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
Introduction
Core formation resulted in almost complete sequestration of the Highly Siderophile Elements (HSE) from the Earth’s mantle (cf. Newsom, 1990). However, abundances of HSE in the Earth’s mantle are substantially higher than expected from metal-silicate partition coefficients experimentally determined at low pressures (Capobianco et al., 1993, Newsom, 1990). Furthermore, most of the HSE occur in broadly chondritic relative proportions, although there is significant variation in measured HSE ratios of mantle peridotites (cf. Meisel et al., 2001, Becker et al., 2006, Brandon et al., 2006). These observations have been taken as evidence for the addition of a ‘late veneer’ of chondritic material to Earth’s mantle after core formation (cf. Morgan, 1986), which would have elevated mantle HSE abundances and established their chondritic relative proportions. Although core segregation under high pressure and/or high temperature (high P-T) conditions may explain the mantle abundances of many slightly and moderately siderophile elements and possibly some HSE such as Pd (cf. Righter et al., 2008, Righter, 2011), it is uncertain whether high P-T core segregation can account for the near- chondritic relative abundances of a wide range of HSE.
Elemental ratios are susceptible to modification by processes such as melt-infiltration and metasomatism and/or weathering and alteration. This makes reliable estimation of Primitive Upper Mantle (PUM) compositions using measured elemental ratios subject to large uncertainties reflecting inherent scatter in the data resulting from the complicated, multi-stage history of many peridotites. For example, Becker et al. (2006) estimated Re/OsPUM ≈ 0.096 ± 0.029 (1 SE) by extrapolating Re/Os ratios in variably depleted peridotites to primitive mantle compositions (Al2O3 ~ 4.45 wt.%; McDonough and Sun, 1995), but the sizable error in this estimate reflects the large inherent scatter in mantle peridotite Re/Os ratios. Long-lived radiogenic isotope ratios (e.g. 187Os/188Os) can potentially provide more precise constraints on some primitive mantle elemental ratios as they reflect time-integrated parent/daughter ratios and are largely immune to recent modification. Extrapolation of PUM parent/daughter ratios from isotope systematics in peridotites also provides an independent check on estimates based on elemental abundances. Because 187Re decays to form 187Os [t1/2 ~ 42 Ga (Selby et al., 2007)], 187Os/188Os ratios in peridotites can constrain long-term Re/Os ratios in the mantle. Previous studies (cf. Meisel et al., 2001) have inferred that the 187Os/188Os of the PUM is ≈ 0.1296 ± 9. From this estimate, Re/OsPUM is calculated to be ≈ 0.090 ± 0.001 (1 SE), which is consistent with but significantly more precise than estimates utilizing measured elemental ratios (cf. Becker et al., 2006). This estimate is statistically indistinguishable from the estimated Re/Os of ordinary and enstatite chondrites (cf. Horan et al., 2003, Brandon et al., 2006). Thus, the Re/Os of the silicate Earth appears to be broadly chondritic, consistent with the late veneer model (cf. Becker et al., 2006).
Because 190Pt undergoes alpha decay to form 186Os [t1/2 ~ 468 Ga; Begemann et al., 2001], 186Os/188Os can be similarly used to infer the Pt/Os ratio of the PUM. Previous studies have examined 186Os/188Os in a variety of upper mantle materials. This dataset includes Os-Ir alloys (Walker et al., 2005) and chromitites (cf. Brandon et al., 1998, Walker et al., 2005), most of which have sub-chondritic 187Os/188Os. Other analyzed upper mantle materials include a small set of abyssal peridotites (AP) from a single section of the Kane Fracture Zone (KFZ). These peridotites span a narrow range in isotopic composition (187Os/188Os ≈ 0.1226-0.1276) and extent of melt depletion (whole rock Al2O3 ≈ 1.2-1.8 wt.%) (Brandon et al., 2000), which makes it difficult to evaluate correlations between 186Os/188Os and peridotite composition or melt-depletion history.
In this paper, we report 186Os/188Os in peridotites from continental (Rio Grande Rift and Colorado Plateau) and oceanic (Lena Trough and Hawaiian Islands) settings that span a wide range in fertility and 187Os/188Os. Correlations between fertility and 186Os/188Os are utilized to investigate the extent to which long-term Pt/Os fractionation in the upper mantle is controlled by melt depletion and to constrain the 186Os/188Os and thus the Pt/Os ratio of the PUM. Pt/OsPUM inferred from 186Os/188Os of fertile peridotites is combined with PUM estimates for other HSE ratios to evaluate the late veneer hypothesis. Because the addition of a late veneer alone cannot explain a combination of chondritic (e.g., Pt/Os) and suprachondritic (e.g., Pd/Ir) mantle HSE ratios, we examine alternate models involving minor metal or sulfide segregation concurrent with late veneer addition. Finally, the degree of isotopic (186Os/188Os and 187Os/188Os) and parent/daughter (Pt/Os and Re/Os) variability observed in upper mantle peridotites is utilized to constrain the timescale of mantle mixing and homogenization.
Section snippets
Samples
Mantle peridotites were selected from both continental (Rio Grande Rift and Colorado Plateau) and oceanic (Hawaiian Island and Lena Trough) settings that span a wide range in fertility and 187Os/188Os. Details of each of the sampling locations are given below.
- a)
Lena Trough
Abyssal peridotites (AP) from the Lena Trough were recovered from deep-sea dredges during the Polarstern ARK-XV-2 (1999) and Polarstern ARK-XX-2 (2004) cruises. Samples have undergone variable degrees of seafloor weathering and
Analytical methods
Detailed description of all analytical methods including standard values can be found in the supplementary materials. Most of the samples selected for this study have been previously analyzed for whole rock and mineral major and trace element compositions and detailed procedures are reported elsewhere (cf. Bizimis et al., 2007, Byerly and Lassiter, 2012, Lassiter et al., 2014). For this study, whole rock major and trace element compositions were analyzed via XRF and ICP-MS at the Geo-Analytical
Results
Bulk rock major and trace element concentrations are reported in Table S.1. Mineral major element compositions are reported in tables S.2-S.3 and clinopyroxene trace element concentrations are reported in Table S.4. Measured peridotites span a wide range of compositions. Bulk rock Al2O3 ranges from 0.67 to 4.42 wt.%. Spinel Cr# [molar Cr/(Cr + Al)] ranges from 8.2 to 50. Clinopyroxene Cr# varies from 5 to 25. Whole rock Al2O3, spinel Cr#, clinopyroxene Cr# and clinopyroxene Yb contents are all
Effect of melt depletion on mantle 190Pt-186Os evolution and constraints on the Pt/Os ratio of the PUM
Extrapolation of 186Os/188Os-fertility trends can be used to constrain PUM 186Os/188Os and Pt/Os in the same way previous studies (cf. Meisel et al., 2001) have utilized 187Os/188Os–fertility trends in peridotites to constrain PUM 187Os/188Os and Re/Os. In addition, if Pt/Os in peridotites is controlled by melt depletion, fertile peridotites may extend to higher Pt/Os, and therefore higher 186Os/188Os values than the refractory peridotites analyzed to date (Brandon et al., 2000). Previous
Conclusions
186Os/188Os ratios in variably melt-depleted peridotites from continental (Rio Grande Rift and Colorado Plateau) and oceanic (Lena Trough and Hawaiian Islands) settings correlate with various indices of peridotite fertility. This suggests that melt depletion largely controls 186Os/188Os variations in the upper mantle. Extrapolation of 186Os/188Os-fertility trends to PUM compositions suggests a mean PUM 186Os/188Os of 0.1198378 ± 23 (2 SD), which is slightly lower than H-chondrites [≈ 0.1198398 ± 16
Acknowledgements
This work was supported by NSF Grant EAR-1321937 and by the Jackson School of Geosciences, University of Texas at Austin. This manuscript benefitted from constructive comments by Laurie Reisberg, an anonymous reviewer and editor Catherine Chauvel as well as comments by Graham Pearson on an earlier draft.
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