Hydrothermal origin of apatite, monazite and xenotime

Experimental studies of accessory mineral solubility in aqueous fluids suggests that apatite and monazite solubility in pure H2O is low, and increases with decreasing pH (Ayers and Watson 1991). Ayers et al. (1999), in an experimental study of the coarsening kinetics of monazite in the presence of aqueous fluids, found that monazite either recrystallizes and moves as host grain boundaries migrate, or is trapped as inclusions within the host phase. Inclusion monazites, once isolated from the rock matrix, should record the time of entrapment whereas matrix monazite should record the time of final recrystallization, or, if growth zoned, a complex age spectrum. 

To realize the potential of monazite (and xenotime) as a geochronometer, it is necessary to identify the monazite-producing reactions, and, ideally, to locate them in P-T space. It is clear from the summary above that there are a number of reactions by which monazite may grow during metamorphism, and no simple generalizations are possible. 

The first appearance of monazite. The uncertainty surrounding monazite-producing reactions is perhaps best illustrated by the question of what reaction produces the first appearance of metamorphic monazite. The fact that monazite is sometimes detrital in origin is not in dispute, but the conditions at which the first metamorphic monazite appears are controversial. 

The past decade has seen an enormous growth of information about metamorphic phosphates. Particularly encouraging is the wealth of information contained in these minerals regarding metamorphic histories. These studies point towards a number of fruitful areas for further research  

The goal of these studies is to link the paragenesis of these (and other) phases to events in a rock s history which, when combined with texture-sensitive geochronology (e.g., SIMS, LA-ICP-MS, and EMP), will enable the determination of the time of these events. These data will allow direct inferences of fundamental tectonic parameters such as the duration of metamorphic events, the duration of melting in the crust, the rates of heating/cooling and burial/exhumation of orogenic belts, and the time and duration of hydrothermal activity. 

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