Phosphorus and the rare earth elements in felsic magmas: an assessment of the role of apatite

Abstract

The solubility of fluorapatite in 17 silica-rich melts in the system Na2O-K2O-Al2O3-SiO2 (with and without CaO or CaF2) was determined at 1 kbar water pressure and 750 900°C. Apatite saturation occurs at levels of dissolved P2O5 ranging between 0.04 (± 0.02) and 0.28 (± 0.13) wt%. with only 4 values outside the 0.09–0.20 wt% range.The results demonstrate not only that apatite is a common liquidus phase in felsic melts, but also that, under most circumstances, it remains in the residue during episodes of partial fusion of the crust. Given a solubility limit of 0.14 wt% dissolved P2O5 (the mean of the experimental values) a source containing as little as 0.05% P2O5 must be 35% melted before apatite is lost from the residue and no longer buffers the melt P2O5 concentration at the saturation value. Higher abundances of P2O5 in the source postpone the loss of residual apatite to still higher degrees of melting, and if the source P2O5 content exceeds 0.14 wt%, apatite must be residual for all degrees of melting, increasing in abundance as melting proceeds.The generally secondary influence of apatite on the rare earth element (REE) patterns of melt and residue is most apparent when garnet and/or amphibole is minor or lacking in the residue. Fractional crystallization of intermediate (e.g. andesitic) magmas toward felsic compositions invariably results in saturation in apatite and some consequent depletion of REE in the melt.