Chronology and petrogenesis of a 1.8 g lunar granitic clast:14321,1062

Abstract

A study was undertaken to determine the chronology of a pristine granite clast (1062) from Apollo 14 breccia 14321 using Rb-Sr, Sm-Nd and 39Ar- 40Ar methods. The genesis of the granite as constrained by the isotopic results and trace element characteristics is discussed. Chronology: The Rb-Sr internal isochron is slightly disturbed and yields an age of 4.09 ± 0.11 AE ( λ( 87Rb) = 0.0139 AE −1 ) and an imprecise initial I( Sr) = 0.702 ∓ .008. If two data are excluded, the age becomes 4.13 ± 0.03 AE and I( Sr) = 0.698 ∓ .003. The whole rock and mineral separates are extremely radiogenic; they yield model ages which are relatively well-defined. The average model age is 4.12 ± 0.03 AE (relative to BABI = 0.69898). The Sm-Nd internal isochron is also slightly disturbed and gives an age of 4.11 ± 0.20 AE ( λ( 147Sm) = 0.00654 AE −1). The 39Ar- 40Ar average age of the non-magnetic fraction of the sample yields a slightly younger age of 3.88 ± 0.03 AE (K-Ar constants from Steiger and >a ̈ , 1977). The concordancy of Rb-Sr and Sm-Nd internal isochrons with the Rb-Sr model age strongly suggests that the granitic clast formed at 4.1 AE ago in the shallow crust and was later excavated and brecciated about 3.88 AE ago. Petrogenesis: Isotopic and trace element data of the lunar granite show large K/La and Rb/Sr fractionations, small Sm/Nd fractionation and the distinct V-shaped REE distribution pattern at the time of crystallization. A two-stage model involving crystal fractionation followed by silicate liquid immiscibility (SLI) is proposed for lunar granite genesis. We propose that the granite can be the immiscible acidic liquid produced by SLI from a residual liquid which underwent fractionation of ca, 3% of phases with REE distribution coefficients similar to those of phosphate minerals from a highly evolved parental magma with REE contents about twice those of the 15405,85 quartz monzodiorite (QMD). The extreme scarcity of lunar granitic samples and their young formation ages suggest that they are probably not directly crystallized from the differentiation of the primordial magma ocean. Our isotopic results and trace elements data from other workers suggest that granites, QMD and probably Mggabbronorites may be genetically related and may have formed in a plutonic environment similar to gabbro-granophyre associations in terrestrial layered intrusions such as the Skaergaard Intrusions.