HR: 0800h
AN: V31C-0622    [Abstracts]
TI: The Solubility of Diopside in Water at 10 to 15 kbar and 650 to 900 C
AU: * Macris, C A
EM: cmacris@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: Subduction zone fluids play a critical role in mass transfer and mantle-wedge metasomatism, yet little is known about their composition and chemical behavior. One way to better understand these important fluids is to investigate the solubility of minerals at subduction zone conditions. The solubility of diopside in H₂O was measured at 10 to 15 kbar, 650 to 900 °C using a piston-cylinder apparatus with NaCl-graphite furnaces. A single gem-quality diopside crystal was placed in an inner Pt capsule, which then was perforated to allow fluid ingress and added with H₂O to an outer Pt capsule, which was then sealed. The solubility was determined by the weight loss of the diopside crystals. All charges were carefully examined by SEM for composition and textural characteristics of quench material. SEM analysis revealed that in several experiments small diopside crystals nucleated and grew in the outer capsule or on the walls of the inner capsule due to fluid convection within the charge. These experiments yielded erroneously high solubilities and were omitted from final plots and equation calculations. Diopside was found to dissolve incongruently to forsterite + dissolved species at all conditions investigated. The forsterite occurs as euhedral crystals in pits etched from the diopside grain covering less than 5% of its surface. In addition, long thin wollastonite blades and a layer of SiO₂ "mud" form upon quenching of the run. Experiments at 700 °C and varying times showed that diopside plus forsterite plus fluid reached equilibrium by 12 hours. The solubility of diopside + forsterite in H₂O increases with increasing pressure and temperature. At 10 kbar, solubility increases from 0.004 molal at 650 °C to 0.012 molal at 900 °C. At 800 °C and 10 to 15 kbar, solubility increases from 0.008 to 0.015 molal. These preliminary data yield the equation: log m_di+fo = -0.7539 + -2135.7/T + 0.6355P, where T is in K and P is in GPa. Because temperature enhances solubility more than pressure in this system, we can predict that as a fluid moves from slab to wedge, which leads to decompression and heating, di+fo solubility will increase. The resulting fluid will be enriched in Ca and Si, but low in Mg. These results are consistent with theoretical predictions on the composition of fluids in equilibrium with eclogites (Manning 1998), experimental investigations on fluid compositions in equilibrium with high-pressure mantle rocks (Schneider and Eggler 1986; Ayers et al. 1997), and vein-mineral assemblages in blueschists and eclogites (e.g., Gao and Klemd 2001; Becker et al. 1999). Ayers J., Dittmer S.K., Layne G.D. (1997) Earth Planet. Sci. Lett. 150:381-398; Becker H., Jockum K.P., Carlson R.W. (1999) Chem. Geol. 160:291-308; Gao J., Klemd R. (2001) Contrib. Mineral. Petrol. 142:1-14; Manning C.E. (1998) Swiss Bull. Mineral. Petrol. 78:225-242; Schneider M.E., Eggler D.H. (1986) Geochim. Cosmochim. Acta 50:711-724
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