钙同位素在高温地质过程中的应用及前景

Application and prospects of calcium isotopes in high temperature processes

制约高温过程中Ca同位素研究的因素主要有分析技术、储库端元和分馏机理。随着质谱的发展和双稀释剂的应用,分析技术已经能很好地满足高温过程同位素示踪的要求,但是目前对于储库端元的研究还比较薄弱,而分馏机理也存在较大争议。因此,完善不同储库端元的Ca同位素组成,厘清Ca同位素分馏机理势在必行。全硅酸岩地球作为最重要的Ca储库,前人研究表明,其Ca同位素组成在0.94‰~1.05‰之间变化。Ca—O化学键能的强弱是造成矿物间Ca同位素组成差异的主要因素,此外,温度和矿物组成差异也会引发Ca同位素分馏。热扩散和化学扩散都会引起Ca同位素分馏,但热扩散只发生在特殊的环境中。化学扩散受控于CaO化学势梯度,而化学势梯度受控于体系的成分、温度和压力。部分熔融和熔体提取过程中轻同位素趋向于富集在熔体相中。由于不同含钙矿物的Ca同位素组成存在差异,因此,在分离结晶过程中,矿物晶出顺序不同会引起Ca同位素分馏。变质作用过程中流体交代作用或碳酸盐岩沉淀都会造成Ca同位素分馏。K-Ca衰变体系使得Ca同位素既可以对古老的富钾岩石或矿物进行定年,也可以利用放射性成因Ca对源区和壳幔物质循环进行示踪。

The main factors restricting the development of Ca isotopes in high temperature processes are analytical techniques,Ca isotopic composition of the end members,and fractionation mechanisms.With the application of mass spectrometry and double spikes,the analytical method can meet the requirements of high temperature process tracing.However,research on the Ca isotopic composition of the end members is insufficient,and the fractionation mechanisms are controversial.Therefore,it is imperative to identify the Ca isotopic composition of the end members and to clarify the mechanisms of Ca isotope fractionation.As the most important end member,the Ca isotopic composition of bulk silicate Earth varies from 0.94‰to 1.05‰.The inter-mineral fractionation of Ca isotopes is primarily controlled by the strength of the Ca—O bond.In addition,the difference in temperature and mineral composition also leads to the fractionation of Ca isotopes.Both thermal and chemical diffusion can cause Ca isotope fractionation,but thermal diffusion only occurs in special environments.Chemical diffusion is controlled by the chemical potential of CaO,and the chemical potential is controlled by the composition of the system,temperature,and pressure.Light isotopes tend to be enriched in the melt during partial melting and melt extraction.The Ca isotopic heterogeneity of felsic minerals leads to Ca isotopic fractionation in the process of fractional crystallization.Both fluid metasomatism and carbonate precipitation during metamorphism lead to Ca isotopic fractionation.The K-Ca decay system makes it possible to date the ancient K-rich rocks or minerals.Radioactive Ca can also be used to trace the source region and crust-mantle material cycle.