Mössbauer study of redox processes in the evolution of chondrites

Abstract The evolution of Fe-containing phases of carbonaceous chondrites heated under various oxidation-reduction conditions was investigated by means of Mössbauer spectroscopy. Heating of the lower petrological types of chondrites (CM2) released gases which initially produced oxidizing conditions (∼450°C) and then reducing conditions (> 700°C Phase transformation occurred rapidly (during 1–5 minutes) at all temperatures. During heating the Fe-bearing phyllosilicate phases in CM2-chondrite converted to Fe-bearing olivine, metallic iron and troilite (pentlandite). These phases resemble those of CO3, CV3 and EH-chondrites. Iron distribution resembling that of ordinary chondrites (some additional Fe2+ in pyroxene) was obtained only by heating a mixture of oxidized matter (CM2) with reduced matter (EH). A phase transition discovered at 1050°C is probably the temperature boundary between conditions of formation of the two main components of ordinary chondrites: matrix and chondrules. Chondrules of ordinary chondrites may be formed at temperatures > 1050°C while the matrix forms at temperatures < 1050°C For the carbonaceous chondrite Kainzas (CO3) these temperatures are approximately 1000°C and < 900°C The experimental conditions determined for the evolution of chondrites do not contradict the theoretical two-component model of Wood-Anders-Ringwood and may further its development.