[S-CG53] Petrology, Mineralogy & Resource Geology

[SCG53-10] Geochemical characteristics of the Aitutaki peridotite xenoliths from the Cook Islands and Sr-Nd isotopic features of their constituent clinopyroxenes

*Masako Yoshikawa¹, Norikatsu Akizawa¹, Akihiro Tamura², Tomoaki Morishita² (1.Graduate School of Advanced Science and Engineering, Hiroshima University, 2.Institute of Science and Engineering Faculty of Natural System, Kanazawa University)

Keywords:Aitutaki peridotite xenolith, depleted MORB source mantle, small scale mantle convection, mantle metasomatism, Sr-Nd isotopic ratio, Geochemical characteristics

Aitutaki Island in the Cook Islands, Southwest Pacific, has a lagoon covering ∼85 % of its 80 km², with the remaining ∼15% covered by lava mainly composed of olivine nephelinite and basanite (Turner & Jarrad, 1982). Mantle peridotite xenoliths (<5 cm), mainly spinel-lherzolites, have been reported from the olivine nephelinites (e.g. Wood, 1978). Akizawa et al. (2024) observed lherzolite containing fine mineral aggregate (FMA) of decomposed garnet surrounding spinel, suggesting that FMA lherzolites experienced an increase in pressure and subsequent decrease in temperature from the spinel to the garnet stability fields and these phenomena were caused by small-scale mantle convection. In contrast, the zoning profile of pyroxene in spinel harzburgite exhibited a simple temperature decrease feature (Ohshima et al., 2020), suggesting that spinel harzburgite was derived from lithospheric mantle.
We report on the major element compositions of 3 FMA-bearing lherzolites, 2 spinel lherzolites, and a harzburgite along with trace element and Sr-Nd isotopic compositions of their clinopyroxenes. The lherzolites are distributed in the region of primitive mantle and MORB source mantle in the MgO-Al₂O₃ diagram. The harzburgite exhibits the characteristics of residual peridotite, that is higher MgO and lower Al₂O₃ contents than lherzolite. Sr-Nd isotopic ratios of clinopyroxenes of them are linearly distributed between the MORB of the East Pacific Ocean Rise and the Pacific-Antarctic Ridge and the host olivine nephelinite. Sr-Nd isotopic ratios of clinopyroxenes in harzburgite is similar to those of the host rocks. These results suggest that the lithospheric mantle beneath the Aitutaki Island is a residual peridotite, which were suffered metasomatism during/just before incorporated into the host rock, and that the athenospheric mantle, which caused small-scale convection beneath the ocean, exhibits the chemical and Sr-Nd isotopic composition of the MORB source mantle.