Composition and clues to the origin of refractory metal nuggets extracted from chondritic meteorites

Abstract

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

Corundum–hibonite inclusions and the environments of high temperature processing in the early Solar System

“…This nanoparticle may have actually acted as nuclei for corundum condensation. The RMN inclusion is free of W and Mo in comparison to RMNs found in situ from ALHA 77307 AOAs (Han et al, 2014) and recovered from acid residues of Murchison, Allende, and Leoville (Schwander et al, 2014). A Re, Os, Ir, Ru-rich metal alloy that condensed from a cooling solar composition gas is expected to contain significant fractions of W and Mo because these elements are less refractory than Re and Os, but more refractory than Ru.…”

Synthesis of refractory metal nuggets and constraints on the thermal histories of nugget‐bearing Ca, Al‐rich inclusions

“…On one hand, several meteoritic RMNs (e.g., Schwander et al. ) have somewhat higher Mo and W concentrations than those observed in RMNs from run #6 (Table ; Fig. b).…”

“…However, recent more detailed studies of RMN compositions by Schwander et al. () and Schwander () and comparison with predictions from the condensation model of Palme and Wlotzka () show that the observed compositions are extremely difficult, to say the least, to achieve in this way. Further, Rudraswami et al.…”

In situ analysis of Refractory Metal Nuggets in carbonaceous chondrites

“…An exhaustive literature search was conducted in an attempt to produce a library of all previous RMN compositional data (Berg et al, 2009;Bischoff & Palme, 1987;Blander et al, 1980;Wark & Lovering, 1976;Wark, 1986;Palme et al, 1982;Croat et al, 2013;Schwander et al, 2014a;Palme et al, 1994;Wark & 140 Lovering, 1978;Harries et al, 2012). We also include compositions of RMNs thought to be derived from crystallisation processes (Schwander et al, 2015;Rudraswami et al, 2014).…”

“…However, later work by Schwander et al (2014a) on RMNs from Murchison, Allende and Leoville, show that some but not all RMNs from these residues have been affected by metamorphic oxidation and sulphidation on the 55 parent body resulting in the loss of W and Mo.…”