Effects of water, depth and temperature on partial melting of mantle-wedge fluxed by hydrous sediment-melt in subduction zones. (15th December 2016)

• Record Type: Journal Article
• Authors: Mallik, Ananya; Dasgupta, Rajdeep; Tsuno, Kyusei; Nelson, Jared
• Is Part Of: Geochimica et cosmochimica acta. Volume 195(2016:Dec. 15)
• Page Start: 226
• Page End: 243
• Publication Date: 2016-12-15
• Subjects: Subducted sediments -- Mantle-wedge hybridization -- Arc magmatism -- Hydrous partial melting -- Phase equilibria; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2016.08.018
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: This study investigates the partial melting of variable bulk H2 O-bearing parcels of mantle-wedge hybridized by partial melt derived from subducted metapelites, at pressure–temperature ( P – T ) conditions applicable to the hotter core of the mantle beneath volcanic arcs. Experiments are performed on mixtures of 25% sediment-melt and 75% fertile peridotite, from 1200 to 1300 °C, at 2 and 3 GPa, with bulk H2 O concentrations of 4 and 6 wt.%. Combining the results from these experiments with previous experiments containing 2 wt.% bulk H2 O (Mallik et al., 2015 ), it is observed that all melt compositions, except those produced in the lowest bulk H2 O experiments at 3 GPa, are saturated with olivine and orthopyroxene. Also, higher bulk H2 O concentration increases melt fraction at the same P – T condition, and causes exhaustion of garnet, phlogopite and clinopyroxene at lower temperatures, for a given pressure. The activity coefficient of silica ( ϒ SiO 2 ) for olivine-orthopyroxene saturated melt compositions (where the activity of silica, a SiO 2, is buffered by the reaction olivine + SiO2 = orthopyroxene) from this study and from mantle melting studies in the literature are calculated. In melt compositions generated at 2 GPa or shallower, with increasing H2 O concentration, ϒ SiO 2 increases from <1 to ∼1, indicating a transition from non-ideal mixing as OH − in the melt ( ϒ SiO 2 <1) to ideal mixing as molecular H2 O ( ϒ SiO 2 ∼1). At pressures >2 GPa, ϒ SiO 2 >1 at higher H2 O concentrations in the melt, indicate requirement of excess energy to incorporate molecular H2 O in the silicate melt structure, along with a preference for bridging species and polyhedral edge decorations. With vapor saturation in the presence of melt, ϒ SiO 2 decreases indicating approach towards ideal mixing of H2 O in silicate melt. For similar H2 O concentrations in the melt, ϒ SiO 2 for olivine-orthopyroxene saturated melts at 3 GPa is higher than melts at 2 GPa or shallower. This results in melts generated at 3 GPa being more silica-poor than melts at 2 GPa. Thus, variable bulk H2 O and pressure of melt generation results in the partial melts from this study varying in composition from phonotephrite to basaltic andesite at 2 GPa and foidite/phonotephrite to basalt at 3 GPa, forming a spectrum of arc magmas. Modeling suggests that the trace element patterns of sediment-melt are unaffected by the process of hybridization within the hotter core of the mantle-wedge. K2 O/H2 O and H2 O/Ce ratios of the sediment-melts are unaffected, within error, by the process of hybridization of the mantle-wedge. This implies that thermometers based on K2 O/H2 O and H2 O/Ce ratios of arc lavas may be used to estimate slab-top temperatures when (a) sediment-melt from the slab reaches the hotter core of the mantle-wedge by focused flow (b) sediment-melt freezes in the overlying mantle at the slab-mantle interface and the hybridized package rises as a mélange diapir and partially melts at the hotter core of the mantle-wedge. Based on the results from this study and previous studies, both channelized and porous flow of sediment-melt/fluid through the sub-arc mantle can explain geochemical signatures of arc lavas under specific geodynamic scenarios of fluid/melt fluxing, hybridization, and subsequent mantle melting.