Geochronology of high-K aluminous mare basalt clasts from Apollo 14 breccia 14304

Abstract

RbSr, KAr and SmNd age determinations were undertaken for two aluminous mare basalt clasts (,127 and,128) of high potassium abundances from Apollo 14 breccia 14304. Genesis of the basalts is discussed in the context of the ages and the RbSr and SmNd isotopic evidence. The RbSr isochrons for these rocks are well-defined and yield slightly different old ages of 3.95 ± 0.04 Ga and 3.99 ± 0.02 Ga (relative to λ( 87 Rb) = 0.0139 Ga −1 ) and identical low initial 87 Sr 86 Sr , I( Sr) , of 0.69938 ± 0.00011 and 0.69938 ± 0.00007, respectively. The age and I(Sr) results suggest that these basalts probably represent two related flows derived from a common source. The SmNd internal isochron for the basalt clast 14304,128 is also well-defined but yields a less precise age of 4.04 ± 0.11 Ga and an initial ϵ Nd of 0.27 ±2.61. The average 39Ar 40Ar plateau age of the whole rock sample of 14304,127 yields a slightly younger age of 3.85 ± 0.04 Ga. The concordancy of RbSr and SmNd isotopic ages for these rocks strongly suggest that they were crystallized at 3.95−3.99 Ga ago. The younger 39Ar 40Ar age was probably reset by the Imbrium event ~3.85 Ga ago. The RbSr and SmNd age data also show that these basalts are younger than most of the Apollo 14 mare basalts but distinctly older than the very high potassium (VHK) mare basalts. There seems to be an anti-correlation between potassium abundances and ages of aluminous mare basalts at the Apollo 14 site. The age and the I(Sr) value for these basalts show an approximately ten-fold fractionation of Rb Sr for a two-stage evolution model. However, the age and the ϵ Nd values indicate no significant fractionation of Sm Nd . The large Rb Sr enrichment but minimum Sm Nd fractionation relative to their source materials can be explained by the assimilation model proposed recently for VHK basalt genesis involving granitic material and aluminous basaltic magma. Aluminous mare basalt volcanism was probably derived from shallow mantle sources underlying thick highland crust, and these magmas are more likely to be contaminated by crustal materials on ascent than are other types of mare magmas.