(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).

Weyrauch et al., 2018
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.

Leave a Reply