Around 2:00 on a July afternoon, people in Haute Garonne, France heard detonations and observed the fall of a meteor. A meteorite weighing ~14 kg was recovered 1.5 km south of Saint-Sauveur, Saint-Mengaud, and Saint-Mourié. Saint-Sauveur is a member of the high-Fe group of enstatite chondrites, one of a very small number classified as petrologic type 5. It is considered to be an impact-melt breccia, and has been weakly shock metamorphosed to stage S3 corresponding to a shock pressure of ~10 GPa. Shock features include planar fractures and twinned clinoenstatite lamellae within orthopyroxene, and the occurrence of opaque veins of kamacite and troilite.
Enstatite chondrites are the most reduced meteorites among chondrites as evidenced by their extremely low FeO content, and by the presence of rare sulfide minerals such as oldhamite, daubréelite, and alabandite (EL) or niningerite (EH). Moreover, metal occurs primarily as low-Ni kamacite in both the EH and the EL groups. Surprisingly, it has been demonstrated by Macke et al. (2009) that these two groups do not actually differ in their iron content, and that they are indistinguishable in density, porosity, and
magnetic susceptibility as well. However, differences in siderophile, chalcophile, and other mineralogical abundances can be employed to distinguish the two groups. The EH and EL groups are clearly resolved from each other based on compositional, textural, and mineralogical differences, as well as by O-isotopic data and formation intervals, indicating that they were derived from separate, but closely related parent bodies. In addition, both Fe- and Zn-isotopic compositions are fractionated to different degrees between EL and EH chondrites; EL chondrites are heavier than EH chondrites, indicating that they experienced higher volatilization while forming closer to the Sun (Mullane et al., 2005). The nonrefractory siderophile, chalcophile, and alkali elements in Saint-Sauveur clearly establish it as a member of the EH group.
Within the EH group, a distinction can be readily made between EH3 and EH4,5 petrologic types based on mineral compositions. One difference is evident in their respective Ni content in kamacite (EH3: 24–33 mg/g Ni; EH4,5: 65–79 mg/g Ni), which may be explained by the depletion of Ni by the formation of high Ni perryite at the surface of kamacite grains in the EH3 chondrites. Perryite formation was induced through hot nebular exchange reactions in which metal was converted to FeS, thus freeing up Ni to form perryite. In contrast to the unmetamorphosed E chondrites, this mineral did not survive subsequent metamorphic heating in E chondrites of higher petrologic types. Since elemental abundances in E chondrites of petrologic types 4 and 5 are practically the same, it is only from observations of mineralogical changes, produced by varying degrees of thermal metamorphism, that a distinction can be made between them.
Enstatite chondrites have O-isotope compositions that plot along the terrestrial fractionation line, suggesting that they may have formed within the Mercury–Venus region in the inner solar system, and that they were subsequently perturbed into the inner regions of the asteroid belt. In such a case, the strongly reducing conditions under which they were formed could have been promoted by an excess of H and C, maintained by a hot, dusty environment close to the Sun. Utilizing Mn–Cr isotope systematics, Shukolyukov and Lugmair (2004) concluded that the E chondrites formed at a location closer to the Sun—between at least 1 AU outward to 1.4 AU—than at the location within the asteroid belt they now occupy.
However, if the region between ~1.0 and 1.4 AU were truly the formation location of E chondrites, they should have highly elliptical orbits; but this is not what is observed. In fact, reflectance spectrometry has identified asteroids similar to E chondrites in stable orbits between 1.8 and 3.2 AU, suggesting that the inner asteroid belt is the actual location where they originated. In addition, a heliocentric distance of ~2.0–2.9 AU was calculated for two E chondrites on the basis of their implanted solar noble gas concentrations (Nakashima et al., 2004). Furthermore, an isotopically anomalous Xe-containing component, associated with an anomalous light N component, is found proportionately in both carbonaceous and enstatite chondrites, but not on Earth. Since this component is almost certainly of nucleosynthetic origin, it follows that the carbonaceous and enstatite chondrites should share a similar heliocentric formation location. In this case, the strongly reducing conditions under which E chondrites formed could have been promoted by the loss of refractory oxides prior to condensation from the local nebula. Details of a computer-based model (Blander et al., 2009) of the formation history of E chondrites can be found on the Sahara 97096 page.
The possibility of two subgroups constituting the EH group has been proposed. Within the subgroups, the cooling rate and MnS content in niningerite are correlated. This correlation is not adequately explained by burial depth or impact-generated differences, and therefore, formation on two separate bodies has been suggested. The thermal history of Saint-Sauveur is consistent with inclusion into the subgroup that experienced fast cooling with a low MnS content in niningerite. Data from Rb–Sr systematics infer a formation age for Saint-Sauveur of 4.516 (±0.029) b.y., with indications of a high-temperature shock event occurring 60–200 m.y. after formation, consistent with the presence of the high-pressure silica polymorph cristobalite. This cristobalite was preserved through rapid cooling (Kimura et al., 2005). The occurrence in Saint-Sauveur of the mineral keilite, produced from melting of niningerite and troilite, is also indicative of an impact melt event accompanied by rapid quenching. Cosmic-ray exposure ages are generally lower for EH chondrites than for EL chondrites, 0.5–7 m.y. and 4–18 m.y., respectively.
A xenolith that was found in the carbonaceous chondrite Kaidun, named Kaidun III, has been determined to be an EH5 inclusion, one that underwent hydration on the Kaidun parent body. Interestingly, another clast found in a Kaidun sample is a rare EL3, named Kaidun IV. In addition to three Antarctic EH5 members, the St. Mark's meteorite is the only other non-Antarctic EH5 sample in our collections. The specimen of Saint-Sauveur shown above is a 1.34 g partial slice obtained in a trade with the Muséum National d'Histoire Naturelle, Paris, France, by International Meteorite Brokerage.
