Whole rock compositional variations in an upper mantle peridotite (Horoman, Hokkaido, Japan): Are they consistent with a partial melting process?

Whole rock major and trace element abundances of the Horoman peridotites we re used to understand processes forming lithological and compositional vari ations in the upper mantle. Similar to other orogenic peridotites, Horoman peridotites range from fertile lherzolites (3 to 4% Al2O3 and CaO) to deple ted harzburgites (similar to 0.5% Al2O3 and CaO). Abundances of major oxide s and compatible to moderately incompatible elements vary systematically wi th variations in MgO content. Such trends are commonly interpreted as indic ating that the peridotites formed as residues from varying degrees of parti al melting. The fertile end of these trends coincides with estimates of pri mitive mantle composition. Because of a mismatch between experimental melti ng trends for spinel peridotite, especially the Na2O-MgO trend, the composi tional variations 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 precl ude a two-component mixing model. Recent melting experiments on garnet peri dotite demonstrate that at 3 GPa the near-solidus peridotite has a large am ount of subcalcic clinopyroxene (ca. 27%) coexisting with small amount of g arnet (ca. 2%). Residues from polybaric melting of such garnet peridotite a re consistent with the abundance variations of major and moderately incompa tible elements, such as Na and heavy rare-earth elements, in the Horoman pe ridotites. A similar conclusion is applicable to other orogenic peridotites such as the Ronda peridotite because their major element compositional var iations are similar to the Horoman peridotite. Copyright (C) 2000 Elsevier Science Ltd.