Cesium fractionation in miarolitic pegmatites: a reevaluation of k-feldspar data

A review of K-feldspar compositions from miarolitic pegmatites shows that in most pegmatites the pocket K-feldspars are enriched in Rb and Cs relative to exopocket K-feldspar within the same body. Rayleigh modeling of simultaneous crystallization of K-feldspar from a melt and coexisting aqueous solution predicts that the Cs content of K-feldspar falls to nil, which implies that rare-alkali enriched (up to 2190 ppm Cs) pocket K-feldspar must have crystallized from a fluid-undersaturated pegmatite melt. However, most petrologists contend that miarolitic cavities develop after exsolution of an aqueous phase from a pegmatite melt. To investigate the process responsible for the high uptake of Cs and Rb in pocket K-feldspar we determined the rare-alkali content of synthetic K-feldspars that crystallized at 500 degrees C from a supercritical aqueous fluid in a granitic melt + fluid system. The K/Cs ratio of the synthetic K-feldspar was compared to modeled K/Cs ratios for K-feldspars formed from a water-saturated melt in which the initial Cs concentration (C-o) of the melt was identical to the starting glass used in experiments. Our results show that the K/Cs ratios of synthetic K-feldspar are orders of magnitude lower than that predicted using the Rayleigh fractionation model. We attribute the high uptake of Rb and Cs in K-feldspar to kinetic effects associated with rapid crystal growth in an undercooled water-saturated melt. Therefore, we propose that Rb- and Cs-rich K-feldspars that line the pockets of natural miarolitic pegmatites are the products rapid growth in an aqueous fluid that coexists with a highly fractionated residual melt.