Quantitative in situ Raman analysis of formate salt solutions at 25-350 °C and 0.1-40 MPa: implications for formate...

Formate (HCOO^-) is widely detected in diverse high-temperature geological environments, including hydrothermal vents and the deep lithosphere, where it plays a crucial role in supporting microbial communities. However, the thermal stability of formate remains poorly constrained, and a robust in situ Raman quantitative model for determining HCOO^- concentrations across a range of temperatures and pressures is lacking. In this study, we developed a Raman quantitative model for formate applicable at 25-350 °C and 0.1-40 MPa. In situ formate concentrations were calibrated using the linear relationship A_Formate/A_H2O = 0.0095C_Formate (R² = 0.999). Using this model, we investigated the thermal stability of 50-300 mmol/kg sodium formate (NaHCOO) solutions under conditions of 25-350 °C and 0.1-40 MPa. In situ results demonstrate that NaHCOO solutions remain stable at temperatures of ≤300 °C, but decompose primarily to H₂ and CO₂ production above 350 °C. Particularly, formate decomposition rates at 350 °C were several orders of magnitude slower than previously reported. This study indicates that abiotic formate is likely more widespread in diverse geological settings than previously recognized and serves as a significant hydrogen source within the deep lithosphere.