The behavior of apatite during crustal anatexis: Equilibrium and kinetic considerations

Abstract

The solubility and dissolution kinetics of apatite in felsic melts at 850°–1500°C have been examined experimentally by allowing apatite crystals to partially dissolve into apatite-undersaturated melts containing 0–10 wt% water. Analysis of P and Ca gradients in the crystal/melt interfacial region enables determination of both the diffusivities and the saturation levels of these components in the melt. Phosphorus diffusion was identified as the rate-limiting factor in apatite dissolution. Results of four experiments at 8 kbar run in the virtual absence of water yield an activation energy ( E) for P diffusion of 143.6 ± 2.8 kcal- mol −1 and frequency factor ( D 0) of 2.23 +2.88 −1.26 × 10 9 cm 2- sec −1. The addition of water causes dramatic and systematic reduction of both E and D 0 such that at 6 wt% H 2O the values are ~25 kcal-mol −1 and 10 −5 cm 2-sec −1, respectively. At 1300°C, the diffusivity of P increases by a factor of 50 over the first 2% of water added to the melt, but rises by a factor of only two between 2 and 6%, perhaps reflecting the effect of a concentration-dependent mechanism of H 2O solution. Calcium diffusion gradients do not conform well to simple diffusion theory because the release of calcium at the dissolving crystal surface is linked to the transport rate of phosphorus in the melt, which is typically two orders of magnitude slower than Ca. Calcium chemical diffusion rates calculated from the observed gradients are about 50 times slower than calcium tracer diffusion. Apatite solubilities obtained from these experiments, together with previous results, can be described as a function of absolute temperature ( T) and melt composition by the expression: In D apatite/ melt P = [(8400 + (( SiO 2 − 0.5)2.64 × 10 4))/ T] − [3.1 + (12.4( SiO 2 − 0.5))] where SiO 2 is the weight fraction of silica in the melt. This model appears to be valid between 45% and 75% SiO 2, 0 and 10% water, and for the range of pressures expected in the crust. The diffusivity information extracted from the experiments can be directly applied to several problems of geochemical interest, including I) dissolution times for apatite during crustal anatexis, and 2) pileup of P, and consequent local saturation in apatite, at the surfaces of growing major-mineral phases.