Forty-seven stones totaling ~28 kg were found at an unpublished location in the Sahara Desert by the Labenne Family, the largest of which weighs 6,140 g (Sah 97091). The 2.52 kg stone designated Sahara 97096 is only slightly weathered to a grade of W1, and it contains some localized shock-melt veins representative of shock stage S34. The Van SchmusWood (1967) scheme for petrographic type has been modified for enstatite chondrites, establishing both a textural type (37), reflecting peak metamorphic temperature, and a mineralogical type (αδ), pertaining to the cooling history (Zhang and Sears, 1996; Quirico et al., 2011). Sahara 97096 is a highly primitive, highly reduced, EH3.13.4α,β chondrite that retains features of the primary nebula in its sulfide- and metal-rich chondrules.
Weyrauch et al. (2018) analyzed the mineral and chemical data from 80 enstatite chondrites representing both EH and EL groups and spanning the full range of petrologic types for each group. They found that a bimodality exists in each of these groups with respect to both the Cr content in troilite and the Fe concentration in niningerite and alabandite (endmembers of the [Mn,Mg,Fe] solid solution series present in EH and EL groups, respectively). In addition, both the presence or absence of daubréelite and the content of Ni in kamacite were demonstrated to be consistent factors for the resolution of four distinct E chondrite groups: EHa, EHb, ELa, and ELb (see table below).
ENSTATITE CHONDRITE SUBGROUPS
Weyrauch et al., 2018
Cr <2 wt%
Cr >2 wt%
Cr <2 wt%
Cr >2 wt%
Fe <20 wt%
Fe >20 wt%
Fe <20 wt%
Fe >20 wt%
Ni <6.5 wt%
Ni >6.5 wt%
Ni <6.5 wt%
Ni >6.5 wt%
A few other E chondrites with intermediate mineralogy have also been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 (E-an), and PCA 91020 (possibly related to LEW 87223). Studies have determined that these meteorites were not derived from the EH or EL source through any metamorphic processes, and some or all of them could represent separate E chondrite asteroids. The revised E chondrite classification scheme of Weyrauch et al. (2018) including selected examples from their 80-sample study can be found here. It was determined that the Sahara 97096 pairing group is part of the EHa subgroup.
Spherical metalsulfide nodules are abundant (37 vol%) in this meteorite and contain a wide variety of sulfides (e.g., Ca-bearing oldhamite, a mineral which might have formed as the highest-temperature condensate during cooling of a solar gas under reducing conditions), phosphides (e.g., schreibersite and perryite), and silicate mineral phases (Weisberg et al., 2006; Lehner et al., 2014). Moreover, it was revealed that a continuum exists for these spherical nodules, in which one end member consists of metalsulfide nodules that contain no silicates, and the other end member consists of pyroxene-rich chondrules that contain no sulfides (Lehner et al., 2014). They suggest that when taken together, the compositions of each of these components (transitional objects and matrix) manifest a complementary relationship. Trace element studies of Sahara 97096 by Jacquet et al. (2015) led them to conclude that formation of these opaque nodules was the result of an extreme sulfidizing event in which oldhamite (CaS) was expelled from chondrules, thus negating the need for enstatite chondrite silicates to have initially condensed under unique reducing conditions associated with high C/O ratios.
A characteristic which is unique to this meteorite compared to other E chondrites is the presence in some portions of abundant refractory inclusions. It has been shown by Tagle and Berlin (2007) that the E chondrites are typically depleted in those refractory elements that 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 that associated with 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 meteorite 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 the differences that now exist in their degree of reduction.
Rubin et al. (2009) identified a clastic matrix component incorporated among the chondrules, chondrule fragments, and opaque assemblages in EH chondrites. The matrix is typically present in relatively low abundance in EH chondrites, and constitutes ~21.5 vol% in Sah 97096 (Lehner et al., 2014). It consists of coarse angular particles of silicate (2030 vol%) and opaque (2530 vol%) minerals having a similar mineralogy to the minerals common in the bulk EH chondrite; therefore, it has been considered likely that the matrix component represents a disaggregated component of these same mineral phases. Contrariwise, Lehner et al. (2011) determined that the matrix and chondrule compositions of Sah 97096 are different from each other and are not complementary. They found that the matrix is not composed of a mixture of components from the bulk meteorite, and that the silicate component of the matrix is more depleted in refractory elements than are the chondrules. Instead, Lehner et al. (2014) determined that the composition of the matrix consists primarily of pulverized pyroxene-rich chondrules that have undergone sulfidation in a hot nebula environment, along with a minor component of metal clasts. Utilizing transmission electron microscopy (TEM), Weisberg et al. (2014) found that the matrix material in Sah 97096 and other studied EH chondrites is composed of a unique, fine-grained, reduced component that was not derived from chondrules, but rather from primary dust and debris inherent to the EC formation region. Employing TEM and other advanced techniques, Lehner et al. (2014) ascertained that the matrix consists of both amorphous and crystalline grains of enstatite (~45 vol%) and cristobalite (up to 30 vol%), along with typical opaque phases (1520 vol%, as kamacite and troilite), as well as minor oldhamite, niningerite, and C-rich spherules. The glassy silica grains were shown by Zolensky et al. (2014) to contain inclusions of sulfide, plagioclase, schreibersite, and enstatite.
Present in the matrix is a presolar nebular dust component (4550 vol%) which occurs as fine-grained (nm- to sub-µm-sized), amorphous or finely crystalline, Al- or Mg-rich silicate particles, and also includes SiC grains and C-anomalous grains. The matrix constitutes only 25 vol% of EH chondrites 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 comprise the minerals kamacite, troilite, niningerite, oldhamite, Cu-rich sulfide, schreibersite, enstatite, and silica; FeFeS 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, but are different in significant ways from the matrix material in EL chondrites; e.g., the abundance of silica is higher and the abundance of albitic plagioclase is lower in EH compared to EL chondrites. Schreibersite particles in EH chondrites are observed to occur separatly from FeNi-metal, and they are thought to represent an early condensate (Lehner and Buseck, 2010).
An anomalous olivine grain was identified in Sah 97096 that has a unique texture and composition compared to other chondrule components (Weisberg et al., 2011). Its O-isotopic ratios are more similar to those of R chondrites, and it is considered that this olivine possibly represents a relict grain acquired from a different generation and distinct reservoir of chondrules that was preserved due to incomplete melting.
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 which was conducive to the formation of FeO. They contend that a cloud of supercooled liquid droplets in equilibrium with the nebular gas of solar compositionµat a pressure of ~0.11.0 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 (CAIs) were gravitationally removed from the condensation region as is reflected by the composition of the later formed enstatite chondrites. Weisberg et al. (2011) concluded that unlike the locally-formed chondrules, all CAIs presently associated with the various chondrule groups were formed in a distinct nebular location and were subsequently redistributed to diverse accretion regions.
As temperatures continued to decrease below ~1325°C, Fe was 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, consisting primarily of near-pure enstatite (58 wt%) with lesser 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, the formation of niningerite [(Mg,Fe,Mn)S], troilite (FeS), and oldhamite (CaS) occurred through the sulfidation of ferromagnesian silicates as Mg became volatilized within an H-poor, C- and S-rich gaseous reservoir (Lehner et al., 2013). 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 can be inferred that Mercury is composed 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 540 µm-sized metallic and silicate fragments by an Fe metalsulfide 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. However, the chemical composition of Sah 97096 as exhibited in its high Fs content in pyroxene, high Ti content in troilite, and low Cr content in olivine may be more consistent with a low degree of metamorphism (Komatsu et al., 2011). In a study of 16 different E chondrites conducted by Macke et al. (2009), Sah 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 there was a wide variation in oxygen fugacity (related to the partial pressure of available oxygen) 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 that existed during formation of the E chondrite parent body, and are indicative of a highly reduced environment (Ebata and Yurimoto, 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.
Some earlier 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. Studies have determined that the E chondrites formed at a location closer to the Sunat a distance of at least 1 AU outward to 1.4 AUthan the current location in the asteroid belt which they now occupy.
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 did resolve the EH group from both 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 ~45 m.y. younger than the EH4 chondrites Abee and Indarch, possibly representing an extended cooling history for Khairpur on a common parent body, or perhaps an origin on a distinct parent body. In a similar manner, age data based on IXe for EL and EH chondrites was attained by Hopp et al. (2013, 2015). They found significant age variations exist for meteorites both among members of the same group and between the two groups, so this precise chronometer does not resolve the two groups; e.g., the age of EH3 Sah 97096 at ~4.5544 b.y. vs. EH4 Abee at ~4.5618 b.y. vs. EH5 St. Marks at ~4.5612 b.y. vs. EL6 Neuschwanstein at ~4.5584 b.y. vs. EL6 LON 94100 at ~4.5579 b.y.
Employing a broader range of EL and EH petrologic types exemplifying differences in formation temperatures, Hopp et al. (2014) were able to resolve these two groups using the KAr system. They determined a lower corrected age range for metamorphic cooling of EL5 and EL6 meteorites of 4.484.51 b.y., and also found evidence of a more complex thermal history for the EH parent body indicative of multiple impact resetting events and homogenization during the period 24.5 b.y. ago. Their studies of Sah 97096 and other EH chondrites revealed a late partial metamorphic resetting event ~2.2 b.y. ago, while the oldest ArAr age of ~4.53 b.y. was measured for the EH parent body in the LAP 02225 impact melt.
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 either 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 that 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 petrologic 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 chondritesEL chondrites are heavier than EH chondrites, indicating that they experienced higher volatilization during formation 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 in 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 in E chondrites appear to be segregated based on petrologic type instead of genetic (parent body) relationships. For example, all E chondrites of petrologic type 46 have both a Q and a subsolar gas 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 E46 are similar. Moreover, since the subsolar component in E46 chondrites is less fractionated than the Q component present in E3 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 inferred scenario calls for solar-type and Q-type noble gases to be incorporated into 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 incurred during terrestrial weathering.
In addition to those parent body distinctions which can be made through studies of trapped noble gas compositions, other characteristics also suggest an independent nebular origin for the E3 and the E46 chondrites; e.g., O-isotopes, Ni content in kamacite, Si content in metal, and the lack of an anticorrelation between 36Ar and petrologic type (Hopp et al., 2014 and references therein). These characteristics, along with the trapped noble gas data, are consistent with a separate formation of E3 and E46 chondrites on separate parent bodies. This scenario would be a radical departure from the commonly cited onion-skin model which serves as the basis for petrologic type divisions in other chondrite groups.
Piani et al. (2009) employed multiple techniques to study the insoluble organic matter (IOM) component of Sah 97096, located primarily within matrix material surrounding chondrules and in opaque nodules. They identified only a small number of aromatic compounds, including benzene and naphthalene, compared to the wide diversity of both aromatic and aliphatic compounds present in CI and CM carbonaceous chondrites. Many differences exist between IOM in primitive carbonaceous chondrites compared to Sah 97096, including a low abundance of IOM compounds in Sah 97096, and these can be attributed to the higher temperature conditions experienced by Sah 97096 prior to accretion or during parent body metamorphism.
Utilizing Raman micro-spectroscopy, Robin et al. (2008) analyzed the maturity (degree of structural order) of the IOM in a fine-grained matrix component and in inclusions within metal nodules of several E chondrites in order to better resolve the metamorphic grades and to assign petrologic types. 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 3.13.4; this is among the lowest found thus far in members of the EH group. The thermal history of both EH and EL enstatite chondrite asteroids, including the relationship between petrologic type and the closure temperature of opaque phases, is consistent with a simple onion shell model (Quirico et al., 2011).
Notably, a new type of meteorite classified as a forsterite chondrite/achondrite with EH3 affinities has been discovered in a pairing of Sah 97096 (Boyet et al., 2011; abstract). The photo above shows a 19.4 g partial slice of Sah 97096 that was cut from the 2,516 g mass.