New geochemical constraints on hydrothermal fluid fluxes at mid-ocean ridge axes and ridge flanks

这个研究项目解决了地球化学的一个非常基本的问题——有多少水经过海底热液系统的处理并从世界各地的海洋中排出？这是一个重要的问题，因为热液过程对许多元素（包括温室气体二氧化碳）的全球地球化学收支具有深远的影响，因此它们对海洋、大气的组成以及通过改变的海洋地壳的俯冲作用对上地幔的组成产生强烈影响。一般认为，由海洋地壳热模型得出的热液能量输出估计比对热液流体通量的直接地球化学估计更可靠，其中许多估计是基于单个元素和同位素系统（例如Mg和87Sr/86Sr）的海洋质量平衡约束不强。然而，将能量输出转化为热液通量充满困难，主要是由于不同温度的流体之间的热量分配未知（因此化学反应性也不同），以及可用热量在多大程度上被用来驱动海洋中脊轴的热液循环存在不确定性。这是不幸的，因为热液系统的地球化学影响与水通量直接相关，而不是与能量输出有关。在这项研究中，我们将使用一种新的地球化学方法来获得从海洋中脊轴的海底热液系统和存在于脊侧的较冷热液系统排放到海洋中的全球水通量的精确约束。该方法利用相关样品中铊（Tl）元素的浓度和同位素组成数据。最近的一项初步研究表明，新方法比其他地球化学方法更加可靠。然而，试点研究的结果价值有限，因为它们仅以少量分析数据为基础。本研究将通过对蚀变海洋基底岩石的Tl浓度和同位素组成的全面表征，利用来自不同构造环境和扩展速率的代表性样品，以及一套全球代表性的热液流体，来解决这一缺点。由于Tl同位素系统的独特性质，这些分析将提供迄今为止最可靠的热液流体通量的地球化学估计。

This research project addresses a very fundamental issue of geochemistry - how much water is processed through and expelled from the sub-seafloor hydrothermal systems that are found throughout the world's oceans? This is an important question because hydrothermal processes have a profound effect on the global geochemical budgets of numerous elements (including the greenhouse gas carbon dioxide) and they thus exert a strong influence on the composition of the oceans, the atmosphere and, through subduction of altered ocean crust, the upper mantle. Estimates of hydrothermal power outputs that are derived from thermal models of the oceanic crust are generally thought to be more reliable than direct geochemical estimates of hydrothermal fluid fluxes, many of which are based on the poorly constrained marine mass balances of individual elements and isotope systems (e.g., Mg and 87Sr/86Sr). The conversion of power outputs into hydrothermal water fluxes is fraught with difficulties, however, primarily due to the unknown partitioning of heat between fluids of different temperature (and hence different chemical reactivity) and uncertainties about the extent to which the available heat is used to drive hydrothermal circulation at mid-ocean ridge axes. This is unfortunate because the geochemical impact of hydrothermal systems is directly related to the water fluxes rather than the power outputs. In this study, we will use a novel geochemical approach to obtain precise constraints on the global water fluxes that are expelled into the oceans from both the hot sub-seafloor hydrothermal systems of mid-ocean ridge axes and the cooler hydrothermal systems that exist at ridge flanks. The method utilizes concentration and isotope composition data for the element thallium (Tl) in relevant samples. A recent pilot study demonstrated that the new approach is significantly more robust than other geochemical methods. The results of the pilot study are only of limited value, however, because they are based on only a small number of analytical data. The present investigation will address this shortcoming, through a thorough characterization of the Tl concentrations and isotopic compositions of altered oceanic basement rocks, using representative samples from different tectonic settings and spreading rates, and a suite of globally representative hydrothermal fluids. Due to the unique properties of the Tl isotope system, these analyses will provide the most reliable geochemical estimates of hydrothermal fluid fluxes available to date.