186Os/188Os variations in upper mantle peridotites: Constraints on the Pt/Os ratio of primitive upper mantle, and implications for late veneer accretion and mantle mixing timescales

Abstract

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). We report new 186Os/188Os data for mantle peridotites from continental (Rio Grande Rift and Colorado Plateau) and oceanic (Lena Trough and Hawaiian Islands) settings that span a wide range in fertility (Al2O3 ≈0.67-4.42 wt.%) and 187Os/188Os ratios (0.1138-0.1305). Although peridotite 186Os/188Os values span only a narrow range (from 0.1198345 to 0.1198384), 186Os/188Os broadly correlates with indices of melt depletion including bulk rock Al2O3, spinel Cr#, and clinopyroxene Cr#, consistent with Pt depletion in residual peridotites.PUM 186Os/188Os is estimated to be 0.1198378±23 (2 SD) based on extrapolation of 186Os/188Os-fertility trends, which is very slightly lower than H-chondrites [≈0.1198398±16 (2 SD); Brandon et al., 2006]. This value is consistent with a PUM Pt/Os of 1.7±0.2, similar to average Pt/Os ratios of fertile continental peridotites. The inferred PUM Pt/Os is slightly lower than but within error of Pt/Os values measured in several classes of chondrites [Carbonaceous ≈1.8±0.2, Ordinary ≈1.9±0.1, and Enstatite ≈1.9±0.1 (Brandon et al., 2006)] indicating that PUM Pt/Os is broadly chondritic. In contrast, estimates for PUM Ru/Ir and Pd/Ir (cf. Becker et al., 2006) are suprachondritic. The addition of a chondritic late veneer alone cannot create a combination of chondritic and suprachondritic HSE ratios for the PUM. Instead, minor core segregation occurring concurrently with the addition of a late veneer may explain the observed mantle HSE abundances and ratios.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. In contrast, combined Hf-Nd isotopic and Lu/Hf and Sm/Nd variability in peridotites indicate much shorter homogenization timescales (<0.4 Ga), potentially reflecting enhanced homogenization by melt-rock interaction to which the Pt-Os and Re-Os systems are relatively immune. The mechanical mixing timescale inferred from Os isotopes is consistent with timescales predicted for whole mantle convection.