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

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 H 2 O contents. At water undersaturated conditions and fO 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 Ocontents B9 wt% at 950°C and is replaced by amphibole at lower temperature. The slopes of the plagioclase-in boundaries are moderately negative in T-X H 2 O 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. Garnetcontrolled 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.

Magmatic garnet in the Cordilleran-type Galiléia granitoids of the Araçuaí belt (Brazil): Evidence for crystallization in the lower crust

“…Mg# in the cores varies between 0.50 and 0.39, while Mg# from the rims have a lower and more restricted range of values (0.36-0.41). As described by Alonso-Perez et al (2009), this decrease in Mg# is related to decreasing temperature during magma crystallization. In contrast, Al 2 O 3 is depleted in the cores (7.68-12.58 wt.%) compared with the respective rims (11.41-14.02 wt.%).…”

“…Schmidt and Thompson (1996), this textural information is consistent with pressure conditions in the range of 0.8-1.0 GPa. Thus the experiments of Alonso-Perez et al (2009), Green (1992, and Schmidt and Thompson (1996), constrain the stability of garnet + epidote within the Galiléia granitoids to pressures between ca. 0.8 and 1.0 GPa, corresponding to depths of approximately 25-30 km in the lower crust.…”

“…Crystallization experiments and studies on natural metaluminous igneous rocks have shown that garnet with CaO contents above 4 wt.% (and in general with Grs N 10 mol%) records crystallization at pressures above approximately N 0.8 GPa (Alonso-Perez et al, 2009;Bach et al, 2012;Barnes and Allen, 2006;Dawes and Evans, 1991;Day et al, 1992;Green, 1972Green, , 1977Green, , 1992Ringwood, 1968, Green, 1972;Harangi et al, 2001;Samadi et al, 2014 and others). Schmidt and Thompson (1996) investigated garnet stability within a granodiorite and established that the garnet-in reaction is located at a pressure as high as 1.3 GPa and around 800°C.…”

“…Crystallization experiments (e.g. Alonso-Perez et al, 2009;Green, 1972Green, , 1977Green, , 1992Green and Ringwood, 1968;Schmidt, 1993;Schmidt and Thompson, 1996) on metaluminous andesitic and tonalitic to granodioritic compositions show that garnet is stabilized at high pressures and at low temperatures, in magmas that are water-rich (typically in the range of 5-10 wt.% H 2 O). The rare occurrence of garnet in such compositions in the plutonic record suggests that these conditions are rarely achieved (or preserved) in nature.…”

Origin of mafic and ultramafic cumulates from the Ditrău Alkaline Massif, Romania

“…Empirical and experimental studies indicate that the composition of amphiboles can record the crystallization conditions (P, T, fO 2 , fH 2 O) of magmas (e.g., Johnson and Rutherford, 1989;Ernst and Liu, 1998;Alonso-Perez et al, 2009;Ridolfi et al, 2010;Krawczynski et al, 2012). The studied cumulates contain abundant amphibole crystals suggesting that amphiboles most likely records the temperature and pressure (depth) of the cumulate formation and perhaps one of the main differentiation level in the Ditrău Alkaline Massif.…”