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81Krkr Cosmic Ray Exposure Ages of Individual Chondrules from Allegan

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81Krkr Cosmic Ray Exposure Ages of Individual Chondrules from Allegan Meteoritics & Planetary Science 48, Nr 12, 2430–2440 (2013) doi: 10.1111/maps.12228 81Kr-Kr cosmic ray exposure ages of individual chondrules from Allegan I. STRASHNOV1,2* and J. D. GILMOUR1 1School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester M13 9PL, UK 2Present address: School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK *Corresponding author. E-mail: [email protected] (Received 21 January 2013; revision accepted 12 September 2013) Abstract–81Kr-Kr cosmic ray exposure (CRE) ages of individual chondrules (6–10 mg) and adjacent matrix samples (5–10 mg) from the Allegan H5 chondrite have been measured using a new highly sensitive resonance ionization mass spectrometer. No conclusive evidence of variations among the CRE ages of individual chondrules or between chondrules and matrix has been observed—average CRE ages of 5.90 Æ 0.42 Ma (81Kr-78Kr) and 5.04 Æ 0.37 Ma (81Kr-80+82Kr) are identical within error to those determined for the matrix (7.42 Æ 1.27 Myr, 81Kr-80+82Kr) and agree well with the literature value for bulk Allegan. If any accumulation of cosmogenic krypton in the early solar system took place, either it was below our detection limit in these samples (<100 atoms), or any such gas was lost during parent body metamorphism. However, this demonstration that useful 81Kr-Kr ages can be obtained from few milligram samples of chondritic material has clear relevance to the analysis of samples returned by planned missions to asteroids and to the search for a signature of pre-exposure in other, less processed meteorites. INTRODUCTION chondrules today includes some produced in the early solar system, derived CRE ages will be too long and Chondrules are distinct clasts of mostly silicate may differ both among chondrules and between material that clearly existed as discrete entities before chondrules and adjacent matrix. While this would be incorporation into the rock in which we observe them the smoking gun for pre-exposure, interpreting such today. Some models propose that they originated in the variations in terms of durations of exposure in the solar solar nebula, while others consider formation on the nebula would require knowledge of the cosmic ray flux surfaces of asteroids and protoplanets as a result of at that time, which is lacking. Extension to cases of collisions (Boss 1996). If chondrules were present in the multiple recent exposure intervals such as required by solar nebula as small objects for a significant interval complex exposure histories is straightforward. before accreting to a planetesimal, they might be Previous work on H chondrites has reported expected to contain cosmogenic isotopes produced differences between the inferred CRE ages that may before accretion unless these isotopes were lost during indicate pre-exposure. For instance, Polnau et al. (2001) metamorphism on the parent body. This can be termed studied several large (20–50 mg) chondrules separated “pre-exposure.” from eight chondrites of a high petrologic type, using the In considering what evidence may be present for 3He, 21Ne, and 38Ar chronometers, and reported a slightly pre-exposure, we use a simple model in which cosmic older apparent CRE age for chondrules than matrix. The ray exposure (CRE) ceased when the chondrule accreted same group reported the CRE age determined for to a planetesimal, and restarted only when the fragment individual chondrules from the Allan Hills (ALH) 76008 destined to be recovered as a meteorite separated from H6 chondrite (Polnau et al. 1999). Although the CRE age its parent asteroid. CRE ages are calculated from the of the bulk meteorite is 1.72 Æ 0.11 Myr, the 3He, 21Ne, concentration of cosmogenic isotopes and the recent and 38Ar CRE ages of its chondrules turned out to be production rate. If the cosmogenic isotope content of higher by 31%, 67%, and 55%, respectively. © The Meteoritical Society, 2013. 2430 81Kr-Kr CRE ages of Allegan chondrules 2431 Hohenberg et al. (1990) studied meteorite grains 81Kr-Kr exposure ages would correspond to variations from the CM regolith breccias Murchison, Murray, and in total cosmic ray fluence. As each adjacent sample has Cold Bokkeveld containing VH (very heavy, Z > 20) experienced the same recent fluence, such variation must ion tracks from solar flares, indicating that they had have occurred in the early solar system before been exposed to energetic particles prior to accretion to lithification of the meteorite. the parent body. They identified a significant excess in Until recently, it has been impossible to apply the spallation-produced neon, and inferred that the flux of 81Kr-Kr chronometer to samples the size of individual solar cosmic rays in the early solar system was much chondrules due to the low concentration of 81Kr in higher than at present. meteorites. It has been applied mainly to approximately Roth et al. (2011) reported that the approximately 100 mg samples of eucrites, which have the highest 20% of Murchison chondrules that exhibit solar flare concentrations of the target elements; once equilibrium tracks have higher cosmogenic neon concentrations than between production and decay is attained, an average those chondrules without such tracks. However, because eucrite contains a few thousand 81Kr atoms mgÀ1. their samples contained lower concentrations of Chondrites have lower concentrations of target elements spallation-produced neon than the chondrules analyzed for krypton production than eucrites, so the analytical by Hohenberg et al. (1990), they suggested exposure to challenge of quantifying 81Kr concentrations from galactic cosmic rays in the regolith for about 30 Myr. chondrule-sized samples is correspondingly more severe. The previous data of Hohenberg et al. (1990) would We have developed a high sensitivity resonance have required regolith exposure of 145 Myr. ionization mass spectrometer (RIMSKI—resonance Wieler et al. (2000) presented data obtained by in ionization mass spectrometer for krypton isotopes) vacuo etching of the howardite Kapoeta, showing that capable of determining Kr isotope ratios at this level plagioclase separates that were rich in solar gases (Strashnov et al. 2011). It combines a laser resonance contained small excesses of cosmogenic neon compared ionization ion source with a high transmission time-of- with those low in solar gases. This can only be attributed flight mass spectrometer and a cryogenic sample to irradiation of Kapoeta by galactic cosmic rays in the concentrator. The method allows rapid determination of regolith, as its ultimate source is igneous activity on the isotope ratios with precision comparable to conventional parent body. A similar explanation has been advanced mass spectrometry, but on much smaller samples. We for Allende chondrules (Roth et al. 2011) that show have demonstrated its utility for 81Kr-Kr CRE age dating nominal exposure ages identical to each other within in a study of small (<5 mg) eucrite samples (Strashnov uncertainties of a few hundred thousand years. et al. 2012). Here, we present the 81Kr-Kr CRE age data In deducing apparent exposure ages from neon obtained in a study of individual chondrules and matrix isotopes, it is necessary to calculate neon production of the Allegan H5 chondrite. We are not aware of any rates from major element compositions using a physical evidence that Allegan is a regolith breccia, and so it may model of cosmogenic nuclide production (taking be expected that no evidence of pre-exposure in regolith shielding into account), such as that developed for would be found. Our goals in this study were to examine carbonaceous chondrites by Leya and Masarik (2009). whether this high petrologic type sample exhibits any In contrast, the 81Kr-Kr method (Eugster et al. 1967; evidence of pre-exposure similar to that previously Marti 1967) uses the equilibrium between production reported (Polnau et al. 2001). In addition, our results 81 5 81 and decay of Kr (T1/2 = 2.29 9 10 yr) to determine demonstrate that useful Kr-Kr ages can be measured the (recent) production rate for each sample. from few milligram samples of ordinary chondrites and Theoretical calculations for H chondrites using to establish the precision with which CRE ages on excitation functions of nuclear reactions and primary individual chondrules can be measured using our and secondary particle spectra show that the method is technique. The demonstrated ability to apply the 81Kr-Kr insensitive to changes in concentration of target system for such small samples has clear relevance for elements; for Rb, Sr, Y, and Zr concentrations varying analysis of asteroidal regolith samples returned by from 0.1 to 10 times those of typical H chondrites, the planned missions. expected variation in 81Kr-Kr CRE age is less than 3& (Leya et al. 2004a). The concentration of 81Kr can thus EXPERIMENTAL be assumed to be a function of the concentration of target elements and the recent cosmic ray flux Resonance Ionization Mass Spectrometer for Krypton experienced by the sample. For adjacent samples, the Isotopes recent cosmic ray flux can be considered identical, so variations in 81Kr concentrations can be used to Our instrument is described in detail in a previous account for compositional differences, and variations in publication (Strashnov et al. 2011). Briefly, laser heating 2432 I. Strashnov and J. D. Gilmour is used for extraction of krypton from the samples. (a) After exposure to a Zr-V-Fe alloy getter for several minutes, the gas is admitted into the time-of-flight (TOF) mass spectrometer (MS). In the spectrometer, atoms continuously condense onto a cold spot held at 75 K in the back plate of the Wiley-McLaren ion source (Wiley and McLaren 1955). They are repeatedly released from the cold spot with a 10 Hz duty cycle by a pulsed 1064 nm laser, and are resonantly ionized in the evaporation plume.
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