9 Discussion
9.3 Ureilite vein metal as a primary component
9.3.3 Modeling of batch melting on the ureilite parent body
9.3.3 Modeling of batch melting on the ureilite parent body
Description of the batch melting process
In contrast to fractional melting, batch melting requires simultaneous monitoring of sulfur and carbon in the system. Unfortunately, little information is available on the behavior of carbon and sulfur‐rich systems at temperatures below 1100°C (RAGHAVAN 1988, WANG et al.
1991, OHTANI and NISHIZAWA 1987). The available data agree on the presence of a carbon‐rich and a sulfur‐rich immiscible liquid at temperatures above ~1100°C. GOODRICH and BERKLEY (1986) have adapted a ternary Fe‐FeS‐Fe3C‐diagram from VOGEL and RITZAU (1931) that is also used in this study.
It is assumed that melting commences near the eutectic temperature of the Fe‐S‐System at about ~950°C (GOODRICH and BERKLEY 1986, RANKENBURG et al. 2008, VAN ORMAN et al. 2009).
The first melt formed is a eutectic sulfur‐rich melt with ~31 wt% S. Melt generation continues until sulfur is exhausted in the residual metal. At that point, the solidus temperature of the system increases, as the amount of S in the melt decreases.
At ~1100°C, the formation of a C‐rich melt starts. The composition of the C‐ and the S‐rich melt has been taken directly from the ternary Fe‐FeS‐Fe3C‐diagram in GOODRICH and BERKLEY (1986). The S‐rich melt contains ~28 wt% S and ~0.3 wt% C, while the C‐rich melt contains
~1 wt% S and ~4 wt% C. As most of the S of the system is contained in the S‐rich melt, the C‐
rich melt is formed partly at the expense of the Sr‐rich melt. The formation of the C‐rich eutectic melt will continue until C is depleted in the residual metal.
At that point, the solidus temperature increases again, while S‐ and C‐concentrations in the respective liquids decrease. The solidus temperature will increase until the whole system is molten, or the estimated peak temperature of ~1300°C for the ureilite parent body is reached. As ureilites have lost >95 wt% of their initial metal component, it is assumed that the melt is removed when 95% of the primary metal component is molten.
Starting compositions rich in carbon and sulfur
In a C‐ and S‐rich parent body (Fig. 81, Fig. 82), similar to a CI1‐chondrite, partitioning
behavior is dominated by S as has been proposed by a number of authors (e.g. WARREN et al.
2006, RANKENBURG et al. 2008). In case of a reduced parent body (Fig. 81), only small amounts of C‐rich melts are formed (~15 wt%) until Fe is exhausted, leaving a carbon residue. An oxidized parent body (Fig. 82) might not produce a C‐rich melt at all, as the system contains sufficient S to completely melt all of the metal at temperatures lower than 1100°C.
In both modeled systems the inferred Ni and Co concentrations of the residual metal are far greater than those observed in our ureilite vein metal. The calculated Ni/Co‐ratios argue against S‐rich melt as origin for the vein metal. The C‐rich liquid is, although not a perfect fit, similar to the vein metal with respect to Ni‐ and Co‐concentrations as well as Ni/Co‐ratio.
Fig. 81 Concentrations of Ni and Co, solidus temperatures and Ni/Co‐ratio during batch melting of the metal fraction (Fe‐
Ni‐S‐C‐Co) of a reduced parent body (e.g. ALH84136) with a carbon‐ and sulfur‐rich CI1 chondritic composition. The concentrations of Ni and Co have been normalized to pure Fe‐Ni‐Co‐metal.
Fig. 82 Concentrations of Ni and Co, solidus temperatures and Ni/Co‐ratio during batch melting of the metal fraction (Fe‐
Ni‐S‐C‐Co) of an oxidized parent body (e.g. GRA95205) with a carbon‐ and sulfur‐rich CI1 chondritic composition. The concentrations of Ni and Co have been normalized to pure Fe‐Ni‐Co‐metal.
Starting compositions with moderate sulfur and carboncontents
Models based on a CV3‐chondritic parent body with intermediate sulfur and carbon‐
concentrations produce different results based on the degree of oxidation assumed. On a reduced parent body (Fig. 83), a Fe‐S eutectic melt is formed until S is exhausted in the residual metal at ~25% melting. The solidus temperature of the system increases, while S becomes depleted in the melt, until it reaches ~1100°C. At this temperature the eutectic C‐
rich melt is formed. Formation of the Fe‐C‐eutectic melt persists until C is depleted in the residue at ~88% melting. At that point, the solidus temperature of the system increases again, until the whole system is molten at ~1270°C.
In contrast, a more oxidized parent body contains higher relative amounts C and S in the metal portion. After the formation of the Fe‐S‐eutectic melt, Fe‐C eutectic melt is generated and Fe is exhausted at ~99% of melting, generating a carbon rich residue.
This starting composition does not produce a metal residue or a metal liquid that constitutes a suitable candidate for the ureilite vein metal.
Fig. 83 Concentrations of Ni and Co, solidus temperatures and Ni/Co‐ratio during batch melting of the metal fraction (Fe‐
Ni‐S‐C‐Co) of a reduced parent body (e.g. ALH84136) with intermediate carbon‐ and sulfur‐concentrations similar to a CV3‐chondritic composition. The concentrations of Ni and Co have been normalized to pure Fe‐Ni‐Co‐metal.
Fig. 84 Concentrations of Ni and Co, solidus temperatures and Ni/Co‐ratio during batch melting of the metal fraction (Fe‐
Ni‐S‐C‐Co) of an oxidized parent body (e.g. GRA95205) with intermediate carbon‐ and sulfur‐concentrations similar to a CV3‐chondritic composition. The concentrations of Ni and Co have been normalized to a pure Fe‐Ni‐Co‐metal.
Starting compositions with low sulfur and carboncontents
Batch melting on a parent body of H‐chondritic composition will produce similar results, regardless of the degree of oxidation (Fig. 85, Fig. 86). At ~980°C, the Fe‐S‐eutectic melt forms, followed by the Fe‐C‐eutectic melt at ~1100°C. Both liquids will become progressively depleted in carbon and sulfur as the solidus temperature rises, until the whole metal is molten. The calculated solidus temperatures of the more reduced system are slightly above the estimated maximum temperature of the ureilite parent body. Neither the melts nor the metal residues resemble the ureilite vein metal.
Fig. 85 Concentrations of Ni and Co, solidus temperatures and Ni/Co‐ratio during batch melting of the metal fraction (Fe‐
Ni‐S‐C‐Co) of a reduced parent body (e.g. ALH84136) with low carbon‐ and sulfur‐concentrations similar to a H‐chondritic composition. The concentrations of Ni and Co have been normalized to pure Fe‐Ni‐Co‐metal.
Fig. 86 Concentrations of Ni and Co, solidus temperatures and Ni/Co‐ratio during batch melting of the metal fraction (Fe‐
Ni‐S‐C‐Co) of an oxidized parent body (e.g. GRA95205) with low carbon‐ and sulfur‐concentrations similar to a H‐
chondritic composition. The concentrations of Ni and Co have been normalized to pure Fe‐Ni‐Co‐metal.
Is the vein metal the result of a batch melting process on the UPB?
Residual metal: Similar to fractional melting, batch melting will not produce residual metal that resembles the ureilite vein metal. In all calculated scenarios, Ni concentrations of residual metal were higher (factor ~1.5‐3+) than observed in ureilite vein metal. Similarly, Co‐concentrations are too high for most scenarios, except scenarios with low to moderate sulfur‐ and carbon‐concentrations and a reduced parent body (Fig. 83, Fig. 85). Ni/Co ratios of the residual metal are generally higher than observed ratios, except for very carbon‐ and sulfur‐rich parent body compositions (Fig. 81, Fig. 82). In order to represent a suitable model for the origin of ureilite vein metal, all three conditions (Ni and Co‐concentrations and Ni/Co‐
ratio) must be met.
Residual carbon: Both oxidized and reduced parent bodies produce large carbon residues of 10‐20 wt% of the initial model core metal, for sulfur‐ and carbon‐rich starting compositions.
An oxidized parent body with moderate concentrations of carbon and sulfur would produce a small carbon residue of 1‐3 wt%, which is consistent with carbon concentrations observed in bulk ureilites.
Sulfur‐rich metal melt: Generated sulfur‐rich melts are generally too high in Ni and too low in Co to be suitable candidates for ureilite vein metal. The calculated Ni/Co‐ratio is generally a factor of 2 – 4 higher than in vein metal.
Carbon‐rich metal melt: Calculated carbon‐rich melts are generally too high in Ni as well as Co and Ni/Co‐ratio. An exception is a reduced parent body with high sulfur‐ and carbon‐
concentrations. The composition of calculated carbon‐rich melts in such a UPB are close to the composition of vein metal (Fig. 81). Although not a perfect fit, fine tuning of sulfur and carbon‐concentrations in the starting composition can produce a carbon‐rich melt that fits the ureilite vein metal. Decreasing the assumed sulfur‐concentrations by ~4 wt% would, for example, result in a perfect fit for the Ni‐ and Co‐concentrations and Ni/Co‐ratios. Such a melt contains ~4 wt% of carbon which is consistent with ureilite vein metal.
Again, carbon‐rich melts that resemble ureilite vein metal are only produced on reduced parent bodies. They therefore do not represent an explanation for the origin of vein metal in the majority of ureilites with fa>18.