The Oxidation State Of Iron In Chromite As A Record Of Deep Earth Processes

Abstract:

Recent work on podiform chromitite from the Luobusa massif, Tibet, suggests that a lithospheric slab containing the chromitite was driven into the Transition Zone (>400 km) after primary crystallisation at shallow depth [1]. Exsolution of coesite and pyroxenes from chromite is believed to reflect conversion from the spinel structure to the ultra-high pressure (UHP) calcium ferrite (CF) polymorph below 400 km [2]. Experimental studies report that UHP polymorphs can have a high affinity for Fe³⁺, leading to disproportionation of Fe²⁺ to Fe³⁺ + Fe⁰ [3], despite low fO₂ as evidenced by inclusions of diamond, native metals and alloys, moissanite, and Cr²⁺-bearing chromite. Luobusa chromitites may be the first natural example of this phenomenon; one study reported a massive chromitite with higher Fe³⁺/∑Fe than nodular or disseminated varieties with lower modal chromite [4]. The absence of indicators of oxidation implies that the high Fe³⁺/∑Fe was not produced by formation or alteration of chromite at Earth’s surface. A study using samples from Luobusa and the low-pressure Antalya Complex, Turkey was carried out to investigate relationships between pressure, fO₂ and Fe³⁺/∑Fe. In the first application of μ-X-ray absorption near edge structure (μ-XANES) spectroscopy to measure chromite Fe³⁺/∑Fe, we constructed calibration curves for the pre-edge centroid and main edge maximum features using Fe³⁺/∑Fe (from Mössbauer spectroscopy) in synthetic and natural spinels. Pre-edge results show that Fe³⁺/∑Fe increases with vol.% chromite in chromitites from Luobusa, but not from Antalya (fig. 1). High Fe³⁺/∑Fe thus appears to be a consequence of crystallographic stabilisation of Fe³⁺ in the UHP polymorph stable below 400 km, despite low-fO₂ conditions. The rapid upwelling of the Luobusa chromitite to the uppermost mantle (<10 Ma) has preserved the high Fe³⁺/∑Fe in samples where re-equilibration with olivine was limited.

[1] McGowan et al., Geology (2015) 43, 179–182; [2] Chen et al., PNAS (2003) 100, 14651–14654; [3] McCammon, Science (2005) 308, 807-808; [4] Ruskov et al., J. Metamorphic Geol. (2010) 28 551–560.