A METHOD FOR SEPARATION AND ISOTOPIC ANALYSIS OF PRIMARY AND SECONDARY CARBONATE MINERALS FOUND IN ULTRAMAFIC MINE TAILINGS

Atmospheric carbon dioxide may react with ultramafic minerals found in mine tailings to form secondary magnesium carbonate precipitates (e.g. hydromagnesite, dypingite; see Wilson et al.; Dipple et al., this session), thus sequestering CO₂. These carbonate minerals form as complex, intergrown mixtures of primary (bedrock, e.g. magnesite) and secondary (e.g. nesquehonite, dypingite, hydromagnesite) minerals. We have developed a sequential extraction method using weak acids to separate and analyze carbon from different carbonate minerals. Powdered mine tailings containing both primary and secondary carbonate minerals are weighed into BD® Vacutainer tubes, which are then evacuated to a pressure of <10^-5 mbar. Either 85% or 99% H₃PO₄ is injected into the tubes using a gas tight needle, and is allowed to react with samples at lab temperature (~ 25°C) for 20 minutes. The resulting CO₂ is cryogenically purified and trapped in glass ampoules for subsequent stable isotope (δ¹³C and δ¹⁸O) or radiocarbon (¹⁴C) analysis. The remaining mixture of carbonate minerals and H₃PO₄ is evacuated to a pressure of <10^-4 mbar and heated at 70°C for 2 hours, after which the CO₂ released is cryogenically purified. Experiments utilizing weighed mixtures of magnesite and hydromagnesite indicate that this technique allows essentially 100% separation of these minerals. This method of sequential gas extraction makes it possible to verify CO₂ sequestration in mine tailings using bulk samples, while bypassing lengthy procedures for grain separation and sample preparation.

Stable isotopes are measured using a Deltaplus XL at The Pacific Centre for Isotopic and Geochemical Research (PCIGR), UBC, while ¹⁴C is measured at The Australian National University radiocarbon facility. Blank measurements indicate that contamination by modern atmosphere is minimal (<<1%) during vacuum line sample preparation. Stable isotope measurements allow us to elucidate different mechanisms for secondary carbonate mineral formation, including biological (depleted δ¹³C) and evaporative (elevated δ¹⁸O) pathways. Our results allow us to determine that bedrock carbonate minerals contain zero atmospheric CO₂ (0 to 2% pMC) and to confirm CO₂ sequestration within secondary precipitates, which contain between 30 and 99% modern atmospheric CO₂ (30 to 102% pMC).