Precise determination of lithium isotopic composition in low concentration natural samples

An improved procedure for lithium isotope analysis using Li₃PO₄ as the ion source has been investigated for application to geological samples. The ⁷Li/⁶Li ratio is measured using double rhenium filament thermal ionization mass spectrometry in which isotopic fractionation is minimized at high temperatures. The method produces a stable, high intensity Li⁺ ion beam that allows measurement of nanogram quantities of lithium. This results in a reduction in sample size of up to 1000 times relative to that required for the established Li₂BO⁺₂ method while maintaining a comparable precision of better than ∼1%o (1σ). Replicate analyses of the NBS L-SVEC Li₂CO₃ standard yielded a mean value of 12.1047 ± 0.0043 (n = 21), which is close to the reported absolute value of 12.02 ± 0.03. Intercalibration with a wide range of geological samples shows excellent agreement between the Li₃PO₄ and Li₂BO⁺₂ techniques. Replicate analyses of seawater and a fresh submarine basalt display high precision results that agree with previous measurements. Taking advantage of the high ionization efficiency of the phosphate ion source, we have made the first measurements of the lithium concentration (by isotope dilution) and isotopic composition of calcareous foraminiferal tests and other marine carbonates. Preliminary results indicate that substantial lithium exchange occurs between carbonate sediments and their interstitial waters. In addition, a possible link between lithium paleoceanography and paleoclimate during the last 1000 ky may be derived from planktonic foraminiferal tests. This highly sensitive technique can be applied in the examination of low lithium reservoirs and thereby provide insight into some fundamental aspects of lithium geochemistry.