Platinum-group element abundances and Re–Os isotopic systematics of the upper continental crust through time: Evidence from glacial diamictites

Abstract

The fine-grained matrix of glacial diamictites, deposited periodically by continental ice sheets over much of Earth history, provides insights into the average composition and chemical evolution of the upper continental crust (UCC) , and references therein). The concentrations of platinum-group elements (PGEs, including Os, Ir, Ru, Pt and Pd) and the geochemically related Re, as well as 187 Re/ 188 Os and 187 Os/ 188 Os ratios, are reported here for globally-distributed glacial diamictites that were deposited during the Mesoarchean, Paleoproterozoic, Neoproterozoic and Paleozoic eras. The medians and averages of PGE concentrations of these diamictite composites decrease from the Mesoarchean to the Neoproterozoic, mimicking decreases in the concentrations of first-row transition elements (Sc, V, Cr, Co and Ni). By contrast, Re concentrations are highly variable with no discernable trend, owing to its high solubility. Assuming these diamictites are representative of average UCC through time, the new data are fully consistent with the previous inference that the Archean UCC contained a greater proportion of mafic-ultramafic rocks relative to younger UCC. Linear regressions of PGEs versus Cr and Ni concentrations in all the diamictite composites from the four time periods are used to estimate the following concentrations of the PGEs in the present-day UCC: 0.059 ± 0.016 ng/g Os, 0.036 ± 0.008 ng/g Ir, 0.079 ± 0.026 ng/g Ru, 0.80 ± 0.22 ng/g Pt and 0.80 ± 0.26 ng/g Pd (2r of 10,000 bootstrapping regression results). These PGE estimates are slightly higher than the estimates obtained from loess samples. We suggest this probably results from loess preferentially sampling younger UCC rocks that have lower PGE concentrations, or PGEs being fractionated during loess formation. A Re concentration of 0.25 ± 0.12 ng/g (2r) is obtained from a regression of Re versus Mo. From this, time-integrated 187 Re/ 188 Os and 187 Os/ 188 Os ratios for the UCC are calculated, assuming an average UCC residence duration of $2.0 Ga, yielding ratios of 20 ± 12 and 0.80 ± 0.38 (2r), respectively.

Tungsten-182 in the upper continental crust: Evidence from glacial diamictites

“…Further, Garçon et al (2017) examined the 143 Nd/ 144 Nd and 176 Hf/ 177 Hf isotopic compositions of Archean metasedimentary rocks from the Barberton area, southern Africa. Consistent with the conclusions of Gaschnig et al (2016) and Chen et al (2016), that study also concluded that the Archean UCC sampled by the metasedimentary rocks was more mafic than modern UCC.…”

“…The diamictite composites can, therefore, be considered representative of the glacially sampled UCC at each location at the time of deposition. Additional details regarding the preparation of the diamictite composites, major and trace element compositions, as well as highly siderophile element (HSE) abundances and Li, O, Ba and Re-Os isotopic compositions of individual diamictites, as well as the composites were previously reported (Gaschnig et al, 2014(Gaschnig et al, , 2016Li et al, 2016;Chen et al, 2016;Nan et al, 2018).…”

“…For example, major and trace element variations in glacial diamictites through time suggest that average Archean UCC had a larger proportion of mafic, and especially ultramafic (komatiitic) inputs, compared to glacial diamictites with younger provenance (Gaschnig et al, 2014(Gaschnig et al, , 2016. This difference is particularly apparent in the higher concentrations of Ni and highly siderophile elements (HSE: e.g., Re, Os, Ir, Ru, Pt, Pd) in diamictites from older provenances compared with younger (Chen et al, 2016).…”

“…Here, we report 182 W isotopic data for glacial diamictites that were deposited during the Archean through the Phanerozoic. We analyzed a subset of composite samples studied for major and trace elements including HSE, as well as oxygen, lithium and barium stable isotopes by Gaschnig et al (2016), Li et al (2016), Chen et al (2016) and Nan et al (2018). The primary objective is to assess whether or not W isotopic anomalies are present in these rocks, and if so, whether the anomalies can be correlated with other chemical or isotopic characteristics.…”

High‐precision Determination of Gold Mass Fractions in Geological Reference Materials by Internal Standardisation

“…The mixture of concentrated HCl and HNO 3 (aqua regia or reverse aqua regia) is widely used to digest geological silicate samples in high-pressure asher or Carius tubes for determination of PGE and Re mass fractions (e.g., Shinotsuka and Suzuki 2007, Fischer-G€ Li et al 2014, 2015a, Puchtel et al 2014, Chu et al 2015, Chen et al 2016, Liu et al 2016, Meisel and Horan 2016. We digested samples using reverse aqua regia in Carius tubes, and the results for Au, Re and PGEs are listed in Table 3 and Table S1.…”