Equilibrated ordinary chondrites: constraints for thermal history from iron-magnesium ordering in orthopyroxene

Iron-magnesium ordering was determined in orthopyroxenes from two suites of unshocked (shock stage S1, S2), equilibrated L- and LL-chondrites (10 grains from 5 meteorites and 7 grains from 4 meteorites, respectively) by means of single crystal x-ray diffraction (SCXRD). This study, together with a previous investigation of H-chondrites (13 grains from 8 meteorites), produces an internally consistent data set about the thermal record in equilibrated ordinary chondrites (EOCs). The major feature outlined by cation ordering in EOC orthopyroxenes is that H-, L- and LL-chondrites share a common low-temperature record, that is, a common range of similar cooling rates in the 340-480 degrees C interval for the petrographic types 4 to 6. As a consequence, the thermal evolution of EOCs consists of at least two subsolidus stages; the first stage occurred at temperatures > 480 degrees C where petrographic types were established in distinct environments; the second stage occurred when EOCs, irrespective of chemical class and type, cooled through 340-480 degrees C in environments characterized by close temperature-time conditions. Quantitative estimates of minimal cooling rates for EOCs range from a few degrees C/ka to similar to 10(2) degrees C/ka in the 340-480 degrees C interval. Possibly, final ordering was attained in environments where moderate radiative heat-loss was possible and, thus indicating shallow burial depths in the parent body