[meteorite-list] Thermoluminescence - Part 3

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:52 2004
Message-ID: <DIIE.0000004000001C96_at_paulinet.de>

S.W.S. McKEEVER and D.W. SEARS (1980) Meteorites
That Glow (Sky and Telescope, July 1980, pp. 14-16):


A number of small red CL grains have been identified as the minor phosphate
mineral chlorapatite*. It is important as the host mineral for certain radioactive
elements, and once contained the now-ex-tinct isotope plutonium 244. The decay
of these elements by nuclear fission releases energetic particles, leaving tracks
of damaged material in the crystal, and these may be used to date the meteorite.
The difficult task of locating the phosphates is greatly simplified by spotting
the red CL against the blue background of feldspar.

Thermoluminescence studies are important to meteorite research in several ways.
The amount of TL shown by a meteorite just after falling to Earth is governed
by its exposure to high-energy radiation and by the temperatures it experienced
in space. These act in opposite senses - radiation increases TL, heat lowers it.
In most meteorites the TL level has settled to a uniform level, but we have dis-
covered several in which it is lower than expected. They act as if they had been
heated prior to our laboratory studies.

The fiery heat of the plunge through the atmosphere does not penetrate the meteorite
more than four or five millimeters, so that it cannot effect the TL at greater depths.
They may have been heated some other way - perhaps by passing close to the sun. TL
provides a promising means of recognizing these meteorites.

On reaching Earth, meteorites are protected from the high-radiation environment of space
and the TL subsides to a new, much lower level. Theoretically, this decrease in TL provides
a means to estimate how long ago a meteorite fell. This idea has been tested by examining
a great many meteorites, some of which fell recently and some many thousands of years ago.
The results are consistent with computer predictions of how TL drops with time. This, in turn,
helps in estimating the terrestrial lifetimes of meteorites in the two parts of the world where
finds are most common - the American Prairies and Antarctica. In the United States, it seems,
meteorites can lie undiscovered on the ground for up to 20,000 years; in Antarctica, most
samples are at least 10,000 years old.

Some meteorites not only have low TL when examined, but seem incapable of displaying much even
when artificially irradiated in a laboratory. These samples are usually black or almost white
instead of the usual pale gray, and they also have very low radiometric ages. Chondrites typically
were formed about 4.6 billion years ago, but the black ones seem to be a mere half-billion years
old. Therefore, about that time some meteorites suffered a violent event which caused them to
blacken and lose their TL and CL. Laboratory experiments have demonstrated that these effects
can result from intense shock. The event suffered by the black chondrites is commonly believed
to be the shock and heating attending the breakup of the parent body of those meteorites.

These studies are barely a beginning. The luminescence of meteorites may not be a new discovery,
but we believe it offers many possibilities for new ways to study these enigmatic bodies from space.

*Chlorapatite = Ca5(PO4)3Cl
Received on Tue 23 Mar 2004 10:37:17 AM PST

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