[S-CG41] Hard-Rock Drilling Science: From Continental to Deep Sea Drilling, and Oman Project

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

*Tsuyoshi Ishikawa¹, Kazuya Nagaishi², Kyoko Kanayama³, Keitaro Kitamura⁴, Shigeyuki Wakaki¹, Yuki Kusano⁵, Susumu Umino⁶ (1.Kochi Institute for Core Sample Research, JAMSTEC, 2.Marine Works Japan Ltd., 3.San'in Kaigan Geopark Museum of the Earth and Sea, 4.Asia Air Survey Co., Ltd., 5.Geological Survey of Japan, AIST, 6.College of Science and Engineering, Kanazawa University)

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.