کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
6427834 | 1634724 | 2015 | 11 صفحه PDF | دانلود رایگان |
- Experimental H-annealing of olivine at high pressure and temperature.
- Secondary modification of initial OH information in many natural olivines.
- Much smaller storage capacity of water in the reducing upper mantle.
- Pervasive hydrous melting in the reducing upper mantle appears unlikely.
Experimental studies of OH solubility in peridotite minerals are of crucial importance for understanding some key geochemical, geophysical and geodynamical properties of the upper mantle. In reducing depths of the upper mantle, C-O-H fluids are dominated by CH4 and H2O. However, available experimental H-annealing of olivine concerning water storage capacity in the reducing upper mantle has been exclusively carried out by equilibrating olivine with H2O only. In this study, OH solubility in olivine has been investigated by annealing natural olivine crystals under peridotite-bearing and CH4-H2O-present conditions with piston cylinder and multi-anvil apparatus. Experiments were performed at 1-7 GPa and 1100-1350â°C and with oxygen fugacity controlled by Fe-FeO buffer, and OH solubilities were measured from polarized infrared spectra. The olivines show no change in chemical composition during the experiments. The infrared spectra of all the annealed olivines show OH bands in the range 3650-3000 cmâ1, at both high (>3450 cmâ1) and low (<3450 cmâ1) frequency, and the bands at â¼3400-3300 cmâ1 are greatly enhanced above â¼3 GPa and 1300â°C. The determined H2O solubility is â¼90-385 ppm for the olivine coexisting with H2O (1-7 GPa and 1100â°C), and is â¼40-380 ppm for the olivine coexisting with CH4-H2O (1-7 GPa and 1100-1350â°C). When CH4 is present in the equilibrium fluid, the H2O solubility is reduced by a factor of â¼2.3 under otherwise identical conditions, indicating a strong effect of CH4 on the partitioning of water between olivine and coexisting fluid. The storage capacity of water in the reducing upper mantle is, modeled with the measured solubility of olivine and available partition coefficients of water between coexisting minerals, up to â¼2 orders of magnitude lower than some previous estimates. Considering the temperature along the geotherm in the reducing oceanic upper mantle, the required H2O concentration to trigger hydrous melting is 250 and 535 ppm at â¼100 and 210 km depth, respectively, and is even larger at greater depths. These values exceed the typical H2O abundance (â¼100±50 ppm) in the upper mantle, suggesting that pervasive hydrous melting at reducing depths of the oceanic upper mantle is not likely. Similar arguments may also be casted for the reducing deep upper mantle in the continental regions.
Journal: Earth and Planetary Science Letters - Volume 432, 15 December 2015, Pages 199-209