HSE Abundances Research Articles

Evidence for the early differentiation of the core from Pt–Re–Os isotope systematics of 2.8-Ga komatiites

Komatiite samples from the 2.8-Ga Kostomuksha greenstone belt in the Baltic Shield define Pt–Os and Re–Os isochrons with ages of 2816±190 and 2880±83 Ma, respectively. The mean initial 186Os/188Os and 187Os/188Os ratios calculated at T=2821 Ma are 0.1198341±7 and 0.11042±69 (γ187Os=+2.5±0.6), respectively. This Os isotopic composition characterizes that of the source of the Kostomuksha komatiite and is 53±6 ppm and 2.5±0.6% more radiogenic in 186Os/188Os and 187Os/188Os, respectively, than the putative contemporary convecting mantle. The coupled 186Os–187Os enrichment cannot be the result of incorporation of the material of aged recycled oceanic crust into the source of the plume, as five to six times more crustal component is required to create the 186Os-enrichment than the 187Os-enrichment observed. Moreover, the radiogenic 186Os/188Os ratio combined with the subchondritic Pt/Os in this source calculated from the HSE abundance data is inconsistent with any model of Os isotopic evolution that involves crustal recycling, as these models require substantially suprachondritic Pt/Os in the hybrid sources. The coupled 186Os–187Os enrichment in the source of the Kostomuksha komatiite is best explained via derivation of most of the Os from the outer core. This is most consistent with the Kostomuksha komatiites originating from a mantle plume that arose from the core–mantle boundary (CMB). If this interpretation is correct, the data provide minimum constraints on the 186Os/188Os and 187Os/188Os ratios of the outer core at 2.8 Ga. The existing models of core crystallization based on experimentally determined solid metal–liquid metal partition coefficients for Pt, Re, and Os can adequately explain the Os isotopic composition of the Kostomuksha mantle source, although require the onset of inner core crystallization several hundred million years prior to formation of the Kostomuksha plume. Based on thermodynamic models, such an early onset of inner core crystallization would require a relatively low heat flux across the CMB of ∼3 TW and the presence of >100 ppm K in the core. The results of this study combined with the HSE abundance data for the source of komatiites at Kostomuksha indicate that core–mantle interaction, at least in this instance, could occur in the form of isotopic exchange without significant mass transfer from the core to the mantle.

Highly siderophile element geochemistry of 187Os-enriched 2.8 Ga Kostomuksha komatiites, Baltic Shield

New analyses of highly siderophile elements (HSE; Re, Os, Ir, Ru, Pt, and Pd) obtained by Carius tube digestion isotope dilution inductively coupled plasma mass-spectrometry (ID-ICPMS) technique are reported for 187Os-enriched 2.8 Ga komatiites from the Kostomuksha greenstone belt. As a result of a significant improvement in the yield over our previous digestions by the NiS fire-assay technique, these komatiites have now been shown to contain 22 to 25% more Os, Ir, and Pt and 34% more Ru. The emplaced komatiite lavas at Kostomuksha thus had siderophile element abundances comparable to those of the Abitibi belt. The discrepancies observed between the two techniques are interpreted to be the result of incomplete digestion of HSE carriers (particularly chromite) during the NiS fire-assay procedure. Our results for UB-N peridotite reference material agree well with those obtained by the high-pressure ashing digestion ID-ICPMS technique reported in the literature. Two types of komatiite lavas have been distinguished in this study based on the IPGE (Os, Ir, and Ru) behavior during lava differentiation. The Kostomuksha type is unique and is characterized by an incompatible behavior of IPGEs, with bulk solid-liquid partition coefficients for IPGEs being close to those for olivine. Cumulate zones in this type of komatiite lava occupy <20% of the total thickness of the flows. The Munro type exhibits a compatible behavior of IPGEs during lava differentiation. The cumulate zone in this type of komatiite occupies >20% of the total thickness of the flows. The calculated bulk partition coefficients indicate that, as with the other Munro-type komatiite lavas, the bulk cumulate contained an IPGE-rich minor phase(s) in addition to olivine. The non-CI chondritic HSE pattern for the source of the Kostomuksha komatiites calculated here is similar to that of Abitibi komatiites and to average depleted spinel lherzolite (ADSL) and supports the hypothesis of a non-CI chondritic HSE composition of the Earth’s mantle. The absolute HSE abundances in the source of the Kostomuksha komatiite have been demonstrated to be comparable to those of the source of Abitibi komatiites, even though the two komatiites contrast in their Os isotopic compositions. This supports the earlier hypothesis that if core-mantle interaction produced the 187Os/ 188Os radiogenic signature in the Kostomuksha source, it must have occurred in the form of isotope exchange at the core-mantle boundary. Other explanations of the radiogenic Os signature are similarly constrained to conserve the elemental abundance pattern in the mantle source of Kostomuksha komatiites.

Partitioning of trace amounts of highly siderophile elements in the Fe–Ni–S system and their fractionation in nature

The partitioning of highly siderophile elements (HSEs: Re, Os, Ir, Pt, Pd, and Au) between solid metal and liquid metal sulfide in the Fe–Ni–S system at 1000 to 1200°C and low pressure has been investigated at abundances close to those in nature by using neutron activation analysis. Partition coefficients (D) are high for the most refractory metals (Re, Os, and Ir), intermediate for Ru and Pt, and near unity for the least refractory metals (Pd, Ni, and Au), results broadly consistent with literature data. The partitioning of HSE seems to be reasonably independent of the concentration of these elements (trace to minor), temperature (1000 to 1900°C), and pressure (1 bar to 11 GPa), but varies markedly with variation in bulk composition, particularly for the refractory metals. We demonstrate fractionation of Re and Os in the Fe–S system [molar D(Os)/D(Re) = 1.15 ± 0.11] consistent with fractionation patterns for Re and Os in magmatic iron meteorites and qualitatively consistent with that hypothesised for 187Os-enrichment in some oceanic island basalt (OIB) sources. Laboratory partition coefficients for HSE in the Fe–Ni–S system correlate closely with log HSE vs. log Ni regression slopes of the magmatic iron meteorites. The selective partitioning behavior of HSE is related to the extreme range in melting point of the pure elements, and is unlikely to be erased completely by pressures in the earth’s core, even in the hypothetical absence of S. Also, the abundances of HSE in mantle peridotites and OIB could not have been influenced by partitioning between large quantities of solid metal and liquid metal sulfide.

Major and trace element associations in carbonates of Bhima Basin, Karnataka South India

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