Petrologie und Geochemie

Ferric iron in majorite at conditions of Earth's upper mantle as a function of oxygen fugacity, pressure, temperature and bulk composition

Christopher Beyer

Collaboration: Robert Myhill (University of Bristol), Katharina Marquardt (BGI Bayreuth)

Funding Support: DFG Core Facility (BGI Bayreuth).
 

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Figure 1: Mineral fraction as a function of pressure of a tholeiite. Modified after Irifune et al. (1986). Cpx: clinopyroxene; Grt: garnet-majorite solid-solutions; St: stishovite

Garnet transforms to majorite (Mg3(MgSi)Si3O12) with increasing depth in Earth’s mantle. At pressures, corresponding to the Earth’s transition zone (14 – 23 GPa) majorite is one of the most abundant phases (Figure 1). Majorite is composed of different end-members, depending on the coexisting pyroxene, which is incorporated into the garnet structure (e.g. jadeite -> Na-substitution, enstatite -> Mg-substitution). Breaking down the effect of pressure, temperature, fO2 and end-member composition is crucial to setup a model that describes the oxygen distribution in Earth’s upper mantle. The model will be helpful to reconstruct the redox conditions of majoritic inclusions in diamonds and in mantle xenoliths. This has far reaching consequences for the speciation of carbon and the onset of incipient melting.

 

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Figure 2: Backscattered electron image of a recovered experiment in a Re capsule. The starting material was a synthetic basalt. Run conditions: 14 GPa, 1800 °C, Re-capsule with ReO2 in a sealed Pt-capsule.

3+/ΣFe ratios in high pressure experiments is challenging due to the small crystal size and difficult to obtain equilibrium. In this study we do multi capsule experiments with different metal – metal oxide buffers (Fe-saturated, Mo-MoO2, Re-ReO2) to conduct experiments with basaltic and peridotitic bulk composition simultaneously to identify the parameters influencing Fe3+-partitioning (Figure 2).

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Figure 3: Fe-edge energy loss spectra of peridotitic (KLB-1) and basaltic (MORB) majorite.