Reaction between silica or alkali-rich melts and mafic minerals - small scale kinetic fractionation with applications to global-scale mantle metasomatism (TP11)

二氧化硅或富碱熔体与镁铁质矿物之间的反应 - 小规模动力学分馏及其在全球规模地幔交代作用中的应用（TP11）

• 批准号: 33853719
• 负责人: Privatdozent Dr. Ralf Milke
• 金额: --
• 依托单位: Institut für Geologische Wissenschaften
• 依托单位国家: 德国
• 项目类别: Research Units
• 财政年份: 2007
• 资助国家: 德国
• 起止时间: 2006-12-31 至 2013-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/33853719?language=en
• 关键词: Reaction; between; silica; alkali; rich

Mantle metasomatism occurs by interaction of fluids (melts) with olivine-dominated mantle wallrock. The metasomatising agents are thought to be silicic or carbonatitic melts, with the first group prevailing. Non-basaltic silicic melts may be derived from partial melting of eclogite, i.e. deeply subducted oceanic crust. Metasomatised mantle rocks in ophiolites or xenoliths within volcanic systems contain information about the scales and mechanisms of chemical re-distribution within the Earth s mantle (like, f. ex., diffusion vs. reacting flow). Metasomatised mantle may melt at lower temperatures than pristine mantle rocks and thus have a strong imprint on the composition of directly observable volcanic rocks. In this proposal I ask for support of experiments that aim at the mechanisms of metasomatic exchange between melt and mafic minerals. The experiments will be focused on silicate melts that are out of equilibrium with mafic mantle minerals in two different ways: - SiO2-rich melt that reacts with olivine; - alkali-rich and Mg-poor melt that reacts with olivine and orthopyroxene. SiO2 metasomatism is a process that apparently occurs worldwide in various tectonic settings. Alkali-metasomatism with formation of Na(K)-amphibole, mica, diopside and other minerals at the expense of original mantle phases also occurs widespread as deduced from mantle xenoliths. The motivation for the suggested research comes from two different directions: - In experiments in the system olivine-quartz it was shown that the chemical composition of the evolving orthopyroxene rim is kinetically controlled and largely depends on grain boundary diffusion. Major and trace element concentratios in the orthopyroxene rims may deviate substantially from the values expected in bulk equilibrium between Opx and Ol. In these experiments the Opx/Qz interface is closed for element exchange other than the SiO2 component. This will be clearly different in the situation more relevant to mantle where the source of SiO2 is a silicic melt, but kinetic fractionation in such melt-olivine rim growth experiments is still expected; - Reaction rims between olivine and orthopyroxene and phonolite melt are recently investigated from the Heldburg phonolite, Thuringia. They consist of assemblies dominated by K-richterite-magnesian mica-diopsidic clinopyroxene that are very similar to several types of mantle metasomatites. The Heldburg phonolite plug is considered a natural experimental petrology lab . Controlled experimentation may answer open questions concerning these reaction rims, but first of all will give insight into the systematics of element exchange between alkaline melt and mafic mantle wallrock. In our experiments we will start out with chemically simple starting materials and single dopants and successively will introduce more chemical complexity. We will focus on the grain scale and grain boundary effects. This approach is complementary to earlier attempts of the experimental replication of mantle metasomatism in the lab using natural starting materials.

地幔交代作用是由流体（熔体）与橄榄石为主的地幔围岩相互作用产生的。交代剂被认为是硅质或碳酸盐熔体，以第一组为主。非玄武质硅熔体可能来源于榴辉岩的部分熔融，即深俯冲洋壳。火山系统中蛇绿岩或捕虏体中的交代地幔岩石包含有关地球地幔内化学物质再分配的规模和机制的信息（例如，扩散与反应流）。交代地幔可能在比原始地幔岩石更低的温度下熔化，因此对直接可见的火山岩的组成有强烈的印记。在这一建议中，我请求支持旨在研究熔体和基性矿物之间交代交换机制的实验。实验将集中在与基性地幔矿物不平衡的硅酸盐熔体上，有两种不同的方式：-与橄榄石反应的富含sio2的熔体；-富碱贫镁熔体，与橄榄石和正辉石反应。SiO2交代作用是一个明显发生在世界范围内各种构造背景下的过程。从地幔捕虏体中推断，碱交代作用也广泛发生，形成Na(K)角闪孔、云母、透辉石等矿物，破坏了原始地幔相。本研究的动机来自两个不同的方向：-在橄榄石-石英体系的实验中，表明演化中的正辉石边缘的化学成分受动力学控制，并且在很大程度上取决于晶界扩散。正辉二烯环中的主要元素和微量元素浓度可能大大偏离Opx和Ol之间的体积平衡所期望的值。在这些实验中，Opx/Qz界面是封闭的，用于交换除SiO2组分以外的元素。在与SiO2来源为硅熔体的地幔相关的情况下，这将明显不同，但在这种熔体-橄榄石边缘生长实验中，动力学分馏仍然是值得期待的；-最近研究了图林根州Heldburg的橄榄石与正辉石和phonolite熔体之间的反应环。它们由以钾辉石-镁云母-透辉石斜辉石为主的组合组成，与几种类型的地幔交代岩非常相似。Heldburg phonolite塞被认为是一个天然的实验岩石学实验室。控制实验可以回答有关这些反应边缘的开放性问题，但首先将深入了解碱性熔体与基性地幔围岩之间元素交换的系统。在我们的实验中，我们将从化学上简单的起始材料和单一掺杂剂开始，并逐步引入更多的化学复杂性。我们将重点研究晶粒尺度和晶界效应。这种方法是对早期在实验室中使用天然起始材料实验复制地幔交代作用的尝试的补充。