ERG CHECH 002


Achondrite, ungrouped
Felsic gabbro 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 was described by Irving and Carpenter (MetBull 109) and Carpenter et al. (2021 #2205) as an unbrecciated gabbroic (medium- to coarse-grained) igneous rock containing scattered mm- to cm-scale greenish pyroxene megacrysts (orthopyroxene, pigeonite, and augite xenocrysts), some exhibiting embayed crystals extending for a considerable length (cm-scale; see photos below). Both pyroxene and olivine xenocrysts were studied by Barrat et al. (2021), and it was concluded that these were likely sourced from one or more associated, but more magnesian magmas. The groundmass is light tan in color with some areas of reddish staining, and is composed primarily of fine- to medium-grained pyroxene (~62 vol%) and sodic plagioclase (~33 vol%) (Nicklas et al., 2021 #1074; Hamann et al., 2021 #6236). Accessory phases observed by Irving and Carpenter (MetBull 109) include Ti-chromite, ilmenite, troilite, merrillite, and silica polymorph; the latter was shown to be cristobalite through Raman Spectroscopy conducted by Hoffmann et al. (2022 #6420). Rare Ni-poor metal and rare carbon phases were also identified through Raman spectra by Hoffmann et al. (2022 #6420). In a paleomagnetic study of Erg Chech 002, Maurel et al. (2022 #6053) measured a kamacite content of ~0.2 wt%, which records a natural remanent magnetization of 15–30 µT probably acquired from the early solar nebula field. A low degree of terrestrial weathering has transformed a minor amount of troilite and metal to secondary goethite and has produced tiny calcite veinlets; however, some primary calcite and/or aragonite was identified through Raman spectra by Hoffmann et al. (2022 #6420).

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

An analysis of Erg Chech 002 published by Yamaguchi et al. (2021 #1892) and Barrat et al. (2021) revealed slightly different mineral proportions, with samples consisting of 45 vol% albitic feldspar with associated K-feldspar, 38 vol% augitic and low-Ca pyroxene, 8 vol% pore space, 5 vol% interstitial silica (cristobalite and tridymite), and minor phases similar to those listed above. They determined a bulk rock composition which plots in the andesitic field on a Total Alkali vs. Silica (TAS) diagram (see diagram below). Barrat et al. (2021) showed that the parental melt of Erg Chech 002 at 1224°C (see Fig. S10) experienced a high cooling rate of ~5°C/yr in the temperature interval 1200°C to 1000°C, consistent with either a thick lava flow or a shallow sill or dike. This is a similar petrogenesis as that proposed for the brachinite-related trachyandesite GRA 06128/9 (e.g., Day et al., 2009 #2012, 2009; Day et al., 2012; Lunning et al., 2017; Lunning et al., 2020) and several other trachyandesitic (ureilite-related MS-MU-011/35), andesitic (ungrouped NWA 11119), and basaltic andesitic (ungrouped NWA 11575) achondrites. Yamaguchi et al. (2021) also recognized that the lack of quartz in Erg Chech 002 indicates an even faster cooling rate of >0.1–1°C/day below 900°C, likely related to quenching during impact ejection. Similar cooling rates over the temperature range 1200°C to 800°C were obtained by Mikouchi and Zolensky (2021 #2457) using Fe–Mg zoning profiles of orthopyroxene megacrysts.

Total Alkali vs. Silica (TAS) Plot
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Diagram credit: Yamaguchi et al., 52nd LPSC, #1892 (2021)

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 (see top diagram below). 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 achondrite from the NC region of the disk that is unrelated to typical HEDs (see bottom 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; Mittlefehldt et al., 2021) determined that the much lower than normal pyroxene Fe/Mn ratios of the EET 87542 and QUE 94484 basaltic eucrites (ratios even lower than that of EC 002) were caused by reduction of FeO by S 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).

Pyroxene Fe/Mg vs. Fe/Mn Plots for Erg Chech 002
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Diagram credit: Mikouchi and Zolensky, 52nd LPSC, #2457 (2021)

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

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

The Al–Mg age for Erg Chech 002 presented in Chaussidon et al. (2021 #2222) and Barrat et al. (2021) was calculated to be 4.5650 b.y., or ~2.255 (±0.013) m.y. after CAIs. This age is based on the assumption that the canonical ratio of 26Al to 27Al (5.2 [±0.2] × 10–5) existed at the time of crystallization. This assumption was verified by Desch et al. (2022 #2567; 2022) who found that 26Al, and virtually all other short-lived radionuclides present in meteorites (e.g., 10Be; Dunham et al., 2022), were homogeneously distributed in the molecular cloud prior to the start of the Solar System, the abundances of which were the result of ongoing stellar nucleosynthesis within the Galactic host spiral arm. It is noteworthy that an older age of 4.5661 b.y., or 1 m.y. after CAIs, was calculated by Barrat et al. (2021) when anchored to the D'Orbigny angrite; this age is accordant with that determined by Zhu et al. (2022) (see below).

Another of the short-lived chronometers applied to the dating of Erg Chech 002 is the 146Sm–142Nd system. Using the the more consistent, best fit 146Sm half life of 103 (±5) m.y. (as opposed to 68 [±7] m.y. in accordance with IUPAC–IUGS), and utilizing a hybrid mass spectrometer (MC-ICP-MS), Fang et al. (2022) calculated an even earlier and more precise crystallization age of 1.80 (±0.01) m.y. after CAIs. In addition to that, they established a new initial Solar System 146Sm/144Sm ratio of 0.00840 (±0.00032).

Furthermore, Chaussidon et al. (2021 #2222) and Barrat et al. (2021) deduced from the initial δ26Mg* (= 26Mg excesses due to 26Al decay [see full description]) value that partial melting of the parental source lithology of Erg Chech 002 occurred only 1.4 m.y. after CAIs (see diagram below). This chronometry attests to the earliest petrogenesis of an achondrite known to date, and the timing is concordant with the formation of the earliest magmatic iron meteorites from the terrestrial planet region; e.g., metal–silicate segregation for IVA irons at 1.5 (±0.6) m.y. after CAIs (Lichtenberg et al., 2021 Tab. S1). A younger Ar–Ar age of 4.51 (±0.04) b.y. was calculated by Takenouchi et al. (2021 #6162), possibly representing the impact ejection event ~50 m.y. after crystallization.

Timing of EC 002 Differentiation Based on Mg Isotopic Evolutionary Curves
given:  (i) chondritic 27Al/24Mg ratio of 0.101   (ii) (26Al/27Al)0 of 5.23 × 10–5   (iii) δ26Mg*0 of –0.034‰
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Diagram credit: Barrat et al. PNAS, vol. 118, #11, p. 4 (2021)
'A 4,565-My-old andesite from an extinct chondritic protoplanet'
(https://doi.org/10.1073/pnas.2026129118)

Using the most accurate Mn–Cr isotopic data for Erg Chech 002 developed to date, Anand et al. (2022/journal reference: 2022) calculated a 53Mn–53Cr isochron age anchored to the D'Orbigny angrite (Pb–Pb age of 4563.37 [±0.25] m.y.) or 1.73 (±0.96) m.y. after CAIs. In addition, they constrained the chromite model age to between 1.46 (–0.68/+0.78) m.y. and 2.18 (–1.06/+1.32) m.y. after CAIs. These ages are consistent with those calculated in previous analyses.

Utilizing a coupled ε54Cr vs. Δ17O diagram, Anand et al. (2022) demonstrated that a possible genetic link may exist between Erg Chech 002 and the probable pairing group comprising the ungrouped dunitic/lherzolitic breccias NWA 12217, 12319, NWA 12562 and 13954. In addition, Erg Chech 002 may be genetically linked to the unbrecciated anomalous eucrite EET 92023, and/or to the main-group pallasites (MGP; see top diagram below). If any of these various meteorites are genetically linked, the differences in their respective O and Cr isotopes may possibly be due to varying degrees of contamination of their parental V-type asteroid (possibly Vesta) by an H-type chondritic impactor (see bottom diagram below).

ε54Cr–Δ17O Diagram for Meteorites Related to V-type Asteroids
NWA 12217, 12319, and 12562; EET 92023; Main-Group Pallasites
(note that chromites in IIIAB Sacramento Mountains show no genetic linkage)
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Diagram credit: Vaci et al., Nature Communications, vol. 12, #5443 (2021, open access link)
Adapted by Dey and Yin, 53rd LPSC, #2428 (2022)

ε54Cr–Δ17O Diagram for Erg Chech 002 and Possible Relatives
(note that chromites in IIIAB Cape York plot with HEDs but distant from EC 002 and MGP)
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Diagram credit: Anand et al., vol. 57, #11, p. 2011, fig. 6b (2022, open access link)
'53Mn–53Cr chronology and ε54Cr-Δ17O genealogy of Erg Chech 002: the oldest andesite in the Solar System'
(https://doi.org/10.48550/arXiv.2209.05381)
(MAPS: https://doi.org/10.1111/maps.13916)

A high-precision Mn–Cr analysis of Erg Chech 002 was also completed by Zhu et al. (2022), who obtained a ε54Cr value of –0.35 (±0.06) for a bulk sample (see diagram below). This value is significantly higher than the –0.65 (±0.10) average value obtained by Anand et al. (2022) for Erg Chech 002 samples. Using the ε54Cr value obtained by Zhu et al. (2022) coupled with the accepted Δ17O value (~ –0.13 ‰), the meteorite plots within the brachinite field (ε54Cr = –0.44 [±0.13]; Δ17O = –0.23 [±0.14] ‰) rather than near the HEDs as in the diagram of Anand et al. (2022) above. In addition, Zhu et al. (2022) calculated a 53Mn–53Cr isochron age anchored to the D'Orbigny angrite of 4.5666 (±0.0006) b.y., or 0.7 (±0.6) m.y. after CAIs; this is within error of the Al–Mg age anchored to D'Orbigny as determined by Barrat et al. (2021) (see above). Such an old age for the crystallization of Erg Chech 002 is accordant with the oldest two NC-reservoir iron groups, IC and IIAB, which have Hf–W model core formation ages of 1.0 (±0.7) and 1.4 (±0.7) m.y. after CAIs, respectively (see Hf–W age diagram by Spitzer et al. [2021]). However, Anand et al. (2022) note that the NC magmatic iron groups do not plot near Erg Chech 002 in ε54Cr–Δ17O space. Furthermore, Anand et al. (2022) suggest that the difference in the ε54Cr values obtained in the two studies is due to the much smaller sample used by Zhu et al. (2022) which likely included significant xenolithic material with anomalous ε54Cr.

ε54Cr–Δ17O Diagram for Erg Chech 002
(Erg Chech 002 = Red Star)
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Diagram credit: Zhu et al., Monthly Notices of the Royal Astronomical Society: Letters, vol. 515, #1, fig. S4 (2022, open access link)
'Radiogenic chromium isotope evidence for the earliest planetary volcanism and crust formation in the Solar system'
(https://doi.org/10.1093/mnrasl/slac061)

The significant excess radiogenic 26Mg accumulated prior to crystallization is attributed by Barrat et al. (2021) to the highly viscous nature of an andesitic melt leading to a protracted ascent to the surface spanning multiple half-lives of 26Al (Collinet and Grove, 2020). Equally important, in modeling the early thermo–mechanical evolution of planetesimals, Lichtenberg et al. (2016, 2018/journal reference: 2019) found that the dominant factor controlling the permeability and magma transport rate inside a small planetesimal (120 km-diameter) that accreted early (≈1 m.y. after CAIs) is the grain size, in which a size > 0.1–1 mm is necessary for melt segregation from the residual rock; this grain size is typical for primitive achondrites such as the winonaites, brachinites, and the acapulcoite–lodranite clan. Importantly, Monnereau et al. (2022/journal reference: 2023) presented a more complete treatment of melt migration as a function not only of grain growth, but also of the liquid/solid silicate viscosity and the parent body size (the latter controlling the cooling rate), each of which affects the silicate liquid percolation rate. They found that for bodies that accreted in the first ~1.15 m.y. after CAIs, this percolation rate is not high enough to extract 26Al to the surface and prevent the formation of a global magma ocean (see diagram below).

Five Types of Evolution of Small Bodies
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Diagram credit: Monnereau et al., Icarus, vol. 390, art. 115294, fig. 7 (2023, open access link)
'Differentiation time scales of small rocky bodies'
(https://hal.archives-ouvertes.fr/hal-03794734)
(Icarus: 10.1016/j.icarus.2022.115294)

Of secondary importance in a dry melt scenario is the melt–rock density contrast that is controlled by the oxidation state, where a higher melt–rock density contrast corresponds to lower oxygen fugacities which produce more Fe-poor, buoyant silicate melts; these lower oxygen fugacities are characteristic of planetesimals that accreted in the inner Solar System (Lichtenberg et al., 2018). Nevertheless, Collinet and Grove (2020) contend that the extraction of a highly viscous SiO2 and alkali-rich melt is a complex process, and that a trapped gas phase may have increased excess pressures to overcome the tensile strength of the rock and facilitate the formation of dikes. The rapid cooling experienced by Erg Chech 002, which has been attributed to possible melt extrusion at the surface, could also reflect melt intrusion into a cold, porous upper lid (Lichtenberg et al., 2018).

Kaminski et al. (2020) and Sturtz et al. (2021 a/b, 2022) developed a thermal evolution model of early accreted planetesimals (within the first few m.y.) involving a convecting magma ocean (onset at 40% partial melt) underneath a stable, conductive, viscous chondritic lid only a few km thick (see schematic illustrations below). These conditions will effectuate crystal–melt segregation and eventually metal–silicate segregation. During the time period in which the volume fraction of crystals in suspension remains below 60%, they will segregate according to buoyancy properties (e.g., crystal radius, relative density and volume fraction) to form an olivine-rich basal cumulate and a silica- or plagioclase-rich flotation crust in a volume ratio of ~ 90:10; this flotation crust is andesitic in the case of Erg Chech 002. Metal–silicate segregation will ensue rapidly once the volume fraction of crystals in suspension reaches 60% and the viscosity is reduced to a liquid-like state. Repeated cycles of crustal remelting and recrystallization can occur before internal heat generation ceases. Based on this planetary differentiation model and some basic assumptions about the timing of events in the petrogenesis of a planetesimal, Sturtz et al. (2022) used the calculated crystallization age (~1.8 m.y. after CAIs) and cooling rate (5°C/yr) of Erg Chech 002 to constrain the size of its source parent body to a diameter of 140–260 km.

Planetesimal Differentiation With Magma Ocean Episode
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Illustrations credit: Sturtz et al., Icarus, vol. 385, art. 115100, figs. 3a,b (2022, open access link)
'Structure of differentiated planetesimals: a chondritic fridge on top of a magma ocean'
(https://doi.org/10.1016/j.icarus.2022.115100)

The 3He and 21Ne cosmogenic noble gas data of Barrat et al. (2021) provide a CRE age for Erg Chech 002 of 26.0 (±1.6) m.y. and 25.6 (±1.0) m.y., respectively. An older CRE age of ~62–65 m.y. was calculated by Takenouchi et al. (2021 #6162) based on Ar–Ar dating; the age difference between chronometers is tentatively attributed to heterogeneity in the meteorite. Barrat et al. (2021) also showed that the unfractionated composition of Erg Chech 002 is consistent with a primitive or primary melt, and when compared to experimental partial melts of ordinary chondrites (see Figs. S8 and S9), its composition is consistent with a high degree (~25%) of partial melting and separation of an early metallic fraction as evidenced by the low abundances of certain siderophile elements (e.g., W, Ni, Co, Ga, and Zn).

In a comparison of reflectance spectra for Erg Chech 002 with those obtained through photometric surveys of asteroids, Barrat et al. (2021) demonstrated that the meteorite has a signature unlike that of any asteroid taxonomic class (see diagram below and Fig. S14 color-based taxonomic diagram). In fact, no known asteroid class can be matched to its spectral signature even when accounting for space weathering effects or for the addition of a reasonable quantity of xenocrystic olivine or mantle component (mixture of olivine and pyroxene).

Comparison of Erg Chech 002 Spectra with Known Asteroids
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Diagram credit: Barrat et al. PNAS, vol. 118, #11, Fig. S13 (2021)
'A 4,565-My-old andesite from an extinct chondritic protoplanet'
(https://doi.org/10.1073/pnas.2026129118)

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 top photo below shows a sampling of smaller stones with weights ranging from 0.4 to 10 g, courtesy of Miguel Angel Contreras Gomez, and the bottom photo below shows the largest known mass with a weight of 4,140 g, courtesy of Ben Hoefnagels.

Erg Chech 002 Small Stones
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Erg Chech 002 Main Mass
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Photos shown courtesy of Miguel Angel Contreras Gomez (top) and Ben Hoefnagels (bottom)