HR: 0800h
AN: V31C-0620    [Abstracts]
TI: Rutile solubility in H₂O-NaAlSi₃O₈ fluids at High T and P: Implications form HFSE mobility in Subduction zones
AU: * Antignano, A
EM: aaiv@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AB: The trace element signatures of arc magmas are characterized by HFSE depletion relative to the LILE. Rutile, a common accessory phase in high-pressure assemblages, is an important reservoir for the HFSE and is often invoked to explain the HFSE depletion of arc magmas. This model is in part based on experimental studies, which show that rutile has very low solubility in pure H₂O. However, rutile is also a common accessory to eclogite-facies vein assemblages of albite, paragonite and quartz, which likely precipitated from slab-derived fluids. This observation requires either that fluid fluxes were unrealistically high, or that current estimates of Ti solubility are too low. A possible solution to this problem is that dissolved silicate components can enhance Ti solubility via complexing. To test this, we measured the solubility of rutile in H₂O-NaAlSi₃O₈ (albite) bearing fluids at high T and P. Experiments were conducted using a piston-cylinder apparatus with NaCl-graphite furnaces. A single synthetic rutile crystal was loaded into a 1.6 mm OD Pt inner capsule, which was lightly crimped and then placed in a 3.5 OD Pt outer capsule with ultra pure H₂O and powdered Amelia albite. Solubility was determined by the weight loss of the rutile grain after 10 hrs. A time series demonstrates that equilibrium is achieved after 8-10 hrs. Preliminary results at 800°C, 1.0 GPa, show that rutile solubility rises with increasing NaAlSi₃O₈ concentration from 1.15(12) millimolal at 2.18 wt% NaAlSi₃O₈ to 3.77(13) at 8.80 wt% NaAlSi₃O₈. Corundum mats + fluid are observed in 3.4-8.80 wt% NaAlSi₃O₈ and are interpreted to be the result of incongruent dissolution of albite. Quenched melt spheres where observed in an experiment containing ~15 wt% NaAlSi₃O₈, but not at 8.80 wt%. At 8.80 wt% NaAlSi₃O₈, rutile solubility is higher by a factor of 6 relative to that in pure H₂O. Our results suggest that TiO₂ solubility is increased by complexing with Na-Al-Si-bearing fluid components. It has been proposed that polymerized aqueous albite-like molecules occur in high-P fluids (Manning, 2004). Since field observations show that subduction zone fluids contain significant quantities of dissolved Na, Al and Si, then silicate complexing almost certainly controls Ti solubility in natural settings. Substitution of Ti into such complexes could explain the presence of rutile in the fluid-derived veins found in subduction zone complexes. This suggests that models of HFSE mobility which are based on rutile solubility in pure H₂O may significantly underestimate the ability of water-rich fluids to transport these elements.
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005