APATITE SATURATION IN BASIC TO INTERMEDIATE MAGMAS

The solubility of fluorapatite in magmatic liquids ranging in composition from olivine melilitite to andesite was determined by fusion of powdered rock samples in fluorapatite capsules and subsequent analysis of the quenched glasses with an electron microprobe. Over the temperature range 1165° ‐ 1275°C at 1 atmosphere pressure, saturation of the melts in apatite is controlled by three primary variables: 1) concentration of P2O5, 2) concentration of SiO2, and 3) temperature. As SiO2 concentration increases from 30 to 55 wt% at 1250°C, the amount of dissolved P2O5 required to saturate a melt in apatite decreases from 12 to less than 4 wt%. At 1165°, the saturation level is uniformly ∼ 36% lower than at 1250°C. By comparison of these results with P2O5 concentrations in natural basalts, it is clear that few basic lavas are saturated in apatite at the time of extrusion. Extrapolation of the data to high‐pressure conditions in the upper mantle is tenuous at the present time, but the low‐pressure data suggest that any apatite present in regions of basalt production may be consumed during melting. Copyright 1979 by the American Geophysical Union.