The Noble Gas Systematics of Subduction

地球的大气层和表面含有丰富的水和气体，这些挥发性物质维持着我们星球的可居住性。这些不相容的元素，包括惰性气体和卤素，通过大洋中脊和弧火山的岩浆活动不断从地球释放出来。直到最近，人们才接受这种流动是单向的;挥发物只是被释放出来，永远不会返回地幔。然而，随着新的高灵敏度测量技术的出现，很明显，地球地幔的同位素/元素组成包含与海水相同的Ar/Kr/Ar特征。由于这是太阳系中独特的成分，唯一可信的解释是，一种未被破坏的类似海水的惰性气体特征在俯冲过程中幸存下来，通常认为排除了98%以上的挥发物。由于稀有气体在流体中的溶解度是地壳矿物的100到100，000倍，因此即使是少量的流体迁移也会使俯冲岩石的稀有气体成分破裂。那么，惰性气体和卤素是如何在俯冲过程中逃脱被移除和分馏的呢？挥发相存在于下行板中的什么位置？它们通过变质流体流被搬运了多远？而且，关键的是，有多少惰性气体和卤素返回地幔？我提议对俯冲板块的惰性气体和卤素元素/同位素组成进行首次系统的分析表征。我将在牛津大学的惰性气体实验室进行这些最先进的挥发性测量，该实验室是全球少数几个拥有所需专业知识和分析技术的实验室之一。将对从几个关键的高压变质岩石中收集的岩石样品组中的矿物和流体包裹体进行测量，这些岩石样品组代表了板片的不同部分，并保留了不同程度的挥发损失。一旦确定了主相，将使用热力学计算稀有气体和卤素组合物平衡时的压力和温度。还将在几个有代表性的岩性边界收集岩石样本，以确定化石惰性气体浓度分布，保存俯冲过程中流体流动和反应的相对作用的直接记录。我将构建一个数值模型去卷积这些配置文件到关键的传输参数，量化的程度，惰性气体是从板中的富水流体的运输解耦。当与现有的俯冲脱水系统学模型相结合时，这将首次允许计算俯冲到地幔的惰性气体通量的估计。这一新颖的跨学科项目将是第一个将惰性气体和卤素与俯冲板块的变质演化联系起来的项目。因此，拟议的研究有可能在我们对俯冲过程中挥发物如何处理的理解中带来一个步骤的变化。未来的研究方向包括利用全尺度地幔对流模型研究这些回流通量对地幔化学和物理演化的影响。

The Earth's atmosphere and surface contain abundant water and gas, volatile species that sustain the habitability of our planet. These incompatible elements, including the noble gases and halogens, are continually released from the Earth through magmatism at mid-ocean ridges and arc volcanoes. Until recently, it has been accepted that this flux was one-way; volatiles are only released and never returned back into the Earth's mantle. However, with the advent of new high-sensitivity measurement techniques, it has become apparent that the isotopic/elemental composition of the Earth's mantle contains an Ar/Kr/Xe signature that is identical to seawater. Because this is a unique composition in the solar system, the only credible explanation is that a non-disrupted seawater-like noble gas signature survives the subduction process, generally thought to exclude more than 98% of input volatiles. Given that the noble gases are between 100 and 100,000 times more soluble in fluids than crustal minerals, even small amounts of fluid transport would be expected to fractionate the noble gas composition of the subducting rock. How then do noble gases, and halogens, escape removal and fractionation during subduction? Where do the volatile phases reside in the downgoing slab? Over what distances are they transported by metamorphic fluid flow? And, critically, what quantities of noble gas and halogens are returned back into the mantle? I propose to undertake the first systematic analytical characterization of the noble gas and halogen elemental/isotopic composition of the subducting slab. I will conduct these state-of-the-art volatile measurements at Oxford's Noble Gas Laboratory, one of only few laboratories worldwide with the required expertise and analytical technology. Measurements will be performed on minerals and fluid inclusions from a rock sample suite collected from several key high-pressure metamorphic terranes, representing different portions of the slab and preserving differing degrees of volatile-loss. Once the host phases have been identified, the pressures and temperatures that the noble gas and halogen compositions equilibrated at will be calculated using thermodynamics. Rock samples will also be collected across several representative lithological boundaries to determine fossil noble gas concentration profiles, preserving a direct record of the relative roles of fluid flow and reaction during subduction. I will construct a numerical model to deconvolve these profiles into key transport parameters, quantifying the degree to which the noble gases are decoupled from the transport of water-rich fluids in the slab. When combined with existing models of subduction dehydration systematics, this will allow, for the first time, estimates of the subducted noble gas flux to the mantle to be calculated. This novel interdisciplinary project will be the first to link the noble gases and halogens with the metamorphic evolution of the subducting slab. Accordingly, the proposed research has the potential to deliver a step change in our understanding of how volatiles are processed during subduction. Future research avenues include investigating the effects of these return fluxes on the chemical and physical evolution of the mantle, using models of whole-scale mantle convection.