Claydon, J L

Crowther, S A

Gilmour, J D

English

Vestan eucrites carried on losing xenon for longer than anomalous eucrites. Was extended activity associated with a larger parent body

The University of Manchester - Institutional Repository

THE I-PU-XE SYSTEM IN ANOMALOUS AND VESTAN EUCRITES: WAS VESTA UNUSUALLY LARGE? J. L. Claydon1,2, S. A. Crowther2 and J. D. Gilmour2, 1Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, U.K. 2School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, UK (j.claydon@nhm.ac.uk).  Introduction: Eucrites are basaltic meteorites with oxygen isotope signatures [1] indicating formation on a single parent body, thought to be the asteroid 4 Vesta [2]. However, several meteorites that have similar ma-jor element chemistry and mineralogy to eucrites devi-ate from the oxygen isotope mass fractionation line [3-5], suggesting a non-Vestan origin. Of these six anom-alous eucrites [3, 4] two may originate on one body [3] whilst the other four appear to have originated on sep-arate differentiated bodies. This suggests that differen-tiated asteroids chemically and geologically similar to Vesta, were once more common in the Solar System but have since been catastrophically disrupted. Xenon is a useful trace element as its scarcity in solid materials makes isotopic anomalies easy to de-tect. 131-136Xe are produced by spontaneous fission of 238U and 244Pu, whilst 124-130Xe are produced by cos-mic-ray spallation on target elements: Ba and light rare earth elements (LREE). Previous work [6-9] on Xe in eucritic meteorites (Vestan and anomalous) has fo-cused on the Pu-Xe dating system, which utilises the spontaneous fission of 244Pu to 131-136Xe with a half-life of 80 Myr. This work also showed the presence of excesses of 129Xe* in several eucritic meteorites. 129Xe* is produced by radioactive decay of 129I with a half-life of 16 Myr and has been shown to be a reliable early Solar System chronometer [10-11]. By irradiat-ing samples with thermal neutrons, stable 127I is con-verted to 128Xe*, allowing simultaneous measurement of 129Xe* and 127I (via 128Xe*). A consistent relation-ship between 129Xe* and 128Xe* during step-heating indicates the 129Xe* has remained in situ in iodine sites and therefore the 129Xe*/127I ratio is representa-tive of the age of closure to Xe. I-Xe ages are refer-enced to a standard, enstatite from the aubrite Shal-lowater, which has an absolute age of 4562.3 ± 0.4 Myr [12]. Here we examine the isotopic signatures of Xe produced from short-lived isotopes 244Pu and 129I in the anomalous eucrites Ibitira [3] and Bunburra Rockhole [4] and the Vestan eucrites Juvinas and Béréba, in order to understand more about the history of differentiated asteroids in the early Solar System. Methods: Xenon isotopic ratios in two sets of whole-rock samples (one unirradiated, one irradiated) from Ibitira, Bunburra Rockhole, Juvinas and Béréba were measured using the RELAX (Refrigerator En-hanced Laser Analyser for Xenon) mass spectrometer [13-15]. Sample masses ranged from 1.0-4.5 mg. The gases were released by laser step-heating. Absolute amounts of gas were calculated, and a sensitivity cor-rection made, by reference to measurements of terres-trial air interspersed throughout analyses. The proce-dural blank was measured throughout and subtracted from the calculated isotopic abundances of the sam-ples. Before Xe analyses, the irradiated samples were wrapped in aluminium foil and placed in glass vials that were evacuated and sealed before being exposed to thermal neutrons (6.82×1018 n cm-2) at the Petten nuclear reactor, Netherlands, to covert 127I to 128Xe*. Samples of Shallowater enstatite were included in the same irradiation to monitor conversion of 127I to 128Xe. During this process heavy Xe isotopes are also pro-duced from neutron-induced fission of 235U.  Results and discussion: Xenon components Unir-radiated samples show excesses of 124-130Xe consistent with spallation of material with a chondritic Ba/LREE ratio [16], and excesses of 131-136Xe indicating fission of 244Pu and/or 238U (the data do not allow distinction between the two). 244Pu is thought to dominate the fission component in ancient samples [17] so the fis-sion-Xe seen here has been attributed to 244Pu, con-sistent with previous work [10-13]. A small contribu-tion from trapped Xe was seen (0.2-5.6×10-12 cc atoms 132Xe at STP g-1) only at low-temperatures (except in Ibitira). This is consistent with adsorption of terrestrial 00.10.20.30.40.50.60.70 2 4 6 8130Xe/129Xe128Xe/129XeSpallation-XeAirPure-iodineBunburra RockholeLow-T releases  Figure 1. Isochron plots for irradiated samples of Bunburra Rockhole and Ibitira. Uncorrected 130Xe and 128Xe are nor-malized to 129Xe. Data from both meteorites form coherent isochrons (dashed lines) between spallation-Xe and pure-iodine end-members. The initial iodine ratios correspond to I-Xe ages of 7 ± 9 Myr and 7 ± 1 Myr after Shallowater for Bunburra Rockhole and Ibitira respectively. Shallowater has an absolute age of 4562.3 ± 0.4 Myr [12]. Samples of Juvinas and Béréba failed to form coherent isochrons.  2173.pdf44th Lunar and Planetary Science Conference (2013)atmospheric Xe. High-temperature gas releases were dominated by spallation and fission Xe. I-Xe system All samples showed excesses of 129Xe*. To determine whether the excesses were pro-duced by in situ decay of 129I the I-Xe system in the irradiated samples was examined. Ordinarily 128Xe, 129Xe and an iodine-free isotope (usually 132Xe or 130Xe), corrected for contributions from spallation and fission are used. In these samples, iodine is released in high temperature steps that contain an insignificant amount of trapped Xe, so uncorrected 130Xe (to which, only spallation-Xe contributes) was used. In the 128Xe, 129Xe, 130Xe system the data reveal mixing between spallation and iodine-derived Xe (Figure 1). All sam-ples showed data points that fell to the right of the spallation-air mixing line indicating the presence of 127I in the samples. The high temperature data appear to form mixing lines between spallation and iodine end-members that can be interpreted as isochrons orig-inating at the spallation composition. Bunburra Rock-hole and Ibitira produced more well-defined trends than Juvinas and Béréba (not shown in Figure 1). The initial iodine ratios obtained from York fits correspond to I-Xe ages of 7 ± 1 Myr (Ibitira) and 7 ± 9 Myr (Bunburra Rockhole) after Shallowater. This gives an absolute age of 4555 ± 1 Myr for Ibitira which agrees well with Mn-Cr [18], Pb-Pb [19] and Pu-Xe ages [9]. 244Pu-129I relationship: It was observed (Figure 2) that in these samples 129Xe* produced from 129I corre-lates with fission -Xe from 244Pu, indicating co-location of the parent elements. Comparison of the systematics of iodine-derived isotopes and spallation-Xe (from Ba and LREE) confirms that the iodine re-tained in these rocks behaved as an incompatible ele-ment during their formation. Analysis of irradiated samples showed that iodine concentrations and initial I/Pu ratios were comparable among the various sam-ples, demonstrating that variations in 129Xe*/136Xe* seen among the meteorites (Figure 2) correspond to differences in the closure time to Xe loss – lower ratios require later resetting – rather than differences in vola-tile content. Ibitira and Bunburra Rockhole closed ear-lier to Xe loss than the Vestan eucrites, suggesting an extended period of processing on Vesta compared to the parent body of the anomalous eucrites. If a simple open-model is assumed, based on the 129I/244Pu ratios and decay rates of 129I and 244Pu, Xe loss on Vesta continued for ~50-100 Myr after geolog-ical activity on the parent body of Ibitira ceased. The faster cooling rate of the anomalous eucrite parent bodies can be explained if they were smaller than Ves-ta. The larger size of Vesta may therefore be the reason it has escaped catastrophic disruption from collisions that destroyed the other differentiated asteroids in the early Solar System. Acknowledgements: The authors would like to thank P. A. Bland of Imperial College London for providing samples of Bunburra Rockhole. Samples of Ibitira, Béréba, Juvinas and Shallowater were provided by the Natural History Museum, London. This work was funded by the STFC. References: [1] Clayton R. N. & Mayeda T. K. (1996) GCA 60, 1999-2017. [2] McCord T. B. et al. (1970) Science 168, 1445-1447. [3] Scott E. R. D. et al. (2009) GCA 73, 5835-5853. [4] Bland P. A. et al. (2009) Science 325, 1525. [5] Mittlefehldt D. W. (2005) MAPS 40, 665-677. [6] Miura Y. (1993) GCA 57, 1857-1866. [7] Miura Y. et al. (1998) GCA 62, 2369-2387. [8] Park J. & Nagao K. (2005) Antartc. Meteorite Res. 18, 213-224. [9] Shukolyukov A. & Begemann F. (1996). GCA 13, 2453-2471. [10] Brazzle R. H. et al. (1999) GCA 63, 739-760. [11] Gilmour J. D. et al. (2006) MAPS 41, 19-31. [12] Gilmour J. D. et al. (2009) MAPS 44, 573-579. [13] Gilmour J. D. (1991) Mass. Sci. Technol. 2, 589-595. [14] Gilmour J. D. et. al. (1994) Rev. of Sci. Inst. 65, 617-625. [15] Crowther S. A. et al. (2008) J. Anal. Atom. Spectrom. 23, 921-1044. [16] Hohenberg C. M. et al. (1981) GCA 45, 1909-1915. [17] Turner G. et al. (2004) Science 306, 89-91. [18] Lugmair G. W. and Shu-kolyukov A. (1998) GCA 62, 2863-2886. [19] Chen J. H. and Wasserburg G. J. (1985) LPSC XVI 119-120. 00.511.522.533.544.50 1 2 3 4 5 6 7136Xe*/132 XeAIR129Xe*/132XeAIRBunburra RockholeIbitiraBereba244Pu129IClosed to Xe loss earlierClosed to Xe loss laterJuvinas (data shown on plot below)  01020304050607080900 5 10 15 20 25136Xe*/132 XeAIR129Xe*/132XeAIRBunburra RockholeIbitiraBerebaJuvinas244Pu129IClosed to Xe loss earlierClosed to Xe loss later  Figure 2. Three isotope plot showing 136Xe* (from 244Pu), 129Xe* (from 129I) normalised to fission-corrected 132XeAIR in unirradiated samples. Each sample shows a distinct 129I/244Pu ratio with the anomalous eucrites showing high-er 129I/244Pu ratios. Given that 129I has a shorter half-life (16 Myr) than 244Pu (80 Myr) this suggests that anomalous eucrites closed to xenon loss earlier than Vestan eucrites. 2173.pdf44th Lunar and Planetary Science Conference (2013)

The I-Pu-Xe System in Anomalous and Vestan Eucrites: Was Vesta Unusually Large?

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