. zone is the most likely tectonic setting where hydrous

melt can be formed and upwell. The Iwanaidake peri-dotite may have been emplaced in the mantle wedge ata subduction zone when the dunites were formed.

COMPARISON WITH PERIDOTITESIN KNOWN TECTONIC SETTINGSData compilations for peridotites from known tectonicsettings have been reported previously (Dick & Bullen, ACKNOWLEDGEMENTS1984; Arai, 1994). Arai (1994) demonstrated the re- Thanks are due to Professor Hiroko Nagahara andlationship between Mg# of olivine and Cr# of spinel for Assistant Professor Hikaru Iwamori for their guidanceperidotite xenoliths from known tectonic settings (ocean and encouragement, and to Professors Richard J. Arculusfloor, fore-arc, oceanic hotspot, Japan arcs, continent or and David H. Green for critical readings of the manu-African craton). For abyssal peridotites, the Mg# of script. I am also grateful to Professors Eiichi Takahashiolivine ranges from 89·5 to 91·5 and Cr# of spinel ranges and Kazuhito Ozawa for valuable suggestions. I thankfrom 8 to 60 (Dick & Bullen 1984; Arai, 1994). These Dr Kyoko Matsukage and Dr Natsue Abe for discussion.values are lower than most of the Iwanaidake peridotites. Thanks are also due to Mr Hideto Yoshida for hisThe most fertile harzburgites in Iwanaidake correspond assistance with EPMA analysis.to the most depleted abyssal peridotite. Therefore, thehost peridotite (= fertile harzburgite in this massif ) mayhave been an abyssal peridotite, but the more depletedharzburgite and dunite may have been formed in another

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Dunite Formation Processes in Highly Depleted Peridotite: Case

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