ERG CHECH 002


Achondrite, ungrouped
Eucrite-like, gabbroic with pyroxene megacrysts and sodic plagioclase

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Found May 2020
26° 1' 55" N., 1° 36' 40" W.

A large number of stones of widely varying weights were found in the Dune seas of the Erg Chech desert region in southwestern Algeria. Only rare remnant fusion crust has been observed on the larger stones (photos courtesy of Ziyao Wang). The total recorded weight of this find is 31.78 kg, but one source puts the total at ~45 kg. Two stones weighing 1,839 g and 207 g were purchased by M. Lyon from R. Chaoui, and samples from the 1,839 g stone were provided to the University of Washington in Seattle (A. Irving), Washington University in St. Louis (P. Carpenter), and the University of New Mexico (K. Ziegler) for analyses and classification. Erg Chech 002 was classified as an ungrouped achondrite.

Erg Chech 002 is an unbrecciated gabbroic (medium- to coarse-grained) igneous rock containing scattered mm- to cm-scale greenish pyroxene (orthopyroxene, pigeonite, and augite) megacrysts, some exhibiting embayed crystals extending for a considerable length (see photos below). The groundmass is light tan in color with some areas of reddish staining, and it is primarily composed of mm-scale grains of pigeonite and sodic plagioclase. Accessory phases include Ti-chromite, ilmenite, troilite, silica polymorph (inferred to be cristobalite), merrillite, and rare Ni-poor metal. A low degree of terrestrial weathering has transformed a minor amount of troilite and metal to secondary goethite and has produced tiny calcite veinlets.

Backlit Pyroxene Megacryst in Erg Chech 002
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Photo courtesy of Charles Hassen

Pyroxene Megacryst Cluster in Erg Chech 002
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Photo courtesy of Marcin Cimala

Elongated Pyroxene Megacryst in Erg Chech 002
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Photo courtesy of Alan Mazur

Compared to typical eucrites, the groundmass pyroxene in EC 002 has much lower FeO/MnO ratios (~30 [±2] and ~24, respectively) and the plagioclase is sodic rather than calcic. In addition, the orthopyroxene in the megacrysts is unlike that of typical diogenites in having more magnesian and Cr-rich cores as well as lower FeO/MnO ratios. The oxygen isotope values for four EC 002 groundmass samples (Δ17O: –0.142, –0.143, –0.137, –0.123 [ave.: –0.13625]) plot in a distinct field compared to the anomalous eucrites Emmaville, Bunburra Rockhole, Asuka 881394 and EET 92023, but EC 002 is another O-anomalous mafic achondrite unrelated to typical HEDs (see diagrams below). In addition, the sodic plagioclase and lower pyroxene FeO/MnO ratios in EC 002 exclude a common origin with any of the other O-anomalous meteorites or the HEDs. It should be noted that Mittlefehldt et al. (2016 #1240) has determined that the much lower than normal pyroxene Fe/Mn ratios of the EET 87542 and QUE 94484 eucrites (ratios even lower than that of EC 002) were caused by late-stage reduction of FeO on the HED parent asteroid, an origin which is also consistent with their normal eucrite O-isotopic compositions. The ε54Cr value for EC 002 should help determine whether its parent body derives from a distinct isotopic reservoir or one similar to the other anomalous eucrites, and whether the meteorite's isotopic composition could be due to incorporation and mixing of material from an alternate chondritic source object (Sanborn et al., 2016 #2256).

Δ17O for Eucrite-like Achondrites and Erg Chech 002 (purple)
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Diagram adapted from Mittlefehldt et al., 49th LPSC, #2700 (2018)

O-isotopes for Erg Chech 002, O-anomalous Achondrites, and HEDs
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Diagram adapted from Mittlefehldt et al., 47th LPSC, #1240 (2016)

The photo of Erg Chech 002 shown above is a 6.1 g partial slice sectioned from the 1,839 g type specimen in the possession of Mark Lyon. The left photo below shows the largest known mass with a weight of 4,140 g, courtesy of Aziz Habibi, while the right photo shows a sampling of smaller stones with weights ranging from 0.4 to 10 g, courtesy of Miguel Angel Contreras Gomez.

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Photos courtesy of Aziz Habibi (left) and Miguel Angel Contreras Gomez (right)