HR: 0800h
AN: V31A-1405    [Abstracts]
TI: Solubility of apatite in H$_{2}$O-NaCl and silicate-bearing solutions at 0.7-3.0 GPa, $800\deg$C
AU: * Antignano, A
EM: aaiv@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90049 United States
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90049 United States
AB: Apatite is a major reservoir for the rare-earth elements (REE) in the earth's crust. However, little is known about its solubility in metamorphic fluids. We measured the solubility of apatite in H$_{2}$O-NaCl and silicate bearing fluids at $800\deg$C and 1.0-2.0 GPa using a piston-cylinder apparatus with NaCl-graphite furnaces. A single Durango fluor-apatite crystal was loaded into a 1.6 mm OD Pt inner capsule, which was crimped and then placed in a 3.5 OD Pt outer capsule with ultra pure H$_{2}$O and NaCl or powdered albite. Solubility was determined by the weight loss of the apatite grain after 24 hrs. In the H$_{2}$O-NaCl experiments, total dissolved solids (TDS) were initially below detection (0.4 millimolal) between X$_{NaCl}$= 0 and X$_{NaCl}$= 0.025. At X$_{NaCl}$= 0.035, solubility was 3.3(0.2) millimolal (errors are 1s), and it increased to 57.5(0.4) millimolal at X$_{NaCl}$= 0.526. Our results show that there is an enhancement in apatite solubility with increasing pressure in pure H$_{2}$O. Solubility is initially below detection at $<$1.0 GPa, but increases to 0.8(0.2) and 1.2(0.2) millimolal at 1.5 GPa and 2.0 GPa respectively. At 1.0 GPa, apatite solubility in H$_{2}$O-albite (Ab) fluids increases from 0.5(0.2) millimolal at X$_{albite}$= 0.004 to 1.4(0.2) at X$_{albite}$= 0.007. Monazite occurs as $<$1 $\mu$m subhedral grains on apatite surfaces in all runs, but its weight is insignificant and does not effect the solubility measurements. Our results demonstrate that, at a given P and T, apatite solubility rises with increasing concentrations of NaCl and albite, suggesting that the additional components promote mass transport of dissolved apatite components during metamorphism. Concentrated brines may explain the origin of apatite veins associated with in high-grade granulite terranes and ore bodies. The formation of monazite demonstrates that the REE are less soluble in H$_{2}$O-NaCl and silicate bearing solutions. This probably explains textures in which monazite mantles apatite, which are common in granulite metamorphic terranes, such as the Kiirunavaara magnetite-apatite ore.
DE: 3630 Experimental mineralogy and petrology
DE: 1020 Composition of the crust
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting