DIRECT OBSERVATIONS OF MAGNESITE FORMATION FROM MIXED OXIDES AT MANTLE PRESSURES AND TEMPERATURES

We have made in situ observations of magnesite formation from mixed oxides (MgO and CO₂) using laser-heated diamond anvil cells (DACs) at mantle pressures and temperatures. Specifically, we used high-pressure gas-loading techniques to fill DAC sample chambers with liquid CO₂ and powdered MgO to make experimental observations at pressures ranging from 5 to 40 GPa and temperatures between 1,000 to 3,000 K. Synchrotron X-ray diffraction patterns were moderately obscured by the wide range of high-pressure CO₂ polymorphs and their poorly constrained bulk moduli yet, as expected, the magnesite-forming reaction proceeded at all conditions examined. No other pressure-quenchable crystalline materials were observed to form. A large body of previous work has documented the stability of magnesite at elevated pressures and temperatures but, to our knowledge, this is first study to document the formation of magnesite, rather than just the lack of a breakdown reaction, at mantle pressures. Accordingly, this work further strengthens the notion that carbonate phases are indeed the primary storage vehicle for carbon in the deep Earth.