SOMBRERETE

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 silicate inclusions, and it was routinely included in studies of this group. However, the silicate inclusions in Sombrerete have an O-isotope composition that plots far away from those of IIE irons, and it exhibits other unique characteristics as well. It can be argued that Sombrerete followed a similar petrogenetic path as the silicate-bearing IIE irons and possibly other silicated irons, but represents a separate parent body. A taxonomic revision of the IAB-IIICD iron group was proposed by Wasson and Kallemeyn (2002), which has led to the tentative inclusion of Sombrerete into the newly defined IAB complex, and resolved on a Ni-Au diagram as a member of the high-Au, low-Ni subgroup (sHL).

Sombrerete contains 7.3 vol% highly fractionated, rounded silicates, 1–10 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).

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, along with other constituents. Still another type of glass inclusion, which may be Na-, K-, or Na–K-rich, contains a complex mixture of mineral constituents, including titanean kaersutite, ilmenite, plagioclase, chromite, merrillite, and tridymite. Some plagioclase present in inclusions of the latter type, which is intergrown with the glass at inclusion margins, 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 metal–silicate 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) envisage a two-stage formation scenario leading to the observed high fractionation of silicates:

First, a low-degree (~4–8%) partial melting phase occurred, likely within a CR-like chondritic protolith, which was the result of endogenic heating by 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.

Next, 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 between inclusions. This flow was likely generated by an impact event or a gravitational interaction, which may have also resulted in the tidal disruption and reaccretion of the planetesimal*, thereby separating the solid and molten phases, and moving the molten metal-silicate mixture near to the surface where it was rapidly cooled. The observed compositional variation 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, non-magmatic irons such as the evolved members of the IIE group, with Colomera showing very close similarities to Sombrerete.

An Ar–Ar age of 4.541 (±0.0012) b.y. was established for Sombrerete, older than the radiometric ages of the IIE irons. It was inferred that no resetting event had occurred since crystallization (Bogard et al., 2000). The CRE age for Sombrerete was calculated to be 278–819 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 that was cut from the 433 g section shown in the top photo below, courtesy of J. Piatek. The 433 g section was previously part of the J. Schwade Collection, originally obtained from M. Cilz. The lower photo shows the crusted side of the 49.72 g partial slice.

Photo courtesy of J. Piatek

For additional information on collisional dynamics, read the PSRD article by G. Jeffrey Taylor - "Tagish Lake—Hit-and-Run as Planets Formed", Nov. 2006.