高温Ca同位素地球化学研究进展与双稀释剂校正

High temperature calcium isotope geochemistry in progress and double spike technique

近年来随着质谱和分析技术的进步,Ca同位素的分析精度获得了很大的提高,出现了很多高温条件下Ca同位素地球化学的研究成果。双稀释剂的合理使用在Ca同位素的分析中起关键作用。本文系统地阐述了前人对Ca同位素测试时所使用的双稀释剂技术,提供相应的计算程序,并且系统地归纳近年来高温条件下Ca同位素地球化学研究的新成果,包括单矿物对之间Ca同位素的分馏、陨石和地幔Ca同位素组成的不均一性和陨石的εCa异常及K-Ca定年等。斜方辉石和橄榄石相比于共生的单斜辉石更富集重Ca同位素,并且Δ44/40 CaOpx-Cpx受斜方辉石Ca含量和Ca—O键长的影响。地幔矿物之间具有不相同的Ca同位素组成说明地幔Ca同位素组成的不均一性。陨石及其内部难熔包裹体的Ca同位素组成变化范围很大,并且普遍存在εCa的异常。顽火辉石球粒陨石与地球具有相近的Ca同位素组成,表明它们可能具有相同的起源。但是高温条件下Ca同位素的分馏研究仍处于起步阶段,未来对于地球主要储库的Ca同位素组成、岩浆过程(部分熔融和分离结晶)和洋壳俯冲过程的Ca同位素分馏仍然需要更多地研究。

With the development of mass spectrometers and analytical advances, Ca isotopic analytical precision has been greatly improved. Many research papers focused on Ca isotopic fractionation under high temperature were published. That rational use of double spike technique plays a key role in the analysis of Ca isotopes. Based on previous studies, we compile a program for Ca double spike data reduction algorithm. We also summarize the research progress on high temperature Ca isotopic fractionation including Ca isotopic fractionation between mineral pairs, Ca isotopic heterogeneity of meteorites and of the mantle, nucleosynthetic isotope anomalies and K-Ca dating. Orthopyroxene and olivine enrich heavier Ca isotopes than co-clinopyroxene. A44/4~ Caop^cpx is influenced by Ca content and thus by Ca-O bond length in the orthopyroxene. These demonstrates that Ca isotopic composition of the mantle inclusions exhibit a wide range in Ca isotopic composition is heterogeneous. Meteorites and their refractory and widely exist ~Ca anomaly. Enstatite chondrites and Earth have similar Ca isotopic composition, which indicates that they may have the same origin. However, research on high temperature Ca isotopic fractionation is still at its early stage. Future research involves Ca isotopic composition in Earth＇s building blocks, Ca isotopic fractionation during magma differentiation （partial melting and fractional crystallization） and oceanic crust subduction.