& Planetary Science 48, Nr 12, 2430–2440 (2013) doi: 10.1111/maps.12228

81Kr-Kr cosmic ray exposure ages of individual 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 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 0.42 Ma (81Kr-78Kr) and 5.04 0.37 Ma (81Kr-80+82Kr) are identical within error to those determined for the matrix (7.42 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 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 and to the search for a signature of pre-exposure in other, less processed .

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 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 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 separated from H6 chondrite (Polnau et al. 1999). Although the CRE age its parent . CRE ages are calculated from the of the bulk meteorite is 1.72 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 , 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 contains a few thousand 81Kr atoms mg1. 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 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. Two resonant transitions are employed. One (at 116.5 nm) involves photons produced by four-wave frequency mixing in Xe (Strashnov et al. 2009a); the second is at 558.1 nm, which is generated by a dye laser. These resonant steps are followed by single photon ionization at 1064 nm (b) (Strashnov et al. 2009b). Ions are accelerated into a flight tube and detected using a pair of chevron- mounted microchannel plates (MCP). Multiple analyses of air aliquots and <5 mg samples of eucrites, both containing approximately 105 total krypton atoms, show precision of approximately 1% for major isotope ratios. They are reproducible to this level through a day’s analyses. Typical blanks are approximately 5800 Kr atoms (approximately 3300 84Kr atoms) having atmospheric isotopic composition. During measurements, no build up (increase in the signal after the MS is isolated from the 2 pumps) is observed—the dominant process is removal of krypton from the MS volume by the ionization process. Blanks generally indicate the absence of isobaric Fig. 1. a) 84Kr “pump out curve” for a sample containing interference at any mass. This is expected for this 2 9 105 total krypton atoms derived from the terrestrial resonance ionization process, where several laser beams atmosphere: the Kr ionization signal decreases as ionized “gently” excite krypton atoms into higher energy levels atoms are removed from the ion source and implanted into from where they are ionized by a strong 1064 nm pulse. the detector. A fast signal decrease indicates efficient ionization. The mean lifetime of a 84Kr atom in the The resonance excitation process itself does not require spectrometer is 75 s. b) A time-of-flight spectrum of krypton high intensity laser pulses (<1 lJand<500 lJin from a 10 mg chondrule, collected during the second heating approximately 10 ns pulses for the first and second steps, step (11 W, 1064 nm). The inset shows enlarged the mass respectively). This amount of power at these wavelengths region around 81Kr approximately 1000 atoms. is usually not sufficient for other species to be ionized nonresonantly (the remaining 1064 nm photon is of low energy and usually does not produce a significant signal air calibrations can be successfully applied and that the even when high power Nd:YAG beams are used). One ionization efficiency of 81Kr (not present in the air exception is mass 78, where interferences are observed calibration) can be inferred from that of 83Kr. This is from ionization of benzene (C6H6). The procedure for expected because isotopes of odd nuclear mass correcting for this is described below. experience the same fractionation due to nuclear effects In our previous study, we demonstrated the routine such as hyperfine splitting of the atomic levels used in detection of <1000 atoms of 81Kr with an average the laser ionization process. precision of approximately 9% during measurements of Data reduction protocols are described in detail in a 81Kr-Kr exposure ages of mg-size samples of eucrites previous publication (Strashnov et al. 2011). Briefly, the (Strashnov et al. 2012). These measurements were in data are continuously collected for 5 min with a 10 Hz agreement with those determined by other groups (for duty cycle. The software sums spectra in blocks of 100 those samples where literature data were available), consecutive duty cycles to produce 30 summed spectra where samples approximately 100 times larger had been from the 5 min analysis (as shown in Fig. 1). The area analyzed. This demonstrated that in the case of laser under each krypton peak is determined and used to ionization, corrections for mass discrimination based on calculate an isotope ratio. Because different isotopes 81Kr-Kr CRE ages of Allegan chondrules 2433 have different evaporation rates from the cold spot, the isotope ratios change with time. A linear least squares fit to the 30 time-ordered ratios is used to determine the isotope ratios at inlet time.

Allegan Analyses

A few grams of the Allegan H5 (Allegan USNM215) were provided to our group by the Smithsonian National Museum of Natural History, Washington, DC. Chondrules were separated from the matrix using a gentle crushing technique and the remaining matrix was carefully removed under an optical microscope. Seven chondrules of <10 mg were prepared in this way. Matrix powder was collected l using a 25 m sieve. In principle, the collected matrix is Fig. 2. A small amount of benzene (C6H6) is always present in not guaranteed to be free of crushed chondrules; the system. It condenses onto the cold spot throughout the 78 however, this material can be generally regarded as measurement day and interferes with the Kr peak. Filled circles indicate the magnitude of the benzene interference chondrule-depleted. inferred from the excess over the expected signal (based on A typical single chondrule is expected to contain at 84Kr) in a measurement of a standard sample derived from the most a few thousand 81Kr atoms, so it was important terrestrial atmosphere. They indicate that the build-up is linear that RIMSKI was operating at peak sensitivity for these throughout the day. Open dots are interpolated estimates of the benzene contribution in analyses of samples and can be analyses. This was established by monitoring the pump 78 — used to correct the Kr measurement for the interference. out curve the krypton ionization signal measured at a The calculated contribution from benzene did not exceed 5.5% single isotope as a function of time. An example of such of the total m/z = 78 peak in any chondrule or matrix a curve obtained during these analyses from an air analysis. calibration sample containing 2 9 105 total krypton atoms is presented in Fig. 1a. The Kr signal decreases as the ionized atoms are “pumped out” of the MS corresponding spectrum acquired at high-gain enlarged volume and get implanted into the MCP detector. As and inset. Both spectra are the sum of 100 individual can be seen, almost all the sample atoms become TOF spectra collected over 10 s. ionized within 5 min of data acquisition. The lifetime Correction for the interference at mass 78 attributed against detection during these analyses (the time to benzene was made with reference to the bracketing constant of the exponential decay approximated by the air calibrations. The interference signal in air data) is approximately 75 s. calibrations was observed to gradually increase through During the analyses reported here, three air the day (Fig. 2). We attribute this to accumulation on calibrations (approximately 105 krypton atoms each) the cold spot because it reduced again overnight as the were measured before and after each sample cold spot was cycled to room temperature. This linear measurement to allow correction for a contribution at increase was not affected by the samples run between mass 78 from benzene and account for instrumental some air calibrations, indicating that parent compounds effects such as mass discrimination associated with the originated from the instrument itself (possibly oil vapors resonance ionization process. These arise in part from from the turbo-molecular pumps used for rough- spectroscopic effects, e.g., a hyperfine structure and pumping the system after it has been exposed to power broadening of the atomic spectra. As in our atmosphere) and not from the Allegan samples. Because previous eucrite analyses, noise from the analog to the 78Kr/84Kr ratio of air is known, the height of the digital converter (ADC) was the major source of baseline interference peak at mass 78 can be calculated from the variation. For this reason, data were acquired with two 78Kr/84Kr ratio in each air calibration, and the height of ADC settings—spectra used for determination of 81Kr the interference in a sample analysis can be deduced by were acquired for 10 s, then a lower gain ADC scale was interpolation. In Fig. 2, the initial C6H6 contribution used for 10 s to determine the stable isotope abundances. was calculated for all the air aliquots (filled dots, Analyses then proceeded with ADC settings alternating measured data). The benzene fraction in the sample according to this pattern. Figure 1b shows a TOF (open dots, interpolated data), which did not exceed spectrum of krypton collected from a 10 mg chondrule 5.5% of the 78 peak, was then determined assuming its (second 11 W heating step) with the 81Kr region from a linear build-up throughout the day. 2434 I. Strashnov and J. D. Gilmour

Table 1. Concentration and isotopic composition of Kr in Allegan chondrules. Heating 78Kr Sample power 84Kra 86Kr ≡1 80Kr 81Kr 82Kr 83Kr 84Kr Allegan chondrule, 10 mg 5 W 0.070 0.208 0.0081 0.805 0.764 3.264 0.001 0.016 0.0016 0.042 0.039 0.148 11 W 0.315 0.757 0.0534 1.654 1.876 4.309 0.0160 0.031 0.0062 0.053 0.023 0.090 16 W 0.072 0.587 0.0322 1.422 1.769 4.010 9.2 0.031 0.035 0.0066 0.071 0.088 0.179 Allegan chondrule, 6.4 mg 12 W 8.6 0.118 0.266 0.0092 0.853 0.895 3.379 0.010 0.010 0.0019 0.022 0.027 0.068 Allegan chondrule, 7.7 mg 8 W 10.8 0.050 0.2031 0.0056 0.765 0.875 3.308 0.005 0.010 0.00147 0.024 0.010 0.078 Allegan chondrule, 9.2 mg 10 W 0.053 0.211 0.0042 0.764 0.792 3.327 0.008 0.010 0.0016 .025 0.029 0.091 17 W 0.092 0.283 0.0085 0.843 0.935 3.317 6.0 0.006 0.009 0.0013 0.021 0.0246 0.069 Allegan chondrule, 3.7 mg 17 W 6.2 0.088 0.263 0.0081 0.879 0.917 3.408 0.006 0.009 0.0026 0.020 .025 0.066 Allegan condrule, 2.8 mg 17 W 11.8 0.067 0.212 0.0060 0.786 0.881 3.347 0.006 0.006 0.0011 0.015 0.0203 0.056 Allegan chondrule, 2.4 mg 17 W 9.0 0.072 0.205 0.00602 0.759 0.806 3.463 0.006 0.006 0.00095 0.016 0.019 0.057 Uncertainties in the isotopic ratios are 1r; uncertainties in the absolute gas amounts are 20%. All the chondrules were melted in a single step except 10 and 9.2 mg samples, for which gases were extracted in several steps using laser heating. For these samples, Kr concentrations are given as a sum of all the steps. ain 1012 cm3 STP g1.

RESULTS AND DISCUSSION In this application, both production rate formulae have limitations. Measurement of spallation 80,82Kr 81 T81, the Kr-Kr CRE age (in Myr) is calculated isotopes can be compromised by neutron capture using the following equations (Eugster et al. 1967; reactions on bromine (although the Br concentration is Marti 1967): low in Allegan, 0.024 lgg1—Morgan et al. 1985), while 78 Kr was subject to the interference correction described P 83Kr above. For this reason, ages based on both methods of T ðMyrÞ¼s 81 ; (1) 81 81 81 determining the production rate ratio are reported here. P83 Kr c where the subscript c refers to the cosmogenic Krypton Isotope Ratios of Allegan Chondrules and Matrix composition, s81 = 0.330 Myr is the mean lifetime of 81Kr—the half-life has recently been revised from Krypton concentrations and isotope ratios of 2.13 9 105 yr (Baglin 1976) to 2.29 9 105 yr (Baglin Allegan chondrules and matrix are presented in Tables 1 and 2 alongside sample masses and the laser 2008)—and P81/P83 is the ratio of the production rates of 81Kr and 83Kr. This ratio is derived from power that was used for heating in each step. Only a 81 measurements of the same sample and calculated as few thousand Kr atoms were released from each 81 84 (Marti and Lugmair 1971): chondrule, hence the errors on Kr/ Kr ratios are rather large (on average approximately 20%); for 78 analyses of matrix, it was not possible to increase the P81 Kr ¼ 1:262 þ 0:381 (2) sample weight due to saturation of the MCP and the P 83Kr 83 c error was approximately 30%. Because the quantum efficiency of the MCP detectors is approximately 50%, or as: and because the interleaving of ADC settings led to only 50% of the 81Kr counts being used to determine 80 þ 82 P81 ¼ : Kr Kr : the abundance, for our experimental protocol, the 0 95 83 (3) P83 2 Kr c theoretical maximum is approximately 6% for 1000 81Kr-Kr CRE ages of Allegan chondrules 2435

Table 2. Concentration and isotopic composition of Kr in Allegan matrix. 80Kr Sample Heating power, W 83Kra 86Kr ≡1 81Kr 82Kr 83Kr Allegan matrix, 9.2 mg 9 0.190 0.0029 0.748 0.755 0.003 0.0007 0.010 0.011 13 0.175 0.0021 0.706 0.691 16.9 (both steps) 0.004 0.0007 0.012 0.013 Allegan matrix, 10 mg 8 15.1 0.143 0.0006 0.673 0.673 0.003 0.0003 0.011 0.011 14 18.2 0.165 0.0019 0.716 0.715 0.004 0.0005 0.011 0.0122 Allegan matrix, 5 mg 14 18.9 0.166 0.0020 0.734 0.737 0.003 0.0003 0.010 0.011 Uncertainties in the isotopic ratios are 1r; uncertainties in the absolute gas amounts are approximately 20%. The 9.2 and 10 mg samples were step-heated, a 5 mg sample was melted in a single step. ain 1012 cm3 STP g1. atoms; the major source of error is baseline noise. The Calculation of Cosmogenic Krypton Ratios major isotope ratios can be determined with better than stbl. 83 81 83 3% precision, and so this was not a major control on The cosmogenic ( Kr/ Kr)c and ( Kr/ Kr)c the precision with which CRE ages were determined. ratios (“stbl.” denotes a stable isotope) are calculated Experimental uncertainty is controlled predominantly from measured ratios (denoted by “msd”) by by the uncertainty in the 81Kr measurement and by the subtracting atmospheric krypton (denoted by “air”) calculation of the cosmogenic fraction of each isotope assuming that all the 86Kr derives from the atmosphere: when a significant trapped component is present and : : from the correction for benzene. On average, the errors stbl:Kr stbl Kr=86Kr stbl Kr=86Kr 78 86 ¼ msd air (4) on the Kr/ Kr ratios are approximately 10% for 83Kr ðÞ83Kr=86Kr ðÞ83Kr=86Kr the measurements of chondrules; this includes the c msd air uncertainty associated with the benzene correction (the 81Kr 81Kr=86Kr amount of C6H6 was usually approximately 5% of ¼ msd (5) = 83 ð83 =86 Þ ð83 =86 Þ the full m/z 78 peak). Kr c Kr Kr msd Kr Kr air In our first experiments, gas from the chondrules was extracted in several steps (see data for the 10 and In principle, some or all of the 86Kr may derive 9.2 mg samples). However, we noted that the from Kr-Q. However, the Kr-Q composition differs cosmogenic isotope ratios did not vary considerably from air by only a few percent (Ozima et al. 1998; from step to step and the majority of a gas was Busmann et al. 2000) and we show below that the extracted already at lower heating steps. Thus, the rest calculated exposure ages are not materially affected if of the samples were melted in one step with the full the source of 86Kr is assumed to be entirely Kr-Q. laser power of 17 W. This is valid even for the matrix, where the For the matrix samples, a high content of trapped cosmogenic component amounted to <10% of the total krypton (terrestrial air or a planetary component) for some samples. For visualization, we present the combined with a factor of 2 lower concentration of three isotope plots of both the chondrules and matrix cosmogenic krypton isotopes was the main limitation. It samples at Fig. 3 and mark the air and Kr-Q values did not prove practical to increase the size of the matrix by triangles. samples, so that excesses due to cosmogenic production The calculated composition of cosmogenic krypton of could be resolved with better precision because the the Allegan chondrules and matrix are presented in amount of gas released led to saturation of the MCP Tables 3 and 4. As noted above, in some cases, the amount detector by the major krypton isotopes at peak of gas released led to saturation of the 84Kr signal, so data sensitivity. Matrix samples of approximately 10 mg for this isotope are not reported for the matrix and some contained enough 81Kr to detect without leading to of the chondrules. We calculate the weighted average saturation of the major isotopes, with the exception of of the cosmogenic ratios for the chondrules— 84 80 83 82 83 the Kr signal. The MCP at m/z = 84 was allowed to ( Kr/ Kr)c = 0.472 0.017 and ( Kr/ Kr)c = 0.727 saturate to better digitize the less abundant isotopes 0.03—which are effectively the production rate ratios of (84Kr is not used in the age calculations). these isotopes. These values are close to (and actually 2436 I. Strashnov and J. D. Gilmour

(a) (b)

(c) (d)

Fig. 3. Three-isotope plots of Allegan chondrule and matrix samples determined in this study. The air and Kr-Q (Ozima et al. 1998) values are marked by triangles. lower than) the values determined previously for eucrites: where 80P/83P = 0.515 0.003, 82P/83P = 0.765 0.003 (Miura 1 et al. 1998); 80P/83P = 0.509 0.003, 82P/83P = 0.766 w ¼ i r2 0.004 (Shukolyukov and Begemann 1996). This is an i 80,82 indication that the Kr derived from cosmic rays is and r is an error of the i-th measurement. dominated by spallation and that the contribution from The error on the weighted average is calculated 79,81 80,82 80,82 neutron capture on Br ( Br (n,c) Br (b ) Kr) is using the following expression: small-to-negligible, as expected given the low bromine content (Morgan et al. 1985). 1 Xn 1 78 ¼ The CRE ages (both those based on Kr and those 2 2 (7) r\ [ r based on 80,82Kr) determined for each chondrule are T81 i¼1 i plotted in Fig. 4, where their weighted averages are r represented by the horizontal line. From this data set, The weighted averages of the chondrule ages (1 78 we conclude that all the chondrules have essentially the errors) are 5.90 0.42 Myr (based on Kr) and 80,82 same CRE age within error (a standard deviation of 5.04 0.37 Myr (based on Kr), while for the 81 78 0.87 Ma and 1.3 Ma can be calculated for the 78Kr and matrix, we obtain 7.42 1.27 Myr ( Kr- Kr). The 80+82Kr CRE age data sets, respectively). We compare ages obtained for both chondrules and matrix are the exposure ages of the chondrules with its matrix by comparable to the bulk Allegan age (6.27 0.96 Myr) calculating their weighted averages as usual: (Leya et al. [2004b], value modified to ensure a consistent production rate; Wieler, personal Pn communication). wiTi In the above calculation, it has been assumed that \ [ ¼ i¼1 ; 86 T81 Pn (6) Kr was entirely sourced from a component with the wi composition of terrestrial air. Adopting the alternative i¼1 assumption that it is sourced from a component with Table 3. Isotope composition and concentration of cosmogenic krypton of Allegan chondrules and their 81Kr-Kr exposure ages.

T81(78), T81(80,82), T81(78), T81(80,82), 78 Heating Kr Myr Myr Myr Myr Sample power, W 81Kra83Kra 83Kr ≡1 80Kr 81Kr 82Kr 84Kr All air All air All Kr-Q All Kr-Q Allegan chondrule, 10 mg 5 0.03 0.477 0.748 0.0778 1.362 – 4.18 4.12 4.31 4.36 – 0.177 0.319 0.0328 0.645 2.00 2.24 2.06 2.03 81

11 <0.01 0.242 0.516 0.0439 0.815 0.8494 5.17 4.61 2437 5.19 4.64 chondrules Allegan of ages CRE Kr-Kr 0.013 0.027 0.0052 0.046 0.0759 0.63 0.57 0.63 0.79 16 <0.01 0.047 0.412 0.0290 0.685 0.6618 5.01 5.75 5.04 5.80 0.8 0.028 0.045 0.0064 0.084 0.1701 1.18 1.36 1.18 1.53 Allegan chondrule, 6.4 mg 12 0.02 0.6 0.419 0.578 0.0394 0.810 0.4373 7.62 5.35 7.74 5.56 0.065 0.078 0.0092 0.132 0.2946 1.92 1.39 1.95 1.46 Allegan chondrule, 7.7 mg 8 0.03 1.1 0.141 0.340 0.0262 0.480 0.1504 7.04 4.75 7.26 5.10 0.027 0.049 0.0069 0.114 0.3641 1.91 1.44 1.96 1.80 Allegan chondrule, 9.2 mg 10 0.01 0.250 0.618 0.0320 0.768 0.3864 7.19 6.59 7.49 7.05 0.082 0.157 0.0143 0.258 0.6944 3.39 3.28 3.53 3.39 17 <0.01 0.264 0.556 0.0308 0.654 0.1503 7.65 5.97 7.78 6.18 0.4 0.033 0.061 0.0056 0.096 0.2522 1.47 1.23 1.49 1.37 Allegan chondrule, 3.7 mg 17 0.01 0.5 0.264 0.518 0.0317 0.841 0.5150 7.44 6.51 7.57 6.74 0.003 0.061 0.0108 0.114 0.2643 2.58 2.30 2.62 2.45 Allegan condrule, 2.8 mg 17 0.02 0.8 0.214 0.373 0.0274 0.562 0.3199 7.83 5.18 8.00 5.49 0.0034 0.045 0.0057 0.088 0.2570 1.73 1.22 1.76 1.41 Allegan chondrule, 2.4 mg 17 0.04 1.0 0.355 0.518 0.0410 0.663 1.2743 6.68 4.38 6.86 4.70 0.063 0.080 0.0084 0.139 0.4270 1.51 1.08 1.55 1.16 Cosmogenic isotope ratios are calculated assuming that all the trapped gas has atmospheric composition. The 81 Kr-Kr exposure ages calculated using these ratios are denoted by “All air.” For comparison “All Kr-Q” ages are also calculated assuming that all trapped gas is Kr-Q. Uncertainties in the isotopic ratios are 1r; uncertainties in the absolute gas amounts are approximately 20%. The indicated concentrations are the sum of all heating steps for 9.2 and 10 mg chondrules. ain 10 12 cm3 STP g 1 . 2438 I. Strashnov and J. D. Gilmour

Table 4. Isotope composition and concentration of cosmogenic krypton of Allegan matrix and its 81Kr-Kr exposure ages. 80 Heating Kr T81(80,82), Myr T81(80,82), Myr Sample power, W 81Kra83Kra 83Kr ≡1 81Kr 82Kr All air All Kr-Q Allegan 9 0.01 0.629 0.0308 0.893 7.51 8.17 matrix, 9.2 mg 0.085 0.0085 0.151 2.25 2.48 13 <0.01 1.456 0.0663 1.378 6.49 7.43 0.3 0.614 0.0354 0.6804 4.06 3.73 Allegan 8 0.01 1.002 0.0427 0.799 6.41 9.67 matrix, 10 mg 0.878 0.0444 0.1032 8.22 8.51 14 <0.01 0.647 0.0356 0.975 6.92 7.91 0.2 0.162 0.0122 0.299 2.78 2.9 Allegan 14 0.01 0.3 0.475 0.0268 0.932 7.97 8.91 matrix, 5 mg 0.085 0.0059 0.194 2.14 2.33 Cosmogenic isotope ratios are calculated assuming that all the trapped gas have atmospheric composition. The 81Kr-Kr exposure ages calculated using these ratios are denoted by “All air.” For comparison “All Kr-Q” ages are also calculated assuming that the all trapped gas is Kr-Q. Uncertainties in the isotopic ratios are 1r; uncertainties in the absolute gas amounts are approximately 20%. The indicated concentrations are the sum of all heating steps for 9.2 and 10 mg chondrules. ain 1012 cm3 STP g1.

(a) the composition of Kr-Q (Busmann et al. 2000) modifies the composition of the cosmogenic component. The effect is to increase the exposure ages by approximately 5% for chondrules and approximately 10% for matrix (which has a higher contribution from trapped krypton). We obtain: 5.97 0.43 Myr (81Kr-78Kr), 5.21 0.44 Myr (81Kr-80+82Kr) for the chondrules and 8.301.34 Myr (81Kr-78Kr) for the matrix. In Fig. 5, we plot CRE ages calculated based on both isotope systems (81Kr-78Kr, 81Kr-80-82Kr) and calculated assuming either air or Kr-Q as the trapped component. r (b) Although there is variation greater than 2 indicating that the matrix is older than the chondrules, evidence for pre-exposure is not compelling. To evaluate the significance of the apparent difference between the two data sets, we consider the errors on the weighted means as the variances of underlying distributions divided by the square root of the number of observations (i.e., as the standard error on the mean) and perform a version of Welch’s t-test using the Welch-Satterthwaite equation to estimate the number of degrees of freedom (Welch 1947). For the ages calculated over an air composition for trapped krypton, this returns a probability of around 37% that the matrix and chondrule data reflect the same

81 underlying distribution (t statistic 1.27, 2 degrees of Fig. 4. Kr-Kr ages determined for seven individual — chondrules separated from the Allegan H5 chondrite (filled freedom 24% if a Kr-Q composition is assumed for the dots). Continuous wave laser heating was used, with one trapped component); the null hypothesis that there is no release for each sample (total fusion) except for two large difference between exposure ages of chondrule and samples (9 mg and 10 mg chondrules) that were step-heated. matrix is thus not falsified based on these data. Polnau For the latter two average ages are also plotted (open dots). 81 81 78 et al. (1999) derived a matrix age of 1.72 0.05 Myr (7 The Kr-Kr ages were derived using a) Kr- Kr and b) r 81Kr-80+82Kr methods (see the text for details). The weighted observations, 1 errors) and a chondrule age of averages are indicated on both figures by the solid lines. The 2.62 0.15 Myr (3 observations, 1r errors) from bulk meteorite value is marked by an asterisk. analyses of a large chondrule and matrix from ALH 81Kr-Kr CRE ages of Allegan chondrules 2439

CONCLUSION

We have demonstrated that useful 81Kr-Kr CRE ages can be obtained from few milligram samples of ordinary chondrites using our new instrument, RIMSKI, opening up the possibility of seeking evidence of pre-exposure in this system from a range of clasts in chondrites. There is clear potential for this technique in the analysis of regolith samples returned from asteroids by current or planned missions and the investigation of possibly varying exposure histories of individual clasts from primitive (and other) meteorites.

Acknowledgments—We thank Dr. M. Schonb€ achler€ and Dr. K. Theis for their help in the sample preparation Fig. 5. 81Kr-Kr exposure ages of Allegan chondrules, matrix, and for communication with the Smithsonian’s National and bulk meteorite. The values are the weighted averages of Museum of Natural History, Washington, DC from the ages determined for the individual chondrules and adjacent which the Allegan USNM215 sample studied in this – 81 78 matrix samples (5 10 mg), derived using both Kr- Kr and study was obtained. This study was funded by the 81Kr-80+82Kr methods. Some samples—especially the matrix— might contain not only trapped air but also Kr-Q. Because STFC, grant number ST/G00306811. Kr-Q fraction is difficult to estimate, the cosmogenic ratios (and 81Kr-Kr ages) have been derived assuming that all 86Kr Editorial Handling—Dr. Ingo Leya is trapped air (filled dots) or all trapped gas is purely Kr-Q (open dots). REFERENCES = 76008 (H6). If we instead test the hypothesis that our Baglin C. M. 1976. Nuclear data sheets for A 81. Nuclear Data Sheets 18:3. data derive from chondrules that are 0.9 0.3 Myr older Baglin C. M. 2008. Nuclear data sheets for A = 81. Nuclear than the matrix (in accord with the Polnau et al. report), Data Sheets 109:2257–2437. the same test yields a probability of around 31% (t Boss A. P. 1996. A concise guide to chondrule formation statistic 1.42, 2 degrees of freedom—21% if a Kr-Q models. In Chondrules and the protoplanetary disk,editedby composition is assumed for the trapped component); Hewins R. H., Jones R. H., and Scott E. R. D. Cambridge, UK: Cambridge University Press. pp. 257–263. the precision of our data is insufficient to rule out the Busmann H., Baur H., and Wieler R. 2000. Primordial noble difference they observe. Applying the same test (to the gases in “phase Q” in carbonaceous and ordinary Polnau et al. data) produces a probability of 3% that chondrites studied by closed-system stepped etching. these were produced from a single distribution, provided Meteoritics & Planetary Science 35:949–973. 81 that all sources of uncertainty have been taken into Eugster O., Eberhardt P., and Geiss J. 1967. Kr in meteorites and 81Kr radiation ages. Earth and Planetary account. Science Letters 2:385–393. It is instructive to consider the precision required to Hohenberg C. M., Nichols R. H., Olinger C. T., and detect the age differences Polnau et al. (1999) report. If we Goswami J. N. 1990. Cosmogenic neon from individual require a 95% confidence level, we would need to measure grains of CM meteorites: Extremely long pre-compaction the exposure ages of chondrules and matrix to a precision exposure histories or an enhanced early particle flux. Geochimica et Cosmochimica Acta 54:2133–2140. of 6.5% to demonstrate the difference they report. Given Leya I. and Masarik J. 2009. Cosmogenic nuclides in stony the ability to measure multiple individual chondrules and meteorites revisited. Meteoritics & Planetary Science multiple matrix samples, it is feasible to reduce the errors 44:1061–1086. arising from 81Kr measurement to this level. The major Leya I., Begemann F., Weber H. W., Wieler R., and Michel other contributory source of error is the need to correct for R. 2004a. Simulation of the interaction of GCR protons with —On the production of 3H and light noble a trapped component, especially in the matrix samples (it is gas isotopes in isotropically irradiated thick gabbro and the dominant error source for these samples in our iron targets. Meteoritics & Planetary Science 39:367–386. analyses). Some form of mineral separation before analysis Leya I., Gilabert E., Lavielle B., Wiechert U., and Wieler R. may be required to achieve this (as employed by Polnau 2004b. Production rates for cosmogenic krypton and 36 36 et al. 1999). Overall, the outcome of this preliminary series argon isotopes in H-chondrites with known Cl- Ar ages. Antarctic Meteorite Research 17:185–199. of analyses is encouraging, especially as we have 81 Marti K. 1967. Mass spectrometric detection of cosmic-ray- demonstrated the ability to measure Kr-Kr exposure produced Kr81 in meteorites and possibility of Kr-Kr ages for individual chondrules. dating. Physical Review Letters 18:264–266. 2440 I. Strashnov and J. D. Gilmour

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