近日，中国科学技术大学夏琼霞团队报道了地壳深熔过程中锆石Zr同位素的分馏。相关论文于2024年6月26日发表在《中国科学：地球科学》杂志上。

据研究人员介绍，锆是一种高场强元素，但由于数据库有限，其在地球化学过程中的同位素行为尚不确定。虽然岩浆岩中的Zr同位素常被用来示踪岩浆分馏结晶的演化，但令人感兴趣的是，在地壳深熔过程中，锆石的生长产生了高度非均质的Zr同位素。本研究通过对中国拉萨带南部和大别山带北部，两个高温变质地体混合岩的原位锆石Zr同位素分析，解决了这一问题。

研究结果表明，δ⁹⁴Zr值分别在-0.30‰至+0.81‰和-0.58‰至+0.49‰之间，变化较大。除岩浆成因的残余锆石外，根据产状、微量元素和δ⁹⁴Zr值等还鉴定出两类新生锆石。包晶型锆石主要赋存于原位浅色体中，其Nb-Ta-Hf-U含量最高，δ⁹⁴Zr值也高于残余锆石。深熔锆石主要赋存于浅色脉中，其Nb-Ta-Hf-U含量高于残余锆石，δ⁹⁴Zr值与之相似。

模型计算表明，新生锆石中可变Zr同位素组成是由锆石和非锆石相分离释放Zr引起的。包晶型锆石的Zr供给主要来源于原位深熔熔体中含锆矿物的分解(非锆石效应)，而深熔型锆石的Zr供给主要来源于演化熔融体中已有锆石的溶解(锆石效应)。混合岩中Zr同位素的显著变化，解释了部分熔融过程中深熔熔体的生成、迁移和聚集。因此，Zr同位素可以作为区分地壳深熔过程中包晶锆石和深熔锆石的有力手段。

附：英文原文

Title: Zircon Zr isotope fractionation during crustal anatexis

Author: Erlin ZHU, Qiongxia XIA, Zhaoya LI, Renxu CHEN, James VAN ORMAN

Issue&Volume: 2024/06/26

Abstract: Zirconium is one of high field strength elements but its isotope behavior during geochemical processes is still uncertain because of the limited database. While Zr isotopes in magmatic rocks are often used to trace the evolution of magmas through fractional crystallization, it is intriguing how highly heterogeneous Zr isotopes were produced by the growth of zircon during crustal anatexis. We address this issue by in-situ zircon Zr isotope analyses of migmatites from two high-temperature metamorphic terranes in the South Lhasa zone and the North Dabie zone, respectively, in China. The results show highly variable δ⁹⁴Zr values from -0.30‰ to +0.81‰ and from -0.58‰ to +0.49‰, respectively. In addition to the relict zircon of magmatic origin, two types of newly-grown zircons were identified in terms of their occurrences, trace elements and δ⁹⁴Zr values. The peritectic zircon, mainly occurring in the in-situ leucosomes, exhibits the highest Nb-Ta-Hf-U contents and variably higher δ⁹⁴Zr values than those of the relict zircon. The anatectic zircon, mainly occurring in the leucocratic veins, shows higher Nb-Ta-Hf-U contents than and similar δ⁹⁴Zr values to those of the relict zircon. Model calculations demonstrate that the variable Zr isotope compositions of newly-grown zircons would result from decoupled release of Zr from zircon and non-zircon phases. The Zr supply of the peritectic zircon is mainly derived from the decomposition of Zr-bearing minerals in the in-situ anatectic melt (the non-zircon effect), whereas the Zr supply of the anatectic zircon is mainly from the dissolution of pre-existing zircons in the evolved melt (the zircon effect). The significant Zr isotope variations in the migmatites well illustrate the generation, migration and accumulation of the anatectic melts during the partial melting. Therefore, Zr isotopes can be used as a powerful means for distinguishing between the peritectic and anatectic zircons during crustal anatexis.

DOI: 10.1007/s11430-024-1350-4

Source: https://www.sciengine.com/SCES/doi/10.1007/s11430-024-1350-4