Abstract
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. Cosmic-ray 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 (Xe129/Xe132=1.18+/−0.03). Two isotopically distinct N components are present, an isotopically heavy component (δN15∼+20‰) released at low temperatures and a major isotopically light component (δN15∼−110‰) at higher temperatures. This disequilibrium in N suggests that the diamonds in Hypatia were formed in space rather than upon impact on Earth (δNatm15=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.
Highlights
In 1996 a very unusual ~30 g sized pebble was found in the Libyan Desert Glass strewn field where abundant fragments of impact-related silica-rich glass are found (Barakat, 2012; Reimold and Koeberl, 2014)
In this work we extend the study by Kramers et al (2013) with isotopic analyses of all five noble gases in several mg-sized fragments of Hypatia in two different laboratories (CRPG Nancy, France and ETH Zürich, Switzerland) and with a nitrogen isotope investigation performed both at CRPG (Nancy) and IPG-Paris
In addition to the clear cut evidence pointed out by Kramers et al (2013) (e.g. 40Ar/36Ar ratios below the atmospheric value), our study shows: 3He/4He, 20Ne/22Ne, Xe isotopes, and likely Kr and Ar close to the isotopic composition of the ubiquitous component phase Q in meteorites, isotopic composition of nitrogen similar to components found in ureilites and maybe in graphite inclusions of iron meteorites or acapulcoites, and small concentrations of cosmogenic 21Ne produced in space
Summary
In 1996 a very unusual ~30 g sized pebble was found in the Libyan Desert Glass strewn field where abundant fragments of impact-related silica-rich glass are found (Barakat, 2012; Reimold and Koeberl, 2014). The combined evidence led them to conclude that Hypatia did not originate in the asteroid belt where chondrites likely formed They suggested instead that it formed in a more external region of the solar accretion disk, such as the Kuiper Belt, where presolar components might be more abundant, i.e., that Hypatia could be of cometary origin. They further proposed that the airburst of the parent comet of Hypatia resulted in the formation of the Libyan Desert Glass. This interpretation was subsequently criticized by Reimold and Koeberl (2014), a cometary origin for the Libyan Desert Glass has been advocated many times, starting with Urey's seminal paper (Urey, 1957)
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