Mantle volatiles: processes, reservoirs and fluxes

我们汇集了一个由英国调查人员和国际合作伙伴组成的财团，其主要目标是提供一种新的基于挥发性元素（例如H2O，C，S，惰性气体和卤素）通量的理解，这些通量进入俯冲带的深部地幔，并在洋中脊和海洋岛屿设置的地幔。地幔是地球上碳、氮和硫的最大硅酸盐储库，也是挥发物的输入和输出（例如，H2O、C、N、S、P和卤素）对气候和生物圈提供了长期的控制。然而，我们对地球深部挥发性循环的理解是粗略的。部分原因是我们对地幔系统中再循环对原始挥发物的相对贡献以及这在不同地幔储层中的变化了解甚少。这部分是因为挥发性元素在喷发过程中大量丢失，许多样品类型使得很难确定开发连贯模型所需的控制过程。为了实现我们的目标，该联合体结合了新样品资源和分析工具方面的若干进展：i）认识到喷发物质中的快速淬火熔融包裹体（MI）通常保留地幔源挥发性成分; ii）除了MI中的主要挥发性成分外，还能够确定硫和硼同位素;（三）发现硼同位素可以追踪俯冲板块挥发性物质损失到地表的程度，并将这种信号保存在更深的地方。地幔; ㈣惰性气体同位素测定方面的创新，使我们能够从吸积过程中捕获的挥发物中分离出再循环的挥发物，并将其与卤素、H2O和C联系起来; ㈤开发非传统的稳定同位素，如Fe、Cu和Se，以确定系统的氧化态（了解硫的关键变量）和亲铜微量元素测定;六）计算能力和技术的进步，使人们能够更好地表示类地幔系统：通过协调联合体在两个热反差大的俯冲区的同一套样品上的专门知识和分析资源（堪察加半岛（冷）和智利南部（热）），我们计划调查的过程和热环境如何控制的效率和地球化学特征（同位素组成和相对丰富的其他挥发物）的挥发性俯冲到地幔深处。这使我们能够考虑到俯冲温度的变化，因为地球冷却的通量模型，运行的地球年龄的发展。在洋中脊和海洋岛屿设置与不同的地球化学来源（如HIMU，EMI，EMII，FOZO），我们将确定的比例和特征的挥发性元素已被回收相比，被纳入地幔在其形成过程中（原始挥发物）。这是建立我们对进入地幔的挥发性通量的理解的一个重要组成部分，需要支持今天地幔中的信号。在财团内开发的新的实验分区和我们跟踪氧化态的能力将使我们能够在理解硫循环方面取得一步变化-迄今为止几乎不了解，但在理解气候和商业矿物存款形成方面至关重要。地幔输运的地球化学元素套件的数值模拟，迭代的地球物理参数，以接近地球化学观测值的匹配，将使我们能够确定关键的地球物理过程中的俯冲带，并在整个地幔对流，控制地球化学分布的俯冲与原始挥发分在地幔中。总之，这些将导致在重建地球历史上的地球深部挥发性通量方面取得重大进展-这是一项重大的科学挑战。

We have brought together a consortium of UK investigators and international partners with the key objective of providing a new process based understanding of volatile element (e.g. H2O, C, S, noble gases and halogens) fluxes into the deep mantle at subduction zones and out of the mantle at mid ocean ridges and ocean island settings. The mantle is by many orders of magnitude the largest silicate reservoir for carbon, nitrogen and sulphur on Earth and the input and output of volatiles (e.g., H2O, C, N, S, P, and halogens) at plate boundaries provides long-term controls on the climate and the biosphere. Nevertheless, our understanding of the deep-Earth volatile cycle is crude. In part because we have a very poor understanding of the relative contribution of recycled to primordial volatiles in the mantle system and how this might vary in different mantle reservoirs. In part this is because volatile elements are extensively lost during the eruptive process from many sample types making it hard to identify the controlling processes necessary to develop coherent models. To address our objective the consortium combines several advances in new sample resources and analytical tools:i) The recognition that rapidly quenched melt inclusions (MIs) within erupted material often preserve mantle-source volatile compositions; ii) The ability to determine sulphur and boron isotopes in addition to major volatiles in the MIs;iii) The discovery that boron isotopes can track the extent of volatile loss to the surface from subducting slabs and preserve this signal in the deeper mantle; iv) The innovations in noble gas isotope determination that allow us to resolve recycled volatiles from those trapped during accretion and provide links to halogens, H2O and C;v) The development of non-traditional stable isotopes such as Fe, Cu and Se to identify system oxidation state (a key variable in understanding sulphur) and chalcophile trace element determinations;vi) The advances in computing power and techniques that allow better representation of mantle-like systems.By coordinating the combined consortium expertise and analytical resources on the same sample suites in two thermally contrasting subduction regimes (Kamchatka (cool) and Southern Chile (hot)) we plan to investigate how both the processes and thermal setting control the efficiency and geochemical character (isotopic composition and relative abundance to other volatiles) of volatile subduction into the deep mantle. This allows us to take into consideration changes in subduction temperature as the Earth cools in the development of flux models that run for the age of the Earth. At mid ocean ridges and ocean island settings with different geochemical provenance (e.g. HIMU, EMI, EMII, FOZO) we will determine the proportion and character of volatile elements that have been recycled compared to those that were incorporated into the mantle during its formation (primitive volatiles). This is an essential component in building our understanding of the volatile flux into the mantle required to support the signals in the mantle today. New experimental partitioning developed within the consortium and our ability to track oxidation state will allow us to make a step change in understanding the sulphur cycle - barely understood to date but critical in understanding climate and commercial mineral deposit formation. Numerical simulations of mantle transport for suites of geochemical elements, iterating the geophysical parameters to approach matches for the geochemical observables, will allow us to identify the key geophysical processes in subduction zones and during whole mantle convection that control the geochemical distribution of subducted vs. primordial volatiles in the mantle. Together, these will lead to a significant advance in reconstructing the deep Earth volatile fluxes over Earth history - a grand science challenge.