DOI: 10.1594/pangaea.712126
title: Pb concentrations, Pb isotopic composition, and amount of metalliferous components in pelagic sediments from the Pacific Ocean

description: The amount of lead annually transferred from oceanic crust to metalliferous sediments was estimated in order to test the hypothesis that a non-magmatic flux of lead causes the Pb surplus in the continental crust. A Pb surplus has been inferred from global crust-mantle lead mass balances derived from lead concentration correlations with other trace elements and from lead isotope systematics in oceanic basalts. DSDP/ODP data on the amount of metalliferous sediments in the Pacific Ocean and along a South Atlantic traverse are used to calculate the mean worldwide thickness of 3 (+/-1) m for purely metalliferous sediment componens. Lead isotope ratios of 39 metalliferous sediments from the Pacific define mixing lines between continent-derived (seawater) and mantle-derived (basaltic) lead, with the most metal-rich sediments usually having the most mantle-like Pb isotope composition. We used this isotope correlation and the Pb content of the 39 metalliferous sediments to derive an estimate of 130 (+/-70) µg/g for the concentration of mantle-derived lead in the purely metalliferous end-member. Mass balance calculations show that at least 12 (+/-8)% of the lead, annually transferred from upper mantle to oceanic crust at the ocean ridges, is leached out by hydrothermal processes and re-deposited in marine sediments. If all of the metalliferous lead is ultimately transferred to the continental crust during subduction, the annual flux of this lead from mantle to continental crust is 2.6 (+/-2.0) * 10**6 kg. Assuming this transfer rate to be proportional to the rate of oceanic plate production, one can fit the lead transfer to models of plate production rate variations through time. Integrating over 4 Ga, hydrothermal lead transfer to the continental crust accounts for a significant portion of the Pb surplus in the continental crust. It therefore appears to be one of the main reasons for the anomalous behavior of lead in the global crust-mantle system.
description: Lead isotopic compositions were measured at the MPI in Mainz. All samples (30 ng loads) were measured at least twice to ensure reproducibility of the results, and the mean values corrected for fractionation by about 1.4 per mil amu**-1 are given. Replicate analysis of 30 ng NBS 982 (n = 33) demonstrate that the external precision (2 sigma) in the isotope ratios is 0.46 per mil amu**-1 for the 206Pb/204Pb and 0.40 per mil amu**-1 for the 207Pb/204Pb and 208Pb/204Pb values. The fractionation factor was calculated using the NBS 982 values given in Todt et al. (1993). We were able to improve the procedural blank during the course of this study from about 150 pg to about 50 pg Pb. Lead concentrations were determined on freeze-dried samples by isotope dilution mass spectrometry using a moderately enriched 205Pb spike and corrected to 105°C dried bulk sediment samples. The reproducibility was determined by duplicate analysis to be better than 1%. The in situ Pb concentrations were calculated using DSDP/ODP data on the wet water content, or the bulk density and porosity for the respective core intervals. The amount of metalliferous components were estimated by microscopic smear slide investigations. The uncertainties are relatively large (20%, relative) and increase towards lower concentrations.
description: Supplement to: Peucker-Ehrenbrink, Bernhard; Hofmann, Albrecht W; Hart, Stanley R (1994): Hydrothermal lead transfer from mantle to continental crust: the role of metalliferous sediments. Earth and Planetary Science Letters, 125(1-4), 129-142