Water Partitioning at the Base of the Transition Zone: No Need for a Lower Mantle Water Filter?

Abstract:

Recent measurements of water in ringwoodite inclusions found in a deep diamond suggests that the diamond formed in a part of the transition zone containing ~1 wt% water. Experimental and computational results consistently show a significantly lower water storage capacity in MgSiO₃ perovskite, the dominant mineral of the Earth’s lower mantle. This disparity in water solubility predicts widespread melting at the base of the transition zone in mantle downwellings or some other process to create a transition zone “water filter.”

We present the results of ab-initio calculations on the hydrogen incorporation in ringwoodite, majorite, ilmenite, calcium silicate perovskite, and magnesium silicate perovskite. We demonstrate a multiplicity of OH defect mechanisms at the base of the transition zone, including vacancies on the Mg and Si sites, as well as coupled substitutions with aluminum. We calculate the partitioning of water between each phase. While the partitioning of water between ringwoodite and MgSiO₃ is >100, we find that partitioning of water between CaSiO₃-perovskite and ringwoodite exceeds unity under the conditions of cold downwellings at the base of the transition zone, suggesting that a significant fraction of the water can be stored in minor phases of the lower mantle instead of requiring a filter at the base of the transition zone. This finding significantly relaxes the constraints on the Earth’s total water budget and suggests a mechanism for sequestering deep water through subduction.