Direct determination (without chromatographic separation) of lithium isotopes in saline fluids using MC-ICP-MS: establishing limits on water chemistry

Abstract

Produced waters from petroleum and geothermal reservoirs contain large amounts of cations including lithium, and while lithium isotopes are time-intensive to measure in the traditional way, they have the potential to reveal information about fluid and solute origins and diagenesis. We tested the effect of added cations that dominate in produced waters (Na, Ca, Mg) on accuracy and precision of lithium isotope measurements by MC-ICP-MS in Li-isotope standard solutions without chromatographic separation. Repeated measurement of Li-isotope standards with no added matrix demonstrate high 2 SD reproducibility: LSVEC (RM8545) had δ⁷Li of 0.04 ± 0.74‰ (n = 19); IRMM16 had δ⁷Li of 0.07 ± 1.2‰ (n = 11); Li7N had δ⁷Li of 30.07 ± 0.12‰ (n = 11); Li6N had δ⁷Li of −8.04 ± 0.58‰ (n = 8). Replicates of standards with matrix and of diluted produced waters had 2 SD reproducibility smaller than ±1.8‰ and ±2.0‰, respectively. Results showed that Na/Li (weight ratio) up to about 500 (analysed aliquot <20 mg kg⁻¹ Na) and Ca/Li or Mg/Li up to about 250 (analysed aliquot <10 mg kg⁻¹ Ca or Mg) do not diminish δ⁷Li accuracy or precision, so long as Li concentration in the measured solution is greater than 20 μg kg⁻¹. Because produced waters are chemically more complex than the added-matrix isotope standards we tested initially, we also compared δ⁷Li in produced waters from which the Li was chromatographically separated, as is the convention, with diluted produced waters. Results show marked Li isotope fractionation occurred in those samples in which Na/Li ranged from 500 to almost 10 000. Geothermal waters and Na–Ca–Cl-type produced waters, therefore, are fluids that are most likely to have cation/Li ratios that will make them amenable to direct determination of δ⁷Li with only sample dilution. We also found that two aliquots of the Li-isotope standard, LSVEC (RM8545), one obtained in 1985 and one in 2012, have δ⁷Li that differs by about 2‰.