Igneous garnet and amphibole fractionation in the roots of island arcs: experimental constraints on andesitic liquids

• Published in: Contributions to mineralogy and petrology Vol. 157; no. 4; pp. 541 - 558
• Main Authors: Alonso-Perez, Raquel, Müntener, Othmar, Ulmer, Peter
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01.04.2009; Springer Nature B.V
• Subjects: Crystallization; Dehydration; Earth and Environmental Science; Earth Sciences; Fractionation; Geology; High temperature; Island arcs; Low temperature; Melting; Mineral Resources; Mineralogy; Original Paper; Petrology; Roots; Silica; Trace elements; Upper mantle; Volcanic rocks; High-pressure crystallization; Garnet fractionation; Experimental petrology; Amphibole; Hydrous andesite liquids
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-008-0351-8

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 H 2 O contents. At water undersaturated conditions and f O 2 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 H 2 O-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 H 2 O contents exceeding 8 wt% and is replaced by spinel at decreasing dissolved H 2 O. 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 H 2 O in silicate liquids and is thus likely to affect trace element compositions of H 2 O-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.