HR: 0800h
AN: V31C-0598    [Abstracts]
TI: Carbonate Solid Solution Models to 6 GPa
AU: * Franzolin, E
EM: ettore.franzolin@erdw.ethz.ch
AF: ETH Zurich IMP, Clausiusstrasse, Zurich, 8092, Switzerland
AU: Schmidt, M
EM: Max.Schmidt@erdw.ethz.ch
AF: ETH Zurich IMP, Clausiusstrasse, Zurich, 8092, Switzerland
AU: Poli, S
EM: Stefano.Poli@unimi.it
AF: Università  degli Studi di Milano Dipartimento di Scienze della Terra, Via Botticelli 23, Milan, 20133, Italy
AB: At convergent boundaries, volatile compounds are transported in the interior of the Earth affecting the chemical evolution of our planet. H2O and CO2 are the most important compounds affecting phase stabilities during subduction. The stability of carbonates at high pressure is proved by the occurrence of coesite and diamond inclusions in dolomite and magnesite from carbonate-bearing eclogite, with the general succession of carbonates with pressure being calcite - dolomite - magnesite. The fate of carbonates, is controlling the residence time of C in the mantle, affecting its global geochemical cycle and contributing to the chemical and, in carbonate rich sediments, also mechanical structure of subducted crust. Nevertheless the phase diagram of CaCO3- MgCO3- FeCO3 is unknown at high pressure and is now investigated to 6 GPa and 1300 °C. Starting materials are natural MgCO3, synthetic CaCO3 and FeCO3 in different proportions. Preliminary experiments have been carried out at 3.5 GPa, 900 °C in Pt-C capsules to fix oxygen fugacity roughly to GCO, thus avoiding siderite oxidation. These runs show a broad miscibility field between CaCO3 and CaMgxFe1-x(CO3)2, which is in agreement with the observation of (Mg,Fe)- calcites with high Mg+Fe contents in experiments on natural bulk compositions. At this P-T conditions ankerite is not been observed, it has been observed a continuous solid solution between dolomite and CaMg0.2Fe0.8(CO3)2 . Between CaMgxFe1-x(CO3)2 and the continuous solid solution siderite-magnesite exist a broad miscibility gap; no three phase field has been noted. Selected runs have been observed with TEM to investigate disordering and reordering after quench, future synchrotron X-ray diffraction will determine in-situ the state of disorder of carbonates and the transition, and quantify the experimental "quenchability" of disordering. Further experiments at different P-T conditions in piston cylinder and multi anvil apparatus are necessary to obtain a full set of thermodynamic properties and a full fledged solid solution model as a function of pressure and temperature.
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting