Fractionation of Stable Oxygen and Clumped Isotope During Acid Digestion of Calcite in the Presence of an External DC Electric Field

Abstract:

Phosphoric acid digestion of carbonate generates CO₂ for stable oxygen and clumped isotope analysis using gas source isotope ratio mass spectrometer. The initial step of digestion reaction is protonation of calcite while the product CO₂ equilibrates with the system allowing further exchange of isotopes to various extents depending on the nature of acid digestion methods. An external electric field is introduced in the break seal method to demonstrate the role of the protonation reaction and the post-digestion isotopic exchanges in the final isotopic composition of product CO₂. Acid digestion experiment following the break seal method was conducted at constant temperature of 25±0.5 °C in presence of a uniform external electric field of 0.5 kV/cm within a specially fabricated corona chamber. Calcite powder of reference standard (MAR J1) is reacted in replicates for 24 hours both in presence and absence of external electric field for varying period of exposure time (6 to 24 hours) and the evolved CO₂ was analyzed with the dual-inlet of a Thermo-Finnigan MAT 253 isotope ratio mass spectrometer. The CO₂ yield from the Phosphoric acid digestion of MAR J1 calcite was 20% lower during the reaction in presence of electric field for an exposure time of 24 hours, while the corresponding δ¹⁸O and Δ₄₇ composition were lighter by 0.3‰ and 0.1‰ respectively compared to the experimental observation without any electric field. We documented systematic control of oxygen and clumped isotope ratio in CO₂ evolved from the carbonate acid digestion reaction with varying exposure time to the external electric field. We provide a new method involving external electric field to manipulate the isotopic fractionation during acid digestion reaction of calcite. The experimental observation enabled theoretical understanding of the reaction mechanism during carbonate with phosphoric acid useful for stable and clumped isotope studies.