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

Abstract

Argon from neutron-irradiated mineral separates and whole rock samples of a metamorphosed breccia (65015) from Apollo 16 has been analysed 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 ⁴⁰Ar–³⁹Ar plateau age of 3.98 × 10⁹ yr, which we attribute to the time of metamorphism, and an age of ∼ 4.5 × 10⁹ yr in the high temperature fraction. Correlation of the release pattern with ³⁷Ar instead of ³⁹Ar permits the association of the ∼4.5 × 10⁹ yr 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.

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