Iodine-xenon analysis of Chainpur (LL3.4) chondrules

Geochimica et Cosmochimica Acta, Vol. 69, No. 1, pp. 189-200, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 ⫹ .00

doi:10.1016/j.gca.2004.06.021

Iodine-xenon analysis of Chainpur (LL3.4) chondrules G. HOLLAND,1,* J. C. BRIDGES,2,† A. BUSFIELD,1 T. JEFFRIES,2 G. TURNER,1 J. D. GILMOUR1 1

School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, M13 9PL UK 2 Department of Mineralogy, Natural History Museum, London, SW7 5BD UK (Received October 21, 2003; accepted in revised form June 14, 2004)

Abstract—We present I-Xe analyses of ten chondrules from Chainpur LL3.4 by IR laser-stepped heating. Five chondrules provided isochrons of varying quality, giving a range of ages from 0.5 Ma before Shallowater to 17.8 after Shallowater. This confirms the extended range of Chainpur chondrule ages determined by previous data. We discuss evidence for fluid alteration, shock, and thermal events in explaining the chondrule ages and suggest that chondrule remelting events, presumably from bombardment of the parent body surface, are responsible for resetting the I-Xe chronometer. Previous data show a negative correlation between 132 Xe/129Xe of the trapped Xe component and 127I/129I of an initial iodine component. This behaviour that requires the presence of a component with trapped 129Xe/132Xe lower than the planetary value has been cited as evidence for closed system evolution of the I-Xe system. We find no evidence of an unambiguous trapped component lower than planetary and no evidence of a negative correlation in our data. Therefore we suggest that open system behaviour more suitably explains the I-Xe systematics of Chainpur chondrules. Copyright © 2005 Elsevier Ltd employed to separate low-temperature iodine, typically with variable, low, or zero 129I/127I ratios, from high-temperature releases (Swindle and Podosek, 1988). The presence of several consecutive high-temperature releases, with uniform derived 129 127 I/ I ratios and a consistent trapped component, is taken as evidence of a well-defined initial iodine ratio. This can be interpreted chronologically, subject to assumptions regarding the process that last led to closure to xenon loss in the sample and to the assumption that 129I was well mixed in the solar nebula. When applied to a whole rock sample, this procedure effectively selects data from a high-release temperature iodine site that may or may not record the events responsible for defining the macroscopic characteristics of what is, in effect, a sedimentary rock. The problem is exacerbated, compared to the Ar-Ar system, because the timescales of metamorphism and aqueous alteration on chondrite parent bodies may be long compared to the resolution of the I-Xe system. In contrast to the problems of whole rock analyses, recent I-Xe analyses of individual clasts (Gilmour et al., 2000; Pravdivtseva et al., 2003a; Pravdivtseva et al., 2003b), chondrules (Swindle et al., 1991a; Swindle et al., 1991b; Gilmour et al., 2000; Pravdivtseva et al., 2002; Whitby et al., 2002), and mineral separates (Brazzle et al., 1999; Pravdivtseva and Hohenberg, 1999; Pravdivtseva et al., 2001) have provided compelling evidence for interpretation of initial 129I/127I as a valid chronometer. In addition, Gilmour (2000) reinterpreted much early whole-rock data and demonstrated that the most primitive meteorites had retained proportionately higher levels of 129Xe than the more-processed samples. This is expected if the moreprocessed meteorites lost 129Xe, both from higher temperature sites and for longer periods; the contrast between this and the high-temperature isochron ages is that total 129Xe*/I is a macroscopic feature of the meteorite, whereas high-temperature 129 127 I/ I records reset in one, potentially minor or trace, phase. The chronological interpretation of the I-Xe system has been reinforced by the agreement in elapsed time between resetting events in the chondrites Ste. Marguerite and Richardton, deter-

1. INTRODUCTION 129

It is well established that the Xe excesses in primitive meteorites arose from the in situ decay of 129I (Jeffery and Reynolds, 1961); this decay scheme forms the basis of the I-Xe dating technique. The earliest whole rock 129I/127I ratios of chondrites clustered around 10-4 and were interpreted as indicating a restricted period of formation, ⬃106 yr (Turner, 1965). With increasing numbers of I-Xe age determinations in the subsequent decade, the range of 129I/127I ratios expanded to cover around one half life of 129I decay, i.e., ⬃16 Ma. Beyond a general implication of an overall 107 yr time scale for some of the major events, such as chondrule formation and early proto-planet formation, this period can be characterised as generating many ages but little understanding. The situation was only made more puzzling by the discoveries of other extinct radionuclides, the chronologies of which did not coincide in a convincing way with that of 129I. However, within the past decade refinement of the technique has allowed its application to single chondrules and single minerals (Swindle et al., 1991a; Swindle et al., 1991b; Brazzle et al., 1999; Pravdivtseva and Hohenberg, 1999; Gilmour et al., 2000; Whitby et al., 2002) in well-characterized samples, and the technique is now providing a convincing early solar system chronology that can be linked to other isotope chronometers. In the I-Xe technique, samples are subjected to a neutron fluence that converts stable 127I to 128Xe via 128I (e.g., Drozd and Podosek, 1977). In our application of the technique, the 129 127 I/ I ratio of the sample can then be determined by a xenon isotopic analysis as 129Xe*/128Xe*, the conversion efficiency of 127 I to 128Xe during the irradiation being determined using a suitable reference sample. Stepwise heating experiments are

* Author to whom correspondence should be addressed ([email protected]). † Present address: Planetary and Space Sciences Research Institute, Open University, Milton Keynes, MK7 6AA UK 189

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Table 1. Physical characteristics, mineralogy and major element chemistry of chondrules selected for xenon analyses. Chondrules were measured and weighed prior to sectioning. Texture

Min. compositions

K/Na, K/Al wt % of mesostasis

CR1 CR2 CR3 CR4

PO, IIA POP POP PP

Fo78–83 Fo75–79, En75–79Fs19–24 Fo72–82, En74–80Fs18–24 En69–84Fs16–27

0–0.6, 0–0.2 0.1–0.3, 0.1 0, 0 0, 0

CR5 CR6 CR7 CR8

50% metal PO, IIA PP PO, IA

En74–78Fs21–24 Fo77–83 En74–83Fs17–24 Fo88–98

0, 0 0–0.4, 0–0.1 0, 0

CR10 CR15

PO, IIA PO(P) IA

Fo65–69 Fo85–96, En95–97Fs2–4

Other features CI-bearing mesostasis ⬍10 vol% mesostasis Some rim attached, ⬍10 vol% mesostasis Fractured, minor pyroxene CI-bearing mesostasis, some rim attached

0–0.4, 0–0.2 Some rim attached

IA, IIA after Scott and Taylor (1983). PO ⫽ porphyritic olivine; PP ⫽ porphyritic pyroxene; POP ⫽ porphyritic olivine pyroxene; Fo ⫽ Forsterite; En ⫽ enstatite; and Fs ⫽ ferrosilite.

mined by the I-Xe and Mn-Cr systems (Gilmour and Saxton, 2001; Polnau and Lugmair, 2001). In addition to time-related variations in 129I/127I, correlated variations in 129Xe/132Xe have been claimed by Swindle et al. (1991a) in a suite of Chainpur (LL3.4) chondrules. This behaviour would be expected if 129I decayed to 129Xe in a closed system, since a decrease in the 129I/127I ratio would be accompanied by an increase in the 129Xe/132Xe ratio; the relative magnitudes of the ratio change depending on the 127I/132Xe ratio. However, in I-Xe experiments, the 129Xe/132Xe ratio of the trapped component is deduced by extrapolating the isochronous high-temperature correlation lines to an end member that contains no iodine. When no releases defining the line are iodine-free, this procedure necessarily produces 129Xe/132Xe ratios lower than any actually observed. Indeed, Gilmour et al. (2001) noted that some derived trapped xenon values contributing to the proposed evolutionary negative correlation possessed 129Xe/132Xe ratios as low as 0.92, much lower than that of planetary xenon (1.04; Lavielle and Marti, 1992). As an alternative explanation, Gilmour et al. (2001) proposed that the negative correlation of derived initial 129I/127I and derived trapped 129Xe/132Xe compositions was an artefact. This was attributed to the presence of a mixed component in which I and Xe are trapped in constant proportion, similar to components observed in the Martian meteorite Nakhla, and attributed to minor shock. One motivation of this work was to increase the dataset of analyses of Chainpur chondrules, seeking further evidence of the correlation and evidence in support of one of the explanations outlined above. Recently, much progress has been made in the detailed chronology of the early solar system. Gilmour and Saxton (2001) demonstrated reasonable agreement among the major chronometers (Al-Mg, Mn-Cr, I-Xe, and Pb-Pb) for all samples except CAIs, where the I-Xe system measures aqueous processes involved in sodalite formation, and the Mn-Cr data suggest some other form of isotopic effect. Since then, more precise Pb-Pb ages for CAI and the first Pb-Pb ages of chondrules have shown that the interval between CAI and chondrule formation recorded by the Al-Mg and Pb-Pb chronometers are consistent (Amelin et al., 2002). Some inconsistencies remain when we attempt to reconcile all the proposed chronometers,

but the overall picture seems to be clearing (Gilmour, 2002). In particular, the redefinition of the CAI Pb-Pb age from the CR chondrite Acfer 059 (Amelin et al., 2002) has firmed the agreement of the Al-Mg and Pb-Pb chronometers when applied between Efremovka and Allende CAIs and feldspar from the chondrites Ste. Marguerite and Forest Vale (Zinner and Göpel, 2002). Chondrule Pb-Pb data define a remarkably small formation time span of ⬃2 Ma, although data are as yet only available from Acfer 059 (Amelin et al., 2002). This is in contrast to the chondrule ages of the I-Xe system; chondrules from ordinary and enstatite chondrites cluster around the same apparent age when calibrated against Ste. Marguerite feldspar— but a spread in excess of 4 Ma is significant compared to the accuracy of individual measurements (Whitby et al., 2002). Furthermore, the less processed meteorites tend to record the largest spreads of chondrule ages, which is puzzling; we return to this in the discussion section. Therefore, a second motivation for this work was to seek verification of the large spread in ages of chondrules from the Chainpur meteorite and to try to identify any other property that may account for this observation, if confirmed. 2. EXPERIMENTAL Two hundred chondrules have been separated from Chainpur (LL3.4), weighed, and their diameters measured before splitting and polishing in resin blocks. Splitting, weighing, and measuring were performed in a clean lab to try to minimize the risk of chemical contamination. Fragments of the chondrules were retained for I-Xe analyses. The large sample number was necessary to provide a representative selection of chondrules in terms of size, mineralogy, and geochemistry. Individual mineral analyses (Table 1) were also made with a JEOL 5400LV ASEM at 20 kV, 1.5 nA to minimise loss of volatiles from phases in the mesostases. Bulk chondrule major element contents were determined by averaging multiple EPMA (Cameca SX50, 20 kV) defocused, 30 ␮m spot analyses. Between 35 and 50 defocused spot analyses were made on the chondrules, apart from Cr5, for which it was only practical to make four defocused analyses. We determined bulk trace element abundances on the chondrules CR1-8, CR10, and CR15 by quadrupole-based laser ablation ICP-MS, using a NewWave Research (Fremont, USA) UP213 laser ablation system coupled to a Thermo Elemental (Winsford, UK) PQ3 ICP-MS with enhanced sensitivity S-option interface, at the Natural History Museum. A rastered beam pattern was used over the chondrule to gain

I-Xe analysis of Chainpur chondrules representative bulk analyses. Beam spot diameter within the rasters was 40 to 70 ␮m. For calibration we used the National Institute of Science and Technology (NIST) standard reference material NIST SRM612. Jeffries (2001) provides an in-depth discussion of the laser ablation ICP-MS technique (see also Heinrich et al., 2003). Further details of the chondrule ICP-MS dataset can be obtained from one of the coauthors ([email protected]). Ten chondrules (CR1-8, CR10 and CR15), selected to reflect the spread in chondrule size and variation in chondrule mineralogy of the wider suite, underwent neutron irradiation for Xe analysis at the Penubaba reactor, South Africa, in the Safari water cooled position. Samples of the enstatite achondrite Shallowater (129I/127I ⫽ 1.125 x 10⫺4; Hohenberg, 1967) were included in the irradiation as an iodine conversion standard, leading to 128Xe*/I ⫽ 5.74 ⫻ 10⫺5. Hornblende Hb3gr flux monitors indicated a neutron fluence of 1.8 x 1018n cm⫺2 fast and 6.6 x 1018n cm⫺2 thermal, allowing nominal conversion factors for generation of Xe isotopes from Te, Ba, and U fission to be calculated. Xenon isotopic analyses of the irradiated samples were made using the RELAX resonance ionization mass spectrometer (Gilmour et al., 1994) in conjunction with laser stepped heating. The laser stepped heating technique allows a very low blank (approx. 1.5 x 10⫺16cm3 132 Xe at STP) compared to a conventional furnace, but accurate temperatures are not available. Step heating experiments were performed by increasing the power of a Nd:YAG continuous wave laser (1064 nm) with an estimated maximum temperature of ⬃2100°C. The contribution to total 132Xe from neutron induced fission of 235U was calculated assuming mixing of a neutron induced end member with a 134 Xe/132Xe ratio of 1.84 (Ozima and Podosek, 1983) and an ordinary chondrite component (134Xe/132Xe ⫽ 0.38; Lavielle and Marti, 1992). Fission contributions at 132Xe were always less than 10% (and absent in some samples). This 132Xep value was then used to calculate excesses of I derived 128Xe* (and other neutron produced Xe isotopes) over ordinary chondrite Xe. No significant spallation component was detected. All quoted errors are one standard deviation, while error bars represent the major and minor axes of ellipses of constant probability enclosing 68% of the expected data distribution. All Xe data are available in an electronic annex. 3. RESULTS

3.1. Mineralogy/Petrography and REEs The 200 chondrules have a log-normal size distribution with a mean diameter of 0.71 mm, which is similar to other calculated average chondrule diameters in L and LL chondrites (Hughes, 1978). They contain typical proportions of chondrule textures: porphyritic, radiating, barred, and metal. The 10 chondrules used for I-Xe analyses are listed in Table 1. They range in mass from 0.06 mg to 2.51 mg and contain porphyritic olivine (po), porphyritic olivine-pyroxene (pop), porphyritic pyroxene (pp), and one metal-troilite rich chondrule. Two of the chondrules are recognizably type IA chondrules, having Fepoor silicate compositions and fine porphyritic textures with generally rounded phenocrysts. Three of the chondrules are Fe-rich type IIA. Three of the chondrules (CR4, CR8, CR15) have some fine-grained rim attached. The mesostasis of the Chainpur chondrules contains a mixture of albite and nonstoichiometric glass. The glassy and albitic parts of the Chainpur mesostases have K/Na wt% ratios varying from 0 to 0.6 (Table 1). This type of alkali variation is thought to be the result of incipient thermal metamorphism in UOCs (Grossman and Brearley, 2003). The silicate mineral compositions are typical for the range of type IA, IIA chondrules, e.g., CR8 having an olivine compositional range of Fo88-98 and CR1 Fo78-83. Two of the chondrules, CR2 (POP) and CR8 (PO, IA), contain Cl-enriched phases within the mesostases. The distribution of Cl is irregular, with contents ranging up to

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3.4 wt% in one irregular veinlet. The size of the vein, a few microns in diameter, prohibited quantitative analyses, but identification as a Cl-bearing iron oxide was possible. Veining is not seen in CR8, instead the Cl (ⱕ1.7 wt%) is associated with the fine-grained mixture of feldspathic glass and pyroxene dendrites. Grossman et al. (2000) demonstrated that Cl-enrichment was a feature of extraterrestrial alteration of Chainpur chondrules in bleached zones, rather than a result of any terrestrial contamination of chondrules. Similarly, Bridges et al. (1997) showed that ⬃7% of Chainpur chondrules contained Cl-rich phases of extraterrestrial origin such as sodalite or scapolite of extraterrestrial origin. The bulk compositions of the silicate chondrules in this study are typical for ferromagnesian chondrules. For instance, Mg/Si, Al/Si, and Na/Si wt. ratios range from 0.5 to 1.0, 0.1 to 0.2, and 0.02 to 0.08 values, which overlap the compositional range for other LL3 chondrules and are close to bulk LL chondrite values (Grossman and Wasson, 1983; Bridges et al., 1998). The chondrules have REE abundances of between 0.1 and 5 x CI, with flat REE patterns from La to Lu (La/Lu ratios vary from 3 to 12), but most have negative Eu anomalies of ⬇0.8 Eu/Eu* (Fig. 1 and Table 2). However, CR2 has a slight positive Eu anomaly of 1.2 and CR6, has no Eu anomaly. These two chondrules also have higher volatile abundances (Rb, Cs, Zn, and Ge) than the other chondrules. CR5 has the highest metal and troilite contents and also the lowest lithophile trace element abundances. Variation in metal and sulphide contents of the chondrules is reflected in the variability of chalcophile and siderophile elements. For instance CR5, which contains 50% metal, has higher Co and Ni abundances than the predominantly silicate chondrules. 3.2. I/Xe Results Five chondrules define high-temperature isochrons of variable quality on 3-isotope plots (Fig. 2a– e). From these data, trapped 132 Xe/129Xe (y-intercept) and initial 127I/129Xe (x-intercept) can be deduced. Based on the initial 129Xe/127I ratio, these chondrules have ages of ⫺17.8 Ma ⫾ 0.5 (CR1), ⫹0.5 Ma ⫾ 0.5 (CR2), ⫺0.9 Ma ⫾ 0.5 (CR3), ⫺11.2 Ma ⫾ 0.3 (CR4), and ⫺9.5 Ma ⫾ 0.9 (CR10) relative to Shallowater. (Note that negative numbers indicate ages after Shallowater. This is opposite to the convention adopted in the past for I-Xe ages, but is the same as that adopted for Mn-Cr ages, and indeed, for all long-lived chronometers, where positive ages are older). We shall discuss the validity of a chronological interpretation, and the criteria by which we assess the presence or absence of an isochron, in sections 4.2 and 4.3. The 129 Xe/132Xe ratio of the trapped component is approximately planetary or slightly higher (1.04 –1.055) in four of the five chondrules. These data are compared with the data from the previous study and the proposed closed system evolution line in Figure 3. Five chondrules did not yield any isochrons (Fig. 2f–j), therefore, neither the initial iodine ratio nor the trapped xenon can be adequately constrained. Most samples, particularly those exhibiting well-defined isochrons, exhibit a high proportion of trapped planetary xenon, so that inferred trapped components are well-defined. This contrasts with relatively unprocessed EH3 chondrites with fairly high-iodine content (Whitby et al., 2002). Although the relative proportions vary from chondrule to chondrule (Table 3), in all chondrules the highest concentrations of (neutron-induced) 131Xe* are accompanied by the largest release of 128Xe*, and the absence of 131Xe* is mirrored

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Fig. 1. Trace element abundances of chondrules CR1 to CR8, 10, and 15 determined on the chondrules by LA-ICPMS using a rastered beam to gain representative bulk analyses. The chondrules show generally unfractionated REE and other trace element abundances. There is, however, some variation in Eu and other volatile element abundances. The lowlithophile element abundances of CR5 is due to the high volume of metal and troilite (Table 1. See text for discussion.)

by very small 128Xe* releases. As 131Xe* is produced by a (n,␤) reaction on 130Te and by neutron capture on 130Ba, a correlation between 128Xe* and 131Xe* strongly suggests that albite is the major iodine carrier (the other mineral phases, olivine and Ca-poor pyroxene, are unlikely to contain significant amounts of Ba or Te). The relative proportions do not exhibit any correlation with chondrule age and presumably reflect differences in the mineralogy of the individual chondrules. Previous work (Caffee et al., 1982) has demonstrated that shock can redistribute I and Xe. Gilmour et al. (2000) analysed a clast from Julesburg (L3.6) that exhibited younger model ages with increasing temperature and increasing iodine component: the opposite trend from that expected. Gilmour et al. (2000) inferred that iodine was mobilised more readily than xenon from the feldspar lattice, the major site of iodine. In contrast to Gilmour et al. (2000), we see no evidence of a systematic change in age with temperature. 4. DISCUSSION

4.1. Is There Evidence of Closed System Evolution in Chainpur Chondrules? Previous Chainpur chondrule analyses (Swindle et al., 1991a) revealed the presence of a negative correlation between 129 Xe/132Xe in trapped planetary xenon and the initial 129I/127I

ratio of the iodine end member and identified it as evidence of closed system evolution. In contrast to Swindle et al. (1991a), we find no evidence for evolution of 129Xe/132Xe in the trapped component amongst those chondrules where it can be determined with the required precision (Fig. 3). CR2 is an exceptional case that will be discussed in the following section. Furthermore, when all data from this study are combined (Fig. 4), there is no evidence for a component with an unusually low 129 Xe/132Xe ratio as required by the proposed isotopic evolution. Thus there is no evidence for closed system evolution of a common component with a consistent ratio of iodine to xenon. Gilmour et al. (2001) put forward an alternative explanation for the negative correlation that had been observed in the previous study. They demonstrated that the presence of mixed components with roughly similar proportions of iodine and xenon in each chondrule could account for the appearance of iodine and xenon evolving in a closed system. While no negative correlation was observed in this study, CR3 exhibited a series of 25 releases isotopically identical within error, which might be evidence of such a component. In addition to the absence of an evolving trapped component, we further argue that I/Xe ratios are inconsistent with closed system evolution. Swindle (1998) considered the similarity between 127I/132Xe of Chainpur chondrules (⬃48,900) and Orgueil bulk composition (⬃33,000) to be evidence of alter-

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Table 2. Major and trace element compositions of Chainpur chondrules calculated as wt% oxides. These are determined by multiple EPMA analyses. Trace element analyses (ppm) are determined by ICPMS. CR1

CR2

CR3

CR4

CR5

CR6

CR7

CR8

CR10

CR15

SiO2 TiO2 Al2O3 Cr2O3 FeO NiO MnO MgO CaO Na2O K2O P2O5 SO2 Total

44.7 0.10 2.11 0.35 12.8 0.04 0.34 37.0 1.28 1.46 0.11 0.07 0.12 100.5

43.9 0.15 2.81 0.51 22.9 1.50 0.35 27.4 2.28 1.20 0.12 0.20 0.91 104.2

48.2 0.18 4.93 0.68 15.5 0.02 0.35 27.0 2.81 2.39 0.02 0.04 0.09 102.2

54.9 0.17 3.89 0.71 12.9 1.01 0.58 21.5 3.03 2.21 0.02 0.02 1.02 101.9

32.1 0.08 1.88 0.03 37.5 1.93 0.26 21.4 0.56 0.78 0.08 0.04 0.05 96.7

51.8 0.17 4.13 0.46 13.5 0.02 0.42 26.0 2.57 2.77 0.22 0.06 0.13 102.3

57.0 0.14 3.06 0.73 9.8 0.02 0.57 25.5 2.25 1.81 0.07 0.02 0.13 101.0

47.1 0.21 5.05 0.47 5.3 0.38 0.12 38.6 3.67 0.56 0.01 0.06 0.56 102.1

42.2 0.13 3.26 0.61 20.3 0.03 0.30 27.4 2.01 2.17 0.15 0.45 0.11 99.1

37.4 0.10 2.50 0.61 19.7 1.13 0.13 30.5 1.38 0.79 0.07 0.24 4.48 99.0

Sc V Co Ni Zn Ge Rb Sr Y Zr Nb Mo Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf W

12 70 33 111 24 0.77 2.73 2.08 1.66 4.22 0.41 1.54 0.13 0.60 0.25 0.76 0.10 0.50 0.16 0.05 0.18 0.04 0.27 0.06 0.18 0.03 0.18 0.03 0.11 0.06

14 84 98 3020 270 4.17 33 11.2 1.84 6.93 0.51 1.85 1.64 3.98 0.27 0.86 0.11 0.55 0.18 0.08 0.20 0.04 0.32 0.07 0.22 0.03 0.23 0.04 0.21 0.04

13 84 22 102 69 0.75 0.38 13.3 1.95 5.41 0.47 1.53

14 86 48 5400 22 4.64 0.40 5.12 2.13 4.88 0.59 2.60 0.03 1.83 0.33 1.08 0.13 0.66 0.21 0.04 0.25 0.05 0.36 0.08 0.24 0.03 0.25 0.04 0.13 0.02

22 48 8200 9070 28 85 1.21 4.15 1.48 15.6 0.56 6.54

11 76 31 103 25 0.83 10.3 2.46 1.96 4.62 0.49 1.57 0.53 0.67 0.31 1.00 0.13 0.62 0.20 0.07 0.24 0.05 0.33 0.07 0.21 0.03 0.22 0.03 0.12 0.08

12 83 18 149 17 0.86 2.78 1.41 2.57 6.45 0.69 1.98 0.12 0.90 0.42 1.37 0.17 0.85 0.27 0.03 0.32 0.06 0.43 0.09 0.27 0.04 0.28 0.04 0.17 0.02

12 100 203 4410 32 1.81 0.85 27.60 2.62 6.49 0.47 0.76 0.04 2.56 0.40 1.35 0.16 0.79 0.28 0.07 0.33 0.06 0.43 0.09 0.27 0.04 0.29 0.04 0.17 0.08

10 77 78 806 94 1.32 4.14 5.84 1.92 7.51 0.51 1.31 0.18 3.10 0.31 0.95 0.12 0.58 0.19 0.06 0.24 0.04 0.33 0.07 0.21 0.03 0.22 0.04 0.20 1.59

9 99 1790 24400 139 40.0 3.94 2.35 1.73 4.28 0.69 3.68 0.17 2.34 0.27 0.86 0.11 0.54 0.17 0.06 0.20 0.04 0.29 0.06 0.18 0.03 0.21 0.03 0.11 0.43

7.78 0.27 0.88 0.11 0.56 0.19 0.06 0.22 0.05 0.34 0.07 0.21 0.03 0.22 0.03 0.14 0.30

ation and exchange between Chainpur chondrules and the parent body. However, closed system evolution that increases 129 Xe/132Xe of the trapped component by ⬃10% requires that evolution of chondrules occurs in an environment with 129I/ 132 Xe ⫽ 0.1 giving 127I/132Xe of ⬃1000 (with the assumption that initial 129I/127I ⫽ 1 ⫻ 10-4). This factor of 50 discrepancy in I/Xe can only be attributed to open system behaviour. We note that this open system environment would also erase any evidence of an evolving trapped component from decay of 129I on the parent body. 4.2. Chondrule CR2 Although a negative correlation between initial iodine and extrapolated trapped 129Xe/132Xe does not exist among the chondrules, a low 129Xe/132Xe ratio can be derived from the near-linear array of data produced from chondrule CR2. Rather

1.57 0.10 0.32 0.05 0.30 0.12 0.03 0.16 0.03 0.27 0.06 0.18 0.03 0.25 0.03 0.50 1.19

than interpreting this linear array as an isochron, we see it as a mixing line between high I/Xe and low I/Xe components with different ratios of 129Xe excess to iodine. We now discuss the origin of these components. In the previous section we note that ICP-MS and petrographical analyses indicate that chondrule CR2 probably underwent aqueous alteration. CR2 contains 4 large gas releases (⬃45% total 129Xe release) that plot closest to the initial iodine end member (labeled 1– 4 in Fig. 2b) and a number of higher temperature releases with less iodine in proportion to xenon. Model ages (calculated over a planetary composition) exhibit a trend from early (high-initial iodine, with high I/Xe) to late (low initial iodine, with low I/Xe), which contrasts with the other chondrules in this study and most other iodine-xenon samples where the evolution is from late to early with increasing temperature. This trend

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Fig. 2. (a-e) Three-isotope plot of all Chainpur chondrule data. Solid data points are included in best fit line (after York, 1969), open data points are excluded from fit. Solid line in CR2 plot is best fit to all data; dashed line excludes the three iodine-rich releases.

can be accounted for if an initially isochronous system was modified either by addition of 129Xe to lower temperature sites or of iodine to higher temperature releases. The four consecutive gas releases dominated by iodine derived gas are accompanied by 131Xe* from Ba, strongly suggesting that the carrier phase for the iodine-rich points is divalent (calcic) plagioclase. As Ca-rich plagioclase is altered with relative ease, it is conceivable that this phase acquired “parentless” 129Xe (that is, 129Xe not accompanied by its quota of 127I). If we remove the three 129Xe-rich data points and fit an isochron through the remaining data points, we

obtain a trapped component of 1.05 ⫾ 0.04 that is completely consistent with the planetary trapped component of the other chondrules where end member identification is possible. Alternatively, late addition of iodine to (or formation of) the phase(s) responsible for the low I/Xe end member would also cause the observed effect if the data were erroneously interpreted as revealing a consistent age. This addition of volatiles might also have been a source of the associated trapped xenon. We note that CR2 exhibits traces of chlorine enrichment characteristic of the bleached margins attributed to aqueous alteration in previous work

I-Xe analysis of Chainpur chondrules

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Fig. 2. (f-j) (continued)

(Grossman et al., 2000). This would be the equivalent of the phase identified by Gilmour et al. (2001) as being responsible for the behaviour interpreted as closed system evolution if present in multiple chondrules. Further evidence of this hypothesis has been provided recently by Hohenberg et al. (2002), who analysed irradiated and unirradiated Allende CAIs and Allende dark inclusions. They determined that the 128Xe/132Xe ratios in the trapped components are identical to OC-Xe, but the isochrons of irradiated samples often pass below OC-Xe, suggesting that the 129Xe/132Xe ratios are lower than OC-Xe. They inferred

that 127I is intimately mixed with trapped xenon to produce a consistent end member in I-Xe experiments, as suggested by Gilmour et al. (2001), and as we consider to be the case for CR2. 4.3. Can Ages be Derived from Mixing Lines? The interpretation of the CR2 “isochron” as a mixing line inevitably leads us to examine whether identifying all of the observed correlations as mixing lines would invalidate the derived initial iodine ratios. Correlation lines have been con-

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G. Holland et al. Table 3. Mineralogy and xenon data from Chainpur chondrules.

Chondrule

Minerals

Mass (mg)

Age (Ma)

129

Xexs

131

Xexs

134

Xexs

132

Xep

Trapped

129

Xe

132

Xe

Iodine (ppb)

CR1 CR2

ol, ab, FeS ol, px ⫹ Fe

1.56 0.17

⫺17.8 ⫾ 0.5 ⫹0.5 ⫾ 0.5

15.0 137.6

16.9 76.5

7.3 7.1

93.3 121.2

1.040 ⫾ 0.008 0.924 ⫾ 0.023

2.4 11.1

CR3 CR4 CR5 CR6 CR7 CR8

ol, px, feld, FeS px, ab, Fe px, ab, ol, Fe ol, px ⫹ ab px, ab, ol ol, px ⫹ Fc

2.51 1.80 0.16 0.96 0.21 0.23

⫺0.9 ⫾ 0.5 ⫺11.2 ⫾ 0.3 ? ? ? ?

89.6 218.9 35.0 15.2 177.1 39.0

62.2 40.1 37.5 17.7 42.9 10.8

7.7 1.6 6.2 6.0 7.1 4.3

480.2 1355.5 267.5 63.3 444.8 177.4

1.044 ⫾ 0.004 1.055 ⫾ 0.004 ? ? ? ?

6.8 18.4 3.2 1.7 27.4 13.0

CR10 CR15

px, ab, ol, Fe ol ⫹ px

0.44 0.06

⫺9.5 ⫾ 0.9 ?

42.5 281.1

30.4 46.3

6.5 19.2

525.0 714.3

1.051 ⫾ 0.016 ?

4.8 73.0

CI enrichment No 3.4wt% in 1 vein No No No No No ⬍1.7wt% in px No No

Chondrules are composed of olivine (ol), pyroxene (px), albite (ab), feldspar (feld), metallic Fe (Fe) and Fe sulphide (FeS). 129Xexs, 131Xexs and Xexs are excesses calculated over a trapped component assumed to be ordinary chondrite xenon. 132Xep is planetary 132Xe (total 132Xe minus fission component). The spread in ages of Shallowater standard is ⫾0.2Ma. Xe concentrations given in units of 10⫺12cc STP g⫺1. Iodine contents are calculated from excess 128Xe and the measured 129Xe/128Xe ratio. Iodine concentration from all iodine releases. Errors on gas concentrations and iodine content are 10%.

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sidered reliable if the derived end members are not significantly (more than 1␴) changed if any one of the releases that define them is excluded. However, this approach may be flawed if each chondrule is similar to CR2 in having simply two end members, one iodine-rich and one xenon-rich. In this case the presence of several points defining a mixing line is an artefact of the experimental procedure, in particular the temperature resolution employed in the analysis, rather than an innate property of the sample. In this two end-member scenario, we have two options. One is to interpret the mixing line as an isochron with a pure iodine end member representing an initial 129 127 I/ I ratio; we have argued in the case of CR2 that this approach is invalid. The other approach is to assume a planetary trapped xenon composition and to derive a model age for releases at each end of the mixing line. In the case of CR2, this leads to clear evidence of two components with distinct Xe/I and 129I/127I ratios. The remaining chondrules exhibiting mixing lines are distinguished from CR2 in that the two approaches yield identical initial iodine ratios, since all releases are consistent with a single iodine component mixing with planetary xenon. For this reason we have confidence that the derived 129 127 I/ I ratios are meaningful. If the derived trapped composition is not planetary, the initial 129I/127I ratio is merely constrained to lie between the model ratio of the most iodine-rich point and the ratio derived from the mixing line. 4.4. Nucleosynthetic Arguments Against Evolution of a Trapped 129Xe/132Xe Component In addition to the lack of experimental evidence for closed system evolution, there are also serious difficulties if one considers closed system evolution of xenon in the light of nucleosynthetic models. 129Xe and 132Xe are produced in both the s-process and r-process nucleosynthesis; the sprocess 129Xe/132Xe ratio is known to be close to 0.12 from calculation and through analysis of xenon in presolar SiC grains (Huss and Lewis, 1995). The low value reflects the well known odd-even mass number effect corresponding to systematic variations in neutron capture cross section. The ultimate r-process ratio is 1.5. This is deduced by subtracting

an s-process xenon signature from solar xenon using the relative abundance of 130Xe, which is produced only in the s-process. Since r-process production of 129Xe proceeds through 129I, the ratio immediately after nucleosynthesis is zero. Thus, to allow the 129Xe/132Xe ratio of solar system material to evolve from 0.9 to 1.04, at least 10% of the total r-process 129Xe (and so 10% of associated r-process xenon isotopes) must have been synthesized in an event immediately before the system corresponding to the earliest data was reset. This contrasts with estimates of recent r-process nucleosynthetic contributions to the solar system, which are of the order of 5 ⫻ 10-3 (Meyer and Clayton, 2000). Another objection arises when the linked iodine isotope systematics are considered. Decay of 129I to 129Xe can cause 129Xe/ 132 Xe to change by ⬃0.1, while 129I/127I changes by ⬃10-4 if 127 132 I/ Xe ⬃1000. Note that the proposed evolution requires trapping of a component when 129Xe/132Xe ⫽ 0.9 and subsequent evolution of the solar ratio to 1.04, hence, it is the elemental ratio of the solar system that needs to be ⬃1000, rather than that trapped in a meteorite parent body. Neither of the known processes of nucleosynthesis achieves this ratio. 132Xe is closer to the s-process peak near barium than is 127I, and the ratio of 132Xe/I is further enhanced by the odd-even isotope effect. Thus, s-process 127I/ 132 Xe is ⬃0.1. Both 127I and 132Xe are also produced in the r-process. In this case, 127I is closer to the r-process peak near tellurium than is 132Xe, and so r-process 127I/132Xe is expected to be 1.04 (Arlandini et al., 1999). However, no ratios higher than 10 relative to 132Xe are observed in r-process material in the “tellurium” peak. Evolution of the solar ratio to that observed today, given it was once 0.9 as recorded in primitive meteorites, thus requires a solar I/Xe ratio ⬃ 1000. Ozima et al. (2002) demonstrate that a negative correlation can be explained by assuming that Xe has been fractionated from I in a solar nebula that was dissipating exponentially with time. However, in the absence of compelling evidence of evolution of 129I-129Xe with time, theories explaining the suggested evolution in terms of nebula processes are moot.

I-Xe analysis of Chainpur chondrules

197

Fig. 3. Plot of the initial trapped 129Xe/132Xe against derived 129I/127I for the five chondrules with well defined trapped components (solid squares), together with Chainpur chondrule data from (Swindle et al., 1991a) (open squares). Each data point represents the initial iodine ratio (as 129I/127I) and deduced initial 129Xe/132Xe ratio from a chondrule. Horizontal dashed line is planetary value 129Xe/132Xe ⫽ 1.04.

4.5. How to Explain the Wide Variation of Chainpur Chondrule Ages? The trace element abundances of the chondrules reflect limited fractionation during chondrule formation. In particular, the

flat REE patterns with negative Eu anomalies are characteristic of UOC chondrules (Nakamura, 1993); loss of this element occurs during chondrule formation because it is the most volatile of the REEs (Taylor, 1992). Of the two chondrules that do

Fig. 4. Three-isotope plot of all Chainpur chondrule data showing that the majority of high-T releases achieve a final value of 132Xe/129Xe 0.9 ⫺ 0.95 and a nonzero iodine component. Solid line is isochron of Shallowater with planetary trapped component (0.9615). Horizontal dashed line is planetary value 132Xe/129Xe ⫽ 0.9615. On the abscissa, 127I/129Xe yields the initial iodine ratio of the pure iodine component. Older ages plot toward the origin.

198

G. Holland et al.

not have negative Eu anomalies (CR2 and CR6), both chondrules have higher Rb and Cs abundances than the other chondrules, indicating that these chondrules gained the more volatile trace elements during formation. The high-Cl content of the CR2 mesostasis might have resulted from an early influx of volatile elements during chondrule formation or cooling. Alternatively, it could have resulted from low-temperature aqueous alteration. Grossman et al. (2000) showed that low-temperature aqueous alteration of the UOCs had left bleached margins to some chondrules. Texturally these zones were characterized by corrosion and pitting of the minerals and mesostasis, together with some replacement of glass by smectite. The chemical changes associated with bleached zones were variable between different samples, but some meteorites, including Chainpur, were characterized by Cl-enrichment. Oxidation was another feature sometimes found with the bleaching. Of our samples, only CR2 and CR8 show possible signs (Cl-enrichment) of bleaching, with only CR2 showing both possible Cl and other volatile (e.g., Rb) enrichment. The presence of some oxidation shown by ferric oxide veining in CR2 suggests that the latter origin is more likely than a high-temperature origin associated with the mobilization of Eu and other volatiles. However, most samples seem to have escaped Cl-enrichment. This suggests that the chondrules experienced different conditions and amounts of alteration on the parent body. This, in turn, suggests a mixing of chondrules from sources across the parent body, which is consistent with the suggestion of Ruzicka (1990) (based on the variation in shock levels between chondrules) that Chainpur is an agglomerate composed of chondrules with widely diverse histories before compaction. It is notable that, of our samples, only CR2 exhibits a mixing line requiring phases with different 129Xe*/I ratios, which is suggestive of a contribution of iodine and planetary xenon from more than one event. The spread of ages of Chainpur chondrules is significantly wider than that observed for other ordinary and enstatite chondrites and persists to later apparent ages. Either chondrule formation continued over a far more extended period, or resetting events persisted for longer than other chondrites. If chondrules were created in a single, well-defined epoch, the large spread of ages must be attributed to processing on the ordinary chondrite parent body. Aqueous processing may be feasible, though the ⬃18 Ma range of chondrule ages suggests that liquid water is unlikely to have been maintained by thermal metamorphism of 26Al decay (t1/2 ⫽ 0.73 Ma). More realistically, sporadic water flows may have occurred near the parent body’s surface as a result of impacts by icy planetesimals or the remelting of near surface ice by impact heating. Delivery of water ice by planetesimals is also suggested by Busfield et al. (2004). There is, however, little mineralogical evidence of aqueous alteration in the chondrules (with the exception of CR2) and no sign of aqueous effects in the albite implicated as the host of correlated 129Xe*. Of the two chondrules exhibiting possible signs of alteration (CR2 and CR8), CR2 provides the oldest age of the five chondrules that have good isochrons (although with the caveat of section 4.2). The chondrules with the later ages appear to be unaltered. As discussed in detail in section 4.1, Swindle (1998) considered 127I/132Xe data to be evidence of alteration and exchange between Chainpur chondrules and the parent body. Inheritance of parentless 129Xe was

deemed responsible for increasing 129Xe/132Xe of the trapped component over time. As we see no evidence of an evolving trapped component and a much lower 127I/132Xe in the chondrules (⬃1000 from I correlated releases, ⬃14000 from all releases) than in a hypothesized parent body regolith (⬃33000), we conclude that I exchange between a chondritic I/Xe parent body reservoir common to all chondrules has not occurred our samples. Therefore, we do not comment further on the possibility of extended periods of aqueous alteration. Similarly, there is no evidence for shock features in any of our Chainpur chondrules. Although Ruzicka (1990) found at least one chondrule had suffered significant shock deformation, there is no requirement that any of the chondrules studied here should have experienced comparable levels of shock, given that Chainpur appears to be an agglomerate. In addition, shock is predominantly a process that is magnified at grain boundaries (Stöffler et al., 1988; Stöffler et al., 1991). Slight shock, undetectable by optical and microprobe methods, would not completely reset the initial iodine ratio in feldspar. Rather, an evolution of age with extraction temperature would be expected, as is the case for a shocked Julesburg clast (Gilmour et al., 2000). Therefore we dismiss impact shock without melting as a resetting mechanism. We now consider a third possibility—thermal metamorphism. Resetting of the I-Xe chronometer over ⬃18 Ma using a radioactive decay heat source is not valid: decay of the Al-Mg system can be invoked as the parent body wide heat source only for the first few Ma after parent body formation. Furthermore, the wide variation in olivine and pyroxene content, coupled with constant feldspar content, strongly suggests that partial melting / crystal settling did not take place (Walter and Dodd, 1972). Also, the degree of fractionation is not related to chondrule size; therefore, vapour fractionation cannot be due to secondary heating after formation, which would require fractionation to be a function of chondrule mass (Walter and Dodd, 1972). Grossman and Brearley (2003) used K/Al and Na/Al wt. ratios to trace the incipient effects of thermal metamorphism in UOCs. They suggested that superchondritic K/Na wt. ratios in chondrule glass (⬃2 for Chainpur) were the result of albite crystallisation, leading to the net removal of Na from glass. The chondrules studied here also show some variation that may be linked to this partial metamorphism, having K/Na/CI wt. ratios varying from 0.1 to 1.3 and K/Al/CI wt. ratios varying from 0.1 to 1.9. However, no correlation is evident between K/Na, K/Al wt. ratios and the calculated I-Xe ages, suggesting that varying influence of thermal metamorphism has not controlled the I-Xe systematics. In the absence of any correlation between chondrule chemistry (including evidence for alteration) and chondrule age, we are required to invoke a different mechanism that is capable of resetting the I-Xe chronometer over a period of ⬃18 Ma. Formation seems to us the most likely event to have set the chronometer, but an ⬃18 Ma age range observed in the chondrules from this study is thought to be inconsistent with a nebular hypothesis of chondrule formation. Therefore, is formation from impacts on the parent body a feasible mechanism for generation of Chainpur chondrules? We note that all chondrules with good isochrons contain a trapped component of approximate planetary composition. During high-temperature formation and subsequent cooling, iodine and xenon are incor-

I-Xe analysis of Chainpur chondrules

porated into progressively less retentive sites. This is unraveled by the reverse of this process—step heating. This results in gas releases that indicate an unfractionated trapped component, i.e., a component identical to that of the planetary reservoir at the time of trapping. In contrast, fluid alteration or shock mobilisation would site iodine and xenon differently to the kinetically controlled release of step heating, leading to possible nonplanetary high-T trapped components. This has been shown experimentally to occur during shock remobilisation (Caffee et al., 1982). In summary, the absence of abundant evidence for aqueous, shock, or thermal processes, and lack of major obstacles for a formation hypothesis, lead us to suggest that chondrule formation by impacts on the parent body surface can feasibly explain the range of I-Xe ages. We envisage a scenario where the first formed chondrules become buried progressively deeper in the regolith to the extent that they are protected from impact melting events, although perhaps not from disturbance of their I-Xe isochrons. In contrast, near surface material is remelted and chondrules formed during the 10 to 100 Ma of planetesimal formation and accretion (Weidenschilling, 2000). Ongoing gardening of the regolith during this period continues to cycle younger chondrules into the near surface, where some are shielded from future impacts (chondrules with isochrons) and some partially reset (chondrules without isochrons). At some undefined later time, the chondrules of different ages are mixed and incorporated into the Chainpur matrix, presumably by a lower energy impact. Is this scenario plausible? Larger asteroidal bodies that accreted in the early solar system tended to have faster rotation rates than the earliest smaller bodies; therefore, it is likely that at one stage of solar system evolution there was a trend in the evolution of chondritic parent bodies towards higher relative differences in velocity (␦V) between colliding bodies. This may produce a paradox in our model because the formation and then preservation and mixing of chondrules by, as we suggest, impact requires a general decrease in ␦V conditions. However, collisions of small (e.g., 1 km diameter) bodies with larger (e.g., 10 to 100 km diameter planetesimals) will also act to reduce the original angular momentum of the larger planetesimal and progressively lower ␦V values for subsequent impacts (e.g., Lewis, 2004). The number of such collisions between early swarms of chondritic parent bodies and large planetesimals will have been much greater than the collisions between the larger planetesimals, so higher ␦V conditions were probably localised and rare enough to allow for the preservation of chondrules. Therefore, we believe the apparent chondrule ages are most likely real ages that have recorded this extended period of bombardment, with the earliest chondrule age providing a limit for the duration of the chondrule forming process that can be compared with the data from other chronometers. This scenario is more consistent with the “onion shell” configuration of petrologic type in ordinary chondrite parent bodies (Trieloff et al., 2003) and may also explain the puzzling observation that Chainpur chondrules have later I-Xe ages than LL4-LL6 chondrites (Bernatowicz et al., 1988). 5. SUMMARY

● We observe a ⬃18 Ma spread of ages in Chainpur chondrules from 0.5 Ma before to 17.8 Ma after Shallowater (4564.3 Ma).

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● In contrast to Swindle et al. (1991a), we find no evidence of systematic evolution of a trapped xenon component and an initial iodine component. ● Chondrule CR2 can be interpreted as having a 129Xe/132Xe lower than the planetary ratio. It has also undergone aqueous alteration. We argue that two-component mixing (addition of a high I/Xe component to low T points and/or a low I/Xe component to higher T points) is a better explanation for this spurious trapped component end member. ● Interpretation of a linear array as mixing leads us to doubt the validity of isochrons: a cluster of gas releases of constant trapped composition accompanied by only a handful of iodine rich gas releases may also be a result of mixing. However, the consistency of the trapped component suggests alteration is minimal (except in CR2). ● The consistency of the trapped component and the absence of ubiquitous evidence for shock or aqueous alteration lead us to conclude that formation due to impact is responsible for the extended range of Chainpur chondrule ages. Acknowledgments—We thank D. J. Blagburn and B. Clementson for technical support. This work was supported by PPARC (G.H., J.C.B., T.J., A.B.) and the Royal Society through a University Research Fellowship (J.D.G.). Ali Korotona (Nuffield Science Bursary, 2001) is thanked for his help in separating chondrules. We also thank A. Ruzicka, O. Pravdivtseva, and M. Ozima for their thorough reviews and U. Ott for his additional valuable comments. Associate editor: U. Ott REFERENCES Amelin Y., Krot A. N., Hutcheon I. D., and Ulyanov A. A. (2002) Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions. Science 297, 1678 –1683. Arlandini C., Käppeler F., Wisshak K., Gallino R., Lugaro M., Busso M., and Straniero O. (1999) Neutron capture in low-mass asymptotic giant branch stars: cross sections and abundance signatures. Astrophys. J. 525, 886 –900. Bernatowicz T. J., Podosek F. A., Swindle T. D., and Honda M. (1988) I-Xe systematics of LL-chondrites. Geochim. Cosmochim. Acta 52, 1113–1121. Brazzle R. H., Pravdivtseva O. V., Meshik A. P., and Hohenberg C. M. (1999) Verification and interpretation of the I-Xe chronometer. Geochim. Cosmochim. Acta 63, 739 –760. Bridges J. C., Alexander C. M. O’D., Hutchison R., Franchi I. A., and Pillinger C. T. (1997) Sodium-, chlorine-rich mesostases in Chainpur (LL3) and Parnallee (LL3) chondrules. Meteorit. Planet. Sci. 32, 555–565. Bridges J. C., Franchi I. A., Hutchison R., Sexton A. S., and Pillinger C. T. (1998) Correlated mineralogy, chemical compositions, oxygen isotopic compositions and size of chondrules. Earth Planet. Sci. Lett. 155, 183–196. Busfield A., Gilmour J. D., Whitby J. A., and Turner G. (2004) Iodine-Xenon analysis of ordinary chondrite halite: implications for early solar system water. Geochim. Cosmochim. Acta 68, 195–202. Caffee M. W., Hohenberg C., Horz F., Hudson B., Kennedy B. M., Podosek F. A., and Swindle T. D. (1982) Shock disturbance of the I-Xe system. Proc. 13th Lunar Planet Sci Conf. 87, Suppl. A318 – A330. Drozd R. J. and Podosek F. A. (1977) Systematics of iodine-xenon dating. Geochem. Journ. 11, 231–237. Gilmour J. D. (2000) The extinct radionuclide timescale of the early solar system. Space. Sci. Rev. 92, 123–132. Gilmour J. D. (2002) The solar system’s first clocks. Science 297, 1658 –1659. Gilmour J. D. and Saxton J. M. (2001) A time-scale of formation of the first solids. Phil. Trans. R. Soc. Lond. A 359, 2037–2048.

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