Incongruent dissolution of REE- and Sr-rich apatite in peraluminous granitic liquids; differential apatite, monazite, and xenotime solubilities during anatexis

The relative solubilities of monazite (Mnz), xenotime (Xno), and apatite (Ap: REE- and Sr-rich, and REE- and Sr-poor) have been studied in peraluminous granitic liquids in month-long experiments at 750 °C and 200 MPa (PH20). In contrast with the high solubility of the apatite (0.7 wtOjoP205) in strongly peraluminous liquids, monazite and xenotime have much lower solubilities «0.05 wt°joP205). In mildly to strongly peraluminous compositions, P205 in the liquid is 0.03-0.04 wt°joat xenotime saturation and 0.02-0.05 wt°jo at monazite saturation; in keeping with the low P in the glasses, RE203 contents are below EMP detection thresholds (:50.08 wt°jo) for all conditions of experiments (saturation of liquid at equilibrium and local saturation around apatite). Apatite dissolves incongruently, crystallizing REE-rich monazite on its surface (1-4 ~m-Iong grains), resulting in similar low REE contents in liquids. Monazite precipitation occurs along the margins of dissolving apatite crystals, even though the bulk liquid is not monazite-saturated. The abundance of monazite microcrystals increases with the REE content of the apatite and the degree of apatite dissolution. The reaction relationship (Ap + LiqI -+ Mnz + Liq2), stemming from differences in relative solubilities (greater than an order of magnitude) between apatite and monazite, results in the dissolution of much smaller amounts of REE into peraluminous liquids than expected by simple evaluation of apatite REE contents. The amount of REE contributed from apatite directly to peraluminous granitic liquid is related to the amount of apatite dissolved by simple mass balance only if the total REE content of the apatite is sufficiently low that monazite saturation in liquid (50-100 ppm RE203 at 750 °C) is avoided. During dissolution of Sr-rich apatite, the Sr partitions into the liquid, and, at 750 °C and 200 MPa (PH20),the diffusion coefficient of Sr in liquid is ---2 x 10-10 cm2js (R2 = 0.659). The reaction relationships described above may have application to some textural features observed in natural igneous rocks. For example, clusters of monazite microcrystals might be indicators of dissolved apatite; monazite morphology can be used to distinguish the source of its REE and the general petrological process (rock anatexis or magma crystallization) under which the monazite formed. In addition, monazite microcrystals could serve as nucleation sites for other minerals, which might explain their common inclusion in biotite and amphibole within granitoids.