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Saint Sauveur

EH5
Impact-melt breccia
(EHb5 in Weyrauch et al., 2018)

standby for saint-sauveur photo
Fell July 10, 1914
43° 44′ N., 1° 23′ E. 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. The owner of the field in which it fell, Antoine Esculie, donated the stone free of charge to the Museum of Toulouse (R. Mathieu). standby for saint-sauveur impact pit photo
Photo courtesy of Société d’Histoire Naturelle de Toulouse
Originally published in the Bulletin de la Société d’Histoire Naturelle de Toulouse, vol. 93 (1958), by G. Astre.
Pictured L–R: Gaston Astre, geologist and naturalist, director of the Museum of Toulouse (1944–1962); Guillaume Champagne, priest of Saint-Sauveur; unidentified neighbor; Barthélémy Cazemajou, mayor of Saint-Sauveur 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 EL chondrites experienced higher volatilization due to its formation closer to the Sun (Mullane et al., 2005), or alternatively, due to elemental fractionation during impact shock events (Rubin et al., 2009). 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 might 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.

The Van Schmus–Wood (1967) scheme for petrographic type has been modified for enstatite chondrites, establishing both a textural type (3–7), reflecting peak metamorphic temperature, and a mineralogical type (α–δ), pertaining to the cooling history (Zhang and Sears, 1996; Quirico et al., 2011). Under this classification scheme, Saint-Sauveur has thermometers that give it a classification of EH5γ.

The possibility that the EH group is comprised of two distinct subgroups has been considered. Within these two 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 by some. The thermal history of Saint-Sauveur is consistent with inclusion in the subgroup that experienced fast cooling with a low MnS content in niningerite. 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
EHa EHb ELa ELb
Troilite Cr <2 wt% Cr >2 wt% Cr <2 wt% Cr >2 wt%
(Mn,Mg,Fe)S Fe <20 wt% Fe >20 wt% Fe <20 wt% Fe >20 wt%
Daubréelite Abundant Missing Abundant Missing
Kamacite 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 Saint-Sauveur is a member of the EHb subgroup.

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.

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 melting event accompanied by rapid quenching (Hill et al., 2014). The presence of fluor-richterite grains also attests to an impact-melt history. 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. More in-depth information on the complex thermal history of the EH chondrites can be found on the Sahara 97096 page.

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. The photo below is the 14 kg main mass of Saint-Sauveur in the Muséum de Toulouse. saint-sauveur main mass
click on photo for a magnified view
Photo courtesy of Didier Descouens—Muséum de Toulouse


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SAH 97096

EH3.1–3.4
(EHa3 in Weyrauch et al., 2018)
standby for sahara 97096 photo
Found 1997
y° 01′ 43′ N., x° 32′ 21′ W.

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 S3–4. The Van Schmus–Wood (1967) scheme for petrographic type has been modified for enstatite chondrites, establishing both a textural type (3–7), reflecting peak metamorphic temperature, and a mineralogical type (α–δ), pertaining to the cooling history (Zhang and Sears, 1996; Quirico et al., 2011). Sahara 97096 is a highly primitive, highly reduced, EH3.1–3.4α,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
EHa EHb ELa ELb
Troilite Cr <2 wt% Cr >2 wt% Cr <2 wt% Cr >2 wt%
(Mn,Mg,Fe)S Fe <20 wt% Fe >20 wt% Fe <20 wt% Fe >20 wt%
Daubréelite Abundant Missing Abundant Missing
Kamacite 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 metal–sulfide 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 metal–sulfide 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 (20–30 vol%) and opaque (25–30 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 (15–20 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 (45–50 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 2–5 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; 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, 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 ~0.1–1.0 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 (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 5–40 µm-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. 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 Sun—at a distance of at least 1 AU outward to 1.4 AU—than 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 ~4–5 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 I–Xe 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 K–Ar system. They determined a lower corrected age range for metamorphic cooling of EL5 and EL6 meteorites of 4.48–4.51 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 2–4.5 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 Ar–Ar 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 chondrites—EL 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 4–6 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 E4–6 are similar. Moreover, since the subsolar component in E4–6 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 E4–6 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 E4–6 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 3.1–3.4; 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.


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Indarch

EH4
(EHa4 in Weyrauch et al., 2018)
standby for indarch photo
Fell April 7, 1891
39° 45′ N., 46° 40′ E. At 8:10 P.M., people in Azerbaydzhan, SSR, USSR, witnessed a fireball accompanied by detonations. The next morning, a single 27 kg stone was recovered. Indarch has experienced minimal impact alteration, exhibiting weak shock metamorphism (S3) at shock pressures of 5–10 GPa. This produced planar fractures in olivine and twinned clinoenstatite, along with metallic shock veins.

The Van Schmus–Wood (1967) scheme for petrographic type has been modified for enstatite chondrites, establishing both a textural type (3–7), reflecting peak metamorphic temperature, and a mineralogical type (α–δ), pertaining to the cooling history (Zhang and Sears, 1996; Quirico et al., 2011). Under this classification scheme, Indarch has thermometers that give it a classification of EH4β,γ.

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
EHa EHb ELa ELb
Troilite Cr <2 wt% Cr >2 wt% Cr <2 wt% Cr >2 wt%
(Mn,Mg,Fe)S Fe <20 wt% Fe >20 wt% Fe <20 wt% Fe >20 wt%
Daubréelite Abundant Missing Abundant Missing
Kamacite 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 Indarch is a member of the EHa subgroup.

Indarch contains both silicate-rich and metal-rich chondrules embedded within a sulfide-rich matrix. A two-stage cooling history has been suggested to explain the reversed zoning in the chondrules. Other minerals present in Indarch include sub-µm-sized, presolar SiC grains in a concentration of 1.3 ppm (Huss, 1990). These have a grain size closely matching that of unprocessed circumstellar grains, and S-isotopic compositions consistent with an origin on AGB stars (Gyngard et al., 2012). The silicon-containing nitride, nierite, also occurs, which some investigators concluded was formed by exsolution of kamacite, perryite, and schreibersite during parent body metamorphism (Alexander et al., 1994). However, an advanced isotopic study conducted by Leitner et al. (2018) led to their contention that the majority of the silicon nitride was formed by nebular condensation/precipitation processes (a rare component is presolar) prior to its incorporation into the EC parent body. Micro- and nano-scale diamonds are present in Indarch at a concentration of ~17 ppm, and the sulfide minerals oldhamite and niningerite have also been identified.

By using 53Mn/53Cr ratios as a chronometer for absolute ages, Shukolyukov and Lugmair (2004) estimated the age of Indarch to be 4.565 b.y. A similar age of ~4.563 b.y. was determined by Busfield et al. (2008) based on I–Xe systematics, while Moseley et al. (2011) calculated an age based on Mn–Cr systematics and anchored to D’Orbigny of ~4.5674 b.y. These ages are similar to that of EH4 Abee, but slightly older than EL6 Khairpur, possibly reflecting Khairpur’s extended cooling history. The K–Ar closure age as determined by Bogard et al. (2010) occurred ≥4.35 b.y., and evidence indicates a later impact-degassing event 4.25 b.y. ago. A Rb–Sr isochron gives an age of 4.52 (±0.15) b.y., while a corrected Rb–Sr age gives 4.50 b.y. Indarch has a matrix CRE age based on 3He, 21Ne, and 38Ar of 12.1 (±2.5) m.y. (Eugster et al., 2007).

Although E chondrites and aubrites share a common O-isotopic signature, some chemical and mineralogical differences exist which had previously cast doubt on their formation on a common parent body. Some of these differences include the higher abundance of Ti and forsterite in aubritic sulfides than in E chondrites. A scenario reconciling these differences has been presented in light of an experiment in which an E chondrite was systematically melted in a highly reducing, oxygen-depleted environment. In the experiment, as the silicate-melt reached a temperature range of 1000–1300°C having a degree of partial melting of 20%, the metal–sulfide component began to migrate out of the silicate. At 1450°C, a completely separated metal component could have begun to establish a metallic core on its parent body. Since the sulfides melted at temperatures as low as 1000°C, it is demonstrated that aubritic sulfides cannot be a product of nebular synthesis as previously speculated. Instead, the tranfer of S and Ca from the S-rich silicate melts resulted in magmatic crystallization of oldhamite (CaS). Additionally, a phase was reached at 1500°C in which tectosilicate was reduced to Si within the metallic melt, with the subsequent crystallization of forsterite. Moreover, Ti-rich troilite crystallized from a combination of an Fe-rich sulfide melt, and a mixed-sulfide melt. All of the results of the experiment are consistent with a derivation of the aubrites from an E chondrite-type precursor in a strongly reducing, oxygen-depleted environment. Previous studies employing multiple lines of evidence including chemical, petrographic, metamorphic, and cosmic-ray exposure age data, suggest that the EL and EH chondrites originated in different layers of the same asteroidal parent body. More recently, very precise isotopic measurements were made of a statistically larger sampling of E chondrites and aubrites. Although their O-isotopic data were indeed identical, a three-isotope plot distinguished the EH group from the EL and aubrite groups by its slightly steeper slope; the plots of the EL and aubrite groups were co-linear with the terrestrial fractionation line. A third grouplet with intermediate mineralogy has recently been identified, represented by the meteorite Y-793225. Studies have determined that it was not derived from material associated with the EH or the EL groups through any metamorphic processes, and therefore could represent a unique enstatite parent body. The Shallowater meteorite is also widely considered to originate from a unique enstatite parent object.

The iron-rich, oxygen-poor composition of Indarch, as well as its greater depletion of refractories than is found on the Earth, has led to speculation that E chondrites might have once been a part of the pre-differentiated outer layer of Mercury. However, reflectance spectrometry has determined that E-type and M-type asteroids are similar to E chondrites, and that they occupy stable orbits between 1.8 and 3.2 AU. These findings suggest that the asteroid belt is where they originated, or more likely, to where they were collisionally and/or gravitationally relocated. 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). By utilizing Mn–Cr isotopic 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 that which they now occupy. Furthermore, an anomalous light N component found proportionately in carbonaceous and E chondrites but not on Earth, and which is almost certainly of nucleosynthetic origin, attests to a similar heliocentric location for the formation of these bodies.

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 specimen of Indarch shown above is a 1.0 g cut fragment.


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Itqiy

EH7-anomalous or Meta-EH-an
(partial melt residue)
standby for itqiy photo
Fell ~1990
26° 35.45′ N., 12° 57.13′ W. Following a detonation with accompanying light, a stone weighing 410 g was found by a nomad in Saguia el Hamra, Western Sahara. In July of 2000, the Labenne family searched for meteorites in the same area near Itqiy, and Luc found a second stone that weighed 4,310 g. The large, smooth stone was covered with a thin, black fusion crust still exhibiting flow lines. Although this meteorite has a low weathering grade (W1–2) consistent with a recent fall, 14C results infer a terrestrial age for Itqiy of 5,800 (±500) years. Initial classification of Itqiy was completed at the Lunar & Planetary Laboratory, University of Arizona.

Itqiy consists of ~78 vol% equigranular silicates composed of coarse-grained enstatite with a size range of 0.5–4 mm. These are chemically similar to silicates in EL chondrites but have a significantly higher CaO content (Patzer et al., 2001). These pyroxene grains form 120° triple junctions which are consistent with an extended annealing process and a high degree of recrystallization. Undulose extinction, irregular fractures, and occasional mosaicism within the grains reflect severe shock exposure consistent with a shock stage of S2–4, while evidence of deformation and a lack of twinning suggests a shock classification of S3. Based on a Raman spectroscopic analysis of enstatite crystals, Zhang et al. (2018) derived a shock stage for Itqiy of S4–5.

Kamacite forms 0.2–2 mm diameter grains and vein networks comprising ~22 vol% of the meteorite, with a compositional range similar to the EH chondrites (Patzer et al., 2001). In contrast, kamacite spherules embedded within sulfide have a composition similar to EL chondrites. No taenite is present and only rare troilite occurs. Patzer et al. (2001) also reported that the Mg–Mn–Fe-sulfides present in Itqiy are compositionally different from those in both EH or EL chondrites, and the Fe–Cr sulfides are unusual as well. Moreover, the Mg/Si and Fe/Si ratios are significantly higher than those in EH or EL chondrites. Plagioclase and relict chondrules are absent.

An absence of radiogenic gases in Itqiy probably reflects a recent loss through an impact-melting event, likely related to shock heating during its excavation. The signature of trapped noble gases in Itqiy shows a subsolar component similar to that of E chondrites of petrologic grades 4–6 (as opposed to the sub-Q signature of type-3 E chondrites), which suggests a possible genetic relationship to equilibrated E chondrites. Moreover, from the similar CRE ages between Itqiy (30.1 [±3.0] m.y.) and E chondrites (28.8 m.y.), as well as by their corresponding O-isotopic compositions, it could be concluded that they formed in a similar region of the solar nebula.

While similarities do exist between Itqiy and the EH and EL chondrites, the many inconsistencies make a definitive assignment tenuous—the assignment of Itqiy to the EH group is followed here as recommended in the Meteoritical Bulletin Database. Patzer et al. (2001) found the compositional and textural characteristics of Itqiy to be analogous to those observed in the lodranites, i.e., derivation from a residual melt from which an ~20% basaltic partial melt rich in plagioclase and sulfide had been removed. In a similar scenario, Bouvier et al. (2016) found that Itqiy is the most incompatible element-depleted crustal sample known, consistent with a residue after a LREE-rich partial melt extraction. This event occurred under highly reducing conditions on a metal-enriched E chondrite parent body, where subsequent cooling over a long period allowed extensive equilibration to occur. Other mineralogical features of Itqiy, including its shock features, are consistent with a late impact-heating event to temperatures below 900°C, followed by rapid cooling. An Ar–Ar study by Bouvier et al. (2016) indicates a late-stage impact event <1.3 b.y. ago.

Studies of the 42.9 g enstatite achondrite NWA 2526 by Keil and Bischoff (2008) concluded that this meteorite, containing ~10–15% metal, shares many textural and mineralogical characteristics with Itqiy (both partial melt residue after ~20% partial melt extraction) and possibly QUE 94204, potentially forming a grouplet of meteorites. Moreover, metal in Mount Egerton and in the anomalous iron meteorite Horse Creek (as well as the anomalous irons LEW 85369, LEW 88055, and LEW 88631) has been described as being compositionally similar (i.e., having complementary HSE patterns in metal) to metal in NWA 2526 (Keil and Bischoff, 2008; Humayun et al., 2009; M. Humayun, 2010). Along with Itqiy, these meteorites might share a common origin on an E chondrite-like parent body unique from the Shallowater, EH, EL, and main-group aubrite parent bodies (Keil and Bischoff, 2008; Izawa et al., 2011).

It is noteworthy that the enstatite achondrite inclusions MS-MU-019 and MS-MU-036 recovered from the Almahata Sitta fall have been compared to Itqiy (Bischoff et al., 2016), and continued studies could help resolve potential genetic links among these anomalous meteorites. The photo above is a 1.3 g interior slice of Itqiy, and the pictures below show the complete Itqiy mass in situ.

standby for itqiy discovery photo

standby for itqiy photo
Photos courtesy of Luc Labenne—Labenne Meteorites


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Abee

EH Impact-Melt Breccia
(EH4 in MetBull 8)
(EHb IMB in Weyrauch et al., 2018)

standby for abee photo
Fell June 9, 1952
54 ° 13′ N., 113 ° 0′ W. At 11:05 P.M., people in Alberta, Canada witnessed a fireball accompanied by detonations. Five days later, a single 107 kg stone was recovered from a hole approximately 1 m in diameter and 2 m deep in a wheat field near the town of Abee, 90 km north of Edmonton. On December 9, the meteorite was purchased from the finder, Harry Buryn, for $10/kg by the Geological Survey of Canada.

The Van Schmus–Wood (1967) scheme for petrographic type was modified for enstatite chondrites, establishing both a textural type reflecting peak metamorphic temperature (3–7), and a mineralogical type pertaining to the cooling history (α–δ) (Zhang and Sears, 1996; Quirico et al., 2011). Under this classification scheme, Abee has high-temperature thermometers consistent with a classification of EH4γ.

Employing multiple lines of evidence including chemical, petrographic, metamorphic, and cosmic-ray exposure age data, previous studies had suggested that the EL and EH chondrites originate from different layers on the same parent body. However, subsequent studies utilizing very precise isotopic measurements were made of a statistically larger sampling of E chondrites and aubrites. Although the O-isotopic data for the samples were indeed identical, a three-isotope plot distinguished the EH group from the EL and aubrite groups by its slightly steeper slope; the plots of the EL and aubrite groups were colinear with the terrestrial fractionation line. A third grouplet with intermediate mineralogy has recently been identified, represented by the meteorite Y-793225; an EH-an classification has been proposed (Rubin and Wasson, 2011). Studies of Y-793225 have determined that it is not derived from the EH or EL groups through any metamorphic proccesses, and thus could represent a unique enstatite parent body. The Shallowater and Itqiy meteorites are also considered by some to have originated from two additional unique enstatite parent bodies.

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

EHa EHb ELa ELb
Troilite Cr <2 wt% Cr >2 wt% Cr <2 wt% Cr >2 wt%
(Mn,Mg,Fe)S Fe <20 wt% Fe >20 wt% Fe <20 wt% Fe >20 wt%
Daubréelite Abundant Missing Abundant Missing
Kamacite 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 Abee is a member of the EHb subgroup.

The formation of Abee began with the accretion of chondrules that were ~5.6 m.y. younger than carbonaceous chondrites (based on Mn systematics). It is thought that Abee experienced sulfurization of metal within the protoplanetary nebula (Lavrentjeva et al., 2006), and thereafter, the rock was transformed in a high-temperature impact event in which up to 90% of the chondrules were melted or resorbed. This shock produced diamonds in Abee in a concentration of ~100 ppm, likely derived from primary graphite.

Euhedral enstatite grains crystallized from the silicate melt and kamacite-rich rims formed around the clasts and relict EH material. The high-temperature silica polymorphs cristobalite and tridymite were formed from the chondrule melt and preserved through rapid cooling/quenching. Presolar SiC is present in Abee at a concentration of 6 ppb (Huss, 1990). The mineral keilite (Fe+2,Mg)S crystallized from the melt phase of niningerite and troilite (Rubin, 2008). Large kamacite nodules crystallized from C-rich metal–sulfide melt regions along with the precipitation of graphite laths, while F was incorporated into fluor-richterite grains. A dark, fine-grained, oldhamite-rich, plagioclase-rich component (~0.2 vol%) was also an igneous product of the partial melt. As envisioned by Rubin and Scott (1997), repeated impacts shattered the homogeneous igneous lithology and produced partial melting of metal and silicate. Thereafter, a second major melting event occurred, probably from impact, producing an enstatite melt that flooded and absorbed the smallest clasts and relict chondrules. Intermixing of the larger silicate clasts and relict chondrules with the metal–sulfide component occurred, followed by rapid quenching and annealing.

An investigation of the compositional variation that exists among the components in Abee was conducted by Higgins and Martin (2018). They propose a less complex process for the observed variability (i.e., clasts enriched in metal compared to the matrix)–a mechanical form of differentiation which they termed ‘smithing’, from the analogous technique of ancient iron working. Higgins and Martin (2018) contend that this smithing process occurred between the initial partial melting and secondary major melting events envisioned by Rubin and Scott (1997) (see preceding paragraph). Starting from a brecciated but broadly homogeneous igneous lithology, impact-generated shockwaves occurring over an extended period produced fracturing of the brittle phases (silicates, sulfides, etc.) and enabled the migration of these small fragments from the surface of clasts and into the open matrix, thus leaving the clasts enriched in the more malleable kamacite. The source lithology of the Abee meteorite subsequently underwent heating and recrystallization giving rise to its current compact texture. The authors speculate that the formation of aubrites may also be attributed to such a smithing process.

Although E chondrites and aubrites do share a common O-isotopic signature, certain chemical and mineralogical differences exist which had previously cast doubt on their formation on a common parent body. Some of these differences include a higher abundance of Ti and forsterite in aubritic sulfides than in E chondrites. A scenario reconciling these differences has been presented in light of an experiment in which an E chondrite was systematically melted in a very reducing, oxygen-depleted environment.

In the experiment, as the silicate melt reached a temperature range of 1000–1300 °C, and the degree of partial melting reached 20%, the metal-sulfide component began to migrate out of the silicate. At 1450 °C, a completely separated metal component could have established a metallic core on its parent body. Since the sulfides melted at temperatures as low as 1000 °C, it was demonstrated that aubritic sulfides cannot be a product of nebular synthesis as previously speculated. Instead, tranfer of S and Ca from the S-rich silicate melts resulted in magmatic crystallization of oldhamite (CaS). Moreover, a phase was reached at 1500 °C in which SiO2 was reduced to Si within the metallic melt, with the subsequent crystallization of forsterite. In addition, Ti-rich troilite crystallized from a combination of an Fe-rich sulfide melt and a mixed sulfide melt. All of the results of the experiment are consistent with a derivation of the aubrites from an E-type chondritic precursor in a strongly reducing, oxygen-depleted environment.

Abee’s iron-rich, oxygen-poor composition, as well as its greater depletion of refractories than that of the Earth, has led to speculation that E chondrites might have once been a part of the pre-differentiated outer layer of Mercury. However, reflectance spectrometry has determined that E-type and M-type asteroids are similar to E chondrites, and they occupy stable orbits between 1.8 and 3.2 AU, suggesting that the asteroid belt is where they originated, or more likely, where they were relocated following a collisional/gravitational perturbation. 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). By utilizing Mn–Cr isotopic 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 the location within the asteroid belt which they now occupy.

An anomalous light N component that is found proportionately in carbonaceous and E chondrites but not on Earth, which is almost certainly of nucleosynthetic origin, points to a similar heliocentric location for the formation of these bodies. The Ar–Ar age was determined by Bogard et al. (2010) to be 4.52 ( ±0.02) b.y., or 4.5621 b.y. calculated relative to Shallowater (Hopp et al., 2011).

An unusual D-depleted, highly disordered, insoluble organic matter component was recovered in an acid residue of Abee, thought to be hosted by the late-stage accretion of dark inclusions (Remusata et al., 2012). The specimen of Abee shown above is a 4.9 g partial slice showing the brecciated nature of this meteorite, including a metallic-rimmed clast (bottom center) and a dark inclusion of unique enstatite chondritic material (upper right). A superb large slab of Abee can be seen on display at the Smithsonian Institution, Washington D.C. A complete slice measuring 374 × 260 × 7 mm and weighing 1,675 grams is in the collection of Edwin Thomson.