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Fell August 25, 2020
31° 49' 35" N., 4° 40' 46" W.

The appearance of a bright yellow fireball accompanied by detonations was seen and heard by numerous people around 2:30 P.M. as it approached from the southwest over southern Morocco (Chennaoui Aoudjehane et al., 2021). Landfall occurred a short distance to the west of Tarda village, resulting in a strewn field ~3 km long situated near the main road between Goulmima and Errachidia. This serendipitous location provided ready access to hundreds of meteorite hunters over the following days, enabling the recovery of thousands of individual fusion-crusted stones ranging in size from 0.1 g to 99 g having a total combined weight of ~4 kg.

Samples of the meteorite were analyzed for classification at numerous institutions including the University of Washington in Seattle (A. Irving), Washington University at St. Louis (P. Carpenter), Arizona State University (L. Garvie), Florida State University (D. Sheikh), the University of New Mexico (C. Agee, K. Ziegler), Hassan II University of Casablanca (H. Chennaoui Aoudjehane), NASA Johnson Space Center (M. Zolensky), and Helmholtz Zentrum München (P. Schmitt-Kopplin). Tarda was determined to be an ungrouped, matrix-rich (~80 vol%) carbonaceous chondrite breccia of petrologic type 2. It contains small chondrules and chondrule fragments, very fine-grained AOAs, and forsterite grains all embedded in a fine-grained matrix. The matrix is composed predominantly of phyllosilicates along with lesser abundances of olivine, magnetite, pyrrhotite, pentlandite, troilite, carbonates, chromite, and very rare kamacite. In addition, Hoffmann et al. (2021) employed Raman spectroscopy in their Tarda analysis and have identified the phosphate mineral apatite.

Modal abundances for Tarda determined by King et al. (2021) are 72 vol% phyllosilicates, 10 vol% olivine, 8 vol% magnetite, 8 vol% Fe-sulfides, and 2 vol% dolomite. They employed thermogravimetric analysis to derive an estimate of the water abundance in Tarda of ~13 wt%. In addition, King et al. (2021) applied the aqueous alteration scale of Howard et al. (2015) and derived a phyllosilicate fraction (PSF) of 0.88, which defines a petrologic subtype of 1.3; this is equivalent to subtype 2.2 on the alteration scale of Rubin et al. (2007) as used herein. Garvie and Trif (2021) ascertained that the phyllosilicates consist of smectite, serpentine, and interstratified serpentine/smectite, and that the carbonates consist of the dolomite-ankerite solid-solution series along with siderite and calcite.

An oxygen isotope analysis conducted at the University of New Mexico (K. Ziegler) showed that Tarda has a bimodal distribution of δ18O values which plot near the fields for CI and CY groups, but with lower Δ17O values than either of these two groups (see top diagram below). Dey et al. (2021) obtained additional isotopic data which shows that Δ17O values for Tarda plot in a unique space between the CI and CR chondrites consistent with a new type of meteorite. Utilizing a coupled ε54Cr vs. Δ17O diagram, they demonstrated that Tarda and the CR group have indistinguishable ε54Cr values which may indicate the existence of a genetic relationship (see bottom diagram below, green-shaded field). Alternatively, the similarity between Tarda and C2-ungrouped Tagish Lake in their ε54Cr values (particularly the data of Petitat et al., 2011) could be consistent with a genetic relationship despite the discrepancy in Δ17O values (see bottom diagram below, blue-shaded field).

Bimodal Distribution of δ18O Values for Tarda
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Diagram credit: Chennaoui Aoudjehane et al., 52nd LPSC, #1928 (2021)
'Tarda (C2-Ung): A New And Unusual Carbonaceous Chondrite Meteorite Fall From Morocco'

Potential Common O–Cr Source Reservoirs for Tarda
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Diagram credit: Dey et al., 52nd LPSC, #2517 (2021)
'Exploring the Planetary Genealogy of Tarda – A Unique New Carbonaceous Chondrite'

Tunney et al. (2021, 2021) investigated the soluble organic matter in two Tarda stones along with that in the associated terrestrial sand for comparison. Their modern extraction techniques employing DCM and MTBSTFA resulted in the recovery of numerous extraterrestrial compounds including cyclic octaatomic sulfur, hexathiane, propanedioic acid, L-proline, glycine, urea, fumaric acid, methylmaleic acid, and dl-theronine. In addition, they identified dozens of organic contaminants from known terrestrial sources in Tarda samples with missing crust and in the local sand sample. The only terrestrial compounds common to both the Tarda samples and the local sand sample were hexadecanoic acid and octadecanoic acid. Tunney et al. (2021) assert that the many contaminants are likely related to the fall surfaces or to inadequate safeguards during handling prior to lab analysis.

It has been shown that Tarda and Tagish Lake have much in common, including their modal abundance of magnetite, which is present in both meteorites in the form of framboids, plaquettes, and spherules (Chennaoui Aoudjehane et al., 2021; Sridhar et al. (2021). In addition, both Tarda and Tagish Lake contain carbonates that precipitated in the form of siderite, while CM2 chondrites contain carbonates that precipitated in the form of calcite (Hicks and Bridges, 2021). Then too, spectroscopic analyses by Yesiltas et al. (2021) revealed very similar water decomposition percentages for Tarda and Tagish Lake. However, VNIR reflectance spectra obtained for Tarda, Tagish Lake, and WIS 91600 by Hiroi et al. (2022) demonstrate that only Tagish Lake is consistent with D-type asteroids (see diagram below).

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Diagram credit: Hiroi et al., 53rd LPSC, #1149 (2022)
'Tagish Lake Is Still The Only Possible Meteorite Sample From D-type Asteroids'

A spectral reflectance analysis was conducted by Applin et al. (2022) for Tarda and the Tagish Lake-like meteorites, including Tagish Lake, WIS 91600, and MET 00432. It was demonstrated that the reflectance spectra of Tarda is most consistent with that of the low-albedo P-type asteroids of the Tholen taxonomic scheme, such as asteroids (65) Cybele and (76) Freia. However, they note that many complex factors contribute to the spectral variations observed among asteroids, which can affect the taxonomic classification (see diagram below).

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Diagram credit: Applin et al., 53rd LPSC, #2218 (2022)
'Spectral Reflectance Properties Of The C2-ungrouped Tarda Meteorite'

Based on magnetic FORC (first order reversal curve) diagrams, principal component analysis, and scanning electron microscopy of a wide variety of carbonaceous chondrites, Sridhar et al. (2021) posited that CI chondrites, Tagish Lake, and WIS 91600 experienced similar conditions during aqueous alteration (e.g., presence of ammoniated ice and higher alkalinity) and formed in a similar region of the outer protoplanetary disk; it could be inferred that Tarda also formed in a similar region. However, an analysis of the primordial soluble organic matter (SOM) in Tarda and Tagish Lake conducted by Isa et al. (2021) showed significant differences exist between them; while Tarda SOM is enriched in S-bearing compounds, that of Tagish Lake is composed of mostly CHNO compounds.

The specimen of Tarda shown above is a 1.14 g specimen acquired from Juan A. Poblador of Jurassic Dreams. As is typical of the smaller recovered stones, this one exhibits partial fusion crust and a broken section with white chondrules visible within.

For additional information and videos about the Tarda meteorite fall, visit the excellent webpage by Karmaka Meteorites.

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