YILMIA


EL6
(ELa6 in Weyrauch et al., 2017)
standby for yilmia photo
Found 1969
31° 11' 30" S., 121° 32' E.

In 1969, numerous fragments were found in a shallow depression during nickel exploration in Western Australia. They were not recognized as meteoritic until 1971, when a single large mass with ablation characteristics, along with smaller fragments totaling 24 kg, was found 400 m from the first find. Although originally described as petrologic type 5, all current research concludes that Yilmia is a type 6. 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, Yilmia has thermometers that indicate a classification of EL6β. A rapid cooling phase was initiated consistent with 30,000°C/day (Kissin, 1989).

Weyrauch et al. (2017) 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., 2017
  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. (2017) including selected examples from their 80-sample study can be found here. It was determined that Yilmia is a member of the ELa subgroup.

Planetary-type noble gases, including Ar, Kr, and Xe, have been identified in Yilmia, the carrier of which is thought to be a nanometer-sized phase designated phase "Q" (for ‘quintessence’). Noble gases are adsorbed at low nebular pressures onto this phase, or precursors of this phase, likely consisting of rare graphite grains, kerogen, or carbon blacks. An alternate scenario, proposed by Matsuda et al. (2010), argues that an amorphous phase of carbon experienced implantation through ion irradiation of planetary noble gases (the "plasma model") and now constitutes the carrier of phase Q. These Q-gases are released through oxidation processes resulting in a rearrangement of the carbon structure.

The higher incidence of impact shock events for EL chondrites is attested by the higher prevalence of impact-melt breccias among the more metamorphosed members, as well as by the occurrence of abundant silica in the form of tridymite, cristobalite, and sinoite, the latter mineral known to crystallize from an impact melt. Following the impact-shock events, most EL6 chondrites experienced an extended period of annealing to shock stage S1, which was followed by a period of less severe impact-shock events resulting in a shock stage of S2 (Rubin et al., 2009).

While taenite is uncommon in other E chondrites, it is present in Yilmia. The photo above shows a 1.25 g partial slice of this terrestrially weathered, low-metal, enstatite chondrite.