Composition and Pressure Effects on Partitioning of Ferrous Iron in Iron-Rich Lower Mantle Heterogeneities

Susannah M Dorfman,Farhang Nabiei,James Badro,Vitali B Prakapenka,Marco Cantoni,Philippe Gillet,Charles-Edouard Boukaré

doi:10.3390/min11050512

Abstract

Both seismic observations of dense low shear velocity regions and models of magma ocean crystallization and mantle dynamics support enrichment of iron in Earth’s lowermost mantle. Physical properties of iron-rich lower mantle heterogeneities in the modern Earth depend on distribution of iron between coexisting lower mantle phases (Mg,Fe)O magnesiowüstite, (Mg,Fe)SiO3 bridgmanite, and (Mg,Fe)SiO3 post-perovskite. The partitioning of iron between these phases was investigated in synthetic ferrous-iron-rich olivine compositions (Mg0.55Fe0.45)2SiO4 and (Mg0.28Fe0.72)2SiO4 at lower mantle conditions ranging from 33–128 GPa and 1900–3000 K in the laser-heated diamond anvil cell. The resulting phase assemblages were characterized by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy. The exchange coefficient between bridgmanite and magnesiowüstite decreases with pressure and bulk Fe# and increases with temperature. Thermodynamic modeling determines that incorporation and partitioning of iron in bridgmanite are explained well by excess volume associated with Mg-Fe exchange. Partitioning results are used to model compositions and densities of mantle phase assemblages as a function of pressure, FeO-content and SiO2-content. Unlike average mantle compositions, iron-rich compositions in the mantle exhibit negative dependence of density on SiO2-content at all mantle depths, an important finding for interpretation of deep lower mantle structures.

Highlights

Iron-enrichment has been suggested to explain seismic observations of dense regions in the deep lower mantle such as large low shear velocity provinces (LLSVPs) (e.g., [1]) and ultra-low velocity zones (ULVZs) (e.g., [2,3])
Diffraction patterns recorded at heating spot centers after temperature quench exhibited peaks corresponding to bridgmanite, magnesiowüstite, stishovite, and a single peak attributed to CaIrO3 -type post-perovskite (Figure 1, Supplementary Figure S4)
2+ -rich thanThe those explored in previous studies of partitioning in Earth’smore deep Fe lower mantle, and experiments presented here examine substantially compositions provide systematic constraints on effects of compositional differences on partitioning than those explored in previous studies of partitioning in Earth’s deep lower mantle, and in iron-rich

Summary

Introduction

Iron-enrichment has been suggested to explain seismic observations of dense regions in the deep lower mantle such as large low shear velocity provinces (LLSVPs) (e.g., [1]) and ultra-low velocity zones (ULVZs) (e.g., [2,3]). ULVZs may be more dramatically enriched in Fe to generate density differences of 10% or more The origin of these regions is unknown, but possibilities include segregation of dense material in a basal magma ocean beneath the crystallizing early mantle [5], accumulation of dense material from subducted slabs [6], or partial melt due to less refractory composition and/or high local temperatures [7]. Understanding these geophysical observations requires dependence of physical properties 4.0/). The distribution of elements in minerals in the modern mantle is moderated by subsolidus chemical partitioning

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Results

Discussion

Conclusion