[S-CG63] 沈み込み帯へのインプット：海洋プレートの進化と不均質

[SCG63-10] Origin of ultra-refractory mantle domain with ancient osmium isotope signature in the Pacific lithosphere constrained by mantle xenoliths from Tahiti and Moorea Islands, Society Islands

*秋澤 紀克^1,2、石川 晃³、小木曽 哲⁴、鈴木 勝彦⁵、三宅 亮⁶、三津川 到⁶ (1.東京大学 大気海洋研究所 海洋底科学部門、2.Macquarie University、3.東京工業大学 理学院 地球惑星科学系、4.京都大学 大学院人間・環境学研究科、5.国立研究開発法人海洋研究開発機構・海底資源センター 、6.京都大学 大学院理学研究科 地質学鉱物学教室 )

キーワード：マントルプリューム、マントル捕獲岩、リソスフェア-アセノスフェア境界、Os同位体、古溶融イベント

The residual harzburgite layer is expected to be perched atop the fertile lherzolite layer in a hypothetical melting column beneath the mid-ocean ridge. In contrast, it is known that world-wide oceanic peridotites record both recent melt extraction beneath the mid-ocean ridge, and ancient melt exhaustion. Thus, melt depletion histories recorded in the oceanic mantle are thus complex. To better constrain the evolutional process of the oceanic mantle, functional mechanism inducing the mantle heterogeneity in terms of the timing of melt depletion is desired to be deciphered. In the oceanic region, numerous mantle plumes thermally and chemically modify the above oceanic lithosphere and cause compositional heterogeneity due to the thermal erosion and successive “heterogeneous” plume impingement on its base. Here, we aim at evaluating the presence of ancient refractory mantle domain derived from such heterogeneous mantle plume using mineral and bulk chemistry with osmium isotopes.
The rock samples used herein are mantle xenoliths from Tahiti and Moorea Islands, members of Society Islands. These oceanic islands are emplaced on ca. 70 Myr-old oceanic lithosphere, where seismically determined lithosphere thickness decreases from 90 km to 70 km due to thermal erosion by the Society mantle plume. The alkali basalts from Tahiti and Moorea Islands are dated at ca. 1.1–0.5 Ma and 1.7–1.5 Ma, respectively. A total of 38 mantle xenoliths were used: 3 lherzolites, 10 harzburgites, 20 dunites, 2 wehrlites, 1 olivine clinopyroxenite, 1 olivine websterite, and 1 orthopyroxenite. The harzburgites and lherzolites show unradiogenic ¹⁸⁷Os/¹⁸⁸Os compositions, whereas the other lithologies show radiogenic ¹⁸⁷Os/¹⁸⁸Os compositions. This contrast results in a bimodal distribution of ¹⁸⁷Os/¹⁸⁸Os compositions throughout all the mantle xenoliths. Because the dunite and wehrlite are similar in compositions of clinopyroxene trace elements and bulk Nd and Sr isotopes (literature data) with the basalts from the Society Islands, we suggest that they are the products after lithospheric mantle and plume-derived melt reactions. Although some of the harzburgites and lherzolites with unradiogenic ¹⁸⁷Os/¹⁸⁸Os are similarly metasomatized by such plume-derived melt infiltrations, “original” ultra-refractory harzburgites with unradiogenic ¹⁸⁷Os/¹⁸⁸Os were recognized from our sample collection. We will present additional investigations targeted at these original ultra-refractory harzburgites, and discuss their origin considering detailed petrography, pressure-temperature histories, and chemical compositions.