The uniformly hexagonal structures of the 15 Murchison RMNs studied strongly suggest that the alloys initially formed as a compositional continuum of single, hcp structured phases. The Mo-dominant grains of our study match the compositions, structure, and approximate lattice parameters of the hexamolybdenum phase previously described from CAIs in the Allende and NWA 1934 CV3 chondrites, where it occurs in association with Sc-Zr-rich ultrarefractory oxides (allendeite), perovskite, and Os-rich alloys (Ma et al. 2009) and krotite (CaAl2O4)/grossite (Ma et al. 2011), respectively. Particularly interesting is that the hexagonal structure in these samples appears to be preserved even for compositions with just about 20–30 atom % hcp metals (Ru + Os). Our findings of solely hexagonal metal alloys are also in accord with previous TEM observations of Eisenhour and Buseck (1992), who investigated five Mo- and W-bearing RMNs hosted in spinel of a fluffy type-A CAI from the Allende CV3 meteorite. Also, these grains were euhedral, hcp structured, and without detectable internal compositional heterogeneity. Recently, Croat et al. (2012) reported three hcp structured, Mo- and W-bearing RMNs from graphite grains of likely presolar origin, supporting the direct condensation from a gas phase. As mentioned above, low vapor pressure is the only common property of all the diverse metals in these metal alloys. This requires high temperatures for the formation of the metal nuggets, either as condensates from a gas of solar composition or as residues from evaporation. We will first describe to what extent evidence from the structure of the alloys constrains their origin and then discuss their possible formation as condensate and as evaporative residues in more detail.
Structural Constraints on the Origin of Refractory Metal Nuggets
Considering the large number of constituents and the highly variable compositions, it is surprising that without exception, all yet sampled RMNs from either Allende or Murchison are hcp structured, homogeneous alloys. One would rather expect to find a variety of different structures and exsolution features. Hence, to constrain the formation of RMNs, it is important to understand whether the hcp structure formed stably or metastably in the relevant temperature interval. Unfortunately, no experimental data exist on the phase relation in the quaternary system Mo-Ru-Os-Ir, and less so in even more extensive systems involving additional Fe, Pt, W, etc. The Mo-Ru binary system clearly shows a fairly wide miscibility gap between bcc and hcp solid solutions at temperatures below 1600 K. Furthermore, the system contains an intermetallic compound (tetragonal σ-phase, approximately Mo5Ru3) at temperatures between 1416 and 2188 K (Okamoto 2000). Also, the Ir-Os, Ir-Ru, and Mo-Os binary systems show miscibility gaps between fcc and hcp alloys at temperatures below 2000 K (Massalski et al. 1986; Okamoto 1992, 1994). Consequently, based on these binary phase diagrams alone, one could expect to find structural diversity among Mo-Ru-Os-Ir alloys. However, the Mo-Ir, Mo-Pt, Mo-Rh, and Fe-Ir binary systems of bcc and fcc metal endmembers show relatively wide homogeneity ranges of nonordered (i.e., superstructure-free) hcp phases at intermediate compositions (Massalski et al. 1986). These phases, known as ε-phases, occur, for example, as complex alloys in spent nuclear fuel, where the hcp structure can prevail over large compositional ranges (e.g., in the Mo-Pd-Ru system; Rand and Potter 1981). Although the temperature ranges of hcp fields in the binary bcc-fcc alloy systems; are either too high (>1880 K for Mo-Ir, >1750 K for Mo-Pt) or too low (<900 K for Fe-Ir) with respect to RMN condensation temperatures, this perspective offers at least a possibility that rather extended, stable ε-phase fields might exist in the multicomponent systems and compositional ranges relevant to RMNs (Fig. 7). Grain 02, which is unusually poor in Mo and essentially composed of Fe, Os, Ru, and Ir (Table 1) likely adopted a hcp structure due to extended hcp fields in the Fe-Ru and Fe-Os binary systems (Swartzendruber and Sundman 1983) and the occurrence of the ε-phase in the Fe-Ir system (Massalski et al. 1986).
Figure 7. Pseudoternary plot of the RMN compositions measured in this study by TEM-EDX. Elements are grouped based on their crystal structure in pure form. Gr 02 is Fe-rich and appears anomalous. However, if Fe is grouped to the bcc metals, Gr02 will fall to the center and little will change among the positions of the other grains. On the left and lower edges, selected 2-phase fields of relevant binary phase diagrams are shown (for references see text); on the right edge, 1-phase fields of hcp structured ε-phases within bcc-fcc systems are shown. Unless noted, the boundaries are for approx. 1500 to 1600 K.
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The possibility that the hcp structure was retained metastably during the growth of RMNs appears rather unlikely: Os-rich and therefore hexagonal metal is the first condensate at high temperatures (>1600 K) expected from equilibrium condensation calculations, and therefore, the nucleation of hcp structured alloy is reasonably the first step of RMN formation. In the hypothetic scenario that a cubic structure would be the thermodynamically stable alloy form at lower temperatures, hexagonally structured RMNs may continue to grow metastably by topotaxy on the previously condensed grains. This process could be favored by hindered nucleation of new cubic alloy crystals in the ambient gas and the resulting oversaturation of condensable metal species. However, such a mechanism would likely work only if the actually stable alloy is bcc structured, because a fcc structure might easily heterogeneously nucleate on the hexagonally close-packed (001) faces of the hcp alloy. Furthermore, in any of such cases, strong compositional zoning and/or deviation from the calculated condensation sequence would be expected, which is not observed in our RMNs. Diffusion calculations based on extrapolated interdiffusion coefficients of hexagonal Mo-Os alloys (Erley and Wagner 1973) show that homogenization of 400 nm sized RMNs at 1500 K is likely to occur within hours or days (D ∼ 3 × 10−17 m2 s−1). Considering the cooling rate of 0.5 K yr−1 suggested by Berg et al. (2009), homogenization as well as equilibration with the surrounding gas would be well feasible in the RMN forming environment. Under such conditions of high atomic mobility within the alloys, it is unlikely that a metastable phase or zoning pattern would persist or that exsolution would be kinetically inhibited. Furthermore, it is probable that any defects within the metal structures would be obliterated almost instantaneously, adding to the argument of high stacking fault energies as discussed above. All in all, it seems plausible that the hcp-structured RMNs formed as thermodynamically stable ε-phase alloys.
From a microstructural perspective, it can be expected that condensation from a gas phase will result in extended planar defects, such as stacking faults or microtwins, which originate from misarranged addition of atoms to the dense-packed atomic layers on the surface of the growing grains. Defect densities are expected to be high when fault energies are low and growth rates are rapid. In the case of presolar silicon carbide and meteoritic nanodiamond grains, abundant stacking faults and microtwins are microstructural indicators of their condensation origin (e.g., Daulton et al. 1996, 2003). The formation of stacking faults in SiC is particularly favored by rather low stacking fault energies of <40 mJ m−2 (Ning and Ye 1990; Hong et al. 2000). In contrast, metals like Mo and Ru have stacking fault energies in the range of 200–400 mJ m−2 (Hirschhorn 1963; Igarashi et al. 1991). This fact may explain the observed scarcity of planar defects in the RMNs studied, considering that cooling and, therefore, condensation rates, were likely relatively low (Berg et al. 2009). In addition, microstructures might have been erased by annealing through secondary thermal processing, e.g., during heating of the CAIs that hosted RMNs.
In summary, we find that (1) a hexagonal ε-phase alloy of refractory metals appears to be a thermodynamically stable phase. (2) Microstructural indicators expected for condensates are lacking. (3) Elemental zoning patterns, which may be expected during fractional condensation, are not present. The latter two points may simply reflect high-temperature formation and processing, which would erase structural defects and compositional gradients by diffusion.
Refractory Metal Nuggets as Condensates
The uniform and likely thermodynamically stable structures of RMNs are compatible with a condensation origin, although there are no definitive signs that would point to condensation as the only possible mechanism of RMN formation. As outlined above, high temperatures have likely erased or prevented such evidence. The main arguments for condensation come from the chemical composition of RMNs. The compositions of RMNs closely follow predictions by condensation calculations, assuming ideal solid solution of all metals involved, as shown by Berg et al. (2009). Particularly noteworthy is the correlation of the hcp metal Ru with the bcc metal Mo and the anticorrelation of the two hcp metals Os and Ru (Fig. 1). Such correlations are predicted by condensation calculations, assuming condensation into a single alloy, which has been confirmed by the TEM work described above. Single alloy condensation has been assumed by Palme et al. (1994) in explaining the patterns of opaque assemblages in some CAIs. Sylvester et al. (1990) suggested condensation of refractory metals in three distinct phases according to the structure of the individual metals. The final metal assemblage is then a mechanical mixture of hcp-, bcc-, and fcc-metal alloys. The results of this study and of the Berg et al. (2009) calculations refute such a model.
The presence of Mo and W in the proportions predicted by condensation calculations in RMNs also supports condensation models. Both elements can be easily oxidized and are then lost from the alloy as both Mo and W oxides are highly volatile. The calculations of Fegley and Palme (1985) show that at an H2O/H2 ratio of 5 × 10−4, which is a factor of ten higher than the canonical solar ratio, a significant Mo-anomaly is present in condensing metal. This oxygen fugacity is still 6 orders of magnitude below the IW-buffer, reflecting the very reducing conditions in the H2-dominated early solar nebula. As a Mo-anomaly is not present in our suite of nuggets, formation at extremely reducing conditions is required. The fully condensed W and Mo in the RMN grains studied here also excludes losses of W and Mo by parent-body processes, probably because nuggets were fully enclosed in mineral grains.
Wark (1986) reported data on RMN in a CAI of the Allende meteorite. Nuggets in the core of the CAI have a more refractory composition with enhanced Re, W, Os, Ir, and Mo, whereas nuggets near the rim of the CAI are higher in the less refractory metals, such as Ru, Pt, Rh, Fe, and Ni, clear signatures of condensation. Moreover, evidence for condensation is not limited to refractory metals. Rare earths elements and other refractory lithophile elements provide evidence for condensation of refractory elements at high temperatures, recorded in CAIs (Boynton 1975; Davis and Grossman 1979).
Refractory Metal Nuggets as Residues from Evaporation
The calculation of RMN composition was carried out by assuming thermodynamic equilibrium between gas and solid. A given solid metal–gas equilibrium can be either reached by condensation (cooling) or by evaporation (heating) in a very reducing environment. As pointed out by Cameron and Fegley (1982), several cycles of evaporation and recondensation of refractory metals might have occurred. The preservation of Mo and W in the metallic state within RMNs requires low oxygen and sulfur fugacities as expected for a solar gas (Blum et al. 1989), because otherwise these elements would volatilize in the form of their oxides (Palme et al. 1998) or convert to sulfides (particularly MoS2).
RMNs could represent the last solid after evaporation of lower temperature phases. Prime candidates for such materials would be Fe-Ni metal grains with solar siderophile trace element abundances and sizes in the order of several tens of micrometers. CAIs as evaporation precursors appear less likely, because RMNs can occur abundantly in single CAIs (amounting to a hundred or more, e.g., Schwander et al. 2012), which would imply that either the precursor CAIs were extremely metal-rich (which is not observed in preserved CAIs), or somehow residues of many CAIs accumulated in few remaining CAIs (which is rather difficult to envision as other residual solids would accumulate as well in those CAIs). Anyhow, the evaporation model works realistically only if a canonical solar gas is present:
Evaporation of RMN precursors in dust- or ice-enriched regions by shock-wave or radiation heating will occur at higher oxygen fugacities compared with the canonical solar gas and will likely lead to extensive or complete loss of W and Mo from the metal (Fegley and Palme 1985
Heating of chondritic material after partial dissipation of the H2
-rich nebula would imprint a much higher oxygen fugacity on the system compared with the solar nebula and, hence, would inevitably lead to losses of W, Mo, and eventually Re and Os (Palme et al. 1998
Equilibrium between solid-metal grains and gas requires that metals must not evaporate into vacuum. For example, a Ru atom evaporating from a refractory metal alloy must subsequently contribute to the partial pressure of Ru above the solid metal. Thus, evaporation into vacuum would not be sufficient to account for the apparent equilibrium compositions observed.
In the condensation scenario, equilibration with the surrounding gas is assumed to have ceased at different temperatures, presumably by the incorporation of RMNs into contemporaneously forming silicate or oxide phases, e.g., spinel as shown by Eisenhour and Buseck (1992), or spinel and hibonite as described by Wark (1986). This eventually resulted in the observed compositional variations. It is not clear how the sequence of alloys reflecting a continuum of formation temperatures (Berg et al. 2009) could be produced by evaporation instead of condensation. For example, if the RMNs originated as residues from evaporation of individual, and necessarily tens of μm large, Fe-Ni metal grains, then this process must have happened in a region characterized by a relatively stable temperature gradient bracketing the calculated equilibrium temperatures, or a mechanism must have existed that provided isolation at some point during locally rising temperatures (obviously, it cannot have been the condensation of a host phase). Both cases are very difficult to reconcile with realistic scenarios.
One could also envision a model where refractory metals form solid alloys by precipitation from liquid silicates during evaporative mass loss. This model could be attractive, because it is now clear that many CAIs were once molten and kinetic isotope fractionations indicate rapid evaporation (e.g., Shahar and Young 2007). Although fO2 was low during CAI melting (Grossman et al. 2008) and probably favorable for keeping Mo in the alloys (most oxidation to form opaque assemblages likely occurred in the chondritic parent bodies), a contemporaneous melting of RMNs during CAI melting or precipitation from a CAI melt is improbable for several reasons:
(Near-) equilibrium crystallization would result in single or only narrowly variable alloy compositions, unlike what is observed among the RMNs sampled.
Within the multicomponent system, fractional crystallization (e.g., by entrapment) is unlikely to mimic the volatility-controlled, calculated compositional trends, which fit our data well. In such a case, it is expected that the geochemical distribution coefficients control the partitioning of refractory siderophile elements between metal and melt.
The platelet shape of the Fe-rich grain 02 suggests that it has never been molten at any time, despite its melting point is likely among the lowest of the observed RMN compositions. If it had melted, a droplet shape or at least a more isometric crystal habit could be expected due to the high interface energy between metal and silicate melt.
As outlined above (including the difficulty of isolation), evaporation of Fe-Ni metal grains could produce the same sequence of RMN compositions during rising temperatures, if the starting material had a solar relative abundances of refractory metals. However, if the evaporating material had been fractionated previously, particularly by oxidation and volatilization of Mo and W (Palme et al. 1998) or, more applicable to molten/sintered CAIs, by partitioning between metal and sulfides, oxides, or silicates, an evaporation origin of the observed compositional variations would be unlikely to comply with condensation calculations invoking solar element ratios in the gas phase.
In principle, it is possible to postulate a perfect equilibrium evaporation scenario under reducing conditions of a canonical solar gas that is the exact reversal of the equilibrium condensation assumed in the calculations of Berg et al. (2009). In this case, ignoring the difficulties discussed above, the resulting RMNs would be exactly the same and it would be impossible to test one scenario against the other. However, if we assume that RMNs are indeed formed by such evaporation, then we have to assume that their precursor (e.g., μm-sized Fe-Ni metal grains), before evaporation, are formed by equilibrium condensation—otherwise evaporation would not end up with the same results as indicated by equilibrium condensation calculations (e.g., any metal processed in chondrule-forming environments or parent bodies would have likely lost substantial amounts of Mo and W due to oxidation). All the evaporation scenarios outlined above are rather complex and appear less likely to happen than a direct formation of RMNs by condensation from a solar gas.