Carbon and oxygen isotope measurement of carbonate molecule in spurrite 〔Ca5Si2O8(CO3)〕

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  • Carbon and oxygen isotope measurement of carbonate molecule in spurrite [Ca<sub>5</sub>Si<sub>2</sub>O<sub>8</sub>(CO<sub>3</sub>)]
  • Carbon and oxygen isotope measurement of carbonate molecule in spurrite Ca5Si2O8 CO3

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Spurrite [Ca5Si2O8(CO3)], a calc-silicate mineral with a carbonate molecule, is a product of high-temperature contact metamorphism, a typical example of which is found at the Fuka contact aureole, Okayama Prefecture, Japan. We report here carbon and oxygen isotope composition of CO2 released from the reaction of spurrite with phosphoric acid in the temperature range of 25°C to 100°C. While reacting with the acid carbon isotopes did not fractionate, while oxygen isotope shows considerable isotope fractionation with varying reaction temperature, similar to other carbonate minerals. This suggests that the carbon and oxygen isotope measurements can be carried out using acid extraction method. In the Fuka contact aureole, carbon isotope fractionation of about 3‰ is observed between coexisting spurrite and coarse-grained calcite. If in equilibrium the carbon isotope fractionation between carbonate molecule in spurrite and calcite has the potential application as an isotope geothermometer. However, oxygen isotope distribution between calcite and spurrite could not be critically evaluated because of the uncertainties of source oxygen isotopes values of infiltrating fluids during the skarn formation and the fractionation of it between silicate and carbonate phases.

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