HR: 13:55h
AN: T22D-02    [PDF]
TI: Phase transition of CaCO3 in the lower mantle
AU: * Ono, S
EM: sono@jamstec.go.jp
AF: Institute for Frontier Research on Earth Evolution (IFREE), Japan Marine Science & Technology Center (JAMSTEC), 2-15 Natsushima-cho, Yokosuka-shi, 237-0061 Japan
AB: Calcite is the dominant carbon-bearing phase in the Earth_fs crust, and acts as a buffer for the long-term cycling of CO2 between the atmosphere, oceans, and solid Earth. It is unsurprising, therefore, that the high-pressure stability and behaviour of CaCO3 and related phases has attracted considerable interest. It is generally known that calcite transforms to aragonite, which often occurs in high-pressure metamorphic rocks, at high P-T that correspond to the lower crust and the uppermost upper mantle. It is unknown whether aragonite transforms to new high-pressure phase or dissociates into CaO and CO2. Therefore, the high-pressure stability limit of aragonite was investigated. High-pressure X-ray diffraction experiments were performed using a laser-heated diamond anvil cell. The samples were heated with a YAG laser to overcome potential kinetic effects on possible phase transitions. The samples were probed using an angle-dispersive X-ray diffraction technique at the synchrotron beam lines BL10XU, SPring-8 and BL13A, Photon Factory in Japan. In the first set of experiments, the pressure was increased directly to 70 GPa at room temperature, and an X-ray diffraction pattern of the sample was recorded. It is difficult to identify CaCO3 phases at room temperature before the sample was heated. A strain-broadening of the diffraction peaks occurred, because a large differential stress was induced in the diamond anvil cell experiments as pressure increased. After the desired pressure was achieved, the sample was heated to about 2000 K to relax the differential stress and to overcome potential kinetic effects on possible phase transitions. After the heating, some new peaks appeared in the diffraction pattern. This implies that the starting material transformed to a new high-pressure phase. According to additional experiments, the phase transformation from aragonite to a new calcium carbonate form was observed at pressures higher than about 35 GPa, corresponding to the lower mantle. The new carbonate shows a hexagonal symmetry and was confirmed to remain stable to 70 GPa. This indicates that carbon might to be stored in the new carbonate phase in the deep mantle.
DE: 3620 Crystal chemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting