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.

Osmium isotope and highly siderophile element constraints on ages and nature of meteoritic components in ancient lunar impact rocks

“…Krähenbü hl et al (1973) and Ganapathy et al (1974) reported siderophile concentrations for white (0.346 ng/g Re, 3.57 ng/g Ir, 1.06 ng/g Au) and grey (0.67 ng/g Re, 7.33 ng/g Ir, 1.9 ng/g Au) lithologies. Ar-Ar plateau ages between 3.91 and 4.10 Ga have been reported (Kirsten et al, 1973;Marti et al, 1983).…”

Origin of sulfide replacement textures in lunar breccias. Implications for vapor element transport in the lunar crust

“…Further, it occurred after the crystallization of shallow Mg-suite plutonic rocks. This could be tied to the 4.03-4.1 Ga Ar-Ar age of the troctolite clast in 67915,149 (Venkatesan and Alexander, 1976;Marti et al, 1983). Although this age could also represent an age reset by impact or other post-crystallization event.…”

“…Only Ar-Ar age dates have been determined for these clasts. The troctolitic anorthosites have Ar-Ar ages of 4.03-4.1 Ga (Venkatesan and Alexander, 1976;Marti et al, 1983).…”

The geochemistry and provenance of Apollo 16 mafic glasses

“…er Mg 0 (30), and lower Al 2 O 3 (10.2 wt% vs. 12.9 wt%) than this glass group (Lindstrom and Salpas, 1983;Marti et al, 1983). Quartz monzogabbros typically have lower TiO 2 (<2 wt%), higher FeO (usually >14 wt%), and usually much higher P 2 O 5 (commonly upwards of several wt%).…”

“…No material is an exact match, although Warren's (1989) high-K KREEP and the Apollo 16 alkali gabbronorite (AGN; Jolliff, 1998) have similar shapes. Apollo 16 sodic ferrogabbro (SFG) data are from Marti et al (1983) and Lindstrom and Salpas (1983). All other data are from Jolliff (1998) …”