Site Section Links
Effects of Volatility On Rubidium-Strontium Dating
Some estimates of the age of moon-rock specimens have been based on the ratio between rubidium and strontium. It should be pointed out that under the conditions of temperature and pressure known to exist at the surface of the moon, unequal migration of these two elements must result.
Examination of the vapor-pressure curves for the elements shows that pressure of rubidium is more than 107 and up to 108 times that of strontium at a temperature of the lunar surface reached during the long lunar day (+150° C), and the vapor of rubidium at this temperature reaches a value of .01 Torr; the inevitable result would be for a substantial amount of rubidium to vaporize and migrate freely. Even if the rubidium were to be chemically combined in the form of less volatile compounds, the constant bombardment of the surface by hydrogen ions in the solar wind would reduce the compounds to free the metallic rubidium.
The metal vapor would tend to migrate to locations of lower temperature, where it would recondense, unless it were to achieve sufficient thermal energy to reach escape velocity and to leave the lunar scene completely. Recondensed rubidium might be found concentrated in clefts of shadowed areas, and there might be a systematic gradient in concentration with significant enrichment in the polar regions and accompanying depletion in the equatorial regions.
Strontium, having a vapor pressure more than ten million times lower than that of rubidium, would be far less affected by this mechanism. The result might be that the estimate of age based on the ratio of these elements would be strongly affected by their local origin on the lunar surface. Vapor migration is a mechanism that may cast doubt on the elemental ratio dating, at least when pairs with widely different volatility are employed.
Robert C., Wright is Senior Development Engineer with the Princeton Applied Research Corporation.
PENSEE Vol. I