Abstract
Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle are key to understanding the chemical composition and thermal structure of the deep Earth, but their origins have long been debated. Bridgmanite, the most abundant lower-mantle mineral, can incorporate extensive amounts of iron (Fe) with effects on various geophysical properties. Here our high-pressure experiments and ab initio calculations reveal that a ferric-iron-rich bridgmanite coexists with an Fe-poor bridgmanite in the 90 mol% MgSiO3–10 mol% Fe2O3 system, rather than forming a homogeneous single phase. The Fe3+-rich bridgmanite has substantially lower velocities and a higher VP/VS ratio than MgSiO3 bridgmanite under lowermost-mantle conditions. Our modeling shows that the enrichment of Fe3+-rich bridgmanite in a pyrolitic composition can explain the observed features of the LLSVPs. The presence of Fe3+-rich materials within LLSVPs may have profound effects on the deep reservoirs of redox-sensitive elements and their isotopes.
Highlights
Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle are key to understanding the chemical composition and thermal structure of the deep Earth, but their origins have long been debated
In situ XRD measurements coupled with a diamond anvil cell (DAC) show that this Fe-rich Aki phase transforms to Bdg phase at 23.5 ± 1.0 GPa and 300 K (Supplementary Fig. 1)
Combining elastic data from previous studies[20,45,46,47] with our results, we modeled the density and velocity anomalies caused by the presence of Fe3+-rich Bdg relative to the pyrolitic composition, which can effectively reproduce the reference seismic velocities and density of PREM37,38
Summary
Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle are key to understanding the chemical composition and thermal structure of the deep Earth, but their origins have long been debated. A study using Stoneley modes suggested an overall negative density anomaly within LLSVPs, without excluding the possibility of a high-density anomaly within the lowermost LLSVPs15 It is still unknown whether the regional differences in density anomaly are caused by the choice of observations used to constrain density models or reflect the nature of LLSVPs associated with their origins. Dense Fe–Ni–S liquid, for instance, was proposed to explain the LLSVPs28, but the amount of this liquid remaining in the deep mantle, which depends on the drainage of melt to the core[29], is under debate Consistent with both efficient drainage of metallic melt and a primordial origin of the LLSVPs is chemical heterogeneity produced by redox reactions in the magma ocean. The conditions of formation of such Fe3+-rich Bdg in a mantle phase assemblage and its elastic properties at lower-mantle-relevant temperatures are vital to test this hypothesis, but these questions remain unclear
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