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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 ophioliteWe 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).