Interactions among magmas and rocks in subduction zone regions: experimental studies from slab to mantle to crust

Abstract

Experiments have been conducted to study contamination of magmas in four environments : (1) above subducted slabs, (2) below and (3) above the mantle-continental crust boundary, and (4) within shallow crust. Reaction couples of peridotite or amphibolite with a sharp boundary against hydrous silicic melts define (1) diffusion profiles within contaminated melts and (2) crystalline reaction zones between the two materials. Mixtures of rocks peridotite-tonalite-granodiorite-granite with water define phase boundaries delineating the extent of contamination of slab-derived melts rising through mantle and of crustal melts in contact with peridotites {e.g., cumulates). Chemical diffusion in granite melts at 1.5 kbar, 810 °C is slow ; diffusion between hot, hydrous basalt-rhyolite at 10 kbar is faster. Convection occurs in H20-undersaturated granite melt at 10-15 kbar, 920-1050 °C, during passage of a melting front through crystalline granite adjacent serpentinized peridotite, the source of H_2O; the convective motions transport contaminated melt away from the diffusion zone, which therefore remains narrow. Rates of contamination and mixing of magmas near the mantle-crust boundary are likely to be much faster than in shallow crustal plutons. Slab-derived melts are richer in Ca/Mg than calc-alkaline rocks, and contamination with overlying mantle at depth makes them richer in Mg/Ca than calc-alkaline rocks. Near the mantle crust boundary, Ca/Mg in contamined melts is appropriate for calc-alkaline rocks.