Apatite in felsic rocks: a model for the estimation of initial halogen concentrations in the Bishop Tuff (Long Valley) and Tuolumne Intrusive Suite (Sierra Nevada batholith) magmas

Abstract

Variations in the concentration of the halogens in apatite can be used to deduce changes in fugacity ratios during the crystallization of intrusive and extrusive felsic rocks. In addition, a model initial concentration of Cl and F in the melt and the associated magmatic volatile phase (MVP) can be calculated if the temperature of apatite crystallization in the system can be approximated. Model apatite saturation temperatures (AST) can be calculated, given appropriate apatite solubility data in silicate melts, by assuming that the initial melt composition is equal to the whole rock composition and that apatite has crystallized from the melt. The calculations suggest that apatite began to crystallize at 860°C in the plinian phase of the Bishop Tuff based on published whole rock P2O5 data. This AST of 860°C has been used in conjunction with published apatite analyses to yield estimates of initial aqueous phase Cl (31 000-42 000 ppm) and F (38-60 ppm) concentrations. These values suggest 700-960 ppm Cl and 160-300 ppm F in the initial melt given reasonable estimates of the melt/volatile partitioning of Cl and F. These calculations were applied to the Tuolumne Intrusive Suite and the results discussed. -from Authors