Abstract
We present a new approach to determine in situ CO2 and H2O concentrations in apatite via attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). Absolute carbon and hydrogen measurements by nuclear reaction analysis (NRA) and elastic recoil detection (ERD) are used to calibrate ATR-FTIR spectra of CO2 and H2O in apatite. We show that CO2 and H2O contents in apatite can be determined via linear equations (r2 > 0.99) using the integrated area of CO2 and H2O IR absorption bands. The main benefits of this new approach are that ATR-FTIR analyses are non-destructive and can be conducted on polished sample material surfaces with a spatial resolution of ~ 35 μm. Furthermore, the wavenumber of the phosphate IR absorption band can be used to determine the crystallographic orientation of apatite, which allows for accurate quantification of CO2 and H2O in randomly orientated apatite grains. The limit of quantification of H2O in apatite is ~ 400 ppm and ~ 100 ppm for CO2. Via two examples, one from a carbonatite and one from a metasedimentary rock, we show that this new technique opens up new possibilities for determining volatile concentrations and behavior in a wide range of hydrothermal, igneous, and metamorphic systems.
Highlights
Volatiles, especially C O2 and H 2O, are key agents for igneous and metamorphic processes that impact the Earth’s atmosphere and climate over millions of years
Our study presents a new, effective and broadly applicable approach to accurately measure C O2 and H 2O in apatite via attenuated total reflection (ATR)-Fourier transform infrared spectroscopy (FTIR), contributing to the further development of using C O2 and H2O concentrations in apatite as a tool to estimate the concentration of these volatiles in igneous, hydrothermal, and metamorphic systems
The results show that the H 2O content in apatite is relatively uniform at ~ 0.30–0.33 wt%, which suggests that apatite attempted to reach equilibrium in the presence of a fluid at ~ 620 °C, which agrees with the findings of Hammerli et al (2014). CO2 contents in 19JH-03 apatite vary between ~ 100 and ~ 600 ppm (Fig. 13C) and in light of the uniform H2O content, variability of CO2 concentrations likely suggests variable aCO2 in metamorphic fluids on a cm-scale
Summary
Especially C O2 and H 2O, are key agents for igneous and metamorphic processes that impact the Earth’s atmosphere and climate over millions of years. They affect mineral stability and metamorphic reactions, and are, directly and indirectly controlling mass transfer (e.g., Phillips and Evans 2004; Williams-Jones and Heinrich 2005; Berkesi et al 2012) as well as the melting and differentiation of the Earth’s crust (e.g., Rubatto et al 2009; see Weinberg and Hasalova 2015 for a review). Given the fugitive behavior of CO2 and H2O, measuring and quantifying these components in igneous, hydrothermal, and metamorphic systems is challenging. While H2O is readily incorporated as a major or a trace component in many common igneous and metamorphic minerals, very few of them can incorporate both of these volatile components in their crystal structure to -detectable concentrations (well above the ppm level)
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