Consortium breccia 73255 - Laser /Ar-39/-/Ar-40/ dating of aphanite samples

Laser (Ar-39)-(Ar-40) studies of clasts and groundmass in breccia 73255 are presented. They show a variety of ages indicating that some materials lost argon from sites that outgas at low temperature, while other materials have retained argon generated before the breccia-forming event. The estimate for the time of the breccia-forming event from thermal release studies is compared to the upper limit of the event from the laser studies; this work also shows that ages in excess of 4.0 G.y. determined on rocks containing devitrified maskelynite clasts probably have no chronological significance.