The temperature dependence of Raman intensity of aqueous species (SO42− and H3PO40): Implication for in situ fluid composition investigation at elevated temperature

Abstract

Raman spectroscopy is a crucial technology in geoscience and often employed in in situ quantification experiments to investigate fluid compositions at elevated temperatures. Raman spectroscopic quantification is fundamental to understanding the hydrothermal behaviors of aqueous species, such as SO42− and H3PO40. Considering that SO42− and PO43− are closely associated with metal transportation and deposition, investigating the hydrothermal behaviors of these species, via Raman spectroscopic quantification, can give an assistance for the comprehension of relative ore-forming processes. During Raman spectroscopic quantification, the accuracy of the calibration coefficient (k), which bridges the concentration of a species (Cspecies) and its Raman peak area (Anormalized) in the form of Cspecies = k · Anormalized, is the key to accurate measurement of fluid composition. For high-temperature experiments, quantification of fluid composition is complicated because the coefficient k varies with varying temperatures. In this study, the temperature dependence of the calibration coefficients is retrieved, based on experiments on solutions containing SO42− and H3PO40 using the fused silica capillary capsules. The results show a well-established linear relationship between the k and 1/T (absolute temperature in Kelvin) for both SO42− and H3PO40. The reported linear k − 1/T relationship simplifies Raman-based fluid composition investigation at elevated temperatures: the k values throughout the whole temperature range of interest can be retrieved by measuring the room-temperature Raman intensity of a standard sample (i.e., with known composition) and one single data point at high temperature. We speculate that this linear k − 1/T relationship method may be established for other species (e.g., carbonate ions) and can be applied for experiments using other technics (e.g., hydrothermal diamond anvil cell) or retrieving the compositions of natural or synthetic fluid inclusions. For example, the empirical linear relationship can facilitate the measurements of gas concentrations (e.g., CO2) in natural fluid inclusions.