Mantle volatiles: processes, reservoirs and fluxes

我们召集了一个由英国研究人员和国际合作伙伴组成的联盟，主要目标是提供基于新过程的对挥发性元素(例如H2O、C、S、惰性气体和卤素)在俯冲带进入地幔深处和在大洋中脊和大洋岛屿环境下流出地幔的理解。地幔是地球上碳、氮和硫的最大硅酸盐储藏库，板块边界的挥发物(如H2O、C、N、S、P和卤素)的输入和输出对气候和生物圈提供了长期的控制。然而，我们对地球深处的波动循环的理解是粗略的。部分原因是我们对地幔系统中循环挥发分对原始挥发分的相对贡献以及这种贡献在不同的地幔储集层中可能会如何变化的了解非常有限。这在一定程度上是因为在喷发过程中，许多样本类型的挥发性元素大量丢失，因此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，该联盟结合了在新的样品资源和分析工具方面的几个进展：i)认识到喷发物质中的快速淬火熔融包裹体(Mis)通常保存了地幔来源的挥发性成分；ii)除了mis中的主要挥发物外，还能够确定硫和硼同位素；iii)发现硼同位素可以跟踪俯冲板块向地表挥发损失的程度，并在地幔更深的地方保存这一信号；iv)稀有气体同位素测定的创新，使我们能够解决那些在吸积过程中捕获的循环挥发物，并提供与卤素、H2O和C的联系；五)开发非传统稳定同位素，如铁、铜和硒，以确定系统氧化状态(了解硫的一个关键变量)和亲硫微量元素测定；六)计算能力和技术的进步，使之能够更好地代表地幔系统。通过协调两个热对比强烈的俯冲体制(堪察加(冷)和智利南部(热))的同一样品组的联合财团专门知识和分析资源，我们计划调查过程和热环境如何控制挥发物质俯冲到地幔深处的效率和地球化学性质(同位素组成和相对于其他挥发分的相对丰度)。这使我们能够在开发针对地球年龄运行的通量模型时考虑到地球冷却时俯冲温度的变化。在大洋中脊和具有不同地球化学来源(如HIMU、EMI、EMII、FOZO)的海洋岛屿环境中，我们将确定与地幔形成过程中并入的挥发性元素(原始挥发物)相比，被循环的挥发性元素的比例和特征。这是建立我们对进入地幔的挥发性通量的理解的重要组成部分，这是支持今天地幔中的信号所需的。在该财团内开发的新的实验性分配以及我们跟踪氧化状态的能力将使我们能够在理解硫循环方面取得阶段性变化-到目前为止，人们对硫循环知之甚少，但对理解气候和商业矿藏形成至关重要。对一整套地球化学元素的地幔输运进行数值模拟，迭代地球物理参数以接近地球化学观测数据的匹配，将使我们能够识别俯冲带和整个地幔对流过程中控制俯冲与原始挥发分在地幔中的地球化学分布的关键地球物理过程。总而言之，这些将导致在重建地球历史上地球深处的挥发性通量方面取得重大进展--这是一个巨大的科学挑战。

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.