Effects of Iron Enrichment on Chemistry and Physical Properties of Deep Lower Mantle Silicates

Abstract

Variations in seismic wave speed and density in the Earth's deep lower mantle have been linked to chemical heterogeneities. In order to identify the compositions of these regions and determine their roles in Earth history and dynamics, experimental measurements are needed of the effects of compositional variation, particularly major elements Fe and Al, on phase equilibria and physical properties of mantle minerals. The experiments that comprise this dissertation provide new constraints on the chemistry and compressibility of mantle silicates.

Experiments were conducted at mantle pressure-temperature conditions using the laser-heated diamond anvil cell. Determination of pressure in the diamond anvil cell requires internal pressure calibrants which suffer from uncertainty as high as 10\% at Mbar pressures. A series of experiments were performed to test the reliability and agreement of pressure scales for Au, Mo, MgO, NaCl B2, Ne and Pt. These data were used to determine a new comprehensive pressure scale for use in experiments on mantle materials.

The lower mantle's dominant phase is (Mg,Fe,Al)(Fe,Al,Si)O$_3$ perov\-skite. At pressure-temperature conditions comparable to the deep lower mantle, perovskite undergoes a transition to a post-perovskite phase. I synthesized perovskites and post-perovskites from a series of Fe-rich (enstatite--ferrosilite, (Mg$_{1-x}$,Fe$_x$)SiO$_3$, $0<x<74$) and Fe,Al-rich (pyrope--almandine, (Mg$_{1-x}$,Fe$_x$)$_3$Al$_2$Si$_3$O$_{12}$, $0<x<100$) compositions. These experiments have shown that as much as 75\% FeSiO$_3$ is soluble in perovskite at 70--80 GPa. Fe was observed to lower and broaden the pressure range of the post-perovskite transition. Volume data were collected over a range of pressures for all compositions to constrain the effects of Fe and Al on the equations of state of these phases. Fe and Al incorporation were observed to increase the unit cell volume of perovskite but have a weak effect on its compressibility.

The electronic behavior of Fe in perovskite is complex due to multiple possible valence and spin states. Synchrotron M\"{o}ssbauer spectroscopy was used to determine the electronic states of Fe in almandine-composition perovskite and glass at pressures up to 180 GPa. Unlike some previous studies, no evidence was observed for disproportionation of Fe$^{2+}$ to Fe$^{3+}$ and Fe metal. However, multiple structural sites and/or spin states were observed.

Based on equation of state measurements of Fe and Fe,Al-rich perovskite and post-perovskite, I modeled the effects of composition on observable properties including density and seismic velocity. Experimental observations and density functional theory calculations for seismic properties of the perovskite phase as a function of Fe content are highly consistent. However, the properties of the post-perovskite phase are more poorly constrained. The systematic analysis presented in this work allows us to constrain the compositions of observed heterogeneities based on their densities. Large low shear velocity provinces and ultra-low velocity zones may be consistent with Fe-enrichment to Mg\#78--88 and $<$50, respectively.