In order to constrain the large ion lithophile (LIL) element distribution for subduction-zone environments, exchange coefficients

\(K^{phe-fluid}_{D}\ {=}\ (\mathit{X}^{phe}_{Rb,Cs}\ {\cdot}\ \mathit{X}^{fluid}_{K}){/}(\mathit{X}^{phe}_{K}\ {\cdot}\ \mathit{X}^{fluid}_{Rb,Cs})\)
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.

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