Geochemical evolution of the Horoman peridotite complex: Implications for melt extraction, metasomatism, and compositional layering in the mantle

Major and trace element compositions and isotopic ratios of Sr and Nd were determined for bulk rocks and their constituent clinopyroxenes from the Horoman peridotite complex, Japan. Al₂O₃, CaO, and heavy rare earth elements (HREE) contents of peridotites generally decrease from plagioclase lherzolite through spinel lherzolite to spinel harzburgite, indicating simple melt extraction from a single source. However, the extremely large variations in isotopic (⁸⁷Sr/⁸⁶Sr = 0.7019 to 0.7066, ε_Nd = + 110 to -10) and trace element compositions ([Ce/Yb]_N = 0.006 to 4.0) cannot be explained by a simple melt extraction mechanism. The samples can be divided into two groups: one suite has depleted isotopic and light REE (LREE) characteristics (DP), while the other suite shows enriched isotopic and LREE signatures (EP). Sm-Nd isotope systematics of whole-rock DP samples yield an isochron age of 833 ± 78 Ma with an initial ¹⁴³Nd/¹⁴⁴Nd ratio of 0.5119 ± 2, which is identical to the isotopic composition of mid-ocean ridge basalt (MORB) source mantle at that time. The relationship between MgO and Yb abundances of whole rocks shows that melt extraction was initiated at pressures near the garnet and spinel lherzolite transition. Peridotites that formed at different depths presently occur in close proximity to each other, sometimes within tens of meters. The chemical and isotopic signatures of the EP samples can be explained by mixing between mantle residue and an isotopically and more incompatible element enriched fluid derived from a subducted slab. These observations suggest that the small-scale compositional layering observed in the complex may have formed in a wedge mantle by water-enhanced thinning and folding of metasomatized peridotites which had previously developed large-scale simple stratification as a result of melt extraction beneath a mid-ocean ridge.