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

Abstract

This is a follow-up study of a work by Kramers et al. (2013) on an unusual diamond-rich rock found in the SW side of the Libyan Desert Glass strewn field. This pebble, called Hypatia, is composed of almost pure carbon. Transmission Electron Microscopy and X-ray diffraction results reveal that Hypatia is made of defect-rich diamond containing lonsdaleite and deformation bands. These characteristics are compatible with an impact origin on Earth and/or in space. We analyzed concentrations and isotopic compositions of all five noble gases and nitrogen in several mg sized Hypatia samples. These data confirm that Hypatia is extra-terrestrial. The sample is rich in trapped noble gases with an isotopic composition close to the meteoritic Q component. 40Ar/36Ar ratios in individual steps are as low as 0.4. Concentrations of cosmic-ray produced 21Ne correspond to a nominal cosmic-ray exposure age of ca. 0.1 Myr if produced in a typical m-sized meteoroid. Such an atypically low nominal exposure 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 extinct 129I) is observed (129Xe/132Xe = 1.18 +/- 0.03). Two N components are present, an isotopically heavy component (δ15N = +20 permil) released at low temp. and a major light component (δ15N = -110 permil) at higher temp. This disequilibrium in N suggests that the diamonds in Hypatia were formed in space. Our data are broadly consistent with concentrations and isotopic compositions of noble gases in at least three different types of carbon-rich meteoritic materials. However, Hypatia does not seem to be 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 that led Kramers et al. to propose that Hypatia is a remnant of a comet that impacted the Earth.