A single mass of ~10 kg was found in Sombrerete, Zacatecas, Mexico. Sombrerete was initially considered to be an anomalous iron related to the small non-magmatic IIE group, some members of which contain similar globular silicate inclusions, and it was routinely included in studies of this group. However, the silicate inclusions in Sombrerete have O-isotopic compositions that plot far away from those of IIE irons, and furthermore, the Δ17O is significantly more negative than those for typical IAB irons; this suggests an origin for the sHL subgroup on a different asteroid (Wasson, 2011). Notably, the Δ17O of Sombrerete is nearly identical to that of the silicate-bearing NWA 468, the two showing only a small difference in mass fractionation on an oxygen three-isotope diagram. As demonstrated by its plot on a Δ17O vs. ε54Cr diagram, NWA 468 is now strongly considered to be an anomalous, metal-rich lodranite (Sanborn et al., 2014).
Oxygen Isotope Compositions of Silicate-bearing Irons
Diagram credit: A. Ruzicka, Chemie der Erde, vol. 74, no. 1, p. 6 (Mar 2014)
'Silicate-bearing iron meteorites and their implications for the evolution of asteroidal parent bodies'
Abbreviations: TF = terrestrial fractionation line, CCAM = carbonaceous chondrite anhydrous materials mixing line; silicated iron meteorites include IAB, IIICD, IIE fractionated (IIE fr.) and IIE unfractionated (IIE unfr.), IVA, and IIIAB Puente del Zacate (PdZ); ungrouped irons (Ungr.) include Guin (G), Enon (E), NWA 468 (468), Sombrerete (S), Tucson (T), Mbosi (Mb), Bocaiuva (B), and NWA 176 (176); other meteorites include H, L and LL chondrites, winonaites, mesosiderites (meso.), main-group pallasites (MG pall.) Eagle Station pallasites (ES pall.), and pyroxene pallasites (px pall.)
With reference to the many mineral and textural similarities in their silicates, it can be argued that Sombrerete followed a similar petrogenetic path as the silicate-bearing IIE irons, and possibly other silicated irons. A taxonomic revision of the IABIIICD iron group was proposed by Wasson and Kallemeyn (2002), which led to the tentative inclusion of Sombrerete into the newly defined IAB complex and resolution on a NiAu diagram as a member of the high-Au, low-Ni subgroup (sHL). The fact that a large concentration of sHL members has been found near Erfoud, Morocco, and that four of them might be paired (Hassi-Jekna and NWA-series members 3200, 4706, and 4710), provides evidence for the presumption that the IAB subgroups derive from separate impact-melt pools on a single unique asteroid.
Although no other sHL members have undergone O-isotopic analyses, Sombrerete has a very different O-isotopic value than the other measured IAB irons (Δ17O = 1.39 vs. the typical ~ 0.5). Therefore, it may be more plausible that Sombrerete, and potentially the other sHL subgroup members, formed on a separate asteroid, perhaps related to the CR chondrite clan (Ruzicka et al., 2006; Ruzicka, 2014). In their study of the Mo isotope systematics among IAB complex irons, Dauphas et al. (2002) reported that the sHL subgroup iron Magnesia is distinct from the IAB main group. Further evidence supporting the hypothesis for separate parent bodies for Sombrerete and possibly the entire sHL subgroup was presented by Worsham and Walker (2015, 2016). They studied the Mo-isotopic compositions of representative meteorites from the IAB iron complex, and it was ascertained that Sombrerete is clearly resolved from members of the MG, sLL, and sLM subgroups of the IAB complex. They also recognized that the latter three subgroups share very similar W- and Mo-isotopic values and have Mo-isotopic values indistinguishable from that of the Earth. Notably, these subgroups represent the Earth's closest genetic relatives. Furthermore, they found that a member of the sHH subgroup (ALHA80104) was also clearly resolved from other members of the IAB complex (MG, sLL, sLM) with respect to its Mo- and W-isotopic values and by its older metalsilicate segregation age as determined by HfW systematics, and therefore it was concluded that both the sHL and sHH subgroups might derive from distinct parent bodies located in separate nebular regions from other members of the IAB complex irons. Therefore, because the Δ17O value, HSE data, and Mo-isotopic composition determined for Sombrerete are significantly different from that of all other IAB complex irons, Worsham et al. (2017) suggest that it no longer be classified as a IAB iron.
Sombrerete contains 7.3 vol% highly fractionated, rounded silicates, 110 mm in size (mostly ~2 mm), located mainly along metal grain boundaries (Prinz et al., 1982). The silicates show evidence of rapid quenching from a flowing melt, exemplified by the presence of crystal alignment and skeletal crystals. These silicates are highly enriched in alkalis, with compositions ranging from trachybasalt (~48 wt% silica) to alkali-rich basaltic andesite (~55 wt% silica) to andesite (~60 wt% silica) to dacite (~65 wt% silica). The plagioclase in Sombrerete is unusually Ca-rich compared to that in most other silicated irons, a likely consequence of a fractional crystallization process (Ruzicka, 2014).
A number of different types of silicate inclusions have been distinguished by Ruzicka et al. (2006). Some inclusions are composed primarily of albitic glass, comprised of equal amounts of plagioclase and quartz with varying amounts of chlorapatite and very fine-grained orthopyroxene. Others are composed of glass containing significant amounts of the rare mineral yagiite, a mineral which otherwise has only been reported in the IIE iron Colomera. The occurrence of yagiite infers its crystallization from an immiscibly separated K-rich melt (Ruzicka, 2014). Still other types of glass inclusions, which may be Na-, K-, or NaK-rich, are thought to be derived from immiscible melt fractions. These also contain a complex mixture of mineral constituents, including titanean kaersutite, ilmenite, plagioclase, chromite, merrillite, and tridymite. Some plagioclase present in inclusions of the latter type exhibits a porous texture ("spongy"), produced through the crystallization of an immiscible, quartz-enriched melt. Other inclusions of this same type contain P-rich crescent-shaped regions, with orthopyroxene and plagioclase grains showing preferential alignment to these regions suggestive of flow. The metallic host phase is composed of a plessitic intergrowth of kamacite and taenite, along with troilite and schreibersite.
The globular silicate inclusions, considered by some to reflect metalsilicate liquid immiscibility (Prinz et al., 1983; Ruzicka and Hutson, 2003), are now presumed to reflect a filter-press fractionation mechanism (Ruzicka and Hutson, 2005; Ruzicka et al. 2006). Based on their studies, Ruzicka et al. (2006) hypothesize a two-stage formation scenario leading to the observed high fractionation of silicates:
Initially, a CR-like chondritic protolith experienced low-degree (~48%) partial melting as a result of endogenous heating from the decay of short-lived radionuclides. A CR-like protolith is consistent with the measured O-isotopic compositions, as well as the P content of Sombrerete. This partial melting phase produced a phosphoran basaltic andesite.
Thereafter, the partially molten metallic host acted as a filter to separate the emergent silicate crystals (primarily chlorapatite and orthopyroxene) from the residual silicate melt as it flowed in-between inclusions. This flow was likely generated by an impact event or a close gravitational interaction, which may have also resulted in the tidal disruption and re-accretion of the planetesimal, thereby separating the solid and molten phases and moving the molten metalsilicate mixture nearer the surface where it was rapidly cooled. A possible period of slower cooling may have followed re-accretion. The compositional variation observed among the silicate inclusions (trachybasalt to dacite) is the result of the variable loss of chlorapatite and orthopyroxene from the Si-poor, P-rich parental liquid. A similar chain of events may have occurred in other silicate-bearing, nonmagmatic irons such as the evolved members of the IIE group, with Colomera showing very close similarities to Sombrerete.
The absolute IXe age for Sombrerete, calculated relative to Shallowater (4.5623 [±0.0004] b.y.), was determined to be 4.5619 (±0.0010) b.y. (Bogard and Garrison, 2009). This age is considered to reflect the differentiation of the silicate and its admixture with the metal phase during parent body disruption. Application of the HfW chronometer gave a metal segregation age of 2.1 (±0.9) m.y. after CAI formation (Worsham et al., 2014). This is ~2.4 m.y. earlier than the onset of metal segregation in the IAB main group and members of the sLL subgroup. On the other hand, Worsham and Walker (2016) reported W-isotopic values for Sombrerete that are consistent with those of IAB MG members. Notably, they also determined a W-isotopic composition for sLM subgroup member Persimmon Creek that corresponds to a younger metal#&150;silicate segregation age than that of MG members, which provides support for an impact-melt-pool formation scenario on a common parent body.
An ArAr age of 4.541 (±0.012) b.y. was established for Sombrerete, indicating closure occurred only 20 m.y. later than for the IXe system. However, since it was inferred that no resetting event had occurred since crystallization (Bogard et al., 2000), an ArAr age correction of ~20 m.y. was applied based on improved 40K decay parameters calculated by Vogel and Renne et al. (2008); this correction brings the two chronometers into agreement. These chronometers are therefore most consistent with the scenario of rapid cooling after formation. Both of these ages are older than the corresponding radiometric ages of the IIE irons. The CRE age for Sombrerete was calculated to be 278819 m.y. (based on 21Ne and 38Ar, respectively), also older than that of the IIE irons.
The photo shown above is a 49.72 g partial slice of Sombrerete, while the top photo below shows the crusted side. This specimen is from the 433 g section shown in the bottom photo below. The 433 g section was previously part of the J. Schwade Collection, originally obtained from M. Cilz.