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

大洋中脊轴和洋脊侧翼热液流体通量的新地球化学约束

基本信息

批准号：
NE/E005616/1
负责人：
Mark Rehkamper
金额：
$ 26.58万
依托单位：
Imperial College London
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2007
资助国家：
英国
起止时间：
2007 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FE005616%2F1
关键词：
New geochemical constraints hydrothermal fluid

项目摘要

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.

这一研究项目解决了一个非常根本的地球化学问题--通过世界各地海洋中发现的海底下热液系统处理和排出多少水？这是一个重要的问题，因为热液作用对许多元素(包括温室气体二氧化碳)的全球地球化学收支有深远的影响，从而对海洋、大气的组成产生重大影响，并通过洋壳的俯冲作用对上地幔的组成产生重大影响。根据洋壳热力模型得出的热液发电量估计通常被认为比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个别元素和同位素系统(例如，镁和87Sr/86Sr)的海洋质量平衡限制较差的。然而，将电力输出转化为热液水通量充满困难，主要是因为热在不同温度的流体之间的分配未知(因此具有不同的化学反应活性)，以及关于可用热在多大程度上被用来驱动大洋中脊轴线上的热液循环的不确定性。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，而不是与电力输出有关。在这项研究中，我们将使用一种新的地球化学方法来获得对从大洋中脊轴线的炎热海底热液系统和存在于海脊侧翼的较冷热液系统排入海洋的全球水通量的精确限制。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，新方法明显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，因为它们只基于少量的分析数据。本次研究将通过使用来自不同构造背景和扩张速率的代表性样品和一套具有全球代表性的热液流体，彻底表征蚀变洋底岩的TL浓度和同位素组成，从而解决这一缺点。由于TL同位素体系的独特性质，这些分析将提供迄今可用的最可靠的热液流体通量的地球化学估计。