[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〜

[SCG41-09] Volcanism along the eastern margin of the Ontong Java Plateau: More reasons to test the Ontong Java Nui hypothesis by scientific ocean drilling

★Invited Papers

*Maria Luisa Tejada¹、Takashi Sano²、Takeshi Hanyu¹、Anthony A. P. Koppers³、Masao Nakanishi⁴、Akira Ishikawa⁵、Takashi Miyazaki¹、Kenichiro Tani²、Shoka Shimizu⁶、Qing Chang¹、Bogdan Stefanov Vaglarov¹ (1.Volcano and Earth Interior Research Center, Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka 237-0061, Japan、2.Department of Geology and Paleontology, National Museum of Nature and Science, Amakubo 1-1-1, Tsukuba, Ibaraki 305-0005, Japan、3.College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, U.S.A.、4.Graduate School of Science, Chiba University, Chiba 263-8522, Japan、5.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8550, Japan 、6.Graduate School of Science and Engineering, Chiba University, Chiba 263-8522, Japan)

キーワード：Ontong Java Nui, Ontong Java Plateau, Manihiki Plateau, Hikurangi Plateau

The Ontong Java Nui hypothesis, which states that three Pacific oceanic plateaus, Ontong Java, Manihiki and Hikurangi, once formed a single super plateau has been around for more than a decade now. However, there are critical pieces of the Ontong Java Nui puzzle that either do not fit or are sorely missing. Two of the outstanding inconsistencies are the contrasting crustal thickness between Manihiki Plateau (MP) and Ontong Java Plateau (OJP) and the rather distinct geochemical composition of the former which is not found on the latter so far. During the R/V Kairei KR16-04 cruise, we conducted geological surveys and dredging on the ridges protruding from the eastern margin of the OJP, as well as from the capping seamounts. We found that the dredged rocks have a variety of chemical composition ranging from tholeiitic to transitional to alkalic affinities. The tholeiites show similar middle to heavy rare earth abundances to OJP tholeiites but have elevated highly incompatible element (Rb, Th, Nb, Ta) contents, somewhat forming U-shaped trace element patterns that resemble those of Danger Island Trough basalts. Alkalic basalt composition mimics the trace element patterns of the 44 Ma, 65 Ma, and 90 Ma transitional to strongly alkalic basalts from the Solomon Islands and the Lyra Basin that form the southern and western margins of the OJP, respectively. Indicative older ~96-116 Ma ⁴⁰Ar-³⁹Ar ages of the ridge samples dredged near or below the base of the seamounts contrast with the 67-68 Ma plateau ages of samples from a seamount atop one of the ridges. These ages indicate later formation of the ridges compared to the main OJP (~122 Ma) but match with those of dated samples from Rapuhia Scarp on the northern margin of Hikurangi Plateau. Isotope and trace element data provide the missing compositional link while the new age data tie together the younger ages among Ontong Java, Manihiki and Hikurangi plateau volcanism. These findings support but add complexity to the Ontong Java Nui jigsaw fit. Thus, drilling the basement around the Eastern Salient of the OJP is required to further probe the extent or existence of the Ontong Java Nui super plateau.