Evidence for an approximately 4.5 aeon age of plagioclase clasts in a lunar highland breccia

Argon from neutron-irradiated mineral separates and whole rock samples of a metamorphosed breccia (65015) from Apollo 16 has been analyzed with a large number of gas extraction steps in order to obtain a high resolution in the apparent ages and to identify the gas released from different sources. The results on plagioclase show a Ar-40/Ar-39 plateau age of 3.98 b.y. which is attributed to the time of metamorphism, and an age of about 4.5 b.y. in the high-temperature fraction. Correlation of the release pattern with Ar-37 instead of Ar-39 permits the association of the approximate 4.5 b.y. age with relict plagioclasts which were demonstrated in previous petrographic and Rb-Sr studies as being unequilibrated. This result suggests that it is possible to identify lithic components which represent the early lunar crust.