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Raman spectroscopy offers new insights into the CO2 contents of magmas

A new calibration for micro-Raman spectroscopy paves the way for easy and accurate quantification of CO2 dissolved in volcanic glasses

CO2 is an important volcanic volatile. It is commonly the second most abundant dissolved gaseous species in a molten rock (after H2O) and it can have a dramatic effect on the phase relations and rheology of degassing magmas. The release of CO2 dissolved in magmas is also a vital part of the global carbon cycle. Thus, there has been considerable experimental effort dedicated to measuring CO2 solubility in silicate melts.

Raman laser

Laser path of the micro-Raman spectrometer in the School of Earth Sciences at the University of Bristol

Raman spectroscopy is a non-destructive spectroscopic technique that harnesses the scattering of light to provide information about the molecular structure of sample, e.g., CO2 content. Micro-Raman has advantages over other comparable techniques because it can analyse <10 μm spot sizes and it requires relatively minimal sample preparation; however, the analysis requires a compositionally dependent calibration.

To this end, Morizet and co-authors present a new calibration for the quantification of CO2 in geologically relevant glass compositions by micro-Raman. The study collected micro-Raman CO2 data for an extensive database of synthetic and natural samples, whose CO2 content had previously been quantified by bulk analysis, and found a relationship between the spectral features in the high-frequency region of aluminosilicate glasses and the spectral peak associated with dissolved carbonate. This new calibration is found to be accurate to better than ±0.4 wt% CO2.

Morizet Y, Brooker RA, Iacono-Marziano G, & Kjarsgaard BA (2013) Quantification of dissolved CO2 in silicate glasses using micro-Raman spectroscopy. American Mineralogist, 98(10), http://dx.doi.org/10.2138/am.2013.4516

SCIENTIFIC ABSTRACT

This study investigates the potential use of confocal micro-Raman spectroscopy for the quantification of CO2 in geologically relevant glass compositions. A calibration is developed using a wide range of both natural and synthetic glasses that have CO2 dissolved as carbonate (CO32−) in the concentration range from 0.2 to 16 wt%. Spectra were acquired in the 200 and 1350 cm−1 frequency region that includes the ν1 Raman active vibration for carbonate at 1062–1092 cm−1 and the intensity of this peak is compared to various other peaks representing the aluminosilicate glass structure. The most precise and accurate calibration is found when carbonate peaks are compared to aluminosilicate spectral features in the high-frequency region (HF: 700–1200 cm−1), which can be simulated with several Gaussian peaks, directly related to different structural species in the glass. In some samples the “dissolved” CO32− appears to have two different Raman bands, one sharper than the other. This may be consistent with previous suggestions that CO32− has several structural environments in the glass, and is not related to any precipitation of crystalline carbonate from the melt during quenching. The calibration derived using the HF peaks appears linear for both the full range of glass composition considered and the range of CO2 concentrations, even when multiple carbonate peaks are involved. We propose the following, compositionally independent linear equation to quantify the CO2 content in glass with micro-Raman spectroscopy Formula

where CO3/HF is the area ratio of the fitted ν1 carbonate peak(s) at 1062–1092 cm−1 to the remaining area of the fitted aluminosilicate envelope from 700–1200 cm−1. This is similar to the Raman calibration developed for water, but is complicated by the overlapping of these two fitted components. Using error propagation, we propose the calibration accuracy is better than ±0.4 wt% CO2 for our data set.

The ν1 Raman peak position for carbonate is not constant and appears to be correlated with the density of the melt (or glass) in some way rather than the chemical composition.

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