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1 GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany
In order to constrain the large ion lithophile (LIL) element distribution for subduction-zone environments, exchange coefficients Kphe-fluidD = (XpheRb,Cs • XfluidK)/(XpheK • XfluidRb,Cs) for K, Rb, and Cs between aqueous fluids and phengite (phe) have been determined experimentally at 2.0 and 4.0 GPa. Derived KD values for the Rb-K exchange slightly increase from 1.62 ± 0.10 at 2 GPa, 600 °C, to 1.84 ± 0.15 at 4 GPa, 700 °C. For the Cs-K exchange, much lower KD values of 0.22 ± 0.06 (at 2 GPa, 600 °C) and 0.37 ± 0.10 (at 4 GPa, 700 °C) were determined. The results show that, for the pressure-temperature range investigated, Rb preferentially fractionates into phengite, whereas Cs partitions into the fluid.
Assuming a one-step model of perfect Rayleigh fractionation for 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 behavior between fluids and phengites determined in this study. Our data indicate that previously derived models for metasomatic mass transfer during subduction processes need to be reconsidered.
Key Words: phengite • LIL-fractionation • subduction zone • exchange coefficients
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