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 sample s. The Li-7/Li-6 ratio is measured using double rhenium filament therm al ionization mass spectrometry in which isotopic fractionation is min imized at high temperatures. The method produces a stable, high intens ity Li+ ion beam that allows measurement of nanogram quantities of lit hium. This results in a reduction in sample size of up to 1000 times r elative to that required for the established Li2BO2+ method while main taining a comparable precision of better than similar to 1 parts per t housand (1 sigma). Replicate analyses of the NBS L-SVEC Li2CO3 standar d 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 wit h a wide range of geological samples shows excellent agreement between the Li3PO4 and Li2BO2+ techniques. Replicate analyses of seawater and a fresh submarine basalt display high precision results that agree wi th previous measurements. Taking advantage of the high ionization effi ciency of the phosphate ion source, we have made the first measurement s of the lithium concentration (by isotope dilution) and isotopic comp osition of calcareous foraminiferal tests and other marine carbonates. Preliminary results indicate that substantial lithium exchange occurs between carbonate sediments and their interstitial waters. In additio n, a possible link between lithium paleoceanography and paleoclimate d uring the last 1000 ky may be derived from planktonic foraminiferal te sts. This highly sensitive technique can be applied in the examination of low lithium reservoirs and thereby provide insight into some funda mental aspects of lithium geochemistry.