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

地幔交代作用是通过流体(熔体)与橄榄石为主的地幔围岩相互作用而发生的。交代剂被认为是硅质或碳酸盐熔体，第一类占优势。非玄武岩硅质熔体可能来源于榴辉岩的部分熔融，即深俯冲洋壳。火山岩系蛇绿岩或捕虏体中的交代地幔岩石包含有关S地幔内化学再分布的规模和机制的信息(例如，扩散与反应流)。交代地幔可以在比原始地幔岩石更低的温度下熔融，因此对直接可观察到的火山岩的组成有很强的印记。在这项提议中，我要求支持旨在研究熔体和镁铁质矿物之间交代交换机制的实验。实验将集中在以两种不同方式与镁铁质地幔矿物失去平衡的硅酸盐熔体：-与橄榄石反应的富二氧化硅熔体；-与橄榄石和斜方辉石反应的富碱和贫镁熔体。二氧化硅交代作用是一种明显发生在世界各地不同构造环境中的过程。根据地幔包体的推断，碱交代作用也普遍发生，形成Na(K)角闪石、云母、透辉石等矿物，损害了原始地幔相。这项研究的动机来自两个不同的方向：-在橄榄石-石英系统中的实验表明，正在演化的斜方辉石边缘的化学成分是受动力学控制的，并且很大程度上取决于晶界扩散。斜方辉石轮缘中的常量元素和微量元素浓度可能与OPx和Ol之间总体平衡时的预期值有很大偏离。在这些实验中，OPX/QZ界面关闭，以进行除SiO_2组分以外的元素交换。在与地幔更相关的情况下，这将明显不同，在那里，二氧化硅的来源是硅质熔体，但在这种熔体-橄榄石边缘生长实验中仍有望发生动力学分馏；-橄榄石与斜方辉石和声石熔体之间的反应边缘最近在图林根州的赫尔德堡响应岩中进行了研究。它们由钾镁云母透辉单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。赫尔德堡响石塞被认为是一个天然的实验岩石学实验室。对照实验可能会回答有关这些反应圈的公开问题，但首先将深入了解碱性熔体和镁铁质地幔围岩之间元素交换的系统学。在我们的实验中，我们将从化学简单的起始材料和单一的掺杂剂开始，然后依次引入更多的化学复杂性。我们将重点研究晶粒度和晶界效应。这种方法是对实验室中使用天然起始材料进行地幔交代实验复制的早期尝试的补充。

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.