Argon-40/ Argon-39 Dating of Lunar Rock Samples

Abstract

Seven crystalline rock samples returned by Apollo 11 have been analyzed in detail by means of the ⁴⁰Ar-^-39Ar dating technique. The extent of radiogenic argon loss in these samples ranges from 7 percent to ≥ 48 percent. Potassium-argon ages, corrected for the effects of this loss, cluster relatively closely around the value of 3.7 x 10⁹ years. Most of the vulcanism associated with the formation of the Mare Tranquillitatis presumably occurred around 3.7 x 10⁹ years ago. A major cause of the escape of gas from lunar rock is probably the impact event which ejected the rock from its place of origin to its place of discovery. Upper limits for the times at which these impact events occurred have been estimated.