Refractory metal nuggets within presolar graphite: First condensates from a circumstellar environment

Abstract

Abstract-Transmission electron microscope (TEM) investigations have revealed Os, Ru, Mo-rich refractory metal nuggets within four different presolar graphites, from both the high-density (HD) Murchison (MUR) and low-density (LD) Orgueil (ORG) fractions. Microstructural and chemical data suggest that these are direct condensates from the gas, rather than forming later by exsolution. The presolar refractory metal nugget (pRMN) compositions are variable (e.g., from 8 < Os atom% < 77), but follow the same chemical fractionation trends as isolated refractory metal nuggets (mRMNs) previously found in meteorites (Berg et al. 2009). From these compositions one can infer a temperature of last equilibration with the gas of 1405-1810 K (e.g., Berg et al. [2009] at approximately 100 dyne cm À2 pressure), which implies that the host graphites form over roughly the same range (in agreement with predictions) and that the pRMNs are chemically isolated from the gas when captured by graphite. Further, the pRMN compositions give evidence that HD graphites form at a higher T than LD ones. Chemical and phase similarities with the isolated mRMNs suggest that the mRMNs also condense directly from a gas, although from the early solar nebula rather than a presolar environment. Although the pRMNs themselves are too small for detection of isotopic anomalies, NanoSIMS isotopic measurements of their host graphites confirm a presolar origin for the assemblages. The two pRMN-containing LD graphites show evidence of a supernova (SN) origin, whereas the stellar origins of the pRMNs in HD graphite are unclear, because only less-diagnostic 12 C enrichments are detectable (as is commonly true for HD graphites).

In situ analysis of Refractory Metal Nuggets in carbonaceous chondrites

“…RMNs have been identified in pre-solar graphite (Croat et al, 2013) grains. We anticipate that pre-solar RMNs will be present elsewhere in primitive meteorites.…”

“…The assumption that has arisen in prior studies is that RMNs are unique to CAIs (Berg et al, 2009;Bischoff & Palme, 1987;Blander et al, 1980;Eisenhour & Buseck, 1992;El Goresy et al, 1978;Fegley 60 & Kornacki, 1984;Grossman et al, 1977;Palme & Wlotzka, 1976;Palme et al, 1982;Sylvester et al, 1990;Wark, 1986;Wark & Lovering, 1976) with one exception: recent work by Croat et al (2013) described RMNs contained within pre-solar graphite. These are the only RMNs that can be interpreted as having a pre-solar origin with relative certainty.…”

“…Our interpretation is that the Solar Nebula inherited an initially compositionally diverse population of RMNs from the giant molecular cloud; that a variety of Solar System processes have acted on that population; but none have completely homogenised it. Most RMNs have experienced disk and asteroidal processing, but some may have retained a primordial composition.RMNs have been identified in pre-solar graphite (Croat et al, 2013) grains. We anticipate that pre-solar RMNs will be present elsewhere in primitive meteorites.…”

“…If there is sufficient evidence to suggest a mechanism for how the RMN formed, a suitable precursor shall be attached to the start of the word i.e. presolar-RMN for an RMN with a demonstrably presolar origin similar to the naming scheme adopted by Croat et al (2013). This can be extended to any process e.g.…”

“…With recent work suggesting 30 a mechanism of formation of pre-solar-RMNs in asymptotic giant branch stars (Schwander et al, 2014b), following the observation of RMNs found within pre-solar graphite (Croat et al, 2013).…”