Collaborative Research: Hf and Pb Isotope Ratios in Seawater - Indicators of Paleo-Hydrothermal Activity

本研究旨在研究Hf和Pb在海水和热液系统中的行为，目的是利用海洋沉积物中这些元素的同位素比率作为过去火山活动速率的指标。海相Hf和Pb的同位素组成表明，这些元素的海洋库存中有很大一部分来自“地幔”来源：俯冲的年轻火山岩。造山带和洋中脊。由于Hf和Pb在海洋中的停留时间不同，它们是互补的。Hf在海洋中的停留时间与海洋混合时间相似，使其在整个海洋中的同位素相对均匀。新的分析发展使得现在可以在一定体积的天然水中测量Hf的浓度和同位素组成。它的海洋化学现在可以更好地定义。Pb在海水中的停留时间较短，来自洋中脊的热液通量局部主导其同位素组成，从而可以确定该通量的区域变化。地幔通量变化模型将基于同位素系统学，使地幔Hf和Pb与海洋的比例得以明确确定。将Pb和Hf与Sr通量模型结合起来，可以比单独使用Sr模型更准确地确定过去地幔流向海洋的通量变化。这样就有可能检验这样一种假设，即更快的扩张速度、火山活动速度和增加的地幔向海洋的通量与其他扩张速度变化的测量方法是等同的。这反过来将使其他元素（如碳）的全球地球化学循环模型得以改进。

Collaborative Research: Hf and Pb Isotope Ratios in Seawater: Indicators of Paleo-Hydrothermal Activity This study is to investigate the behavior of Hf and Pb in seawater and hydrothermal systems with the goal of using the isotope ratios of these elements in marine sediments as indicators of the past rates of volcanism. The isotopic compositions of marine Hf and Pb indicate that large proportions of the oceanic inventory of these elements are derived from 'mantle' sources: young volcanic rocks of subduction/.orogenic zones and mid-ocean ridges. Hf and Pb are complementary because of their different oceanic residence times. Hf has a residence time in the oceans similar to ocean mixing times, making it relatively isotopically homogeneous throughout the oceans. New analytical developments make it now possible to measure the concentration and isotopic composition of Hf in reasonable volumes of natural waters. Its marine chemistry can now be better defined. Pb has a short residence time in seawater, and the hydrothermal flux from mid-ocean ridges locally dominates its isotopic composition, thus allowing regional variations in this flux to be determined. Models of mantle flux variations will be based on isotope systematics that allow the proportion of mantle Hf and Pb to the oceans to be determined unequivocally. Combination of Pb and Hf with Sr flux models will make it possible to define past changes in mantle fluxes to the oceans more accurately than has been possible by using Sr alone. It will then be possible to test the hypothesis that faster spreading rates, volcanism rates, and increased mantle fluxes to the oceans are synonymous by comparison with other measures of spreading rate variations. This in turn will allow a refinement of models of global geochemical cycles of other elements, such as carbon.