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

Abstract

Ar analyses of several bulk fragments of related meteorites A-881757, Y-793169 and MIL 05035 give crystallization ages of 3.763 ± 0.046 Ga, 3.811 ± 0.098 Ga and 3.845 ± 0.014 Ga, which are comparable with previous age determinations by Sm-Nd, U-Pb Th-Pb, Pb-Pb, and Rb-Sr methods. These three meteorites differ in the degree of secondary 40 Ar loss with Y-793169 showing relatively high Ar loss probably during an impact event ~200 Ma ago, lower Ar loss in MIL 05035 and no loss in A-881757. Bulk and impact melt glass-bearing samples of LAP 02205 gave similar ages (2.985 ± 0.016 Ga and 2.874 ± 0.056 Ga) and are consistent with ages previously determined using other isotope pairs. The basaltic portion of EET 96008 gives an age of 2.650 ± 0.086 Ga which is considered to be the crystallization age of the basalt in this meteorite. The Ar release for fragmental basaltic breccia EET 96008 shows evidence of an impact event at 631 ± 20 Ma. The crystallization age of 2.721 ± 0.040 Ga determined for NWA 479 is indistinguishable from the weighted mean age obtained from three samples of NWA 032 supporting the proposal that these meteorites are paired. The similarity of 40 Ar-39 Ar ages with ages determined by other isotopic systems for multiple meteorites suggests that the K-Ar isotopic system is robust for meteorites that have experienced a significant shock event and not a prolonged heating regime.

Petrogenesis of the Northwest Africa 4734 basaltic lunar meteorite

“…The younger Pb-Pb ages of the LaPaz lunar meteorites ($3.0 Ga) are consistent with not only the U-Pb age (2929 ± 150 Ma; Anand et al, 2006) determined in situ from single crystals, but also those determined from the whole rock by Ar-Ar (2985 ± 16 Ma;Fernandes et al, 2009b), Rb-Sr (3020 ± 30 Ma;Nyquist et al, 2005;2990 ± 36 Ma;Rankenburg et al, 2007) and Sm-Nd (2992 ± 170 Ma;Rankenburg et al, 2007) methods. It would be highly unlikely that different dating techniques yield almost identical ages if the system was severely disturbed.…”

“…There are only a few conference abstracts available for its petrology, mineralogy, and geochemistry (Chennaoui and Jambon, 2008;Fernandes et al, 2009a;Jambon and Devidal, 2009;Korotev et al, 2009). The reported crystallization ages for NWA 4734 vary from 2720 ± 40 (Fernandes et al, 2009a) to 3190 ± 190 Ma (2r) (Jambon and Devidal, 2009), the latter being marginally compatible with that of the LaPaz lunar meteorites (Nyquist et al, 2005;Anand et al, 2006;Rankenburg et al, 2007;Fernandes et al, 2009b). To get a deeper understanding of the petrogenesis of NWA 4734 and its implications for lunar magmatism, and to place refined constraints on the genetic relationship between NWA 4734 and the LaPaz lunar meteorites, we have carried out detailed petrological, mineralogical, and geochemical studies on NWA 4734, and made in situ Pb-Pb dating of baddeleyite and zirconolite from NWA 4734 and LAP 02205/02224.…”

“…The Ar-Ar age of NWA 4734 ($2.7 Ga, Fernandes et al, 2009a) is $10% younger than its Pb-Pb age, which was probably resulted from the susceptibility of Ar-Ar age to shock events. Similarly, the Ar-Ar age of LAP 02205 determined from an impact melt glass-bearing sample (2874 ± 56 Ma) is also distinctly younger than that from the bulk sample (2985 ± 16 Ma; Fernandes et al, 2009b). The Pb-Pb age of LAP 02205/02224 (3039 ± 12 Ma) obtained in this work is almost identical to ages previously obtained by Pb-Pb (3005 ± 17 Ma;Zhang et al, 2010), U-Pb (2929 ± 150 Ma; Anand et al, 2006), Rb-Sr (3020 ± 30 Ma;Nyquist et al, 2005;2990 ± 36 Ma;Rankenburg et al, 2007), Sm-Nd (2992 ± 170 Ma;Rankenburg et al, 2007), and Ar-Ar (2985 ± 16 Ma;Fernandes et al, 2009b) dating methods.…”

“…Preliminary studies show that NWA 4734 and the LaPaz lunar meteorites share many chemical and petrologic characteristics and are potentially paired (Connolly et al, 2007;Chennaoui and Jambon, 2008;Fernandes et al, 2009a). Petrology, mineralogy, geochemistry and geochronology of the LaPaz lunar meteorites have been extensively studied (e.g., Righter et al, 2005;Zeigler et al, 2005;Anand et al, 2006;Day et al, 2006a;Joy et al, 2006;Rankenburg et al, 2007;Fernandes et al, 2009b;Hill et al, 2009;Zhang et al, 2010). They are an important representative of young ($3.0 Ga), evolved, incompatible element-enriched lunar lavas, unlike any samples available in the Apollo or Luna collections (Anand et al, 2006;Day et al, 2006a;Joy et al, 2006).…”

Petrogenesis and chronology of lunar meteorite Northwest Africa 4472: A KREEPy regolith breccia from the Moon

“…Clasts with a KREEPy basaltic composition have also been reported in the Calcalong Creek meteorite (Hill and Boynton, 2003) and Yamato 983885 (Arai et al, 2005), and several of the basaltic lunar meteorites are reported to have assimilated a KREEP melt component (e.g. NWA 733: Fagan et al, 2003;Jolliff et al, 2003;Borg et al, 2004Borg et al, , 2009, and the LAP clan of lunar meteorites: Zeigler et al, 2005;Anand et al, 2006;Day et al, 2006;Joy et al, 2006;Fernandes et al, 2009, andDhofar 287a: Anand et al, 2003).…”