A simplified, accurate and fast method for lithium isotope analysis of rocks and fluids, and δ7Li values of seawater and rock reference materials

We report a new approach to conduct fast and accurate lithium isotope ratio measurements by MC-ICP mass spectrometry after wet chemical sample preparation. In contrast to most previously published methods our MC-ICP-MS set-up did not use a desolvating system to achieve appropriate ion beam intensities and, therefore, was less affected by matrix-induced shifts of the instrumental mass bias. As the total lithium background and build-up in the sample introduction system was low, previous sample residues could be washed out by an extended uptake of the new sample. Elimination of a nitric acid rinse step increased the sample throughput by a factor of two and allowed the instrumental mass bias drift to be tracked more precisely. delta Li-7 values of powdered silicate rock reference materials and seawater obtained in this study revealed good accuracy and an overall analytical uncertainty of typically 0.5 parts per thousand (2s). On the basis of a comparison between our lithium isotope data and compiled literature data, we recommend preliminary average delta Li-7 values for seawater (+30.8 parts per thousand) and several silicate rock reference materials (BHVO-1: +5.0 parts per thousand; JA-1: +5.6 parts per thousand; JB-2: +4.8 parts per thousand). The compilation of published delta Li-7 values for seawater suggests that the observed large lithium isotope differences are due to inter-method and/or interlaboratory bias. Most recently published delta Li-7 values for seawater show little variation and confirm a constant lithium isotope composition (at the sub parts per thousand level) of seawater in well mixed ocean basins.