Al-bearing dense hydrous minerals under mantle transition zone and uppermost lower mantle conditions
Abstract
Subducting slabs are supplying water into the deep mantle, and some hydrous minerals may be present in the mantle transition zone and even in the uppermost lower mantle. The water storage capacities of mantle minerals are supposed to be significantly coupled with Al by a substitution with Mg2+, Si4+ or Mg2+ + Si4+, because Al3+ is the trivalent cation, and H+ is the monovalent cation. In this study, we have examined the effect of Al for the water content and the stability of some hydrous phases in the system MgO - Al2O3 - SiO2 - H2O in the mantle transition zone and the uppermost lower mantle condition. The experiments were conducted at 20 - 25 GPa and 1873 - 2273K by MA8-type (KAWAI-type) high pressure apparatus. The samples were sealed by AuPd capsule to prevent the loss of water, and the temperatures were measured by WRe thermocouple. The charges were heated for 1 hour in the desired P-T conditions, and then quenched at the rate of ~1000K/sec by tuning off the electric power supply. After releasing the press load for several hours, the run products were recovered, and then the recovered samples were polished to measure the chemical composition and the H2O contents.. We succeeded to synthesize the Al-bearing hydrous minerals (Al-bearing phase B, Al-bearing superhydrous phase B, Mg-Si bearing δ-AlOOH), and found that the significant coupling of H and Al were occurred in these minerals. The chemical compositions and the lattice parameters are examined by SEM-EDS and microfocus-XRD, and the H2O contents were measured by SIMS. The present chemical formula and the lattice parameters were compared to discuss about the difference between the pure chemical forms. Still more, the substitution mechanisms were considered basically by the chemical compositions and the information of the crystal structures. Our study suggests that water can be stored in the mantle transition zone and the uppermost lower mantle in the form of Al-bearing hydrous minerals or nominally anhydrous minerals.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMMR21A2315I
- Keywords:
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- 1025 GEOCHEMISTRY Composition of the mantle;
- 1042 GEOCHEMISTRY Mineral and crystal chemistry;
- 3630 MINERALOGY AND PETROLOGY Experimental mineralogy and petrology;
- 3620 MINERALOGY AND PETROLOGY Mineral and crystal chemistry