Chondrule thermal history from unequilibrated H chondrites: a transmission and a

2019-12-01 13:10:25

temperature electron deg microscopy rims

责任者: Ferraris, C.;Folco, L.;Mellini, M. 单位: Dipt. di Sci. della Terra, Siena Univ., Italy 来源出处: Meteoritics Planetary Science(Meteorit. Planet. Sci. (USA)),2002/10/,37(10):1299-321 摘要: 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°C. Protoenstatite crystallizes at temperatures higher than 1350-1200°C; it later inverts to clinoenstatite in the 1250-1200°C range. Enstatite is surrounded by pigeonitic or (less frequently) augitic rims; the minimal crystallization temperature for the rims is 1000°C; high pigeonite later inverts to low pigeonite, between 935 and 845°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°C. Cooling rate (determined using the speedometer based upon ortho-clinoenstatite intergrowth) was in the order of 50-3000°C/h at the clinoenstatite-orthoenstatite transition temperature (close to 1250-1200°C), but decreased to 5-10 degC/h or slower at the exsolution temperature (between 1000 and 650°C), thus revealing nonlinear cooling paths. Nanoscale observations indicate that the individual chondrules formed and cooled separately from 1500°C down to at least 650°C. Accretion into chondritic parent body occurred at temperatures lower than 650°C 关键词: electron microscopy;meteorites;chondrule thermal history;unequilibrated H chondrites;transmission electron microscopy;analytical electron microscopy;FeO-rich chondrules;Brownfield;Frontier Mountain 90003;Frontier Mountain 90032;FRO 90003;FRO 90032;optical microscopy;scanning electron microscopy;thermal evolution;textural groups;Protoenstatite;clinoenstatite;pigeonitic rims;augitic rims;minimal crystallization temperature;augite;sigmoidal precipitates;exsolution temperature;cooling paths;accretion;chondritic parent body