Dar al Gani 734


EL4
(ELa4 in Weyrauch et al., 2018)
standby for dar al gani 734 photo
Found Winter 1996/97
27° 7.91' N., 16° 3' E.

Several stones weighing a total of 1,378 g were found in the Libyan Sahara. Studies performed at the Max–Planck–Institut für Chemie in Mainz, Germany by Frank Wlotzka have led to the classification of this meteorite as an EL4 chondrite. The chondrule texture and average size of 0.45 (±0.2) mm, and the size of the matrix enstatite crystals, indicate that this is an EL4 meteorite. Among EL chondrites, the petrologic type 4 sample is the most poorly represented in our collections, with the 1.2 g QUE 94368 being the first such meteorite found (although initially classified as an E5, QUE 94368 was subsequently determined to be an EL4; A. Rubin, 1997). Dar al Gani 734 is a highly weathered (W4) and friable meteorite.

Enstatite chondrites were formed in a highly reducing environment. Therefore, they contain virtually no metal in the oxide form—an amount much less than other chondrites, and even the terrestrial planets. The mineral sinoite (silicon oxynitride) has been found to occur in many EL chondrites that have high bulk N contents, and is associated with crystallization from an impact melt. This suggests that there was a period of high-temperature, melt-forming conditions followed by annealing, and subsequently, a late shock to stage S2.

Previously, employing multiple lines of evidence including chemical, petrographic, metamorphic, and cosmic-ray exposure age data, studies suggested that the EL and EH chondrites originated from different layers on the same 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 collinear 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 the EH or EL groups through any metamorphic processes, and thus might represent a unique enstatite parent body. Likewise, the Shallowater meteorite is thought to have originated on a unique enstatite parent body.

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 been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 [EL3-an; abs], 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 DaG 734 is a member of the ELa subgroup.

Oxygen isotopic studies place the formation of enstatite chondrites on the terrestrial fractionation line, which is taken by some to mean that they formed within the inner Solar System. Based on Mn–Cr isotope systematics and its correlation with heliocentric distance, Shukolyukov and Lugmair (2004) concluded that E chondrites originated ~1.0–1.4 AU from the Sun before being perturbed into their present locations in the asteroid belt. Similarly, Nakashima et al. (2006) calculated a heliocentric distance of >1.1 and 1.3 AU for two EL3 chondrites (ALH 85119 and MAC 88136, respectively) on the basis of their implanted solar noble gas concentrations. In contrast, the identification of the E-asteroid group, including Hungaria at 1.94 AU, Nysa at 2.42 AU, and Angelina at 2.68 AU, suggests that the actual solar region of formation may lie at a greater heliocentric distance.

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 240–420 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
standby for el planetesimal schematic illustration
click on image for a magnified view
Schematic illustration credit: Avdellidou et al., Astronomy & Astrophysics, vol. 665, #L9, fig. 2 (2022 open access link)
'Athor asteroid family as the source of the EL enstatite meteorites'
(https://doi.org/10.1051/0004-6361/202244590)

The specimen of DaG 734 pictured above is a 5.2 g slice displaying an advanced stage of weathering due to its extended terrestrial residence, consistent with the extreme rarity of this meteorite. An abundance of large chondrules is a characteristic that distinguishes DaG 734 from the smaller-sized chondrules found in the EH group. The complete lack of visible free metal in this meteorite is likely a reflection of its advanced stage of weathering.