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I-Xe Analysis of Enstatite Meteorites and a Eucrite

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I-Xe Analysis of Enstatite Meteorites and a Eucrite 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 Rights Copyright © The Meteoritical Society Download date 26/09/2021 21:32:45 Item License http://rightsstatements.org/vocab/InC/1.0/ 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.
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