Elastic geobarometry yielding a faithful sublithospheric depth for a ferropericlase inclusion in diamond

Abstract

Diamonds offer a valuable record of mantle geological activity. Residual pressures in inclusions can be used to constrain diamond formation depths, although they are typically underestimated. Here, we show an instance where a ferropericlase inclusion in diamond yields an unexpectedly faithful estimate of diamond growth pressure using simple elastic geobarometry. This example uses recently published and new single-crystal X-ray diffraction data for a ferropericlase inclusion in a sublithospheric or super-deep diamond from the Karowe mine, Botswana. Previously, growth conditions for this diamond were established as ∼23.5 GPa and 1650 ± 200 °C based on the mineral inclusion assemblage and thermodynamic calculations using the FeMg partitioning coefficient, KD, between touching ferropericlase and bridgmanite inclusions. With elastic geobarometry, we used the unit-cell volume measurement for a ferropericlase inclusion within the same diamond to estimate the original pressure it must have been trapped at, giving an independent constraint for the depth of diamond formation. Comparing the measured unit-cell before and after releasing the inclusion from the confinement of the diamond host reveals that it was held at a 5.2(±0.5) GPa residual pressure. Elastic geobarometry constrains its minimum entrapment pressure to 22.1(±2.1) GPa, within the experimental uncertainty of the thermodynamically obtained pressure of diamond formation. Although the effects of plastic deformation and fractures around inclusions often result in variable loss of remnant pressure, especially in super-deep diamonds, this example shows that ferropericlase can sometimes retain high pressures and provide a realistic measure of formation conditions with elastic geobarometry. If future studies encounter ferropericlase with high residual pressure, this simple approach may be a valuable tool to assess formation depth, which is important because ferropericlase is the most common type of inclusion observed in super-deep diamonds.