⁴⁰Ar/³⁹Ar dating and cosmic‐ray exposure time of desert meteorites: Dhofar 300 and Dhofar 007 eucrites and anomalous achondrite NWA 011

Abstract

Abstract-We performed high-resolution 40 Ar-39 Ar dating of mineral separates and whole-rock samples from the desert meteorites Dhofar 300, Dhofar 007, and Northwest Africa (NWA) 011. The chronological information of all samples is dominated by plagioclase of varying grain size. The last total reset age of the eucrites Dhofar 300 and Dhofar 007 is 3.9 ± 0.1 Ga, coeval with the intense cratering period on the Moon. Some large plagioclase grains of Dhofar 007 possibly inherited Ar from a 4.5 Ga event characteristic for other cumulate eucrites. Due to disturbances of the age spectrum of NWA 011, only an estimate of 3.2-3.9 Ga can be given for its last total reset age. Secondary events causing partial 40 Ar loss ≤3.4 Ga ago are indicated by all age spectra. Furthermore, Ar extractions from distinct low temperature phases define apparent isochrons for all samples. These isochron ages are chronologically irrelevant and most probably caused by desert alterations, in which radiogenic 40 Ar and K from the meteorite and occasionally K induced by weathering are mixed, accompanied by incorporation of atmospheric Ar. Additional uptake of atmospheric Ar by the alteration phase(s) was observed during mineral separation (i.e., crushing and cleaning in ultrasonic baths). Consistent cosmic-ray exposure ages were obtained from plagioclase and pyroxene exposure age spectra of Dhofar 300 (25 ± 1 Ma) and Dhofar 007 (13 ± 1 Ma) using the mineral's specific target element chemistry and corresponding 38 Ar production rates.

Shergottites Dhofar 019, SaU 005, Shergotty, and Zagami: 40Ar‐39Ar chronology and trapped Martian atmospheric and interior argon

“…Fig. 1), while 37 Ar degasses differently (maximum release at 1480 °C, Table A3), similar to other pyroxene separates (Trieloff et al 1997(Trieloff et al , 2003aKorochantseva et al 2005). A comparison of the K concentration of the pyroxene separate computed from Kderived 39 Ar (328 ppm) (Table 2) with the K content of pyroxene mineral grains measured by electron microprobe analyses (EMPA-Taylor et al 2002)-generally below the detection limit-demonstrates that most 39 Ar degasses not from pyroxene mineral grains, but from K-bearing impurities.…”

“…This particularly concerns Dhofar 019, which has a long terrestrial residence time. The influence of desert weathering on argon systematics is described in detail by Korochantseva et al (2005). The significant terrestrial contamination complicates the identification and decomposition of Martian argon components.…”

“…12-14). The details of plotting CRE age spectra and the advantages of this kind of presentation are described in Korochantseva et al (2005). Tables 1 and 3 summarize obtained CRE ages and applied production rates in this study and CRE ages from other studies.…”

“…Our previous high resolution 40 Ar stepheating studies of terrestrial and extraterrestrial plagioclase, pyroxene, and olivine mineral separates (e.g., Trieloff et al 1997Trieloff et al , 2003aKorochantseva et al 2005;Hopp and Trieloff 2005;Hopp et al 2007) have demonstrated that neutron induced argon isotopes (besides K-derived 39 Ar, also Caderived 37 Ar and Cl-derived 38 Ar) can be used to identify and resolve radiogenic argon from various trapped argon components which can be hosted by main minerals or accessory phases. In this paper, we present comprehensive high-resolution 40 Ar stepwise heating analyses on whole rock and handpicked mineral separates of several Martian shergottites (Shergotty, Zagami, Sayh al Uhaymir [SaU] 005, Dhofar 019).…”

Advances in 40 Ar/ 39 Ar dating: from archaeology to planetary sciences – introduction

“…Jourdan et al 2005;Verati et al 2007) or pyroxene (e.g. Korochantseva et al 2005;Kennedy et al 2013) can be dated. As such, the 40 Ar/ 39 Ar dating technique has become one of the most trusted dating techniques with routine employment for over 40 years.…”