HR: 1330h
AN: V22D-0611    [PDF]
TI: Very Low Solubility of Rutile in H$_{2}$O at High P and T
AU: * Tropper, P
EM: Peter.Tropper@uibk.ac.at
AF: Institute of Mineralogy and Petrography, University of Innsbruck Innrain 52, Innsbruck, A-6020 Austria
AU: Manning, C
EM: manning@ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AB: Rutile (TiO$_{2}$) is a common accessory mineral in high-pressure rocks and contains high concentrations of high field strength elements (HFSE) such as Nb and Ta. Understanding the solubility behavior of rutile is necessary in order to interpret the characteristic HFSE depletion of island arc basalts. Previous study (Ayers and Watson, 1993, Contrib. Mineral. Petrol., 114, 321) suggests increasing solubility in H$_{2}$O with T at 900-1100 $\deg$C, 1 and 2 GPa, reaching 0.246 molal at 1100 $\deg$C, 1 GPa. These results imply significant solubility and mobility of aqueous Ti at mantle conditions. However, this conclusion depends on the interpretation that newly formed rutile crystallites in the experiments grew during quench rather than during the runs. In order to place additional constraints on the solubility of rutile in H$_{2}$O at these P-T conditions, we carried out weight-loss experiments on synthetic rutile single crystals in a piston-cylinder apparatus. Our main objective was to explore the effects of minor temperature gradients in the charges on rutile crystal growth. T gradients were minimized by using thick-walled graphite furnaces and small, horizontal Pt capsules (OD=3.5 mm). Experiments were conducted with and without inner Pt envelopes to contain crystals. Preliminary results showed that in the absence of the Pt-envelopes, significant aqueous Ti transport and newly formed, blocky rutile crystals occur in the capsule ends, presumably due to slight temperature gradients in the charges. Longer run times result in increased mass of these rutile crystallites. By contrast, new rutile growth is suppressed in the presence of a Pt envelope containing the crystal. Evidently, the inner capsule has a thermal baffling effect which reduces temperature gradients. In all runs, morphologically distinct rutile needles form on the capsule walls upon quench. Only experiments yielding no growth of new, blocky crystals during the run can provide accurate solubility measurements from the weight change of the starting crystal. At 1 GPa, we find rutile solubility at 800 $\deg$C and 900 $\deg$C to be $<$0.0007 molal, and at 1100 $\deg$C, it is 0.0028 molal. At 2.0 GPa, the solubility at 1000 and 1100 $\deg$C is also very low (0.0018 and 0.0030 molal, respectively). These data indicate that the solubility of rutile in H$_{2}$O at $\geq$1000 $\deg$C is up to 75 times lower than previously reported. Our results reduce significantly the solubility gradient that would be experienced by aqueous fluids in the mantle wedge above subduction zones, implying that such fluids are inefficient agents for the redistribution of HFSE in this environment.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting