ASSIMILATION OF XENOCRYSTIC APATITE IN GRANITE MAGMAS

Apatite is a ubiquitous magmatic phase in granite plutons. Contamination of a granite magma with wall-rock material also contributes foreign apatite that subsequently undergoes textural and compositional changes to reach physical and chemical equilibrium (perfect assimilation) in the melt. Our experiments replicate the conditions in such contaminated granites.

The starting materials consist of a peraluminous synthetic SiO₂-Al₂O₃-Na₂O-K₂O (SANK) granite gel with A/NK of 1.3, synthetic F-apatite and Cl-apatite, and natural Durango apatite. The 8-day experiments were conducted in an Ar-pressurized internally heated pressure vessel with a rapid-quench setup at 1200 °C, and 200 MPa, conditions which are above the liquidus for quartz and feldspar. Apatite is the only solid phase in the run products. The starting composition of each run was 90 wt.% SANK granite gel and 10 wt.% crushed apatite (consisting of one, two, or three varieties), with and without 4 wt.% added water.

Run products were examined by SEM and by EMPA. The synthetic granite composition contains no Ca, F, Cl, or REEs. Thus, in every run, apatite was initially undersaturated in the melt. In all experiments, most large apatite grains consisted of anhedral shards with rounded corners, most small apatite grains were round, and a small proportion of apatite grains developed one or more crystal faces. In experiments with two or three apatite compositions, the run-product apatite had compositions intermediate between those of the starting apatites, and they were homogeneous with respect to Cl, and probably F, but not with respect to REEs.

The processes to reach textural equilibrium consist of dissolution until the melt is saturated in apatite, followed by Ostwald ripening to eliminate small grains and to develop crystal faces on larger ones. The processes to reach chemical equilibrium consist of dissolution of apatite, diffusion of cations (Ca, P, REE) and anions (F, Cl, OH) through the silicate melt, and solid-state diffusion in the undissolved apatite grains. The halogens approached chemical equilibrium in all experiments, but in the experiments containing Durango apatite, the REEs have not. We conclude that the rate of chemical equilibrium for the halogens is greater than that for the REEs, and is also greater than the rate to achieve textural equilibrium.