Igneous garnet and amphibole fractionation in the roots of island arcs: experimental constraints on andesitic liquids |
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| Authors: | Raquel Alonso-Perez Othmar Müntener Peter Ulmer |
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| Institution: | (1) Institute of Mineralogy and Petrology, ETH, Zurich, Switzerland;(2) Institute of Mineralogy and Geochemistry, University of Lausanne, Lausanne, Switzerland |
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| Abstract: | To evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the
roots of island arcs, a series of experiments were performed on a synthetic andesite at conditions ranging from 0.8 to 1.2 GPa,
800–1,000°C and variable H2O contents. At water undersaturated conditions and fO2 established around QFM, garnet has a wide stability field. At 1.2 GPa garnet + amphibole are the high-temperature liquidus
phases followed by plagioclase at lower temperature. Clinopyroxene reaches its maximal stability at H2O-contents ≤9 wt% at 950°C and is replaced by amphibole at lower temperature. The slopes of the plagioclase-in boundaries
are moderately negative in  space. At 0.8 GPa, garnet is stable at magmatic H2O contents exceeding 8 wt% and is replaced by spinel at decreasing dissolved H2O. The liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for
the 1.2 GPa series, but show substantial enrichment in FeO/MgO for the 0.8 GPa series related to the contrasting roles of
garnet and amphibole in fractionating Fe–Mg in derivative liquids. Our experiments indicate that the stability of igneous
garnet increases with increasing dissolved H2O in silicate liquids and is thus likely to affect trace element compositions of H2O-rich derivative arc volcanic rocks by fractionation. Garnet-controlled trace element ratios cannot be used as a proxy for
‘slab melting’, or dehydration melting in the deep arc. Garnet fractionation, either in the deep crust via formation of garnet
gabbros, or in the upper mantle via formation of garnet pyroxenites remains an important alternative, despite the rare occurrence
of magmatic garnet in volcanic rocks. |
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| Keywords: | Experimental petrology Hydrous andesite liquids High-pressure crystallization Amphibole Garnet fractionation |
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