Bell, E A

Crowther, S A

Gilmour, J D

Harrison, T M

Turner, G

English

Xenon in &gt;4 Ga zircons yields estimates of original Pu/U that deviate from chondritic estimates. Xenon loss is resolved for some; other causes are less certain

The University of Manchester - Institutional Repository

ORIGINS OF PU/U VARIATIONS IN >4 Ga TERRESTRIAL ZIRCONS. E. A. Bell1, J. D. Gilmour2, T. M. Harrison1, G. Turner2, S. A. Crowther2; 1Dept. of Earth and Space Sciences, UCLA (ebell21@ucla.edu), 2School of Earth, Atmospheric, and Environmental Science, University of Manchester.  Introduction:  Examination of radiogenic xenon in chondritic meteorites led to an estimation of solar sys-tem initial 244Pu/238U of ~0.0071.  However, apparent initial Pu/U ratios in >4 Ga terrestrial zircons from the Jack Hills, Western Australia, yield variations both above2 and below2,3 this value.  Given the highly varia-ble behavior of Pu and U under even mildly oxidized aqueous conditions4, this variation could be an indica-tor of aqueous alteration in the precursors to Jack Hills zircon magmas.  This could be a second line of evi-dence (after high δ18O values in the same zircons) for a Hadean hydrosphere5, contrasting with the earlier par-adigm of a dry Hadean Earth.  However, the lack of extant natural plutonium has limited  insights into its behavior in terrestrial geologic settings.  Thus an aque-ous history may not be the only potential cause of Pu/U variations, and the potential effects of magmatic evolu-tion (similar to that seen in evolving zircon U/Th rati-os) and secondary alteration of the zircons need to be considered.  In order to unravel the causes of apparent Jack Hills Pu/U variations, we have collected a multi-variate dataset on 11 zircons consisting of analyses of xenon isotopes along with U-Pb age, trace element contents, and oxygen isotopes.  Consideration of the xenon-derived Pu/U estimates with other geochemical information on a wider suite of Jack Hills Hadean zir-cons suggests that Pu/U in the zircons reflects both secondary alteration and primary zircon chemistry de-rived from various processes operating on the Hadean Earth.  Understanding these processes will yield in-sights for the interpretation of Pu/U signatures on plan-etary bodies in the early solar system. Methods:  U-Pb age, oxygen isotope analyses, and trace element analyses of 32 zircons were made on CAMECA ims1270 ion microprobes.  Ages and δ18O for 23 of the sampled zircons are reported in [6].  11 Zircons were irradiated to induce fission of 235U (simi-lar to the procedure documented by [2]) at a nominal neutron fluence of ~5-6x1018 n/cm2.  Xenon isotopic analyses were made using the Refridgerator-Enhanced Laser Analyzer for Xenon (RELAX) instrument7,8.  Zircons were heated with an infrared laser at various temperatures to extract xenon, resulting in one or sev-eral xenon release steps per zircon. Results:  Mixture models based on other variables:  Finite mixture modeling of a larger dataset of 32 Hade-an Jack Hills zircons using variables relevant to both aqueous alteration (δ18O, U/Th due to the solubility of U)4,5 and magmatic differentiation (Zr/Hf, Yb/Gd and U/Th, Txlln)9,10 along with 207Pb/206Pb age, Nd/U (due to similarities of Pu and Nd chemistry in some meteorite studies11), and Ut (to check for uranium abundance effects on its various ratios; the subscript ‘t’ refers to the ratio at the time of zircon crystallization) suggests two groups of zircons, separated most readily by Nd/Ut.  Group A have Nd/Ut > 0.009, range 4.16-4.09 Ga in age (average 4.12 Ga), and have significantly lower (U/Th)t and Yb/Gd.  Group B ranges 4.2-4.0 Ga in age (average 4.07 Ga) and has significantly higher (U/Th)t and Yb/Gd.  There is no appreciable difference in Ut or δ18O between the groups.  Group B contains all of the zircons with multiple xenon release steps as well as all of the zircons with (Pu/U)o > 0.001 in this dataset (the subscript ‘o’ refers to the ratio projected to the beginning of the solar system at 4.568 Ga).  Fig. 1: Hadean zircon data in xenon three isotope space.  All but one xenon release fall within error of a ternary defined by the end members produced by fis-sion of 244Pu, 238U, and 235U.  Xenon Results:  Fig. 1 shows our 11 Hadean zircons in three xenon isotope space.  All fall within a ternary consisting of the two radiogenic end-members (238U and 244Pu) and nucleogenic xenon from 235U.  The zir-cons’ (Pu/U)o show only a weak correlation with δ18O (see Fig. 2b; when only the highest-Pu/U release step in each zircon is considered, R2~0.4) and no obvious cor-relations with indicators for melt evolution such as Ti-in-zircon crystallization temperature (Txlln), (U/Th)t, Zr/Hf, or Yb/Gd.  However, (Pu/U)o does show a rough inverse trend with several LREE/actinide ratios, seen most clearly with Nd/Ut (Fig. 2a).  High Nd/Ut values are associated exclusively with low (Pu/U)o, and low Nd/Ut with both high and low (Pu/U)o.  This contrasts with the similar Pu and Nd chemistry in meteorites11. Discussion:  The younger Group B set contains all zircons in this dataset with apparent (Pu/U)o>0.001.  It 2313.pdf44th Lunar and Planetary Science Conference (2013)also appears to include zircons from more magmatical-ly evolved systems (e.g., later-stage melts or more fel-sic granitoids).   Earlier investigations of Jack Hills zircon xenon also revealed higher Pu/U generally fall-ing among younger zircons2, although there is no in-formation on these zircons’ trace element chemistries.  The lack of (Pu/U)o correlations with other variables in this dataset may be due to the small sample size.   Fig. 2: Hadean Jack Hills zircons in several geo-chemical variables.  Negative (Pu/U)O estimtes are shown here as zero.  The zircons’ apparent (Pu/U)o vs. Nd/Ut and the aqueous indicator δ18O (A,B); Hadean zircons’ 207Pb/206Pb ages vs. Nd/Ut (C) reveals that the high-Nd/Ut zircons are temporally restricted to ~4.15-4.1 Ga.  Yb/Gd, an indicator for magmatic tempera-ture and evolution, increases with time (D).  Effects of recrystallization on Pu/U: A notable fea-ture of the older Group A is the bimodal nature of their relative U-Xe ages: Group A contains both one of the oldest and two of the youngest relative U-Xe ages (see Fig. 1).  This higher degree of xenon loss compared to many younger Group B zircons could also explain some of the variations in Pu/U.  An earlier study found that Pu/U divergence from a chondritic estimate of ~0.0071 increases in Jack Hills zircons for increasing discordance between 207Pb/206Pb and U-Xe ages, indi-cating xenon loss as a method for generating Pu/U di-versity. A late loss of xenon due to heating or recrystalliza-tion would be expected to lead to both lower U-Xe age and apparent Pu/U.  However, certain types of solid-state recrystallization12 concentrate compatible trace elements in the zircon lattice, and such a process oper-ating while Pu was still live would form Pu/U-enriched recrystallized regions.  Lower U-Xe ages coupled with higher Pu/U for some release steps from multi-release zircons are observed both here and in a previous study2.  This may point to such Hadean recrystalliza-tion events, although the timing remains unclear. Secondary aqueous alteration:  One explanation for the weak trend (R2~0.4) between the highest-(Pu/U)o releases and the δ18O of their respective zir-cons may be the production of both low-Pu/U and low-δ18O regions in the zircons by later reaction with a hy-drous fluid.  Of our samples only one (ANU 31-15.8) displays the high, flat LREE pattern often interpreted as resulting from aqueous alteration9.  However, both lower δ18O and enriched LREE (for which we use Nd/Ut as a proxy) are expected results of secondary aqueous alteration, and so the association of lower δ18O and Nd/Ut > 0.01 exclusively with low-Pu/U zircons may indicate  that the low-Pu/U signature in these zir-cons is due to later aqueous alteration.  The relatively young U-Xe ages resulting from later xenon loss in several Group A zircons provides further support for this interpretation.  The timing of this proposed aque-ous alteration event is difficult to estimate but an upper bound of ~4.0 Ga is constrained based on the youngest crystallization ages of zircons similar in relative U-Xe age to young Group B samples. Primary variation in Group B:  The differing Pu/U in different xenon release steps of three Group B zir-cons suggests secondary alteration, in general apparent-ly including an enhancement of Pu/U in the younger release (and thus likely due to Hadean recrystallization of portions of the zircons).  The rest of the (Pu/U)o variation may well be primary.  The lack of any but weak trends between (Pu/U)o (highest release steps only) and various indicators for both aqueous activity and magmatic differentiation is either due to small sample size or the contribution of several processes to the primary Pu/U diversity.  A larger sample set will be needed to determine the cause(s) of primary Pu/U vari-ations in Hadean zircons. References: [1] Hudson, G.B., et al. (1989)  Proc. 19th Lunar and Planetary Sci. Conf., 547-557.  [2] Turner, G., et al. (2007) Earth. Planet. Sci. Lett. 261, 491-499.  [3] Turner, G., et al. (2004) Science 306, 89-91.  [4] Langmuir, D. (1978) Geochim. Cosmochim. Acta 42, 547-69.  [5] Harrison, T.M. (2009) Annu. Rev. Earth. Planet. Sci. 37, 479-505.  [6] Trail, D., et al. (2007) Geochem.Geophys.Geosyst.8, Q06014.  [7] Gilmour, J.D., et al. (1994) Rev. Sci. Instrum. 65, 617-625.     [8] Crowther, S. A., et al. (2008) J. Anal. At. Spectrom. 23, 938–947. [9] Hoskin, P.W.O. and Schaltegger, U., (2003), In Hanchar, J.M., Hoskin, P.W.O., eds., Zircon. Rev. Mineral. 53, 343-385.  [10] Claiborne, L.L., et al. (2010) Contrib. Mineral. Petrol. 160, 511-531.  [11]  Jones, J.H, Burnett, D.S. (1987) Geochim. Cosmochim. Acta 51, 769-782.  [12] Hoskin, P.W.O., Black, L.P. (2000)  J. Meta. Geol. 18, 423-439. 2313.pdf44th Lunar and Planetary Science Conference (2013)

Origins of Pu/U Variations in >4 Ga Terrestrial Zircons

Abstract

Similar works

Full text

Available Versions

The University of Manchester - Institutional Repository

Origins of Pu/U Variations in &gt;4 Ga Terrestrial Zircons

Abstract

Similar works

Full text

Available Versions

The University of Manchester - Institutional Repository

Origins of Pu/U Variations in >4 Ga Terrestrial Zircons