HR: 17:20h
AN: V34A-05    [Abstracts]
TI: WATER SOLUBILITY IN FORSTERITE AND ENSTATITE: A KEY FOR UDERSTANDING MANTLE RHEOLOGY
AU: * Kovacs, I
EM: istvan.kovacs@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, 61 Blg, Mills Road, Canberra, ACT 0200, Australia
AU: Hermann, J
EM: joerg.hermann@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, 61 Blg, Mills Road, Canberra, ACT 0200, Australia
AU: O`Neill, H S
EM: hugh.oneill@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, 61 Blg, Mills Road, Canberra, ACT 0200, Australia
AB: The temperature dependence of solubility of H₂O in forsterite and enstatite has been studied experimentally using two different starting compositions in the system MgO-SiO₂- H₂O to produce the parageneses per+fo+fluid and fo+en+fluid (per = MgO; fo = Mg₂SiO₄; en = MgSiO3). All experiments were run at 2.5 GPa for at least 24 hours, with temperature varied from 1000 to 1400 °C. Recovered samples were examined for the types of H2O substitution and their quantification by FTIR spectroscopy and SIMS. In the per-buffered experiments, peaks in fo at 3612, 3589, 3566, 3555, 3533 and 3480 cm^-1 were identified, which are related to silica vacancies due to low silica activity. In the en-buffered experiments, the IR spectrum of fo shows two peaks at 3160 and 3220 cm^-1 in addition to those present in the per-buffered experiments. These peaks indicate Mg vacancies. Peak intensities decrease with increasing temperature in both the per- and en-buffered systems, showing that water solubility is inversely proportional to temperature in fo. For en, two peaks at 3360 and 3060 cm^-1 were found. These peaks are close to the peaks in fo at 3160 and 3220 cm^-1 that are associated with higher silica activity, suggesting that H₂O only substitutes in en by a Mg vacancy mechanism. The H₂O content of en increases with increasing temperature. The results imply that the partitioning of H₂O between olivine and orthopyroxene in the mantle is a very strong function of temperature. Because of their much higher H₂O contents, pyroxenes control the water budget of the upper mantle, but olivine controls its rheology. Cooling of the mantle will transfers H₂O from orthopyroxene (and presumably clinopyroxene) into olivine. We calculate that transfer from pyroxenes increases the H₂O content of olivine by a factor of 2.5 for a 100 °C decrease in temperature. Since H₂O causes marked weakening in olivine, this increase in H₂O may "dampen" the effect of decreasing temperature on mantle viscosity, so that the net effect of secular cooling on mantle convection may be much less than currently estimated.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly