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.
