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106. Distinct 238 U/ 235 U Ratios and REE Patterns in Plutonic And Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 213 (2017) 593–617 www.elsevier.com/locate/gca Distinct 238U/235U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes Franc¸ois L.H. Tissot a,b,⇑, Nicolas Dauphas a, Timothy L. Grove b a Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL, USA b Department of the Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Received 28 December 2016; accepted in revised form 28 June 2017; Available online 8 July 2017 Abstract Angrites are differentiated meteorites that formed between 4 and 11 Myr after Solar System formation, when several short- lived nuclides (e.g., 26Al-26Mg, 53Mn-53Cr, 182Hf-182W) were still alive. As such, angrites are prime anchors to tie the relative chronology inferred from these short-lived radionuclides to the absolute Pb-Pb clock. The discovery of variable U isotopic composition (at the sub-permil level) calls for a revision of Pb-Pb ages calculated using an ‘‘assumed” constant 238U/235U ratio (i.e., Pb-Pb ages published before 2009–2010). In this paper, we report high-precision U isotope measurement for six angrite samples (NWA 4590, NWA 4801, NWA 6291, Angra dos Reis, D’Orbigny, and Sahara 99555) using multi- collector inductively coupled plasma mass-spectrometry and the IRMM-3636 U double-spike. The age corrections range from À0.17 to À1.20 Myr depending on the samples. After correction, concordance between the revised Pb-Pb and Hf-W and Mn- 53 55 Cr ages of plutonic and quenched angrites is good, and the initial ( Mn/ Mn)0 ratio in the Early Solar System (ESS) is recal- culated as being (7 ± 1) Â 10À6 at the formation of the Solar System (the error bar incorporates uncertainty in the absolute age of Calcium, Aluminum-rich inclusions – CAIs). An uncertainty remains as to whether the Al-Mg and Pb-Pb systems agree in large part due to uncertainties in the Pb-Pb age of CAIs. A systematic difference is found in the U isotopic compositions of quenched and plutonic angrites of +0.17‰. A difference is also found between the rare earth element (REE) patterns of these two angrite subgroups. The d238U values are consistent with fractionation during magmatic evolution of the angrite parent melt. Stable U isotope fractionation due to a change in the coordination environment of U during incorporation into pyroxene could be responsible for such a fractionation. In this con- text, Pb-Pb ages derived from pyroxenes fraction should be corrected using the U isotope composition measured in the same pyroxene fraction. Ó 2017 Elsevier Ltd. All rights reserved. Keywords: Angrites; U stable isotopes; Pb-Pb ages; Short-lived chronometers; U stable isotope fractionation 1. INTRODUCTION ⇑ Angrites are differentiated meteorites (achondrites) of Corresponding author at: Department of the Earth, Atmo- basaltic composition. They are of either volcanic (a.k.a. spheric and Planetary Sciences, Massachusetts Institute of Tech- quenched) or plutonic origin and display minimal post- nology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. crystallization alteration, metamorphism, shock or impact E-mail address: [email protected] (F.L.H. Tissot). http://dx.doi.org/10.1016/j.gca.2017.06.045 0016-7037/Ó 2017 Elsevier Ltd. All rights reserved. 594 F.L.H. Tissot et al. / Geochimica et Cosmochimica Acta 213 (2017) 593–617 brecciation (e.g., Keil, 2012 and references therein). Their assumption of Pb-Pb dating. Because variation in the abso- old U-Pb (e.g., Amelin, 2008a, 2008b) and Pb-Pb ages lute 238U/235U ratio will impact absolute ages, both Pb and (e.g., Wasserburg et al., 1977; Lugmair and Galer, 1992; U isotopic compositions need to be measured to obtain pre- Baker et al., 2005; Connelly et al., 2008) make angrites cise and accurate Pb-Pb ages. The first goal of the present some of the earliest volcanic rocks known in the Solar Sys- paper is to provide high-precision U isotope data on a large tem, with crystallization ages for the oldest samples of 4 array of angrites in order to correct their Pb-Pb ages. These Myr after the formation of Calcium, Aluminum-rich inclu- U-isotope-corrected ages will be used to assess the degree of sions (CAIs, the oldest known solids in the Solar System). concordance between short-lived nuclides systems As such, these achondrites provide insights into early stages (26Al-26Mg, 53Mn-53Cr, 182Hf-182W) and the absolute Pb- of planetary melting and differentiation. They also play an Pb clock. important role as anchors for early solar system (ESS) The second aim of this study is concerned with identify- chronology. Indeed, the quenched angrite specimens have ing whether all angrites have a similar U isotopic composi- estimated cooling rate between 7 and 50 °C/h (Mikouchi tion, and, if not, what is(are) the process(es) responsible for et al., 2000, 2001), which means that it took 18–127 h this variability. Brennecka and Wadhwa (2012) measured a for them to cool from a liquidus temperature of 1190 °C series of angrite samples and suggested that all angrites had (Longhi, 1999) to below the closure temperature of a homogeneous U isotopic composition. They reached this 300 °C relevant to the decay systems discussed in this con- conclusion because they propagated the uncertainties on tribution (Pb: in clinopyroxene Cherniak, 1998, in plagio- the U isotopic composition of the two U double spikes that clase, Cherniak, 1995, in apatite, Cherniak et al., 1991; they used onto the final 238U/235U ratio of each sample. Cr: in olivine, Ito and Ganguly, 2006, in spinel, Posner However, the same double spike (IRMM-3636) was used et al., 2016; Mg: in olivine, Chakraborty, 1997, in plagio- for analysis of all bulk samples, while another spike (in- clase, Muller et al., 2013, in cpx, Van Orman et al., 2014; house ASU spike) was only used to measure a replicate of W: in olivine and model pyroxene, Cherniak and Van D’Orbigny and two mineral separate fractions (D’Orbigny Orman, 2014). Hence, most radiochronometric systems, pyroxenes, AdoR phosphates). Any error on the spike com- and in particular those commonly used in ESS chronology, position will shift the 238U/235U ratios of the samples by a closed simultaneously in quenched angrites (Dodson, 1973). constant value, so that difference in the U isotopic compo- Their mineralogy is also diverse, which makes them amen- sitions of samples corrected by the same double spike are able to dating with various chronometers. Because few sam- better known than one would be led to believe if uncertain- ples meet the requirements of non-disturbance, ties on the spike composition are propagated. When only synchronous isotope closure and phase diversity like the the isotope measurement error is considered, some variabil- quenched angrites, they have become important samples ity in the angrite 238U/235U ratio data set of Brennecka and to cross-calibrate short-lived dating techniques (e.g., Wadhwa (2012) is visible. It is presently unknown whether 26Al-26Mg, 53Mn-53Cr, 182Hf-182W) with absolute dating igneous processes can fractionate U isotopes but the U iso- techniques (i.e., U-Pb, Pb-Pb) (e.g., Nyquist et al., 2009; topic variability of 0.50‰ measured among igneous rocks Dauphas and Chaussidon, 2011; Kleine et al., 2012). As (see compilation in Fig. 6 of Tissot and Dauphas, 2015) the precision of isotope measurement improves, it is possi- raises the possibility that magmatic differentiation on the ble to test at finer levels the concordance of these angrite parent-body could have fractionated U isotopes. chronometers. For instance, and based on a 1.5 Myr dis- Here, we report high-precision U isotopic data for six crepancy between the Al-Mg and Pb-Pb ages of CAIs and angrite samples: NWA 4590, NWA 4801, NWA 6291, quenched angrites, Larsen et al. (2011) proposed that 26Al Angra dos Reis, D’Orbigny, and Sahara 99555. Using this was not uniformly distributed in the Solar System, thus data set and previously published measurements questioning one of the pillars of ESS chronology. To settle (Brennecka and Wadhwa, 2012; Connelly et al., 2012; the vigorous debate around this issue, improvement of the Goldmann et al., 2015), we question the homogeneity of absolute and relative ages of ESS anchors are therefore the U isotope composition of angrite specimens, and discuss critical. the possible processes by which different angrite samples can The most precise absolute ages of ESS objects are acquire different U isotopic compositions. After correcting obtained using the Pb-Pb dating method (Patterson et al., the Pb-Pb ages of angrites, we test their age concordance 1955; Patterson, 1956): a dual chronometer since 238U with short-lived chronometers and discuss the implications decays into 206Pb with a half-life of 4468.3 ± 4.8 Myr while for the distribution of short-lived radionuclides in the ESS. 235U decays into 207Pb with a half-life of 703.81 ± 0.96 Myr (Jaffey et al., 1971). In the late 1970s, to ensure that abso- 2. SAMPLES AND METHODS lute ages could be compared between laboratories, and based on the observed constancy of the 238U/235U ratio in All Teflon labware used in this study (i.e., PFA vials and natural samples, a ‘‘consensus” value of 238U/235U= beakers from Savillex) was pre-cleaned with boiling aqua 137.88 was adopted for use in geochronology (Steiger and regia (3:1 mixture of HCl:HNO3) three times, followed by Jager, 1977).
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