The Effect of Pulsed Laser Heating on the Stability of Ferropericlase at High Pressures

Georgios Aprilis,Leonid Dubrovinsky,Natalia Dubrovinskaia,Maxim Bykov,Caterina Melai,Anna Pakhomova,Catherine Mccammon,Saiana Khandarkhaeva,Aleksandr I Chumakov,Egor Koemets,Timofey Fedotenko,Stella Chariton,Elena Bykova,Michael Hanfland

doi:10.3390/min10060542

Abstract

It is widely accepted that the lower mantle consists of mainly three major minerals—ferropericlase, bridgmanite and calcium silicate perovskite. Ferropericlase ((Mg,Fe)O) is the second most abundant of the three, comprising approximately 16–20 wt% of the lower mantle. The stability of ferropericlase at conditions of the lowermost mantle has been highly investigated, with controversial results. Amongst other reasons, the experimental conditions during laser heating (such as duration and achieved temperature) have been suggested as a possible explanation for the discrepancy. In this study, we investigate the effect of pulsed laser heating on the stability of ferropericlase, with a geochemically relevant composition of Mg0.76Fe0.24O (Fp24) at pressure conditions corresponding to the upper part of the lower mantle and at a wide temperature range. We report on the decomposition of Fp24 with the formation of a high-pressure (Mg,Fe)3O4 phase with CaTi2O4-type structure, as well as the dissociation of Fp24 into Fe-rich and Mg-rich phases induced by pulsed laser heating. Our results provide further arguments that the chemical composition of the lower mantle is more complex than initially thought, and that the compositional inhomogeneity is not only a characteristic of the lowermost part, but includes depths as shallow as below the transition zone.

Highlights

The Earth’s lower mantle constitutes more than half of the volume of the planet, from the transition zone at the depth of 660 km to the core-mantle boundary (CMB) at 2900 km [1]
We investigate the effect of pulsed laser heating on the stability of ferropericlase, with a geochemically relevant composition of Mg0.76 Fe0.24 O (Fp24) at pressure conditions corresponding to the depth from 800 km up to 1200 km within the upper part of the lower mantle [23,24], and at a temperature range of 1550 K to 3400 K
While crystal C03 have persisted upon the continuous wave (CW) laser heating experiment, diffraction patterns of crystals

Summary

Introduction

The Earth’s lower mantle constitutes more than half of the volume of the planet, from the transition zone at the depth of 660 km to the core-mantle boundary (CMB) at 2900 km [1]. The expected percentage of iron (expressed as Fe/(Mg + Fe)) in (Mg,Fe)O in the lower mantle is 10–25%, as follows, from studies of Mg-Fe partitioning between bridgmanite and ferropericlase [7,8]. Ferropericlase with such a composition is so far considered to be stable in a NaCl-type (B1) structure Fm3m throughout the lower mantle [9] (in contrast to bridgmanite that is replaced by post-perovskite near the CMB [10]); above ~50 GPa, iron in ferropericlase undergoes a spin crossover from a high-spin to a low-spin state [11]. In contrast to the earlier reports on decomposition, subsequent studies in laser-heated diamond anvil cells (LHDACs) did not observe any segregation between iron and magnesium [9,14,15,16]

Methods

Results

Conclusion