Sources of osmium to the modern oceans: new evidence from the super(190)Pt- super(186)Os system

• Published in: Geochimica et cosmochimica acta 2004-03, Vol.68 (6), p.1243-1252
• Main Authors: McDaniel, D K, Walker, R J, Hemming, SR, Horan, M F, Becker, H, Grauch, R I
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 0016-7037
• DOI: 10.1016/j.gca.2003.08.020

High precision Os isotope analysis of young marine manganese nodules indicate that whereas the composition of modern seawater is radiogenic with respect to super(187)Os/ super(188)Os, it has super(186)Os/ super(188)Os that is within uncertainty of the chondritic value. Marine Mn nodule compositions thus indicate that the average continental source of Os to modern seawater had long-term high Re/Os compared to Pt/Os. Analyses of loess and freshwater Mn nodules support existing evidence that average upper continental crust (UCC) has resolvably suprachondritic super(186)Os/ super(188)Os, as well as radiogenic super(187)Os/ super(188)Os. Modeling the composition of seawater as a two-component mixture of oceanic/cosmic Os with chondritic Os compositions and continentally-derived Os demonstrates that, insofar as estimates for the composition of average UCC are accurate, congruently weathered average UCC cannot be the sole continental source of Os to seawater. Our analysis of four Cambrian black shales confirm that organic-rich sediments can have super(187)Os/ super(188)Os ratios that are much higher than average UCC, but super(186)Os/ super(188)Os compositions that are generally between those of chondrites and average-UCC. Preferential weathering of black shales can result in dissolved Os discharged to the ocean basins that has a much lower super(186)Os/ super(188)Os than does average upper crust. Modeling the available data demonstrates that augmentation of estimated average UCC compositions with less than 0.1% additional black shale and 1.4% additional ultramafic rock can produce a continental end-member Os isotopic composition that satisfies the requirements imposed by the marine Mn nodule data. The interplay of these two sources provides a mechanism by which the super(187)Os/ super(188)Os of seawater can change as sources and weathering conditions change, yet seawater super(186)Os/ super(188)Os varies only minimally.