2:45 PM - 3:00 PM
[SCG60-11] Exploring more asthenospheric melt: the preliminary report of KS-18-9 cruise, R/V Shinseimaru
Keywords:petit-spot, asthenosphere, Pacific plate, alkali-basalt, outer rise
We newly discovered the directly ascending petit-spot melt from asthenosphere (Sato et al., 2018, Int’l. Geol. Rev. 60, 1231). The lavas, silica-poor and strongly alkaline with phenocrystic olivines, are completely discriminated from the majority of previously reported petit-spot. They selectively erupt atop the outer-rise wherein the lithosphere behaves entirely in an extensional regime during the ascent of melt from asthenosphere. We conducted several dredge samplings in order to access the asthenosphere more directly, by the research cruise KS-18-9, R/V Shinseimaru, in August 2018, supported by AORI, Tokyo University, and the JSPS Japanese-German Graduate Externship. Special thanks to shipboard scientific party (S. Machida, N. Akizawa, T. Ishii, A. Matsumoto, M. Kaneko, Y. Sato, Y. Nakano, T. Yutani, K. Azami, S. Sakai, Y. Katsuragi), captain A. Tsuji, and crews on KS18-09, R/V Shinsei.
The petit-spot rocks are dredged at a lava field displayed by multibeam survey without any topographic highs, which are small blocks surrounding very fresh glass with less palagonitic alteration and manganese coating, implying the lavas are quite young. The quenched glass rinds were analysed for major and trace elements by an electoron microprobe at University of Bayreuth, Germany (co-work with T. Yutani), and an inductively coupled plasma–mass spectrometer (LA-ICP-MS) at Nanyang Technological University, Singapore (co-work with F. Rodriguez, J. Oalmann, Y. Katsuragi), respectively. As we gained “directly ascending petit-spots” from several volcanoes on this cruise, it is possible to recognize their source and melting process from their geochemical variations. The strongly alkaline lavas, directly ascending petit-spot, show clear trend in Zr/Hf, Sm/Hf ratios and HREEs, implying the carbonatite addition with the depth in source mantle. We present here the several processes of lithospheric flexure, metasomatized lithosphere, magma-fractionation, and finally, upwelling directly from asthenosphere occur prior to trench-oceanward slope, which would be significant components into the incoming oceanic plate.