Limits to resolution in composition and density in ultra high-pressure experiments on natural mantle-rock samples

We compressed and analyzed an undepleted natural peridotite—thought to be representative of the Earth’s upper mantle—from ∼25 to 110 GPa and heated to 2000–2500 K. Scatter in zero-pressure unit-cell volumes observed for high-pressure phases synthesized from peridotite, of ±0.4% for orthorhombic perovskite and ±0.8% for magnesiowüstite, can be ascribed to compositional heterogeneity in the natural starting material and its transformation products (calcium perovskite was also observed at high pressures, but does not quench to zero pressure). The scatter is larger than the measurement uncertainties, which are as little as ±0.1%, but is less than would be expected from the variation in composition among distinct minerals if the starting material had failed to equilibrate upon transformation to the high-pressure assemblage (e.g., up to ±0.6–0.8% in volume for orthorhombic perovskite). The equilibrium unit-cell volume of orthorhombic perovskite calculated from an average of electron microprobe analyses on the quenched samples, 163.3 (±0.6) Å3, agrees to within 0.4% with the average value measured from the same samples by X-ray diffraction at zero pressure (163.9±0.7 Å3). Similarly, values of zero-pressure density for the whole-rock assemblage of high-pressure phases are identical (4.13±0.03 g/cm3), whether derived from electron microprobe or X-ray diffraction measurements.