Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite

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1 Meteoritics & Planetary Science 43, Nr 5, (2008) Abstract available online at AUTHOR S PROOF Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite A. BUSFIELD, G. TURNER, and J. D. GILMOUR * School of Earth, Atmospheric and Environmental Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK * Corresponding author. jamie.gilmour@manchester.ac.uk (Supplementary tables and figures are available online at supplements.htm) (Received 06 October 2006; revision accepted 21 November 2007) Abstract We have determined initial 129 I/ 127 I ratios for mineral concentrates of four enstatite meteorites and a eucrite. In the case of the enstatite meteorites the inferred ages are associated with the pyroxene-rich separates giving pyroxene closure ages relative to the Shallowater standard of Indarch (EH4, 0.04 ± 0.67 Ma), Khairpur (EL6, 4.22 ± 0.67 Ma), Khor Temiki (aubrite, 0.06 Ma), and Itqiy (enstatite achondrite, 2.6 ± 2.6 Ma), negative ages indicate closure after Shallowater. No separate from the cumulate eucrite Asuka (A-) yielded a consistent ratio, though excess 129 Xe was observed in a feldspar separate, suggesting disturbance by thermal metamorphism within 25 Ma of closure in Shallowater. Iodine-129 ages are mapped to the absolute Pb-Pb time scale using the calibration proposed by Gilmour et al. (2006) who place the closure age of Shallowater at ± 0.4 Ma. Comparison of the combined 129 I-Pb data with associated 53 Mn ages, for objects that have been dated by both systems, indicates that all three chronometers evolved concordantly in the early solar system. The enstatite chondrites are offset from the linear array described by asteroid-belt objects when 53 Mn ages are plotted against combined 129 I-Pb data, supporting the suggestion that 53 Mn was radially heterogeneous in the early solar system. INTRODUCTION Iodine-129 was incorporated into meteoritic material in the early solar system, where it decayed to 129 Xe with a halflife of 16 Ma (Jeffrey and Reynolds 1961; Reynolds 1960). A relative chronology based on this decay scheme is now well established. In addition, recent work shows a good correlation between the I-Xe and Pb-Pb systems, suggesting that both provide valid chronological information (Busfield 2004; Busfield et al. 2004; Gilmour et al. 2006). In the I-Xe technique (recently reviewed by Gilmour et al. 2006), samples are neutron-irradiated, transmuting 127 I to 128 Xe. Step heating experiments are performed and xenon isotopic analyses made on the gas released in each of a series of sequentially increasing temperature steps. Data representing a mixture between a trapped xenon component and an iodinerich component with a consistent 129 Xe * /I ratio (where 129 Xe * indicates the excess 129 Xe over trapped 129 Xe) are identified by an isochron technique, whereupon 129 Xe * /I corresponds to 129 I/ 127 I on isotopic closure. The event dated by the I-Xe system is constrained by the mineral phase(s) responsible for the isochron. Since iodine can be present in both primary minerals such as pyroxene (Hohenberg 1967) and secondary minerals (e.g., halide, Busfield et al. 2004), the system may in principle date formation or subsequent processing. By assuming consistency among the Pb-Pb, Mn-Cr, and I-Xe systems in the Ste. Marguerite ordinary chondrite Gilmour and Saxton (2001) identified a 1 2 Ma discrepancy between the accepted Pb-Pb age of Acapulco phosphate (Göpel et al. 1992) and the Mn-Cr/I-Xe ages for this meteorite. Understanding the reason for this apparent inconsistency between the chronometers is important because Acapulco phosphate has traditionally been used as the absolute time anchor for the I-Xe system. Subsequently, a re-examination of data from Acapulco phosphates (Amelin 2005) gave an age in line with the adjustment in the calibration of the I-Xe system (Gilmour et al. 2006). Thus there is good evidence that all three chronometers showed general consistency in the early solar system. Limited data from ordinary chondrites also suggest coherence of the Mn-Cr and I-Xe systems (Busfield 2004; Gilmour et al. 2006). However, Shukolyukov and Lugmair (2004) have presented evidence of a heterogeneous distribution 883 The Meteoritical Society, Printed in USA.

2 884 A. Busfield et al. of 53 Mn across the early solar system, with variation in 53 Mn/ 55 Mn between the ordinary and enstatite chondrite source regions. This was based on variations in ε 53 Cr among the terrestrial, Martian, enstatite chondrite, and ordinary chondrite reservoirs. (ε units indicate a deviation in the isotopic composition of a sample relative to a standard in parts per ten thousand such that ε = 10,000 [(smpl-std)/std]). The enstatite meteorites are thought to originate from much closer to the Sun than the other chondrites and eucrites (Baedecker and Wasson 1975), whose source regions are considered to be within the asteroid belt. Birck et al. (1999) argued that the same data could be explained by the volatility differences between Mn and Cr. If 53 Mn was indeed heterogeneous in the early solar system then its use as a chronometer might be limited to obtaining relative ages within small, well-defined source regions. Shukolyukov and Lugmair (2004) have suggested a correction that can be applied to enstatite meteorite 53 Mn data to allow their data to be interpreted chronologically. One goal of this work has been to examine whether direct evidence of variation in 53 Mn/ 55 Mn in the early solar system can be found by comparing the I-Xe and Mn-Cr chronometers between the enstatite and ordinary chondrites. The approach adopted has been to extend the data set of mineral-specific 129 I ages so that the processes responsible for setting or resetting the chronometer can be constrained. We have also attempted to extend the I-Xe system to achondrites (enstatite and eucrite), motivated by the need to provide more points of comparison between I-Xe and other potential chronometers. The samples analyzed in this work are the two enstatite chondrites Indarch (EH4) and Khairpur (EL6), the aubrite Khor Temiki, an anomalous enstatite achondrite Itqiy and the cumulate eucrite Asuka (A-) In this work, negative relative ages indicate setting of the chronometer after the standard which, for the 129 I- 129 Xe system, is the non-magnetic fraction of the anomalous enstatite achondrite Shallowater. Absolute ages can be obtained by adopting an absolute age for the monitor. In this work we adopt an age for Shallowater of ± 0.4 Ma, proposed by Gilmour et al. (2006). MINERAL CONCENTRATION, SAMPLE CHARACTERIZATION, AND DATA REDUCTION Mineral separates were made by crushing samples in an agate pestle and mortar and hand-picking. The mortar was cleaned by twice crushing clean quartz before being wiped out with acetone between each sample. In all cases, the amount of processing and crushing was kept as low as possible to minimize the possibility of contamination. Samples were handpicked under a binocular microscope. After picking, samples were weighed out and a further few grains were mounted and polished for electron-probe and scanning electron microscopy (SEM) characterization. This work was not intended to be a detailed petrographic study of the meteorites, but was designed to allow the different phases in each separate to be identified. The abundances of these phases were estimated from backscatter electron (BSE) images. Electron-probe analyses were carried out on the separates to determine the composition of minerals and to assess the homogeneity of each phase. Three to five spot analyses were measured in all but the smallest grains. Where grains or inclusions were very tiny, only 1 spot measurement was made. Phase abundances are given in Table 1. Samples were tightly wrapped in small aluminium foil packets which were sealed for irradiation. Small quantities of the irradiation monitor, the non-magnetic fraction of the anomalous enstatite achondrite Shallowater, were wrapped and sealed in the same way. Foil packets were weighed before and after addition of the sample material, to aid in later identification of irradiated samples, and then sealed in evacuated quartz tubes. Samples were irradiated in irradiation Mn19 at the Penubaba Reactor, South Africa (fast fluence n cm 2 ; thermal fluence ncm 2 ). Analysis of the Shallowater standards indicated that the 128 Xe/ 127 I conversion factor (i.e., the efficiency of conversion of 127 I to 128 Xe) was (6.320 ± 0.008) 10 5, assuming a 129 I/ 127 I ratio of in Shallowater (Brazzle et al. 1999). Xenon measurements were conducted using the RELAX (Refrigerator Enhanced Laser Analyser for Xenon) mass spectrometer (Gilmour et al. 1994). Samples were unwrapped from their foil packets, loaded into the RELAX sample port and baked overnight. Gas was released by a laser stepped-heating technique as described in Gilmour et al. (1995). Heating steps lasted for 2 min and then the evolved gas was gettered for an additional minute to remove active gases before being admitted to the mass spectrometer. Analysis proceeded for 5 min and the data were subsequently blank corrected and reduced as described in Gilmour et al. (1998, 2000) who give full details of the correction for fission Xe and a trapped Xe component. Throughout this work the trapped component is assumed to be equal to Q-Xe (Busemann et al. 2000), except for the 129 Xe/ 132 Xe ratio; details specific to each sample are given below. Thus 132 Xe corrected for fission is indicated by the subscript p (for planetary) and excess Xe over the trapped component is denoted by an asterisk (as in 128 Xe * ). The 129 I/ 127 I ratio is calculated from the 129 Xe * / 128 Xe * ratio and reference to the 128 Xe * / 127 I conversion factor. Gas concentrations were calculated with reference to analyses of an air aliquot of known volume. Cosmogenic effects on 128 Xe and fissiogenic contributions to 132 Xe were negligible. Cold (no laser) procedural blanks were typically cm 3 of 132 Xe and are negligible except in the case of A where the blank amounts to ~1% of the sample gas. Data are summarized in Table 1 and full Xe data are available as supplementary data tables at meteoritics.org/online supplements.htm. Our technique does

3 Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite 885 Table 1. Summary of mineral separates and Xe data. All errors are 1σ. Approximate vol% Mass (mg) [Total I] ppb [ 132 Xep] 2 cc g 1 STP ( ) Total 129 Xe*/ 127 I (/10 5 ) 129 Xe * / 127 I 3 Age relative to Shallowater (Ma) IndB Enstatite 85 EH4 Other sulfide ± ± ± 0.01 Fe-sulfide 5 IndC Enstatite 60 EH4 Metal * ± ± 20 1 (1.074 ± 0.006) ± 0.67 IndD Enstatite 80 EH4 Sulfide * 254 ± ± 20 1 Metal 10 KPA Enstatite 50 EL6 Metal ± ± 20 (8.9 ± 0.06) ± 0.67 Sulfide 5 KPB Enstatite 95 EL6 Sulfide ± ± ± 0.03 KTD Enstatite ~99 Aubrite Sulfide < ± ± ± 0.3 (1.07 ± 0.02) 10 4? 0.06 to 1.5 ItA Enstatite Metal ± ± 3 Achondrite Enstatite 25 ItB Enstatite Enstatite ± ± 35 Achondrite Metal 25 ItC Enstatite Enstatite ~99 Achondrite Metal ~ ± ± ± 0.6 (9.58 ± 1.10) ± 2.6 AsA Anorthite 80 Cumulate Silica 15 Eucrite Pyroxene < ± ± 70 Max < 25 Chromite <1 AsB Pyroxene 85 Cumulate Anorthite ± ± 80 1 ± 30 Eucrite Silica 5 Ind = Indarch; KP = Khairpur; KT = Khor Temiki; It = Itqiy; As = Asuka * Denotes maximum mass as a small amount of sample was lost during loading. 1 Indicates a maximum concentration due to sample loss during loading. 2 Subscript p denotes planetary and indicates the [ 132 Xe] has been corrected for contribution from fission Xe. See text for discussion. 3 Ratio of correlated or maximum 129 Xe * / 127 I used to define age of sample. See text for how this ratio was obtained.

4 886 A. Busfield et al. not allow us to determine release temperatures. We do however state the current of the heating laser as a proxy for temperature, although it is not clear if there is a linear relationship between current and heating temperature. Iodine-Xenon Analyses RESULTS AND DISCUSSION This section provides a brief description of each sample, the techniques employed in mineral separation, and any relevant previous chronological data. We also discuss the Xe results obtained for each sample and, where necessary, compare them to other results. Results of xenon analyses are summarized in Table 1 and full Xe isotopic data are available in a supplementary table. Indarch Indarch (EH4, Sears et al. 1982) consists of silicate-rich chondrules, metal-rich chondrules, and a heterogeneous matrix. Keil (1968) determined the modal composition of Indarch to be 73 wt% silicate, 17 wt% Ni-Fe metal, 7 wt% troilite with minor schreibersite, niningerite, oldhamite, daubréelite, and graphite. The silicate phase is dominated by pure MgSiO 3 (clinoenstatite, Leitch and Smith 1982). Kennedy et al. (1988) determined a whole rock I-Xe age for Indarch of 1.8 ± 0.4 Ma before Shallowater and Shukolyukov and Lugmair (2004) calculated an initial 53 Mn/ 55 Mn ratio of (2.7 ± 0.2) Our sample was rusted all over its surface. Crushing and picking took place under ethanol so that rust particles floated free and rust-free grains could be selected. Three mineral concentrates were prepared. IndB was dominated by subhedral, medium sized ( µm), dark grey enstatite crystals. IndC nominally consisted of magnetic grains, but SEM analysis revealed enstatite to be a significant contaminant. IndD consisted of the residue from repeated crush-pick cycles. It consisted of fine-grained magnetic powder with occasional grains up to ~100 µm diameter. Silicate, metal, and sulfide were present in IndD in the approximate proportions of the bulk sample (Keil 1968). Pyroxene composition was enstatite with up to ~4% Fe. Sulfides were dominated by Fe-sulfide although IndB contained some more Mn-rich sulfides. Thus the purest mineral separate was IndB; IndC contained a significantly higher proportion of metal than the other separates, and IndD most closely resembles a bulk rock composition (Table 1). Indarch Xe data are displayed in Fig. 1 and summarized in Table 1. Complete xenon isotopic data are available in Supplementary Tables 1 3. Iodine concentrations from all separates are high and similar. The slightly lower abundance of iodine in IndC may be related to the significantly lower concentration of enstatite in this separate. All concentrates released iodine (unassociated with 129 Xe * ) at low temperature, which is typical of I-Xe analyses. At higher temperatures excess 129 Xe * was also released. The large number of releases preclude numbering temperature steps in Fig. 1, but generally points lying further to the right were obtained at lower temperature; a step-release diagram is available as supplementary Fig. A1. None of the separates define a consistent 129 Xe * / 127 I ratio over consecutive steps but a well-defined maximum in the 129 I/ 127 I ratio is observed. Closer examination of the data reveals that the maximum involves temperature steps from IndB and IndD but not IndC. Comparison of data from IndB (enstatite) and IndC (enstatite + metal) suggests that pyroxene is the host phase that defines the upper limit while metal contains more recent/uncorrelated iodine. The maximum is best defined by data from fine grained IndD, which can be understood statistically; a wider range of chondrules can contribute to release from a fine grained sample than to a coarse grained sample of similar mass, so each extraction approximates the average more closely than extractions from a coarser separate. A similar effect is observed in larger samples of meteorites, which often produce isochrons even when small samples or chondrule-by-chondrule analyses record a range of initial 129 I/ 127 I ratios (Gilmour et al. 1997). Alternatively, this sample may exhibit superior separation of correlated from uncorrelated iodine because the larger pyroxene crystals of IndB are contaminated with iodine in occluded grains, this component being more effectively separated in the step heating of finer grained samples since once-occluded grains have been exposed. The scatter in data from pyroxene ages may reflect pyroxene crystals from separate chondrules retaining different initial iodine ratios. This would suggest that chondrule ages were not reset by the thermal metamorphism of the Indarch parent body. Alternatively, a consistent high initial iodine ratio may be disguised by different amounts of uncorrelated iodine contributing to different releases. Bearing these alternatives in mind, we have characterized the lower limit on initial iodine ratio of our data by performing York fits (York 1969) to data selected in order of decreasing apparent initial iodine ratio until the resulting reduced χ 2 (chi-squared per degree of freedom) approaches 1. This yields a trapped 129 Xe/ 132 Xe ratio of 1.04 ± 0.03 consistent with Q-Xe. Patzer and Schultz (2002) showed that noble gas ratios in Indarch are similar to Q-xenon, though Kennedy et al. (1988), reported an anomalously low trapped 129 Xe/ 130 Xe ratio in bulk Indarch. Relatively high 132 Xe concentrations in fine-grained IndD suggest a matrix host for trapped xenon, suggesting either planetary xenon or implantation of terrestrial atmosphere during crushing. The iodine-derived end member has a 129 I/ 127 I ratio of (1.074 ± 0.006) 10 4, corresponding to an age of 0.04 ± 0.67 Ma relative to Shallowater. This is consistent with the spread of initial iodine ratios reported by Whitby et al. (2002) in their study of EH3 meteorites, suggesting a range of chondrule ages has survived to be sampled by our analysis.

5 Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite 887 Fig 1. Three-isotope plot of Indarch data. Inset shows that data are clustered close to the pure iodine component and contain little trapped xenon. For full sample compositions refer to Table 1 and full Xe data are available in supplemental Tables A1 A3. Temperature steps are not identified on this figure due to the large number of points. In general, lower temperature releases lie on the right of the figure. No consistent 129 I/ 127 I ratio is identified in consecutive steps (a plateau), thus no unique age is defined by the data. The fit shown by the dotted line was obtained by a York regression to the oldest points selected in order of decreasing 129 I/ 127 I ratio until the reduced χ 2 of the fit exceeded 1, and gives an initial iodine ratio which corresponds to an age of 0.04 ± 0.67 Ma relative to Shallowater. This age range may represent to a real variation in chondrule ages, but we cannot exclude a contribution from low-temperature iodine contamination to each release. In this case, the fit corresponds to the lower limit on the initial iodine ratio in this meteorite. See text for discussion. The trapped 129 Xe/ 132 Xe composition indicated by this fit is 1.04 ± Khairpur Khairpur (EL6) consists of metallic Ni,Fe (grains and irregular patches) and troilite (in smaller grains) set in a crystalline enstatite matrix. Large grains of oldhamite are also present (Prior 1916). Silicate material is predominantly enstatite with some albitic feldspar. The modal composition of Khairpur is 75 wt% silicate, 13 wt% Ni,Fe-metal, 10 wt% troilite, with minor schreibersite, daubreelite, oldhamite, ferroan alabandite, and graphite (Keil 1968). Kennedy et al. (1988) reported an I-Xe age of 2.1 ± 0.7 Ma relative to Shallowater, while Shukolyukov and Lugmair (2004) reported 53 Mn/ 55 Mn = Our sample was very hard with black, shiny, metallic areas. Repeated crushing was needed to separate material. At each crushing step small silicate grains and single grains of magnetic material were retained, the remainder being crushed further. Nominally, KPA consisted of blue-black grains of magnetic material, while KPB was the non-magnetic (silicate) residue. Later SEM analyses revealed that both separates were dominated by enstatite, though KPA also contained Fe,Ni-metal (Table 1). Both KPA and KPB contained a sulfide phase as small discrete grains. Silicate grains were quite small, on the order of 200 µm. Silicate was almost exclusively irregularly shaped enstatite which showed little internal structure except for the presence of tiny sulfide inclusions. There was no evidence of a rust phase or other contamination. Pyroxene composition was dominated by enstatite, and the few feldspar spots identified were albite rich. We consider KPB as the enstatite phase and KPA as the more metal-rich phase, despite significant enstatite contamination. Complete xenon data are available in supplementary Tables A4 and A5. The xenon data display a clear correlation between 129 Xe * and 127 I in a 3 isotope plot (Fig. 2) but, as for Indarch, the line is not defined by consecutive releases. Again we have not labelled the temperature steps for practical reasons but a step-release diagram is available as supplementary Fig. A2. The arguments presented above

6 888 A. Busfield et al. Fig. 2. Three-isotope plot of Khairpur data. For full sample compositions refer to Table 1 and full Xe data are available in supplemental Tables A4 and A5. As for Indarch (Fig.1), temperature steps are not identified on this figure due to the large number of points. Again, no consistent 129 I/ 127 I ratio is identified in consecutive steps. As for Indarch, a York regression has been performed, giving an initial iodine ratio which corresponds to 4.2 ± 0.7 Ma with a trapped composition of ± This age range may represent a real variation in chondrule ages or can alternatively be interpreted as the lower limit on the initial iodine ration if a component of low-temperature iodine contamination is present. for Indarch are also applicable to Khairpur, and the lowest plausible initial 129 I/ 127 I ratio derived as for Indarch is (0.893 ± 0.006) 10 4, corresponding to an age of 4.2 ± 0.7 Ma. The trapped Xe component has a 129 Xe/ 132 Xe p ratio of ± 0.007, higher than the equivalent ratio of Q-xenon. An increased trapped 129 Xe/ 132 Xe p is consistent with evolution over time in an environment with high I/Xe, assuming a closed system with ~constant I/Xe ratio where the radiogenic 129 Xe * is allowed to mix with ambient xenon. This was also observed and discussed by Kennedy et al. (1988) in the enstatite chondrites where the effect is most pronounced due to the high atomic I/Xe ratio. The ages obtained here by regression to the oldest releases in Indarch and Khairpur of 0.04 ± 0.67 Ma and 4.22 ± 0.67 Ma are significantly younger than those previously obtained by Kennedy et al. (1988) of 1.8 ± 0.4 Ma and 2.1 ± 0.7 Ma, respectively (our signing convention has been used). However, the intervals between the ages we obtain and those of Kennedy et al. (1988) are similar (4.26 ± 0.95 Ma versus 3.9 ± 0.81). Some of this variation between the two studies may be attributable to the different irradiation monitors employed. Gilmour et al. (2006) reported variation in small samples of the Bjurböle monitor used by Kennedy et al. of up to 2.5 Myr, though the larger samples required by analyses using a previous generation of mass spectrometer would be expected to increase reproducibility. While heterogeneity can account for variations between separate samples of the same meteorite, it is hard to see how this can maintain the observed difference between the meteorites when our separate sample suites are analyzed. We note, however, that our data reduction process where releases with progressively lower model ages are incorporated into the fitted data set until the chi-squared criterion is matched is likely to include releases with small contributions of uncorrelated iodine. It is plausible that this led to similar offsets from the true I-Xe age in each sample, and it is for this reason that we describe them as a lower limit and the lowest plausible initial 129 I/ 127 I ratio above. Khairpur belongs to the EL6 meteorite group that shows textural evidence for a greater degree of metamorphism than the EH4 meteorites, while mineralogical thermometers suggest that the EH4 and EL6 meteorites experienced similar equilibration temperatures ( C, Zhang et al. 1996), i.e., the temperature at which their mineral systems underwent closure. Following this, the EHs and ELs apparently experienced very different thermal histories, with very rapid cooling of EHs (>6 C/h, Skinner and Luce 1971) and slower cooling of ELs ( C/Ma, Rubin 1984). Thus the ~4 Ma age difference observed between the EHs and ELs (Kennedy et al. 1988) may reflect the duration of very slow cooling on the EL parent body. However, the preservation of a range of enstatite ages, thought to have originated from

7 Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite 889 different individual chondrules, observed here in both Indarch and Khairpur suggests that the I-Xe system in pyroxene was not reset by this metamorphism and hence ages reflect a difference in age of formation for these meteorites. Khor Temiki The aubrite Khor Temiki consists of large, blue-grey, unbrecciated, unveined enstatite crystals embedded in a friable white matrix consisting of fine-grained forsterite, diopside, and trace enstatite (Hey and Easton 1967). Occasional REE enriched dark clasts are brecciated and consist of large (up to 1 mm) and homogeneous enstatite crystals set in a finegrained matrix. Newsom et al. (1996) suggested the dark coloration was caused by shock darkening and the highly variable REE concentrations are a result of rare oldhamite in some of the clasts. There is no chronological information on Khor Temiki available in the literature. Our sample had a light-colored, powdery surface with occasional rust-colored spots. During crushing most of the sample powdered immediately, revealing many cream to grey prismatic chips. A small number of magnetic grains were also observed. Due to the limited sample size, only one separate suitable for I-Xe study was obtained KTD, a very pure enstatite concentrate (Table 1). The large (~2 mm) enstatite crystals were easily separated from the friable matrix and the BSE images reveal very little contamination from matrix particles. The enstatite had a uniform composition with little Fe and only a very low abundance of tiny sulfide inclusions (Table 1). Each grain was composed only of enstatite with no matrix silicates; these are the blue-grey enstatites described by Hey and Easton (1967) and not the shock clasts described by Newsom et al. (1996). It is expected that enstatite is a major host phase for iodine as it is in the anomalous enstatite achondrite Shallowater, and this proved to be the case. A 3-isotope plot of the KTD data is shown in Fig. 3. With many fewer points than Indarch and Khairpur, it has been possible to label the data with the step numbers (see supplementary Table A6). The isotopes have been normalized to 132 Xe p in this figure rather than to total 129 Xe as in the previous figures. This is purely to allow visualization of the correlation and is mathematically identical to the other 3-isotope plots. Using the step-release diagram (shown as an inset in Fig. 3) it is possible to distinguish a distinct change in the 129 I/ 127 I ratio with increasing temperature. Regression to only the high-temperature data gives a 129 I/ 127 I = (1.07 ± 0.02) 10 4 and a trapped 129 Xe/ 132 Xe p component of 0.05 ± 0.19, which is unusually low. It is conceivable that the trapped 129 Xe/ 132 Xe p in a closed system can evolve to values greater than Q (1.04) but there is no plausible thermal mechanism to decrease this ratio. Caffee et al. (1982) first investigated the effect of shock on I-Xe systematics and Gilmour et al. (2001) have suggested that the apparent low 129 Xe/ 132 Xe p seen in some samples is due to mixing with a xenon endmember residing in an iodine-bearing phase with a low or zero initial 129 I/ 127 I ratio ( dead iodine) and a well-defined 127 I/ 132 Xe ratio. If data are then interpreted as if a xenon-only component is present, an incorrect low 129 Xe/ 132 Xe ratio is derived. Gilmour et al. (2001) envisage a mechanism whereby lightly bound Xe and I are injected into more retentive crystallographic sites by the effect of local adiabatic heating caused by shock. Khor Temiki is known to have suffered severe shock as recorded by the dark clasts (Newsom et al. 1996). There is also evidence for less intense shock recorded in other components of this meteorite. It is therefore plausible that shock could be responsible for the apparent low trapped 129 Xe/ 132 Xe observed in Khor Temiki. Least squares regression to the data leads to a calculated age for KTD of 0.06 ± 0.42 Ma. Although the evidence for shock disturbance complicates chronological interpretation of the data, some tentative conclusions can be drawn. If it is assumed that shock redistribution did not affect the 129 Xe * / 127 I ratio of the high-temperature component (the gradient of the regression line), then the age of the sample is 0.06 Ma. Alternatively, if Khor Temiki pyroxenes formed in the presence of Q-Xe, and assuming that the oldest point (step 11) is least disturbed by shock, a model age of 1.5 ± 0.2 Ma is implied (where the error is determined from the uncertainty in the 129 Xe*/ 127 I ratio of step 11 only). These place upper and lower limits on the age of Khor Temiki pyroxene. The age of Khor Temiki is very similar to that of both the enstatite chondrite Indarch (0.04 Ma) and enstatite achondrite Shallowater, although it is somewhat older than the type 6 Khairpur ( 4.2 Ma). By obtaining an age for Khor Temiki it is possible to fit the aubrite parent body into the picture of evolution in the enstatite meteorite forming region. There is strong evidence that Shallowater originated from a parent body distinct from the aubrites (Keil 1989). The age inferred here demonstrates that igneous parent bodies in this region were undergoing development at the same time. Very little chronological information is available for the aubrites. Recently Shukolyukov and Lugmair (2004) attempted to obtain Mn-Cr ages for aubrites. They showed that the breccia Peña Blanca Spring contained excess 53 Cr, but that it was not correlated with Mn/Cr, indicating early formation of this aubrite followed by secondary disturbance, which Shukolyukov and Lugmair (2004) associated with the breccia-forming event. This scenario is very similar to the interpretation of the I-Xe data presented here, and it seems likely that both Mn-Cr and I-Xe in the aubrites have been severely disturbed by shock. Itqiy Itqiy is a coarse-grained metal-rich enstatite meteorite with achondritic texture. Modal analysis yields 78% silicate, 14% metal, and 8% rust (Patzer et al. 2001). Silicate (En 96.8 Fs 0.2 Wo 3.0 ), which has been extensively recrystallized, forms subhedral, equigranular grains (0.5 4 mm), while the metal grains range from 0.2 to 2 mm. Metal also passes through silicate as a network of veinlets. Patzer et al. (2001)

8 890 A. Busfield et al. Fig. 3. Three-isotope plot of KTD (enstatite) data and the associated regression line. The regression indicates a trapped 129 Xe/ 132 Xe p of 0.05 and an age of 0.06 ± 0.42 Ma. Data included in the regression are shown in black and have been selected with reference to the step-release diagram (inset). This shows a distinct difference between the 129 Xe * / 127 I ratios at low and high temperature. Step numbers are shown adjacent to points. The anomalously low trapped composition suggests the possibility of shock disturbance of the system. An alternative chronological interpretation is to calculate the model age of the oldest point (step 11), which is 1.5 ± 0.2 Ma. See text for discussion. also observed regions of sulfide and metal intergrowths consisting of intergranular assemblages of 3 different sulfides and metal globules. Different features in pyroxene indicate shock stages between 2 and 4 (Patzer et al. 2001). Itqiy enstatite shows no evidence for zoning and has Na 2 O, FeO and MnO abundances similar to EL chondrite pyroxenes, though CaO content is considerably higher than in ELs, whereas kamacite is compositionally identical to that of EH chondrites. The sulfide assemblages consist of a Mg-Mn-Fe sulfide host with oldhamite, Fe-Cr sulfide (consistent with ELs) and metal droplets embedded in it. The host sulfide has a very variable composition that falls within neither the EL or EH fields (Patzer et al. 2001). Itqiy therefore shows no clear affinity to either the EH or EL groups, and hence is classed as anomalous. It has also clearly experienced a complicated thermal history demonstrated by the recrystallized nature of the enstatite grains combined with the presence of the heterogeneous sulfide assemblage. No chronological information is available for this meteorite. This sample was very hard and appeared dark with shiny metallic grains. There was some rusting on the surface. When crushed the rust was pervasive and left fine dust on grain surfaces. Shiny metal grains were separated and designated ItA. The rest of the material consisted of dark, sometimes euhedral, crystals (ItB) and clear grains with occasional black magnetic inclusions (ItC). SEM images showed that ItA contained grains up to 0.5 mm in diameter, and was a mixture of metal and silicate. As well as forming discrete grains the metal in this concentrate was present as veins and inclusions within the enstatite grains. ItB and ItC were originally distinguished by their visual appearance, and microscopic and BSE observation showed that ItC was almost pure enstatite whereas ItB consisted of enstatite with a high abundance of dark (probably metal) inclusions (Table 1). Grain size and appearance were very similar to the previous description of this meteorite (Patzer et al. 2001). ItA is therefore a metal-rich separate, ItB, although enstatite-rich, contains significant contamination by metal, while ItC is a very pure enstatite separate. Complete Xe data are available in supplementary Tables A7 A9. The I-Xe analysis of ItA revealed no 129 Xe * in the majority of releases (Fig. 4a), although there is evidence for some 129 Xe * in the very highest temperature steps with a maximum observed 129 Xe * / 127 I of ~ (for details of the step-release pattern refer to supplementary Fig. A3). The trapped 129 Xe/ 132 Xep composition in ItA is 1.12 ± 0.02,

9 Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite 891 Fig. 4. Three-isotope plots of Itqiy data. Full Xe data available in supplemental Tables A7 A9. a) ItB and low-temperature ItC data lie on a mixing line with an implied age of 85 Ma. b) High-temperature ItC data define an isochron of 2.6 ± 2.6 Ma. The trapped 129 Xe/ 132 Xe p is well defined at For full sample compositions refer to Table 1. The transition from low temperature to high temperature involved a significant change in the 129 I/ 127 I ratio, observed at 13.5A (Step 5) for ItC and 18A (step 11) for ItB. The highest ItA steps, containing 129 Xe *, fall closest to the y-axis and are visible in (b). higher than Q (1.04). As discussed in the Khairpur section the elevated trapped 129 Xe/ 132 Xe p composition is consistent with evolution over time. A 3-isotope plot of ItB and ItC data reveals that they can be clearly separated into two discrete subsets defining two apparent isochrons. The later of these is dominated by ItB and low-temperature ItC data. The earlier isochron is defined predominantly by ItC data. These distributions are illustrated in Fig. 4. As was the case for Indarch and Khairpur data we have not shown the release temperature steps on Fig. 4 for practical reasons and a step-release diagram is available as supplementary Fig. A3. The transition from the lowtemperature correlation to the high-temperature correlation (i.e., a significant change in 129 I/ 127 I) occurred in step 5 (13.5A) for ItC and at step 11 (18A) for ItB. The lowtemperature ItC points are clearly visible on Fig. 4a lying on

10 892 A. Busfield et al. the ItB line and in Fig. 4b lying above the ItC line. Least squares regressions to these data sets yield slightly elevated trapped ratios of ± (ItB) and ± (ItC). A value of 1.06 has therefore been used to calculate excess 129 Xe *. The fit to ItC high-temperature points yields an isochron with a 129 Xe * / 127 I ratio of (9.58 ± 1.10) 10 5 (Fig. 4). The highest temperature ItB steps are consistent with the ItC ratio of The majority of the ItB data and lowtemperature ItC points describe a regression line with a 129 Xe * / 127 I ratio of (2.50 ± 0.47) These ratios correspond to ages of 2.6 ± 2.6 Ma (ItC) and 85 ± 4 Ma (ItB). The petrology of Itqiy indicates a complex thermal history recording at least two major events: crystallization of large silicate grains and later heating indicated by the mixed sulfide assemblage (Patzer et al. 2001). The most enstatite rich separate, ItC, shows the best defined isochron which is associated with early silicate formation or recrystallization. The age, 2.6 ± 2.6 Ma, is entirely consistent with previously obtained ages for enstatite achondrites (i.e., Shallowater and Khor Temiki) and other igneous meteorites. No sulfide separate was made from this meteorite, but kamacite should also have been affected at the temperatures sufficient to cause sulfide melting (McCoy et al. 1999). The metal separate, ItA, has a similar (order of magnitude) iodine concentration to the other Itqiy separates (Table 1), so the lack of a 129 Xe * excess cannot be attributed to the absence of iodine and must be the result of resetting, probably by the sulfide-forming event. 129 Xe * observed in the highest ItA temperature steps was probably derived from enstatite contamination of this separate. These steps are visible in Fig. 4b, lying closest to the y-axis. The ItB correlation could be a result of mixing between the iodine released from the metal and the 129 Xe * released from the silicate at each temperature step since ItB contains significant metal. In this case, the isochron has no chronological significance. However, the ItC data points that fall on the ItB correlation account for about 50% of the iodine released from this separate, invoking the possibility of either a late-stage, low-temperature resetting event or the presence of a minor low-temperature phase associated with the enstatite that has not been identified in the grain mounts. With the present data, we favour the latter interpretation. Asuka The cumulate eucrite A is a coarse-grained igneous rock consisting of granular pyroxene crystals ( mm) with interstitial plagioclase. Nyquist et al. (2003) determined modal mineral abundances to be pyroxene 49%, plagioclase 45%, silica 5% and chromite 0.5%. Pyroxene (Ca 2 Mg 54 Fe 44 ) shows well-developed exsolution with herring-bone texture and is well separated from the lamella augite (Ca 42 Mg 39 Fe 19 ). Opaques are mostly chromite with rare troilite. Plagioclase is very calcic with a composition of An Chromite composition (Chr 72 Ulv 10 Her 18 ) suggests formation below 800 C (Sack and Ghiorso 1991). Oxygen isotopes group this meteorite with the HED clan (Nyquist et al. 2003). A is an unusual cumulate eucrite in that it contains magnesian pigeonite with a metamorphic, granular texture, and the thickness of its exsolution lamellae suggests relatively fast cooling. The poikilitic texture of chromite indicates that a metamorphic event is responsible for the observed textures. A granulitic origin was proposed for this meteorite based on the rounded form of the pyroxene grains and granoblastic texture of plagioclase (Nyquist et al. 2003). A possible thermal history is one of recrystallization as extensive as that of other cumulate eucrites, but with faster subsequent cooling. Rb-Sr and Sm-Nd systems indicate an age of ~ Ga, the 146 Sm- 142 Nd chronometer gives an age of 4 ± 26 Ma relative to the Lewis Cliff (LEW) angrite, 53 Mn yields an age of 6 ± 2 Ma before LEW and the 26 Al formation interval in plagioclases is 3.95 ± 0.13 Ma relative to CAIs (Nyquist et al. 2003). The postulated thermal history of this meteorite makes it an ideal candidate for study with multiple chronometers, as fast cooling should mean the different closure temperatures are encountered in quick succession. Our sample of A was relatively coarse-grained (up to ~5 mm diameter) and consisted predominantly of green/brown (pyroxene) grains, and clear or white (plagioclase) grains. Minor amounts of shiny black grains were observed and removed from the silicate separates. Where this material was observed as inclusions within the white silicate grains the grains were rejected, although the presence of very small inclusions cannot be ruled out in this separate. The phases identified by SEM in A were feldspar, pyroxene, silica, and chromite, with individual grains ranging up to ~0.75 mm. Both pyroxene and feldspar showed evidence of exsolution, as previously described for this meteorite (Nyquist et al. 2003). The only opaque phase identified was chromite. The large grain size of this meteorite allowed the phases to be separated relatively easily resulting in pure concentrates (see Table 1). There was no evidence of rusting or low-temperature alteration from the SEM images. Probe data confirmed two discrete pyroxene compositions with augite lamellae exsolved from the low Ca host pyroxene. Feldspar was very anorthite-rich. The two concentrates were designated AsA (plagioclase-rich) and AsB (pyroxene-rich). Complete Xe data are available in supplementary Tables A10 and A11. The iodine and xenon concentrations measured in both separates were significantly lower than in the enstatite meteorites (Table 1). AsA shows a clear excess of 129 Xe *, although no consistent 129 Xe * / 127 I ratio is implied. In the steprelease diagram for AsA (Fig. 5), 129 Xe * / 127 I evolves to higher values with increasing step-release temperature to a maximum of ~ Using the maximum observed 129 Xe * / 127 I in

11 Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite 893 Fig. 5. AsA (feldspar) step-release plot. Although no 129 Xe * / 127 I plateau is seen the 129 Xe * / 127 I increases with release temperature to a maximum of This pattern is indicative of resetting due to thermal metamorphism and indicates that the I-Xe system was reset within 25 Ma after Shallowater. AsA gives an age for the onset of closure to Xe after metamorphism of <~25 Ma after Shallowater. Despite a similar I/ 132 Xe ratio in the AsB (pyroxene) separate, there was little evidence of any 129 Xe * and no correlation of 129 Xe * with iodine. A is an unusual eucrite both petrologically and in terms of short-lived isotopes. Unlike other eucrites, it has a metamorphic granulitic texture and appears to have crystallized very early with a 26 Al age of ± 0.6 Ma and a 53 Mn age of 4564 ± 2 Ma (Nyquist et al. 2003). Additionally, initial 146 Sm/ 144 Sm ratios indicate an ancient age of 4562 Ma (Nyquist et al. 2001a). Our data suggest that 26 Mg escaped redistribution during the metamorphic event that formed the granulitic texture, and that this event significantly post-dates crystallization. It is noteworthy that the Mn-Cr and I-Xe chronometers that behave concordantly among the enstatite chondrites have responded differently in this case; however, this is not unexpected since they are not recording events from a common host phase in A The less precise Rb- Sr and Sm-Nd systems record later ages of 4370 ± 60 Ma and 4490 ± 20 Ma (Nyquist et al. 2001a), suggesting that they were set later than the onset of xenon retention in some sites. CORRELATION OF SHORT-LIVED ISOTOPE TIME SCALES Our data demonstrate that silicate is the phase responsible for the high-temperature I-Xe isochron observed in the enstatite chondrites Indarch and Khairpur. We obtain a lowest possible initial iodine ratio corresponding to a latest possible formation age for each meteorite: 0.04 ± 0.67 Ma for Indarch and 4.2 ± 0.7 Ma for Khairpur. Our derived ages are similar, but not identical, to those of Kennedy et al. (1988), suggesting possible heterogeneity in these meteorites. The age difference between Indarch and Khairpur is ~4 Ma in both our data set and those of Kennedy et al. (1988). Shukolyukov and Lugmair (2004) obtained a Mn-Cr isochron from Indarch silicate and chromite, which yielded an initial 53 Mn/ 55 Mn at time of isotopic closure of (2.7 ± 0.2) Since both the 129 I and 53 Mn systems date closure in silicate phases, we might expect to find closure intervals measured by these two systems to agree. Further Mn-Cr analyses revealed an age difference between Indarch and Khairpur of 4.5 ± 0.3 Ma (Shukolyukov and Lugmair 2004), very similar to the age difference defined here by the I-Xe clock. We now investigate whether this consistency is maintained across a wider range of samples. In previous work (Busfield 2004; Gilmour et al. 2006), we have shown that a plausible correlation exists between I-Xe formation intervals and model ages derived from the Pb-Pb system. This lends support to the chronological interpretation of data from these two systems and allows us to consider the consistency of Mn-Cr data with the candidate unified system based on Pb-Pb and I-Xe data. In Fig. 6 we present a summary plot of all samples of which we are aware for which data exist in either the Pb-Pb or I-Xe systems (Gilmour et al. 2006) and the Mn-Cr system. Mn-Cr formation intervals are calculated relative to LEW and mapped to the absolute time scale using the Pb-Pb age of this meteorite ( ± 0.5 Ma; Lugmair and Galer 1992). A credible correlation between Mn-Cr and the combined

12 894 A. Busfield et al. Fig Mn ages (calibrated by Pb age of LEW 86010) against Pb-Pb or 129 I ages (calibrated via Gilmour et al. 2006: absolute Shallowater age = ± 0.4 Ma). Solid squares are samples included in the regression, open squares are excluded samples, light grey boxes are uncorrected enstatite chondrites, dark grey boxes are enstatite chondrites corrected for a 53 Mn heterogeneity (see text). The dotted line represents a Williamson least-squares regression to the included points. Rich. indicates Richardton and Ste. Marg. indicates Ste. Marguerite. Earliest chondrules are Chainpur ( 53 Mn and 129 I) and Allende (Pb-Pb). Enstatite chondrites are shown by an age range that extends from the older ages of Kennedy et al. (1988) to the age obtained here which may represent the lower limit of the initial iodine ratio of the relative meteorites (see text for discussion). The solid symbol indicates the model age of the release with the highest initial iodine ratio. Corrected enstatite chondrite ages were obtained by applying the Shukolyukov and Lugmair (2004) correction for 53 Mn heterogeneity. Earliest chondrules: Nyquist et al. (2001b), Amelin et al. (2004), and Swindle et al. (1991); Ste. Marg.: Polnau and Lugmair (2001), Göpel et al. (1994), and Brazzle et al. (1999); A : Nyquist et al. (2003), Amelin et al. (2006); D Orbigny: Glavin et al. (2004) and Zartman et al. (2006); Indarch and Khairpur: Shukolyukov and Lugmair (2004), Kennedy et al. (1988), and this work; Forest Vale: Göpel et al. (1994) and Polnau et al. (2000); LEW 86010: Lugmair and Galer (1992) and Lugmair and Shukolyukov (1998); Ibitira: Lugmair and Shukolyukov (1998) and Amelin et al. (2006); Richardton: Polnau and Lugmair (2001), Brazzle et al. (1999), Göpel et al. (1994), Pravdivtseva et al. (1998), and Amelin (2001); Acapulco: Zipfel et al. (1996), Amelin (2005), and Brazzle et al. (1999). Pb-I chronology is apparent, suggesting the chronometers are coherent in a large subset of the samples. The earliest chondrules points represents the earliest chondrule ages obtained in each system: Chainpur for 53 Mn (Nyquist et al. 2001b) and 129 I (Brazzle et al. 1999), and Allende for Pb-Pb (Amelin et al. 2004). The notable outliers represent the range of Pb-Pb and I-Xe ages recorded by the different phases in Acapulco and the ordinary chondrites, since many of these meteorites have only a single (whole rock) Mn-Cr age. These samples, along with the enstatite chondrites, are excluded from the regression (Williamson 1968) to the data shown in Fig. 6, which has a slope of 1.07 ± 0.12 and intercept of 300 ± 500, and is shown in bold. A gradient of 1 and intercept of 0 are what would be expected for coherent chronometers and this regression confirms the angrites as a robust calibration for the Mn-Cr system. Examination of the correlation with respect to the ordinary chondrites and Acapulco reveals that the whole rock Mn-Cr system was reset by the events that formed feldspar (in the case of Richardton) and phosphate (in the case of Acapulco). This is unlike the enstatite chondrites in which Mn-Cr and I-Xe both remain intact in the high-temperature pyroxene phase. This result is not unexpected as Mn is partially hosted by olivine in ordinary chondrites and Acapulco (Allen and Mason 1973; Zipfel et al. 1996) and closure temperatures for Cr in olivine can be a lot lower than in enstatite (Ganguly et al. 2007). The enstatite chondrites are shown on Fig. 6 by an age range for each meteorite. This extends from the Kennedy et al. (1988) age to the lower limits on age derived in this work. The maximum age corresponding to the highest initial iodine ratio observed in any release from each meteorite is shown as a solid symbol. The enstatite chondrites are offset from the regression line. Based on the relationship between ε 53 Cr and initial 53 Mn/ 55 Mn ratios, Shukoluykov and Lugmair (2004) proposed radial heterogeneity in 53 Mn/ 55 Mn. They suggested a correction should be applied to derived initial 53 Mn/ 55 Mn ratios to allow enstatite chondrite and ordinary chondrite ages to be compared. Their correction factor converts the initial 53 Mn/ 55 Mn ratio in the region of the ordinary chondrites (i,oc) to the expected initial ratio in the enstatite chondrite formation zone (i,ec) such that