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Some nuclides that were produced in supernovae are heterogeneously distributed between different meteoritic materials. In some cases these heterogeneities have been interpreted as the result of interaction between ejecta from a nearby supernova and the nascent solar system. Particularly in the case of the oldest objects that formed in the solar system – Ca–Al rich inclusions (CAIs) – this view is confirm the hypothesis that a nearby supernova event facilitated or even triggered solar system formation. We present Hf isotope data for bulk meteorites, terrestrial materials and CAIs, for the first time including the low-abundance isotope 174 Hf (∼0.16%). This rare isotope was likely produced during explosive O/Ne shell burning in massive stars (i.e., the classical “p-process”), and therefore its abundance potentially provides a sensitive tracer for putative heterogeneities within the solar system that were introduced by supernova ejecta. For CAIs and one LL chondrite, also complementary W isotope data are reported for the same sample cuts. Once corrected for small neutron capture effects, different chondrite groups, eucrites, a silicate inclusion of a IAB iron meteorite, and terrestrial materials display homogeneous Hf isotope compositions including 174 Hf. Hafnium-174 was thus uniformly distributed in the inner solar system when planetesimals formed at the <50 ppm level. This finding is in good agreement with the evidently homogeneous distributions of p-process isotopes 180 W, 184 Os and possibly 190 Pt between different iron meteorite groups. In contrast to bulk meteorite samples, CAIs show variable depletions in p-process 174 Hf with respect to the inner solar system composition, and also variable r-process (or s-process) Hf and W contributions. Based on combined Hf and W isotope compositions, we show that CAIs sampled at least one component in which the proportion of r- and s-process derived Hf and W deviates from that of supernova ejecta. The Hf and W isotope anomalies in CAIs are therefore best explained by selective processing of presolar carrier phases prior to CAI formation, and not by a late injection of supernova materials. Likewise, other isotope anomalies in additional elements in CAIs relative to the bulk solar system may reflect the same process. The isotopic heterogeneities between the first refractory condensates may have been eradicated partially during CAI formation, because W isotope anomalies in CAIs appear to decrease with increasing W concentrations as inferred from time-integrated 182 W/ 184 W. Importantly, the 176 Lu– 176 Hf and 182 Hf– 182 W chronometers are not significantly affected by nucleosynthetic heterogeneity of Hf isotopes in bulk meteorites, but may be affected in CAIs. [ABSTRACT FROM AUTHOR]

Analytical protocols for measurements of isotope ratios involving low-abundance isotopes 174 Hf (~ 0.16%), 180 W (~ 0.12%), and 190 Pt (~ 0.014%) using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) at ca. tenfold improved precision compared to previous studies are presented. The improvements in precision result in part from the use of Faraday amplifiers equipped with 10 12 Ω resistors for collection of the isotope beams of interest, as well as for the isotope beams used to monitor isobaric interferences. For ion currents > 5 × 10 − 13 A (> 50 mV on an amplifier with 10 11 Ω resistor), uncertainties (2 s.d.) of better than 100 ppm are typically achieved for isotope ratios involving 174 Hf, 180 W and 190 Pt. Terrestrial isotope abundances (atomic percent ± 2 s.d.) of 174 Hf (0.16106 ± 0.00006; n = 15); 180 W (0.11910 ± 0.00009; n = 10); and 190 Pt (0.01286 ± 0.00005; n = 9) were determined on reference solutions throughout multiple analytical sessions over ~ 3 years. The results, however, rely on the accuracy of the reference value chosen for the isotope ratios employed for mass bias correction and, to a lesser extent, on the accuracy of the exponential mass fractionation law used for mass bias correction. For the precise determination of isotope ratios involving 174 Hf, 180 W and 190 Pt in natural materials, efficient chromatographic separation schemes for Hf and W from silicate-rich matrices, and for Pt from iron meteorite matrices, are presented that provide effective separation of the element of interest from interfering elements. First results for Pt suggest that IAB, IIAB and IIIAB iron meteorites exhibit terrestrial 190 Pt abundances within uncertainty. Three chondrite samples exhibit indistinguishable 174 Hf from terrestrial Hf. The isotope abundance of 180 W in meteorites is more variable due to radiogenic 180 W that was produced by 184 Os-decay. [ABSTRACT FROM AUTHOR]

Abstract: The decay of the rare nuclide 184Os by alpha emission to 180W has been theoretically predicted, but was previously never observed in experiments. Variable excesses of 180W were recently observed for iron meteorites, but the contribution to these excesses by 184Os-decay was regarded as insignificant. Here, we present combined 180W and Os–W concentration data for meteorites and terrestrial rocks, now indicating that the 180W heterogeneities can be explained by α-decay of 184Os. A combined 184Os–180W isochron for iron meteorites and chondrites yields a decay constant value of of (half life yr), in good agreement with theoretical estimates. The 184Os–180W decay system may constitute a viable tracer and chronometer for important geological processes like core formation, silicate differentiation or late accretion processes. This is illustrated by a measured 180W-deficit in terrestrial basalts relative to chondrites by parts in 10 000, consistent with core formation ∼4.5 Ga ago. [Copyright &y& Elsevier]

It has long been recognized that the stable oxygen isotope 18O/16O ratios of chemical sediments show a systematic decrease with increasing age. This continuous, 3.8 Ga long secular trend has been attributed to 80 °C hot early Earth oceans, a ~15‰ lower 18O/16O ratio of Archean seawater, or diagenetic obliteration of the original isotope signal. We demonstrate that high-precision triple oxygen isotope measurements including the rare third stable oxygen isotope 17O on silica rocks (cherts) can be used to shed light on this major problem in geochemistry. Our triple oxygen isotope data do not support 80 °C hot oceans in the Archean. The data also exclude that the 18O/16O of seawater was much lower in the Archean. Also, a simple freshwater-origin of cherts is not supported by the new data. We discuss the effects of diagenetic alteration on the triple oxygen isotope compositions of cherts and show that the Archean and Proterozoic cherts could have equilibrated with fluids with higher 18O/16O than today's oceans. For the Archean cherts, the high 18O/16O fluids required to explain the observations could reflect the composition of ambient seawater or of modified hydrothermal fluids. Oceans with higher 18O/16O ratio would be expected for an Archean "water world" without large continents. High 18O/16O fluids, however, could also have formed locally by reactions between seawater and the oceanic crust. Our study shows that high-precision triple oxygen isotope data provide new understandings of processes deep in time. [ABSTRACT FROM AUTHOR]

The δ 18 O value of seawater is presently buffered by high-temperature hydrothermal alteration and low-temperature weathering of lithospheric rocks. It is much debated whether the δ 18 O of seawater has been buffered to this steady-state value throughout Earth's history or, alternatively, whether it gradually increased towards the present value since the Archean. A third possibility is that the δ 18 O of seawater was buffered at a higher value before continents emerged, and has been buffered at its current value (δ 18 O ∼ −1‰) since the emergence of continents. In this contribution, we reconstruct the δ 18 O of Archean seawater from triple oxygen isotope (δ 17 O, δ 18 O) variations in >2.7 Ga ultramafic rocks that reacted with fluids that, in turn, had been derived from seawater in the Archean. The samples that were studied are peridotites from the Ivnartivaq complex, an ultramafic lens in the Archean Kuummiut terrane (Rae craton, southeast Greenland) that have unusually low δ 18 O values for peridotites (in olivine, 1.7 ‰ ≤ δ 18 O ≤ 4.6 ‰ , relative to VSMOW). Bulk rock trace element concentrations, mineral compositions and U-Pb dating of zircon grains in the peridotites indicate that these samples are metamorphic peridotites, that formed by the dehydration of serpentinites ∼2.7 Ga ago. The serpentinite protoliths, in turn, had formed by alteration of ultramafic cumulate rocks at high temperatures (250–450 °C) by fluids that were derived from seawater. Triple oxygen isotope variations in olivine from the peridotites indicate that the fluids could not have been derived from seawater with a significantly higher or lower δ 18 O value than that of seawater in the Phanerozoic, but could have been derived from seawater with a value of δ 18 O ∼ −1‰, i.e., the modern (ice-free) seawater composition. We conclude that the δ 18 O of seawater had reached its current steady state value by 2.7 Ga ago. • Archean peridotites from southeast Greenland contain olivine with 1.7‰ ≤ δ 18 O ≤ 4.6‰. • They are crystal cumulates that were altered by seawater to low- δ 18 O serpentinites. • The serpentinites became dehydrated during metamorphism ∼2.7 Ga ago. • We reconstruct the δ 18 O of >2.7 Ga seawater from triple O isotope variations in olivine. • Seawater had reached its current steady state value (δ 18 O ∼ −1‰) by 2.7 Ga ago. [ABSTRACT FROM AUTHOR]

Abstract: Heavy, proton-rich stable isotopes belong to the least abundant isotopes in the solar system. Their formation mechanisms and their stellar sources are most likely different from those of neutron-capture generated r- and s-process nuclei that comprise the majority of nuclides heavier than iron. Heavy p-nuclide abundances in meteorites are therefore potentially useful in deciphering distinct stellar contributions to the nascent solar system. We therefore conducted the first high-precision measurements of the heavy p-process isotope 180W, achieving a typical precision of ±0.7 ε-units for ca. 300ngW. Measured samples comprise metals from magmatic- and non-magmatic iron meteorites, as well as metal from one H4 chondrite (NWA 926) and two reduced terrestrial basalts (from Disko Island, Greenland and the Dzheltul''ski massif from Eastern-Siberia, Russia). The analyzed iron meteorites show clearly resolvable 180W anomalies of up to +6 ε-units. Conversely, the chondritic metal and both terrestrial samples exhibit 180W abundances indistinguishable from the standard value. As cosmogenic effects during space exposure of the meteoroids may have affected the 180W budget, cosmic-ray exposure of the meteorites has to be critically evaluated. We therefore propose a method to approximate cosmogenic contributions to the 180W signatures in order to unravel nucleosynthetic 180W abundance anomalies. Our study reveals significant cosmogenic effects only for the longest exposed meteorites, shifting 180W anomalies always to lower values (average cosmic-ray correction-factors can be estimated to lie between 0.01 and 0.30 ε 180W-units per 100Myr of exposure). Cosmogenic effects for most of the analyzed meteorites therefore appear to be negligible with respect to the analytical precision achieved for iron meteorites. In addition to cosmic-ray exposure, radiogenic effects can be caused by putative decay of 184Os or by decay of 180Ta in its ground state. Whereas potential alpha decay of 184Os could shift 180W anomalies to higher values (but only up to levels that are within the analytical error of ∼0.5 ε-units for most samples), no significant production of 180W could have occurred from 180Ta decay. Notably, we identified significant and systematic abundance variations in 180W between different iron meteorite groups, indicating that these isotope anomalies are characteristic for their entire parent asteroids. Our finding of decreasing excesses in 180W from early formed magmatic iron meteorites (+3.8±1.2 ɛ-units) towards later formed non-magmatic iron meteorites (+0.6±0.5 ɛ-units), the analyzed chondrite and both terrestrial rocks (−0.3±0.7 ε-units) may thus mirror progressive homogenization of 180W in the early solar nebula. This overall trend is also supported by a co-variation between 180W and metal segregation ages for the different iron meteorite groups as well as by a co-variation between 180W deviations and the respective asteroidal accretion ages. Such an interpretation would suggest progressive homogenization of the solar nebula within about ∼2.5 to ∼6Myr. [Copyright &y& Elsevier]

To expand the newly developed ARM glasses as reference materials for in situ microanalysis of isotope ratios and iron oxidation state by a variety of techniques such as SIMS, LA‐MC‐ICP‐MS and EPMA, we report Li‐B‐Si‐O‐Mg‐Sr‐Nd‐Hf‐Pb isotope data and Fe2+/ΣFe ratios for these glasses. The data were mainly obtained by TIMS, MC‐ICP‐MS, IR‐MS and wet‐chemistry colorimetric techniques. The quality of these data was cross‐checked by comparing different techniques or by comparing the results from different laboratories using the same technique. All three glasses appear to be homogeneous with respect to the investigated isotope ratios (except for B in ARM‐3) and Fe2+/ΣFe ratios at the scale of sampling volume and level of the analytical precision of each technique. The homogeneity of Li‐B‐O‐Nd‐Pb isotope ratios at the microscale (30–120 μm) was estimated using LA‐MC‐ICP‐MS and SIMS techniques. We also present new EPMA major element data obtained using three different instruments for the glasses. The determination of reference values for the major elements and their uncertainties at the 95% confidence level closely followed ISO guidelines and the Certification Protocol of the International Association of Geoanalysts. The ARM glasses may be particularly useful as reference materials for in situ isotope ratio analysis. [ABSTRACT FROM AUTHOR]

Abstract The stepwise acid digestion of primitive chondritic meteorites allows the identification of nucleosynthetic isotope anomalies that are otherwise hidden on the bulk rock scale. Here, we present combined Hf and W isotope data for acid leachates, residues, and bulk rock aliquots of several primitive chondrites that include highly precise analyses of the heavy p-process isotopes 174Hf and 180W. Including data for these two p-process isotopes enables, for the first time, the clear-cut discrimination between s- and r-process contributions to the Hf and W isotope inventory. Our analyses reveal Hf and W isotopic homogeneity at the bulk rock scale, but significant Hf and W isotope anomalies that are complementary between acid leachates and residues. Since both r- to p-process isotope ratios are invariant in leachates and residues, the observed anomalies can unambiguously be tied to variable contributions of carrier phases enriched in s-process nuclides, as previously inferred for, i.e., Mo and Ru in leaching experiments. Hafnium and W isotope anomalies co-vary in leachate and residue fractions from CM chondrites, whereas CO and CV chondrites are characterized by distinctly larger Hf isotope anomalies compared to W. This observation is most likely explained by more efficient homogenization of s-process W carrier(s) or, alternatively, by local redistribution of anomalous W into secondary less resistant phases during parent body and/or nebular processing. This implies the presence of different s-nuclide carrier phases for Hf and W. Several carriers of s-process-material appear to have been selectively dissolved by our leaching protocol, while contributions from r- and p-process Hf and W carrier phases appear invariant, possibly due to the generally more labile nature of their carrier phases during solar nebula and/or parent body processing. [ABSTRACT FROM AUTHOR]