Physics of Mantle and Core Minerals

Citation

Jeanloz, Raymond (1980) Physics of Mantle and Core Minerals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/dzss-k041. https://resolver.caltech.edu/CaltechTHESIS:07072023-223914164

Abstract

Shock-wave equation-of-state (Hugoniot) data for initially porous and nonporous samples of iron provide experimental support for theoretically calculated properties of the earth's core, and show that whereas both densities and bulk moduli in the outer core are less than those of Fe under equivalent conditions (by about 10% and 12% respectively) their gradients with pressure are consistent with gross chemical homogeneity through the outer core; simple dynamic models of the core are allowed. New Hugoniot data for wüstite show that oxygen (~11 wt. %) can be the element which reduces the density of the outer core below that of Fe. The properties of the inner core are consistent with those of iron, suggesting that the inner core-outer core boundary is both a phase and a compositional boundary. The minimum estimated temperature at the top of the outer core is ~2800K, whereas subsolidus phase equilibria of olivine indicate a temperature near 2000K in the transition zone. Hugoniot data for porous and nonporous MgO and SiO₂ (phases considered representative of the lower mantle) provide an experimentally­ constrained (adiabatic) geothermal gradient through the lower mantle which implies the presence of one or (for a more consistent result) more thermal boundary layers in the lower mantle. These suggest that the core is a major heat source for the mantle and that a barrier to convection occurs in (or near the top of) the lower mantle: a chemical discontinuity would be a likely cause. This inference is consistent with new shock­ wave data for Cao which show that calcium could be substantially enriched in the lower mantle, as suggested by inhomogeneous accretion theories. A thermal equation of state is determined for anorthite from porous and nonporous Hugoniot data which, however, show that this refractory mineral can probably not be a major Ca-bearing phase in the lower mantle, except perhaps near the core-mantle boundary. Diamond-cell and Hugoniot data show that CaO undergoes a B1/B2 transition at 70 GPa with properties well predicted theoretically. FeO undergoes a similar transition (at ~70 GPa) and these results suggest that transformation in magnesiowüstite may be important in the lowermost mantle. New Hugoniot data for bronzite are combined with previous shock-wave measurements for olivines and pyroxenes. These data are consistent with static high­ pressure results, but suggest the occurrence of post-perovskite phases (density ~5% greater than perovskite) and they also provide evidence of nonequilibrium effects under shock to pressures above 100 GPa. Spectroscopic and microscopic studies of shock-compressed olivines support this evidence: the structure of olivine achieved under shock is apparently far from equilibrium, as is indicated by phase-transformation theory. Although the bulk properties measured under shock are consistent with the attainment of thermodynamic equilibrium, these properties apparently represent highly transient and nonequilibrium states.

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