The Noble Gas Systematics of Subduction

俯冲惰性气体系统学

基本信息

批准号：
NE/L01095X/1
负责人：
Andrew Smye
金额：
$ 65.53万
依托单位：
University of Oxford
依托单位国家：
英国
项目类别：
Fellowship
财政年份：
2015
资助国家：
英国
起止时间：
2015 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FL01095X%2F1
关键词：
Noble Gas Systematics Subduction

项目摘要

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.

地球的大气和表面含有丰富的水和气体，挥发性物种可维持我们地球的可居住性。这些不兼容的元素，包括贵重气体和卤素，通过中山山脊和弧形火山的岩浆不断从地球上释放。直到最近，人们已经接受了这种通量是单向的。挥发物仅被释放，再也没有回到地球的地幔中。但是，随着新的高敏性测量技术的出现，显然，地球地幔的同位素/元素组成包含与海水相同的AR/KR/KR/XE签名。因为这是太阳系中的独特组成，所以唯一可信的解释是，非中断的海水样贵重气体签名幸存下来的俯冲过程，通常被认为排除了超过98％的输入挥发物。鉴于贵重气体的溶解性比地壳矿物多100至100,000倍，因此即使是少量的流体运输也有望将俯冲岩石的贵重气体组成分馏。那么在俯冲过程中如何逃脱和卤素逃避分馏呢？挥发性阶段在何处位于下游的平板中？它们在什么距离上通过变质流体流动？而且，至关重要的是，返回地幔的多少数量贵重气体和卤素又回到了地幔中？我建议对俯冲板的贵气体和卤素元素/同位素组成进行第一个系统的分析表征。我将在牛津的诺布尔天然气实验室（Oxford's Noble Gas Laboratory）上进行这些最先进的波动测量，这是全球少少数具有所需的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地层收集的岩石样品套件的矿物质和流体夹杂物进行测量，该套件代表了平板的不同部分，并保留了不同程度的挥发性损失。一旦确定了宿主相，将使用热力学计算出平衡的高贵气体和卤素成分的压力和温度。还将在几个代表性的岩性边界中收集岩石样品，以确定化石贵重的气体浓度曲线，并保留俯冲过程中流体流量和反应的相对作用的直接记录。我将构建一个数值模型，以将这些概况解析为关键的传输参数，从而量化了与平板中水的富含水的流体传输的程度。当与现有的俯冲脱水系统模型结合使用时，这将首次允许估算到地幔上的俯冲降压气通量。这个新颖的跨学科项目将是第一个将贵重气体和卤素与俯冲板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如何处理挥发物处理的理解方面做出改变。未来的研究途径包括研究这些返回通量对地幔的化学和物理演化的影响，使用全尺度披风对流的模型。