Oxygen stable isotopic analysis of calcite by Raman microprobe spectrometry

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Virginia Tech


The oxygen stable isotopic compositions of minerals give valuable insight into the processes that govern many low- and high-temperature geologic systems. Much research has focused on the ability to analyze the fine-scale oxygen isotopic zonation in many minerals using techniques such as the laser probe and ion microprobe. In this study, I examine the feasibility of making high-resolution, nondestructive oxygen stable isotopic analyses with precision comparable to current microbeam techniques (i.e. <1‱).

Five natural and two synthetic ¹⁸O doped calcites were analyzed using the laser Raman microprobe (LRM) to determine whether the intensity ratio of the C¹⁸O¹⁶O₂²⁻ and C¹⁶O₃²⁻ symmetric stretching bands can be directly related to isotopic compositions measured by ratio mass spectrometry (RMS). The calcites were cooled to near liquid nitrogen temperatures (78-90K) to remove the background from Mn²⁺ fluorescence and minimize the overlap of the symmetric stretching bands. At near liquid nitrogen temperatures, the presence of the C¹⁸O¹⁶O₂²⁻, C¹⁷O¹⁶O₂²⁻, and C¹⁶O₃²⁻ symmetric stretching bands are clearly visible.

Initially, LRM isotopic values showed poor correlation with RMS isotopic values; however, the difference between LRM and RMS isotopic values was shown to be a linear function of the halfwidth of the symmetric stretching band. Corrected LRM isotopic values for all of the natural calcites agree with RMS values to within 3.8‱.

Currently, the precision and accuracy of LRM isotopic measurements on calcite are several times poorer than for RMS measurements. The major limitation on the precision of the measurements is counting statistics. The strong Mn²⁺ fluorescence background of many natural calcites may also be a limiting factor in the precision of LRM isotopic measurements. However, the use of alternate laser excitation wavelengths and analysis at lower temperatures may allow the Mn²⁺ fluorescence background to be eliminated in these calcites.