Metasomatism of the upper mantle beneath the Hyblean Plateau (Sicily): Evidence from pyroxenes and glass in peridotite xenoliths
C. Perinelli, G. T. Sapienza, P. Armienti, L. Morten, 2008. "Metasomatism of the upper mantle beneath the Hyblean Plateau (Sicily): Evidence from pyroxenes and glass in peridotite xenoliths", Metasomatism in Oceanic and Continental Lithospheric Mantle, M. Coltorti, M. Grégoire
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Coarse-grained, protogranular spinel peridotite xenoliths in the Upper Miocene Valle Guffari diatreme display a complex history for the shallow mantle underneath the Hyblean Plateau (SE Sicily). The mineral assemblage and composition (olivine Fo89–91, orthopyroxene En88–91, Cr-diopside En48–49Fs4–6Wo45–48, Cr-rich spinel with cr-number=25–39) record at least one depletion event caused by melt extraction, followed by metasomatic enrichment. One of these samples (HYB40) hosts a fresh glass vein. Rare earth elements (REE) in clinopyroxenes from these peridotites show three patterns: (1) light REE-enriched (Lan/Ybn=7–17); (2) spoon-shaped (Lan/Ybn=18–20; Lan/Smn=21–34; Smn/Ybn<1); (3) nearly flat (Lan/Ybn ∼3). Whole-rock and clinopyroxene trace elements indicate that these patterns are associated with more or less complete equilibration with at least two distinct metasomatic melts: an alkaline silicate melt resembling the host basalt and a hawaiitic melt (for peridotite HYB40). P–T estimates yield 0.9–1.2 GPa and 870–1050 °C, suggesting that refertilization by metasomatizing melts occurred at the Crust–Mantle boundary or just below. In addition, the P–T data coincide with the palaeogeotherm reported by an earlier worker that is consistent with a high geothermal gradient. However, this thermal regime does not fit with the occurrence of an active mantle plume beneath the Hyblean area because of the deduced mantle potential temperatures, which are almost 200°C lower than those typical for a mantle plume. fO2 calculation gives a redox state above the fayalite–magnetite–quartz buffer FMQ (up to +1.7 Δlog units) related to melt-driven metasomatism.
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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.