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

Abstract

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 90 GPa and 2,400 K. 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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Figure 1: Pressure versus volume of bridgmanite at room temperature, with corresponding Birch–Murnaghan equation of state curves.
Figure 2: Elemental mapping of Mg, Si, Fe, Al and Ga of quenched sample cross sections.
Figure 3: Density of Bm at ambient conditions as a function of Fe content.
Figure 4: Computed assemblage density, S-wave speed (VS), and P-wave speed (VP) as computed by BurnMan.
Figure 5: Process of segregation between reduced and oxidized material caused by intrinsic density differences.
Figure 6: Fraction of oxidized (orange, dashed) and reduced (blue, solid) material entrained into the upper mantle for four different models.

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Acknowledgements

We thank R. Weber and S. Tumber for glass synthesis; Z. Du, A. McNamara and N. J. Planavsky for discussions; G. Amulele, J. Eckert, Z. Jiang, M. Rooks, F. Camino and Y. Yang for technical support. We thank the staff and beamline scientists at GSECARS and HPCAT—in particular, V. Prakapenka, C. Prescher and S. Tkachev. We thank L. Zhang and H. K. Mao for sharing beam time. This work was funded by an NSF CAREER grant to K.K.M.L. (EAR-0955824). M.L. is supported by the NSF grant EAR-1338810. We thank CIDER 2014 for providing this opportunity for multi-disciplinary collaboration (NSF FESD grant 1135452). FIB use was supported by YINQE (NSF MRSEC DMR 1119826) and by the Center for Functional Nanomaterials, Brookhaven National Laboratory (US DOE-BES under Contract No. DE-AC02-98CH10886). EPMA was funded by the NSF (EAR-0744154) and Yale University. Portions of this work were performed at GSECARS (NSF EAR-1128799, DOE DE-FG02-94ER14466, and NSF EAR 11-57758 for gas loading system), and HPCAT (DE-NA0001974, DE-FG02-99ER45775, with partial instrumentation funding by NSF). This research used resources of the APS, a US DOE Office of Science User Facility operated for the DOE Office of Science by ANL under Contract No. DE-AC02-06CH11357.

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T.G. and K.K.M.L. designed the experiments and conducted the analysis; T.G. performed the experiments, and designed geodynamical models with M.L., who also performed the geodynamical simulations. C.M. performed the Mössbauer analyses and interpretation. All authors contributed in the writing of the manuscript. K.K.M.L. supervised the project.

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Correspondence to Tingting Gu or Mingming Li.

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Gu, T., Li, M., McCammon, C. et al. Redox-induced lower mantle density contrast and effect on mantle structure and primitive oxygen. Nature Geosci 9, 723–727 (2016). https://doi.org/10.1038/ngeo2772

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