Rhenium-osmium isotopes and highly siderophile elements in ultramafic rocks from the Eoarchean Saglek Block, northern Labrador, Canada: implications for Archean mantle evolution

摘要: Abstract We determined highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) concentrations 187Os/188Os ratios for ultramafic rocks distributed over the Eoarchean gneiss complex of Saglek-Hebron area in northern Labrador, Canada order to constrain what extent variations HSE abundances are recorded Early Archean mantle that have well-resolved 182W isotope anomalies relative present-day (∼+11 ppm: Liu et al., 2016). The samples analysed here been previously classified into two suites: mantle-derived peridotites occurring as tectonically-emplaced slivers lithospheric mantle, metakomatiites comprising mostly pyroxenitic layers supracrustal units dominated by amphibolites. Although previous Sm-Nd Pb-Pb studies provided whole-rock isochrons indicative ∼3.8 Ga protolith formation both suites, our Re-Os data on a similar set yield considerably younger errorchrons with ages 3612 ± 130 Ma (MSWD = 40) 3096 ± 170 Ma (MSWD = 10.2) metakomatiite respectively. respective initial 187Os/188Os = 0.10200 ± 18 0.1041 ± 18 within range chondrites. Re-depletion Os model unradiogenic from suites consistent (metakomatiite TRD = 3.4–3.6 Ga; TRD = 2.8–3.3 Ga). These observations suggest not coeval. However, estimated sources ultramafics terms their broadly chondritic evolution patterns. In detail, show small excess Ru/Ir modern primitive but ∼20% deficit absolute 74 ± 18% PUM; 82 ± 10% PUM), Isua source Neoarchean komatiite around world (∼70–86% PUM). This demonstrates lower unique komatiites, rather might be ubiquitous feature convecting mantle. tentatively suggests late accretion components boosted inventory after core separation Hadean, was depleted its content today. Further investigation is required explore this hypothesis.