K–Rb–Cs partitioning between phlogopite and fluid: experiments and consequences for the LILE signatures of island arc basalts

The distribution of Rb–K and Cs–K between phlogopite and 1- to 2-m aqueous (K,Rb,Cs)–chloride solutions was investigated at 800 °C and pressures of 0.2, 2 and 4 GPa. Phlogopite of the solid solution binaries KAlMg3Si3O10(OH)2 (phlogopite)–RbAlMg3Si3O10(OH)2 (Rb–phlogopite) and KAlMg3Si3O10(OH)2–CsAlMg3Si3O10(OH)2 (Cs–phlogopite) formed within the experiments according to the chemical exchange vectors XIIK−1+XIIRb1+ and XIIK−1+XIICs1+ involving the interlayer sites. The compositions of phlogopite and coexisting fluids were determined from EMP and ICP analyses, respectively. The following K–Rb and K–Cs exchange coefficients KDphl–fluid between phlogopite and fluid were derived by extrapolating the measured Rb and Cs concentrations to the parts per million range, for which Henryʹs law is valid: KDphl–fluid(Rb–K): 1.71±0.06 at 0.2 GPa, 2.73±0.10 at 2 GPa and 2.76±0.15 at 4 GPa and KDphl–fluid(Cs–K): 0.57±0.05 at 0.2 GPa, 0.73±0.09 at 2 GPa and 0.93±0.26 at 4 GPa. Based on the contrasting and slightly pressure-dependent fractionation behaviour of Rb and Cs relative to K between phlogopite and hydrous fluids—Rb preferentially partitions into phlogopite, whereas Cs fractionates into the fluid—variations in the large ion lithophile element (LILE) ratios within rocks and infiltrated fluids, as a result of formation or breakdown of phlogopite, are discussed by applying models of Rayleigh fractionation and ion exchange processes operating in a one-dimensional chromatographic rock column. Assuming significant amounts of metasomatically formed phlogopite within the mantle wedge, calculations using the chromatographic column model, led to Cs/K ratios significantly lower than those observed for island arc basalts (IAB). We therefore propose that metasomatic formation of phlogopite within the mantle wedge should be limited and does not significantly influence the LILE characteristics of IAB.