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OXYGEN ISOTOPE INHERITANCE AND STRUCTURAL TRANSITION OF SPINEL IN PERIDOTITES

Li Yiliang;Wang Zhengrong;Zheng Yongfei (Laboratory for Chemical Geodynamics, Department of Earth and Space Sciences,University of Science and Technology of China, Hefei 230026)  
Using oxygen isotope data offered by laserprobe and fractionation curves for mantle minerals, oxygen isotope geothermometry has been carried out for mineral pairs containing spinel from mantle-derived peridotites. Using increment method of the mineral pair O-isotope fractionation, the calculated temperatures between the normal spinel and coexisting minerals are above the equilibria temperature of peridotite assemblages, whereas the temperatures calculated by using inverse spinel-mineral pairs are approximated to the equilibria temperature of the peridotite assemblages. Thus, it is inferred that the oxygen isotope feature of the spinel from mantle peridotite shows the nature of its precursor inverse spinel. It is the inverse spinel that has equilibrated oxygen isotope with coexisting minerals in the peridotites. Although the inverse spinel was transformed to the normal spinel afterwards, the oxygen isotopes of inverse spinel was not reequilibrated. A thermodynamic analysis of the non-convergent order of Mg2+and Al3+ in MgAl2O4 shows that the normal spinel and the inverse spinel have their stable fields at different temperatures. The calculated discontinuity s=1/ 3 is the critical point between the two polymorphs. The corresponding temperature is estimated between 1550℃ and 1782℃, which is in accordance with the oxygen isotope temperatures calculated from fractionations of the inverse spinel-mineral pair.
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