Natsue Abe

GEOCHEMICAL CHARACTERISTICS OF THE UPPERMOST MANTLE BENEATH THE JAPAN ISLAND ARCS : IMPLICATIONS FOR UPPER MANTLE EVOLUTION

Abstract Geochemical characteristics of clinopyroxene in peridotite xenoliths from three volcanos, Megata, On-yama and Kurose, in the Japan arcs are important for understanding igneous and metasomatic processes within the mantle wedge. The clinopyroxenes in peridotite xenoliths from the Japan arcs are clearly different from those in abyssal peridotites and the peridotite xenoliths from other tectonic settings, such as continental rifts and oceanic hotspots. Geochemical characteristics of the sub-arc clinopyroxenes are not apparently related either to degree of hydration or to degree of refractoriness, but are consistent from one sample to another. The REE patterns vary from LREE-depleted pattern to flat or slightly LREE-enriched patterns. Then their (Ce/Yb) N (subscript N=chondrite-normalized) vary widely from 0.04 to 4.0. On the other hand the Ti/Zr ratio is rather constant in each sample, around 100. Clinopyroxenes in the Japan arcs peridotite xenoliths are intermediate for Ce and Sr contents, and (Ce/Yb) N and Ti/Zr ratios. Furthermore, the most fertile peridotites from Japan arcs are similar for the clinopyroxene chemistry to the most fertile abyssal peridotites. The peridotite xenoliths from Japan arcs had possibly evolved through different process from common source peridotite to abyssal peridotite. The Japan arc mantle peridotites had been polluted by the metasomatic agent with consistent chemical characteristics due to regional mantle wedge metasomatism.

Reaction of orthopyroxene in peridotite xenoliths with alkali-basalt melt and its implication for genesis of alpine-type chromitite

Abstract The Kawashimo alkali basalt of the southwest Japan arc has peridotite xenoliths with a wide lithological range, from lherzolite {Fo of olivine, 89; Cr [=Cr/(Cr + Al) atomic ratio] of spinel O.IO}to harzburgite (Fo of olivine, 91; Cr of spinel, 0.54). Reaction zones between orthopyroxene and alkali-basalt melt are low-pressure analogues of mantle-melt interaction products and consist of two subzones: a fine-grained inner subzone (adjacent to orthopyroxene) and a relatively coarse-grained outer subzone. In both xenolith types the reaction products are olivine + diopsidic clinopyroxene ± spinel ± glass, but the spinel concentrations are remarkably different around lherzolite from those around harzburgite. Cr-bearing spinel is concentrated only in the outer subzone on harzburgite orthopyroxene (Cr > 0.14); the inner subzone on harzburgite orthopyroxene and both the inner and outer subzones on lherzolite orthopyroxene (Cr = 0.05) are almost free of spinel. The remarkable enrichment of spinel in the outer subzone on the harzburgite orthopyroxene suggests a mechanism of spinel concentration, Le., the origin of podiform chromitite is related to interaction between Cr-rich orthopyroxene and basaltic melt. This observation for the Kawashimo xenoliths is concordant with the near absence of podiform chromitite in Iherzolitic mantle. Chromian spinel could be concentrated if a relatively silica-rich secondary melt, produced by interaction between pyroxene-undersaturated magma and harzburgite orthopyroxene, is mixed with a primitive magma in the upper mantle.

Podiform chromitite in the arc mantle: chromitite xenoliths from the Takashima alkali basalt, Southwest Japan arc

Chromitite xenoliths from the Takashima alkali basalt in the Southwest Japan arc are classified into two types: Type 1 chromitite in thin layers in dunite or wehrlite xenoliths; and Type 2 chromitite in discrete xenoliths which has an orbicular texture, previously documented only from podiform chromitites in ophiolites. Type 1 may be equivalent to layered chromitites in ophiolitic cumulates and Type 2 to podiform chromitites in the transition zone of ophiolites. This example of podiform chromitite from the Southwest Japan arc suggest that these podiform chromitites may exist in the upper mantle beneath an arc, where their formation is favored.