Retentivity of CO₂ in fluid inclusions in mantle minerals

To assess the capacity of fluid inclusions in mantle minerals for CO₂ retention, annealing experiments were conducted for two mantle xenoliths with CO₂ inclusions for 8 days at 1000 °C under atmospheric pressure and fO₂ of 10⁻¹¹ MPa. The results show no marked decrease in the CO₂ density of the CO₂ inclusions for any examined minerals – olivine, orthopyroxene, clinopyroxene, or spinel. The CO₂ density of CO₂ inclusions in olivine in the present mantle xenoliths is lower than that in pyroxenes or spinel. Results of previous studies indicate that the low CO₂ density in olivine is attributable to plastic deformation of olivine around CO₂ 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 CO₂ 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 CO₂ inclusions. The orthopyroxene used for this study showed a sparse distribution of dislocations around a CO₂ inclusion, whereas olivine showed dense dislocations around CO₂ inclusions, implying that the low CO₂ density of the CO₂ inclusions in olivine results from volume expansion of the CO₂ inclusions through plastic deformation of the host mineral during annealing of the xenoliths in ascending magma.

In this respect, constancy of CO₂ density during the annealing experiments for all minerals is an interesting finding. Regarding olivines, the reduction of internal pressure of the CO₂ inclusions or interaction of the dense dislocations possibly inhibits decrepitation or further volume expansion of the CO₂ inclusions during annealing experiments. However, pyroxenes and spinel show higher and similar CO₂ density, which reflects the resistance to plastic deformation and which indicates the effectiveness of CO₂ inclusions in these minerals as a depth probe for mantle xenoliths.