HYDROTHERMAL LEAD TRANSFER FROM MANTLE TO CONTINENTAL-CRUST - THE ROLE OF METALLIFEROUS SEDIMENTS

The amount of lead annually transferred from oceanic crust to metallif erous sediments was estimated in order to test the hypothesis that a n on-magmatic flux of lead causes the Pb surplus in the continental crus t. A Pb surplus has been inferred from global crust-mantle lead mass b alances derived from lead concentration correlations with other trace elements and from lead isotope systematics in oceanic basalts. DSDP/OD P data on the amount of metalliferous sediments in the Pacific Ocean a nd along a South Atlantic traverse are used to calculate the mean worl dwide thickness of 3(+/-1) m for purely metalliferous sediment compone nts. Lead isotope ratios of 39 metalliferous sediments from the Pacifi c define mixing lines between continent-derived (seawater) and mantle- derived (basaltic) lead, with the most metal-rich sediments usually ha ving the most mantle-like Pb isotope composition. We used this isotope correlation and the Pb content of the 39 metalliferous sediments to d erive an estimate of 130(+/-70) mug/g for the concentration of mantle- derived lead in the purely metalliferous end-member. Mass balance calc ulations show that at least 12(+/-8)% of the lead, annually transferre d from upper mantle to oceanic crust at the ocean ridges, is leached o ut by hydrothermal processes and re-deposited in marine sediments. If all of the metalliferous lead is ultimately transferred to the contine ntal crust during subduction, the annual flux of this lead from mantle to continental crust is 2.6(+/-2.0) x 1(0)6 kg. Assuming this transfe r rate to be proportional to the rate of oceanic plate production, one can fit the lead transfer to models of plate production rate variatio ns through time. Integrating over 4 Ga, hydrothermal lead transfer to the continental crust accounts for a significant portion of the Pb sur plus in the continental crust. It therefore appears to be one of the m ain reasons for the anomalous behavior of lead in the global crust-man tle system.