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

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.