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Distinct ^(238)U/^(235)U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes

Tissot, François L. H. and Dauphas, Nicolas and Grove, Timothy L. (2017) Distinct ^(238)U/^(235)U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes. Geochimica et Cosmochimica Acta, 213 . pp. 593-617. ISSN 0016-7037. https://resolver.caltech.edu/CaltechAUTHORS:20180614-103635044

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Abstract

Angrites are differentiated meteorites that formed between 4 and 11 Myr after Solar Systemformation, when several short-lived nuclides (e.g., ^(26)Al-^(26)Mg, ^(53)Mn-^(53)Cr, ^(182)Hf-^(182)W) 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 ^(238)U/^(235)U 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 −0.17 to −1.20 Myr depending on the samples. After correction, concordance between the revised Pb-Pb and Hf-W and Mn-Cr ages of plutonic and quenched angrites is good, and the initial (^(53)Mn/^(55)Mn)_0 ratio in the Early Solar System (ESS) is recalculated as being (7 ± 1) × 10^(−6) 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 δ^(238)U values are consistent with fractionationduring 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 context, Pb-Pb ages derived from pyroxenes fraction should be corrected using the U isotope composition measured in the same pyroxene fraction.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/j.gca.2017.06.045DOIArticle
ORCID:
AuthorORCID
Tissot, François L. H.0000-0001-6622-2907
Alternate Title:Distinct 238U/235U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes
Additional Information:© 2017 Elsevier Ltd. Received 28 December 2016, Accepted 28 June 2017, Available online 8 July 2017. Constructive criticisms from Greg Brennecka, two anonymous reviewers, and editor Yuri Amelin greatly helped improve the manuscript. This work was funded by NSF (grants CSEDI EAR1502591 and Petrology and GeochemistryEAR1444951) and NASA (grants LARS NNX17AE86G, EW NNX17AE87G, and SSW NNX15AJ25G) to ND, NASA grant NNX16AD29G to TG, and a Crosby Postdoctoral Fellowship to FT. The Robert A. Pritzker Center for Meteoritics and Polar Studies at the Field Museum and P. Heck, the Museum National d’Histoire Naturelle (Paris) and the Smithsonian Institute are thanked for providing meteorite samples. G. Brennecka is thanked for providing REE, Th and U concentrations data for some angrites. Authors Contributions: ND and FT designed the research; FT performed the research; FT, ND and TG interpreted the data and wrote the paper.
Funders:
Funding AgencyGrant Number
NSFEAR-1502591
NSFEAR-1444951
NASANNX17AE86G
NASANNX17AE87G
NASANNX15AJ25G
NASANNX16AD29G
Crosby Postdoctoral FellowshipUNSPECIFIED
Subject Keywords:Angrites; U stable isotopes; Pb-Pb ages; Short-lived chronometers; U stable isotope fractionation
Record Number:CaltechAUTHORS:20180614-103635044
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20180614-103635044
Official Citation:François L.H. Tissot, Nicolas Dauphas, Timothy L. Grove, Distinct 238U/235U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes, Geochimica et Cosmochimica Acta, Volume 213, 2017, Pages 593-617, ISSN 0016-7037, https://doi.org/10.1016/j.gca.2017.06.045. (http://www.sciencedirect.com/science/article/pii/S0016703717304040)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:87099
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:14 Jun 2018 18:32
Last Modified:03 Oct 2019 19:52

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