Chondrule thermal history from unequilibrated H chondrites: A transmission and analytical electron microscopy study

Abstract

Sixteen texturally different (porphyritic, barred, radial, cryptocrystalline) FeO-rich chondrules from the unequilibrated ordinary chondrites Brownfield, Frontier Mountain (FRO) 90003 and FRO 90032 were characterized by optical and scanning electron microscopy and then thoroughly studied by transmission and analytical electron microscopy. Nanotextural and nanochemical data indicate similar thermal evolution for chondrules of the same textural groups; minor, yet meaningful differences occur among the different groups. Olivine is the earliest phase formed and crystallizes between 1500 and 1400 degrees C. Protoenstatite crystallizes at temperatures higher than 1350-1200 C; it later inverts to clinoenstatite in the 1250-1200 degrees C range. Enstatite is surrounded by pigeonitic or (less frequently) augitic rims; the minimal crystallization temperature for the rims is 1000 degrees C; high pigeonite later inverts to low pigeonite, between 935 and 845 degrees C. The outer pigeonitic or augitic rims are constantly exsolved, producing sigmoidal augite or enstatite precipitates; sigmoidal precipitates record exsolution temperatures between 1000 and 640 degrees C. Cooling rate (determined using the speedometer based upon ortho-clinoenstatite intergrowth) was in the order of 50-3000 degrees C/h at the clinoenstatite-orthoenstatite transition temperature (close to 1250-1200 degrees C), but decreased to 5-10 degrees C/h or slower at the exsolution temperature (between 1000 and 650 degrees C), thus revealing nonlinear cooling paths. Nanoscale observations indicate that the individual chondrules formed and cooled separately from 1500 down to at least 650 degrees C. Accretion into chondritic parent body occurred at temperatures lower than 650 degrees C.