TY - JOUR

T1 - High spatial resolution analysis of the iron oxidation state in silicate glasses using the electron probe

AU - Hughes, Ery C.

AU - Buse, Ben

AU - Kearns, Stuart L.

AU - Blundy, Jon D.

AU - Kilgour, Geoff

AU - Mader, Heidy M.

AU - Brooker, Richard A.

AU - Balzer, Robert

AU - Botcharnikov, Roman E.

AU - Genova, Danilo Di

AU - Almeev, Renat R.

AU - Riker, Jenny M.

N1 - Funding information: We thank Richard Hinton for his assistance at the NERC ion microprobe facility at the University of Edinburgh, U.K. (IMF560/0515). We thank Priscille Lesne, Charlotte Stamper, Peter Ulmer, and Liz Cottrell for providing samples. E.C.H. is supported by a NERC GW4+ DTP studentship from the Natural Environment Research Council (NE/L002434/1) and is thankful for the support and additional funding from CASE partner GNS Science, New Zealand. G.K. acknowledges support from the New Zealand Strategic Science Investment Fund. H.M.M., R.A.B., and D.D.G. were supported by the NSFGEO-NERC “Quantifying disequilibrium processes in basaltic volcanism” (NE/N018567/1). R.R.A. was supported by the German Science Foundation (DFG project AL1189/6?1). We thank John Donovan and two anonymous reviewers for their helpful comments.

PY - 2018/9/25

Y1 - 2018/9/25

N2 - The iron oxidation state in silicate melts is important for understanding their physical properties, although it is most often used to estimate the oxygen fugacity of magmatic systems. Often high spatial resolution analyses are required, yet the available techniques, such as μrXANES and μMössbauer, require synchrotron access. The flank method is an electron probe technique with the potential to measure Fe oxidation state at high spatial resolution but requires careful method development to reduce errors related to sample damage, especially for hydrous glasses. The intensity ratios derived from measurements on the flanks of FeLα and FeLβ X-rays (FeLβf/FeLαf) over a time interval (time-dependent ratio flank method) can be extrapolated to their initial values at the onset of analysis. We have developed and calibrated this new method using silicate glasses with a wide range of compositions (43-78 wt% SiO2, 0-10 wt% H2O, and 2-18 wt% FeOT, which is all Fe reported as FeO), including 68 glasses with known Fe oxidation state. The Fe oxidation state (Fe2+/FeT) of hydrous (0-4 wt% H2O) basaltic (43-56 wt% SiO2) and peralkaline (70-76 wt% SiO2) glasses with FeOT > 5 wt% can be quantified with a precision of ±0.03 (10 wt% FeOT and 0.5 Fe2+/FeT) and accuracy of ±0.1. We find basaltic and peralkaline glasses each require a different calibration curve and analysis at different spatial resolutions (∼20 and ∼60 μm diameter regions, respectively). A further 49 synthetic glasses were used to investigate the compositional controls on redox changes during electron beam irradiation, where we found that the direction of redox change is sensitive to glass composition. Anhydrous alkali-poor glasses become reduced during analysis, while hydrous and/or alkali-rich glasses become oxidized by the formation of magnetite nanolites identified using Raman spectroscopy. The rate of reduction is controlled by the initial oxidation state, whereas the rate of oxidation is controlled by SiO2, Fe, and H2O content.

AB - The iron oxidation state in silicate melts is important for understanding their physical properties, although it is most often used to estimate the oxygen fugacity of magmatic systems. Often high spatial resolution analyses are required, yet the available techniques, such as μrXANES and μMössbauer, require synchrotron access. The flank method is an electron probe technique with the potential to measure Fe oxidation state at high spatial resolution but requires careful method development to reduce errors related to sample damage, especially for hydrous glasses. The intensity ratios derived from measurements on the flanks of FeLα and FeLβ X-rays (FeLβf/FeLαf) over a time interval (time-dependent ratio flank method) can be extrapolated to their initial values at the onset of analysis. We have developed and calibrated this new method using silicate glasses with a wide range of compositions (43-78 wt% SiO2, 0-10 wt% H2O, and 2-18 wt% FeOT, which is all Fe reported as FeO), including 68 glasses with known Fe oxidation state. The Fe oxidation state (Fe2+/FeT) of hydrous (0-4 wt% H2O) basaltic (43-56 wt% SiO2) and peralkaline (70-76 wt% SiO2) glasses with FeOT > 5 wt% can be quantified with a precision of ±0.03 (10 wt% FeOT and 0.5 Fe2+/FeT) and accuracy of ±0.1. We find basaltic and peralkaline glasses each require a different calibration curve and analysis at different spatial resolutions (∼20 and ∼60 μm diameter regions, respectively). A further 49 synthetic glasses were used to investigate the compositional controls on redox changes during electron beam irradiation, where we found that the direction of redox change is sensitive to glass composition. Anhydrous alkali-poor glasses become reduced during analysis, while hydrous and/or alkali-rich glasses become oxidized by the formation of magnetite nanolites identified using Raman spectroscopy. The rate of reduction is controlled by the initial oxidation state, whereas the rate of oxidation is controlled by SiO2, Fe, and H2O content.

KW - electron beam damage

KW - Electron probe microanalysis (EPMA)

KW - flank method

KW - iron (Fe) oxidation state

KW - oxidation

KW - Raman spectroscopy

KW - reduction

KW - silicate glass

UR - http://www.scopus.com/inward/record.url?scp=85053317275&partnerID=8YFLogxK

U2 - 10.2138/am-2018-6546CCBY

DO - 10.2138/am-2018-6546CCBY

M3 - Article

AN - SCOPUS:85053317275

VL - 103

SP - 1473

EP - 1486

JO - American Mineralogist

JF - American Mineralogist

SN - 0003-004X

IS - 9

ER -