Preservation of Primary Si Isotope Signatures in Devonian Lahn‐Dill‐Type Iron Ores as Revealed by Femtosecond Laser Ablation

Abstract Silicon isotopic ratios (expressed as δ 30 Si) become established as a powerful tool to decipher the formation processes of quartz deposits throughout the Earth's history. In this study, we established a protocol for matrix‐independent in situ Si isotopic analysis using ultraviolet femtosecond laser ablation (UV fs LA) coupled to multi‐collector inductively coupled plasma mass spectrometry and applied this method to microcrystalline quartz in Lahn‐Dill‐type iron ores. Obtained results on “MPI‐DING” silicate glasses using NIST SRM 610 as the calibration standard agree with excepted values within uncertainty, revealing an external reproducibility of ±0.2‰ (2 SD) for δ 30 Si. Analysis of quartz and quartz‐hematite microdomains in iron ores (Fortuna Mine, Rhenish Massif, Germany) show little differences in average δ 30 Si ranging between −3.66 ±0.97‰ (2 SD) and −3.06 ±0.90‰ (2 SD) but exhibit an overall large variability between −4.56 and −2.04‰ on the micrometer‐scale. Together with detailed petrographic and geochemical investigations (Schmitt et al., 2023, https://doi.org/10.1007/s00126‐023‐01218‐3 ; 2024, https://doi.org/10.1007/s00126‐024‐01307‐x ), we conclude that strong kinetic effects during absorption of seawater Si on Fe‐(oxyhydr)oxides of hydrothermal origin caused very negative and variable δ 30 Si values in quartz precursors, which were preserved during diagenesis. The derived lower limit for Devonian seawater is −0.9‰ by assuming that the less negative δ 30 Si values reflect precipitation closest to equilibrium conditions. This implies a prevalence of volcanogenic‐derived Si in the Rhenohercynian Ocean, a back‐arc basin characterized by high volcanic activity. Our results demonstrate the capability of matrix‐independent Si isotope analysis by UV fs LA to unravel quartz formation processes.