Northwest Africa 17020 (NWA 17020)

(Northwest Africa)

Find: 2020

Classification: Ordinary chondrite (H3-6)

History: Cascadia received a package from Fabien Kuntz on January 11, 2023. Inside were several meteorite samples and a note saying "Specimens sent as per request of Mustapha Oulkouch." A sealed bag labeled OM 770 contained three individual samples (15.8, 9.1, and 11.5 g). No other information was included. Cascadia contacted Mr. Oulkouch, who responded that these were pieces from 4 kg of fragments found in Mali in 2020. Mr. Oulkouch has distributed the material and does not remember the original number of fragments. All three pieces sent to Cascadia were sectioned and analyzed to confirm they are pieces of a single stone.

Physical characteristics: Physical Characteristics: Three individual fragments were examined. All were covered by an identical orange-brown weathering patina. The 15.8 g fragment has remnant fusion crust along one face and has a dark gray cut face with visible metal, including what appears to be a discontinuous metal vein, as well as two lighter gray small (~2-3 mm exposed area) clasts. The 9.1 g fragment has remnant fusion crust along one side, and a medium brown-gray and porous interior. The cut face of the 11.5 g fragment is a medium brown-gray and is slightly more brown than the cut faces of the other two fragments.

Petrography: (M. Hutson, C. Stinson, A. Ruzicka, Cascadia): Both the 9.1 and 11.5 g fragments have approximately 70-75% replacement of metal by terrestrial weathering product, consistent with a weathering grade of W3. A large weathered vein cuts across the center of the 11.5 g fragment and appears to be a weathered shock vein. In thin section, both fragments grade from areas with more distinct chondrules to areas where chondrule and fragment margins are less distinct. Plagioclase feldspar grains in both sections are typically ~50 µm across, although there are enough grains up to 70 µm across to suggest a mix of petrographic types 5 and 6 material. Olivine grains were slightly more deformed in the 11.5 g fragment than the 9.1 g fragment, consistent with a shock stage of S4 versus S3 for the two fragments. In thin section, the 15.8 g fragment is clearly a breccia, with a sharp contact between a lighter colored clast with barely discernible chondrules, and a clast with distinct chondrules set in a dark matrix; the latter grades into a lighter material in which the chondrules are also distinct. BSE images show that olivine in the clast with distinct chondrules has large variations in gray scale, indicating that it is unequilibrated. Approximately 30-40% of the metal in the section has been replaced by terrestrial weathering product, consistent with a weathering grade of W2; deformation in olivine is consistent with shock stage S4.

Geochemistry: Fa and Fs in olivine and orthopyroxene grains in the 9.1 g, 11.5g, and equilibrated part of the 15.8 g fragments are indistinguishable and hereafter referred to as the host lithology. Host lithology: Olivine Fa_19.6±0.4, N=50; orthopyroxene Fs_17.2±0.2Wo_1.4±0.2, N=34; plagioclase feldspar Ab_80.6±1.5Or_6.5±1.2An_13.0±0.6, N=22. Unequilibrated clast: Olivine Fa_18.3±6.4, N=37; orthopyroxene Fs_14.1±6.0Wo_1.3±1.2, N=26.

Classification: Ordinary chondrite (H3-6). Host lithology grades from a high type 5 to a low type 6 based on mineral chemistry of olivine and orthopyroxene, distinctness of chondrules and fragments, and size of feldspar grains. Unequilibrated clast is consistent with H3.4-H3.6 based on olivine compositions.

Specimens: Cascadia holds 30.1 g in four pieces, as well as three polished thin sections and material in epoxy butts; Mustapha Oulkouch holds the main mass.