Redox-induced lower mantle density contrast and effect on mantle structure and primitive oxygen

Posted: Wed, 2016-08-03 13:42

Tingting Gu, MingMing Li, Catherine McCammon and Kanani M. Lee

The mantle comprises nearly three-quarters of Earth’s volume and through convection connects the deep interior with the lithosphere and atmosphere. The composition of the mantle determines volcanic emissions, which are intimately linked to evolution of the primitive atmosphere. Fundamental questions remain on how and when the proto-Earth mantle became oxidized, and whether redox state is homogeneous or developed large-scale structures. Here we present experiments in which we subjected two synthetic samples of nearly identical composition that are representative of the lower mantle (enstatite chondrite), but synthesized under different oxygen fugacities, to pressures and temperatures up to 90GPa and 2,400K. In addition to the mineral bridgmanite, compression of the more reduced material also produced Al2O3 as a separate phase, and the resulting assemblage is about 1 to 1.5% denser than in experiments with the more oxidized material. Our geodynamic simulations suggest that such a density difference can cause a rapid ascent and accumulation of oxidized material in the upper mantle, with descent of the denser reduced material to the core–mantle boundary. We suggest that the resulting heterogeneous redox conditions in Earth’s interior can contribute to the large low-shear velocity provinces in the lower mantle and the evolution of atmospheric oxygen.

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