[S-CG41] ハードロック掘削科学〜陸上掘削から深海底掘削そしてオマーン〜

[SCG41-P04] Sr-Nd-Pb isotope systematics of proto-arc volcanic rocks in the Oman ophiolite

*石川 剛志¹、永石 一弥²、金山 恭子³、北村 啓太朗⁴、若木 重行¹、草野 有紀⁵、海野 進⁶ (1.海洋研究開発機構 高知コア研究所、2.マリンワーク・ジャパン、3.山陰海岸ジオパーク海と大地の自然館、4.アジア航測、5.産業技術総合研究所 地質調査総合センター、6.金沢大学 理工研究域地球社会基盤学系)

In the Oman ophiolite, spreading-ridge-derived lavas are covered by arc-like lavas after a short interval, possibly related to subduction initiation. The lavas with arc affinity include boninite, which is generally known to represent one of the most important products of proto-arc magmatism. Thus, the occurrence of boninites and related volcanic rocks in the Oman ophiolite has a key to understanding subduction initiation and concomitant proto-arc magmatism. However, currently only limited geochemical data are available for these volcanic rocks. In this study, we present Sr, Nd, and Pb isotope compositions of boninites in the Oman ophiolite, together with those of proto-arc tholeiites extruded prior to the boninites. The proto-arc tholeiites show slightly to moderately enriched Sr and Pb isotope compositions but almost identical Nd isotope compositions compared to axial lavas, consistent with melting of residual depleted mantle metasomatized by amphibolite-facies slab fluids. On the other hand, boninites show distinctly more radiogenic Sr, Nd, and Pb isotope compositions than axial lavas, which requires addition of slab fluids formed at higher temperatures as well as small amounts of mafic-sedimentary hybrid slab melt to the residual highly depleted mantle. The observed isotope variations suggest involvement of hotter slab in boninite magma genesis than in the case of proto-arc tholeiites, probably due to successive subduction of the slab after subduction initiation.