ORIGIN OF PB ISOTOPE VARIATIONS IN FE-MN CRUSTS FROM THE IZU-BONIN FOREARC

Deciphering the relative importance of the various potential sources of Pb to the oceans (e.g. aeolian, volcanic, riverine, hydrothermal) is complicated primarily because of anthropogenic Pb pollution and is still debated. Most studies have focused on Pb isotopes records from sediments and Fe-Mn nodules from the deep oceans. In this study, we examine temporal records of Pb isotope variations in Fe-Mn crusts recovered using the Shinkai 6500 submersible on cruise YK04-05 from the shallow shores of the Ogasawara Ridge in the Izu-Bonin fore-arc region in order to determine distinct sources of Pb to this area. Two crusts displaying dense laminations were measured, photographed, thin-sectioned, and mounted on a series of 1” round epoxy moulds. XRD techniques showed that the mineralogy is dominated by vernadite with minor amounts of quartz and feldspar, indicating a hydrogenetic origin of the crusts. Major element data measured by ICP-AES confirm a hydrogenetic origin (high Fe/Mn 1.5-2.0; high Co/Zn 2-4; high Co+Ni+Cu 4000-6000 ppm). Pb isotopes were measured by laser ablation MC-ICP-MS at Oregon State University. The crusts have high 208Pb/206Pb (2.08-2.09) compared to deep water hydrogenetic Pacific crusts (2.04-2.08). In detail, the Pb isotope compositions rule out any significant role for Pb derived from local hydrothermal or volcanic sources. Instead, the closest match is with published data for the easily leached component that coats Chinese loess sediments, suggesting a predominantly aeolian source for Pb to these forearc Fe-Mn crusts. Spatial variations in Pb isotope composition are similar in both crusts implying similar growth rates. However, the mechanisms producing these temporal changes in Pb isotope composition are still uncertain.