Northwest Africa 11745 (NWA 11745)

(Northwest Africa)

Purchased: 2004

Classification: Ordinary chondrite (L5)

History: sample was purchased in Morocco from a meteorite collector by Dr Mahesh Anand (OU) some years ago. In July 2016, thin sections of the sample were passed to Craig Walton (UStA) for preliminary classification work.

Physical characteristics: One large piece is present in the collection (554.16). Sample predominantly has a light color, but has a dark fusion crust on one side. Sample contains abundant complex shock features - mainly micro-fault shear textures, associated with melt pools.

Petrography: (Craig Walton, UStA) Overall, sample has a chondritic brecciated texture. Main phases are olivine and pyroxene with opaque metal + troilite. Porphyritic olivine, radial pyroxene, barred olivine and porphyritic olivine pyroxene chondrule types are all common, along with rare cryptocrystalline and glassy chondrules. Chondrule average diameter is 0.56 mm, ranging from 1.25 mm to 0.25 mm. Chondrules comprise ~ 70% of the sample. Opaque phases are metal (Fe, Ni _ in exsolution as kamacite and taenite) and sulfide (troilite). A recrystallized and shock-darkened matrix of fine-grained lithic fragments comprises ~ 20 % of the sample area. Some isolated euhedral olivine grains (up to ~ 1mm) occur embedded in this matrix. Olivine contains planar deformation features and has undulose extinction. Planes across which shear displacement has occurred (microfaults) crosscut chondrule and brecciated matrix textures. Fine-grained silicate melt-pools bearing globules of immiscible metal and sulfide are hosted within some sections of these microfaults. Microfault textures crosscut chondrules, matrix and metal, with individual faults running along the length of the slide. Offset on the faults is on the order of 500 μm. ~50 μm-wide shock-melt pools/veins occur within the faults.

Geochemistry: Mineral compositions and geochemistry: (Craig Walton, UStA; Mahesh Anand, Sam Hammond, Richard Greenwood, OU) Microprobe (Cameca SX100 EMPA) Olivine (n=15): MgO = 38.4, MnO = 0.5, FeO = 23.4, SiO = 38.3 P₂O₅ = 0.1 Pyroxene (n=7): MgO = 28.7, CaO = 0.9, MnO = 0.5, FeO = 14.5, Al₂O₃ = 0.2, Cr₂O₃ = 0.1, SiO₂ = 55.7, TiO₂ = 0.2. Plagioclase: Na₂O = 10.7, K₂O = 1.1, CaO = 2.3, FeO = 0.8, Al₂O₃ = 22.1, SiO₂ = 65.8 ; Plagioclase Ab_76.1Or_7.71An_16.2 (N=11).. Oxygen Isotopes (Laser fluorination CO₂ laser, Thermo Scientific MAT 253 gas source mass spectrometer) δ¹⁸O = 4.931, δ¹⁷O = 3.693, Δ¹⁷O = 1.129, Δ¹⁷O linear = 1.105 (all ‰).

Classification: L5. Shock stage is S4/5 as evidenced by shear microfault textures, associated shock-melt veins, shock darkening of matrix and undulose extinction and planar deformation features in olivine. Oxygen isotopes are indiscriminate between L and LL, however Fs (PX) and Fa (Ol) data indicate an L chondrite. Potentially paired with NWA 11744.

Specimens: Main mass at OU