Elsevier

Geochimica et Cosmochimica Acta

Volume 303, 15 June 2021, Pages 110-136
Geochimica et Cosmochimica Acta

The composition and redox state of bridgmanite in the lower mantle as a function of oxygen fugacity

https://doi.org/10.1016/j.gca.2021.02.036Get rights and content

Abstract

The chemistry of bridgmanite (Brg), especially the oxidation state of iron, is important for understanding the physical and chemical properties, as well as putting constraints on the redox state, of the Earth’s lower mantle. To investigate the controls on the chemistry of Brg, the Fe3+ content of Brg was investigated experimentally as a function of composition and oxygen fugacity (fo2) at 25 GPa. The Fe3+/∑Fe ratio of Brg increases with Brg Al content and fo2 and decreases with increasing total Fe content and with temperature. The dependence of the Fe3+/∑Fe ratio on fo2 becomes less steep with increasing Al content. Thermodynamic models were calibrated to describe Brg and ferropericlase (Fp) compositions as well as the inter-site partitioning of trivalent cations in Brg in the Al–Mg–Si–O, Fe–Mg–Si–O and Fe–Al–Mg–Si–O systems. These models are based on equilibria involving Brg components where the equilibrium thermodynamic properties are the main adjustable parameters that are fit to the experimental data. The models reproduce the experimental data over wide ranges of fo2 with a relatively small number of adjustable terms. Mineral compositions for plausible mantle bulk compositions can be calculated from the models as a function of fo2 and can be extrapolated to higher pressures using data on the partial molar volumes of the Brg components. The results show that the exchange of Mg and total Fe (i.e., ferric and ferrous) between Brg and Fp is strongly fo2 dependent, which allows the results of previous studies to be reinterpreted. For a pyrolite bulk composition with an upper mantle bulk oxygen content, the fo2 at the top of the lower mantle is −0.86 log units below the iron–wüstite buffer (IW) with a Brg Fe3+/∑Fe ratio of 0.50 and a bulk rock ratio of 0.28. This requires the formation of 0.7 wt.% Fe–Ni alloy to balance the raised Brg ferric iron content. With increasing pressure, the model predicts a gradual increase in the Fe3+/∑Fe ratio in Brg in contrast to several previous studies, which levels off by 50 GPa. Oxygen vacancies in Brg decrease to practically zero by 40 GPa, potentially influencing elasticity, diffusivity and rheology in the top portion of the lower mantle. The models are also used to explore the fo2 recorded by inclusions in diamonds, which likely crystallized as Brg in the lower mantle, revealing oxygen fugacities which likely preclude the formation of some diamonds directly from carbonates, at least at the top of the lower mantle.

Introduction

The ability to determine the chemical compositions of lower mantle minerals is important not only for understanding transport properties (Xu et al., 1998, Holzapfel et al., 2005, Keppler et al., 2008, Manthilake et al., 2011, Okuda et al., 2019) but also for placing constraints on the redox state and the speciation of volatile elements in the lower mantle and exploring the origin of mineral inclusions in diamonds proposed to originate there (McCammon et al., 1997, McCammon et al., 2004c, Walter et al., 2011, Kaminsky et al., 2015). Furthermore, in order to interpret seismic observations of the lower mantle to potentially obtain constraints on its composition and temperature, we need to understand how this composition may be distributed between mineral phases as a function of depth (Mattern et al., 2005, Bina and Helffrich, 2014, Mao, 1997). The Earth’s lower mantle likely consists dominantly of bridgmanite (Brg) with lesser amounts of ferropericlase (Fp) and CaSiO3 perovskite but the compositions of Brg and Fp can change as a function of depth due to interphase Fe2+–Mg partitioning (Kobayashi et al., 2005, Auzende et al., 2008, Sinmyo et al., 2008, Sakai et al., 2009, Irifune et al., 2010, Lin et al., 2013, Sinmyo and Hirose, 2013) and changes in mineral Fe3+/∑Fe ratios (Prescher et al., 2014, Piet et al., 2016, Shim et al., 2017, Andrault et al., 2018). In addition, the specific mechanism through which trivalent cations such as Fe3+ and Al are accommodated in the Brg structure, may also have implications for mineral, and therefore lower mantle, properties.

There are two cation sites in Brg: the larger A site occupied by Mg and the smaller octahedral B site occupied by Si. While ferrous iron, Fe2+ substitutes for Mg on the A site, trivalent cations M3+ (M3+ = Fe3+ or Al3+) can enter both sites through at least two mechanisms (Andrault et al., 1998, McCammon et al., 1992, Lauterbach et al., 2000). A charge-coupled substitution (CC) can occur, that forms compositions along the MgSiO3–M2O3 join and is described, using Kröger–Vink notation (Kröger and Vink, 1956), byMgMgX+SiSiXMMg+MSi'and also an oxygen vacancy (OV) forming mechanism can occur, that results in compositions along the MgSiO3–MgMO2.5 join, i.e.,2SiSiX2MSi'+VO

The latter mechanism will also depend on the nature of coexisting phases that buffer the silica activity, such as Fp. The oxidation state of Fe in Brg, in addition to its site occupancy along with Al, will have strong effects on properties such as elasticity (Andrault et al., 2001, Andrault et al., 2007, Walter et al., 2004, Saikia et al., 2009, Glazyrin et al., 2014, Mao et al., 2017), rheology (Holzapfel et al., 2005), and electrical (Xu et al., 1998) and thermal conductivity (Goncharov et al., 2009, Goncharov et al., 2010) as well as determining whether iron spin transitions occur in Brg under lower mantle conditions (Catalli et al., 2010, Catalli et al., 2011, Hsu et al., 2011, Lin et al., 2012, Lin et al., 2016) and whether charge disproportionation of Fe2+ may lead to the formation of iron-rich alloy in the lower mantle (Frost et al., 2004). Understanding the controls on the chemistry of Brg is, therefore, an important first step in ultimately modelling the mineral physics, redox state and seismic properties of the lower mantle.

In spite of its importance the Fe3+/∑Fe ratio in Brg at lower mantle conditions is poorly understood. The Brg Fe3+/∑Fe ratio and Fe–Mg partitioning between Brg and Fp as a function of pressure obtained in different diamond anvil cell (DAC) studies are generally in poor agreement (Sinmyo et al., 2011, Prescher et al., 2014, Piet et al., 2016, Shim et al., 2017). Even in multi-anvil studies at uppermost lower mantle conditions, reported Brg Fe3+/∑Fe ratios vary significantly (McCammon, 1997, Lauterbach et al., 2000, Frost and Langenhorst, 2002, Frost et al., 2004, McCammon et al., 2004b, Irifune et al., 2010, Stagno et al., 2011) and while it is qualitatively apparent that these variations are dependent on Brg Al content (Frost et al., 2004, McCammon et al., 2004b), and to some extent on oxygen fugacity (fo2) (McCammon et al., 2004b, Nakajima et al., 2012), there is currently no framework through which to understand or predict these variations. This makes it also very hard to understand the differences observed between higher pressure DAC studies where further complications potentially arise from the occurrence of iron spin crossover transitions involving Fe2+ and Fe3+ (see Lin et al., 2013 for a review). Experiments on Brg at deep lower mantle conditions are extremely challenging and involve inherently large uncertainties in temperature and composition. It is, therefore, essential to have a thermodynamic model based on a rigorous set of experiments performed at well constrained experimental conditions that can be used to assess and interpolate between high-pressure data sets.

By varying the fo2 over wide ranges, tight constraints can be placed on the thermodynamic properties of Fe3+-bearing Brg components. In this study, this has been achieved using a variety of different oxygen buffering assemblages and the effects of varying Al and bulk Fe contents have also been examined. By first deriving thermodynamic equations based on equilibria involving Brg components in simple Fe-free and Al-free systems it was possible to derive a thermodynamic model to describe the composition and site speciation in Fe and Al-bearing Brg as a function of composition and fo2 at 25 GPa and 1973 K. Based on this model, the compositions of phases at the top of the lower mantle and the amount of metal formed through Fe2+ charge disproportionation can be calculated for various bulk compositions. Moreover, using the volumes and equations of state of different Brg components from previous studies, an understanding of how the composition of Brg may change at higher pressures in the lower mantle can be obtained. This can then be used to evaluate the results of DAC studies performed at higher pressure conditions.

Section snippets

Experiments

Five pyroxene compositions (A)–(E) with different Fe and Al contents as shown in Table 1 were prepared from dried oxide mixtures of reagent grade MgO, SiO2, Al2O3 and Fe2O3. To ensure chemical homogeneity, the oxides were first made into glass by grinding the weighed oxide powders together under ethanol, then, after drying, fusing them at 1650 °C followed by rapidly quenching into cold water. The obtained glass (Table 1) was then powdered and cold pressed into pellets and fired in a CO2–CO

Phase assemblages and compositions

Recovered phase assemblages are listed in Table 2 and full chemical analyses of Brg and coexisting phases are given in Table 3. Typical back-scattered electron (BSE) images of run products are shown in Fig. 1. In all experiments, coexisting Brg and Fp were successfully recovered together with the buffering phases which were dispersed throughout the charge. In the synthesis experiments performed with Re + ReO2 or Ru + RuO2 oxygen buffers, both phases were present (Fig. 1c, d). For experiments in

The Al–Mg–Si–O system

A thermodynamic model that aims to reproduce the trivalent cation concentration in Brg needs to take both the CC and OV substitution mechanisms into account. The OV mechanism cannot be overlooked because it appears to be still important in Fe-bearing systems, particularly at low oxygen fugacities, which are likely most typical for the lower mantle. The concentration of OVs is the highest and the most well constrained in the Al–Mg–Si–O system, as determined in recent studies (Fig. 3b), which is,

Summary

Experiments performed at 25 GPa and 1973 K–2373 K allow the relationship between fo2 and Brg Fe3+/∑Fe ratio to be quantitatively described in terms of the Brg Al and total Fe contents in the presence of Fp. Al-free data reveal a steep relationship between fo2 and the Brg Fe3+/∑Fe ratio. This relationship becomes less steep with increasing Brg Al content, which also raises the Brg Fe3+/∑Fe ratio at a constant fo2. There is a modest decrease in Fe3+/∑Fe ratio with total Brg Fe content.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We would like to thank H. Fischer for the machining of multi-anvil assembly parts, R. Njul for sample preparation and D. Krauße and A. Potzel for assistance with EPMA analysis. We are also very grateful for the constructive comments of two anonymous reviewers and the associate editor. This work was supported by DFG grant FR1555/11. The Titan G2 TEM and Scios FIB at BGI were financed by a DFG grant No. INST 91/315-1 FUGG and INST 91/251-1 FUGG, respectively.

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