Northwest Africa 7135: An Unusual Reduced, Unequilibrated Chondrite Containing Oldhamite, Daubreelite, Schreibersite and Djerfisherite, and with a Unique Oxygen Isotopic Composition

Northwest Africa

Purchased: 2010 Oct

Classification: Ungrouped chondrite

History: Purchased by Fabien Kuntz in October 2010 from a Moroccan dealer at the Munich Show.

Physical characteristics: Brown stone (51.3 g) lacking fusion crust and with evident light-colored, round chondrules.

Petrography: (A. Irving and S. Kuehner, UWS) Composed of separated, mostly spherical, relatively small chondrules (mean 0.69±0.53 mm, n=40) in a matrix (~30 vol.%) rich in altered kamacite. Some chondrules contain interstitial feldspathic glass. Minerals are olivine, zoned orthopyroxene, pigeonite, diopside, subcalcic augite, Cr-rich chromite, pyrrhotite, pentlandite, daubreelite, oldhamite, schreibersite and rare barite. The presence of highly reduced sulfides and phosphides suggest an atypical ordinary chondrite.

Geochemistry: Olivine, predominantly Fa4.8-6.3 (N = 15), with some more ferroan grains Fa19.8-19.9 (N = 2); orthopyroxene, Fs7.0-21.8Wo0.4-1.5 (N = 3); pigeonite, Fs17.3Wo6.9; subcalcic augite, Fs13.3-22.2Wo25.4-29.9 (N = 2). The mean olivine composition (Fa6) is much more magnesian than for H chondrites of similar subtype. Oxygen isotopes (K. Ziegler, UNM): analyses of acid-washed subsamples by laser fluorination gave, respectively, δ17O = 3.207, 3.649, 3.262, 3.379, 3.859; δ18O = 5.175, 6.072, 5.326, 5.333, 6.450; Δ17O = 0.475, 0.443, 0.450, 0.563, 0.453 (all per mil). Oxygen isotopes plot in a region above the TFL and at relatively high δ18O values.

Classification: Ungrouped chondrite, high metamorphic type 3 or type 4.

Specimens: 10.44 g including one polished thin section at PSF; main mass with Kuntz.