A comprehensive study of noble gases and nitrogen in “Hypatia”, a diamond-rich pebble from SW Egypt

Guillaume Avice (1) , Matthias M.M. Meier (1, 2) , Bernard Marty (1) , Rainer Wieler (2) , Jan Kramers (3) , Falko Langenhorst (4) , Pierre Cartigny (5) , Colin Maden (2) , Laurent Zimmermann (6) , Marco Andreoli (7)

1 CRPG - Centre de Recherches Pétrographiques et Géochimiques
2 D-ERDW - Department of Earth Sciences [Swiss Federal Institute of Technology - ETH Zürich]
3 Department of Geology [University of Johannesburg]
4 Institut für Geowissenschaften
5 IPGP - Institut de Physique du Globe de Paris
6 CERILAC (EA_4410) - Centre D'Etude et de Recherche Interdisciplinaire de l'UFR LAC
7 WITS - University of the Witwatersrand [Johannesburg]

This is a follow-up study of a work by Kramers et al. (2013) on a very unusual diamond-rich rock fragment found in the area of south west Egypt in the south-western side of the Libyan Desert Glass strewn field. This pebble, called Hypatia, is composed of almost pure carbon. Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD) results reveal that Hypatia is mainly made of defect-rich diamond containing lonsdaleite and multiple deformation bands. These characteristics are compatible with an impact origin on Earth and/or in space. We also analyzed concentrations and isotopic compositions of all five noble gases and nitrogen in several ~mg sized Hypatia samples. These data confirm the conclusion by Kramers et al. (2013) that Hypatia is extra-terrestrial. The sample is relatively rich in trapped noble gases with an isotopic composition being close to the Q component found in many types of meteorites. 40Ar/36Ar ratios in individual steps are as low as 0.4 ± 0.3. Cosmicray produced "cosmogenic" 21Ne is present in concentrations corresponding to a nominal cosmic-ray exposure (CRE) age of roughly 0.1 Myr if produced in a typical meter-sized meteoroid. Such an atypically low nominal CRE age suggests high shielding in a considerably larger body. In addition to the Xe-Q composition, an excess of radiogenic 129Xe (from the decay of short-lived radioactive 129I) is observed (129Xe/132Xe = 1.18 +/- 0.03). Two isotopically distinct N components are present, an isotopically heavy component (δ15N ~ +20‰) released at low temperatures and a major isotopically light component (δ15N ~ -10‰) at higher temperatures. This disequilibrium in N suggests that the diamonds in Hypatia were formed in space rather than upon impact on Earth (δ15Natm = 0 ‰). All our data are broadly consistent with concentrations and isotopic compositions of noble gases in at least three different types of carbon-rich meteoritic materials: carbon-rich veins in ureilites, graphite in acapulcoites/lodranites and graphite nodules in iron meteorites. However, Hypatia does not seem to be directly related to any of these materials, but may have sampled a similar cosmochemical reservoir. Our study does not confirm the presence of exotic noble gases (e.g. G component) that led Kramers et al. (2013) to propose that Hypatia is a remnant of a comet nucleus that impacted the Earth.

Domaines: Géochimie

Guillaume Avice, Matthias M.M. Meier, Bernard Marty, Rainer Wieler, Jan Kramers, et al.. A comprehensive study of noble gases and nitrogen in “Hypatia”, a diamond-rich pebble from SW Egypt. Earth and Planetary Science Letters, 2015, 432, pp.243-253. ⟨10.1016/j.epsl.2015.10.013⟩. ⟨hal-01346149⟩