The origin of aubrites: Evidence from lithophile trace element abundances and oxygen isotope compositions

Type Article
Date 2016-11
Language English
Author(s) Barrat Jean-Alix1, Greenwood R. C.2, Keil K.3, Rouget M. L.4, Boesenberg J. S.5, 6, Zanda B.7, 8, Franchi I. A.2
Affiliation(s) 1 : UBO IUEM, CNRS UMR Domaines Ocean 66538, Pl Nicolas Copern, F-29280 Plouzane, France.
2 : Open Univ, Dept Phys Sci, Planetary & Space Sci, Walton Hall, Milton Keynes MK7 6AA, Bucks, England.
3 : Univ Hawaii Manoa, Sch Ocean & Earth Sci & Technol, Hawaii Inst Geophys & Planetol, Honolulu, HI 96822 USA.
4 : IUEM, CNRS UMS 3113, Pl Nicolas Copern, F-29280 Plouzane, France.
5 : Amer Museum Nat Hist, Earth & Planetary Sci, New York, NY 10024 USA.
6 : Brown Univ, Geol Sci, Providence, RI 02912 USA.
7 : Museum Natl Hist Nat, Lab Mineral, 61 Rue Buffon, F-75005 Paris, France.
8 : Cosmochim Museum, CNRS UMR7202, 61 Rue Buffon, F-75005 Paris, France.
Source Geochimica Et Cosmochimica Acta (0016-7037) (Pergamon-elsevier Science Ltd), 2016-11 , Vol. 192 , P. 29-48
DOI 10.1016/j.gca.2016.07.025
WOS© Times Cited 16
Keyword(s) Aubrite, Trace elements, Oxygen isotopes, Oldhamite
Abstract

We report the abundances of a selected set of "lithophile" trace elements (including lanthanides, actinides and high field strength elements) and high-precision oxygen isotope analyses of a comprehensive suite of aubrites. Two distinct groups of aubrites can be distinguished: (a) the main-group aubrites display flat or light-REE depleted REE patterns with variable Eu and Y anomalies; their pyroxenes are light-REE depleted and show marked negative Eu anomalies; (b) the Mount Egerton enstatites and the silicate fraction from Larned display distinctive light-REE enrichments, and high Th/Sm ratios; Mount Egerton pyroxenes have much less pronounced negative Eu anomalies than pyroxenes from the main-group aubrites. Leaching experiments were undertaken to investigate the contribution of sulfides to the whole rock budget of the main-group aubrites. Sulfides contain in most cases at least 50% of the REEs and of the actinides. Among the elements we have analyzed, those displaying the strongest lithophile behaviors are Rb, Ba, Sr and Sc. The homogeneity of the Delta O-17 values obtained for main-group aubrite falls [Delta O-17 = +0.009 +/- 0.010% (2 sigma)] suggests that they originated from a single parent body whose differentiation involved an early phase of large-scale melting that may have led to the development of a magma ocean. This interpretation is at first glance in agreement with the limited variability of the shapes of the REE patterns of these aubrites. However, the trace element concentrations of their phases cannot be used to discuss this hypothesis, because their igneous trace-element signatures have been modified by subsolidus exchange. Finally, despite similar O isotopic compositions, the marked light-REE enrichments displayed by Mount Egerton and Larned suggest that they are unrelated to the main-group aubrites and probably originated from a distinct parent body.

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