[PPS06-P03] Estimate of Chondrule Cooling Rates from Fe-FeS Cooling Experiments
Keywords:chondrule, cooling rate, metallic iron, iron sulfide, eutectic solidification texture
Chondrules are major constituents of chondrites, but their formation mechanism is still debated [1]. Understanding of the thermal history of chondrules is a key to constraint chondrule-formation events. The peak temperatures should have been above the silicate solidus, and may have been even above or below the silicate liquidus depending on chondrules. Crystallization experiments to reproduce chondrule textures showed that the the cooling rates should have been 100-1000 °C for porphyritic olivine [3, 4] and 500-2300 °C /h for barred olivine [5, 6]. These cooling rates are those above the silicate solidus, and little or no constraint has been given for cooling rates below the silicate solidus except for that below 600°C [7]. In this study, we focus on the eutectic solidification texture of Fe-FeS, which are common opaque phases in chondrules, to constrain the cooling rates of chondrules below ~1000°C.
Powders of Fe metal and FeS with a mixing ratio close to the Fe-FeS eutectic composition was heated at 1400°C in an evacuated silica glass tube with graphite for 3 hours, quenched in water, and ground into 50-400 micron-sized powder. The powder was dispersed in silica wool, and sealed in a silica glass tube with FeS and graphite under vacuum. The sealed tube was heated at 1330°C for 3 hours, and cooled down to room temperature with different cooling rates of 25, 100, 500 K/h. The texture of run products was observed with FE-SEM (JEOL JSM-7000F).
The eutectic solidification texture of Fe-FeS contains Fe metal blobs in a FeS matrix. The typical size of Fe metal blobs is smaller and their number density is higher for samples cooled at higher rates. The distance to the nearest neighbor (d) was measured for individual Fe metal grains to quantify the relationship between the texture and the cooling rate. We found that the frequency distribution of d (number × d2) is well well-explained with a log-normal distribution. The parameters of log-normal distributions (m and s2) are (1.53, 0.31), (1.16, 0.20), and (0.18, 0.14) for the cooling rates of 25, 100, and 500 K/h, respectively, and are clearly different for different cooling rates. This distribution of the nearest neighbor distance could thus be applicable to the estimate of cooling rates of chondrules below the silicate solidus.
References: [1] Desch S. J. et al. (2012) Meteorit. Planet. Sci., 47, 1139–1156 [2] Herzberg C.T. (1979) GCA, 43, 1241-1251 [3] Hewins R. H. & Radomsky P. M. (1990) Meteoritics, 25, 309-318 [4] Lofgren G. (1989) GCA, 53, 461-470 [5] Radomsky P. M. & Hewins R. H. (1990) GCA, 54, 3475-3490 [5] Lofgren G. & Lanier A. B. (1990) GCA, 54, 3537-3551 [6] Tsuchiyama A. et al. (2004) GCA, 68, 653-672 [7] Schrader D. L. et al. (2008) GCA, 72, 6124-6140
Powders of Fe metal and FeS with a mixing ratio close to the Fe-FeS eutectic composition was heated at 1400°C in an evacuated silica glass tube with graphite for 3 hours, quenched in water, and ground into 50-400 micron-sized powder. The powder was dispersed in silica wool, and sealed in a silica glass tube with FeS and graphite under vacuum. The sealed tube was heated at 1330°C for 3 hours, and cooled down to room temperature with different cooling rates of 25, 100, 500 K/h. The texture of run products was observed with FE-SEM (JEOL JSM-7000F).
The eutectic solidification texture of Fe-FeS contains Fe metal blobs in a FeS matrix. The typical size of Fe metal blobs is smaller and their number density is higher for samples cooled at higher rates. The distance to the nearest neighbor (d) was measured for individual Fe metal grains to quantify the relationship between the texture and the cooling rate. We found that the frequency distribution of d (number × d2) is well well-explained with a log-normal distribution. The parameters of log-normal distributions (m and s2) are (1.53, 0.31), (1.16, 0.20), and (0.18, 0.14) for the cooling rates of 25, 100, and 500 K/h, respectively, and are clearly different for different cooling rates. This distribution of the nearest neighbor distance could thus be applicable to the estimate of cooling rates of chondrules below the silicate solidus.
References: [1] Desch S. J. et al. (2012) Meteorit. Planet. Sci., 47, 1139–1156 [2] Herzberg C.T. (1979) GCA, 43, 1241-1251 [3] Hewins R. H. & Radomsky P. M. (1990) Meteoritics, 25, 309-318 [4] Lofgren G. (1989) GCA, 53, 461-470 [5] Radomsky P. M. & Hewins R. H. (1990) GCA, 54, 3475-3490 [5] Lofgren G. & Lanier A. B. (1990) GCA, 54, 3537-3551 [6] Tsuchiyama A. et al. (2004) GCA, 68, 653-672 [7] Schrader D. L. et al. (2008) GCA, 72, 6124-6140