Mantle volatiles: processes, reservoirs and fluxes

我们汇集了一个由英国研究人员和国际合作伙伴组成的联盟，其主要目标是提供一种新的基于过程的挥发性元素（例如H2O、C、S、惰性气体和卤素）通量，这些元素在俯冲带进入深部地幔，在洋中脊和海洋岛屿环境中离开地幔。地幔是地球上碳、氮和硫的最大硅酸盐储存库，在板块边界输入和输出挥发物（如H2O、C、N、S、P和卤素）对气候和生物圈提供了长期控制。然而，我们对地球深处的挥发循环的理解还很粗糙。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的地幔储层中如何变化的理解非常有限。在某种程度上，这是因为挥发性元素在许多样品类型的喷发过程中大量丢失，这使得很难确定开发连贯模型所必需的控制过程。为了实现我们的目标，该联盟结合了新样品资源和分析工具的几项进展：i)认识到喷发材料中快速淬火的熔体包裹体（MIs）通常保留了地幔源挥发性成分；ii)除MIs中的主要挥发物外，还能测定硫和硼同位素；iii)硼同位素可以追踪俯冲板块向地表挥发损失的程度，并将这一信号保存在更深的地幔中；iv)惰性气体同位素测定的创新，使我们能够从吸积过程中捕获的循环挥发物中分离出来，并提供与卤素，H2O和C的联系；v)发展非传统稳定同位素，如Fe、Cu和Se，以确定系统氧化态（理解硫的关键变量）和亲铜微量元素的测定；（vi）计算能力和技术的进步，可以更好地表示地幔状系统。通过对两个热对比俯冲区（堪察加（冷）和智利南部（热））相同样品组的联合专家和分析资源进行协调，我们计划研究过程和热环境如何控制挥发性俯冲进入深部地幔的效率和地球化学特征（同位素组成和相对于其他挥发物的丰度）。这使我们能够在发展地球年龄的通量模型时考虑到地球冷却时俯冲温度的变化。在具有不同地球化学来源的洋中脊和洋岛环境中（例如，HIMU， EMI， EMII， FOZO），我们将确定已回收的挥发性元素与在其形成过程中被纳入地幔的挥发性元素（原始挥发物）的比例和特征。这是建立我们对地幔挥发性通量的理解的一个重要组成部分，而这些通量是支持今天地幔中的信号所必需的。在联盟内部开发的新的实验划分和我们跟踪氧化态的能力将使我们在理解硫循环方面做出一步改变-迄今为止几乎不被理解，但对理解气候和商业矿床形成至关重要。对地球化学元素组合的地幔迁移进行数值模拟，迭代地球物理参数以接近地球化学观测值的匹配，将使我们能够确定俯冲带和整个地幔对流期间的关键地球物理过程，这些过程控制着地幔中俯冲挥发物与原始挥发物的地球化学分布。总之，这些将在重建地球历史上的深层地球挥发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.