The I-Xe chronometer and its constraints on the accretion and evolution of planetesimals

Transcription

1 Geochemical Journal, Vol. 51, pp. 69 to 80, 2017 doi: /geochemj The I-Xe chronometer and its constraints on the accretion and evolution of planetesimals J. D. GILMOUR* and S. A. CROWTHER School of Earth and Environmental Science, University of Manchester, Manchester M13 9PL, United Kingdom (Received January ; Accepted June 9, 2016) We report a 1.6 ± 2.6 Myr (1s error) I-Xe age of the ungrouped achondrite NWA 7325 relative to the Shallowater standard. We re-evaluate the calibration of the relative I-Xe dating system against the absolute Pb-Pb chronometer in the light of this and other recently reported analyses, and taking into account revisions to the Pb-Pb system, deriving a new absolute age for the Shallowater standard of ± 0.3 Ma. With this calibration, the oldest chondrule I-Xe ages overlap the oldest Pb-Pb chondrule ages and the Pb-Pb ages of Calcium Aluminium-rich inclusions. Literature data for large aliquots of equilibrated ordinary chondrites suggest iodine loss during metamorphic processing and show some evidence that bulk 129 Xe*/I ratios decrease with increasing petrologic type. However, the range of ratios at each petrologic type suggests that thermal evolution was affected by changes in thermal insulation with time, perhaps by impact processing of the parent planetesimals. Literature I-Xe ages for chondrules from Bjurböle (L/LL4) and pyroxene from Richardton (H5) suggest closure shortly after the peak of metamorphism, consistent with a high closure temperature in mafic minerals. The extended range of ages reported for chondrules from the LL3.4 chondrite Chainpur is interpreted as a product of collisional processing of material near the surface of the parent body, and may record a decline in the rate of collisions in the asteroid belt over the first 100 Myr of solar system history. Keywords: extinct radioisotopes, iodine-xenon, chondrites, early solar system, chronology INTRODUCTION Monoisotopic anomalies of 129 Xe were first detected in primitive meteorites in 1960 (Reynolds, 1960). They were proposed to be the decay product of short-lived 129 I. This was the first evidence of a short-lived radioisotope in the early solar system, and represented the birth of a new field: dating early solar system events based on the decay of extinct, short-lived radioisotopes. The origin of the anomalies was confirmed by a correlation between excesses of 129 Xe and 128 Xe that had been produced from stable 127 I by artificial neutron irradiation in a reactor (Jeffery and Reynolds, 1961). The I-Xe dating scheme emerged in close to its current form in Fish and Goles (1962). It was based on sample-to-sample differences in the 129 I/ 127 I ratios on closure to xenon loss as measured by the 129 Xe*/ 128 Xe* ratio (the ratio of radiogenic 129 Xe to 128 Xe produced by artificial neutron irradiation). The I-Xe dating scheme was initially applied in step heating studies of whole rock samples of chondritic meteorites, with results (in the form of isochrones over sev- *Corresponding author ( Jamie.gilmour@manchester.ac.uk) Copyright 2017 by The Geochemical Society of Japan. eral high-temperature releases) that were found puzzling. With improved technology it became possible to establish closure ages of individual components of chondritic meteorites and, eventually, to calibrate the chronometer against the Pb-Pb scheme. However, it remains underutilised. In this paper we review the development of the scheme and the status of the calibration against the Pb- Pb timescale, summarise the constraints it imposes on the evolution of chondritic planetesimals, and suggest how it might be further developed. METHODS At its simplest, the I-Xe system involves subjecting aliquots of a set of samples to the same neutron fluence, typically ~10 19 n cm 2. Atoms of 127 I in the samples capture a neutron to form 128 I, which then decays to 128 Xe with a half life of 25 minutes. After decay of 128 I is complete, each aliquot is analysed by step pyrolysis it is heated to each of a series of increasing temperatures, and the gas evolved at each temperature is measured by noble gas mass spectrometry. Neutron irradiation also produces 131 Xe from 130 Ba and 130 Te, and 131,132,134,136 Xe (4 isotopes) from neutron induced fission of 235 U; these reactions provide information on the chemistry of sites releasing xenon at each temperature step (e.g., Claydon et 69

2 al., 2015). After corrections for fission or spallation components, if present, a correlation is sought between 129 Xe and 128 Xe by examining ratios among these and an isotope not produced from iodine, most often 132 Xe. From such correlations, the 129 Xe*/ 128 Xe* ratio can be calculated. Correlations tend to be observed in high temperature releases; lower temperature steps often contain iodine without its full complement of 129 Xe*, perhaps indicating terrestrial contamination. This can be particularly apparent in analyses of finds. Since the conversion factor between 127 I and 128 Xe depends only on the neutron fluence, the 129 Xe*/ 128 Xe* ratios are related to 129 Xe*/ 127 I ratios by a constant. Thus for sample 1 and sample 2 ( ) ( ) 129 * 128 * Xe Xe sample * 128 * Xe Xe sample 2 ( 129 * 127 * Xe I ) sample 1 = 129 * 127 * ( Xe I ) sample 2 ( ) () = exp -ldt Where Dt 12 is the interval between closure of the I-Xe system in sample 1 and in sample 2, l is the decay constant of 129 I, and the subscripts to the parentheses indicate that the ratios are those measured in the two different samples. In this work it has been assumed that the half life of 129 I is exactly 16.1 Myr based on a reported half life of 16.1 ± 0.07 Myr (Chechev and Sergeev, 2004). The error on the half life introduces a systematic error to all I-Xe ages, and so should not be included when reporting the age of a particular sample. The decay constant is then Myr 1 to three significant figures. Other half lives are used in the literature and where necessary a conversion has been applied. In extracting the 129 Xe*/ 128 Xe* ratio, most commonly a correlation is sought between 129 Xe/ 132 Xe and 128 Xe/ 132 Xe over a series of consecutive steps. In this case the desired 129 Xe*/ 128 Xe* ratio is the gradient of the correlation line. Alternatively, a correlation is sought between 132 Xe/ 129 Xe and 128 Xe/ 129 Xe or 128 Xe*/ 129 Xe. 128 Xe* for any release can be calculated over an assumed underlying trapped composition, such as Q-Xe (Busemann et al., 2000), using the normalising isotope. (In practice corrections for spallation or fission products may need to be made first.) In this case the desired ratio is the reciprocal of the value of 128 Xe/ 129 Xe (or, equivalently, 128 Xe*/ 129 Xe) where 132 Xe/ 129 Xe = 0 on the correlation line; this is informally referred to as the inverse isochron approach, though both correlation lines are isochrones. An illustration of the two approaches applied to a common dataset can be found in the review by Gilmour et al. (2006). In either case, the correlation line should be extracted with an appropriate algorithm that takes into account the covariance between the input ratios for each data point introduced by the common denominator: a York fit (York, 1969) or a maximum likelihood method (Titterington and Halliday, 1979) such as minimization of the sum of the squares of the Mahalanobis distances of the data from the line (Gilmour, 2015). If all the uncertainties were normally distributed, the 129 Xe*/ 128 Xe* ratios and associated errors derived by the two different approaches would agree within the rounding error of the calculation since they are based on the same underlying data. When this is not the case, it is necessary to consider which approach best reports the uncertainty of the measurement. Differences between the two approaches can arise when there are releases that contribute to an isochron that have 132 Xe contributions within error of the blank; that is to say, when there is no compelling evidence of a contribution to 132 Xe from the sample. In these circumstances the conventional isochron approach is invalid. Fitting algorithms are predicated on a normal distribution of the uncertainties around the central value. This is only approximately true for isotope ratios, which are the ratios of quantities that are themselves Poisson distributions approximated by normal distributions. The approximation that isotope ratios are normally distributed breaks down when there is a significant probability that the denominator isotope was not present (Gilmour, 2015). It should also be noted that error bars based on propagated errors do not accurately represent confidence intervals in ratios where the denominator is within error of zero. Equation (1) allows the determinations of relative closure intervals for a set of samples that each had aliquots included in the same irradiation. In order to compare closure intervals across different irradiations, it is necessary to monitor the conversion efficiency from 127 I to 128 Xe in each irradiation. This is achieved by including one or more aliquots of a standard. Ideally, a standard should have a consistent, reproducible 129 Xe*/I ratio that can be determined with high precision from an isochron; this requires a high I/Xe ratio and formation in the first ~10 Myr of solar system history. It should also be available in sufficient quantities to allow consumption through its use in many irradiations. The first standard widely used as an irradiation monitor was whole rock from the L/LL4 chondrite Bjurböle (Hohenberg and Kennedy, 1981). This has now been replaced by aliquots of enstatite from the anomalous aubrite Shallowater, which appears to be more reproducible than Bjurböle when small (<1 mg) quantities are used (Gilmour et al., 2006). For this reason, closure of the system in Shallowater enstatite is adopted as the zero of the I-Xe relative chronometer; I-Xe ages relative to Shallowater enstatite are the end product of any analytical campaign. Brazzle et al. (1999) measured the closure age of Bjurböle whole rock as 0.47 ± 0.15 Myr relative to closure of Shallowater (positive values indi- 70 J. D. Gilmour and S. A. Crowther

3 Fig. 1. I-Xe data for NWA7325. Two aliquots sourced from the consortium study of Weber et al. (2016) were included in irradiation MN14a alongside aliquots of Shallowater enstatite. Both NWA7325 and Shallowater enstatite were analysed by continuous wave laser step heating following the protocol described by Claydon et al. (2015), using the RELAX mass spectrometer (Crowther et al., 2008). Data were corrected for mass discrimination using interspersed analyses of an air standard, and for procedural blank. A correction was made for production of 132 Xe by neutron induced fission of 235 U based on the 134 Xe/ 132 Xe ratio. The first aliquot (1.69 mg) revealed a relatively consistent 129 Xe/ 132 Xe ratio alongside variation in the elemental Xe/I ratio, suggesting the presence of an evolved trapped component, as was observed in analysis of an unirradiated aliquot (Weber et al., 2016; data from this analysis of an unirradiated sample are shown as filled squared). In addition to this, higher temperature analyses of the second aliquot (0.17 mg) demonstrated the presence of an iodine-rich component with a consistent 129 Xe*/ 128 Xe* ratio that corresponded to closure 1.6 ± 2.3 Myr after Shallowater enstatite. (MSWD for the line fitted to NWA7325 was 0.47; for Shallowater, 0.60; the highest release temperature xenon component was consistent with no 132 Xe being released from the sample, necessitating this inverse isochron presentation.) The area between the origin and the dotted line in (a) is shown on an expanded scale in (b), where the correlation lines for the irradiated data and Shallowater are also shown. Data corrected for blank and mass discrimination are presented in Table 1; full data may be obtained from the University of Manchester data repository. Grey diamonds are labelled with potential xenon end members air and Q-Xenon (Busemann et al., 2000). cate earlier closure throughout, so Bjurböle closed earlier than Shallowater), allowing literature ages determined relative to Bjurböle to be directly compared to literature ages determined relative to Shallowater. CALIBRATING THE I-Xe SYSTEM The procedure described in the previous section is sufficient to allow an extensive database of relative closure ages for early solar system materials in the I-Xe system. This is supplemented by two separate and unrelated forms of absolute calibration: determination of the actual 129 Xe*/I ratio in a sample today, which is equivalent to the 129 I/ 127 I ratio on closure; determination of absolute closure ages by calibration against the Pb-Pb chronometer. The 129 I/ 127 I ratio of the early solar system Determining absolute 129 Xe/ 127 I ratios requires independent knowledge of the conversion efficiency of 127 I to 128 Xe. This is best achieved by irradiating a known amount of iodine and measuring the amount of 128 Xe produced. The iodine in samples used for I-Xe dating is typically present in trace concentrations (ppm - ppb); independent (i.e., not relying on an irradiation) determination of such iodine concentrations is challenging. For this reason, absolute calibration has been made by inclusion of potassium iodide in the irradiation so that the iodine content can be determined from the mass of KI irradiated. Macroscopic quantities of KI are required so that accurate mass measurements can be used to determine iodine content. This introduces two complications (Hohenberg and Kennedy, 1981; Hohenberg et al., 2000). Approximately half the conversion of 127 I to 128 Xe in a typical irradiation proceeds via resonant absorption of an epithermal neutron. In a sample of potassium iodide, the amount of iodine is high enough for self-shielding to be important neutrons at the resonant energies are depleted through interaction with iodine in the sample, effectively reducing the flux in parts of the KI aliquot below that The iodine-xenon chronometer and planetesimals 71

4 Table 1. Xenon isotope data for analysis of two aliquots of the ungrouped achondrite NWA7325. All errors are 1s. Sample mass (mg) Laser current (A) Atoms 129 Xe ( cc STP g -1 ) 124 Xe/ 129 Xe 126 Xe/ 129 Xe 128 Xe/ 129 Xe 130 Xe/ 129 Xe 131 Xe/ 129 Xe 132 Xe/ 129 Xe 134 Xe/ 129 Xe 136 Xe/ 129 Xe n.d. 0.1 (2) n.d. n.d. 0.9 (8) 0.6 (9) 0.8 (8) n.d. n.d n.d. 0.0 (6) n.d. n.d. n.d. 3 (7) 4 (7) 0 (2) n.d n.d. 0.1 (3) 0.1 (2) n.d. 0.4 (7) n.d. 0.6 (8) 0.6 (8) 0.2 (5) (1) 0.00 (3) n.d. 100 (10) n.d. 11 (1) 0.02 (7) 0.01 (6) 0.01 (5) (9) 0.03 (7) 0.03 (7) 80 (20) n.d. 50 (10) 0.5 (3) n.d. n.d (8) 0.6 (9) 0.2 (1) 90 (30) n.d. 120 (40) 1.7 (6) n.d. 0.2 (2) (2) n.d (8) 15 (1) 0.03 (2) 32 (2) 1.03 (9) 0.29 (4) 0.21 (3) (3) n.d (3) 4.6 (2) 0.12 (2) 9.4 (4) 0.89 (5) 0.56 (4) 0.49 (3) (2) (1) (2) 1.93 (4) (6) 1.72 (3) 0.83 (2) 0.53 (1) 0.44 (1) (1) 0.00 (1) 0.02 (2) 0.79 (9) 0.17 (5) 0.9 (1) 0.75 (9) 0.40 (7) 0.36 (6) (1) n.d. n.d. 1.0 (2) 0.08 (8) 0.9 (2) 0.6 (1) 0.3 (1) 0.4 (1) (1) (5) n.d (4) 0.11 (2) 0.85 (5) 0.72 (4) 0.41 (3) 0.33 (3) (1) (6) (6) 0.99 (6) 0.06 (2) 0.75 (5) 0.46 (4) 0.24 (3) 0.23 (2) (7) n.d. n.d. 2 (2) n.d. 2 (2) 1 (1) 0.1 (6) 0.0 (5) (2) (5) n.d (7) 0.06 (2) 0.77 (5) 0.31 (3) 0.13 (2) 0.11 (2) (1) 0.02 (2) n.d. 1.5 (2) 0.11 (6) 0.7 (1) 0.12 (7) 0.17 (7) 0.14 (6) Total 376 (5) experienced by the aliquots of other samples. Secondly, irradiation of such KI samples produces amounts of 128 Xe vastly in excess of those that can be measured in a noble gas mass spectrometer, requiring dilution procedures before analysis, and the dilution process is also capable of introducing systematic errors. By addressing these issues, Hohenberg and Kennedy (1981) determined that the absolute initial iodine ratio of Bjurböle whole rock was (1.095 ± 0.029) Taking into account the age of Bjurböle relative to Shallowater and the half life of 129 I, the 129 I/ 127 I ratio for Shallowater is (1.07 ± 0.03) 10 4 (Brazzle et al., 1999), where the error is dominated by the uncertainty in the 129 I/ 127 I ratio of Bjurböle. Converting I-Xe ages to absolute ages Converting relative I-Xe ages into absolute ages is an entirely separate problem, which requires the determination of both an I-Xe age and an absolute age in at least one sample where it is reasonable to think that they date the same event. As for other short-lived radioisotopes, the absolute chronometer based on lead isotopes produced by decay of 235 U and 238 U is the only one with precision comparable to the I-Xe system. No Pb-Pb age has been determined for Shallowater enstatite, but its equivalent age in the Pb-Pb system can be determined from samples that have I-Xe ages determined relative to Shallowater and Pb-Pb ages. The first samples to yield ages in both systems were phosphate grains from Acapulco (Nichols et al., 1994) and ordinary chondrites (Brazzle et al., 1999). Gilmour et al. (2006) supplemented these data with other candidates from the literature and, by comparing Pb-Pb ages with I-Xe ages across a range of candidate samples, proposed an absolute age for the Shallowater standard. Gilmour et al. (2009) revisited this in the light of new data and, using likelihood to exclude outliers, proposed an absolute age for the Shallowater standard of ± 0.4 Ma. Since this publication, a correction has been made to the Pb-Pb chronometer. It was recognised that decay of 247 Cm required a revision of the 235 U/ 238 U ratio used to calculate ages (Brennecka et al., 2010). Further work has shown significant variation in the 235 U/ 238 U ratio of chondrites, including those on which the I-Xe calibration was based (Goldmann et al., 2015). Pravdivtseva et al. (2016) incorporated the effects of these changes and, including a new datum for a chondrule from the CB chondrite Hammadah al Hamra (HaH) 237, proposed an absolute Pb-Pb age for the Shallowater standard of ± 0.2 Ma. This calibration can be considered in the light of new I-Xe and Pb-Pb data. The earliest Pb-Pb chondrule ages ( ± 0.2, 1s error) are now contemporaneous with CAI formation (Connelly et al., 2012) and, as originally proposed by Gilmour et al. (2006), we consider them comparable with the earliest I-Xe chondrule age (Swindle et al., 1991b). Two chips (1.6 mg and 4.6 mg) of the anomalous eucrite Ibitira yielded a whole rock I-Xe age of 7 ± 1 Myr (Claydon, 2012), which can be compared to a Pb-Pb age of ± 0.6 (1s error, Iizuka et al., 2014). Finally, the ungrouped achondrite NWA 7325 (Irving et al., 2013; Weber et al., 2016) has been reported to have a Pb-Pb age of ± 1.3 (1s error, Koefoed et al., 2016). In Fig. 1, and in Table 1, we present I-Xe data for aliquots of this sample, which yields an I-Xe age of 1.6 ± 2.3. In Table 2, which extends table 2 of Pravdivtseva et al. (2016), we summarise the data potentially available for calibration of the I-Xe system as originally proposed by Gilmour et al. (2006). In this approach: absolute Pb- Pb ages are plotted against I-Xe ages relative to Shallowater; a line is fit through the data; the intercept of the line corresponds to the absolute age of Shallowater, 72 J. D. Gilmour and S. A. Crowther

5 Table 1. (continued) Sample mass (mg) Laser current (A) Atoms 129 Xe ( cc STP g -1 ) 124 Xe/ 129 Xe 126 Xe/ 129 Xe 128 Xe/ 129 Xe 130 Xe/ 129 Xe 131 Xe/ 129 Xe 132 Xe/ 129 Xe 134 Xe/ 129 Xe 136 Xe/ 129 Xe n.d. 0.1 (5) n.d. n.d. n.d. n.d. 2 (3) 1 (2) n.d n.d. n.d. 0.0 (1) n.d. n.d. 0.3 (5) 0.5 (4) 0.3 (3) 0.4 (3) (1) n.d (2) 67 (6) n.d. 12 (1) 0.27 (7) 0.07 (5) 0.08 (4) (3) n.d. n.d. 56 (5) n.d. 24 (2) 0.41 (7) 0.09 (3) 0.05 (3) (3) 0.03 (2) 0.03 (2) 56 (5) n.d. 48 (5) 0.7 (1) 0.06 (4) 0.00 (3) (2) n.d (2) 63 (8) n.d. 139 (17) 1.6 (3) n.d (4) (2) 0.02 (3) n.d. 53 (7) n.d. 165 (22) 1.5 (3) 0.04 (6) 0.04 (5) (2) 0.03 (5) 0.06 (4) 60 (10) n.d. 228 (37) 2.2 (4) 0.03 (9) 0.2 (1) (2) n.d. n.d. 60 (10) 0.4 (1) 219 (35) 2.4 (4) 0.09 (8) n.d (4) 0.02 (1) (7) 23 (2) 0.08 (3) 57 (4) 1.2 (2) 0.22 (4) 0.21 (4) (2) n.d (4) 60 (10) 0.1 (1) 130 (26) 1.8 (4) n.d. 0.1 (1) (2) n.d (3) 52 (8) n.d. 118 (19) 2.1 (4) 0.3 (1) 0.18 (9) (2) 0.02 (3) 0.00 (2) 32 (4) n.d. 103 (14) 1.8 (3) 0.19 (8) 0.16 (7) (2) n.d (1) 24 (2) n.d. 63 (6) 1.3 (2) 0.25 (6) 0.19 (5) (2) 0.02 (1) 0.05 (3) 16 (1) 0.13 (4) 39 (3) 1.1 (1) 0.26 (5) 0.22 (4) (2) 0.01 (1) n.d. 16 (1) 0.12 (3) 28 (2) 0.95 (8) 0.30 (4) 0.25 (4) (1) n.d (7) 12.4 (6) 0.09 (2) 16.2 (8) 0.97 (6) 0.36 (3) 0.30 (3) (2) 0.01 (1) n.d. 9.1 (6) 0.14 (3) 16 (1) 0.83 (8) 0.34 (4) 0.29 (4) (2) (7) n.d. 6.8 (3) 0.10 (2) 13.0 (6) 0.82 (6) 0.32 (3) 0.28 (3) (1) (6) (6) 5.1 (2) 0.10 (2) 7.9 (3) 0.86 (5) 0.38 (3) 0.31 (3) (2) (7) (4) 5.8 (3) 0.11 (2) 24 (1) 0.97 (7) 0.35 (4) 0.33 (3) (4) (5) (4) 6.3 (3) 0.03 (1) 18.8 (8) 0.81 (5) 0.37 (3) 0.32 (3) (6) (2) (2) 6.8 (2) (8) 10.9 (3) 0.76 (3) 0.37 (2) 0.32 (2) (5) (4) (3) 6.3 (3) 0.08 (1) 10.1 (4) 0.87 (5) 0.46 (3) 0.41 (3) (4) (3) (2) 4.6 (2) 0.10 (1) 6.9 (2) 0.81 (4) 0.43 (2) 0.35 (2) (4) (3) (3) 4.4 (2) 0.10 (1) 6.2 (2) 0.79 (4) 0.44 (3) 0.36 (2) (3) (3) (2) 4.3 (1) (9) 5.9 (2) 0.79 (3) 0.45 (2) 0.40 (2) (3) n.d (2) 3.9 (1) (8) 4.3 (1) 0.84 (3) 0.47 (2) 0.40 (2) (2) (2) (2) 3.65 (9) (7) 4.25 (9) 0.79 (2) 0.47 (2) 0.42 (1) (2) (1) (1) 2.82 (6) (6) 2.83(5) 0.83 (2) 0.58 (1) 0.51 (1) (2) (2) (2) 2.68 (8) (9) 2.70 (8) 0.79 (3) 0.53 (2) 0.47 (2) (2) (3) (3) 2.49 (8) 0.09 (1) 2.37 (7) 0.76 (3) 0.45 (2) 0.39 (2) (2) n.d (4) 2.9 (1) 0.07 (1) 3.2 (1) 0.70 (4) 0.44 (3) 0.37 (3) (2) n.d (6) 2.5 (1) 0.06 (2) 2.3 (1) 0.74 (5) 0.45 (4) 0.40 (3) (2) (4) n.d. 3.0 (1) 0.09 (2) 2.7 (1) 0.74 (4) 0.48 (3) 0.44 (3) (2) (2) (2) 3.07 (7) (6) 3.80 (7) 0.92 (2) 0.65 (2) 0.56 (1) (3) (1) (7) 3.22 (5) (4) 3.54 (4) 0.96 (1) 0.66 (1) (9) (2) (3) (3) 2.62 (9) 0.10 (1) 2.55 (9) 0.81 (3) 0.50 (3) 0.41 (2) (2) n.d (3) 2.42 (8) 0.11 (1) 1.77 (6) 0.75 (3) 0.41 (2) 0.35 (2) (3) (2) (1) 2.51 (6) (7) 1.77 (4) 0.73 (2) 0.38 (1) 0.33 (1) (2) (2) (2) 2.31 (6) (8) 1.43 (4) 0.69 (2) 0.38 (2) 0.36 (1) (3) (1) (1) 2.35 (5) (5) 1.75 (3) 0.63 (1) 0.31 (1) (9) (2) (3) n.d (6) 0.09 (1) 1.30 (5) 0.64 (3) 0.29 (2) 0.23 (1) (2) (6) n.d (9) 0.09 (2) 1.09 (7) 0.59 (4) 0.25 (3) 0.23 (3) (1) 0.02 (1) n.d. 2.1 (2) 0.16 (4) 1.7 (2) 0.74 (8) 0.29 (5) 0.28 (5) (2) (1) (2) 0.96 (2) (6) 0.73 (2) 0.64 (1) (9) (7) (2) (4) n.d (7) 0.12 (2) 0.89 (5) 0.67 (4) 0.24 (2) 0.22 (2) (1) 0.00 (1) 0.00 (1) 2.3 (2) 0.05 (3) 1.2 (1) 0.65 (6) 0.31 (4) 0.22 (3) (1) (7) (5) 1.00 (6) 0.09 (2) 0.84 (5) 0.69 (4) 0.27 (3) 0.22 (2) (2) (1) (1) 1.01 (3) (6) 0.94 (2) 0.67 (2) 0.27 (1) (8) (1) 0.01 (4) 0.02 (3) 1.1 (2) 0.17 (9) 1.0 (2) 0.6 (1) 0.4 (1) 0.33 (9) (1) 0.02 (2) n.d. 1.0 (1) 0.17 (5) 1.0 (1) 0.8 (1) 0.37 (7) 0.24 (5) Total 358 (2) and the gradient of the line tests whether ages are varying concordantly. This approach tests the assumptions of contemporaneous closure across a range of samples rather than relying on an argument based on the petrologic history of a single sample. In light of the larger dataset now available, we do not include I-Xe ages that were derived via the Mn-Cr chronometer in the original publication (Gilmour et al., 2006). When using the dataset of Pravidivtseva et al. (2016) we derive an age for the Shallowater standard of ± 0.2 Ma (consistent with their ± 0.2 Ma within a rounding error), a gradient of 1.01 ± 0.12 (their 1.01 ± 0.11) and MSWD = 2.2. For the complete dataset of our Table 2 we obtain a Shallowater age of ± 0.2 Ma, a gradient of 1.02 ± 0.09, and an MSWD = 2.4. In each case, the MSWD indicates more scatter than would be expected based on reported measurement errors, indicating a significant breakdown in the assumption that the The iodine-xenon chronometer and planetesimals 73

6 Table 2. Candidate samples for calibration of the I-Xe system (extended from Pravdivtseva et al. (2016)) Sample I-Xe age Pb-Pb age M2 b M2 c (Myr) (Ma) a Earliest chondrule 4.3 ± 0.6 Swindle et al. (1991b) ± 0.2 Connelly et al. (2012) Richardton Px 1.1 ± 2.0 Pravdivtseva et al. (1998) ± 0.6 Amelin (2001) Richardton chondrule ± 0.1 Gilmour et al. (2006) ± 0.8 Gilmour et al. (2006) Richardton chondrule ± 0.6 Gilmour et al. (2006) ± 1.4 Gilmour et al. (2006) Acapulco Fspar -3.8 ± 1.5 Brazzle et al. (1999) ± 0.0 et al. (1994) Ste Marguerite 0.7 ± 0.4 Brazzle et al. (1999) ± 0.3 et al. (1994) 7.2 æ Kernouve phosphate 43.3 ± 6.0 Brazzle et al. (1999) ± 0.7 et al. (1994) HaH 237 chondrule -0.3 ± 0.2 Pravdivtseva et al. (2016) ± 0.2 Bollard et al. (2015) Ibitira chip -7.0 ± 1.0 Claydon (2012) ± 0.3 Iizuka et al. (2014) NWA 7325 chip -1.6 ± 2.3 This work ± 1.3 Koefoed et al. (2016) a Recalculated where necessary using the uranium isotope date of Goldmann et al. (2015). b Sum of squares of the Mahalanobis distances relative to the best fit line to all data. c Sum of squares of the Mahalanobis distance relative to the best fit line excluding Ste Marguerite. Fig. 2. The relationship between I-Xe closure age (relative to Shallowater enstatite) and Pb-Pb age (after Gilmour et al., 2006, 2009; Pravdivtseva et al., 2016). Data are listed with sources in Table 2. Pb-Pb ages have been recalculated based on the uranium isotope data of Goldmann et al. (2015) and I-Xe ages have been scaled to a 129 I half-life of 16.1 Myr. The dashed line corresponds to the best fit line to all data excluding Ste Marguerite (see text). The box with a dotted line (a) is the area shown on an expended scale in (b). All errors are 1s. two systems closed at the same time. This in turn suggests that the derived uncertainties should be increased by a factor of at least 2. Closer inspection reveals that the Ste Marguerite datum has a squared Mahalanobis distance of 7.2 from the best fit line (Table 2) the next highest value is for Richardton pyroxene (2.7). The Ste Marguerite datum relies on associating the Pb-Pb age of phosphate with the I-Xe age of feldspar, and there is evidence that the I-Xe and Pb-Pb systems have different closure characteristics in these two minerals (Crowther et al., 2009). We therefore exclude this data point from the fit, leading to an MSWD of 1.5 for the fit to the remaining points. Rather than proceed with further outlier rejection, we scale uncertainty for the intercept and gradient by the MSWD, leading to a best age for the Shallowater standard of ± 0.3 (1s). The associated gradient is 1.02 ± The covariance between the uncertainties on gradient and intercept, which should be considered if using this calibration to derive equivalent Pb-Pb ages, is This fit and the data discussed are displayed in Fig. 2. Further sequential rejection of the outlier with the greatest squared Mahalanobis distance would lead to a Shallowater age of ± 0.2 (MSWD = 1.13), then 74 J. D. Gilmour and S. A. Crowther

7 Fig. 3. In (a) iodine concentration shows a decreasing trend in ordinary chondrites from type 3 (highest) to types 5 and 6, where there is considerable overlap (after Gilmour, 2000). This is as expected for a volatile element that is lost during parent body processing (Jordan et al., 1980). In a simple, onion shell model of instantaneous accretion of a spherical body that then evolves thermally due to 26 Al decay and conductive heat loss, higher peak temperatures (higher petrologic type) are associated with longer cooling timescales. So, in such a model, the ratio of 129 Xe* ( 129 Xe produced by 129 I decay) to iodine should correlate with petrologic type. In (b), there is some evidence of such behaviour across types 4 6, but there is a significant degree of overlap especially between types 4 and 5 suggesting excavation or disruption affected cooling rates on the parent body. Data are tabulated in Table 3, and all errors are 1s ± 0.2 (MSWD = 0.74). As discussed by Pravdivtseva et al. (2016), the proximity of the gradient of the free fit correlation line to unity indicates consistency among the half lives of the radioisotopes to within the tolerance of the fit. In the revised Pb-Pb chronology the age of CAI formation is ± 0.2 (Connelly et al., 2012), which corresponds to an initial iodine ratio 129 I/ 127 I ª It is interesting to compare this to an average galactic background at the time of solar system formation, a calculation first done by Wasserburg et al. (1960). 129 Xe is predominantly an r-process isotope, and thus has been produced via 129 I. Since the age of the galaxy at solar system formation was around 9 Gyr, approximately one part in 10 4 of the current 129 Xe budget was produced over each million years of galactic history before solar system formation. The 129 Xe/I ratio of average solar system material is ª1 (Anders and Grevesse, 1989), so the rate of change of this ratio at the time of solar system formation was ª10 4 Myr 1. This is related to the 129 I abundance at this time by the decay constant, so we have l( 129 I/ 127 I) = 10 4, where l is the decay constant of 129 I as above. This leads to a galactic average 129 I/ 127 I ratio at solar system formation of To reach the initial solar system value would require a free decay period of around 70 Myr. The star formation rate has declined with time over the lifetime of the galaxy, so the supernova rate and the rate of production of r-process isotopes will also have declined, so the production rate of 129 I in the recent history of material from which the solar system was formed was lower, and so this interval is an overestimate. Nonetheless, the calculation demonstrates that there is no requirement for a significant input of 129 I shortly before solar system formation, and thus no grounds to expect heterogeneity of the 129 I/ 127 I ratio across the solar system. Homogeneous distribution of the parent isotope is a requirement before variations in its relative abundance from sample to sample can be interpreted chronologically. THE I-Xe CHRONOLOGY OF THE EARLY SOLAR SYSTEM Bulk rock samples of ordinary chondrites The iodine-xenon system is unusual in that both parent and daughter are isotopes of extremely volatile elements. However, although iodine is severely depleted in the solid phase relative to solar abundances, xenon is even more depleted; based on Anders and Grevesse (1989) the solar 132 Xe/ 127 I ratio is 1.4, whereas in the CI chondrite Orgueil it is Even the ultra-trace amounts of iodine incorporated into mafic minerals at high temperature can thus yield measurable excesses of 129 Xe from 129 I decay. Bulk rock examinations of the chronometer were thus possible for the I-Xe system but impractical for other short-lived chronometers. This allowed I-Xe The iodine-xenon chronometer and planetesimals 75

8 Fig. 4. Age spectrum for chondrules from Bjurböle (dashed line, 21 data from Caffee et al. (1982) and Gilmour et al. (1995)) and Chainpur (solid line, 26 data from Swindle et al. (1991a) and Holland et al. (2005)). Each chondrule age is represented by a normalised Gaussian corresponding to the mean and standard deviation. These are summed together and displayed vs. closure age after CAIs. The small range of ages for Bjurböle chondrules is suggestive of resetting during post-metamorphic cooling. In contrast, the wider range exhibited by Chainpur chondrules suggests chondrule formation may be recorded by the earliest sample (i.e., it was not reset during parent body metamorphism) and collisional modification of the LL chondrite parent body s regolith may have reset individual chondrule ages over a period of ~100 Myr after CAI formation. The vertical broken line indicates the closure age of the higher temperature Xe-P3 sites in an Efremovka nanodiamond-rich separate, and the arrows indicate that lower temperature Xe-P3 sites were degassed still more recently (Gilmour et al., 2016). A double headed arrow labelled FZ indicates the forbidden zone for the I-Xe system during which (in the simple model described in the text) chondrite parent bodies were heating up due to 26 Al decay so the onset of xenon retention is not expected. Whether ages occur within the forbidden zone can be seen as a test of the simple model. correlations to be observed in high temperature analyses of whole rock chondrite samples that typically did not correlate with petrologic type, as was expected at the time. Historically, this led to low confidence being placed in the results of the I-Xe system, partly because the parent isotope is volatile and partly because the data based on high temperature isochrones did not confirm the expectation that more metamorphosed samples would have later closure ages. However, ratios between total 129 Xe* and total iodine based on the data from these early studies are worth further attention. An age calculated from such a ratio would be of dubious significance in itself. But the ratio does reflect the fraction of the initial 129 I incorporated into a parent body on formation that was retained as 129 Xe* at the sample s location. In an onion shell model for a planetesimal that accreted rapidly and was heated by 26 Al decay, the ratio is expected to be lower in samples that were better insulated from the surface, where heat is radiated away. Such samples reached higher peak temperatures than, and cooled later through any specified temperature than, less deeply buried material. If this model is correct there should, therefore, be a correlation; samples with higher petrologic type should have lower bulk 129 Xe*/I ratios. In Fig. 3 the iodine concentrations and 129 Xe*/I ratios for L and H chondrites are plotted against petrologic type. Bjurböle (L/LL4) samples yield 129 Xe*/I ratios of 0.92, 0.83 and 0.69 in three separate studies (Jordan et al., 1980; Shukolyukov et al., 1986; Drozd and Podosek, 1976); this shows that, even for analyses of these relatively large amounts of material, heterogeneity between aliquots of the same sample is the dominant source of uncertainty. For this reason the centre and range of the reported values are taken as indicating the uncertainty on the ratio at each petrologic type. Data are sparse, and it is unlikely that further data on such large aliquots will be produced (the smallest aliquot is 54 mg), but there is a suggestion of a trend. As petrologic type increases from type 4 to 6, the maximum observed 129 Xe*/I ratios decreases. Against this trend, the spread of 129 Xe*/I ratios is larger than might be expected simply from sample heterogeneity. To summarise, there is a first order suggestion that more metamorphosed samples tended to cool through closure temperatures for iodine-xenon more recently than less metamorphosed sam- 76 J. D. Gilmour and S. A. Crowther

9 Table 3. Literature data for whole rock analyses of chondrites, plotted in Fig. 3 Meteorite Class Mass (/g) 129 Xe*/I (/10-4 ) a,b Iodine (/ppb) a,b Source reference Beaver Creak H Jordan et al. (1980) Menow H Jordan et al. (1980) Ambapur Nagla H Jordan et al. (1980) Nadiabondi H Jordan et al. (1980) Kernouve H Jordan et al. (1980) Rakiry L Shukolyukov et al. (1986) L/LL Jordan et al. (1980) L/LL Shukolyukov et al. (1986) L/LL4 (not reported) Drozd and Podosek (1976) L Shukolyukov et al. (1986) Saratov L Shukolyukov et al. (1986) Arapahoe L5 (not reported) Drozd and Podosek (1976) Ausson L Jordan et al. (1980) Khmelevka L Shukolyukov et al. (1986) Tsarev L Shukolyukov et al. (1986) Peetz L Jordan et al. (1980) Stavropol L Shukolyukov et al. (1986) Soko Banja LL Bernatowicz et al. (1988) LL Bernatowicz et al. (1988) Guidder LL Bernatowicz et al. (1988) Olivenza LL Bernatowicz et al. (1988) Tuxtuac LL Bernatowicz et al. (1988) St. Severin LL Podosek (1970) a Data were corrected for a neutron-induced fission component based on 134 Xe/ 132 Xe. 129 Xe* (excess 129 Xe) was then calculated over 129 Xe/ 132 Xe = 1.02 (corresponding to the solar ratio mass fractionated as in the Q-Xe model of Crowther and Gilmour, 2013). 128 Xe* (excess 128 Xe) was calculated over 128 Xe/ 132 Xe = , and converted to 127 I based on the data reported from analyses of standard material in the same irradiation. b Aliquot-to-aliquot variability is the major source of uncertainty, as demonstrated in Bjurböle, so errors are not reported. ples, but the spread of 129 Xe*/I ratios observed within samples of type 4 and 5 suggests a more complex scenario than simple cooling in an onion shell parent body. The data suggest that some samples cooled more rapidly that their petrologic type might predict, perhaps indicating a role for collisional excavation. Against this broad trend, the one measurement for a type 3 ordinary chondrite appears anomalous. It is possible that a different mechanism was responsible for resetting the system in type 3 material, but this cannot be concluded based on a single datum. We now investigate this further. The I-Xe system on a smaller scale Chondritic material is an assembly of individual clasts and grains that has been modified by parent body processing such as thermal metamorphism and/or aqueous alteration. Each clast or grain had a distinct pre-accretionary history. Since the 1980s, advances in technology have made it possible to study the I-Xe system in these individual clasts, chondrules and mineral separates. Chondrules are widespread in chondritic meteorites and clearly formed before accretion, and some have retained their integrity through parent body processing to the present day. Statistical analysis of chondrule ages can provide insight into the probable most recent cause of closure of the I-Xe system. In Fig. 4 we present a summary of the chondrule age spectrum from two meteorites where there are sufficient published analyses for meaningful statistics: Bjurböle L/ LL4 (Caffee et al., 1982; Gilmour et al., 1995) and Chainpur LL3 (Swindle et al., 1991a; Holland et al., 2005). The reproducibility of the high temperature isochron from Bjurböle whole rock at the 10s of milligram scale is reflected in the relatively tight distribution of chondrule ages. In contrast, the Chainpur chondrule ages span more than 60 million years, and range from significantly before Bjurböle to long after. As hinted at by the whole rock data of Fig. 3, the difference between these distributions suggests different processes reset the iodine-xenon system in type 3 material and in type 4 6 material. We can identify two processes that might be recorded in the I-Xe system of a chondrule in type 4 6 material: chondrule formation, and cooling after the peak metamorphic temperature. Of necessity, chondrule formation preceded accretion of the parent body (or, at least, that part of the parent body sampled by the meteorite that the chondrule was selected from), whereas cooling after peak metamorphism requires that the 26 Al incorporated into the The iodine-xenon chronometer and planetesimals 77

10 parent body as it formed had substantively decayed. Assuming (as above) that I-Xe ages record cooling through a closure temperature, there is thus a forbidden zone in the history of a chondritic sample in which we would not expect to find I-Xe ages in this simple model; it is indicated in Fig. 4. The timing and length of this forbidden zone can be estimated by noting that the peak temperature required for metamorphism of chondrites requires elevation of the accreted material to a temperature of ~900 C (Slater-Reynolds and McSween, 2005). This requires accretion with an initial 26 Al/ 27 Al ratio of ~10 5. If the 26 Al concentration was homogeneous between the formation region of CAIs and chondrules, accretion around 2 3 half-lives after CAI formation is required. This 26 Al then generates heat over a period of 2 3 half lives, during which time temperature is increasing and closure of the I-Xe system is precluded. The forbidden zone thus starts around ~2 Myr after CAI formation, and extends for ~2 Myr. It is illustrated in Fig. 4. The earliest Chainpur chondrule significantly predates the peak of the Bjurböle distribution (and its earliest extent), and may record chondrule formation. It is reasonable to associate the Bjurböle chondrule ages with cooling after the peak of metamorphism. As an L/LL4 chondrite, Bjurböle shows evidence of thermal metamorphism which, as discussed above, requires the parent body to have accreted within 2 3 Myr of CAI formation. So, the peak of the chondrule age distribution at 5 Myr cannot date chondrule formation, which must have occurred before accretion of the parent body. The narrow width of the age distribution also suggests setting/resetting of the I-Xe system by a process that, like post-metamorphic cooling, affected the rock as a whole. It is interesting to compare ages from the I-Xe system with thermal evolution models such as those of Gail et al. (2015). In these models, type 4 material evolved through its peak temperature around 5 Myr after the time at which 26 Al/ 27 Al = , consistent with the closure ages of the I-Xe system in Bjurböle. This would suggest closure temperatures for I-Xe in Bjurböle are close to this peak temperature, i.e., in the region of 800 K. The I-Xe age of pyroxene from the H5 chondrite Richardton indicates closure at 1 ± 2 Myr relative to Shallowater (Pravdivtseva et al., 1998); again, this is too late to be consistent with chondrule formation before accretion of the parent body. It is close to the time of peak metamorphic temperature in the model of Gail et al. (2015), indicating closure at ~1000 K. The absolute I-Xe ages of Richardton samples are in good agreement with the Pb- Pb ages of mafic minerals (Table 2), and substantially earlier than the phosphate ages of Amelin et al. (2005: ± 2.6 once corrected using the uranium isotope data of Goldmann et al., 2015), suggesting that the closure temperature of the system in mafic minerals is relatively high. The spread in ages of Chainpur chondrules is not consistent with cooling after metamorphism. To account for it we seek a process capable of resetting the I-Xe system in chondrules on a parent body, which we assume requires an input of energy. It must also be possible to account for the observation that chondrules that are in close proximity in the host meteorite were reset at times separated by tens of millions of years, as found in the studies of Holland et al. (2005) and Swindle et al. (1991a). We propose that collisions with the parent body of the LL3 chondrites are the most plausible energy source for resetting the I-Xe system in Chainpur chondrules. The close proximity of adjacent chondrules with different ages may reflect gardening of the surface that brought together material that had different histories over the first ~100 Myr of the solar system before lithification. Alternatively, it may reflect resetting of the I-Xe system due to the local focusing of energy from nearby impacts as in the model of Bland et al. (2014). Specifically, we suggest that impact-related effects, such as they propose, locally mobilised small amounts of fluid in the matrix that led to the formation of secondary minerals in adjacent chondrules. If this model is correct, further study should be able to correlate the I-Xe system with evidence of secondary processing in specific chondrules. Furthermore, if impact processing of the regolith is the cause of resetting of the I-Xe system in Chainpur, the chondrule age spectrum may record a declining impactor flux in the early solar system; further studies of suites of chondrules from primitive meteorites may confirm this interpretation each parent body should have experienced an impactor flux declining at the same rate. More evidence of late resetting of primitive material may be found in the I-Xe system of meteoritic nanodiamonds (Gilmour et al., 2016). The low temperature release component of xenon, Xe-P3, has been degassed from nanodiamonds to extents that depend on the degree of parent body processing (Huss and Lewis, 1994). Iodine with live 129 I was present alongside this component, but the concentration of 129 Xe* in the separates is more uniform than the concentration of Xe-P3 (Gilmour et al., 2005; Gilmour, 2010); it was retained through the degassing process that led to loss of Xe-P3. This can only be explained if it was present as 129 I during degassing, so the corresponding initial iodine ratio corresponds to the end of degassing. The concentration of 129 Xe* in nanodiamond-rich separates across a range of the most primitive meteorites is consistent, indicating a common average closure time as might be expected for a decline in collisions across the asteroid belt as a whole. Since nanodiamonds are dispersed through the matrix of the meteoritic material, and since the concentration of xenon is much less than one xenon atom per nanodiamond, the initial iodine ratio (and the concentration of 129 Xe*, which is a proxy for the initial iodine ratio) represents an 78 J. D. Gilmour and S. A. Crowther

11 average over the low-temperature resetting history of the material. Assuming concordant evolution of 129 I/ 127 I in nanodiamonds and in the rest of the solar system, the highest release temperature sites that host Xe-P3 degassed ~30 Myr after solar system formation (shown in Fig. 4), with evidence of degassing of an even less robust site continuing for a further 50 Myr (Gilmour et al., 2016). This lends support to the proposal that primitive material was subject to extended processing over the first ~100 Myr and that this is recorded in the I-Xe system. CONCLUDING REMARKS After more than 50 years, the I-Xe system has proved itself capable of providing insights into the first ~100 Myr of solar system evolution. It indicates a relationship between closure age and peak metamorphic temperature, but departures from this simple relationship suggest that the situation was more complex than instantaneous accretion and thermal evolution powered by 26 Al decay with conductive heat loss. Changes in thermal insulation associated with impacts into parent planetesimals may be implicated. Statistics of chondrule spectra show evidence of impact processing of parent bodies over the first ~100 Myr of solar system history, and this is supported by the I-Xe system in meteoritic nanodiamonds. The I-Xe and Pb-Pb systems reveal concordant ages across a range of samples, allowing intercalibration in which the absolute age of the Shallowater enstatite standard is ± 0.3 Ma. To fully realise the potential of this system to constrain the thermal evolution of asteroids independent measurements of closure temperatures of meteoritic minerals would be invaluable. Acknowledgments This work was funded by the Science and Technology Facilities Council, UK, grants ST/M001253/1 and ST/J001643/1. We thank the Institut für Planetologie, Universität Münster, for access to samples of NWA 7325 as part of the consortium study published as Weber et al. (2016). We thank T. Iizuka and O. V. 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