Cosmogenic helium and neon in individual chondrules from Allende and Murchison: Implications for the precompaction exposure history of chondrules

Abstract

Abstract-We analyzed cosmogenic He and Ne in more than 60 individual chondrules separated from small chips from the carbonaceous chondrites Allende and Murchison. The goal of this work is to search for evidence of an exposure of chondrules to energetic particles-either solar or galactic-prior to final compaction of their host chondrites and prior to the exposure of the meteoroids to galactic cosmic rays (GCR) on their way to Earth. Production rates of GCR-produced He and Ne are calculated for each chondrule based on major element composition and a physical model of cosmogenic nuclide production in carbonaceous chondrites (Leya and Masarik 2009). All studied chondrules in Allende show nominal exposure ages identical to each other within uncertainties of a few hundred thousand years. Allende chondrules therefore show no signs of a precompaction exposure. The majority of the Murchison chondrules (the ''normal'' chondrules) also have nominal exposure ages identical within a few hundred thousand years. However, roughly 20% of the studied Murchison chondrules (the ''pre-exposed'' chondrules) contain considerably or even much higher concentrations of cosmogenic noble gases than the normal chondrules, equivalent to exposure ages to GCR at present-day fluxes in a 4p irradiation of up to about 30 Myr. The data do not allow to firmly conclude whether these excesses were acquired by an exposure of the pre-exposed chondrules to an early intense flux of solar energetic particles (solar cosmic rays) or rather by an exposure to GCR in the regolith of the Murchison parent asteroid. However, we prefer the latter explanation. Two major reasons are the GCR-like isotopic composition of the excess Ne and the distribution of solar flare tracks in Murchison samples.

The paradox between low shock-stage and evidence for compaction in CM carbonaceous chondrites explained by multiple low-intensity impacts

“…They examined but rejected an alternative explanation that the chondrules had been shocked in situ but experienced different pressures owing to the heterogeneous nature of the passage of the shock wave through the parent body. The ex situ shock hypothesis of Scott et al (1992) is supported by the findings of Roth et al (2011) that $20% of Murchison chondrules have pre-compaction exposure ages of up to 30 million years, whereas the others yield cosmogenic ages of $1.5 million years. Thus, it may be that the highly shocked subset of Murchison chondrules also spent a long time in the regolith where they were exposed to more impacts and/or higher intensities of shock than the S1 chondrules.…”

“…Thus, it may be that the highly shocked subset of Murchison chondrules also spent a long time in the regolith where they were exposed to more impacts and/or higher intensities of shock than the S1 chondrules. Taken together, the work of Scott et al (1992) and Roth et al (2011) suggests that conventional shock classifications may be inappropriate for quantifying the paleostress histories of meteorites such as the CMs that can have complex regolith histories.…”

Cosmic-Ray Exposure Ages of Meteorites

“…The results reflect the (model-dependent) neon isotopic ratios adopted for SCR and GCR neon in the phases analyzed (feldspars and pyroxenes). The study of this subject continues Roth et al, 2011). We regard this argument as unsettled.…”

“…As a simple, one-stage irradiation cannot explain such wide variation, these observations helped fuel a long-running debate over the nature and location of the irradiation(s) that produced the excesses (see discussion of Kapoeta in the succeeding text). The development of laser heating systems with low blanks has made it possible to analyze samples containing fewer than 10 6 atoms of noble gas isotopes, which in turn has reinvigorated the hunt for early irradiation effects (Das and Murty, 2009;Das et al, 2010Das et al, , 2011Roth et al, 2011). Roth et al measured apparent 21 Ne CRE ages (along with 3 He CRE ages) for chondrules from Allende and Murchison, respectively.…”

Chondrites and Their Components

“…Further laser microprobe analyses of chondrules and other components in the most pristine chondrites are clearly needed to establish which chondrules, if any, acquired solar gases from the nebula or were irradiated by high-energy particles near the inner edge of the disk. Recent measurements of noble gases in the Allende and Murchison chondrules using UV-laser ablation showed no clear evidence of their precompaction exposure to solar energetic particles (Huber et al, 2012;Roth et al, 2011).…”