Forty-seven stones totaling ~28 kg were found at an unpublished location in the Sahara Desert by the Labenne Family. The main mass weighs 6,140 g, with other large paired masses weighing 2,516 g, 1,270 g, and 1,050 g. Sahara 97096 is a highly primitive, highly reduced, EH3 chondrite that retains features of the primary nebula in its sulfide- and metal-rich chondrules. Sulfide–metal nodules are present containing cores of Ca-bearing oldhamite, which may have formed as the highest-temperature condensate during cooling of a solar gas under reducing conditions (Weisberg et al., 2006).
Unique to other E chondrites, portions of this meteorite contain an abundance of refractory inclusions. It has been shown by Tagle and Berlin (2007) that the E chondrites are typically depleted in those refractory elements which condense before Au, but that the CV/CK chondrites are enriched in these same elements to a complementary degree. Below the temperature at which refractory inclusions give way to the formation of chondrules, this complementarity is maintained, although it is reversed. They expressed support for a scenario whereby the refractory component was transferred from the E-chondrite formation region to the CV/CK region.
Sahara 97096 also contains an unusual abundance of FeO-rich silicates, mostly low-Ca pyroxene. However, as shown by Kimura et al. (2003), the O-isotopic compositions of both FeO-rich and FeO-poor silicates are identical, indicating that they both formed from a common oxygen reservoir (the same oxygen reservoir as the Earth and Moon). Notably, FeO-rich silicates with identical O-isotopic values are present in Kakangari (Berlin et al., 2007). These silicates show evidence for reduction processes in both groups, but to a greater degree in E chondrites than in Kakangari. It was suggested that the FeO-rich silicates in both of these groups may have had an early common precursor, despite their present differences in degree of reduction.
Rubin et al. (2009) identified a clastic matrix component incorporated among the chondrules, chondrule fragments, and opaque assemblages in EH chondrites. This matrix generally constitutes between 2 and 15 vol% of the EH chondrites, and consists of coarse angular particles of silicate (20–30 vol%) and opaque (25–30 vol%) minerals having a similar mineralogy to those minerals common to the bulk EH chondrites, and which likely represent a crushed component of these same mineral phases. Also present in the matrix (45–50 vol%) is a nebular dust component which occurs as fine-grained (nm- to sub-µm-sized), amorphous or finely crystalline, Al- or Mg-rich silica particles, and exhibits an enrichment in alkalis such as Na and K possibly due to their recondensation onto nebular dust during chondrule heating. These sub-µm-sized nebular fines constitute 2–5 vol% of EH chondrites and comprise the minerals kamacite, troilite, niningerite, oldhamite, Cu-rich sulfide, schreibersite, enstatite, and silica; Fe-FeS spherules are abundant near shock-melt veins. It was demonstrated that all of the matrix phases originated from the same O-isotopic reservoir as the other EH chondrite components. Schreibersite particles separate from FeNi-metal are thought to represent an early condensate (Lehner and Buseck, 2010).
Computer modeling of the genesis of enstatite chondrite chondrules was conducted by Blander et al. (2009). They demonstrated that high temperature and high pressure conditions initially present in the nebular condensation region created a barrier to the nucleation of Fe, but that they were conducive to the formation of FeO. They contend that a cloud of supercooled liquid droplets in equilibrium with the nebular gas of solar composition, at a pressure of ~0.1–1.0 bar (near the Sun at a distance consistent with the orbit of Mercury) and a high temperature of ~1625°C, resulted in the initial condensation of the more refractory elements such as Ca, Al, Mg, and Si. As the temperature decreased, these Ca,Al,Mg,Si-oxide droplets were gravitationally removed from the condensation region, as is reflected by the composition of the later formed enstatite chondrites.
As temperatures continued to decrease below ~1325°C, Fe was finally precipitated and most of the previously formed FeO underwent reduction. As the temperature reached ~1125°C, the supercooled oxide droplets rapidly solidified to form chondrules of various textures and compositions, primarily consisting of enstatite (58 wt%) and small amounts of olivine (26 wt%), along with a silica-rich liquid phase (16 wt%) that eventually became the chondrule mesostasis. As the temperature decreased below ~400°C, redox reactions between Fe and H-sulfide gas resulted in the production of FeS. The Fe-rich chondrules resulting from this entire process are consistent with those constituting enstatite chondrites, and are similar in composition to the planet Mercury; it may be inferred that Mercury is comprised of these same constituents.
Chondrule-sized, shock-melted, spheroidal lumps have been described in studies of Sah 97096 (Lehner and Buseck, 2009). They were formed in an impact event, probably on the EH parent asteroid, prior to consolidation and lithification of the Sah 97096 host rock. This scenario is evidenced by the sintering of the 5–40 micron-sized metallic and silicate fragments by an Fe metal–sulfide melt phase, and by the presence of melt veins and metal spherules both within the lump and throughout the bulk meteorite. Moreover, the composition of the lumps is similar to bulk Sah 97096. The discovery of these lumps led the investigating team to conclude that Sah 97096 is a primitive breccia. In a study of 16 different E chondrites conducted by Macke et al. (2009), Sahara 97096 was shown to have a higher porosity of 12.6% than all of the others, which typically ranged from 0.3% to 6.4%.
Other compositional details suggest that there was a wide variation in oxygen fugacity during accretion of Sah 97096. However, presolar grains identified in Sah 97096, such as graphite, pyroxene, and grains of C surrounded by
troilite and metal, reflect their stability under the redox conditions of formation of the E chondrite parent body, and are indicative of a highly reduced environment (Ebata1 and Yurimoto1, 2009). Recently, a presolar oxide grain (corundum) was identified in one of the paired fragments of this meteorite, the first ever found in an E chondrite. This oxide grain is isotopically consistent with an origin in a red giant or AGB star.
Previous studies suggested that the EL and EH chondrites originate from different layers on the same parent body. Employing multiple lines of evidence including chemical, petrographic, metamorphic, and cosmic-ray exposure age data, a sequence from the core to the surface of EH6, EH5, EH4, EH3, EL3, EL4, EL5, and EL6 was derived. The theory provides for the inner EH layers to be metamorphosed by internal heating, probably during accretion, while the outer EL layers were metamorphosed by external heating, probably by the Sun's early activities.
More recently, very precise measurements were made of a statistically larger sampling of E chondrites and aubrites. Although their O-isotopic data were identical, a three-isotope plot resolved the EH group from the EL and aubrite groups by its slightly steeper slope. The EL and aubrite groups still plotted on the terrestrial fractionation line. By using 53Mn/53Cr isotope systematics as a chronometer for absolute ages, Shukolyukov and Lugmair (2004) found that the EL6 Khairpur is ~4–5 m.y. younger than the EH4 chondrites Abee and Indarch, possibly representing an extended cooling history for Khairpur. They also concluded that the E chondrites formed at a location closer to the Sun, between at least 1 AU outward to 1.4 AU, than the location within the asteroid belt they now occupy.
A third possible grouplet with intermediate mineralogy has been identified, represented by the meteorite Y-793225. Studies have determined that it was not derived from the EH or EL groups through any metamorphic processes, and thus may represent a unique enstatite parent body. Still, since Y-793225 contains the SiN mineral sinoite, which has only been found to occur in the EL group, this anomalous E chondrite may be related to this group.
Further analyses of many EL3 and EH3 chondrites has identified both regolith breccias containing trapped solar rare gases and those which are solar gas-free. No regolith breccia or solar rare gases have been found in other E-chondrite petrographic types, supporting the theory that EL3 and EH3 members represent the surface material from separate parent bodies. In another study, both Fe- and Zn-isotope 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).
A radically different conclusion about the origins of E chondrites has recently been drawn from studies of trapped noble gases (A. Patzer and L. Schultz, 2002). The trapped primordial noble gases found in these meteorites are present as a mixture of specific components, with each component containing a different ratio of 36Ar, 132Xe, and 84Kr. One component has "solar" ratios of these noble gases, which is typically found in regolith breccias (~30% of E3 chondrites). Another component, which is present in both of the enstatite chondrite groups (EH and EL) as well as in ordinary and carbonaceous chondrites, is called the "Q" component (formerly known as the planetary or common noble gas component). In addition, an unusually Ar-rich component with an elemental composition intermediate between solar and Q ratios has been identified and labeled "subsolar". It has been argued that subsolar gas originates from fractionated solar gas that was implanted into chondrule precursors (Okazaki et al., 2010). Finally, a component with ratios lower than those in Q was identified and given the name "sub-Q".
In contrast to ordinary and carbonaceous chondrite groups, these various noble gas components appear to be segregated in E chondrites based on petrographic type instead of genetic group relationships. For example, all E chondrites of petrographic types 4–6 have a Q and a subsolar component, while all those of type 3 have a Q and a sub-Q component. However, these noble gas compositions do not correspond to variations in thermal metamorphism because subsolar gas abundances throughout the range E4–6 are similar. Moreover, since the subsolar component in petrologic type 4–6 E chondrites is less fractionated than the Q component present in type-3 E chondrites, the subsolar gas cannot be derived through thermal metamorphism of type-3 chondrites. Therefore, these differences in noble gas compositions that exist among E chondrites must have been established at the time of nebular condensation and accretion. The scenario calls for solar-type and Q-type noble gases to be incorporated onto separate parent bodies, with a subsequent metamorphic event fractionating these components into subsolar and sub-Q compositions. It was concluded by Okazaki et al. (2010) that the sub-Q component was derived from fractionation from terrestrial weathering.
Besides those E-chondrite parent body distinctions that can be made based on trapped noble gas compositions, other characteristics also suggest an independent nebular origin for E3 and E4–6 chondrites including O-isotopes, Ni content in kamacite, and Si content in metal. These characteristics, along with the trapped noble gas data, are consistent with a separate formation of E3 and E4–6 chondrites on separate parent bodies. This is a radical departure from the commonly cited onion-skin model, which serves as the basis for petrographic type divisions in other chondrite groups.
Employing multiple techniques to study the insoluble organic matter component of Sah 97096, Piani et al. (2009) found only a small number of aromatic compounds, including benzene and naphthalene, compared to the wide diversity of aromatic compounds present in CI and CM carbonaceous chondrites. This is attributed to the increased thermal conditions experienced by Sah 97096 prior to accretion or during parent body metamorphism. Utilizing Raman spectroscopy, Robin et al. (2008) analyzed the maturity of the carbonaceous component in matrix areas of several E chondrites in order to better resolve the metamorphic grades. Other indicators which measure the degree of thermal metamorphism were also employed, including textural and opaque mineral petrography. Based on their results, and through comparisons with similar studies conducted previously on carbonaceous and ordinary chondrites, an accurate petrologic type for Sah 97096 was determined to be ~3.4, among the lowest found thus far in this group. The photo above shows a 19.4 g partial slice of Sah 97096 that was cut from the 2,516 g mass.
