俯冲带变质岩的钡同位素组成

The large difference of Ba content between crustal materials and the mantle makes Ba isotopes a potentially novel and high sensitive tracer of recycled crustal materials. Besides the Ba isotopic composition of the mantle, two critical questions need to be studied before the utility of Ba isotopes as a tracer of crustal recycling. The first one is the magnitude of Ba isotope fractionation during metamorphic dehydration in subduction zones. The second one is the Ba isotopic compositions of the residual materials after slab dehydration. In order to address these two issues, we measured Ba isotopic compositions of some eclogite samples from the Dabie orogen in the preliminary research. We found that δ137/134Ba of these eclogites varied by 0.36‰, which is 9 times the current analytical uncertainty of 0.04‰ (2SD) on δ137/134Ba. We also observed that eclogites with high δ137/134Ba values had low Ba/Th ratios. This project intends to further investigate and comprehensively analyze the Ba isotopic compositions of 1) metamorphosed rocks of continental origin with varying metamorphic degrees and 2) eclogites of oceanic origin. The findings from these two aspects of this project will provide basic constraints on the behavior of Ba isotopes during subduction-zone processes and on the application of Ba isotopes to tracing recycled crustal materials.

地壳物质与地幔之间Ba含量的巨大差异使得Ba同位素有望成为“高灵敏”示踪地壳物质俯冲再循环的新工具。在开展相关的示踪应用之前，除了地幔的Ba同位素组成以外，另两个需要查明的关键科学问题是：1）俯冲带变质脱水过程中Ba同位素分馏的程度；2）俯冲的地壳物质经历脱水过程后残余相的Ba同位素组成。针对这两个科学问题，我们在预研究阶段测定了一些大别造山带榴辉岩样品的Ba同位素组成，结果发现其δ137/134Ba的变化范围达到0.36‰，是目前分析精度0.04‰(2SD)的9倍；高δ137/134Ba的样品具有较低的Ba/Th比值，表明变质脱水过程中Ba同位素可能会发生显著的分馏。本项目拟进一步系统研究：1）不同变质级别的陆壳俯冲成因变质岩的Ba同位素组成；2）洋壳俯冲成因榴辉岩的Ba同位素组成。两方面的研究结果将为俯冲过程中Ba同位素的分馏行为和利用Ba同位素示踪俯冲再循环的地壳物质提供基础性制约。

地壳岩石相比地幔具有高的Ba含量和大的Ba同位素组成变化，因此Ba同位素是潜在的示踪地壳物质再循环的灵敏工具。俯冲带变质岩和变质脉的Ba同位素特征能够揭示俯冲带变质脱水过程中Ba同位素的分馏行为并限定俯冲的地壳物质经历板片脱水后残余相的Ba同位素组成。本项目研究了秦岭–大别造山带不同变质程度的陆壳俯冲成因变基性岩、西藏松多和西南天山洋壳俯冲成因榴辉岩以及大别造山带碧溪岭地区超高压榴辉岩–高压石英脉体系的Ba同位素组成。主要研究结果和结论是：1.秦岭–大别造山带绿片岩、角闪岩和榴辉岩三类不同变质程度的变基性岩样品的Ba同位素组成基本落在上地壳岩石的Ba同位素组成变化范围内，且三者之间无系统性差别；西藏松多和西南天山榴辉岩样品的Ba同位素组成绝大多数落在蚀变洋壳的Ba同位素组成变化范围内。这些变质岩样品的Ba同位素组成与指示流体活动的指标Ba/Th之间均没有明显的相关性，表明变质脱水过程不会显著地改造俯冲的地壳岩石的Ba同位素组成。2.作为俯冲的地壳物质经历板片脱水后残余相的代表，大别造山带陆壳俯冲成因榴辉岩样品以及西藏松多和西南天山洋壳俯冲成因榴辉岩样品的Ba同位素组成变化范围均显著大于地幔的Ba同位素组成变化范围。结合榴辉岩样品远高于地幔的Ba含量，少量地壳物质（特别是陆壳物质）俯冲进入地幔即有可能导致局部地幔显著的Ba同位素不均一性。3.大别造山带碧溪岭地区高压石英脉及其包裹的榴辉岩样品的Ba同位素组成相比围岩榴辉岩明显偏重，表明围岩榴辉岩变质脱水过程中发生了显著的Ba同位素分馏，成脉流体富集重Ba同位素。岩相学观察和单矿物Ba同位素数据揭示，造成这种分馏的具体机制是多硅白云母退变质分解形成富集轻Ba同位素的黑云母、释放富集重Ba同位素的流体。上述研究结果为更好地理解俯冲带Ba同位素体系和利用Ba同位素示踪地壳物质再循环提供了理论基础。