Retentivity of CO2 in fluid inclusions in mantle minerals

Abstract

To assess the capacity of fluid inclusions in mantle minerals for CO 2 retention, annealing experiments were conducted for two mantle xenoliths with CO 2 inclusions for 8 days at 1000 °C under atmospheric pressure and f O 2 of 10 −11 MPa. The results show no marked decrease in the CO 2 density of the CO 2 inclusions for any examined minerals – olivine, orthopyroxene, clinopyroxene, or spinel. The CO 2 density of CO 2 inclusions in olivine in the present mantle xenoliths is lower than that in pyroxenes or spinel. Results of previous studies indicate that the low CO 2 density in olivine is attributable to plastic deformation of olivine around CO 2 inclusions during annealing in ascending magma. Results of this study present fundamental implications for deformation mechanisms that arise from internal pressure of fluid inclusions in silicate minerals. We calculated the stress field in minerals having a CO 2 inclusion. Results show a steep stress gradient in the host around the inclusion. Such local stress in the mineral induces a local rise in the density of dislocations around the CO 2 inclusions. The orthopyroxene used for this study showed a sparse distribution of dislocations around a CO 2 inclusion, whereas olivine showed dense dislocations around CO 2 inclusions, implying that the low CO 2 density of the CO 2 inclusions in olivine results from volume expansion of the CO 2 inclusions through plastic deformation of the host mineral during annealing of the xenoliths in ascending magma. In this respect, constancy of CO 2 density during the annealing experiments for all minerals is an interesting finding. Regarding olivines, the reduction of internal pressure of the CO 2 inclusions or interaction of the dense dislocations possibly inhibits decrepitation or further volume expansion of the CO 2 inclusions during annealing experiments. However, pyroxenes and spinel show higher and similar CO 2 density, which reflects the resistance to plastic deformation and which indicates the effectiveness of CO 2 inclusions in these minerals as a depth probe for mantle xenoliths.