PILLISTFER


EL6
(ELa6 in Weyrauch et al., 2018)
standby for pillistfer photo
Fell August 8, 1868
58° 40' N., 25° 44' E.

At 12:30 P.M. in Estonian SSR, sonic booms were heard and stones fell at Aukoma, Kurla, Wahhe, and Sawiauk. The weight of these stones was ~14 kg, 7.5 kg, 1.5 kg, and 0.25 kg. These falls are also known by the names of Pilistvere and Pillistvere.

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 Pillistfer is a member of the ELa subgroup.

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, Pillistfer has geothermometers that indicate a classification of EL6β.

Enstatite chondrites were formed in a highly reducing environment. Therefore, they contain virtually no metal in the oxide form—much less by comparison to other chondrites and to the terrestrial planets. Iron in EL6 chondrites is depleted and isotopically fractionated compared to less metamorphosed EL3 and EH chondrites (Wang et al., 2013). A trace element analysis utilizing nonmagnetic micron-scale grains from Pillistfer was conducted by Lavrentjeva and Lyul (2017). They found depletions in siderophile elements and enrichments in lithophile elements, which indicates that nebular metal–silicate fractionation of precursor material occurred, as well as redistribution during parent body metamorphism. The mineral sinoite (silicon oxynitride) has been found to occur in Pillistfer and many other EL chondrites that have a high bulk N content. Sinoite is associated with crystallization from an impact melt, or alternatively, with metamorphic processes. This suggests that Pillistfer experienced a period of high, possibly melt-forming temperatures. A rapid cooling phase was initiated consistent with 0.8°C/day (Kissin, 1989). This was followed by a period of annealing and then a final shock to stage S2.

An isochron age for Pillistfer representing the K–Ar system closure was calculated by Bogard et al. (2010) to be 4,541 (±7) m.y. ago., a similar age to that of several equilibrated E chondrites. A comparison of the younger Ar–Ar ages measured for ordinary chondrites suggests that E chondrites cooled more quickly, possibly reflecting a smaller parent body size, a lower initial heating level, a shallower burial, and/or a collisional disruption prior to K–Ar closure. More recently, employing a broader range of EL chondrite petrologic types (i.e., formation temperatures), Hopp et al. (2013, 2014) determined a lower corrected age range for metamorphic cooling of EL5 and EL6 meteorites of 4.48–4.51 b.y. In a similar manner, the Ar–Ar isochron age for an EL3 chondrite reflected a younger age, possibly representing a late-stage impact ~4.43–4.47 b.y. ago. This better constrained age range would allow for a more extended period of time for parent body cooling and a relaxation of the constraints on the parent body size. However, given that the K–Ar closure for the EL parent body occurred 30 m.y. earlier than that of the H-chondrite parent body, the size of the EL parent body was most likely significantly smaller than the H parent body.

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 could 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 pictured above is a 5.7 g partial slice. It exhibits abundant free metal which characterizes this group.