Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.604y&link_type=abstract
Meteoritics, vol. 30, no. 5, page 604
Mathematics
Logic
4
Chondrule, Textures, Chondrules, Cooling Rates, Elements, Volatile, Meteorites, Semarkona, Olivine Zoning
Scientific paper
Cooling rates for chondrules are among many aspects of chondrule forming events currently under debate and estimates by different authors vary considerably. Calculations based on radiation from isolated chondrules yield an extremely high cooling rate of ~10^5 degrees C/hr [1]. The cooling rates derived from previous petrological and experimental studies are much lower but inconsistent, ranging from 5 - 100 degrees C/hr [2] to ~1000 degrees C/hr [3]. Since cooling rates bear important information about the chondrule-forming environment, they need to be more tightly constrained. Here we re-evaluate the chondrule cooling rates based on the results of our recent flash heating experiments, mainly the volatile loss data, as well as textures, and olivine zoning profiles of the chondrule analog materials. Linear cooling vs. cooling curves. Many previous studies either assumed or used linear cooling rates for chondrules [2,3]. In reality, even with simple radiative cooling, the cooling rates should have followed a non-linear path, according to the Stefan- Boltzmann law. We used non-linear cooling rates throughout our experiments, and our observations show that the initial cooling rate at the high temperature end of a specific cooling curve affects chondrule properties most. Volatile loss results. Our Na and S loss experiments [4] have shown that to reproduce the very high Na contents [5,6] and primary sulfide [7] found in some natural chondrules, heating has to be brief, but fast cooling and relatively high fO2 are also essential. With an fO2 of ~10^(-10) atm, for a type II chondrule flash heated to its liquidus temperature, cooling curves beginning at ~2500 degrees C/hr are necessary to retain >90% of its original Na content or part of its S, unless the ambient gas is very enriched in these elements [8]. Under lower fO2, or for type I chondrule composition, even higher cooling rates are required. Textures and olivine zoning with ~10^1 - ~10^3 degrees C/hr initial cooling rates. Depending on temperature and starting composition, the charges cooled under such cooling rates exhibit either total glass, BO, PO, POP, or relict olivine texture, consistent with previous linear cooling experiments [2,3,9]. For type IIAB chondrule charges with PO, olivine zoning produced by initially cooling between 500 degrees C/hr and 5000 degrees C/hr is very similar to that of Semarkona chondrules [2]. In this cooling rate range, higher cooling rates enhance the zoning. Charges cooled between 10 degrees C/hr and 100 degrees C/hr show very limited olivine zoning. Textures and olivine zoning with ~10^5 degrees C/hr initial cooling rates. A cooling rate this high can only be achieved by quenching the charge in air immediately after the desired temperature is reached. The final charges are composed of glass and numerous small olivine crystals with grain sizes seldom exceeding 20 fm. Repeated heating/quenching cycles at lower temperature slowly coarsen the olivine crystals: 100 heating/quenching cycles can double the size of olivine, but the final olivine crystals are somewhat rounded and have curved embayments. Very limited olivine zoning is produced: the core/rim difference in FeO rarely exceeds 5%. Discussion. Under flash heating conditions, cooling rates of 10 - 100 degrees C/hr will cause extensive volatile loss except with an unusual ambient gas, and produce limited olivine zoning. Cooling rates of as high as 105 degrees C/hr certainly can preserve high volatile contents, but the crystals grown are too small. Repeated heating/extremely fast cooling cycles can coarsen the olivine grains, but not enough, and do not reproduce porphyritic chondrule textures well. In addition, the heating mechanism to heat chondrules hundreds of times with peak temperatures always constrained within a narrow window is unrealistic. The initial cooling rates of the chondrule cooling curves are more likely between these two extremes, and from our volatile loss results, they are probably in the range of several thousand degrees per hour, especially for type II chondrules. The lower temperature part of cooling curve is still uncertain. According to [10], the cooling rate in 1200 - 1300 degrees C range might be as low as ~10 degrees C/hr. Whether or how the cooling rates could have dropped to this low value needs further study. References: [1] Wasson J. T. (1995) in Chondrules and the Protoplanetary Disk, Cambridge Univ., in press. [2] Jones R. H. and Lofgren G. E. (1993) Meteoritics, 28, 213-221. [3] Radomsky P. M. and Hewins R. H. (1990) GCA, 54, 3475-3490. [4] Yu Y. et al. (1995) in Chondrules and the Protoplanetary Disk, Cambridge Univ., in press. [5] Grossman J. N. (1988) in Meteorites and the Early Solar System, 680-696, Univ. of Arizona, Tucson. [6] Hewins R. H. (1991) GCA, 55, 935-942. [7] Zanda B. et al. (1995) GCA, submitted. [8] Lewis R. D. et al. (1993) Meteoritics, 28, 622-628. [9] Tsuchiyama A. and Nagahara H. (1981) Mem. NIPR, Spec. Issue 20, 175-192. [10] Weinbruch S. and Mller W. F. (1995) GCA, in press.
Eiben B. A.
Hewins Roger H.
Yu Yue
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