Noble gases in mineral separates from three shergottites: Shergotty, Zagami, and EETA79001

Abstract

Abstract-This study provides a complete data set of all five noble gases for bulk samples and mineral separates from three Martian shergottites: Shergotty (bulk, pyroxene, maskelynite), Zagami (bulk, pyroxene, maskelynite), and Elephant Moraine (EET) A79001, lithology A (bulk, pyroxene). We also give a compilation of all noble gas and nitrogen studies performed on these meteorites. Our mean values for cosmic-ray exposure ages from 3 He, 21 Ne, and 38 Ar are 2.48 Myr for Shergotty, 2.73 Myr for Zagami, and 0.65 Myr for EETA79001 lith. A. Serious loss of radiogenic 4 He due to shock is observed. Cosmogenic neon results for bulk samples from 13 Martian meteorites (new data and literature data) are used in addition to the mineral separates of this study in a new approach to explore evidence of solar cosmic-ray effects. While a contribution of this low-energy irradiation is strongly indicated for all of the shergottites, spallation Ne in Chassigny, Allan Hills (ALH) 84001, and the nakhlites is fully explained by galactic cosmic-ray spallation. Implanted Martian atmospheric gases are present in all mineral separates and the thermal release indicates a near-surface siting. We derive an estimate for the 40 Ar/ 36 Ar ratio of the Martian interior component by subtracting from measured Ar in the (K-poor) pyroxenes the (small) radiogenic component as well as the implanted atmospheric component as indicated from 129 Xe * excesses. Unless compromised by the presence of additional components, a high ratio of ~2000 is indicated for Martian interior argon, similar to that in the Martian atmosphere. Since much lower ratios have been inferred for Chassigny and ALH 84001, the result may indicate spatial and/or temporal variations of 40 Ar/ 36 Ar in the Martian mantle.

Concordant Rb–Sr, Sm–Nd, and Ar–Ar ages for Northwest Africa 1460: A 346Ma old basaltic shergottite related to “lherzolitic” shergottites

“…Christen et al, 2005). Interestingly, the observed variation of cosmogenic ( 21 Ne/ 22 Ne) cos for subsamples of EET 79001 is the greatest among the shergottites, including, in particular, some values larger than for the other shergottites (Schwenzer et al, 2007). Larger values of ( 21 Ne/ 22 Ne) cos imply greater shielding.…”

“…Reported trapped ( 40 Ar/ 36 Ar) tr ratios in shergottites range from <100 to almost 3000, all with large uncertainties. Most authors have attributed the lower ratios to a component from the Martian interior (e.g., Ott, 1988;Wiens and Pepin, 1988;Bogard and Garrison, 1998;Mathew and Marti, 2001;Schwenzer et al, 2007;Walton et al, 2007). However, the Martian interior ( 40 Ar/ 36 Ar) MI ratio remains poorly defined.…”

“…The CRE age for the paired meteorite NWA 480, from analyses of both cosmogenic noble gases (Mathew et al, 2003) and 10 Be (Nishiizumi and Caffee, 2006), is similar to, if not identical to, CRE ages for ''enriched" basaltic shergottites Zagami, Shergotty, and Los Angeles (Eugster et al, 1997(Eugster et al, , 2002. Several analyses of Shergotty and Zagami in various laboratories gave a range in 36 Ar cos concentrations of 1.77-2.49 Â 10 À9 cm 3 /g for whole rock samples and 1.50-2.71 Â 10 À9 cm 3 /g for feldspar separates (Eugster et al, 1997;Terribilini et al, 1998;Park and Nagao, 2003;Park, 2005;Schwenzer et al, 2007). Variations in these 36 Ar cos values are to be expected because of variable sample composition and space shielding among individual samples.…”

“…5) greatly exceeds the range of possible ratios calculated for galactic cosmic ray spallation according to the production rates of Leya et al (2000) for meteoroid radii up to 85 cm (high mass, high 21 Ne/ 22 Ne end) with three data points exceeding the calculated ''maximum" value. Schwenzer et al (2007) attribute such data to mineralogical variability and fortuitous measurements of pieces with unusual composition, but perhaps EET 79001 does not represent a separate ejection event, but instead experienced a complex exposure history with part of that exposure in a body >85 cm in radius. (Unusually low values of ( 21 Ne/ 22 Ne) cos for most of the EE79001 data compared to the calculated range are attributed by Schwenzer et al (2007) to the action of solar cosmic rays, but this is not the explanation for high ( 21 Ne/ 22 Ne) cos .)…”

“…Schwenzer et al (2007) attribute such data to mineralogical variability and fortuitous measurements of pieces with unusual composition, but perhaps EET 79001 does not represent a separate ejection event, but instead experienced a complex exposure history with part of that exposure in a body >85 cm in radius. (Unusually low values of ( 21 Ne/ 22 Ne) cos for most of the EE79001 data compared to the calculated range are attributed by Schwenzer et al (2007) to the action of solar cosmic rays, but this is not the explanation for high ( 21 Ne/ 22 Ne) cos .) Further, one might consider the possibility that ''lherzolitic" shergottites with ejection ages only $30% older than those of the more common ''enriched" basaltic shergottites of similar crystallization age (cf.…”