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A re-assessment of the oxidation state of iron in MORB glasses

DOI: 10.1016/j.epsl.2017.11.032 DOI Help

Authors: Andrew J. Berry (Australian National University; Imperial College London) , Glen A. Stewart (UNSW Canberra at the Australian Defence Force Academy) , Hugh St.c. O'neill (Australian National University) , Guilherme Mallmann (Australian National University) , J. Fred W. Mosselmans (Diamond Light Source)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Earth And Planetary Science Letters , VOL 483 , PAGES 114 - 123

State: Published (Approved)
Published: February 2018
Diamond Proposal Number(s): 3731 , 6689

Abstract: The oxidation state of Fe, Fe3+/Fe (where Fe=Fe2++Fe3+), in glass samples of mid-ocean ridge basalt (MORB), from a wide range of localities, was determined by XANES spectroscopy to be 0.10(2) (n =42). This value is lower than that reported previously by XANES, 0.16(1)(n =103), but consistent with the most recent value determined by redox titrations, 0.11(2)(n =104), all for similar sets of samples. We attribute the anomalously high XANES value of 0.16 to a calibration error resulting from the interpretation of Mössbauer spectra and the resulting Fe3+/Fe values of the standards. Our alternative interpretation removes the problem of resolving Fe3+/Fe values <∼0.1 in basaltic glasses, produces isomer shift and quadrupole splitting values for Fe3+that are independent of Fe3+/Fe (as is the case for Fe2+), and gives Fe3+/Fe values that are consistent with the thermodynamically expected dependence on oxygen fugacity (fO2). Fe3+/Fe2+is related to fO2for our synthetic MORB composition by the temperature independent expression QFM=4 log(Fe3+/Fe2+) +4.23(5), where QFM is the fO2in log units relative to the quartz–fayalite–magnetite buffer. The average fO2of natural MORB was estimated to be QFM+0.1.

Journal Keywords: Mössbauer spectroscopy; XANES spectroscopy; MORB; oxygen fugacity; Fe3+/Fe2+

Subject Areas: Earth Science, Technique Development

Instruments: I18-Microfocus Spectroscopy