39Ar-40Ar chronology of R chondrites

Abstract

This study presents the first determinations of 39Ar-40Ar ages of R chondrites for the purpose of understanding the thermal history of the R chondrite parent body. The 39Ar-40Ar ages were determined on whole-rock samples of four R chondrites: Carlisle Lakes, Rumuruti, Acfer 217, and Pecora Escarpment #91002 (PCA 91002). All samples are breccias except for Carlisle Lakes. The age spectra are complicated by recoil and diffusive loss to various extents. The peak 39Ar-40Ar ages of the four chondrites are greater than or equal to 4.35, ~4.47 +/- 0.02, 4.30 +/- 0.07 Ga, and greater than or equal to 4.37 Ga, respectively. These ages are similar to Ar-Ar ages of relatively unshocked ordinary chondrites (4.52-4.38 Ga) and are older than Ar-Ar ages of most shocked ordinary chondrites (<<4.2 Ga). Because the meteorites with the oldest (Rumuruti, ~4.47 Ga) and the youngest (Acfer 217, ~4.30 Ga) ages are both breccias, these ages probably do not record slow cooling within an undisrupted asteroidal parent body. Instead, the process of breccia formation may have differentially reset the ages of the constituent material, or the differences in their age spectra may arise from mixtures of material that had different ages. Two end-member type situations may be envisioned to explain the age range observed in the R chondrites. The first is if the impact(s) that reset the ages of Acfer 217 and Rumuruti was very early. In this case, the ~170 Ma maximum age difference between these meteorites may have been produced by much deeper burial of Acfer 217 than Rumuruti within an impact-induced thick regolith layer, or within a rubble pile type parent body following parent body re-assembly. The second, preferred scenario is if the impact that reset the age of Acfer 217 was much later than that which reset Rumuruti, then Acfer 217 may have cooled more rapidly within a much thinner regolith layer. In either scenario, the oldest age obtained here, from Rumuruti, provides evidence for relatively early (~4.47 Ga) impact events and breccia formation on the R chondrite parent body.