A comprehensive study of noble gases and nitrogen in “Hypatia”, a diamond-rich pebble from SW Egypt
Introduction
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). This brittle black stone (Fig. S1) consists of ∼70 wt% carbon, and has a hardness comparable to diamond, reminiscent of carbonados. Kramers et al. (2013) named the stone “Hypatia” in honor of a 4th century female philosopher from Alexandria (Egypt). These authors performed an exploratory analytical study on Hypatia, including XRD, SEM, Raman spectroscopy, TEM, and analyses of C and noble gas isotopes motivated by the fact that this stone was found in the area of the Libyan Desert Glass (LDG), the origin of which remains enigmatic (Reimold and Koeberl, 2014). Noble gas isotope analysis is central to the study of meteorites because these rocks formed from multiple components with distinct noble gas isotopic signatures that help to constrain their origin and evolution. Among these components, the so-called Q phase dominates the budget of heavy noble gases (Ar, Kr Xe) in chondrites originating from the asteroid belt. Although the chemical nature (Marrocchi et al., 2015) and mode of formation (Kuga et al., 2015, Ott, 2014) of Q are debated, this component is chemically and isotopically fractionated relative to the isotopic composition of the Solar Wind (Meshik et al., 2014) possibly due to ionization processes (Marrocchi et al., 2011) and is ubiquitous in pristine to moderately metamorphosed chondrites (Busemann et al., 2000). Other minor noble gas components (e.g. the P3 and G components) are found in presolar materials (for example SiC) trapped in meteorites (Ott, 2014). These components are derived from sources external to the solar system and carry isotopic signatures characteristic of nucleosynthesis in stars. Kramers et al. (2013) concluded that Hypatia is extra-terrestrial, based on 40Ar/36Ar ratios as low as about 40. They noted that O/C ratios (0.19–0.51) in Hypatia are higher than in chondritic Insoluble Organic Matter (IOM). In addition, they reported that the trapped Ne, Kr, and Xe in Hypatia indicate the occurrence of the nucleosynthetic P3 and G components of presolar origin known from meteorites (Ott, 2014), while the Q (and HL) components ubiquitous in chondrites were absent in Hypatia. 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), although a cometary origin for the Libyan Desert Glass has been advocated many times, starting with Urey's seminal paper (Urey, 1957).
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. We also describe results from X-ray diffraction (XRD) experiments and transmission electron microscopy (TEM) observations performed at the University of Jena (Germany). An attempt to determine the oxygen isotopic composition in Hypatia by the Nancy Cameca 1280 ion probe failed because of the reduced size of oxygen-bearing phases and because of the presence of contaminants and important amounts of water. This new study confirms and provides new evidence for the earlier conclusion that Hypatia is a fascinating new type of extra-terrestrial material. In contrast to the exploratory work reported by Kramers et al. (2013), we did find noble gases with isotopic signatures closely resembling the Q component. We also found nitrogen with an isotopic signature clearly distinct from primitive chondrites and closely resembling those of various differentiated meteoritic materials. In particular, we compare our data with noble gas and nitrogen signatures in three known types of carbon-rich extraterrestrial materials: carbon-rich veins in ureilite meteorites, graphite nodules in iron meteorites, and carbon-rich lithologies in acapulcoites and lodranites, and we discuss a possible link of Hypatia with each of these materials.
Section snippets
Samples and methods
X-ray diffraction and Transmission Electron Microscopy (TEM) techniques used in this study are described in the supplementary material.
XRD results and TEM observations
The X-ray diffraction pattern of Hypatia is characterized by broad, low-intensity peaks (Fig. S2) indicating the poor crystallinity of the material. A comparison with the pattern of well-crystallized synthetic diamond shows that the main X-ray reflections in the pattern of the Hypatia sample are fully compatible with the diamond structure. X-ray diffraction tails on both sides of the 111 diamond peak were also observed and can be interpreted as the and peaks of the hexagonal
Discussion
The data presented here undoubtedly confirm and strengthen the conclusion by Kramers et al. (2013) that Hypatia is an extraterrestrial material. Apart from the confirmation of very low 40Ar/36Ar ratios, this is also clearly shown by the isotopic composition of He, Ne, and Xe, which in many extraction steps and bulk samples is essentially identical to the Q component, ubiquitous in many meteorite classes. Furthermore, given that values of δ15N below have never been reported in terrestrial
Summary and conclusions
The analyses presented here confirm conclusions by Kramers et al. (2013) that the enigmatic pebble Hypatia represents an unusual type of extraterrestrial material. 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
Acknowledgments
Yves Marrocchi, David Bekaert, Marc Chaussidon and Léo Martin are gratefully acknowledged for fruitful discussions. We appreciate the constructive comments by two anonymous reviewers. This study was funded in Nancy (France) by the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013 grant agreement No. 267255 to B.M.). M.M. is supported by an Ambizione grant (project PZ00P2_154874) from the Swiss National Science Foundation. Further financial
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