• Cr spinel and chromite as petrogenetic indicators in ordinary chondrites: Equilibration temperatures of petrologic types 3.7 to 6

      Wlotzka, Frank (The Meteoritical Society, 2005-01-01)
      Many equilibrated ordinary chondrites contain (besides chromites of constant composition) Cr spinel with a large spread in Cr/(Cr + Al) ratios. They occur mainly as large grains in chondrules rich in mesostasis, preventing complete equilibration in Cr/Al but not in Fe/Mg. This partially equilibrated Cr spinel turned out to be particularly useful for the selection of an appropriate olivine/spinel thermometer and for the determination of equilibration temperatures. The main results are:1) The H3.7 to 3.8 and the L3.7 to 3.8 chondrites analyzed show temperatures of 625 to 680 C;2) Equilibrated chondrites show a range of olivine/Cr-spinel temperatures between 700 and 820 degrees C, and the same average temperatures for type 4 to 6 (number of analyzed meteorites in brackets): H4 (9) 766 degrees C, H5 (7) 774 degrees C, H6 (3) 775 degrees C, L4 (5) 752 degrees C, L5 (4) 754 degrees C, L6 (1) 754 degrees C. These temperatures are interpreted as equilibration temperatures. One indication is that the measured isotherms are straight lines down to low Cr/(Cr + Al) ratios, which have a higher Fe/Mg interdiffusion coefficient than grains with high ratios. And there is no correlation of measured temperature with grain size of Cr spinel.3) Chromites sensu stricto show temperatures about 50 to 100 degrees C lower than Cr spinel, and a correlation with grain size. This is a closure temperature established during cooling and in situ crystallization.These results can best be interpreted by a "rubble pile" model of parent body evolution. This model cannot explain, however, the absence of type 4 to 6 chondrites with temperatures as low as for type 3.7 to 3.8.
    • Northwest Africa 1500: Plagioclase-bearing monomict ureilite or ungrouped achondrite?

      Goodrich, Cyrena Anne; Wlotzka, Frank; Ross, D. Kent; Bartoschewitz, Rainer (The Meteoritical Society, 2006-01-01)
      Northwest Africa (NWA) 1500 is an ultramafic meteorite dominated by coarse (100-500 micrometers) olivine (95-96%), augite (2-3%), and chromite (0.6-1.6%) in an equilibrated texture. Plagioclase (0.7-1.8%) occurs as poikilitic grains (up to 3 mm) in vein-like areas that have concentrations of augite and minor orthopyroxene. Other phases are Cl-apatite, metal, sulfide, and graphite. Olivine ranges from Fo 65-73, with a strong peak at Fo 68-69. Most grains are reverse-zoned, and also have ~10-30 micrometer reduction rims.In terms of its dominant mineralogy and texture, NWA 1500 resembles the majority of monomict ureilites. However, it is more ferroan than known ureilites (Fo is greater than or equal to 75) and other mineral compositional parameters are out of the ureilite range as well. Furthermore, neither apatite nor plagioclase have ever been observed, and chromite is rare in monomict ureilites. Nevertheless, this meteorite may be petrologically related to the rare augite-bearing ureilites and represent a previously unsampled part of the ureilite parent body (UPB). The Mn/Mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation. However, its petrogenesis differs from that of known augite-bearing ureilites in that: 1) it formed under more oxidized conditions; 2) plagioclase appeared before orthopyroxene in its crystallization sequence; and 3) it equilibrated to significantly lower temperatures (800-1000 degrees C, from two-pyroxene and olivine-chromite thermometry). Formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the UPB than previously sampled. However, its significantly different thermal history (compared to ureilites) may more plausibly be explained if it formed on a different parent body. This conclusion is consistent with its oxygen isotopic composition, which suggests that it is an ungrouped achondrite. Nevertheless, the parent body of NWA 1500 may have been compositionally and petrologically similar to the UPB, and may have had a similar differentiation history.