186Os/188Os Isotopic Compositions of Peridotites: Constraints on Melt Depletion and Pt/Os Evolution of the Upper Mantle

Abstract:

Global correlations between Al₂O₃ and Pt/Os in mantle peridotites suggest that Pt behaves incompatibly relative to Os during partial melting [c.f., 1]. Because ¹⁹⁰Pt decays to ¹⁸⁶Os (t_1/2 = 468 Ga), correlations between ¹⁸⁶Os/¹⁸⁸Os and peridotite fertility can be used to constrain the long-term Pt/Os evolution of the depleted mantle and the initial Pt/Os ratio of the primitive upper mantle (PUM). We examined ¹⁸⁶Os/¹⁸⁸Os in mantle peridotites from continental (Rio Grande Rift/Colorado Plateau) and oceanic (Lena Trough, Hawaiian Islands) settings that span a wide range in fertility (Al₂O₃ ~0.67-4.42 %) and ¹⁸⁷Os/¹⁸⁸Os ratios (0.1138-0.1305). The new data define a narrow range in ¹⁸⁶Os/¹⁸⁸Os (0.1198338 to 0.1198393, 2 SD~24 ppm), placing constraints on long-term Pt/Os variability in the DMM. ¹⁸⁶Os/¹⁸⁸Os is broadly correlated with indices of melt depletion including spinel Cr#, clinopyroxene Cr#, and clinopyroxene Yb content, consistent with the inferred relative compatibility of Pt and Os during partial melting. Extrapolation of the alumina-¹⁸⁶Os/¹⁸⁸Os trend to PUM alumina content (~4.5 wt% Al₂O₃; [2]) suggests a PUM ¹⁸⁶Os/¹⁸⁸Os of ~0.1198380±15, similar to the ¹⁸⁶Os/¹⁸⁸Os of H chondrites (~0.1198398±16; [3]). This ¹⁸⁶Os/¹⁸⁸Os value is consistent with a PUM Pt/Os of 1.8±0.3, similar to Pt/Os values measured in several classes of chondrites (Carbonaceous ~1.9±0.2, Ordinary ~2.0±0.3 and Enstatite ~1.9±0.2; [3]). Whereas ~84% of peridotites worldwide [excluding low-[Os] samples (<1 ppb Os) that may have been compromised by melt-rock reaction and/or weathering and alteration] with measured Pt/Os ratios have Pt/Os between 0.3 and 3.1 (the range permissible from ¹⁸⁶Os/¹⁸⁸Os variations for melt extraction from PUM at ~1.5 Ga), only ~36% fall between 1.3 and 2.2 (a narrower range consistent with an older ~4.5 Ga melt depletion age). This suggests that much of the observed Pt/Os variability in mantle peridotites is relatively recent. Close agreement between our inferred Pt/Os_PUM with previous estimates inferred from chondrites [3] is consistent with the addition of a chondritic late veneer to explain the HSE abundances in the upper mantle. [1] Becker et al., GCA 2006; [2] McDonough et al., Chem Geo 1995; [3] Brandon et al., GCA 2006