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

这个研究项目涉及地球化学的一个非常基本的问题-有多少水是通过遍布世界各大洋的海底以下热液系统处理和排出的？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有着深远的影响，因此对海洋、大气层的组成产生重大影响，并通过蚀变洋壳的俯冲作用对上地幔产生重大影响。一般认为，从洋壳热模型得出的热液功率输出估计值比热液流体通量的直接地球化学估计值更可靠，其中许多估计值是基于单个元素和同位素系统的海洋质量平衡约束不佳（例如，Mg和87 Sr/86 Sr）。然而，将电力输出转换为热液水通量困难重重，主要是因为不同温度的流体之间的热量分配情况不明（因此化学反应性不同），而且不确定可用热量在多大程度上用于驱动大洋中脊轴线的热液循环。这是不幸的，因为热液系统的地球化学影响与水通量而不是电力输出直接相关。在这项研究中，我们将使用一种新的地球化学方法，以获得精确的限制全球水通量，被驱逐到大洋中脊轴的热的海底热液系统和较冷的热液系统，存在于脊侧翼的海洋。该方法利用相关样品中元素铊（Tl）的浓度和同位素组成数据。最近的一项试点研究表明，这种新方法比其他地球化学方法更可靠。然而，试点研究的结果价值有限，因为这些结果仅基于少量分析数据。目前的调查将解决这一缺点，通过一个彻底的表征的Tl浓度和同位素组成的蚀变海洋基底岩石，使用代表性的样品从不同的构造环境和扩展速度，和一套全球代表性的热液流体。由于铊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.