[S-GC33] 固体地球化学・惑星化学

[SGC33-P07] Origin of Cretaceous and Archean komatiitic magma series: implications for source mantle heterogeneity
recorded by highly siderophile elements

井原 悠太¹、*石川 晃¹、清水 健二²、上野 雄一郎¹、横山 哲也¹ (1.東京工業大学　理学院　地球惑星科学系、2.海洋研究開発機構　高知コア研究所)

キーワード：マントル、コマチアイト、強親鉄性元素

The overabundances of highly siderophile elements (HSE) in the modern mantle are commonly attributed to a late veneer of chondritic primitive materials that were accreted after core-mantle differentiation. However, the timing of accretion and the mechanism of homogenization remain unclear. Komatiites, those are abundant in the Archean greenstone belt, provide a unique opportunity to trace the mantle evolution in early Earth. It has been suggested that the HSE content of komatiite sources have increased to PUM composition through the Neoarchean. In order to constrain a long-term chemical evolution of mantle, we analyzed the HSE compositions of mafic-ultramafic rocks (komatiite, picrite, dunite, wehrlite, and olivine gabbro) in Gorgona Island, Colombia, part of the Cretaceous Caribbean-Columbian Oceanic Plateau, which is known to host the youngest komatiites. For comparison, we also analyzed komatiites from the 3.2 Ga Ruth Well Formation in the Pilbara Craton, Australia.
The formation of the igneous rocks in Gorgona Island is related to two magma series with different degree of melt depletion. Both magma series show common initial ¹⁸⁷Os/¹⁸⁸Os within the range of model chondritic mantle composition. In addition, the parental magmas for the two series have similar Ru concentration as shown by that they follow a common spinel crystallization trend. This is consistent with the notion that the magma-mantle partition coefficient is close to unity at high degree of partial melting. The source mantle inferred from Gorgona and Ruth Well komatiites show similar HSE concentrations of about 70% of the PUM composition, as does the many of Archean komatiites, suggesting that the HSE depeleted mantle is still preserved in the Earth's deep interior.