ANALYZING THE MINERAL CHEMISTRY OF THE NWA 8632 METEORITE FOR OLIVINE PHENOCRYSTS

As part of our Mineralogy course, our objective in this research project was to analyze the olivine crystals contained within the basaltic meteorite Northwest Africa (NWA) 8632 to better understand the crystallization process. The bulk chemistry of this meteorite is chemically similar to the Apollo pigeonite basalts and NWA 032/479. However, NWA 8632 is not paired with NWA 032/479 despite having some chemical similarities, therefore it is unique. We collected major and minor element composition data for olivine by utilizing the Cameca SX 100 electron microprobe at the University of Tennessee, Knoxville. NWA 8632 contains two crystal populations of olivine: large phenocrysts (300-1500 µm) and smaller crystals (50-300 µm) resulting from two separate cooling events. The larger olivine phenocrysts have Forsterite numbers (‘Fo’, Mg/[Mg+Fe]*100) with a broader range (Fo26 to Fo70), than that of the smaller crystals (Fo66 to Fo70), but this range covers both core and rim data. When separated out based on the analysis’ location along the olivine core or rim, the data show that the olivine are compositionally zoned. The cores of the phenocrysts are more Mg-rich than the rims (avg. Fo 68 and 51, respectively), as are those of the smaller olivine crystals (avg Fo 55 and 41, respectively). The higher Fo of the phenocrysts is chemical evidence that these grains crystalized in an earlier event, which is supported by the texture. In addition, at least some of the olivine phenocrysts display phosphorus zoning, with P-enriched regions containing up to 349 ppm P, and P-depleted regions containing abundances below the detection limit of the instrument. The P-zoning appears to oscillate, which may indicate complex processes within the magma chamber.