Os isotopic compositions of mantle peridotites and steels : implications for Pt-Re-Os evolution of the Earth's upper mantle and continental crust
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The ¹⁹⁰Pt-¹⁸⁶Os system can be useful to examine a number of different geologic processes including core-mantle interaction, examining the role of pyroxenites or sulfides in mantle melting and dating of Platinum Group Element (PGE) based ores. Limited natural variation of ¹⁸⁶Os/¹⁸⁸Os in the mantle requires extremely high precision to detect natural variations. I identified Johnson noise on baseline integrations to be the largest source of analytical error for the quantities of Os and ¹⁸⁶Os beam intensities utilized in previous ¹⁸⁶Os/¹⁸⁸Os studies. Increasing the duration of baseline measurement significantly reduces the Johnson noise error, allowing to achieve maximum external precision for a given sample size. ¹⁸⁶Os/¹⁸⁸Os variations in mantle peridotites provide constraints on the long-term Pt/Os evolution of the depleted mantle and the Pt/Os ratio of the primitive upper mantle (PUM). Fertility-¹⁸⁶Os/¹⁸⁸Os correlations in mantle peridotites were used to infer PUM ¹⁸⁶Os/¹⁸⁸Os and Pt/Os ratios. I found that PUM Pt/Os ratio is broadly chondritic, consistent with a late veneer addition of chondritic materials and similar to most fertile peridotites. I proposed that the departures in PUM Pd/Ir and Ru/Ir ratios from chondritic values could be explained by minor metal or sulfide segregation to the Earth’s core occurring concurrently with late veneer addition. Combined ¹⁸⁶Os/¹⁸⁸Os-¹⁸⁷Os/¹⁸⁸Os isotopic and Pt/Os and Re/Os variability in peridotites suggest an average mantle homogenization timescale of ~1.2 Ga, consistent with timescales predicted for whole mantle convection. Steels offer a unique opportunity to determine ¹⁸⁶Os-¹⁸⁷Os compositions of crustal and mantle-reservoirs sampled by iron ore, coal and chromitites used in its manufacture. My results indicate that mixing between unradiogenic, upper mantle-like Os derived from chromitites and radiogenic, crustal like Os from iron ore and coal can explain the observed Os concentration and isotope variations in steels. Extrapolation of ¹⁸⁷Os/¹⁸⁸Os-¹⁸⁷Os/¹⁸⁸Os trend in steels allows inference of its mantle and crustal- end member components. Extrapolation of ¹⁸⁷Os/¹⁸⁸Os-¹⁸⁶Os/¹⁸⁸Os trends to the estimated ¹⁸⁷Os/¹⁸⁸Os of chromitites yields a ¹⁸⁶Os/¹⁸⁸Os value consistent with derivation of chromitites from mantle sources during Archean to Proterozoic and subsequent evolution with subchondritic Pt/Os ratios. Extrapolation of steel ¹⁸⁷Os/¹⁸⁸Os-¹⁸⁶Os/¹⁸⁸Os trends to ¹⁸⁷Os/¹⁸⁸Os values typical for banded iron formations and coal yields a ¹⁸⁶Os/¹⁸⁸Os value indistinguishable from present-day upper mantle and within uncertainty of the PUM at the time of BIF and/or coal formation, but systematically lower than bulk upper continental crust (UCC). Previous studies (McDaniel et al., 2004) also report similar unradiogenic (in comparison to upper continental crust) ¹⁸⁶Os/¹⁸⁸Os values for seawater derived marine Mn-nodules and black shales. Preferential weathering of crustal sulfides with low Pt/Os ratios and lower ¹⁸⁶Os/¹⁸⁸Os values than bulk UCC can explain the systematically lower ¹⁸⁶Os/¹⁸⁸Os values in crustal weathering products relative to bulk UCC.