岩浆过程中Ca同位素的分馏尺度及其机制研究

Calcium isotope geochemistry has gained extensive interests from the community for its potential on tracing early processes of the solar system and Ca cycling among different terrestrial geochemical reservoirs. Due to the analytical limitation, our current knowledge on Ca isotope fractionation behavior during magmatic processes is rather scarce. Based on most updated analytical methods, literature data and our own primary research reveal a large whole rock δ44/40Ca variation up to ~0.9‰, about a quarter of the entire range observed in terrestrial samples and overlapping with most terrestrial surface samples. This project will focus on the key issue named ‘Calcium isotope fractionation and its mechanism during magmatic processes’, by investigating the Ca isotope composition and fractionation in i) Laacher See lava series from Germany that has experienced significant crystallization of Ca-enriched minerals; ii) orogenic peridotites from Dabie-Sulu, Tibet and Alps and typical oceanic basalts; iii) intermediate to felsic igneous rocks from Dabie, eastern China, Tibet and typical modern arc areas; iv) co-existing garnet-clinopyroxene, clinopyroxene/amphibole-plagioclase pairs. This project will construct the basic framework about the Ca isotopic systematics of the silica Earth, provide the lithospheric reference that isotope data of terrestrial surface samples can compare with, and ultimately promote the application of Ca isotope tool in Earth System Science.

钙同位素因其在示踪太阳系形成过程和地球各圈层间Ca循环方面的巨大应用潜力而倍受关注。受限于Ca同位素分析技术，迄今为止，岩浆过程中钙同位素的分馏行为研究几乎是空白。基于申请人最新开发的高精度Ca同位素双稀释剂法，文献调研和预研数据揭示，地球岩浆岩全岩δ44/40Ca变化高达0.9‰，占整个地质样品变化范围的1/4左右，并与大多数表生样品重叠。本项目拟针对岩浆过程中钙同位素的分馏尺度及其机制这一核心科学问题，研究：1、德国Laacher See经历显著富钙矿物分离结晶岩浆序列；2、大别-苏鲁、西藏、阿尔卑斯造山带地幔橄榄岩和典型大洋玄武岩；3、大别、中国东部、西藏和现代岛弧区长英质岩石；4、石榴子石-单斜辉石、单斜辉石/角闪石-斜长石矿物对的钙同位素组成和分馏。该项目将构建硅酸盐地球钙同位素示踪体系的基本格架，提供表生环境中Ca同位素数据的比较基准，推动钙同位素在地球系统科学中的应用。

查明岩浆过程中的钙同位素分馏尺度和机制是构建高温钙同位素示踪体系的基础性工作。本项目调查了从超基性到中酸性岩浆岩代表性样品和相关含钙矿物的钙同位素组成，取得了以下主要观察和成果：（1）标定了角闪石–斜长石、石榴子石–单斜辉石间的平衡同位素分馏系数：Δ44/42Ca(hbl–plg) (‰) = 0.10±0.02 × 10^6/T^2 (2SD), Δ44/42Ca(grt–cpx) (‰) = [(–0.0051±0.0029) ×jadeite (%) + (0.37±0.10)] × 10^6/T^2 (2SE)；（2）揭示了基性–酸性岩浆低压分异过程中钙同位素不发生显著分馏，限定了主要含钙矿物与熔体间的钙同位素平衡分馏系数（例如：Δ44/42Ca(cpx–melt) (‰) = 0.04±0.03 × 10^6/T^2 (2SD)）；（3）地幔橄榄岩研究表明熔体提取导致残留相的钙同位素组成逐步变重，但熔体的钙同位素组成仅比地幔源区略轻；（4）根据地幔橄榄岩熔体提取趋势和科马提岩厘定了上地幔的平均钙同位素组成（delta44/40Ca = 0.90±0.03‰; delta44/42Ca = 0.42±0.02‰; ε40/44Ca = –0.41±0.45 ε, 2SE）；（5）大别低镁埃达克质岩和普通花岗岩类的对比研究揭示了低压壳内熔融过程中钙同位素不发生分馏，高压壳内熔融因石榴子石与富硬玉辉石源区残留而产生具有轻钙同位素组成的熔体，发现了钙同位素具有示踪岩浆起源深度的潜力；（6）全球不同地区交代成因地幔包体和洋岛玄武岩研究揭示：碳酸盐与硅酸盐熔体交代都可以产生具有轻Ca同位素组成的地幔源区，洋岛玄武岩Ca同位素组成系统低于洋中脊玄武岩和上地幔，反映了地幔钙同位素组成在不同尺度存在不均一；（7）系统研究了大别–苏鲁造山带变玄武岩的钙同位素组成，揭示了其钙同位素组成在俯冲进变质脱水过程不会发生显著改变。通过上述工作，本项目系统研究了岩浆分异、部分熔融、地幔交代和俯冲脱水等关键地质过程中的钙同位素分馏行为，对于加深对硅酸盐地球钙同位素体系的理解，应用钙同位素示踪壳幔物质循环具有重要意义。