(Table 3) Li concentrations and isotopic compositions in foraminiferal shells, carbonate sediments, and associated pore waters from ODP Sites 131-806 and 138-851

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 Li2BO2+ method while maintaining a comparable precision of better than 1? (1 sigma). 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 Li2BO2+ 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.

本研究针对地质样品的应用场景，开发了以磷酸锂(Li3PO4)作为离子源的锂同位素分析改进方法，并开展了相关系统研究。本方法采用双铼灯丝热电离质谱法(thermal ionization mass spectrometry)测定7Li/6Li比值，可在高温条件下将同位素分馏效应降至最低。该方法可产生稳定且高强度的Li+离子束，能够实现纳克量级锂的定量分析。相较于已成熟的二硼酸锂阳离子(Li2BO2+)分析方法，本方法可将样品用量最多缩减至原用量的1/1000，同时保持优于1（1σ）的相当精度水平。对NBS L-SVEC型碳酸锂(Li2CO3)标准物质进行多次重复分析，得到平均值为12.1047±0.0043（n=21），该结果与已报道的12.02±0.03的绝对参考值较为接近。对多种地质样品开展交叉校准的结果表明，磷酸锂(Li3PO4)法与二硼酸锂阳离子(Li2BO2+)法的分析结果具有极佳一致性。对海水及新鲜海底玄武岩进行重复分析，所得结果精度较高且与既往测量结果一致。
借助磷酸根离子源的高电离效率优势，本研究首次通过同位素稀释法(isotope dilution)测定了钙质有孔虫壳体及其他海洋碳酸盐的锂浓度与同位素组成。初步研究结果显示，碳酸盐沉积物与其孔隙水之间存在显著的锂交换过程。此外，通过浮游有孔虫壳体的分析数据，可推断过去1000千年间锂同位素古海洋学(paleoceanography)与古气候(paleoclimate)之间可能存在关联。这一高灵敏度分析技术可应用于低锂储库的相关研究，从而为锂地球化学的若干基础问题提供新的认知。