Tiny refractory metal nuggets are mainly observed inside Ca, Al-rich inclusions (CAIs) from chondritic meteorites and are commonly assumed to be condensates from a solar composition gas. However, recent detailed studies of metal nugget compositions and their comparison with predictions from condensation show that the observed abundance patterns are extremely difficult to achieve in this way. As a test for the proposed alternative, precipitation from a silicate liquid, we conducted melting experiments, in which nine different refractory metals (nugget components) were equilibrated with each other along with a CAI-like liquid at reducing conditions. When quenched, minerals similar to those in CAIs formed from such liquids including refractory metal nuggets exhibiting compositions and appearances similar to those of the meteoritic nuggets. The run products and their comparison with a meteoritic nugget-bearing CAI is evidence for formation of refractory metal nuggets during cooling of Ca, Al-rich liquids at rates about 1000°/40 s (in the interval from 1900 to 900°C). To achieve the formation of refractory metal nuggets and the textures observed in the host inclusions, during cooling the rate probably changed. Refractory metal nuggets apparently formed during quenching before spinel crystallized.
Atom probe microscopy (APM) is a relatively new in situ tool for measuring isotope fractions from nanoscale volumes (< 0.01 μm3). We calculate the theoretical detectable difference of an isotope ratio measurement result from APM using counting statistics of a hypothetical data set to be ± 4δ or 0.4% (2s). However, challenges associated with APM measurements (e.g., peak ranging, hydride formation and isobaric interferences), result in larger uncertainties if not properly accounted for. We evaluate these factors for Re‐Os isotope ratio measurements by comparing APM and negative thermal ionisation mass spectrometry (N‐TIMS) measurement results of pure Os, pure Re, and two synthetic Re‐Os‐bearing alloys from Schwander et al. (2015, Meteoritics and Planetary Science, 50, 893) [the original metal alloy (HSE) and alloys produced by heating HSE within silicate liquid (SYN)]. From this, we propose a current best practice for APM Re‐Os isotope ratio measurements. Using this refined approach, mean APM and N‐TIMS 187Os/189Os measurement results agree within 0.05% and 2s (pure Os), 0.6–2% and 2s (SYN) and 5–10% (HSE). The good agreement of N‐TIMS and APM 187Os/189Os measurements confirms that APM can extract robust isotope ratios. Therefore, this approach permits nanoscale isotope measurements of Os‐bearing alloys using the Re‐Os geochronometer that could not be measured by conventional measurement principles.
Refractory metal nuggets (RMNs) contain elements, such as Os, Ir, Mo, and Ru, which are predicted to condense from a cooling gas of solar composition simultaneously with CAI-minerals. Berg etal. () identified a large number of RMNs in acid-resistant residues of the Murchison meteorite and suggested that they are pristine condensates. In extending the work of these authors, we have improved the chemical extraction process to enrich the concentration of RMNs in the residue sample and prepared three additional RMN-rich residues from the chondritic meteorites Murchison, Allende, and Leoville. The results show that, while their origin is clearly solar, the compositions in detail of RMNs from all three meteorites do not match well with a simple condensation model based on equilibrium thermodynamics and ideal solid solution of all metals. Thus, we find that a primary formation by direct condensation, as suggested previously, is unlikely for most of the studied grains and that alternative scenarios should be considered in future work. The results also show that several, but not all, alloys from Allende and Leoville have undergone processes, such as metamorphic oxidation and sulfidization in the meteoritic environment, in which they lost, e.g., W and Mo. For Murchison and several Leoville and Allende RMNs, we propose a pristine nature
Refractory metal nuggets (RMNs) contain high abundances of refractory siderophile elements (W, Re, Os, Ir, Mo, Ru, Pt, and Rh) which are the first elements to condense from a gas of solar composition [1]. As some of the first solids formed in the solar nebula RMNs provide important information on the initial isotopic composition of the solar system at a very early stage. Ruthenium is a promising element to study in this regard, because bulk meteorites exhibit nucleosynthetic Ru isotope anomalies [2]. The Ru isotope composition of RMNs extracted from a ∼30g piece of the Allende CV3 meteorite has been measured by MC-ICPMS. The RMN sample reveals large, massindependent isotope anomalies of nucleosynthetic origin. For internal normalization to 99 Ru/ 101 Ru, apparent excesses in all other Ru isotopes are consistent with a deficit in r-process nuclides as calculated using the r-process residuals inferred from the stellar model for sprocess nucleosynthesis of [3]. Since Ru is among the first elements to condense from a solar gas, the most straightforward interpretation of the Ru isotope data is that they reflect an early isotope heterogeneity of the solar nebula. In comparison to the RMNs bulk CAIs exhibit much smaller Ru isotope anomalies and are characterized by a deficit in s-rather than r-process nuclides [2]. To account for these contrasting isotope signatures in RMNs and bulk CAIs, r-and p-process Ru nuclides must have been added to the CAI after formation of the RMNs.
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