TY - JOUR
T1 - A re-assessment of the oxidation state of iron in MORB glasses
AU - Berry, Andrew J.
AU - Stewart, Glen A.
AU - O'Neill, Hugh St C.
AU - Mallmann, Guilherme
AU - Mosselmans, J. Fred W.
N1 - Funding Information:
The Smithsonian Institution is thanked for providing the samples of MORB glass and Diamond Light Source for the award of beamtime for proposals SP3731 and SP6689. A.D. Burnham is thanked for the electron microprobe analyses. A.J.B thanks the Australian Research Council for the award of a Future Fellowship ( FT120100766 ). Dante Canil, an anonymous reviewer, and Catherine McCammon, who provided valuable advice on alternative ways of fitting Mössbauer spectra using two 2D Gaussian-distributed doublets, are thanked for their reviews of the work.
Publisher Copyright:
© 2017
PY - 2018/2/1
Y1 - 2018/2/1
N2 - The oxidation state of Fe, Fe/3+ΣFe (where ΣFe=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 Fe/3+ΣFe values of the standards. Our alternative interpretation removes the problem of resolving Fe/3+ΣFe values <∼0.1 in basaltic glasses, produces isomer shift and quadrupole splitting values for Fe3+ that are independent of Fe3+/ΣFe (as is the case for Fe2+), and gives Fe/3+ΣFe values that are consistent with the thermodynamically expected dependence on oxygen fugacity (fO2). Fe/3+Fe2+ is related to fO2 for our synthetic MORB composition by the temperature independent expression ΔQFM=4log(Fe3+/Fe2+)+4.23(5), where ΔQFM is the fO2 in log units relative to the quartz–fayalite–magnetite buffer. The average fO2 of natural MORB was estimated to be QFM+0.1.
AB - The oxidation state of Fe, Fe/3+ΣFe (where ΣFe=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 Fe/3+ΣFe values of the standards. Our alternative interpretation removes the problem of resolving Fe/3+ΣFe values <∼0.1 in basaltic glasses, produces isomer shift and quadrupole splitting values for Fe3+ that are independent of Fe3+/ΣFe (as is the case for Fe2+), and gives Fe/3+ΣFe values that are consistent with the thermodynamically expected dependence on oxygen fugacity (fO2). Fe/3+Fe2+ is related to fO2 for our synthetic MORB composition by the temperature independent expression ΔQFM=4log(Fe3+/Fe2+)+4.23(5), where ΔQFM is the fO2 in log units relative to the quartz–fayalite–magnetite buffer. The average fO2 of natural MORB was estimated to be QFM+0.1.
KW - Fe/Fe
KW - MORB
KW - Mössbauer spectroscopy
KW - oxygen fugacity
KW - XANES spectroscopy
UR - http://www.scopus.com/inward/record.url?scp=85038420468&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2017.11.032
DO - 10.1016/j.epsl.2017.11.032
M3 - Article
AN - SCOPUS:85038420468
SN - 0012-821X
VL - 483
SP - 114
EP - 123
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
ER -