A XANES calibration for determining the oxidation state of iron in mantle garnet

被引：55

作者：

Berry, Andrew J. ^{[1
]}

Yaxley, Greg M. ^{[2
]}

Woodland, Alan B. ^{[3
]}

Foran, Garry J. ^{[4
,5
]}

机构：

[1] Univ London Imperial Coll Sci Technol & Med, Dept Earth Sci & Engn, Kensington SW7 2AZ, England

[2] Australian Natl Univ, Res Sch Earth Sci, Canberra, ACT 0200, Australia

[3] Goethe Univ Frankfurt, Inst Geowissensch, D-60438 Frankfurt, Germany

[4] Australian Nucl Sci & Technol Org, Australian Synchrotron Res Program, Menai, NSW 2234, Australia

[5] KEK, Australian Natl Beamline Facil, Tsukuba, Ibaraki 3050801, Japan

来源：

CHEMICAL GEOLOGY | 2010年 / 278卷 / 1-2期

基金：

澳大利亚研究理事会;

关键词：

XANES; Garnet; Oxidation state; Iron; Mantle; Oxygen fugacity; K-EDGE; FERRIC IRON; ABSORPTION-SPECTRA; PHASE-RELATIONS; FE; COORDINATION; RESOLUTION; CHEMISTRY; MINERALS; PYROXENE;

D O I：

10.1016/j.chemgeo.2010.08.019

中图分类号：

P3 [地球物理学]; P59 [地球化学];

学科分类号：

070403 [天体物理学]; 070902 [地球化学];

摘要：

Iron K-edge X-ray absorption near edge structure (XANES) spectra were recorded for synthetic garnets of the almandine-skiagite (Fe3Al2Si3O12-Fe3Fe2Si3O12), andradite-skiagite (Ca3Fe2Si3O12-Fe3Fe2Si3O12), Fe-knorringite-skiagite (Fe3Cr2Si3O12-Fe3Fe2Si3O12), and andradite-grossular (Ca3Fe2Si3O12-Ca3Al2Si3O12) solid solutions. Fe3+/Sigma Fe varied systematically in these samples from 0 to 1. The variation in the energy and intensity of spectral features as a function of Fe3+/Sigma Fe was investigated to identify correlations that could be used to determine Fe3+/Sigma Fe of unknowns. The pre-edge energy, which is commonly used for quantification, was found to be relatively insensitive for garnet, particularly at low values of Fe3+/Sigma Fe. The best correlation was for the absorption edge energy, which may provide an accurate and precise method for determining Fe3+/Sigma Fe of garnets in metapelitic rocks. The resulting calibration curve, however, significantly and non-systematically overestimates the Fe3+/Sigma Fe value of mantle garnets (from xenoliths and megacrysts) for which Fe3+/Sigma Fe had been determined previously by Mossbauer spectroscopy. This is probably a result of differences in composition between the synthetic and natural garnets. For the mantle garnets, Fe3+/Sigma Fe is strongly correlated with the intensity ratio of post-edge features at 7138.4 and 7161.7 eV. The deviation of the Mossbauer results from a linear fit is less than 0.01 (1 sigma). This suggests a new method for determining Fe3+/Sigma Fe from XANES spectra that does not require precise energy calibration or spectral fitting. The accuracy and precision for mantle garnets (within the compositional range studied: 0.75 <= Mg/(Mg + Fe) <= 0.86, 3.7-6.3 wt.% CaO, and 03-7.4 wt.% Cr2O3) is comparable to those of Mossbauer spectrosopy, however. XANES spectra can be acquired in similar to 15 min, allowing large numbers of samples to be analysed or the distribution of Fe3+/Sigma Fe to be mapped with micron spatial resolution. (C) 2010 Elsevier B.V. All rights reserved.

引用

页码：31 / 37

页数：7

共 37 条

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Local structure of ferric iron-bearing garnets deduced by IR-spectroscopy [J].