A new phase in the deep lower mantle - Dr. Li Zhang
The Earth’s lower mantle comprises >55% by volume of our planet, extends from 670 to 2900 kilometers in depth. The mantle gets hotter and the materials are squeezed to higher pressures with increasing depth. The pressure and temperature ranges from 237,000 times atmospheric pressure (24 gigapascals) and ~1600 Kevin at the top of the lower mantle and reach 1.3 million times atmospheric pressure (136 gigapascals) and ~2500 K above the coremantle boundary. We actually don’t have any direct access to this depth. Instead, we have two ways to examine the lower mantle: experiments that artificially squeezing minerals to high pressure and simultaneously heating to high temperature to see what they turn into; seismic waves that pass through the Earth, slowing down and speeding up depending on composition and temperature that they interact.
High pressure temperature experiments in the past suggested that the predominant phase was (Mg,Fe)SiO3 with nominally 10 mol% Fe in the orthorhombic perovskite structure until the (Mg,Fe)SiO3 pv transforms to the post-perovskite structure near the D² layer of the lower mantle. Using laser- heated diamond anvil cells, at pressures of 95 to 101 gigapascals and temperatures of 2200 to approximately 2400 Kelvin, the researchers found that such iron-bearing perovskite disassociates into two phases, one an iron-depleted perovskite and a new mineral that is iron-rich and hexagonal in structure, called the H-phase. This discovery fundamentally changes our knowledge of the mineralogical constitution of the lower mantle.
"We still don't fully understand the chemistry of the H-phase," said lead author Li Zhang. "But this finding indicates that all geodynamic models need to be reconsidered to take the H-phase into account. And there could be even more unidentified phases down there in the lower mantle as well, waiting to be identified." The Figure shows a schematic plot of micro-focused synchrotron x-ray probing a sample in a laser-heated diamond anvil cell.
Figure 1. Synchrotron x-ray probes minerals in the deep mantle: Fe-bearing perovskite disproportionates to a nearly Fe-free MgSiO3 perovskite phase and an Fe-rich phase with a hexagonal structure (H-phase).
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