⁴⁰Ar‐³⁹Ar chronology of lunar meteorites Northwest Africa 032 and 773

Abstract

40 Ar incorporated into shock-melt veins. The effects of shock veins were investigated by laser fusion of 20 small samples expected to contain varying proportions of the shock veins. The laser ages show a narrow age distribution between 2.61-2.86 Gyr and a mean of 2.73 ± 0.03 Gyr, identical to the total age of ~2.80 Gyr obtained for the bulk sample. Diffusion calculations based on the stepped heating data indicate that Ar release can be reconciled by release from feldspar (and possibly shock veins) at low temperatures followed by pyroxene at higher temperatures. The exposure age of NWA 032 is 212 ± 11 Myr, and it contains low trapped solar Ar. Stepped heating of cumulate and breccia portions of NWA 773 also give a relatively young age of 2.91 Gyr. The presence of trapped Ar in the breccia makes the age determination of this component less precise, but release of Ar appears to be from the same mineral phase, assumed to be plagioclase, in both lithologies. A marked difference in exposure age between the 2 lithologies also exists, with the breccia having spent 81 Myr longer at the lunar surface; this finding is consistent with the higher trapped Ar content of this lithology. Assuming that 2.80 Gyr and 2.91 Gyr are the crystallization ages of NWA 032 and NWA 773 respectively, these two meteorites are the youngest lunar mare basalts available for study.

40Ar‐39Ar age determinations of lunar basalt meteorites Asuka 881757, Yamato 793169, Miller Range 05035, La Paz Icefield 02205, Northwest Africa 479, and basaltic breccia Elephant Moraine 96008

“…NWA 479 and NWA 032 have indistinguishable Ar-Ar ages of 2.779 ± 0.056 Ga 2.721 ± 0.040 Ga (Fernandes et al 2003), respectively. The Ar-Ar age of NWA 032 is also concordant with the Rb-Sr and Sm-Nd ages (Table 1: Borg et al 2007) making these meteorites among the youngest lunar basalts.…”

“…8), supporting the pairing relationships between these two meteorites. Following the interpretation of Ar data for NWA 032 (Fagan et al 2002;Fernandes et al 2003), the age spectrum of NWA 479 is considered to be influenced by 39 Ar-recoil. On this basis a total age of 2.721 ± 0.040 Ga is calculated for 90% 39 Ar K -release.…”

“…Apparent age and Ca/K spectrum diagrams for two MIL 03035 bulk samples. mean obtained from three samples of NWA 032 at 2.779 ± 0.056 Ga (Fernandes et al 2003). Figure 9 shows the age spectrum for bulk, basalt and breccia samples of EET 96008.…”

“…The majority of Apollo and Luna basalts are mainly low or high Ti basalts with ages in the range of 3.1-3.8 Ga with modes at 3.2-3.3 Ga and 3.6-3.7 Ga. In contrast, lunar basaltic meteorites are low or very low Ti (VLT) basalts with ages between 2.8-4.3 Ga (Fernandes et al 2003;Borg et al 2004;Terada et al 2007a;Shih et al 2008;and Sokol et al 2008). Hagerty et al (2006) have shown that low Ti basalts are from lunar mantle sources with relatively high radioactive heat production, thus enabling them to sustain melting over longer timescales and able to generate basalts of comparatively young age.…”

Mineralogy and petrology of lunar meteorite Northwest Africa 2977 consisting of olivine cumulate gabbro including inverted pigeonite

“…The pairing of NWA 2977 with NWA 773 is supported further by similar isotopic ages that are relatively young for lunar rocks. The radiogenic isotope data from the NWA 773 OC include a 147 Sm/ 143 Nd age of 2.993 ± 0.032 Ga (Borg et al 2009), an 40 Ar-39 Ar step-heating age of approximately 2.91 Ga (Fernandes et al 2003), and 207 Pb analyses of baddeleyite crystals indicating an age of 3.131 ± 0.012 Ga (Shaulis et al 2013). The radiogenic ages of NWA 2977 include whole-rock ages of 2.77 ± 0.04 Ga for Ar-Ar (Burgess et al 2007), 3.10 ± 0.05 Ga for Nd-Sm, 3.29 ± 0.11 Ga for Rb-Sr (Nyquist et al 2009), and 3.12 ± 0.01 Ga for Pb-Pb baddeleyite (Zhang et al 2011).…”