NORTHWEST AFRICA 12945


EH6
standby for nwa 12945 photo standby for nwa 12945 photo
click on photos for a magnified view

Purchased September 2019
no coordinates recorded

A single stone weighing 554 g was found in the desert region of Northwest Africa and subsequently purchased from a dealer in Zagora, Morocco by F. Kuntz. A representative sample was sent for analysis and classification to the University of Washington in Seattle (A. Irving) and Washington University in St. Louis (P. Carpenter), and NWA 12945 was determined to be a rare enstatite chondrite of petrologic type 6.

Northwest Africa 12945 has a recrystallized texture devoid of chondrules and is composed of mostly enstatite along with sodic plagioclase. It also contains accessory kamacite, troilite, niningerite, and schreibersite. The meteorite has experienced moderate terrestrial weathering (W2) and has a low shock stage.

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%

The MetBull description for NWA 12945 does not provide the wt% of Cr in troilite, Fe in niningerite, or Ni in kamacite by which the subgroups in Weyrauch et al. (2018) are determined. However, because the presence of daubréelite is not mentioned in the analysis, the meteorite would be consistent with an EHb classification pending further information. The revised E chondrite classification scheme of Weyrauch et al. (2018) with selected examples from their 80-sample study can be found here.

To better understand the origin of the light Si isotope enrichment in E chondrites, Kadlag et al. (2019) conducted an in situ study of Si isotopes (30Si, 28Si) in Sahara 97072 (EH3) and Indarch (EH4). They analyzed the silicate (chondrule and matrix) and metal (matrix and metal-troilite spherules) components and concluded that a heavy Si-rich silicate component was lost with other refractory elements from the pre-accretionary EH region, so that the precursor material of EH chondrites was already depleted in heavy Si isotopes and was not dependent on a contribution of isotopically light Si in metal. Based on all of the evidence acquired through their study, Kadlag et al. (2019) infer a likely scenario for the formation of the E chondrites as follows (also see schematic diagram below):

(a) Soon after the beginning of the rapid infall stage, high-temperature refractory minerals condense and calcium–aluminum-rich inclusions (CAIs) are formed near the Sun. The CAIs are subsequently transported to the outer region of the protoplanetary disk by viscous expansion and disk winds.

(b) Mg/Si- and 30Si-rich refractory silicates are incorporated into planetesimals that experience runaway growth and quickly reach their isolation mass. These planetesimals and embryos are ultimately accreted by proto-Earth and other inner Solar System bodies.

(c) FeNi-metal and enstatite condense, and the remaining isotopically-light Si-enriched gas and dust undergo further thermal processing and fractionation, ultimately accreting to form E chondrite parent bodies.

(d) According to the 'Grand Tack' model of Walsh et al. (2011), the gas giants enter into resonance causing them to migrate inward and then outward, disrupting the inner disk and driving the E chondrite parent bodies into the asteroid belt. See the Protoplanetary Disk page for further details.

Schematic Representation of the Origin of E Chondrites
standby for ec formation schematic illustration
click on image for a magnified view

Diagram credit: Kadlag et al., GCA, vol. 267, p. 317 (2019)
'The origin of unequilibrated EH chondrites—Constraints from in situ
analysis of Si isotopes, major and trace elements in silicates and metal'
(https://doi.org/10.1016/j.gca.2019.09.010)

In addition, based on the reasoned estimate that Earth's core contains ≤8 mass% Si, Kadlag et al. (2019) calculated the δ30Si and Mg/Si of bulk Earth. These data were used to obtain the proportionate makeup of the pre-accretionary material for bulk silicate Earth (BSE), which they assert is best explained by a mixture of 10–20% lost refractory silicates + 90–80% CC (±OC, ±EC) material, or by a mixture of 30–70% high-temperature equilibrium condensates + 70–30% CC (±OC, ±EC) material.

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) for the formation history of E chondrites can be found on the Sahara 97096 page. The specimen of NWA 12945 shown above is a 27.26 g end section.