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Archaean chromitites show constant Fe 3+ /ΣFe in Earth's asthenospheric mantle since 3.8 GaRollinson, Hugh; Adetunji, Jacob; Lenaz, Davide; Szilas, Kristoffer; University of Derby; University of Trieste; Geological Survey of Denmark and Greenland (Elsevier, 2017-03-29)Theoretical and planetary studies show that the Earth’s upper mantle is more oxidised than it should be. The mechanism by which this took place and the timing of the oxidation is contested. Here we present new Mössbauer spectroscopy measurements of the ionic ratio Fe3+/(Fe3++Fe2+) in the mineral chromite hosted in mantle-derived melts to show that there is no change in mantle Fe3+/(Fe3++Fe2+) ratio before and after the oxidation of the Earth’s atmosphere at ca. 2.4 Ga and over Earth history from 3.8 Ga to 95 Ma. Our finding supports the view that the oxidation of the asthenospheric mantle was very early and that the oxygenation of the Earth’s atmosphere was not directly coupled to mantle processes.
Determination of Fe3+/ΣFe ratios in chrome spinels using a combined Mössbauer and single-crystal X-ray approach: application to chromitites from the mantle section of the Oman ophioliteLenaz, Davide; Adetunji, Jacob; Rollinson, Hugh; University of Derby (Springer, 2014-01-03)We present the results of a comparative study in which we have measured Fe3+/ΣFe ratios in chromites from mantle chromitites in the Oman ophiolite using Mössbauer spectroscopy and single-crystal X-ray diffraction. We have compared these results with ratios calculated from mineral stoichiometry and find that mineral stoichiometry calculations do not accurately reflect the measured Fe3+/ΣFe ratios. We have identified three groups of samples. The majority preserve Fe3+/ΣFe ratios which are thought to be magmatic, whereas a few samples are highly oxidized and have high Fe3+/ΣFe ratios. There is also a group of partially oxidized samples. The oxidized chromites show anomalously low cell edge (a0) values and their oxygen positional parameters among the lowest ever found for chromites. Site occupancy calculations show that some chromites are non-stoichiometric and contain vacancies in their structure randomly distributed between both the T and M sites. The field relationships suggest that the oxidation of the magmatic chromitites took place in association with a ductile shear zone in mantle harzburgites. Primary magmatic Fe3+/ΣFe ratios measured for the Oman mantle chromitites are between 0.193–0.285 (X-ray data) and 0.164–0.270 (Mössbauer data) and preserve a range of Fe3+/ΣFe ratios which we propose is real and reflects differences in the composition of the magmas parental to the chromitites. The range of values extends from those MORB melts (0.16 ± 0.1) to those for arc basalts (0.22–0.28).
Trevorite: Ni-rich spinel formed by metasomatism and desulfurization processes at Bon Accord, South Africa?O'Driscoll, Brian; Clay, Patricia L.; Cawthorn, R. Grant; Lenaz, Davide; Adetunji, Jacob; Kronz, Andreas; Keele University; University of Manchester; University of the Witwatersrand; Trieste University; et al. (Mineralogical Society, 2014-02-01)The 3.5 Ga Bon Accord Ni deposit occurs within the lowest serpentinized mafic ultramafic lavas of the Barberton Greenstone Belt (South Africa). Though now completely mined out, it comprised a suite of rare Ni-rich minerals that led to its interpretation as either an extraterrestrial body or as an oxidized fragment of Fe-Ni alloy originating from the terrestrial core. In this study, we draw on detailed petrographic observation and mineral chemical data, as well as previous work, to re-evaluate these ideas. The balance of evidence, from thin section (<1 mm) to regional (∼10s of km) scales, appears to support an alternative origin for Bon Accord, possibly as an oxidized Ni-sulfide deposit formed in association with ocean floor komatiite eruptions.