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

Abstract

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.