The importance of carbon to the formation and composition of silicates during mantle metasomatism
The importance of carbon to the formation and composition of silicates during mantle metasomatism
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DOI：
10.1016/j.gca.2023.06.025
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发表时间：
2023-06
影响因子：
5
通讯作者：
Michele Rinaldi;S. Mikhail;D. Sverjensky;Joanna Kalita
中科院分区：
文献类型：
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作者：
Michele Rinaldi;S. Mikhail;D. Sverjensky;Joanna Kalita
地幔钻石中的矿物和流体包裹体提供了有关地幔交代作用性质以及流体在地球内部物质传质中的作用的其他信息。我们探索了流体-岩石交代作用过程中碳浓度在产生在次大陆岩石圈地幔钻石中观察到的石榴石和单斜辉石成分范围的作用。我们使用地球深层水模型来预测硅质、碳酸岩和橄榄岩流体与常见地幔岩石（橄榄岩、榴辉岩和辉石岩）在 5 GPa、1000°C、一系列氧化还原条件（logfO2=−2 至 −4 ΔFMQ）以及交代流体中各种初始碳浓度下的交代作用结果。我们的结果表明，交代石榴石和单斜辉石的预测成分受流体和岩石的初始地球化学控制，随后特定矿物的地球化学演化遵循可定义的反应路径。可以形成钻石的富碳交代流体模型（初始碳含量 >5.00 摩尔）会产生富镁石榴石和单斜辉石，是钻石中橄榄岩、榴辉岩和网状辉石包裹体的典型特征。然而，不形成金刚石的贫碳交代流体模型会产生贫镁、富钙的石榴石和单斜辉石。然而，此类石榴石和单斜辉石确实作为钻石内含物存在。在我们的模型中，流体中碳的丰度通过形成水性 Mg-Ca-Fe-C 络合物来控制二价离子的行为，该络合物直接控制交代过程中沉淀的石榴石和单斜辉石的组成。由于富含碳的初始流体可以在钻石中形成镁含量较高的榴辉岩、橄榄岩和网斜岩包裹体，因此这些包裹体可以是同生的（交代的）或可能是原生的。然而，在我们的模型中，在地幔金刚石中发现的相对贫镁、富钙和富铁的榴辉石榴石和单斜辉石包裹体是由不沉淀金刚石的贫碳流体形成的。这些内含物很可能反映了在被纳入其宿主钻石之前的交代事件，或者它们可能代表基于原岩的原生地球化学。因此，用于对钻石进行分类的共生群体不应被视为一种遗传分类，因为流体地球化学的作用似乎比母岩地球化学所发挥的作用更重要。
Mineral and fluid inclusions in mantle diamonds provide otherwise inaccessible information concerning the nature of mantle metasomatism and the role of fluids in the mass transfer of material through the Earth’s interior. We explore the role of carbon concentration during fluid-rock metasomatism in generating the range of garnet and clinopyroxene compositions observed in diamonds from the sub-continental lithospheric mantle. We use the Deep Earth Water model to predict the results of metasomatism between silicic, carbonatitic and peridotitic fluids and common mantle rocks (peridotites, eclogites and pyroxenites) at 5 GPa, 1000 °C, across a range of redox conditions (logfO2= −2 to −4 ΔFMQ), and a wide range of initial carbon concentrations in the metasomatic fluids. Our results show that the predicted compositions of metasomatic garnets and clinopyroxenes are controlled by the initial geochemistry of fluids and rocks, with subsequent mineral-specific geochemical evolution following definable reaction pathways. Model carbon-rich metasomatic fluids that can form diamond (initial C-content >5.00 molal) result in Mg-rich garnets and clinopyroxenes typical of peridotitic, eclogitic, and websteritic inclusions in diamonds. However, model carbon-poor metasomatic fluids that do not form diamond result in Mg-poor, Ca-rich garnets and clinopyroxenes. Such garnets and clinopyroxenes do nevertheless occur as inclusions in diamonds. In our models, the abundance of carbon in the fluids controls the behaviour of the bivalent ions through the formation of aqueous Mg-Ca-Fe-C complexes, which directly govern the composition of garnets and clinopyroxenes precipitated during the metasomatic processes. As the C-rich initial fluids can form the higher Mg-eclogitic, peridotitic, and websteritic inclusions in diamonds, these inclusions can be syngenetic (metasomatic) or possibly protogenetic. However, in our models, the relatively Mg-poor, Ca- and Fe-rich eclogitic garnet and clinopyroxene inclusions found in mantle diamonds formed from C-poor fluids that do not precipitate diamond. These inclusions most likely reflect a metasomatic event prior to being incorporated into their host diamonds, or they could represent protolith-based protogenetic geochemistry. Therefore, the paragenetic groups used to classify diamonds should not be considered a genetic classification, as the role of fluid geochemistry appears to be more important than the one played by host rock geochemistry.