Thermoluminescence and fusion crust studies of meteorites.
thesisposted on 19.11.2015, 08:47 by Derek W. Sears
The thermoluminescence (TL) of meteorites has been examined with apparatus designed with emphasis on linear heating of the sample. The type of TL (i.e. the glow curve) depends on the minerals producing it and on the history of the specimen. The applications that have been made concern three phases of a meteorite's arrival on Earth; its preatmospheric shape, the temperature gradients produced by heating during atmospheric passage, and the terrestrial age of meteorites for which the fall was not observed. It has been found possible to measure terrestrial ages for some meteorites that have been on the Earth several hundred years. It appears probable that shock considerably increases the rate of decay of TL. The extent to which high temperatures experienced by the surface of the meteorite during its atmospheric passage have penetrated into the matrix suggests luminous flight times in the order of 10 seconds, but the gradients tend to be 3 - 5 times less than those predicted theoretically. They appear to have the same dependence on the orientation of the meteorite as the temperature gradients determined from the fusion crust; the steepest gradients being experienced at the front of the meteorite. The fusion crust, besides enabling much of the atmospheric behaviour to be determined, provides a useful source of information complementary to TL work. For example, TL gradients produced by atmospheric heating will only be found when the fusion crust contains an innermost zone. The proatmospheric TL gradients measured in many meteorites suggest that cosmic ray bombardment produces a significant amount of TL, and therefore that TL can be used to measure preatmospheric shape. A comparison with spallogenic nuclide profiles in meteorites suggests that secondary particles play an important part in determining TL. Studies of the Estacado meteorite confirm this expectation and suggest an elongated preatmospheric shape, approximating to an ellipse of eccentricity 0.8. From this it is calculated that the preatmospheric mass of Estacado exceeded 8 tons.