ISLAND-ARC BASALT ALKALI RATIOS: CONSTRAINTS FROM PHENGITE-FLUID PARTITIONING EXPERIMENTS

Abstract

In order to constrain the LIL-element distribution for subduction zone environments, exchange coefficients (KD) for K, Rb and Cs between aqueous fluids and phengite have been determined experimentally at 2.0 and 4.0 GPa. Derived KD–values for the Rb-K exchange slightly increase from 1.62(10) at 2 GPa, 600°C to 1.84(15) at 4 GPa, 700°C. For the Cs-K exchange much lower KD-values of 0.22(6) (at 2 GPa, 600°C) and 0.37(10) (at 4 GPa, 700°C) were determined. The results show that, for the P,T-range investigated, Rb preferentally fractionates into phengite, whereas Cs partitions into the fluid. Assuming a one-step model of perfect Rayleigh fractionation for a continuous decomposition of phengite during subduction, varying alkali-ratios observed for island arc basalts as a function of slab depth may be explained by the LIL-fractionation behaviour between fluids and phengites determined within this study. Our data indicate that previously derived models for metasomatic mass transfer during subduction processes need to be reconsidered.