Pieces of the ancient lunar crust: Ages and composition of clasts in consortium breccia 67915

Abstract

We discuss the composition and chronology of clasts representing three minor lithologies in consortium breccia 67915. The lithologies studied are: sodic ferrogabbro, pristine troctolitic anorthosite (67915,26), and granulitic troctolitic anorthosite (67915,67). These lithologies were presumed to represent samples of the ancient lunar crust. The pristine troctolitic anorthosite, a typical member of the pristine anorthosite suite, contains little or no trapped liquid consistent with its two‐phase mineralogy. The plagioclase separates have 39Ar‐40Ar plateau ages of 4.10±0.06 b.y., and a final rise in the age pattern may represent a ‘memory’ of the earlier evolution. The granulitic troctolitic anorthosite, which has 13 times larger REE abundances, displays a continuous rise of the 40Ar/39Ar ratio. The last 25% of the 39Ar release data establish a minimum age of >3.5 b.y., but the 244Pu/Nd chronology suggests an age of ∼4.3 b.y. The sodic ferrogabbro samples, which appear to be members of the Mg‐gabbronorite group of pristine rocks, show large Ar losses but indicate plateaus in the low‐temperature data, suggesting a metamorphic event more recently than 260 m.y. ago, a time that is consistent with the North Ray impact event 50 m.y. ago.