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Ferrous/ferric (Fe2+/Fe3+) partitioning among silicates in metapelites

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Abstract

Fe3+ and XFe3+, defined as Fe3+/(Fe2+ + Fe3+) on a molar basis, are now recognised as key parameters in phase equilibrium modelling. A hindrance is that it is only possible to routinely measure total Fe, and not Fe3+ and Fe2+, in minerals using the electron microprobe. Charge balance techniques can be used to estimate Fe3+ and Fe2+ for some minerals, but not for those that contain vacancies. Whilst other analytical techniques can determine XFe3+ in minerals, these are not commonly applied by metamorphic petrologists. Therefore, researchers must rely on estimates. This study collates wet chemical, Mössbauer spectroscopy, and X-ray absorption near edge structure (XANES) spectroscopy analyses of XFe3+ in metapelitic minerals and rocks from the literature. The resulting database of 77 studies contains 591 samples, of which 261 have XFe3+ determined for the whole rock. There are XFe3+ measurements for 483 biotites, 192 white micas, 78 chlorites, and 32 staurolites. Average (± 1σ) XFe3+ values in whole rock, biotite, white mica, chlorite, and staurolite are 0.23 ± 0.16, 0.11 ± 0.08, 0.55 ± 0.18, 0.08 ± 0.07, and 0.07 ± 0.06, respectively. The average (± 1σ) number of Fe3+ cations in biotite, white mica, chlorite, and staurolite is 0.28 ± 0.19 (22 O + Ti cations per formula unit, pfu), 0.17 ± 0.13 (22 O pfu), 0.31 ± 0.27 (28 O pfu), and 0.20 ± 0.17 (46 O pfu), respectively. The mean whole rock XFe3+ is similar for metapelites containing ilmenite and magnetite, as well as those that report no Fe-oxide, but is considerably higher for hematite-bearing rocks. Whilst there is little variation with pressure and temperature, there is an increase in the number of Fe3+ cations and XFe3+ of both white mica and biotite with the type of Fe-oxide present. Our observations are compared with the predictions of phase equilibrium modelling using thermodynamic dataset 6.2 (Holland and Powell, J Metamorph Geol 29:333–383, 2011) and the solution models of White et al. (J Metamorph Geol 32:261–286, 2014a) for Fe3+ and XFe3+ in these minerals. The predicted XFe3+ and number of Fe3+ cations in biotite, chlorite, and staurolite broadly match natural observations, but for white mica the predicted mean XFe3+ is underestimated by 0.2–0.4 and the number of Fe3+ cations by 0.05–0.2. Whilst modelling correctly predicted increases in the XFe3+ of white mica and biotite with whole rock XFe3+, it also predicted variations in mineral XFe3+ as a function of pressure and temperature which are not observed in the natural samples.

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Acknowledgements

This research represents a portion of Forshaw's doctoral dissertation at the University of Calgary. We acknowledge the painstaking work of petrologists in obtaining wet chemical analyses of minerals prior to the widespread use of the electron microprobe, especially those involving measurement of Fe2+ and Fe3+. Darby Dyar is thanked for providing unpublished data for the West-central Maine region. Benoît Dubacq and Johann Diener are thanked for their insightful and constructive reviews, and Daniela Rubatto for her editorial handling.

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Funding for this work was provided by a Natural Sciences and Engineering Research Council of Canada Discovery Grant (037233) to D.R.M. Pattison.

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Correspondence to Jacob B. Forshaw.

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Communicated by Daniela Rubatto.

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410_2021_1814_MOESM1_ESM.xlsx

Supplementary file1 Online Resource 1 – supplementary-tables.xlsx. Table S1: List of samples in the database including the following information: locality name, abbreviation, country, reference, sample name, metamorphic zone, facies series, Fe-oxide type, mineral assemblage, minerals analysed, and whether whole rock data are available. Table S2: Bulk rock compositions in weight per cent oxide and mole per cent oxide, with calculated XMg (= Mg/(Mg + Fe2+) in moles) and XFe3+ (Fe3+/(Fe3+ + Fe2+) in moles). Table S3: Biotite weight per cent oxide analyses and cations calculated for a 22 O + Ti cations formula unit, with accompanying XMg and XFe3+. Table S4: White mica weight per cent oxide analyses and cations calculated for a 22 O formula unit, with accompanying XMg and XFe3+. Table S5: Chlorite weight per cent oxide analyses and cations calculated for a 28 O formula unit, with accompanying XMg and XFe3+. Table S6: Staurolite weight per cent oxide analyses and cations calculated for a 46 O formula unit, with accompanying XMg and XFe3+. (XLSX 1017 kb)

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Forshaw, J.B., Pattison, D.R.M. Ferrous/ferric (Fe2+/Fe3+) partitioning among silicates in metapelites. Contrib Mineral Petrol 176, 63 (2021). https://doi.org/10.1007/s00410-021-01814-4

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