A 125.39 g meteorite was found in the Sahara Desert and later purchased by American collector N. Oakes in Rissani, Morocco. A sample was submitted to Arizona State University, Center for Meteorite Studies (G. Huss), and NWA 3132 was determined to be a rare EL3 chondrite composed almost entirely of orthopyroxene. Although almost no metal or sulfide is present, vestigial signs of metal can be seen throughout the meteorite. This enstatite chondrite has a shock stage of S2 and a weathering grade of W4. An unequilibrated petrologic type 3 sample is quite rare among EL-group members, and is thus far represented primarily by Antarctic meteorites including ALH 85119, A-881314, A-882067, EET 90299/90992, MAC 88136 pairing group, MAC 02635, MAC 02837/02839, QUE 93351 pairing group, LAP 03930, and PCA 91020, along with EL3 xenoliths identified in the carbonaceous chondrite Kaidun (designated Kaidun IV) and in the ureilite Almahata Sitta.
With the exception of the transitional EH/L chondrite Y-793225, E chondrites have been historically assigned to one of two distinct groupsa high-Fe, high-siderophile (EH) group, and a low-Fe, low-siderophile (EL) group. Surprisingly, it was 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. From comparisons of elemental abundance ratios between the EH and EL groups, it has been demonstrated that values for all elements except the refractory siderophiles are consistently lower in the EL group than in the EH group. Certain elemental ratios (e.g. La/Sm, Sb/Ir) easily resolve the two groups. Furthermore, the following mineralogical relationships are diagnostic of their distinct parent bodies:
EH group has a higher Si content in kamacite (EH: 1.93.8 wt%; EL: 0.32.1 wt%)
EH group has a lower Mn content in daubreelite (EH: 0.41.1%; EL: 1.44.0%)
EH group has a lower Ni content in schreibersite (EH: <20 wt%; EL: >20 wt%)
EH group has a lower Ti content in troilite (EH: <4.8 wt%; EL: >5.5 wt%)
EH group has a lower An content in plagioclase (EH: <3 mol%; EL: 1317 mol%)
EH group sulfides are enriched in alkali elements (e.g., Na in caswellsilverite, K in djerfisherite), and chondrule mesostasis is enriched in Na relative to EL group
EH group chondrites contain niningerite [(Mg,Fe)S] or keilite [(Fe,Mg)S], the Mg-rich end member of the monosulfide series having the formula [(Mg,Mn,Fe)S]; EL group chondrites contain the Mn-rich end member alabandite [(Mn,Fe)S]
EH group chondrites have higher abundances of the siderophile elements Ni, Fe, Au, and Co
EH group chondrites contain an average of 15 times the abundance of the volatile element Zn
Other good discriminators are Ga, As, Se, and Sb, each of which are found in greater abundances in EH group chondrites
In addition, both Fe- and Zn-isotope compositions are fractionated to different degrees between EL and EH chondrites; EL chondrites are isotopically heavier than EH chondrites, indicating 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). Studies into the origins of EL chondrites conducted by Goresy et al. (2012) determined that petrologic evidence, including the occurrence within FeNi-metal nodules of repeated sinoitegraphite condensation events associated with oldhamite (CaS) in the sequence CaS ⇒ sinoite ⇒ graphite, was indicative of a nebular condensate origin for these chondrites rather than their formation as an impact-melt breccia of preexisting proto-asteroids. On the other hand, later incidences of impact-shock for EL chondrites are demonstrated by the higher prevalence of impact-melt breccias among the more metamorphosed members, as well as by the occurrence of sinoite crystallized from a melt.
Other comparisons demonstrate that EH group chondrites have smaller average chondrule diameters (220 µm vs. 550 µm; Rubin et al., 2000) and smaller average metal diameters than EL-group members as shown in the table below:
Comparison of EH and EL Chondrule and Metal Diameters
0.0451.313 mm (EH3 average 0.278 ±0.229 mm)
0.0852.125 mm (EL3 average 0.476 ±0.357 mm)
The disparity in the size and Na content of chondrules within the EH and EL groups can be reconciled by several possible scenarios, including the one proposed by Schneider et al., 2002:
Chondrules from both groups were formed from similar precursor material in the same nebular region. During accretion, the chondrules underwent a size sorting process induced by volatile flows within the regolith, or alternatively, by the abundance of dust in the prospective accretion regions and by the number of chondrule remelting episodes (Rubin, 2010). Photophoresis, utilizing pressure and particle-size dependence, was also a likely size sorting mechanism of chondrules (Hesse et al.,2011). This resulted in the larger EL chondrules becoming more deeply buried than the smaller EH chondrules on their respective parent bodies. It has been reported that EL3 chondrites usually exhibit a preferred orientation of chondrules and other constituents. The deep burial conditions would support such a foliation as the result of continued impact deformation processes. After burial, lithification of the chondrules into bulk rock was quickly achieved. The shallower EH material experienced more rapid cooling, and thus retained more of its volatile component such as Na, while volatiles were lost during a more extended cooling period in the more deeply buried EL material.
In an attempt to model the precursor material of Earth, it was calculated that approximately 55% of the precursor component could have been of EL chondrite composition, which is the meteoritic material that provides the best match to the Earth in O-isotope composition, bulk Fe/Al weight ratio, and bulk FeO concentration, as well as K-isotopic composition (Burbine and O'Brien, 2004; Zhao et al., 2019). However, it is not possible to model the Earth's precursor material based solely on known chondrites because other factors may have been involved. For example, Earth could have accreted from chondritic material that subsequently underwent differentiation and loss of a basaltic component, or the early Earth might have comprised a significant Si component which was sequestered into the core or lower mantle. Based on isotopic studies, the meteorites of the EL3 subgroup, as opposed to the EL6 subgroup, are thought to be the best candidates for the building blocks of Earth (Boyet et al., 2018).
In a study compiling reflectance spectra for a large number of rare meteorites, Burbine et al. (2020, 2235) demonstrated that the Xc class and Xe class of the Bus-DeMeo taxonomic system are the best asteroid analogs for EL/aubrite and EH chondrites, respectively. The inner main belt Athor asteroid family (Xc-type in the Bus-DeMeo taxonomy), in which the largest member is ~42 km-diameter (161) Athor, has been identified by Avdellidou et al. (2022) as the unique parental source of the EL chondrite meteorites. Utilizing spectrographic (e.g., reflectance spectra, geometric albedo) and isotopic data, as well as thermochronometry and CRE age data, the research team determined that the predecessor of the Athor asteroid family was an EL-type chondritic planetesimal measuring 240420 km in diameter (Trieloff et al., 2022) that accreted within the terrestrial planet region about 4.5 b.y. ago, and which experienced a complex collisional history (see chronological illustration below). An initial severe collisional disruption occurred ~3 b.y. ago which led to the creation of an inferred 64 km-diameter daughter body composed predominantly of type 6 lithologies. This EL-chondrite daughter body ultimately migrated into a stable parking orbit in the inner main asteroid belt. Subsequent collisional fragmentation of this EL asteroid produced a gravitationally-bound association of various sized fragments recognized today as the Athor asteroid family. The identification by Trieloff et al. (2022) of a common CRE age of 33 m.y. for many EL6 chondrites attests to a major impact involving at least one of the Athor family fragments at this time. The location of this impact event is most likely near a dynamical resonance such as the Jupiter 3:1 mean motion resonance at 2.50 AU, which provides ejecta an efficient transfer mechanism into an Earth-crossing trajectory. For example, the EL6 Neuschwanstein meteorite was given a probability of 63 (±13) % of escaping via the Jupiter 3:1 mean motion resonance (Granvik and Brown, 2018). It is noteworthy that one of the three common CRE ages (i.e., major collisional events) among H-type chondrites is also 33 m.y. (Marti and Graf, 1992; Eugster et al., 2006, 2007), and that the H chondrite group is also located near the 3:1 mean motion resonance at 2.50 AU.
Collisional History of the EL Planetesimal
click on image for a magnified view
Schematic illustration credit: Avdellidou et al., Astronomy & Astrophysics, vol. 665, #L9, fig. 2 (2022 open accesslink)
'Athor asteroid family as the source of the EL enstatite meteorites'
The specimen of NWA 3132 shown above is a 0.8 g partial slice. The photo below shows the main mass courtesy of N. Oakes.