ESTIMATING ABUNDANCES OF VOLATILE AND OTHER MOBILE COMPONENTS IN EVOLVED SILICIC MELTS THROUGH MINERAL-MELT EQUILIBRIA

Silicic igneous rocks (granites, pegmatites, and rhyolites) usually as cribed to A- or S-type sources commonly manifest enrichment in some co mbination of the rare alkalis and alkaline earths (Li, Rb, Cs, Be, Sr, Ba) and the fluxing components P, F, and B. Because most of these com ponents are incompatible in rock-forming minerals, they remain mobile up to the subsolidus transition and tend to be dispersed into host roc ks rather than conserved within the igneous body; hence, the igneous w hole rocks do not record the original magmatic abundances of these com ponents. Through mineral-melt equilibria, the abundances of these comp onents can be constrained to variable degrees of accuracy from source (partitioning between residual minerals and anatectic melts), through melt fractionation (partitioning between igneous minerals and residual melts), to the end stages of magma solidification, where higher conce ntrations of normally trace elements may promote saturation in their c rystalline phases. The magmatic abundance of rare alkalis and alkaline earths is controlled largely by reactions among feldspars, micas, and melt. Fluorine in anatectic melts may be buffered at the source by mi cas (and amphiboles), but F is not usually controlled by other silicat e-melt equilibria throughout the remainder of magmatic fractionation. At present, the compositions of micas and of apatite can constrain F c ontents of melts, though only with some important assumptions. The abu ndance of B in melt is dictated by the stability of tourmaline with re spect to other femic aluminosilicates. Equilibria among tourmaline, bi otite, and cordierite (or garnet) in granitic magmas operate at 1-4 wt % B2O3 in melt depending principally on temperature and the activity of Al in melt. The low femic content of evolved B-rich magmas limits t he amount of tourmaline that can crystallize; thus, the buffering reac tions are readily exhausted and B increases unbuffered in melt. Phosph orus is the best constrained of these fluxing components through (1) e quilibria among biotite, garnet, LiAl-silicates and their correspondin g phosphate analogs, (2) solubility models for apatite, and (3) the ca libration of P distribution between the alkali feldspars and melt. In combination with trapped melt inclusions, the mineral equilibria descr ibed here provide useful measures of these petrologically and economic ally important components in high-silica melts.