Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.464y&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 464-464
Other
1
Cooling Rates, Estherville, Guarena, Kernouve, Saint Severin, Tazewell, Zone, Cloudy
Scientific paper
The cloudy zone in the retained taenite of meteoritic metal is composed of two phases, the high-Ni island phase and the low-Ni honeycomb phase [1,2]. There is a concentration gradient through the cloudy zone and the size of the island phase varies with local Ni concentration. We propose that this size variation can be used to estimate the low-temperature cooling rate of the meteorite since the island phase width is controlled by the cooling rate and Ni concentration. The purpose of this study is to develop a relationship between the size of the island phase in the microstructure, the composition of the cloudy zone in the retained taenite of iron, stony-iron, and stony meteorites, and the cooling rate of meteorites obtained by metallographic techniques. A JEOL 6300F high-resolution scanning electron microscope (HRSEM) and a JEOL 733 electron probe microanalyzer (EPMA) were employed to study the microstructure. The island phase size variation was measured using a Micro-Plan II image analysis system (DonSanto Co.). Five meteorites including one mesosiderite, Estherville (ES), one iron meteorite, Tazewell (TA), and three chondrites, Saint Severin (SS), Guarena (GU), and Kernouve (KE),were investigated. The island phase width was measured using high-magnification (50,000X) HRSEM images taken across the cloudy zone of the five meteorites and the local Ni concentration was measured for each image using EPMA. For all the meteorites, the size of the island phase increases with increasing Ni concentration. The Ni concentration of the cloudy zone, which abuts the clear taenite (tetrataenite) rim phase, has the highest Ni, and has the biggest island phases, is almost constant (~42 wt% Ni). The size of the biggest island phase in a meteorite can be used as a measure of the cooling rate. Figure 1 shows the variation of the island phase vs. cooling rate. The metallographic cooling rate data were taken from previous measurements (Estherville [3], Saint Severin [4,5], Guarena [5], Kernouve [4], and Tazewell [6]). A clear trend is observed: the size of the biggest island phase decreases from about 500 nm to 100 nm with increasing cooling rate of the five meteorites studied. Measurements for other meteorites are in progress. Acknowledgments: The suggestions of E. R. D. Scott, University of Hawaii, are greatly appreciated. References: [1] Reuter K. B. et al. (1988) GCA, 52, 617. [2] Yang C. W. et al. (1993) LPSC XXIV, 1557. [3] Wasson J. T. (1985) GCA, 33, 789. [4] Duffield C. E. et al. (1991) Meteoritics, 26, 97. [5] Willis J. and Goldstein J. I. (1983) LPSC, in JGR, 88, B287. [6] Saikumar V. and Goldstein J. I. (1988) GCA, 52, 715. Figure 1, which appears in the hard copy, shows the relationship between the size of the biggest island phase and the cooling rate of meteorites.
Goldstein Joseph I.
Williams Brian D.
Yang Chih-Wen
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