CLASSIFICATION OF TWO ORDINARY CHONDRITES FROM THE OSCAR E. MONNING COLLECTION AT TCU

The chondrite meteorites represent the basic chemical building blocks from which the terrestrial planets were assembled. A thorough knowledge of their composition is crucial to understanding models of planetary formation. In the current work we present an analysis and classification of two meteorites found in Northwest Africa. The meteorites are provisionally numbered NWA 401 and NWA 1171 by the Nomenclature Committee of the Meteoritical Society. The two stones are part of the Oscar E. Manning Meteorite Collection housed at Texas Christian University in Fort Worth, Texas.

Chips from each stone were cut at the NC Museum of Natural Sciences and mounted in epoxy for microprobe analysis. Analysis of each mounted chip was completed using the JEOL JXA-8530F at the Southeastern North Carolina Regional-Microanalytical and Imaging Center (SENCR-MIC). Elemental x-ray maps of each sample were obtained to determine phase distributions and individual phases analyzed by WDS to determine heterogeneity. The phase distributions were used to calculate the metal abundance which determines the L, LL or H sub-classification. The degree of heterogeneity is used to determine petrologic grade (1-6).

NWA 401 and 1171 were determined to be ordinary chondrites based on the presence of visual chondrules and a lack of carbon phases (indicative of carbonaceous chondrites) and enstatite (indicative of enstatite chondrites). Both meteorites have greater than 8 modal percent metal, placing them in the H sub-group. To determine petrologic grade, olivine and pyroxene compositions were measured. Compositional heterogeneity greater than 5% indicates a type 3, less than 5% indicates type 4, while homogeneous compositions indicated 5 or 6. The olivine mg# in NWA 401 shows a 2.09% variation (average of 74.4 and a standard deviation of 1.56) indicate NWA 401 is of petrologic grade 4. NWA 1171 has a variance of 2.56% in olivine mg# (average of 75.7 and a standard deviation of 1.94) indicating that it also has a petrologic grade of 4. Both meteorites are determined to be H4 ordinary chondrites.