⁴⁰Ar‐³⁹Ar studies of whole rock nakhlites: Evidence for the timing of formation and aqueous alteration on Mars

Abstract

Abstract-20-25 mg whole rock samples of the nakhlites Lafayette and Nakhla have been analyzed via the 40 Ar-39 Ar technique, in part to verify their formation ages, but primarily, in an attempt to determine the timing of aqueous alteration in these martian meteorites. As in previous studies, plateaus in apparent age are observed at about 1300 Ma (1322 ± 10 for Lafayette, 1332 ± 10 and 1323 ± 11 for Nakhla), presumably corresponding to crystallization ages. The plateaus are not entirely flat, perhaps reflecting the effects of recoil during creation of 39 Ar in the nuclear irradiation.The first 5-20% of the K-derived Ar released from all three samples give apparent ages <1300 Ma. Coupled with the fact that chronometric isotopic studies of nakhlites typically show some disturbance, we believe the low temperature pattern represents more recent (than 1300 Ma) formation of martian aqueous alteration products such as iddingsite. No low temperature plateaus are observed. This is consistent with petrographic evidence for multiple formation events, although the lack of low temperature plateaus is far from conclusive. On the other hand, if there was a single time of alteration, we believe that it will be difficult, if not impossible, to determine it using the K-Ar system.

39Ar–40Ar ages of martian nakhlites

“…It is not clear why the plateau ages reported by Swindle and Olson (2004) are 24-34 Ma younger than the Nakhla age we report here, but the difference likely is related to whatever produces the downward sloped age spectra. In accounting for higher ages at lower extraction temperatures, Swindle and Olson (2004) favored redistribution of 39 Ar by recoil during neutron irradiation, possibly from the secondary mineral iddingsite. However, this explanation would imply that about half of the K resides in iddingsite (Fig.…”

“…Several workers have reported Ar-Ar analyses for whole rock and mineral samples of Nakhla and Lafayette (Podosek, 1973;Gilmour et al, 1998;Burgess et al, 2000;Swindle and Olson, 2004). The Ar-Ar age spectra for Nakhla measured by Burgess et al (2000) and Gilmour et al (1998) show small or no age peaks at low extraction temperatures, broad age plateaus, and small 39 Ar recoil effects at high temperatures.…”

“…The Ar-Ar age spectra for Nakhla measured by Burgess et al (2000) and Gilmour et al (1998) show small or no age peaks at low extraction temperatures, broad age plateaus, and small 39 Ar recoil effects at high temperatures. Age spectra reported by Swindle and Olson (2004) for both Nakhla and Lafayette did show Fig. 9.…”

“…Anand et al (2006) did not present an age spectra for MIL 03346, and Gilmour et al (1998) and Burgess et al (2000) did not give Ar-Ar ages for their Nakhla data. Swindle and Olson (2004), utilizing only those extractions showing similar ages, derived Ar-Ar ages of 1332 ± 10 and 1323 ± 11 Ma for their two Nakhla samples and 1322 ± 10 Ma for Lafayette. (Whether these age uncertainties represent 1r or 2r was not stated.)…”

“…For Y-000593, Misawa et al (2005) reported ages of Rb-Sr = 1.30±0.02 Ga and Sm-Nd = 1.31 ± 0.03 Ga, and Nakamura et al (2002) reported a Rb-Sr isochron age of 1.269 ± 0.240 Ga. Shih et al (2006) reported a Rb-Sr isochron age of 1.29 ± 0.12 Ga and a Sm-Nd isochron age of 1.36 ± 0.03 Ga for MIL 03346. Carlson and Irving (2004) reported a preliminary Sm-Nd age for NWA 998 of 1.29 ± 0.05 Ga. More recently, Swindle and Olson (2004) reported whole rock Ar-Ar ages, based on 0016-7037/$ -see front matter Published by Elsevier Ltd. doi:10.1016Ltd.…”