Journal of Mineralogical and Petrological Sciences, Volume 109, page 79–84, 2014

LETTER

Formation process of –clinopyroxene cumulates inferred from Takashima , Southwest Japan arc

Ritsuko MUROI and Shoji ARAI

Department of Earth Sciences, Kanazawa University, Kanazawa 920–1192, Japan

Rocks of the –clinopyroxenite series (dunite, wehrlite, olivine clinopyroxenite, and clinopyroxenite) are common as cumulates formed around Moho in arc–related environments, but their formation processes remain unclear. They are common as xenoliths of Group I from Takashima in the Southwest Japan arc, and a descrip- tion of their formation process is provided here. The rocks vary from dunite to clinopyroxenite via wehrlite and olivine clinopyroxenite, and all showing mosaic equigranular to weakly porphyroclastic textures. The rocks are completely free from , and they contain <3 vol% chromian spinel. Some of them contain up to 3 vol% orthopyroxene; these are approximate mixtures of olivine and clinopyroxene. As the change to clinopyroxenites, the Mg# [= Mg/(Mg + total Fe) atomic ratio] varies from 0.93 to 0.84 in olivine, and from 0.92 to 0.87 in clinopyroxene. The Cr# [= Cr/(Cr + Al) atomic ratio] of the chromian spinel varies from 0.8 to 0.2 with decreases in the Mg# of olivine and clinopyroxene. The Mg–Fe distribution relation between olivine and clinopyroxene suggests their subsolidus equilibration is around 800–900 °C. Initial Mg#s expected at a magmatic temperature indicate that their formation proceeded from magma in the order of Mg–rich dunites followed by clinopyroxenites and then less Mg–rich dunite–wehrlite–olivine clinopyroxenite. This suggests a zigzag liquid path, starting from a mantle–derived olivine–oversaturated magma, around the olivine–clinopy- roxene cotectic boundary. Continuous crystallization of olivine or clinopyroxene due to supersaturation could have enabled the magma to straddle the cotectic boundary to form alternately clinopyroxene– and olivine– oversaturated magmas.

Keywords: Dunites, Wehrlites, Clinopyroxenites, Xenoliths, Cumulates, Takashima, Supersaturation

INTRODUCTION enite series rocks (cf., Wandji et al., 2009). Igneous proc- esses for the formation of dunite–clinopyroxenite series Plutonic rocks of the dunite–clinopyroxenite series, i.e., rocks remain unclear despite their common occurrences. dunites, wehrlites, olivine clinopyroxenites, and clinopy- Crystal accumulation during the perfect fractional crys- roxenites, are commonly found in ophiolites (e.g., Ishi- tallization of a mantle–derived basaltic melt, which is oli- watari, 1985; Benn et al., 1988) and as xenoliths (e.g., vine–oversaturated at low pressures, may produce dunite Arai et al., 2000). They are especially important as the followed by olivine clinopyroxenite based on the appro- main constituents of the MTZ (Moho transition zone) of priate phase diagrams (cf., Kushiro, 1969). Wehrlite and arc–derived ophiolites (e.g., Parlak et al., 2002) as well as monomineralic clinopyroxenite cannot be, however, form- of Alaskan–type zoned complexes (e.g., Krause et al., ed by this process. 2007). Dunites and wehrlites distributed around the Moho We describe xenoliths of dunite–wehrlite–clinopy- transition zone are interpreted to be cumulates and/or re- roxenite of Group I (Frey and Prinz, 1978) from Takashima action products between the melt and harzburgite (Koga in the Southwest Japan arc (Arai et al., 2000), and discuss et al., 2001; Negishi et al., 2013). There have been many their formation process as cumulates in the sub–arc con- papers dealing with xenoliths of dunites (e.g., Sen and dition. The analyzed samples are representative of the up- Presnall, 1986) and clinopyroxenites (e.g., Neumann et permost part of the sub–arc mantle (e.g., Takahashi, 1978). al., 1988), but only a few studies on dunite–clinopyrox- doi:10.2465/jmps.131003 GEOLOGICAL BACKGROUND R. Muroi, [email protected] Corresponding author S. Arai, [email protected]–u.ac.jp Large amounts of ultramafic xenoliths from Takashima 80 R. Muroi and S. Arai are found in beach boulders of alkali olivine basalt of 3.0 a microprobe and analyzed the clinopyroxene for REE Ma (Nakamura et al., 1986) on Takashima Island, Kara- (rare earth elements) using La–ICP–MS. See the Appen- tsu City, Saga Prefecture, northern Kyushu, Japan (e.g., dix (available online from http://japanlinkcenter.org/DN/ Arai et al., 2000, 2001). Most of ultramafic xenoliths are JST.JSTAGE/jmps/131003) for analytical details. All Fe less than 30 cm across, and dominated by dunite–wehr- was assumed to be Fe2+ in silicates, and Fe2+ and Fe3+ lite–clinopyroxenite series rocks of both Group I (meta- were calculated assuming a spinel stoichiometry. Mg# morphic, green Cr–rich clinopyroxene bearing) and and Cr# represent Mg/(Mg + Fe2+) and Cr/(Cr + Al) atom- Group II (igneous, black Al–rich clinopyroxene bearing) ic ratios, respectively. Fe# is (1–Mg#). Fo is 100 × Mg# in the sense of Frey and Prinz (1978) (Arai et al., 2000). in olivine. Table 1 shows the results for the selected anal- Arai et al. (2000) presumed a thick ‘cumulus mantle’ lay- yses on the major to minor elements of olivine, clinopy- er (Takahashi, 1978) comprised of dunite–wehrlite–py- roxene, and chromian spinel, whereas results for the REE roxenites of Group I, which could be equivalent to the of clinopyroxene are shown in Table 2 (available online Moho transition zone of ophiolite (Arai and Abe, 1994), from http://japanlinkcenter.org/DN/JST.JSTAGE/jmps/ beneath Takashima. The ‘cumulus mantle’ layer has been 131003). intruded by pyroxenites of Group II (Arai et al., 2000, , clinopyroxenes, and chromian spinels are 2006). See Arai et al. (2001) for more details. homogeneous in chemistry except for rims in contact with different minerals, and homogeneous core composi- PETROGRAPHICAL DESCRIPTIONS tions were used in this study. Olivines were found to be highly Mg–rich (Fo >90) in clinopyroxene–free dunites, Rocks of the dunite–clinopyroxenite series have various and less Mg–rich (Fo >84) in clinopyroxene–rich rocks appearances on alkali olivine basalt boulders (Fig. 1). (wehrlites to clinopyroxenites) (Table 1 and Fig. 2). NiO Nearly homogeneous dunites (Fig. 1a) and olivine–bear- and MnO contents of olivine vary from 0.4 to 0.1 wt% ing clinopyroxenites (Fig. 1b) are frequent, while homo- and 0.1 to 0.2 wt%, respectively, in an approximate rela- geneous wehrlites are less frequent. Various dunite–clino- tion to decreases in Fo. The Mg# of clinopyroxene varies pyroxenite composite xenoliths are commonly found nearly with that of olivine from 0.92 (olivine–rich rocks) (Figs. 1c and 1d); it is noteworthy that the mixture of dun- to 0.86 (clinopyroxene–rich rocks) (Fig. 2). The Al2O3 ite and clinopyroxenite was more common than the homo- content of clinopyroxene varies from 1 to 4 wt% with a geneous wehrlite. For the nearly homogeneous samples, decrease in Mg# (Table 1). Clinopyroxenes are mostly dunites grade to clinopyroxenites with a gap at olivine rich in Cr2O3, containing 0.5 to 0.7 wt% irrespective of clinopyroxenites that could be due to incomplete sam- rock type (Table 1). The Cr# of chromian spinel changes pling. We could not observe fine–scale layering in these from 0.8 to 0.2 with the Fo of olivine. The TiO2 content rocks, which is in contrast to those in some ophiolites of chromian spinel varied from 0.3 wt% in dunites and up (e.g., Ishiwatari, 1985). to 0.8 wt% in clinopyroxene–rich rocks (Table 1). The The dunite–clinopyroxenite series rocks contain Fe3+/(Cr + Al + Fe3+) atomic ratio is around 0.1 (Table 1). small amounts (<3 vol%) of chromian spinel, and some The REE patterns of clinopyroxene, which was nor- of them contain orthopyroxene (<3 vol%), which is malized to the chondritic abundances (Sun and McDo- roughly in proportion to the amount of clinopyroxene. nough, 1989), are strikingly similar. The normalized val- They are completely free from plagioclase. Furthermore, ues exhibit a gentle convex upward trend from Lu to Sm, they commonly exhibit mosaic equigranular to weakly where their levels are also similar in individual samples, porphyroclastic textures. Coarse (<7 mm across) grains and roughly in sympathy with the initial igneous Mg# of olivine and clinopyroxene exhibit deformation fea- (see DISCUSSION) (Fig. 3). No zonation in terms of tures, i.e., kinking and/or undulatory extinction under REE contents was observed in the clinopyroxene grains. the microscope. Some coarse clinopyroxene grains exhib- The patterns were, however, highly variable from Sm to it exsolution lamellae, but the orthopyroxene is free from La, sometimes even in a single (Fig. 3). The them. Chromian spinel is less than 0.5 mm across, and enrichment of light REEs in clinopyroxene is not corre- black to brown in color for the thin sections. Constituents lated with any petrographic or chemical features of the of composite xenoliths are similar in petrography to the host rock. equivalents occurring as discrete xenoliths. Variations of mineral chemistry are discussed below based on the initial recalculated high–temperature Mg# of MINERAL CHEMISTRY olivine and clinopyroxene.

We analyzed minerals for major to minor elements using Formation process of olivine–clinopyroxene cumulates 81

2 (a) (b) 10 Wehrlite (TK-28-2, Mg#=0.898) Clinopyroxenite (TK-02, Mg#=0.883) Dunite (TK-20, Mg#=0.871) Wehrlite (TK-35, Mg#=0.856) 1 10

0 (c) (d) 10 Cpx / C1 chondrite

-1 10 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 3. Chondrite–normalized REE patterns of clinopyroxene in dunite–clinopyroxenite series xenoliths from Takashima. The in- Figure 1. Xenoliths of dunite–clinopyroxenite series from Taka- itial Mg#s (discussed below) are shown in parentheses. Chon- shima, the Southwest Japan arc. (a) Dunite nearly free from drite values from Sun and McDonough (1989). Note the pat- clinopyroxene. (b) Homogeneous olivine clinopyroxenite. (c) terns from Lu to Sm are nearly identical irrespective of the Dunite with clinopyroxenite patches. (d) Clinopyroxenite with rock types and chemistry. The patterns are variable from middle dunite patches. to light REEs between samples and within a single sample.

0.15 Dunite 1:1 ries rocks (Fig. 2), which means that the initial Mg# Wehrlite

) changed for both olivine and clinopyroxene during cool- Olivine clinopyroxenite °C 2+ 700 Clinopyroxenite ing to that temperature (cf., Arai et al., 1988). Some of 0.10 the orthopyroxene and chromian spinel grains associated °C 550 with clinopyroxene appear, at least in part, to have been /(Mg+Fe exsolved from the high–temperature clinopyroxene (e.g., 2+ Kaeser et al., 2006). 0.05 Hawaiian To reconstruct the initial Mg#s of olivine and clino- xenoliths (1000°C) pyroxne at a high igneous temperature, we assumed that Cpx Fe (1) all the Mg and Fe in the rock were preserved during cooling, (2) the Mg–Fe2+ distribution coefficient between 0.05 0.10 0.15 0.20 olivine and clinopyroxene was at unity, and (3) the modal Ol Fe2+/(Mg+Fe2+) ratio of olivine to clinopyroxene was constant during Figure 2. Mg–Fe2+ distribution relations between olivine and cooling. This means the Mg# was the same between oli- clinopyroxene in Takashima rocks. Isotherms are after Obata vine and clinopyroxene, and it is expected to be nearly et al. (1974). Note that nearly all samples are plotted between the same as the bulk–rock Mg# for individual rocks. We 700 and 1000 °C isotherms, which could indicate equilibrium at use the initial Mg#s hereafter in this letter. 800 to 900 °C. All Fe is assumed to be Fe2+. Formation process of the dunite–clinopyroxenite se- DISCUSSION ries rocks from Takashima

Geothermometry and reconstruction of initial compo- The initial Mg# at the magmatic stage is high (>0.90) in sitions of minerals dunites and relatively low (mostly 0.85 to 0.87) in wehr- lites to olivine clinopyroxenites (Fig. 4). It is noteworthy The Mg–Fe2+ distribution between olivine and Ca–rich that clinopyroxenites are intermediate in terms of Mg# clinopyroxene is known to be changeable depending on (mostly 0.87 to 0.89) (Fig. 4). The Mg# of silicates varies the equilibrium temperature (e.g., Obata et al., 1974). in accordance with the other chemical parameters such as Here, the distribution deviate slightly from unity; Fe# is the Cr# of the chromian spinel (Fig. 5a) and Yb content slightly higher in olivine than in coexisting clinopyrox- of clinopyroxene (Fig. 5b). All the rock types examined ene. This is indicative of subsolidus temperatures, i.e., form a crude but continuous fractionation trend in chemi- 800 to 900 °C (Obata et al., 1974), for the olivine–clino- cal space (e.g., Fig. 5), which suggests that they were pairs in Takashima dunite–clinopyroxenite se- formed from a magma, possibly by accumulation of oli- 82 R. Muroi and S. Arai

Table 1. Representative microprobe analyses of main minerals in xenoliths of the dunite–clinopyroxenite series from Takashima, the Southwest Japan arc

Ol, olivine; Cpx, clinopyroxene; Spl, chromian spinel. FeO is total iron as FeO for silicates, and FeO and Fe2O3 were recalculated for Spl, assuming spinel stoichiometry. Mg, Fe, and Ca; atomic fractions of (Mg + Fe + Ca) for . YCr,YAl, and YFe; atomic fractions of Cr, Al, and Fe3+, respectively, of (Cr + Al + Fe3+) for chromian spinel. See text for calculated (Calc.) Mg# at high temperature.

1.0 OSMA 5 (a) (b) (A (a)Yb (clinopyroxene) (b) ra i, 0.95 Dunite 1 9 Dunite 9 4 ) Wehrlite 4 Olivine clinopyroxenite Clinopyroxenite 0.90 0.85 3 0.90 Wehrlite & Olivine clinopyroxenite 0.5 2 Spl Cr/(Cr+Al) Cpx / C1 chondrite

0.85

Mg# (calculated) 0.90 0.85 1

Clinopyroxenite Olivine clinopyroxenite Wehrlite 0.0 0 Dunite Clinopyroxenite 90 85 0.9 0.85 0.80 Ol Fo (calculated) Cpx Mg# (calculated) 0 20406080100 0.90 0.85 Ol / (Ol+Cpx) Mg# (calculated) Figure 5. Relationship between the initial Mg# and other chemi- Figure 4. Rock types and initial Mg#s in the dunite–clinopyrox- cal parameters of the minerals. (a) Relationship between the enite series xenoiths from Takashima. (a) Relations between oli- Mg# of olivine and Cr# of chromian spinel. OSMA, olivine– vine/(olivine + clinopyroxene) and Mg#s. (b) Frequency distri- spinel mantle array (Arai, 1994). (b) Relationship between the bution of Mg#s. One box denotes one sample. The Mg# was Mg# and Yb content, normalized to the chondritic abundance reconstructed assuming a unity in the Mg–Fe2+ distribution (= (Sun and McDonough, 1989) of clinopyroxene. magmatic temperature; Fig. 2) and preservation of the modal proportion between olivine and clinopyroxene; this is the same between olivine, clinopyroxene, and the bulk rock. See text for from the order of formation for cumulates expected for details. olivine–oversaturated magma, i.e., dunite followed by ol- ivine clinopyoxenite on the system forsterite–diopside– silica (e.g., Kushiro, 1969). vine and clinopyroxene. The order of formation was de- Our data, combined with the field observation that termined to be dunite, clinopyroxenite, and wehrlite + dunites (including wehrlites) and clinopyroxenites are olivine clinopyroxenite (Figs. 4 and 5). This is different predominant (>80% in volume) in Group I ultramafic xen- Formation process of olivine–clinopyroxene cumulates 83

Di clinopyroxenites were possibly formed from a range of magma that was straddling the cotectic boundary in a zig- Diss Di zag way (Fig. 6). It is most likely that wehrlites were formed by the mingling of cumulus olivine with cumulus clinopyroxene by some way. Variations of the light REE content of Fo En Qz clinopyroxene (Fig. 3) were possibly reflected by the var- B ious degrees of involvement of the entrapped melt. These values exhibit no clear relationship with the olivine/clino- pyroxene ratio (Fig. 3), which possibly opposes the main role of intercumulus melt in the formation of wehrlites A from olivine cumulates. Unconsolidated cumulus rocks C (e.g., dunite + clinopyroxenite; Fig. 1c) were possibly mechanically mixed and homogenized to form a homo- Foss Enss geneous wehrlite. This mingling of cumulus phases may Fo En be consistent with the absence of clear layered structures in the Takashima dunite–clinopyroxenite series rocks.

Figure 6. Cartoon to show the zigzag liquid path of the magma CONCLUDING REMARKS involved in the formation of cumulates in the dunite–clinopyrox- enite series of Takashima. The forsterite–diopside–silica phase diagram at a specific condition (~ 1 GPa, anhydrous) is shown A series of cumulates of dunite, wehrlites, and clinopy- (Kushiro, 1969). A, a possible initial melt produced at some roxenites are common as deep–seated constituents of the high–pressure condition; B, the first intersection of liquid path arc–related lithosphere, which are possibly distributed – with the cotectic olivine clinopyroxene boundary; C, the invar- around Moho (Takahashi, 1978; Arai et al., 2000; Parlak iant point. et al., 2002). Alternated formation of olivine–oversaturat- ed and clinopyroxene–oversaturated magmas would be oliths from Takashima (Arai et al., 2001), strongly indi- required, and a kind of supersaturation of magma with cates that the melt involved in the formation of dunite– olivine or clinopyroxene around the olivine–clinopyrox- clinopyroxenite series rocks from Takashima was mostly ene cotectic boundary is potentially capable of accumu- either in the olivine liquidus field or in the clinopyroxene lating such series of rocks, although this model is very field. That is, it is not likely that the initial olivine–over- speculative. The concerned magma for the Takashima cu- saturated magma mainly took the olivine–clinopyroxene mulates is of arc affinity based on high Cr# and low TiO2 cotectic boundary after precipitating olivine (= formation content of the chromian spinel (Table 1) (Arai et al., of the Mg–rich dunites). To form clinopyroxene–oversa- 2011); these data are consistent with their mode of occur- turated magma after the dunite formation, a magma rich in rence as xenoliths on the Southwest Japan arc (cf., Arai et the clinopyroxene component was likely formed by con- al., 2000). A much more detailed examination is possible tinuous olivine crystallization across the cotectic bound- on the Moho transition zone of ophiolites of arc origin ary because of a kind of supersaturation (e.g., Donaldson, such as Turkish ones (e.g., Parlak et al., 2002), in which 1974; Toramaru, 2001) (Fig. 6). The resultant highly the spatial relation of individual rocks is much clearer on clinopyroxene–oversaturated magma precipitated only outcrops. clinopyroxene to form clinopyroxenites. This returning melt became oversaturated with olivine in the same man- ACKNOWLEDGMENTS ner after clinopyroxenite precipitation (Fig. 6). Addition- ally, this magma could have precipitated the less Mg–rich We are grateful to T. Mizukami and A. Tamura for the dunite that is lower in Mg# than the clinopyroxenites suggestions and La–ICP–MS analysis, respectively. N. (Figs. 4 and 5). The composite xenoliths, dunite–in–clino- Akizawa helped one of the authors (RM) with the mi- pyroxene, and clinopyroxene–in–dunite (Fig. 1c,d), sup- croprobe analysis. We thank S. Umino and T. Morishita port this interpretation; earlier–formed cumulates were en- for their comments to RM. S. Ishimaru, N. Akizawa, M. closed by later–formed ones as ‘autoliths’. Thus, the entire Miura, and C. Hoshikawa helped us collect the samples liquid path (Presnall, 1969) was possibly a zigzag shape in the field. Lastly, we appreciate M. Obata and A. Ishi- around the olivine–clinopyroxene cotectic boundary (Fig. watari for their critical and careful reviews, and J.–I. Ki- 6). Relatively low–Mg# (0.85 to 0.87) wehrlites to olivine mura for his comments and editorial handling. 84 R. Muroi and S. Arai

DEPOSITORY MATERIALS wehrlite in the Oman ophiolite. Geochemistry, Geophysics, Geosystems, 2, 2000GC000132. Krause, J., Brügmann, G.E. and Pushkarev, E.V. (2007) Accessory Table 2 and Appendix are available online from http:// and rock forming minerals monitoring the evolution of zoned japanlinkcenter.org/DN/JST.JSTAGE/jmps/131003. mafic–ultramafic complexes in the Central Ural Mountains. Lithos, 95, 19–42. Kushiro, I. (1969) The system forsterite–diopside–silica with and REFERENCES without water at high pressures. American Journal of Science, 267–A, 269–294. Arai, S. (1994) Characterization of spinel by olivine– Nakamura, E., McDougall, I. and Campbell, I.H. (1986) K–Ar ages spinel compositional relationships: Review and interpretation. of basalts from the Higashi–Matsuura district, northwestern Chemical Geology, 113, 191–204. Kyushu, Japan and regional geochronology of the Cenozoic Arai, S., Inoue, T. and Oyama, T. (1988) Igneous petrology of the alkaline volcanic rocks in eastern Asia. Geochemical Journal, Ochiai–Hokubo ultramafic complex, the Sangun zone, west- 20, 91–99. ern Japan: a preliminary report. Journal of Geological Society Negishi, H., Arai, S., Yurimoto, H., Ito, S., Ishimaru, S., Tamura, of Japan, 94, 91–102 (in Japanese with English abstract). A. and Akizawa, N. (2013) Sulfide–rich dunite within a thick Arai, S. and Abe, N. (1994) Podiform chromitite in the arc mantle: Moho transition zone of the northern Oman ophiolite: impli- chromitite xenoliths from the Takashima alkali basalt, south- cations for the origin of Cyprus–type sulfide deposits. Lithos, west Japan arc. Mineralium Deposita, 29, 434–438. 164–167, 22–35. Arai, S., Hirai, H. and Uto, K. (2000) Mantle peridotite xenoliths Neumann, E.–R., Andersen, T. and Mearns, E.W. (1988) Olivine from the Southwest Japan arc and a model for the sub–arc clinopyroxenite xenoliths in the Oslo Rift, SE Norway. Con- upper mantle structure and composition of the Western Pacific tributions to Mineralogy and Petrology, 98, 184–193. rim. Journal of Mineralogical and Petrological Sciences, 95, Obata, M., Banno, S. and Mori, T. (1974) The iron–magnesium 9–23. partitioning between naturally occurring coexisting olivine Arai, S., Abe, N., Hirai, H. and Shimizu, Y. (2001) Geological, and Ca–rich clinopyroxene: an application of the simple mix- petrographical and petrological characteristics of ultramafic– ture model to olivine solid solution. Bulletin de la Societé mafic xenoliths in Kurose and Takashima, northern Kyushu, française de Minéralogie et Cristallographie, 97, 101–107. southwestern Japan. Science Reports of Kanazawa University, Parlak, O., Höck, V. and Delaloya, M. (2002) The supra–subduc- 46, 9–38. tion zone Pozanti–Karsanti ophiolite, southern Turkey: evi- Arai, S., Shimizu, Y., Morishita, T. and Ishida, Y. (2006) A new dence for high–pressure crystal fractionation of ultramafic cu- type of orthopyroxenite from Takashima, the South- mulates. Lithos, 65, 205–224. west Japan arc: silica enrichment of the mantle by evolved Presnall, D.C. (1969) The geometrical analysis of partial fusion. alkali basalt. Contributions to Mineralogy and Petrology, American Journal of Science, 267, 1179–1194. 152, 387–398. Sen, G. and Presnall, D.C. (1986) Petrogenesis of dunite xenoliths Arai, S., Okamura, H., Kadoshima, K., Tanaka, C., Suzuki, K. and from Koolau Volcano, Hawaii: Implications for Hawaiian vol- Ishimaru, S. (2011) Chemical characteristics of chromian canism. Journal of Petrology, 27, 197–217. spinel in plutonic rocks: implications for deep magma proc- Sun, S–s. and McDonough, W.F. (1989) Chemical and isotopic esses and discrimination of tectonic setting. Island Arc, 20, systematics of oceanic basalts: Implications for mantle com- 125–137. position and processes. In Magmatism in the ocean basins Benn, K., Nicolas, A. and Reuber, I. (1988) Mantle–crust transition (Sanders, A.D. and Norry, M.J. Eds.). pp. 398, Geological zone and origin of wehrlite magmas: Evidence from the Oman Society Special Publications 42, 313–345. ophiolite. Tectonophysics, 151, 75–85. Takahashi, E. (1978) Petrological model of the crust and upper Donaldson, C.H. (1974) Olivine crystal types in harrisitic rocks of mantle of the Japanese island arcs. Bulletin Volcanologique, the Rhum pluton and in Archean spinifex rocks. Geological 41, 529–547. Society of American Bulletin, 85, 1721–1726. Toramaru, A. (2001) A numerical experiment of crystallization for Frey, F.A. and Prinz, M. (1978) Ultramafic inclusions from San a binary eutectic system with application to igneous textures. Carlos, Arizona: petrologic and geochemical data bearing Journal of Geophysical Research, 106, 4037–4060. on their petrogenesis. Earth and Planetary Science Letters, Wandji, P., Tsafack, P.J.F., Bardintzeff, J.M., Nkouathio, D.G., Ka- 38, 129–176. gou Dongmo, A., Bellon, H. and Guillou, H. (2009) Xenoliths Ishiwatari, A. (1985) Igneous petrogenesis of the Yakuno ophiolite of dunites, wehrlites and clinopyroxenites in the basanites (Japan) in the context of the diversity of ophiolites. Contribu- from Batoke volcanic cone (Mount Cameroon, Central Afri- tions to Mineralogy and Petrology, 89, 155–167. ca): petrogenetic implications. Mineralogy and Petrology, 96, Kaeser, B., Kalt, A. and Pettke, T. (2006) Evolution of the litho- 81–98. spheric mantle beneath the Marsabit Volcani Field (northern Kenya): Constraints from textural, P–T and geochemical stud- Manuscript received October 3, 2013 ies on xenoliths. Journal of Petrology, 47, 2149–2184. Manuscript accepted January 23, 2014 Koga, K.T., Kelemen, P.B. and Shimizu, N. (2001) Petrogenesis of Published online April 5, 2014 the crust–mantle transition zone and the origin of lower crustal Manuscript handled by Jun–ichi Kimura