Elsevier

Earth and Planetary Science Letters

Volume 432, 15 December 2015, Pages 243-253
Earth and Planetary Science Letters

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

https://doi.org/10.1016/j.epsl.2015.10.013Get rights and content

Highlights

  • Hypatia is an unusual stone found in Egypt and rich in diamonds.

  • Noble gases and nitrogen confirm its extra-terrestrial origin.

  • Some similarities are found with carbon-rich extra-terrestrial materials.

  • Its combined features are unique among known extra-terrestrial materials.

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 (δN15110) 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.

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 1010 and 1011 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 40 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

References (49)

  • A. Meshik et al.

    Heavy noble gases in solar wind delivered by Genesis mission

    Geochim. Cosmochim. Acta

    (2014)
  • S. Niedermann

    The 21Ne production rate in quartz revisited

    Earth Planet. Sci. Lett.

    (2000)
  • U. Ott

    Planetary and pre-solar noble gases in meteorites

    Chem. Erde

    (2014)
  • H. Palme et al.

    The Acapulco meteorite: chemistry, mineralogy and irradiation effects

    Geochim. Cosmochim. Acta

    (1981)
  • V.K. Rai et al.

    Noble gases in ureilites: cosmogenic, radiogenic, and trapped components

    Geochim. Cosmochim. Acta

    (2003)
  • V.K. Rai et al.

    Nitrogen components in ureilites

    Geochim. Cosmochim. Acta

    (2003)
  • W.U. Reimold et al.

    J. Afr. Earth Sci.

    (2014)
  • R. Wieler et al.

    Noble gases in “phase Q”: closed-system etching of an Allende residue

    Geochim. Cosmochim. Acta

    (1991)
  • R. Wieler et al.

    Exposure history of the regolithic chondrite Fayetteville: I. Solar-gas-rich matrix

    Geochim. Cosmochim. Acta

    (1989)
  • A.A. Barakat

    The Precious Gift of Meteorites and Meteorite Impact Processes

    (2012)
  • G.K. Benedix et al.

    A petrologic study of the IAB iron meteorites: constraints on the formation of the IAB–Winonaite parent body

    Meteorit. Planet. Sci.

    (2000)
  • R.L. Bjork

    Analysis of the formation of Meteor Crater, Arizona: a preliminary report

    J. Geophys. Res.

    (1961)
  • S.R. Boyd et al.

    Improved techniques for the extraction, purification and quantification of nanomole quantities of nitrogen gas: the nitrogen content of diamond

    Meas. Sci. Technol.

    (1995)
  • H. Busemann et al.

    Primordial noble gases in “phase Q” in carbonaceous and ordinary chondrites studied by closed-system stepped etching

    Meteorit. Planet. Sci.

    (2000)
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