一の目潟と黒瀬のマントルカンラン岩捕獲岩のＲｂ‐Ｓｒ鉱物アイソクロン年代

Abstract

Petrology, geochemistry and Rb-Sr chronology of seven spinel lherzolite-harzburgite xenolith samples from Ichinomegata, northeast Japan and Kurose, southwest Japan were studied. Whole rocks and the constituent minerals were analyzed for Rb and Sr contents and 87Sr/86Sr ratios. The pure minerals were obtained by hand-picking and repetitive washing with hot acids (2M HCl, 5% HF) and distilled water. The peridotite xenoliths from Ichinomegata show wide variations in mineral and chemical compositions; clinopyroxene contents range from 4 to 13 volume%, Cr/(Cr+Al) ratios of spinel range from 0.19 to 0.41 and MgO contents of whole rock range from 38 to 46 weight%. The chondrite-normalized patterns of whole rock vary from LREE-depleted to flat or U-shaped. In spite of above petrographical and geochemical variations, 87Sr/86Sr ratios of the clinopyroxenes are very uniform (0.7033∼0.7034). Rb-Sr mineral isochron ages of 208.9±80.1 Ma, 233.8±8.4 Ma, 252.9±19.8 Ma and 310.8±23.3 Ma are given by olivine, orthopyroxene, clinopyroxene and whole rock from each of four peridotite xenoliths. Since the peridotites have the relatively narrow range of ages and wide range of equilibrium temperatures, it is believed that a large-scale isotopic equilibration has occurred at various depths in the upper mantle beneath Ichinomegata. The peridotite xenoliths from Kurose show narrow ranges in mineral and chemical compositions; clinopyroxene contents range from 7 to 9 vol.%, Cr/(Cr+Al) ratios of spinel range from 0.28 to 0.31 and MgO contents of whole rock range from 41 to 43 wt.%. On the other hand, 87Sr/86Sr ratios of the clinopyroxenes are very diverse (0.7034, 0.7039 and 0.7043), compared with those in Ichinomegata peridotites. The mineral isochrons based on the data of pyroxenes and whole rock also show very variable ages and initial 87Sr/86Sr ratios. The Rb-Sr mineral isochron ages of 129.9±4.8 Ma, 312.8±16.7 Ma and 487.2±9.1 Ma indicate large intervals between the equilibration events.