The record in the meteorites--I: The former environment of stone meteorites as deduced from K 40 -Ar 40 ages

Details The record in the meteorites--I: The former environment of stone meteorites as deduced from K 40 -Ar 40 ages The record in the meteorites--I: The former environment of stone meteorites as deduced from K 40 -Ar 40 ages

Jul 1960

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1960gecoa..19..177g&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 19, Issue 3, pp.177-195

Astronomy and Astrophysics

Astronomy

11

Scientific paper

The K-Ar ages of meteorites are systematically lower than the Rb-Sr and Pb-Pb ages, owing to diffusion losses of argon. An equation is derived that gives the apparent K-Ar age as a function of the "true" age and the parameter D/a 2 , the diffusion constant of Ar 40 divided by the square of the radius of the potassium-bearing grains. The dependence of D /a 2 in stone meteorites on temperature may be obtained from the work of Geiss and Hess, so that given a "true" and an apparent age, it is possible to deduce an upper limit to the mean temperature at which a meteorite was held since the cessation of strong heating in its parent body. The activation energy for the diffusion of radiogenic Ar 40 from meteorites is unexpectedly small (14 kcal/mole), so that for a group of eight dated chondrites, the upper limit to th9 mean environmental temperature is found to be 196°K. This low value implies that the parent body of the chondrites was no closer to the sun than 1.44 astronomical units (a.u.), and no larger than 250 km in radius, for radiation temperatures are higher near the sun, and long-lived radioactivities produce higher temperatures in bodies as large as 300 km in radius. The compatibility of these limits with current theories on the origin of meteorites is examined, and some crucial experiments are proposed. Further diffusion losses will occur after the release of the meteorites from their parent bodies. Consideration of the radiation temperature of a meteorite in space shows that even for perihelia barely within the earth's orbit, some 10.20 per cent of the radiogenic and cosmic-ray induced argon will be lost in times the order of typical exposure ages of chondrites.

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