UCLA

The mineral apatite incorporates OH, F, and Cl into its structure, so volatile concentrations in apatite could be used as a proxy for their abundances in magmas. I investigated this by conducting experiments on F-OH exchange between apatite and a simplified arc basalt at 10 kb and 1100 to 1250 °C for 48 hours. The experiments succeeded in growing, for the first time, apatites that were large enough (25-50 µm) for analysis by secondary ion mass spectrometry (SIMS), measurements were also conducted with the electron probe microanalyzer (EPMA). Results indicated that F is preferentially partitioned into apatite, whereas OH slightly favors the liquid. The exchange of F and OH between apatite and melt can be represented as Fliquid + OHapatite = OHliquid + Fapatite, for which the equilibrium constant is Keq=[Fap][OHliq]/[OHap][Fliq], assuming unit activity coefficients. Weighted least squares fitting to data yielded Keq = 59.