华北克拉通昌乐橄榄岩地幔交代过程中的熔/流体包裹体记录及Li同位素分馏

As mantle metasomatism may largely change the physicochemical properties of the mantle, the related studies play an important role in rebuilding the mantle evolution process and figuring out a series of major issues such as the craton thinning and the recycle of crustal materials. The Changle region in the North China Craton developed various types of mantle peridotites. Although lots of studies have been done, the source of metasomatic agent and the exact metasomatic process are still in debate. The debate also lies in the Li isotopic fractionation during mantle metasomatism, which results in the poor understanding of Li isotopic behaviors during deep geological process. Theoretically, the melt/fluid inclusions hosted in mantle peridotites are able to preserve some direct information about the metasomatic agent. Thus the studies on inclusions have great potential in rebuilding the whole mantle metasomatism and evolution process. The present project plans to carry out detailed in-situ composition analyses on minerals and melt/fluid inclusions from the various Changle peridotites, and then makes strong constraints on the mantle metasomatism process. Based on these achievements, the project plans to measure the Li isotopic compositions of mineral separated from the metasomatism-known samples with solution MC-ICPMS method. This project would provide an excellent opportunity to probe the Li isotopic fractionation during mantle metasomatism as well as the Li isotopic behavior related to the deep geological melts/fluids.

地幔交代可以改变地幔的物理化学性质，对其的相关研究对反演地幔演化以及解决克拉通减薄、地壳物质的再循环等重大科学问题意义重大。华北克拉通昌乐地区广泛出露不同类型的地幔橄榄岩，然而传统方法对于地幔交代介质的来源和交代过程的反演往往出现争议，类似的争议同样出现在地幔交代过程中的Li同位素分馏方面，并导致目前对深部地质过程中的Li同位素行为存在认识上的不足。地幔橄榄岩中存在的熔/流体包裹体理论上可以保留地幔交代介质的直接信息，对其进行的研究有很大的潜力可以全面反演地幔交代和地幔演化过程。本项目计划选取昌乐不同类型地幔橄榄岩进行矿物原位精细分析，熔/流体包裹体的成分分析等并精确约束该地区的地幔交代过程，之后以此为基础挑选已知交代类型的单矿物进行溶液Li同位素的测试，探讨不同类型地幔交代过程的Li同位素分馏，并约束深部地质熔/流体相关的Li同位素行为。

地幔交代是关系俯冲物质再循环以及地表响应的关键过程，而Li同位素有很大的潜力来示踪俯冲物质再循环，但在地幔交代过程的Li同位素分馏仍存在争议。本项目主要对山东昌乐地区玄武岩及所携带的地幔橄榄岩捕虏体中的熔/流体包裹体以及单矿物Li同位素研究。我们发现昌乐橄榄岩中的包裹体成分分为硅酸质和碳酸质两种类型，而橄榄石、斜方辉石和单斜辉石也在矿物层面上记录了两种交代介质的影响。此外，单矿物Li同位素的分析测试发现矿物间存在巨大的Li同位素分馏（可达20‰），进一步分析我们认为硅酸质熔体交代可能会导致斜方辉石和单斜辉石的δ7Li值升高而橄榄石的δ7Li值降低，甚至出现橄榄石δ7Li值低于斜方辉石和单斜辉石的情况，碳酸质熔体的交代会使橄榄石的δ7Li值升高而斜方辉石和单斜辉石的δ7Li值降低，这些过程可能与交代过程中伴随的扩散有关，在全岩尺度上的影响可能不大。最后，沉积碳酸盐和区域玄武岩的Li同位素研究表明古海水的Li同位素演化可能是控制地幔橄榄岩全岩以及玄武岩Li同位素对俯冲再循环沉积碳酸盐响应的关键控制因素。总的来说，Li同位素能够很好区分不同类型地幔交代，后期可以用来示踪地壳物质的再循环。