I-Xe Analysis of Enstatite Meteorites and a Eucrite

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

Item Type Article; text

Authors Busfield, A.; Turner, G.; Gilmour, J. D.

Citation Busfield, A., Turner, G., & Gilmour, J. D. (2008). Testing an integrated chronology: IXe analysis of enstatite meteorites and a eucrite. Meteoritics & Planetary Science, 43(5), 883-897.

DOI 10.1111/j.1945-5100.2008.tb01087.x

Publisher The Meteoritical Society

Journal Meteoritics & Planetary Science

Download date 26/09/2021 21:32:45

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Version Final published version

Link to Item http://hdl.handle.net/10150/656429 Meteoritics & Planetary Science 43, Nr 5, 883–897 (2008) AUTHOR’S PROOF Abstract available online at http://meteoritics.org

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. E-mail: [email protected] (Supplementary tables and figures are available online at http://meteoritics.org/online supplements.htm) (Received 06 October 2006; revision accepted 21 November 2007)

Abstract–We have determined initial 129I/127I 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-) 881394 yielded a consistent ratio, though excess 129Xe 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 4563.3 ± 0.4 Ma. Comparison of the combined 129I-Pb data with associated 53Mn 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 53Mn ages are plotted against combined 129I-Pb data, supporting the suggestion that 53Mn was radially heterogeneous in the early solar system.

INTRODUCTION can be present in both primary minerals such as pyroxene (Hohenberg 1967) and secondary minerals (e.g., halide, Iodine-129 was incorporated into meteoritic material in Busfield et al. 2004), the system may in principle date the early solar system, where it decayed to 129Xe with a half- formation or subsequent processing. life of 16 Ma (Jeffrey and Reynolds 1961; Reynolds 1960). A By assuming consistency among the Pb-Pb, Mn-Cr, and relative chronology based on this decay scheme is now well I-Xe systems in the Ste. Marguerite ordinary chondrite established. In addition, recent work shows a good correlation Gilmour and Saxton (2001) identified a 1–2 Ma discrepancy between the I-Xe and Pb-Pb systems, suggesting that both between the accepted Pb-Pb age of Acapulco phosphate provide valid chronological information (Busfield 2004; (Göpel et al. 1992) and the Mn-Cr/I-Xe ages for this Busfield et al. 2004; Gilmour et al. 2006). meteorite. Understanding the reason for this apparent In the I-Xe technique (recently reviewed by Gilmour et al. inconsistency between the chronometers is important 2006), samples are neutron-irradiated, transmuting 127I to because Acapulco phosphate has traditionally been used as 128Xe. Step heating experiments are performed and xenon the absolute time anchor for the I-Xe system. Subsequently, isotopic analyses made on the gas released in each of a series a re-examination of data from Acapulco phosphates (Amelin of sequentially increasing temperature steps. Data representing 2005) gave an age in line with the adjustment in the a mixture between a trapped xenon component and an iodine- calibration of the I-Xe system (Gilmour et al. 2006). Thus rich component with a consistent 129Xe*/I ratio (where 129Xe* there is good evidence that all three chronometers showed indicates the excess 129Xe over trapped 129Xe) are identified general consistency in the early solar system. by an isochron technique, whereupon 129Xe*/I corresponds to Limited data from ordinary chondrites also suggest 129I/127I on isotopic closure. coherence of the Mn-Cr and I-Xe systems (Busfield 2004; The event dated by the I-Xe system is constrained by the Gilmour et al. 2006). However, Shukolyukov and Lugmair mineral phase(s) responsible for the isochron. Since iodine (2004) have presented evidence of a heterogeneous distribution

883 © The Meteoritical Society, 2008. Printed in USA. 884 A. Busfield et al. of 53Mn across the early solar system, with variation in 53Mn/ work was not intended to be a detailed petrographic study 55Mn between the ordinary and enstatite chondrite source of the meteorites, but was designed to allow the different regions. This was based on variations in ε53Cr among the phases in each separate to be identified. The abundances of terrestrial, Martian, enstatite chondrite, and ordinary these phases were estimated from backscatter electron chondrite reservoirs. (ε units indicate a deviation in the (BSE) images. Electron-probe analyses were carried out on isotopic composition of a sample relative to a standard in the separates to determine the composition of minerals and to parts per ten thousand such that ε = 10,000 × [(smpl-std)/std]). assess the homogeneity of each phase. Three to five spot The enstatite meteorites are thought to originate from much analyses were measured in all but the smallest grains. Where closer to the Sun than the other chondrites and eucrites grains or inclusions were very tiny, only 1 spot measurement (Baedecker and Wasson 1975), whose source regions are was made. Phase abundances are given in Table 1. considered to be within the asteroid belt. Birck et al. (1999) Samples were tightly wrapped in small aluminium foil argued that the same data could be explained by the volatility packets which were sealed for irradiation. Small quantities differences between Mn and Cr. If 53Mn was indeed of the irradiation monitor, the non-magnetic fraction of the heterogeneous in the early solar system then its use as a anomalous enstatite achondrite Shallowater, were wrapped chronometer might be limited to obtaining relative ages and sealed in the same way. Foil packets were weighed within small, well-defined source regions. Shukolyukov before and after addition of the sample material, to aid in and Lugmair (2004) have suggested a correction that can later identification of irradiated samples, and then sealed be applied to enstatite meteorite 53Mn data to allow their in evacuated quartz tubes. Samples were irradiated in data to be interpreted chronologically. One goal of this work irradiation Mn19 at the Penubaba Reactor, South Africa has been to examine whether direct evidence of variation in (fast fluence 1.7 × 1018 n cm−2; thermal fluence 6.7 × 53Mn/55Mn in the early solar system can be found by 1018 ncm−2). Analysis of the Shallowater standards comparing the I-Xe and Mn-Cr chronometers between the indicated that the 128Xe/127I conversion factor (i.e., the enstatite and ordinary chondrites. The approach adopted has efficiency of conversion of 127I to 128Xe) was (6.320 ± been to extend the data set of mineral-specific 129I ages so 0.008) × 10−5, assuming a 129I/127I ratio of 1.072 ×10−4 in that the processes responsible for setting or resetting the Shallowater (Brazzle et al. 1999). chronometer can be constrained. We have also attempted to Xenon measurements were conducted using the RELAX extend the I-Xe system to achondrites (enstatite and eucrite), (Refrigerator Enhanced Laser Analyser for Xenon) mass motivated by the need to provide more points of comparison spectrometer (Gilmour et al. 1994). Samples were unwrapped between I-Xe and other potential chronometers. from their foil packets, loaded into the RELAX sample port and The samples analyzed in this work are the two enstatite baked overnight. Gas was released by a laser stepped-heating chondrites Indarch (EH4) and Khairpur (EL6), the aubrite technique as described in Gilmour et al. (1995). Heating steps Khor Temiki, an anomalous enstatite achondrite Itqiy and lasted for 2 min and then the evolved gas was gettered for an the cumulate eucrite Asuka (A-) 881394. In this work, additional minute to remove active gases before being admitted negative relative ages indicate setting of the chronometer to the mass spectrometer. Analysis proceeded for 5 min and the after the standard which, for the 129I-129Xe system, is the data were subsequently blank corrected and reduced as non-magnetic fraction of the anomalous enstatite achondrite described in Gilmour et al. (1998, 2000) who give full details of Shallowater. Absolute ages can be obtained by adopting an the correction for fission Xe and a trapped Xe component. absolute age for the monitor. In this work we adopt an age Throughout this work the trapped component is assumed to be for Shallowater of 4563.3 ± 0.4 Ma, proposed by Gilmour et al. equal to Q-Xe (Busemann et al. 2000), except for the 129Xe/ (2006). 132Xe ratio; details specific to each sample are given below. Thus 132Xe corrected for fission is indicated by the subscript MINERAL CONCENTRATION, SAMPLE “p” (for planetary) and excess Xe over the trapped CHARACTERIZATION, AND DATA REDUCTION component is denoted by an asterisk (as in 128Xe*). The 129I/ 127I ratio is calculated from the 129Xe*/128Xe* ratio and Mineral separates were made by crushing samples in an reference to the 128Xe*/127I conversion factor. Gas agate pestle and mortar and hand-picking. The mortar was concentrations were calculated with reference to analyses of cleaned by twice crushing clean quartz before being wiped out an air aliquot of known volume. Cosmogenic effects on with acetone between each sample. In all cases, the amount of 128Xe and fissiogenic contributions to 132Xe were negligible. processing and crushing was kept as low as possible to Cold (no laser) procedural blanks were typically 2 × minimize the possibility of contamination. Samples were hand- 10−16 cm3 of 132Xe and are negligible except in the case of picked under a binocular microscope. A-881394 where the blank amounts to ~1% of the sample gas. After picking, samples were weighed out and a further Data are summarized in Table 1 and full Xe data are few grains were mounted and polished for electron-probe and available as supplementary data tables at http:// scanning electron microscopy (SEM) characterization. This meteoritics.org/online supplements.htm. Our technique does Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite 885 1.5 − 0.67 2.6 0.67 ± ± ± 0.06 to 25 − − 4.22 2.6 Age relative to Shallowater (Ma) 0.04 − − < 4 − ? 4 5 − − 5 10 − × 10 10 10 5 × × − × 3 10 I 0.006) × 1.10) 0.02) 127 ± / 0.06) ± ± * ± Xe

–– –– 129 I 127 0.6 (9.58 0.03 0.3 (1.07 0.01 30 – – ) 5 ± ± ± ± ± − Xe*/ Total 129 (/10 (1.074 1 1

200 3.8 35 70 Max 3.8 70 Max 9 7.79 1 8.8 3 20 20 20 (8.9 20 7.20 80 1 2 ) ± ± ± ± ± ± ± ± ± ± ± 13 STP − 1 − Xep] 10 2430 5320 132 × cc g ( [ m fission Xe. See text for discussion. 1 1 . σ 0.10.02 7163 35 0.03 134 0.4 0.1 7690 0.4 2790 1 38600 0.2 8570 13 0.4 3 0.4 3 ± ± ± ± ± ± ± ± ± ± ± 254 text for how this ratio was obtained. 149.5 [Total I] [Total ppb * * Asuka-881394. = 0.88 8 1.00 3.535.12 70.4 2.27 5.30 2.3 3.24 2.07 1.74 0.2 Mass (mg) st during loading. Xe] has been corrected for contribution fro Itqiy; As 132 = of sample was lo I used to define age of sample. See Approximate vol% 127 / e to sample loss during loading. * Khor Temiki; It Khor Temiki; Xe = 129 tary” and indicates the [ Fe-sulfide 5 Sulfide 5 Metal 10 Chromite <1 Khairpur; KT = Indarch; KP = Denotes maximum mass as a small amount Indicates a maximum concentration du Subscript “p” denotes “plane or maximum Ratio of correlated Achondrite Metal 25 ItC Enstatite Enstatite ~99 IndC Enstatite 60 KPB Enstatite 95 EL6KTD Sulfide Enstatite ~99 5 ItB Enstatite Enstatite 75 AubriteItA Enstatite MetalAchondrite Sulfide Enstatite 75 <1 25 KPAEL6 Enstatite Metal 50 45 3.97 51.7 IndBEH4EH4 EnstatiteIndD Other sulfideEH4 10 85 Metal Enstatite Sulfide 1.03 80 40 10 214.2 7.90 AchondriteAsACumulate Metal Silica Anorthite ~1 80 15 Eucrite Silica 5 Ind * 1 2 3 EucriteAsBCumulate Pyroxene Anorthite Pyroxene <5 10 85 2.41 0.2 Table 1. Summary of mineral separates and Xe data. All errors are 1 Table 886 A. Busfield et al. not allow us to determine release temperatures. We do which is typical of I-Xe analyses. At higher temperatures excess however state the current of the heating laser as a proxy 129Xe* was also released. The large number of releases preclude for temperature, although it is not clear if there is a linear numbering temperature steps in Fig. 1, but generally points relationship between current and heating temperature. lying further to the right were obtained at lower temperature; a step-release diagram is available as supplementary Fig. A1. RESULTS AND DISCUSSION None of the separates define a consistent 129Xe*/127I ratio over consecutive steps but a well-defined maximum in the Iodine-Xenon Analyses 129I/127I ratio is observed. Closer examination of the data reveals that the maximum involves temperature steps from This section provides a brief description of each sample, IndB and IndD but not IndC. Comparison of data from IndB the techniques employed in mineral separation, and any (enstatite) and IndC (enstatite + metal) suggests that pyroxene relevant previous chronological data. We also discuss the Xe is the host phase that defines the upper limit while metal results obtained for each sample and, where necessary, contains more recent/uncorrelated iodine. The maximum is compare them to other results. Results of xenon analyses are best defined by data from fine grained IndD, which can be summarized in Table 1 and full Xe isotopic data are available understood statistically; a wider range of chondrules can in a supplementary table. contribute to release from a fine grained sample than to a coarse grained sample of similar mass, so each extraction Indarch approximates the average more closely than extractions from Indarch (EH4, Sears et al. 1982) consists of silicate-rich a coarser separate. A similar effect is observed in larger chondrules, metal-rich chondrules, and a heterogeneous samples of meteorites, which often produce isochrons even matrix. Keil (1968) determined the modal composition of when small samples or chondrule-by-chondrule analyses Indarch to be 73 wt% silicate, 17 wt% Ni-Fe metal, 7 wt% record a range of initial 129I/127I ratios (Gilmour et al. 1997). troilite with minor schreibersite, niningerite, oldhamite, Alternatively, this sample may exhibit superior separation of daubréelite, and graphite. The silicate phase is dominated by correlated from uncorrelated iodine because the larger pure MgSiO3 (clinoenstatite, Leitch and Smith 1982). pyroxene crystals of IndB are contaminated with iodine in Kennedy et al. (1988) determined a whole rock I-Xe age for occluded grains, this component being more effectively Indarch of 1.8 ± 0.4 Ma before Shallowater and Shukolyukov separated in the step heating of finer grained samples since and Lugmair (2004) calculated an initial 53Mn/55Mn ratio of once-occluded grains have been exposed. (2.7 ± 0.2) × 10−6. The scatter in data from pyroxene ages may reflect Our sample was rusted all over its surface. Crushing and pyroxene crystals from separate chondrules retaining picking took place under ethanol so that rust particles floated different initial iodine ratios. This would suggest that free and rust-free grains could be selected. Three mineral chondrule ages were not reset by the thermal metamorphism concentrates were prepared. IndB was dominated by of the Indarch parent body. Alternatively, a consistent high subhedral, medium sized (200–400 µm), dark grey enstatite initial iodine ratio may be disguised by different amounts of crystals. IndC nominally consisted of magnetic grains, but uncorrelated iodine contributing to different releases. SEM analysis revealed enstatite to be a significant Bearing these alternatives in mind, we have characterized the contaminant. IndD consisted of the residue from repeated lower limit on initial iodine ratio of our data by performing crush-pick cycles. It consisted of fine-grained magnetic York fits (York 1969) to data selected in order of decreasing powder with occasional grains up to ~100 µm diameter. apparent initial iodine ratio until the resulting reduced χ2 Silicate, metal, and sulfide were present in IndD in the (chi-squared per degree of freedom) approaches 1. This approximate proportions of the bulk sample (Keil 1968). yields a trapped 129Xe/132Xe ratio of 1.04 ± 0.03 consistent Pyroxene composition was enstatite with up to ~4% Fe. with Q-Xe. Patzer and Schultz (2002) showed that noble gas Sulfides were dominated by Fe-sulfide although IndB ratios in Indarch are similar to Q-xenon, though Kennedy et contained some more Mn-rich sulfides. Thus the purest al. (1988), reported an anomalously low trapped 129Xe/130Xe mineral separate was IndB; IndC contained a significantly ratio in bulk Indarch. Relatively high 132Xe concentrations in higher proportion of metal than the other separates, and IndD fine-grained IndD suggest a matrix host for trapped xenon, most closely resembles a “bulk rock” composition (Table 1). suggesting either planetary xenon or implantation of Indarch Xe data are displayed in Fig. 1 and summarized in terrestrial atmosphere during crushing. The iodine-derived Table 1. Complete xenon isotopic data are available in end member has a 129I/127I ratio of (1.074 ± 0.006) × 10−4, Supplementary Tables 1–3. Iodine concentrations from all corresponding to an age of 0.04 ± 0.67 Ma relative to separates are high and similar. The slightly lower abundance of Shallowater. This is consistent with the spread of initial iodine in IndC may be related to the significantly lower iodine ratios reported by Whitby et al. (2002) in their study concentration of enstatite in this separate. All concentrates of EH3 meteorites, suggesting a range of chondrule ages has released iodine (unassociated with 129Xe*) at low temperature, survived to be sampled by our analysis. 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 129I/127I 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 129I/127I ratio until the reduced χ2 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 129Xe/132Xe composition indicated by this fit is 1.04 ± 0.03.

Khairpur separates were dominated by enstatite, though KPA also Khairpur (EL6) consists of metallic Ni,Fe (grains and contained Fe,Ni-metal (Table 1). Both KPA and KPB irregular patches) and troilite (in smaller grains) set in a contained a sulfide phase as small discrete grains. Silicate crystalline enstatite matrix. Large grains of oldhamite are also grains were quite small, on the order of 200 µm. Silicate present (Prior 1916). Silicate material is predominantly was almost exclusively irregularly shaped enstatite which enstatite with some albitic feldspar. The modal composition showed little internal structure except for the presence of of Khairpur is 75 wt% silicate, 13 wt% Ni,Fe-metal, 10 wt% tiny sulfide inclusions. There was no evidence of a “rust” troilite, with minor schreibersite, daubreelite, oldhamite, phase or other contamination. Pyroxene composition was ferroan alabandite, and graphite (Keil 1968). Kennedy et al. dominated by enstatite, and the few feldspar spots identified (1988) reported an I-Xe age of −2.1 ± 0.7 Ma relative to were albite rich. We consider KPB as the enstatite phase Shallowater, while Shukolyukov and Lugmair (2004) and KPA as the more metal-rich phase, despite reported 53Mn/55Mn = 1.21 × 10−6. significant enstatite contamination. Our sample was very hard with black, shiny, metallic Complete xenon data are available in supplementary areas. Repeated crushing was needed to separate material. At Tables A4 and A5. The xenon data display a clear correlation each crushing step small silicate grains and single grains of between 129Xe* and 127I in a 3 isotope plot (Fig. 2) but, as for magnetic material were retained, the remainder being crushed Indarch, the line is not defined by consecutive releases. further. Nominally, KPA consisted of blue-black grains of Again we have not labelled the temperature steps for magnetic material, while KPB was the non-magnetic practical reasons but a step-release diagram is available (silicate) residue. Later SEM analyses revealed that both as supplementary Fig. A2. The arguments presented above 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 129I/127I 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 1.108 ± 0.007. 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 account for variations between separate samples of the plausible initial 129I/127I ratio derived as for Indarch is (0.893 ± same meteorite, it is hard to see how this can maintain the 0.006) × 10−4, corresponding to an age of −4.2 ± 0.7 Ma. The observed difference between the meteorites when our 129 132 ± trapped Xe component has a Xe/ Xep ratio of 1.108 separate sample suites are analyzed. We note, however, that 0.007, higher than the equivalent ratio of Q-xenon. An our data reduction process where releases with progressively 129 132 increased trapped Xe/ Xep is consistent with evolution lower model ages are incorporated into the fitted data set until over time in an environment with high I/Xe, assuming a the chi-squared criterion is matched is likely to include closed system with ~constant I/Xe ratio where the radiogenic releases with small contributions of uncorrelated iodine. It is 129Xe* is allowed to mix with ambient xenon. This was also plausible that this led to similar offsets from the “true” I-Xe observed and discussed by Kennedy et al. (1988) in the age in each sample, and it is for this reason that we describe enstatite chondrites where the effect is most pronounced due them as a “lower limit” and the “lowest plausible initial 129I/ to the high atomic I/Xe ratio. 127I ratio” above. The ages obtained here by regression to the oldest Khairpur belongs to the EL6 meteorite group that shows releases in Indarch and Khairpur of 0.04 ± 0.67 Ma and −4.22 ± textural evidence for a greater degree of metamorphism than 0.67 Ma are significantly younger than those previously the EH4 meteorites, while mineralogical thermometers obtained by Kennedy et al. (1988) of 1.8 ± 0.4 Ma and −2.1 ± suggest that the EH4 and EL6 meteorites experienced similar 0.7 Ma, respectively (our signing convention has been used). equilibration temperatures (600–800 °C, Zhang et al. 1996), However, the intervals between the ages we obtain and those i.e., the temperature at which their mineral systems underwent of Kennedy et al. (1988) are similar (4.26 ± 0.95 Ma versus 3.9 closure. Following this, the EHs and ELs apparently ± 0.81). Some of this variation between the two studies may experienced very different thermal histories, with very rapid be attributable to the different irradiation monitors cooling of EHs (>6 °C/h, Skinner and Luce 1971) and slower employed. Gilmour et al. (2006) reported variation in small cooling of ELs (103–104 °C/Ma, Rubin 1984). Thus the samples of the Bjurböle monitor used by Kennedy et al. of up ~4 Ma age difference observed between the EHs and ELs to 2.5 Myr, though the larger samples required by analyses (Kennedy et al. 1988) may reflect the duration of very slow using a previous generation of mass spectrometer would be cooling on the EL parent body. However, the preservation of a expected to increase reproducibility. While heterogeneity can range of enstatite ages, thought to have originated from Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite 889 different individual chondrules, observed here in both Indarch ratio. If data are then interpreted as if a xenon-only and Khairpur suggests that the I-Xe system in pyroxene was component is present, an incorrect low 129Xe/132Xe ratio is not reset by this metamorphism and hence ages reflect a derived. Gilmour et al. (2001) envisage a mechanism difference in age of formation for these meteorites. whereby lightly bound Xe and I are injected into more retentive crystallographic sites by the effect of local adiabatic Khor Temiki heating caused by shock. Khor Temiki is known to have The aubrite Khor Temiki consists of large, blue-grey, suffered severe shock as recorded by the “dark clasts” unbrecciated, unveined enstatite crystals embedded in a friable (Newsom et al. 1996). There is also evidence for less intense white matrix consisting of fine-grained forsterite, diopside, shock recorded in other components of this meteorite. It is and trace enstatite (Hey and Easton 1967). Occasional REE therefore plausible that shock could be responsible for enriched dark clasts are brecciated and consist of large (up to the apparent low trapped 129Xe/132Xe observed in Khor 1 mm) and homogeneous enstatite crystals set in a fine- Temiki. grained matrix. Newsom et al. (1996) suggested the dark Least squares regression to the data leads to a calculated coloration was caused by shock darkening and the highly age for KTD of −0.06 ± 0.42 Ma. Although the evidence for variable REE concentrations are a result of rare oldhamite in shock disturbance complicates chronological interpretation of some of the clasts. There is no chronological information on the data, some tentative conclusions can be drawn. If it is Khor Temiki available in the literature. assumed that shock redistribution did not affect the 129Xe*/127I Our sample had a light-colored, powdery surface with ratio of the high-temperature component (the gradient of the occasional rust-colored spots. During crushing most of the regression line), then the age of the sample is −0.06 Ma. sample powdered immediately, revealing many cream to grey Alternatively, if Khor Temiki pyroxenes formed in the prismatic chips. A small number of magnetic grains were also presence of Q-Xe, and assuming that the oldest point (step 11) observed. Due to the limited sample size, only one separate is least disturbed by shock, a model age of −1.5 ± 0.2 Ma is suitable for I-Xe study was obtained—KTD, a very pure enstatite implied (where the error is determined from the uncertainty in concentrate (Table 1). The large (~2 mm) enstatite crystals the 129Xe*/127I ratio of step 11 only). These place upper and were easily separated from the friable matrix and the BSE lower limits on the age of Khor Temiki pyroxene. images reveal very little contamination from matrix particles. The age of Khor Temiki is very similar to that of both the The enstatite had a uniform composition with little Fe and enstatite chondrite Indarch (0.04 Ma) and enstatite achondrite only a very low abundance of tiny sulfide inclusions (Table 1). Shallowater, although it is somewhat older than the type 6 Each grain was composed only of enstatite with no matrix Khairpur (−4.2 Ma). By obtaining an age for Khor Temiki it is silicates; these are the blue-grey enstatites described by Hey possible to fit the aubrite parent body into the picture of and Easton (1967) and not the shock clasts described by evolution in the enstatite meteorite forming region. There is Newsom et al. (1996). It is expected that enstatite is a major strong evidence that Shallowater originated from a parent host phase for iodine as it is in the anomalous enstatite body distinct from the aubrites (Keil 1989). The age inferred achondrite Shallowater, and this proved to be the case. here demonstrates that igneous parent bodies in this region A 3-isotope plot of the KTD data is shown in Fig. 3. With were undergoing development at the same time. many fewer points than Indarch and Khairpur, it has been Very little chronological information is available for the possible to label the data with the step numbers (see aubrites. Recently Shukolyukov and Lugmair (2004) attempted supplementary Table A6). The isotopes have been normalized to obtain Mn-Cr ages for aubrites. They showed that the 132 129 53 to Xep in this figure rather than to total Xe as in the breccia Peña Blanca Spring contained excess Cr, but that it previous figures. This is purely to allow visualization of the was not correlated with Mn/Cr, indicating early formation of correlation and is mathematically identical to the other 3-isotope this aubrite followed by secondary disturbance, which plots. Using the step-release diagram (shown as an inset in Shukolyukov and Lugmair (2004) associated with the Fig. 3) it is possible to distinguish a distinct change in the 129I/ breccia-forming event. This scenario is very similar to the 127I ratio with increasing temperature. Regression to only the interpretation of the I-Xe data presented here, and it seems high-temperature data gives a 129I/127I = (1.07 ± 0.02) × 10−4 likely that both Mn-Cr and I-Xe in the aubrites have been 129 132 ± and a trapped Xe/ Xep component of 0.05 0.19, which severely disturbed by shock. is unusually low. It is conceivable that the trapped 129Xe/ 132 Xep in a closed system can evolve to values greater than Q Itqiy (1.04) but there is no plausible thermal mechanism to Itqiy is a coarse-grained metal-rich enstatite meteorite decrease this ratio. Caffee et al. (1982) first investigated the with achondritic texture. Modal analysis yields 78% silicate, effect of shock on I-Xe systematics and Gilmour et al. (2001) 14% metal, and 8% “rust” (Patzer et al. 2001). Silicate 129 132 have suggested that the apparent low Xe/ Xep seen in (En96.8Fs0.2Wo3.0), which has been extensively recrystallized, some samples is due to mixing with a xenon endmember forms subhedral, equigranular grains (0.5–4 mm), while the residing in an iodine-bearing phase with a low or zero initial metal grains range from 0.2 to 2 mm. Metal also passes 129I/127I ratio (“dead” iodine) and a well-defined 127I/132Xe through silicate as a network of veinlets. Patzer et al. (2001) 890 A. Busfield et al.

129 132 Fig. 3. Three-isotope plot of KTD (enstatite) data and the associated regression line. The regression indicates a trapped Xe/ Xep 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 129Xe*/127I 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 ItA. The rest of the material consisted of dark, sometimes consisting of intergranular assemblages of 3 different euhedral, crystals (ItB) and clear grains with occasional black sulfides and metal globules. Different features in pyroxene magnetic inclusions (ItC). SEM images showed that ItA indicate shock stages between 2 and 4 (Patzer et al. 2001). contained grains up to 0.5 mm in diameter, and was a mixture Itqiy enstatite shows no evidence for zoning and has Na2O, of metal and silicate. As well as forming discrete grains the FeO and MnO abundances similar to EL chondrite pyroxenes, metal in this concentrate was present as veins and inclusions though CaO content is considerably higher than in ELs, within the enstatite grains. ItB and ItC were originally whereas kamacite is compositionally identical to that of EH distinguished by their visual appearance, and microscopic and chondrites. The sulfide assemblages consist of a Mg-Mn-Fe BSE observation showed that ItC was almost pure enstatite sulfide host with oldhamite, Fe-Cr sulfide (consistent with whereas ItB consisted of enstatite with a high abundance of ELs) and metal droplets embedded in it. The host sulfide has dark (probably metal) inclusions (Table 1). Grain size and a very variable composition that falls within neither the EL or appearance were very similar to the previous description of EH fields (Patzer et al. 2001). Itqiy therefore shows no clear this meteorite (Patzer et al. 2001). ItA is therefore a metal-rich affinity to either the EH or EL groups, and hence is separate, ItB, although enstatite-rich, contains significant classed as anomalous. It has also clearly experienced a contamination by metal, while ItC is a very pure enstatite complicated thermal history demonstrated by the recrystallized separate. nature of the enstatite grains combined with the presence of Complete Xe data are available in supplementary Tables the heterogeneous sulfide assemblage. No chronological A7–A9. The I-Xe analysis of ItA revealed no 129Xe* in the information is available for this meteorite. majority of releases (Fig. 4a), although there is evidence for This sample was very hard and appeared dark with shiny some 129Xe* in the very highest temperature steps with a metallic grains. There was some rusting on the surface. When maximum observed 129Xe*/127I of ~1.0 × 10−4 (for details of crushed the rust was pervasive and left fine dust on grain the step-release pattern refer to supplementary Fig. A3). The surfaces. Shiny metal grains were separated and designated trapped 129Xe/132Xep composition in ItA is 1.12 ± 0.02, 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 − − ± 129 132 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 Xe/ Xep is well defined at 1.06. For full sample compositions refer to Table 1. The transition from “low” temperature to “high” temperature involved a significant change in the 129I/127I ratio, observed at 13.5A (Step 5) for ItC and 18A (step 11) for ItB. The highest ItA steps, containing 129Xe*, fall closest to the y-axis and are visible in (b). higher than Q (1.04). As discussed in the Khairpur section the in Fig. 4. As was the case for Indarch and Khairpur data we 129 132 elevated trapped Xe/ Xep composition is consistent with have not shown the release temperature steps on Fig. 4 for evolution over time. practical reasons and a step-release diagram is available as A 3-isotope plot of ItB and ItC data reveals that they can supplementary Fig. A3. The transition from the low- be clearly separated into two discrete subsets defining two temperature correlation to the high-temperature apparent isochrons. The later of these is dominated by ItB and correlation (i.e., a significant change in 129I/127I) occurred in low-temperature ItC data. The earlier isochron is defined step 5 (13.5A) for ItC and at step 11 (18A) for ItB. The low- predominantly by ItC data. These distributions are illustrated temperature ItC points are clearly visible on Fig. 4a lying on 892 A. Busfield et al.

the ItB line and in Fig. 4b lying above the ItC line. Least An97–98. Chromite composition (Chr72Ulv10Her18) suggests squares regressions to these data sets yield slightly elevated formation below 800 °C (Sack and Ghiorso 1991). Oxygen trapped ratios of 1.063 ± 0.003 (ItB) and 1.058 ± 0.003 (ItC). isotopes group this meteorite with the HED clan (Nyquist A value of 1.06 has therefore been used to calculate excess et al. 2003). 129Xe*. A-881394 is an unusual cumulate eucrite in that it The fit to ItC high-temperature points yields an isochron contains magnesian pigeonite with a metamorphic, granular with a 129Xe*/127I ratio of (9.58 ± 1.10) × 10−5 (Fig. 4). The texture, and the thickness of its exsolution lamellae suggests highest temperature ItB steps are consistent with the ItC ratio relatively fast cooling. The poikilitic texture of chromite of 9.6 × 10−5. The majority of the ItB data and low- indicates that a metamorphic event is responsible for the temperature ItC points describe a regression line with a observed textures. A granulitic origin was proposed for this 129Xe*/127I ratio of (2.50 ± 0.47) × 10−6. These ratios meteorite based on the rounded form of the pyroxene grains correspond to ages of −2.6 ± 2.6 Ma (ItC) and −85 ± 4 Ma and granoblastic texture of plagioclase (Nyquist et al. 2003). (ItB). A possible thermal history is one of recrystallization as The petrology of Itqiy indicates a complex thermal extensive as that of other cumulate eucrites, but with faster history recording at least two major events: crystallization of subsequent cooling. Rb-Sr and Sm-Nd systems indicate an large silicate grains and later heating indicated by the mixed age of ~4.4−4.5 Ga, the 146Sm-142Nd chronometer gives an sulfide assemblage (Patzer et al. 2001). The most enstatite age of −4 ± 26 Ma relative to the Lewis Cliff (LEW) 86010 rich separate, ItC, shows the best defined isochron which is angrite, 53Mn yields an age of 6 ± 2 Ma before LEW 86010 associated with early silicate formation or recrystallization. and the 26Al formation interval in plagioclases is −3.95 ± The age, −2.6 ± 2.6 Ma, is entirely consistent with previously 0.13 Ma relative to CAIs (Nyquist et al. 2003). The postulated obtained ages for enstatite achondrites (i.e., Shallowater and thermal history of this meteorite makes it an ideal candidate Khor Temiki) and other igneous meteorites. No sulfide for study with multiple chronometers, as fast cooling should separate was made from this meteorite, but kamacite should mean the different closure temperatures are encountered in also have been affected at the temperatures sufficient to cause quick succession. sulfide melting (McCoy et al. 1999). The metal separate, ItA, Our sample of A-881394 was relatively coarse-grained has a similar (order of magnitude) iodine concentration to the (up to ~5 mm diameter) and consisted predominantly of other Itqiy separates (Table 1), so the lack of a 129Xe* excess green/brown (pyroxene) grains, and clear or white cannot be attributed to the absence of iodine and must be the (plagioclase) grains. Minor amounts of shiny black grains result of resetting, probably by the sulfide-forming event. were observed and removed from the silicate separates. 129Xe* observed in the highest ItA temperature steps was Where this material was observed as inclusions within the probably derived from enstatite contamination of this separate. white silicate grains the grains were rejected, although the These steps are visible in Fig. 4b, lying closest to the y-axis. presence of very small inclusions cannot be ruled out in this The ItB correlation could be a result of mixing between separate. The phases identified by SEM in A-881394 were the iodine released from the metal and the 129Xe* released feldspar, pyroxene, silica, and chromite, with individual from the silicate at each temperature step since ItB contains grains ranging up to ~0.75 mm. Both pyroxene and feldspar significant metal. In this case, the “isochron” has no showed evidence of exsolution, as previously described chronological significance. However, the ItC data points that for this meteorite (Nyquist et al. 2003). The only opaque fall on the ItB correlation account for about 50% of the iodine phase identified was chromite. The large grain size of this released from this separate, invoking the possibility of meteorite allowed the phases to be separated relatively either a late-stage, low-temperature resetting event or the easily resulting in pure concentrates (see Table 1). There was presence of a minor low-temperature phase associated with no evidence of rusting or low-temperature alteration from the the enstatite that has not been identified in the grain mounts. SEM images. Probe data confirmed two discrete pyroxene With the present data, we favour the latter interpretation. compositions with augite lamellae exsolved from the low Ca host pyroxene. Feldspar was very anorthite-rich. The two Asuka-881394 concentrates were designated AsA (plagioclase-rich) and The cumulate eucrite A-881394 is a coarse-grained AsB (pyroxene-rich). igneous rock consisting of granular pyroxene crystals (0.4– Complete Xe data are available in supplementary Tables 1.5 mm) with interstitial plagioclase. Nyquist et al. (2003) A10 and A11. The iodine and xenon concentrations measured determined modal mineral abundances to be pyroxene 49%, in both separates were significantly lower than in the enstatite plagioclase 45%, silica 5% and chromite 0.5%. Pyroxene meteorites (Table 1). AsA shows a clear excess of 129Xe*, 129 * 127 (Ca2Mg54Fe44) shows well-developed exsolution with although no consistent Xe / I ratio is implied. In the step- herring-bone texture and is well separated from the lamella release diagram for AsA (Fig. 5), 129Xe*/127I evolves to higher augite (Ca42Mg39Fe19). Opaques are mostly chromite with values with increasing step-release temperature to a maximum rare troilite. Plagioclase is very calcic with a composition of of ~3.8 × 10−5. Using the maximum observed 129Xe*/127I in Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite 893

Fig. 5. AsA (feldspar) step-release plot. Although no 129Xe*/127I plateau is seen the 129Xe*/127I increases with release temperature to a maximum of 3.8 × 10−5. 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 for Indarch and −4.2 ± 0.7 Ma for Khairpur. Our derived metamorphism of <~25 Ma after Shallowater. Despite a similar ages are similar, but not identical, to those of Kennedy et al. I/132Xe ratio in the AsB (pyroxene) separate, there was little (1988), suggesting possible heterogeneity in these meteorites. evidence of any 129Xe* and no correlation of 129Xe* with iodine. The age difference between Indarch and Khairpur is ~4 Ma A-88194 is an unusual eucrite both petrologically and in in both our data set and those of Kennedy et al. (1988). terms of short-lived isotopes. Unlike other eucrites, it has a Shukolyukov and Lugmair (2004) obtained a Mn-Cr metamorphic granulitic texture and appears to have crystallized isochron from Indarch silicate and chromite, which yielded an very early with a 26Al age of 4563.2 ± 0.6 Ma and a 53Mn age initial 53Mn/55Mn at time of isotopic closure of (2.7 ± 0.2) of 4564 ± 2 Ma (Nyquist et al. 2003). Additionally, initial × 10−6. Since both the 129I and 53Mn systems date closure 146Sm/144Sm ratios indicate an ancient age of 4562 Ma in silicate phases, we might expect to find closure intervals (Nyquist et al. 2001a). Our data suggest that 26Mg escaped measured by these two systems to agree. Further Mn-Cr redistribution during the metamorphic event that formed the analyses revealed an age difference between Indarch and granulitic texture, and that this event significantly post-dates Khairpur of 4.5 ± 0.3 Ma (Shukolyukov and Lugmair 2004), crystallization. It is noteworthy that the Mn-Cr and I-Xe very similar to the age difference defined here by the I-Xe chronometers that behave concordantly among the enstatite clock. We now investigate whether this consistency is chondrites have responded differently in this case; however, maintained across a wider range of samples. this is not unexpected since they are not recording events In previous work (Busfield 2004; Gilmour et al. 2006), from a common host phase in A-88194. The less precise Rb- we have shown that a plausible correlation exists between Sr and Sm-Nd systems record later ages of 4370 ± 60 Ma and I-Xe formation intervals and model ages derived from the 4490 ± 20 Ma (Nyquist et al. 2001a), suggesting that they Pb-Pb system. This lends support to the chronological were set later than the onset of xenon retention in some sites. interpretation of data from these two systems and allows us to consider the consistency of Mn-Cr data with the candidate CORRELATION OF SHORT-LIVED unified system based on Pb-Pb and I-Xe data. ISOTOPE TIME SCALES 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 Our data demonstrate that silicate is the phase responsible systems (Gilmour et al. 2006) and the Mn-Cr system. Mn-Cr for the high-temperature I-Xe isochron observed in the formation intervals are calculated relative to LEW 86010 and enstatite chondrites Indarch and Khairpur. We obtain a mapped to the absolute time scale using the Pb-Pb age of this lowest possible initial iodine ratio corresponding to a latest meteorite (4557.8 ± 0.5 Ma; Lugmair and Galer 1992). possible formation age for each meteorite: 0.04 ± 0.67 Ma A credible correlation between Mn-Cr and the combined 894 A. Busfield et al.

Fig. 6. 53Mn ages (calibrated by Pb age of LEW 86010) against Pb-Pb or 129I ages (calibrated via Gilmour et al. 2006: absolute Shallowater age = 4563.3 ± 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 53Mn 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 (53Mn and 129I) 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 53Mn 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-881394: 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 Mn-Cr and I-Xe both remain intact in the high-temperature coherent in a large subset of the samples. The earliest pyroxene phase. This result is not unexpected as Mn is chondrules points represents the earliest chondrule ages obtained partially hosted by olivine in ordinary chondrites and in each system: Chainpur for 53Mn (Nyquist et al. 2001b) and Acapulco (Allen and Mason 1973; Zipfel et al. 1996) and 129I (Brazzle et al. 1999), and Allende for Pb-Pb (Amelin et al. closure temperatures for Cr in olivine can be a lot lower than 2004). The notable outliers represent the range of Pb-Pb and in enstatite (Ganguly et al. 2007). I-Xe ages recorded by the different phases in Acapulco and The enstatite chondrites are shown on Fig. 6 by an age the ordinary chondrites, since many of these meteorites have range for each meteorite. This extends from the Kennedy et only a single (whole rock) Mn-Cr age. These samples, along al. (1988) age to the lower limits on age derived in this work. with the enstatite chondrites, are excluded from the regression The maximum age corresponding to the highest initial iodine (Williamson 1968) to the data shown in Fig. 6, which has a ratio observed in any release from each meteorite is shown slope of 1.07 ± 0.12 and intercept of −300 ± 500, and is shown as a solid symbol. The enstatite chondrites are offset from in bold. A gradient of 1 and intercept of 0 are what would be the regression line. Based on the relationship between ε53Cr expected for coherent chronometers and this regression and initial 53Mn/55Mn ratios, Shukoluykov and Lugmair confirms the angrites as a robust calibration for the Mn-Cr (2004) proposed radial heterogeneity in 53Mn/55Mn. They system. suggested a correction should be applied to derived initial Examination of the correlation with respect to the 53Mn/55Mn ratios to allow enstatite chondrite and ordinary ordinary chondrites and Acapulco reveals that the whole rock chondrite ages to be compared. Their correction factor Mn-Cr system was reset by the events that formed feldspar converts the initial 53Mn/55Mn ratio in the region of the (in the case of Richardton) and phosphate (in the case of ordinary chondrites (i,oc) to the expected initial ratio in the Acapulco). This is unlike the enstatite chondrites in which enstatite chondrite formation zone (i,ec) such that Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite 895

53 53 meteorites, which determines the event recorded by the high- ⎛⎞Mn ⎛⎞Mn –6 ------= ------– ()4.7± 0.3 × 10 .(1) temperature I-Xe isochron. ⎜⎟55 ⎜⎟55 ⎝⎠Mn iec, ⎝⎠Mn ioc, Table 1 gives the measured iodine concentrations in all separates. The concentration of iodine in enstatite follows We have added an uncertainty to this correction factor the sequence of increasing metamorphic grade, declining which takes account of the errors on the bulk ε53Cr value of by two orders of magnitude from the most primitive to the both ordinary and enstatite chondrites, as well as the most processed meteorites. During metamorphism, enstatite uncertainty in the average value of chondritic 55Mn/52Cr. This homogenizes leading to increased grain size and a decrease allows us to include an uncertainty on “corrected” 53Mn ages in chondrule definition (McCoy et al. 1999; Zhang et al. of enstatite chondrites. Mn-Cr ages for the enstatite meteorites 1995), and it seems that iodine is lost to a large extent during corrected using this factor are displayed in Fig. 6, where it this process. is apparent that taking account of the proposed heterogeneity 129Xe*/I (ratio of totals) is similar among Indarch, between enstatite chondrites and ordinary chondrites markedly Khairpur and Khor Temiki, but a factor of 3 lower in Itqiy—a improves the agreement of the enstatite data with the ordinary reflection of the second, more recent, low-temperature age chondrite correlation line. observed in ItB. In a metamorphic context, the Itqiy data Birck et al. (1999) proposed an alternative explanation demonstrate that xenon loss from the I-Xe host site persists to for the different relationship between ε53Cr and 53Mn/55Mn lower temperatures than iodine loss, as expected. Initial observed by Shukolyukov and Lugmair (2004). They suggest iodine ratios defined by high-temperature releases are similar, that the enstatite chondrites experienced volatility-driven suggesting that loss was completed early, before substantial fractionation of Mn from Cr, relative to the chondritic Mn/Cr decay of 129I. value, resulting in different evolution of initial Cr with 53Mn We thus have a picture of the parent bodies of enstatite decay before formation of the samples analyzed. This would meteorites losing iodine during metamorphism early in their account for the different relationships between ε53Cr and evolution. The somewhat lower iodine concentrations measured 53Mn/55Mn across the sample suites, but this explanation is in EH3 chondrules and matrix by Whitby et al. (2002) does not capable of accounting for the evidence of heterogeneity not fit into this pattern, and as yet we do not have a full presented here since we are directly comparing initial iodine explanation for this. ratios with initial Mn ratios. Neither of these is affected by elemental fractionation. In addition, the whole rock Mn/Cr CONCLUSION ratio in Indarch is very similar to the average of the ordinary chondrites (0.73 versus 0.76), indicating that the Mn/Cr ratio Mineral concentrates obtained from five meteorites, of Indarch has not been fractionated (Shukolyukov and Indarch, Khairpur, Khor Temiki, Itqiy, and A-881394, Lugmair 2004). have been dated using the 129I-129Xe chronometer. Significant We have not approached this work with any assumption scatter was observed in the data of both Indarch (EH4) and that a 53Mn heterogeneity exists, but the result obtained by Khairpur (EL6), which is interpreted as probably combining data from different chronometers supports this representing a spread in chondrule ages. Based on the current hypothesis. The cause of a radial 53Mn heterogeneity is not data, it is not possible to exclude some contamination of the certain, although Meyer and Clayton (2000) have proposed a enstatite data by iodine from a younger phase or mechanism whereby the 53Mn/55Mn ratio may become uncorrelated iodine. Therefore we obtain a lower limit on radially heterogeneous due to a cosmic chemical memory the age of the pyroxene separates to be 0.04 ± 0.67 Ma effect. Whether 53Mn in the solar system derived from a (Indarch) and −4.2 ± 0.7 Ma (Khairpur). Khor Temiki stellar source or was generated within the solar system by (aubrite) was shown to have an exceptionally low trapped irradiation from the young Sun, dynamic modelling indicates 129Xe/132Xe ratio, which is probably due to shock that if it was initially distributed unevenly, heterogeneities can disturbance of the 129I system and makes understanding persist for a significant period of time (Boss 2004). It is the data more complex. However, an age of between important to note, however, that the difference between our −0.06 Ma and −1.5 Ma is suggested. The pyroxene separate data and those of Kennedy et al. (1988) indicate some degree of the anomalous enstatite achondrite Itqiy records an age of of sample heterogeneity. Clearly, an improved understanding −2.6 ± 2.6 Ma. The metal-rich fraction of this meteorite did of the evidence for 53Mn heterogeneity would result from not contain any excess 129Xe* due to the decay of 129I, analyses of I-Xe and Mn-Cr dating of aliquots of the same suggesting very late resetting of this phase. The cumulate mineral separates. eucrite A-881394 contained no excess 129Xe* in the pyroxene fraction, whilst the feldspar separate had 129Xe* which was IODINE AND METAMORPHISM not correlated with 127I. This suggests that A-881394 has been disturbed by a thermal metamorphic event. We now briefly examine the behavior under metamorphism The age data obtained here have been used in conjunction of iodine and xenon associated with pyroxene in enstatite with literature data to compare ages from different chronometers. 896 A. Busfield et al.

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