Comment on: Carbonaceous matter in ∼ 3.5 Ga black bedded barite from the Dresser Formation (Pilbara Craton, Western Australia) – Insights into organic cycling on the juvenile Earth

Abstract

Weimann et al. (2024) offer important insights into carbonaceous matter (CM) preserved within bedded barite from the 3.48 billion-year-old Dresser Formation (Pilbara Craton). However, issues in the fitting of CM Raman spectra and the application of Raman CM thermometry cast doubts on the conclusion that 'CM in quartz veins yielded much lower temperatures of ∼ 220 °C, suggesting that quartz-vein associated CM entered the barite after 3.3 Ga' (Weimann et al., 2024). A primary concern is the inaccurate fitting of the CM's D1 band, whose spectral position (typically in the c. 1335–1355 cm⁻¹ range) and full width at half maximum (FWHM) are indicators of thermal maturity (Kouketsu et al., 2014). This commentary emphasizes the implications of misfitting D1 and other issues in fitting the broader CM signals between 1100 and 1700 cm⁻¹. Specifically, the omission of D4 (c. 1250 cm⁻¹) resulted in a systematic overestimation of D1-FWHM and, consequently, unreasonably low thermal maturity estimates. We show that a more precise fitting including D4 – recognizing that CM spectra with a thermal maturity below 300 °C generally display a corresponding shoulder at c. 1250 cm⁻¹ – yields much higher thermal maturity estimates that appear to align closely with the peak temperature (c. 290 °C) of the Dresser Formation's 3.3 Ga metamorphic event. This suggests a (near) syngenetic origin for the quartz-vein-hosted CM, challenging the notion of post-metamorphic infiltration. Consequently, these results also question Weimann et al.'s (2024) interpretation that the much higher thermal maturity (c. 350 °C) of a distinct CM generation, situated within the barite crystal’s growth zonations, stems from metamorphic overprinting. Instead, it can be reconciled with the thermal maturation of the CM during its transportation within the ancient Dresser hydrothermal system, prior to its concentration within the barite.