Calcic amphiboles in peridotite xenoliths from Avacha volcano, Kamchatka, and their implications for metasomatic conditions in the mantle wedge
S. Ishimaru, S. Arai, 2008. "Calcic amphiboles in peridotite xenoliths from Avacha volcano, Kamchatka, and their implications for metasomatic conditions in the mantle wedge", Metasomatism in Oceanic and Continental Lithospheric Mantle, M. Coltorti, M. Grégoire
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Highly metasomatized parts of peridotite xenoliths from Avacha volcano, Kamchatka, Russia, characteristically contain calcic amphiboles, especially tremolites. They are rich in metasomatic pyroxenes with high Mg-number (= Mg/(Mg+Fe) atomic ratio), up to 0.94–0.98, and contain Cr-poor aluminous spinels. They have the spinel lherzolite mineral assemblage and equilibrium temperatures of 900–1000 °C or higher, beyond the stability field of tremolite. The tremolite was therefore retrogressively formed after the peak of high-temperature metasomatism. The high-Mg-number, low-alkali environment facilitates the formation of tremolite instead of Al-rich calcic amphiboles. A sulphur-bearing silicic melt derived from a slab is a probable agent involved in the metasomatism. High fO2 recorded in the highly metasomatized peridotites is consistent with this process. This type of metasomatism can produce high-Mg-number peridotites and pyroxenites with low-Cr-number spinel within the mantle wedge where the Mg-number of silicates is positively correlated with the Cr-number of spinel in ambient peridotites.
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Twenty years have passed since Menzies & Hawkesworth extended the concept of metasomatism to mantle processes. The aim of this book is to gather together progress made on this topic since then. Most of the 14 papers reported in the volume rely on in situ major and trace element analyses of minerals and glasses in mantle xenoliths, and deal with different kinds of metasomatic agents at variable fluid/rock ratios in tectonic settings as different as intra-plate, mid-ocean ridge (ophiolites) and supra-subduction. The book contributes to the wide debate on the nature of the fluids migrating into the mantle wedge, as well as on the different residential times of the subduction signature. In addition papers on intra-plate settings deal with the problem of relating various metasomatic signatures to one single metasomatic event through an infiltration-reaction process.