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 δ³⁰Si) 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 δ³⁰Si. Analysis of quartz and quartz-hematite microdomains in iron ores (Fortuna Mine, Rhenish Massif, Germany) show little differences in average δ³⁰Si 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 δ³⁰Si 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 δ³⁰Si 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.