WHOLE-ROCK GEOCHEMISTRY, MINERALOGY, PETROGRAPHY AND THERMOMETRY OF AN L6 CHONDRITE FROM PEMBINA COUNTY, NORTH DAKOTA

In 1972, a 4.5 kg suspected meteorite was found in a potato field in Hensel, North Dakota. Preliminary investigation of this rock in 2006 suggested it to be an ordinary H5 chondrite that was probably paired with Drayton, which was also found in the same county in 1982. However, no further detailed mineralogical or geochemical classification was made on this sample. Subsequent detailed analysis of this rock reveals this to be an independent find.

In hand specimen, the sample is dull brown in color and partially covered with fusion crust. The crust displays metal-filled polygonal cracks and fractures. The interior reveals millimeter-sized olivine chondrules embedded in a well developed granular matrix. X ray fluorescence analysis of bulk ground sample shows it is predominantly Fe and Mg (MgO- 22 wt% and FeO(t)-27 wt%) and has a composition similar to picro-basalt. Mg/Si and Al/Si (wt%) ratios vary between 0.69 to 0.70 and 0.07 to 0.08, respectively, which is typical for an ordinary chondrite. The bulk Fe/Si (wt%) ratio varies between 1.09 to 1.1, which clearly identifies this specimen as a L-type low iron-meteorite. Electron microprobe analysis of olivine grains suggests a chrysolite composition (Fo 80±2 and Fa 20±2); the orthopyroxene is hypersthene (En 81±2 and Fs 17±2) with a relatively high concentration of Cr₂O₃ (0.50 wt%) and the clinopyroxene composition is augite. Fa (mole%) and Fs (mole%) ratios also suggest this meteorite to be a L-type. In section, the specimen in heavily brecciated and the matrix is recrystallized. No bronzite was observed. Chromite, troilite, maskelynite, kamacite and taenite have restricted compositions (troilite: Fe 60±3 and S 34±3; Chromite: Cr₂O₃ 57±2, FeO 26±3 and MgO 2±3 wt%; maskelynite: Ab 72±4, An 17±3 and Or 5±3%; kamacite: Fe 92±2 and Ni 4±2; taenite: Fe 58±10 and Ni 30±40 wt%). Olivine-spinel geothermometery models and Fe-Mg exchange models for cpx yield a formation temperature of 960º to 1120º C and a pressure of 8-16 Kbar. The bulk rock geochemistry, petrography and mineral chemistry indicate the meteorite is an L6 chondrite that is chemically and texturally distinct from Drayton, thus it may be considered an independent fall.