Northwest Africa 10731 (NWA 10731)

(Northwest Africa)

Purchased: Feb 2009

Classification: Ordinary chondrite (L3-6)

Petrography: Much of the thin section is composed of a fragmental lithology, which is mostly opaque in transmitted light due to the presence of small melt regions containing metal/sulfide droplets, patches of FeS containing angular silicate fragments, FeS-rims around chondrules, abundant FeS veins and FeS in the interiors of chondrules. BSE imaging shows that the main lithology is an intimate mixture of fragmental material containing a high proportion (~20%) of type 3 material (zoned magnesian and ferroan olivine and pyroxene grains surrounded by glass). With the exception of some small patches of an unusual matrix containing euhedral to subhedral albite crystals intergrown with Fe-rich olivine, the main lithology lacks typical type 3 matrix. Instead, material between chondrules, clasts, and grains is composed of smaller chondrule and mineral fragments. In addition to the main lithology, the section contains two igneously textured light inclusions (one roughly circular and ~3 mm in diameter; the other roughly triangular and ~6 × 3.5 mm in length and width), as well as a large triangular-shaped type 6 inclusion (~6.5 mm in length and width), two small rectangular type 6 inclusions (the larger is ~ 0.5 mm wide and ~3.5 mm long). Metal abundance for the entire section (excluding the two igneous inclusions, which lack metal or sulfide) was estimated from a reflected light mosaic at 3.6 %. Silica polymorph, chlorapatite, merrillite, chromite-plagioclase objects, metallic copper, and an Fe-Ni carbide phase were observed.

Geochemistry: (M. Hutson and A. Ruzicka, Cascadia) Main lithology: (mixture of unequilibrated and equilibrated grains) olivine (Fa_23.0±6.3, N=168, median Fa_25.2), low-Ca pyroxene (Fs_18.6±7.0Wo_1.6±1.2, N= 101, median Fs_21.4). Large triangular type 6 clast: olivine (Fa_25.8±1.2, N=36), low-Ca pyroxene (Fs_21.4±1.3Wo_1.8±0.7, N=20). Rectangular type 6 clast: olivine (Fa_26.0±0.8, N=12), low-Ca pyroxene (Fs_22.0±0.8Wo_2.1±0.4, N=11).

Classification: The main lithology represents an intimate mixture of fragmental material. Type 3 is inferred from presence of zoned magnesian and ferroan olivine grains and abundant glass; type 6 from coarse (>50 µm) plagioclase feldspar grains. Type 6 clasts are inferred from coarseness of feldspar and degree of integration. The most equilibrated clast is right on the boundary between L and LL and other parameters (e.g., metal abundance) are also transitional. Sample is classified as an L3-6 genomict breccia.

Specimens: Cascadia holds 65.7 g in multiple pieces, in addition to one polished thin section and a mounted butt.