Re-Os isotope systematics and weathering of Precambrian crustal rocks: Implications for the marine osmium isotope record

摘要

The effects of weathering of freshly exposed Precambrian rocks on the mobility of osmium were studied on soils developed on a chronosequence of glacial moraines from the western Wind River Range in Wyoming. The Os budget of the Precambrian granitoid source rocks is dominated by Os-rich trace phases, such as magnetite. Amongst the major silicates, biotite, or a trace phase in biotite, carries most of the Os and Re and is also the most radiogenic mineral with ~(187)Os/~(186)Os of ～113 and ~(187)Re/~(186)Os of ～2,000. Re-Os isotope systematics of source rocks and soils indicate that rapid oxidation of magnetite mobilizes Os with an isotopic composition similar to the isotopic composition of the bulk soils. A very radiogenic fraction of Os is mobilized through preferential weathering of biotite. Radiogenic runoff from Precambrian shields, inferred from osmium isotope analyses of freshwater Fe-Mn-nodules suggests that high-latitude Precambrian shields are important source areas of radiogenic Os in seawater. We propose that glacial scouring and weathering of glacial tills exposed after deglaciation of Precambrian shields surrounding the North Atlantic provides a mechanism for the slightly more radiogenic nature of North Atlantic seawater compared to other seawater masses. Glacial-interglacial variations in the osmium isotopic composition of seawater seem plausible and may be triggered by changes in weathering regimes on glacial-interglacial time scales in high-latitude shield areas surrounding the North Atlantic.