The influence of Fe3+ on garnet–orthopyroxene and garnet–olivine geothermometers

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• 作者：Vladimir Matjuschkin (1) (2)
  Gerhard P. Brey (1)
  Heidi E. H?fer (1)
  Alan B. Woodland (1)
  
• 关键词：Ferric iron ; Oxidation state ; Geothermometry ; Garnet ; Orthopyroxene ; Olivine ; Peridotite
• 刊名：Contributions to Mineralogy and Petrology
• 出版年：2014
• 出版时间：February 2014
• 年：2014
• 卷：167
• 期：2
• 全文大小：587 KB
• 作者单位：Vladimir Matjuschkin (1) (2)
  Gerhard P. Brey (1)
  Heidi E. H?fer (1)
  Alan B. Woodland (1)
  
  1. Institut für Geowissenschaften, Goethe-Universit?t Frankfurt am Main, Altenh?ferallee 1, 60438, Frankfurt am Main, Germany
  2. Department of Geology, School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol, BS8 1RJ, UK
  
• ISSN：1432-0967

文摘

Applying Fe2+–Mg exchange geothermometers to natural samples may lead to incorrect temperature estimates if significant Fe3+ is present. In order to quantify this effect, high-pressure experiments were carried out in a belt apparatus in a natural system close to CFMAS at 5?GPa and 1,100-,400?°C. The oxygen fugacity in the experiments was at or below the Re–ReO2 buffer. This is at significantly more oxidized conditions than in previous experiments, and, as consequence, higher Fe3+/Fe2+ ratios were generated. The Fe3+ content of garnet in the experiments was quantified by electron microprobe using the flank method. Making the usual assumption that Fetotal?=?Fe2+, the two-pyroxene thermometer of Brey and K?hler (J Pet 31:1353-378, 1990) reproduced the experimental temperature to ±35?°C and the garnet–clinopyroxene Fe2+–Mg exchange thermometer of Krogh (Contrib Miner Pet 99:44-8, 1988) overestimated the temperatures on average by only 25?°C. On the other hand, application of the garnet–olivine (O’Neill and Wood in Contrib Miner Pet 70:59-0, 1979) and garnet–orthopyroxene (Harley in Contrib Miner Pet 86:359-73, 1984) exchange geothermometers yielded an underestimation in calculated temperatures of >200?°C. However, making explicit accounting for Fe3+ in garnet (i.e. using only measured Fe2+) leads to a vast improvement in the agreement between calculated and experimental temperatures, generally to within ±70?°C for the garnet–orthopyroxene geothermometer as well as noticeable improvement of calculated temperatures for the garnet–olivine geothermometer. Our results demonstrate that the two-pyroxene and garnet–clinopyroxene thermometers are rather insensitive to the presence of Fe3+ whilst direct accounting of Fe3+ in garnet is essential when applying the garnet–olivine and garnet–orthopyroxene thermometers.