Highlights

We recently discovered two achondrites in the North of Chile: a ureilite (Calama 458) from the Antofagasta region and an howardite (Cariquima) from the Tarapaca region. See a more detailed description and some pictures below.

Cariquima

The Cariquima howardite shows a fresh glassy fusion crust on all sides and some flow lines. There is no evident weathering, suggesting a short time on the Earth’s surface. Mass of the single specimen is 280 g.

Petrography: Cariquima is a polymict breccia of sub-mm/mm sized eucrite and diogenite clasts in a fine matrix of related crystalline debris with typical grain size less than 50 μm. Contains aprox. 70 wt% eucrite materials and 30 wt% diogenite materials (based on average Al and Ca contents of the fusion crust). Eucrite components are mostly lithic fragments of basaltic eucrite, with only some clasts of cumulate eucrite. These clasts vary in texture from fine-grained porphyritic to coarse-grained subophitic. One cumulate eucrite clast presents a variolitic texture. The eucrite materials typically contain exsolved ferroan pigeonite and plagioclase, as well as minor and accessory phases such as silica, ilmenite, chromite, troilite, metal, zircon, fayalitic olivine, apatite and merrillite. Diogenite components are essentially orthopyroxene fragments, some with thin chromite veins. Shock evidence include the presence of melt pockets and veins with quenched texture in some pyroxene and silica grains and darkening of pyroxene by small blebs of metal and troilite. One round chondrule-like chromite-plagioclase assemblage was also observed in the studied polished thick section.

Geochemistry: Pyroxene overall Fe/Mn (atomic) 29.8±3.1 (N=77). Diogenitic low-Ca pyroxene Fs_21.7±2.9Wo_2.1±0.7 (N=18), Ca-pyroxene Fs_16.6Wo_38.8 (N=1). Eucritic low-Ca pyroxene Fs_47.5±9.0Wo_5.7±3.2  (N=36), Ca-pyroxene exsolution lamellae Fs_33.7±12.4Wo_34.4±7.6 (N=12). Plagioclase An_87.8±4.8Ab_12.2±4.8 (N=42). Chromite Cr# = 0.77±0.06 (N=8).

Ureilite (Calama 458)

Three meteorite fragments were found within half a meter of each other. They are highly consolidated desert polished rocks, which lack fusion crust. Two fits well together. Density is about 3.3 and total mass is 860 g. Individual masses are 615, 169 and 76 g. Sample cutting with a diamond blade was rather difficult, suggesting the presence of a hard component (diamond).

Thin section in cross-polarized transmitted light (above) and reflected light (below). Sample is 4 mm wide.

Petrography: One thin section examined. Sample dominated by fine to medium-grained (0.5-2 mm) interlocked anhedral-subhedral crystals of olivine (~50 vol.%) and pigeonite (~25 vol.%) with abundant 120º triple junctions and without preferred orientation. Olivine and pigeonite show reduced margins. An interconnected network of Fe hydroxides (~15 vol.%) containing rare kamacite and up to 20 µm thick occur along silicate grain boundaries. Fe hydroxides are also present as µm-size blebs (~5 vol.%) within silicate rims, mostly olivine. Interstitial aggregates of carbon material are also widespread (~5 vol.%). They reach up to 200 µm in size and Raman measurements indicate that they are mostly made of micrometric diamonds and graphite. No large graphite flakes were observed. Some pigeonite and olivine grains show weak undulatory extinction. Twinning is also observed in some silicate grains, mostly in pigeonite and rarely in olivine. Overall, weathering grade is moderate and shock grade is low to moderate.

Geochemistry: Olivine cores (N=10) Fa_21.3±0.7, FeO/MnO=40.2±3.9 (wt. ratio), Cr₂O₃=0.74±0.08 wt.%, CaO=0.33±0.05 wt.%. Olivine rims down to Fa2.5 and FeO/MnO= 5.4. Pigeonite cores (N=7) En_72.3±0.5Wo_9.8±0.2Fs_17.8±0.5, Cr₂O₃=1.3±0.1 wt.%, Pigeonite rims up to En_83.7. Kamacite 4.2 wt.% Ni.

Raman spectra showing the presence of interstitial diamond and graphite.