Interphase REE Partitioning at the Boundary between the Earth’s Transition Zone and Lower Mantle: Evidence from Experiments and Atomistic Modeling

Anastasia P Tamarova,Nikolay N Eremin,Nina G Zinov’Eva,Takafumi Hirata,Ekaterina I Marchenko,Andrey V Bobrov,Yoshiki Makino,Tetsuo Irifune

doi:10.3390/min10030262

Anastasia P Tamarova, Nikolay N Eremin + Show 6 more

Open Access

https://doi.org/10.3390/min10030262

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Abstract

Trace elements play a significant role in interpretation of different processes in the deep Earth. However, the systematics of interphase rare-earth element (REE) partitioning under the conditions of the uppermost lower mantle are poorly understood. We performed high-pressure experiments to study the phase relations in key solid-phase reactions CaMgSi2O6 = CaSiO3-perovskite + MgSiO3-bridgmanite and (Mg,Fe)2SiO4-ringwoodite = (Mg,Fe)SiO3-bridgmanite + (Mg,Fe)O with addition of 1 wt % of REE oxides. Atomistic modeling was used to obtain more accurate quantitative estimates of the interphase REE partitioning and displayed the ideal model for the high-pressure minerals. HREE (Er, Tm, Yb, and Lu) are mostly accumulated in bridgmanite, while LREE are predominantly redistributed into CaSiO3. On the basis of the results of experiments and atomistic modeling, REE in bridgmanite are clearly divided into two groups (from La to Gd and from Gd to Lu). Interphase REE partition coefficients in solid-state reactions were calculated at 21.5 and 24 GPa for the first time. The new data are applicable for interpretation of the trace-element composition of the lower mantle inclusions in natural diamonds from kimberlite; the experimentally determined effect of pressure on the interphase (bridgmanite/CaSiO3-perovskite) REE partition coefficients can be a potential qualitative geobarometer for mineral inclusions in super-deep diamonds.

Highlights

The data on the systematics of interphase partitioning and mechanisms of incorporation of rare-earth elements (REE) in minerals are of key importance for understanding of the geochemical processes in the Earth’s deep mantle
We revealed a fundamentally different nature of the distribution of LREE and HREE between MgSiO3 and CaSiO3
It can be concluded that HREE (Er, Tm, Yb, and Lu) are

Summary

Introduction

The data on the systematics of interphase partitioning and mechanisms of incorporation of rare-earth elements (REE) in minerals are of key importance for understanding of the geochemical processes in the Earth’s deep mantle. Among the major rock-forming phases of the transition zone are the ringwoodite–arensite solid solution Mg2 SiO4 –Fe2 SiO4 [1] and majoritic garnet (Ca,Mg,Fe) Al2 Si3 O12 –. At a depth of 660 km, ringwoodite decomposes to form bridgmanite (Mg,Fe)SiO3 [3]. Minerals 2020, 10, 262 and ferropericlase (Mg,Fe)O. Majoritic garnet breaks down with the formation of CaSiO3 -perovskite and bridgmanite. The data on REE partitioning between the abovementioned phases and melts are quite limited. The concentration of REE in bridgmanite (Brd) is low with a slight enrichment in HREE.

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