Geological reference materials (RMs) with variable compositions and NIST SRM 612 were analysed by isotope dilution mass spectrometry for bulk rock concentrations of chalcogen elements (sulfur, ...selenium and tellurium), rhenium and platinum‐group elements (PGEs: Ru, Pd, Os, Ir and Pt), including the isotope amount ratios of 187Os/188Os. All concentrations were obtained from the same aliquot after HCl‐HNO3 digestion in a high pressure asher at 320 °C. Concentrations were determined after chemical separation by negative TIMS, ICP‐MS and hydride generation ICP‐MS (Se, Te). As in previous studies, concentrations of the PGEs in most RMs were found to be highly variable, which may be ascribed to sample heterogeneity at the < 1 g level. In contrast, S, Se and Te displayed good precision (RSD < 5%) in most RMs, suggesting that part of the PGE budget is controlled by different phases, compared with the chalcogen budget. The method may minimise losses of volatile chalcogens during the closed‐system digestion and indicates the different extent of heterogeneity of chalcogens, Re and PGEs in the same sample aliquot. OKUM, SCo‐1, MRG‐1, DR‐N and MAG‐1 are useful RMs for the chalcogens. NIST SRM 612 displays homogenous distribution of S, Se, Te, Pt and Pd in 30 mg aliquots, in contrast with micro‐scale heterogeneity of Se, Pd and Pt.
Des matériaux géologiques de référence («RMs») avec des compositions variables et le matériel de référence NIST SRM 612 ont été analysés sur roches totales par spectrométrie de masse par dilution isotopique pour obtenir leurs concentrations en éléments chalcogènes (soufre, sélénium et tellure), rhénium et en éléments du groupe du platine («PGEs» : Ru, Pd, Os, Ir et Pt), en incluant les rapports isotopiques 187Os/188Os. Toutes les concentrations ont été obtenues à partir de la même aliquote après une phase de digestion utilisant un mélange HCl‐HNO3 et la méthode de calcination à haute pression à 320 °C. Les concentrations ont été déterminées après séparation chimique en utilisant un TIMS en mode négatif, un ICP‐MS et un ICP‐MS par génération d'hydrure (Se, Te). Comme dans les études précédentes, les concentrations en PGEs dans la plupart des RMs se sont révélées être très variables, ce qui peut être attribuée à l'hétérogénéité des échantillons à l'échelle < 1 g. En revanche S, Se et Te ont montré une bonne précision (RSD < 5%) dans la plupart des RMs, suggérant qu'une partie du budget en PGE est contrôlée par différentes phases, par rapport aux budgets des chalcogènes. La méthode peut réduire les pertes en chalcogènes volatils au cours de la digestion en système clos et indique différent degrés d'hétérogénéité des chalcogènes, du Re et des PGE dans la même aliquote d’échantillon. Les matériaux de référence OKUM, SCo‐1, MRG‐1, DR‐N et MAG‐1 sont utiles pour les chalcogènes. Le NIST SRM 612 affiche une répartition homogène du S, Se, Te, Pt et Pd dans des aliquotes de 30 mg, contrastant avec l'hétérogénéité à micro‐échelle affichée par le Se, le Pd et le Pt.
•Mineralogical mapping at high spatial resolution of small zircons in lunar samples.•Imaging of phase assemblages reveals various different impact related textures.•High spatial resolution U-Pb SIMS ...dating allows to date zircons of <10 μm size.•New zircon 207Pb-206Pb ages are between 4.15 and 4.25 Ga.•Zircon ages likely reflect the early formation of large impact basins on the moon.
Because of their robustness against resetting, in situ U-Pb ages of zircons in lunar impactites have the potential to provide constraints on the lunar bombardment history that may complement the more common K-Ar ages. Most previous work has focused on relatively large zircons that show growth zoning and ages were mostly interpreted as early igneous crystallization ages. Here we combine high-resolution mineralogical imaging and in situ U-Pb dating by ion microprobe to identify, characterize and date <20 μm size zircons in thin sections of lunar impact breccias. Several tens of grains of zircons of this size range were identified in thin sections of impactites from the Apollo 15 and 16 landing sites. Small zircons are more abundant in both noritic and evolved clinopyroxene, SiO2 or K-feldspar bearing lithologies compared to anorthositic bulk compositions. Both granular zircon aggregates and overgrowth on existing zircon or baddeleyite (in breccias 15455 and 67915) are interpreted to reflect high-temperature recrystallization of zircons or its high-temperature-pressure precursor phases, following shock heating events by impact. In contrast, conchoidal or poikilitic zircons <10 μm in Fe-Ni metal bearing noritic clasts or matrix (67915, 67955) crystallized in situ from impact melt. Most U-Pb ages of the 24 analyzed grains are either concordant or reverse discordant with 207Pb-206Pb ages ranging from 4.15 to 4.25 Ga. The small age range, combined with a large textural spectrum and the frequent presence of Fe-Ni metal suggest zircon crystallization from impact melt and recrystallization of pre-existing zirconium-bearing minerals by impact heating. Such ‘impact’ zircons with 4.2 Ga ages have now been reported from most Apollo landing sites, suggesting widespread formation and modification of zircons by basin-forming impacts at this time. The contrast between U-Pb zircon (predominantly 4.2 Ga) and K-Ar feldspar ages (predominantly 3.9 Ga) likely reflects resetting of the latter chronometer by impact heating.
The short-lived
98Tc–
98Ru (Half-life between 4 and ∼10 Ma) and
99Tc–
99Ru (Half-life 0.21 Ma) decay systems have the potential to provide important information on the relative chronology of ...processes that affected metal phases in the early solar system. The proof of extant
99Tc in the solar system would also tightly constrain the time interval between production of the nuclide and injection of
s-process isotopes into the protosolar cloud.
High-precision Ru isotopic data for group IIAB (Negrillos, Bennett County, Coahuila, Filomena, Old Woman) and IIIAB (Casas Grandes, Costilla Peak) iron meteorites, and the chondrites Allende (CV3) and Allegan (H5) indicate that the isotopic compositions of
98Ru and
99Ru in these meteorites overlap with the terrestrial values at the ±0.8 and ±0.3
ε levels (parts in 10,000), respectively. Previous reports of positive deviations of
98Ru in Negrillos and Casas Grandes likely reflect inaccurate measurements.
Evaluation of processes that may fractionate Tc/Ru in solar system objects suggests that this ratio likely varied by only a factor of 2 or less. Using this constraint and solar system initial abundances of
99Tc derived from astrophysical models, evolution models for the
99Tc–
99Ru system predict deviations in
99Ru of >0.3
ε only, if these fractionation events occurred during collapse of the molecular cloud (<10 ka). Thus, positive identification of enriched
99Ru may require better than 0.1
ε unit resolution. Absence of resolvable
98Ru deviations indicates
98Tc/
96Ru
⊙i<2×10
−5.
The effects of melt percolation on the Re–Os systematics of peridotites were studied in garnet- and spinel-bearing high-temperature peridotite bodies in the southern Bohemian massif (lower Austria). ...The mantle rocks occur in the high-grade core of a Carboniferous collision zone. Age constraints, occurrences of calc–alkaline rocks, and the trace element and isotopic composition of garnet pyroxenite layers in the peridotites indicate that the latter must have originated in the hanging wall mantle of a late Devonian–early Carboniferous subduction zone. The garnet pyroxenites are cumulates and their Eu anomalies, radiogenic initial
187Os/
188Os (
γOs
i
up to 450),
87Sr/
86Sr (up to 0.709), and unradiogenic
143Nd/
144Nd (
ϵNd
i
as low as −4.8) were probably inherited from melts that contained slab-derived components. Because the isotopic signatures of the melts were substantially different from normal mantle, these peridotites are particularly suitable for a study of the effects of melt percolation. Layered dunite–pyroxenite rocks resemble replacive dunites in ophiolites, and may have acted as high permeability channels for Mg-rich melts. The dunite–pyroxenite rocks are characterized by suprachondritic
γOs
i
(7.2 to 13.9), indicating addition of radiogenic Os from the melts. Assuming the dunites originally had a composition similar to harzburgites at the same outcrop (
γOs
i
of −2.7 to −3.9), the change in
γOs
i
requires melt/rock ratios between 10 and 400. Osmium and Cr are depleted in the dunites by up to 50–70% compared to normal peridotites, and enriched in associated orthopyroxenites (up to 3.8 ppb Os and 5700 ppm Cr), indicating substantial transfer of peridotitic Os and Cr into melt. Preferred incorporation of Os and Cr into Mg-rich pyroxenites suggests a coupling of the chemical behavior of Os and Cr in Mg-rich igneous silicate systems. Rhenium abundances are uniformly low in these rocks, presumably because of low Re and sulfur contents in the melts. Melt percolation in depleted lherzolites and harzburgites in the vicinity of pyroxenite layers is indicated by Sr–Nd isotopic data and REE patterns.
γOs
i
in these rocks (−5.5 to +0.5) suggest no or only minor addition of radiogenic Os and low melt/rock ratios near 1. However, correlations of mildly incompatible elements such as Al with Re abundances and negative correlation of
143Nd/
144Nd with Re in lherzolites suggest refertilization of previously depleted peridotites, with concurrent addition of significant amounts of Re. Metasomatic addition of radiogenic Os at high melt/rock ratios, and Re at low melt/rock ratios, produced large shifts in the Re–Os model ages of the peridotites.
Constant concentration ratios of trace elements of similar incompatibility in oceanic basalts have been used to determine the abundances and ratios of incompatible elements in mantle sources. ...However, the Mo content in the depleted mantle estimated from Mo/Ce of oceanic basalts is much lower than the estimates based on Mo contents of mantle peridotites (25 ± 7 ng/g versus 113–180 ng/g). This discrepancy partly reflects uncertainties about the geochemical behavior of Mo in the upper mantle. New Mo concentration data obtained on unserpentinized spinel-facies lherzolites, harzburgites, dunites and pyroxenites (n = 47) from the Balmuccia and Baldissero mantle tectonites (Ivrea-Verbano Zone, Italian Alps) provide new insights into the magmatic behavior of Mo and its content in the depleted mantle.
The peridotites and pyroxenites display variable and very low Mo contents (4–16 and 3–11 ng/g, respectively). A slightly serpentinized peridotite shows the highest Mo content of 37 ng/g. The Mo contents show no systematic variation with other incompatible or compatible elements. Chromium spinel in the mantle rocks likely does not control the behavior of Mo because complete or incomplete digestion of spinel did not lead to a noticeable change in Mo contents. The data indicate incorporation of little Mo into olivine, pyroxene and spinel, consistent with the predominant occurrence of Mo6+ and the experimentally-determined very low partition coefficients at typical upper mantle conditions. Bulk rock Mo contents from these mantle rocks, upper limits of Mo contents in sulfides and the lack of variations of Mo concentrations with varying sulfide fractions indicate a minor influence of mantle sulfides on the bulk Mo budget. The incorporation of little Mo into pyroxenes of the mantle rocks contrasts with Ce which is mainly controlled by clinopyroxene. This contrasting behavior suggests a higher incompatibility of Mo than Ce during mantle melting. Modelling polybaric melting of mantle peridotites indicates that melts formed at low degrees of partial melting (e.g., <2–3%) would not retain the Mo/Ce of the mantle sources. In contrast, >5% melting, as in tholeiites and komatiites, leads to extraction of nearly the complete inventory of Mo and Ce into the melts and thus Mo/Ce of the melts should be representative of mean mantle source compositions, even if Mo and Ce show different bulk partition coefficients. Molybdenum contents of mantle rocks from the Ivrea Zone and the data on oceanic basalts and komatiites indicate that the depleted upper mantle has a low Mo content of <30 ng/g.
•Mantle peridotites and pyroxenites from the Ivrea Zone have very low Mo contents.•Little Mo is incorporated into olivine, pyroxene and spinel in mantle rocks, consistent with experimental constraints.•Mantle sulfides play a limited role for the bulk Mo budget of peridotites and pyroxenites.•Mo is more incompatible than Ce in the depleted mantle.•The depleted mantle has a low Mo content of <30 ng/g.
187Re‐187Os systematics, abundances of highly siderophile elements (HSE: Re, PGE, and Au), chalcogen elements (Te, Se, and S), and some major and minor elements were determined in physically ...separated components of the Allende (CV3) and Murchison (CM2) carbonaceous chondrites. Substantial differences exist in the absolute and relative abundances of elements in the components, but the similarity of calculated and literature bulk rock abundances of HSE and chalcogens indicate that chemical complementarity exists among the components, with CI chondrite‐like ratios for many elements. Despite subsequent alteration and oxidation, the overall cosmochemical behavior of most moderately to highly siderophile elements during high‐temperature processing has been preserved in components of Allende at the sampling scale of the present study. The 187Re‐187Os systematics and element variations of Allende are less disturbed compared with Murchison, which reflects different degrees of oxidation and alteration of these meteorites. The HSE systematics (with the exception of Au) is controlled by two types of materials: Pd‐depleted condensates and CI chondrite‐like material. Enrichment and heterogeneous distribution of Au among the components is likely the result of hydrothermal alteration. Chalcogen elements are depleted compared with HSE in all components, presumably due to their higher volatility. Small systematic variations of S, Se, and Te in components bear the signature of fractional condensation/partial evaporation and metal–sulfide–silicate partitioning.
Current models assume that siderophile volatile elements (SVE) are depleted in bulk Earth to the same extent as lithophile elements of similar volatility. The observed additional depletion of many ...SVE relative to lithophile elements in the bulk silicate Earth (BSE) is ascribed to partitioning of SVE into Earth's core. However, the assumption of similar volatility of moderately volatile elements during Earth formation processes as in solar gas is quite uncertain. Here, these assumptions will be tested by assessing abundances and ratios of indium and cadmium in the BSE using new data on mantle rocks, and the application of high- and low-pressure–temperature metal–silicate partitioning data.
New bulk rock abundance data of In and Cd obtained on bulk rocks of peridotite tectonites and xenoliths by isotope dilution refine previous results inferred from basalts and in-situ analyses of silicate minerals in peridotite xenoliths. The CI chondrite-normalized abundance of In in the BSE is similar to zinc and is 3–4 times higher than Cd. New and published low- and high-P–T metal–silicate partitioning data indicate that, during core formation at a range of conditions, In is always more siderophile than Zn and Cd. Adding the fraction of these elements in Earth's core to the BSE results in bulk Earth compositions that yield higher CI chondrite normalized abundances of In in the bulk Earth compared to Zn and Cd. Because In is more volatile than Zn and Cd in gas of solar composition, suprachondritic In/Zn and In/Cd in the bulk Earth suggest that during formation of Earth or its building materials, the volatilities of these elements and perhaps other volatile elements likely have changed significantly (i.e. In became less volatile). The results also suggest that known carbonaceous chondrites likely did not deliver the main volatile element-rich fraction of the Earth. Various arguments suggest that the loss of moderately volatile elements during planetary accretion should be limited, thus, their abundances in the bulk Earth likely reflect the average composition of Earth's building materials. Combined with evidence from nucleosynthetic isotope anomalies, the data suggest that Earth's main building materials originated from compartments of the inner solar system where volatile element abundances evolved differently from the formation area of known chondrites. The materials with nonchondritic volatile element composition may have been used up for building the terrestrial planets.
•New data support previous estimates on In and Cd abundances in bulk silicate Earth.•At high P–T core formation conditions, In is more siderophile than Zn and Cd.•Loss of In, Cd and Zn during accretion and giant impact is limited for Earth.•Indium, Cd and Zn do not follow the assumed volatile depletion trend of the Earth.•Earth's main building materials are now extinct and differ from known chondrites.
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