Solar‐proton‐produced neon in shergottite meteorites and implications for their origin

Abstract

Abstract— Measured Ne isotopes in samples of shergottite ALHA77005 show variations in 21Ne/22Ne ratios and 21Ne abundances that are consistent with the presence of two cosmogenic components: a component produced by nuclear interactions of galactic cosmic rays (GCR) and a component produced at shallow shielding depths (∼0–3 cm) by energetic solar flare protons (SCR). We suggest that the 21Ne/22Ne ratio generally can be used to distinguish between SCR and GCR components in many meteorite types. Analysis of cosmogenic Ne produced in chondrite mineral separates, eucrites, and anorthositic lunar rocks, all having diverse major element compositions, indicate that the GCR 21Ne/22Ne ratio increases modestly with relative Mg content. Data for hundreds of chondrite analyses suggest that SCR Ne is present in no more than a very small fraction of chondrites. Examination of literature data for other shergottites, however, indicate that all of these meteorites contain SCR Ne but that it is apparently absent in other SNC meteorites. The ubiquitous presence of SCR Ne in shergottites, in contrast to most other types of meteorites, suggests that the martian origin of shergottites gave them different orbital parameters compared to other meteorites. This in turn may have contributed to slower entry velocities and lesser surface ablation in the atmosphere or even to higher SCR production rates.

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

“…Second, there could be contributions from solar cosmic ray (SCR) spallation, which is only rarely observed in chondrites (e.g., Wieler 2002), but has been inferred for some shergottites (Garrison et al 1995;Eugster et al 1997). Unfortunately the most up-to-date work on SCR production rates considers the target elements Mg, Al, and Si only (Reedy 1992).…”

“…For further evaluation, whether this is caused by chemistry or SCR, it is necessary to check the influence of chemistry in more detail. We have done so using the modern production rates for GCR-produced Ne as a function of shielding and chemical composition given by Leya et al (2000) that were not yet available to Garrison et al (1995). In contrast to earlier similar "physical models," the model of Leya et al (2000) fairly reliably predicts not only production rates but also Ne isotopic ratios.…”

“…While this agrees well with the distribution of data points for DaG 476 and SaU 005, for EETA79001, Shergotty, and Zagami more than 70% of the data points are lower than the calculated range. Garrison et al (1995) have discussed several possibilities for the apparent prevalence of SCR contributions in shergottites. Short cosmic-ray exposure times may have helped.…”

“…The gray line indicates the range of results for Martian atmospheric Ne given in the literature, which ranges from 20 Ne/ 22 Ne = 7 to 11 (Swindle et al 1986;Wiens et al 1986;Pepin 1991;Ott and Löhr 1992;Bogard and Garrison 1998;Mohapatra et al 2003;Schwenzer et al 2005;Park and Nagao 2006). Also shown is the SCR-Ne signature (after Garrison et al 1995;data from Reedy 1992) as well as the 21 Ne/ 22 Ne ratio derived by Smith and Huneke (1975) for pure sodium spallation. b) Lower right part near spallation composition, enlarged.…”

“…In the following discussion, "c" refers to cosmogenic and "tr" to trapped (+ radiogenic). For this 3 He was taken to be 100% cosmogenic, together with a ratio of ( 4 He/ 3 He) c = 5.2 ± 0.3 (Heymann 1967 (Garrison et al 1995) and found this to result in decreasing the inferred ( 21 Ne/ 22 Ne) c by 2.1% (for bulk samples). However, lower ( 21 Ne/ 22 Ne) c can be due to causes other than low-energy irradiation; differences in chemistry and shielding may cause variations of the ratio similar to or even exceeding the variations due to different types of irradiation (see neon in the discussion).…”