Effect of Bulk Water Concentration on Mantle Wedge Hybridization by Rhyolitic Sediment Melt – Implications for Generation of K-rich Basalts to Andesites in Subduction Zones

Ananya Mallik, University of Arizona, Geosciences, Tucson, AZ, United States, Rajdeep Dasgupta, Rice University, Department of Earth, Environmental and Planetary Sciences, Houston, United States, Jared Michael Nelson, Rice University, Houston, TX, United States; Texas A & M University, Physics and Astronomy, College Station, TX, United States and Kyusei Tsuno, Arizona State University, Eyring Materials Center, Tempe, AZ, United States
Abstract:
Similarities in trace element geochemistry between ocean-floor sediments and arc lavas suggest the involvement of subducted sediments in the mantle source of arc volcanoes. Siliciclastic sediments produce rhyo-dacitic, hydrous partial melts at sub-arc depths, which must react with wedge peridotite during their ascent. In addition to fluids, these sediment melts can be a major carrier of water to the arc source. Here we investigate the effects of bulk water concentration on the phase equilibria of reaction between sediment partial melt and peridotite.

Piston-cylinder experiments were performed using Au-Pd capsules, at 2 and 3 GPa, 1050 – 1350 °C with mixtures of 25% rhyolite + 75% lherzolite, bearing bulk water content of 2 (low-water) and 4 wt.% (high-water). Melting degree is higher in high-water experiments at both 2 and 3 GPa with a sharp increase in melt mode from 31 to 53 wt.% at 1250-1300 °C, 2 GPa and 21 to 49 wt.% at 1225-1250 °C, 3 GPa. This sharp increase in melt mode is accompanied by a corresponding abrupt increase in residual olivine to opx ratio at both pressures (0.11 to 0.53 at 1250-1300 °C, 2 GPa and 0 to 0.71 at 1225-1250 °C, 3 GPa). The stability field of phlogopite, clinopyroxene, and garnet are reduced in high-water experiments due to higher degrees of partial melting. Low-water experiments produce basalts with SiO2, on a volatile-free basis, increasing from 49 to 51 wt.% at 2 GPa and 46 to 48 wt.% at 3 GPa. For high-water experiments, melt SiO2 contents at 2 GPa are slightly higher than those in low-water experiments for a given temperature, varying from 51 to 52 wt.%, and, at 3 GPa, the melts trend towards andesitic compositions with SiO2 ~54 wt.%. These compositional characteristics of the melts can be attributed to the effect of increased olivine to opx ratios in the residue as a function of increasing bulk water concentration.

Our study shows that a spectrum of ultra-potassic, high-Mg arc lavas (MgO varying from 10-16 wt.%) from basaltic to andesitic compositions can be produced by partial melting of sediment melt-fluxed wedge peridotite hybrids with variable water contents.

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