Equilibrated ordinary chondrites: Constraints for thermal history from iron‐magnesium ordering in orthopyroxene

Abstract

Abstract— 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 °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 °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 °C in environments characterized by close temperature‐time conditions.Quantitative estimates of minimal cooling rates for EOCs range from a few °C/ka to ∼102°C/ka in the 340–480 °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.