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16. Evidence for suboceanic small-scale convection from a “garnet”-bearing lherzolite xenolith from Aitutaki Island, Cook Islands

Author

60359172, 10324609, 30816020, Akizawa, Norikatsu, Ozawa, Kazuhito, Kogiso, Tetsu, Ishikawa, Akira, Miyake, Akira, Igami, Yohei, Wallis, R. Simon, Nagaya, Takayoshi, Ohshima, Chihiro, Fujita, Ryo, Kawamoto, Tatsuhiko, Tamura, Akihiro, Morishita, Tomoaki, Arai, Shoji, Yasumoto, Atsushi, 60359172, 10324609, 30816020, Akizawa, Norikatsu, Ozawa, Kazuhito, Kogiso, Tetsu, Ishikawa, Akira, Miyake, Akira, Igami, Yohei, Wallis, R. Simon, Nagaya, Takayoshi, Ohshima, Chihiro, Fujita, Ryo, Kawamoto, Tatsuhiko, Tamura, Akihiro, Morishita, Tomoaki, Arai, Shoji, and Yasumoto, Atsushi

Abstract

Garnet peridotite xenoliths have been rarely reported from suboceanic mantle. Petrographic and geochemical characteristics of garnet-bearing oceanic peridotite xenoliths provide precious information on dynamics of the suboceanic lithosphere and asthenosphere interaction. We examined a lherzolite xenolith included in olivine nephelinite lava from Aitutaki Island, a member of the Cook-Austral volcanic chain. The lherzolite xenolith contains reddish fine-grained (< 5 µm in size) mineral aggregates (FMAs) with size range of 0.5–6 mm, consisting of olivine, calcic and sodic plagioclases, aluminous spinel, native iron, and nepheline. Microstructural observations and chemical data corroborate that the FMA is a decomposed pyrope-rich garnet including chromian spinel grains with an irregular highly indented morphology in the center. The FMA is surrounded by pyroxene-poor and olivine-rich aureole. The spatial and morphological relationships of FMA and chromian spinel with pyroxene-depleted margin suggest a reaction of aluminous spinel + pyroxenes → pyrope-rich garnet + olivine, which requires a compression before decomposition of the garnet to FMA. An orthopyroxene grain shows slight but clear chemical zoning characterized by increase in Al, Ca, and Cr from the grain center to the rim. The zoning patterns of Al and Ca in the orthopyroxene grain can be modeled by diffusion-controlled solid-state reactions induced by pressure and temperature changes, keeping surface concentrations in equilibrium with the other coexisting mineral phases. The results indicate that the mantle, from which the lherzolite xenolith was derived, underwent isothermal decompression followed by a weak heating on a time scale of a few tenths of million years before the xenolith extraction. From the deduced compression and decompression histories, we hypothesize that the mantle beneath Aitutaki Island was once dragged down to a garnet-stable deep mantle region and brought up later by small-scale sublithosph

Published
2024

17. AMPHIBOLE IN ULTRA-DEPLETED PERIDOTITES FROM THE HOROKANAI OPHIOLITE, HOKKAIDO, JAPAN:BONINITE MELT TRAPPED IN THE MANTLE WEDGE

Author

Tamura, Akihiro, Nishio, Ikuya, Iwata, Yuya, Arai, Shoji, Morishita, Tomoaki, Tamura, Akihiro, Nishio, Ikuya, Iwata, Yuya, Arai, Shoji, and Morishita, Tomoaki

Abstract

Ultra-depleted peridotites have been reported from the mantle sections of a number of ophiolitic complexes (e.g., Papua New Guinea and New Caledo-nia). Such rocks are residues derived from boninitic melts in supra-subduction zones, particularly in forearcs. In the mantle wedge, slab-derived fluid-melt containing H2O increases the degree of melting and metasomatizes the peridotites. However, the relationship between high-degree melting under hydrous conditions and metasomatism is not well understood. In this paper, we report petrological features of amphibole-bearing ultra-depleted peridotites from the Horokanai ophiolite, Hokkaido, Japan. The peridotite complex in the Horokanai ophiolite consists of clinopyroxene-free harzburgite, orthopyroxene-poor harzburgite, and dunite. The harzburgites contain high-Cr# spinel (0.70-0.85) and orthopyroxene with very low abundances of incompatible elements (e.g., Y < 0.01 µg/g; Ti ~ 2 µg/g). Pargasitic-edenitic amphibole occurs as spinel-hosted inclusions and as an interstitial phase to olivine and orthopyroxene. The harzburgite is a residue formed by the high-degree melting caused by fluid flux. The hydrous conditions cause incongruent melting of orthopyroxene, producing a depleted Si-rich melt (i.e., a boninitic melt). Because such Si-rich melt can coexist with Cr-rich spinel under these conditions, amphibole is ultimately crystallized in the harzburgite. Such amphibole formation is important in producing secondary sodic amphibole during alteration of ultra-depleted peridotites.

Published
2024

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