PII S0016-7037(99)00346-4 Whole rock compositional variations in an upper mantle peridotite (Horoman, Hokkaido, Japan): Are they consistent with a partial melting process? E. TAKAZAWA, 1, * ,† F. A. FREY, 1 N. SHIMIZU, 2 and M. Obata 3 1 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 USA 2 Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA 3 Department of Geology and Mineralogy, Division of Earth and Planetary Sciences, Kyoto University, Kyoto, 606-8502, Japan (Received October 12, 1998; accepted in revised form September 8, 1999) Abstract—Whole rock major and trace element abundances of the Horoman peridotites were used to understand processes forming lithological and compositional variations in the upper mantle. Similar to other orogenic peridotites, Horoman peridotites range from fertile lherzolites (3 to 4% Al 2 O 3 and CaO) to depleted harzburgites (0.5% Al 2 O 3 and CaO). Abundances of major oxides and compatible to moderately incom- patible elements vary systematically with variations in MgO content. Such trends are commonly interpreted as indicating that the peridotites formed as residues from varying degrees of partial melting. The fertile end of these trends coincides with estimates of primitive mantle composition. Because of a mismatch between experimental melting trends for spinel peridotite, especially the Na 2 O-MgO trend, the compositional varia- tions of Horoman peridotites are not consistent with formation as residues from partial melting of spinel peridotite. Non-linear trends in minor and trace element versus major element abundance diagrams also preclude a two-component mixing model. Recent melting experiments on garnet peridotite demonstrate that at 3 GPa the near-solidus peridotite has a large amount of subcalcic clinopyroxene (ca. 27%) coexisting with small amount of garnet (ca. 2%). Residues from polybaric melting of such garnet peridotite are consistent with the abundance variations of major and moderately incompatible elements, such as Na and heavy rare-earth elements, in the Horoman peridotites. A similar conclusion is applicable to other orogenic peridotites such as the Ronda peridotite because their major element compositional variations are similar to the Horoman peridotite. Copyright © 2000 Elsevier Science Ltd 1. INTRODUCTION Geochemical and petrologic studies of upper mantle perido- tites exposed in the earth’s crust provide information about igneous and metamorphic processes occurring in the upper mantle. In contrast to the small, 1 m, mantle xenoliths en- trained in magmas, the much larger, tens of km 2 , peridotite massifs incorporated into the crust via tectonic processes show geochemical variations on scales of meters to kilometers. In addition, the field relationships between different rock types within a peridotite massif provide important constraints for hypotheses based on geochemical and petrologic data. The mineral compositions of upper mantle rocks typically reflect sub-solidus conditions. Re-equilibration to a changing pressure-temperature environment is commonly incomplete; consequently, the resulting compositional zoning of minerals can be used to understand relatively recent processes that affected the peridotite (e.g., Smith and Boyd, 1989; Shimizu and Sovolev, 1995; Takazawa et al., 1996). However, geo- chemical variations within peridotite massifs also occur on scales much larger than mineral grains; therefore, whole-rock compositions are also useful in understanding upper mantle processes, particularly the igneous processes that preceded the metamorphic events (e.g., Frey et al., 1985; McDonough and Frey, 1989). We are using both mineral and whole-rock com- positions to understand the sequence of upper mantle processes recorded in the Horoman Peridotite which is exposed in Hok- kaido, Japan. This peridotite is relatively large, 8  10  3 km, and consists of several layered lithological sequences of plagioclase lherzolite-lherzolite-harzburgite-dunite-harzbur- gite-lherzolite-plagioclase lherzolite (Komatsu and Nochi, 1966; Niida, 1974; Obata and Nagahara, 1987; Takahashi, 1991). Minor amounts of pyroxenite and mafic layers are concordantly interlayered in the peridotites parallel to their compositional layering and foliation. In the stratigraphically Lower Zone, 2 km thick, the layers define lithological se- quences of 100 to 500 m, whereas in the Upper Zone the layers are thinner and mafic layers are more abundant (Kom- atsu and Nochi, 1966; Niida, 1974; Niida, 1984; Takazawa et al., 1999). Takazawa et al. (1996) used the compositional zoning of clinopyroxenes in the Lower Zone to show that: 1. within the garnet stability field, the peridotite reacted with an exotic melt that had relatively high abundances of in- compatible elements; and 2. subsequently during ascent into the crust the peridotite was a closed system whose mineral compositions record a tran- sition from garnet peridotite at 2.0 –2.4 GPa and 1040 – 1160°C to plagioclase peridotite at 0.7 GPa and 850 – 950°C. Takazawa et al. (1992) and Takazawa et al. (in preparation) use the spatial variations in mineral composi- tion to evaluate the migration pathways of the incompatible element-rich melt and the melt-wallrock reactions that led to the incompatible element enrichment recorded in clinopy- roxene. *Author to whom correspondence should be addressed (takazawa@ sc.niigata-u.ac.jp). † Present address: Department of Geology, Faculty of Science, Niigata University, Niigata, 950-2181 Japan. Pergamon Geochimica et Cosmochimica Acta, Vol. 64, No. 4, pp. 695–716, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 + .00 695