Clues to Nonlinear Thermal Histories from Metallographic Cooling Rate Determinations
Abstract
Introduction: The interior of most differentiated meteorite parent bodies cooled through the temperature range where the Widmanstatten pattern forms at a fairly constant rate and it is therefore generally possible to calculate cooling rates which are representative of the temperature range where the Widmansatten pattern forms. For some meteorite parent bodies, like the mesosiderites and IIF irons, there is, however, evidence that the thermal evolution was significantly nonlinear. We show that it is possible to deduce the thermal histories between 1000 and 600 K based on the Widmanstatten pattern. 1000 K > T > 800 K: The alpha bandwidth can be used to determine cooling rates if alpha bands unaffected by impingement can be located [1]. Due to the rapid decrease in diffusion coefficients with decreasing temperature most of the alpha growth takes place at high temperatures. We find that alpha bandwidth cooling rates, were applicable, define the cooling rate at temperatures above 800 K. 1000 K > T > 650 K: Metallographic cooling rates are, in most cases, based on a suite of measurements of the total width (TW) and the Mid Profile Ni Concentration (MPC) of taenite lamellae [2]. These data should plot along isocooling rate curves in MPCTW diagrams provided the cooling rate was constant and that kamacite nucleation took place without significant undercooling. The MPC of wide taenite lamellae cannot be changed at low temperatures if the widths of the lamellae are larger than the characteristic diffusion length. The wide lamellae are therefore primarily sensitive to the cooling rate at high temperature. In contrast, narrow lamellae can be equilibrated at low temperature and are therefore sensitive to cooling rates at low temperatures. In either case the data points will not plot along isocooling curves in MPCTW diagrams and the deviations from the isocooling rate curves can be used to infer the cooling rate as a function of temperature. 700 K > T > 600 K: The Ni depletion of kamacite close to the kamacitetaenite phase boundary [3] can also be used to estimate cooling rates. The shape and in particular the width of the depletion profile is a function of the cooling rate. The depletion forms at temperatures below 770 K. The inferred cooling rate, which may be determined on the basis of the depletion profile, is characteristic of the cooling rate below 700 K. An advantage of this method is that it is based partly on the same phase diagram and diffusion coefficients as the MPCTW method and the results are therefore comparable. T < 600 K: It has been suggested that the size of the island phase in the cloudy zone [4] and the Mossbauer parameters [5] are low temperature cooling rate indicators (at temperatures ~593 K). Cooling rates based on schreibersite growth have also been used to determine cooling rates down to around 520 K depending on the nucleation temperature of the individual schreibersites [6,7]. The fact that these methods are based on the growth of different phases, makes it, however, difficult to infer relatively small decreases in cooling rate from the MPCTW temperature range to the temperature range where schreibersite and the island phase grow. References: [1] Rasmussen K. L. (1989) Physica Scripta, 39, 410416. [2] Rasmussen K. L. (1989) Icarus, 80, 315325. [3] Agrell S. O. et al. (1963) Nature, 198, 749750. [4] Yang C. W. et al. (1993) Meteoritics, 28, 464. [5] Albertsen J. F. et al. (1980) Meteoritics, 15, 258. [6] Kulpecz A. A. and Hewins R. H. (1978) GCA, 42, 14951500. [7] UlffMuller F. and Rasmussen K. L., this volume. [5] Delaney J. S. et al. (1981) Proc. LPSC 12th 12B, 13151342. [6] Powell B. N. (1969) GCA, 33, 789810. [7] Rasmussen K. L. et al. (1985) Meteoritics, 20, 738739. [8] Haack H. et al., in preparation.
 Publication:

Meteoritics
 Pub Date:
 July 1994
 Bibcode:
 1994Metic..29..470H
 Keywords:

 Cooling;
 Histories;
 Metallography;
 Meteoritic Composition;
 Temperature Profiles;
 Depletion;
 Diffusion Coefficient;
 Nonlinearity;
 Nucleation;
 Phase Diagrams;
 Widmanstatten Structure;
 Lunar and Planetary Exploration;
 IIF; MESOSIDERITES; METALLOGRAPHIC COOLING RATES