Planetary bodies in the Solar System are thought to have formed by the accretion of smaller bodies in a series of catastrophic collisions. The nature of large planets means that material has tended to be reworked over their history, destroying any trace of the original bodies, therefore, in order to get around this planetary scientists use material from smaller bodies such as meteorites, which are thought to have remained relatively unchanged over the history of the Solar System. Meteorites can be sorted into a number of different families, based upon their mineralogy, with each of these different families thought to have derived from a different parent body. One particularly interesting family of meteorites are the Ureilites, which are carbon-rich stony meteorites with numerous graphite and microdiamond inclusions. These meteorites are of interest because of the nature of diamonds, which cannot form at pressures of less than about 2 gigapascals, suggesting that they must have formed in the interior of a body at least a thousand kilometres in diameter.
In a paper published in the journal Nature Communications on 17 April 2018, Farhang Nabiei of the Earth and Planetary Science Laboratory and the Interdisciplinary Center for Electron Microscopy at the Ecole Polytechnique Fédérale de Lausanne, James Badro also of the Earth and Planetary Science Laboratory at the Ecole Polytechnique Fédérale de Lausanne, and of the Institut de Physique du Globe de Paris at Sorbonne Paris Cité, Teresa Dennenwaldt also of the Interdisciplinary Center for Electron Microscopy and of the Electron Spectrometry and Microscopy Laboratory at the Ecole Polytechnique Fédérale de Lausanne, Emad Oveisi and Marco Cantoni, again of the Interdisciplinary Center for Electron Microscopy at the Ecole Polytechnique Fédérale de Lausanne, Cécile Hébert, once again of the Interdisciplinary Center for Electron Microscopy and of the Electron Spectrometry and Microscopy Laboratory at the Ecole Polytechnique Fédérale de Lausanne, Ahmed El Goresy of the Bayerisches Geoinstitut at the Universität Bayreuth, Jean-Alix Barrat of the Institut Universitaire Européen de la Mer at the Université de Bretagne Occidentale, and Philippe Gillet, again of the Earth and Planetary Science Laboratory at the Ecole Polytechnique Fédérale de Lausanne, describe the results of a study which uses mineral inclusions in diamond and graphite layers in the Almahata Sitta Meteorites to try to understand the nature of the Ureilite Parent Body.
The Almahata Sitta meteorites are derived from Asteroid 2008 TC3, an Apollo Group Asteroid which entered the Earth's atmosphere and broke up over the Nubian Desert in northern Sudan on 7 October 2008, the first ever asteroid to be detected before entering the atmosphere, though by only 19 hours. Nabiei et al. examined MS-170, a Almahata Sitta Meteorite whuch has been found to contain clusters of diamonds with common crystal orientations separated by bands of graphite, which are interpreted as being derived from single diamonds as large as 100 μm across, which have been fragmented as a result of shock graphitisation.
Many minerals can contain inclusions of other minerals, smaller crystals around which the mineral has formed. Diamonds are no exception to this (a disadvantage for commercial diamond dealers, since diamonds with inclusions are worth far less than those without), and, due to the very hard nature of diamond, these inclusions are well protected against alteration by outside forces. Nabiei et al. found inclusions within diamonds in MS-170 which shared common orientations with minerals of the same type included in neighbouring diamonds, with both the host diamonds and inclusion minerals separated by graphite layers, lending support to the idea that these features are the result of the break-up of larger diamonds.
Graphitisation of diamond along twinning directions. (a) The high-angle annular dark-field scanning transmission electron microscope image shows two twinning regions indicated as twin 1 and twin 2. Twin 1 is intersecting with two inclusions (indicated by orange arrows) and graphitised, while twin 2 is purely diamond. (b) The graphite-diamond electron energy loss spectroscopy map (from the dashed blue rectangle in panel (a)) indicates that the graphitisation is confined to the twinning region and around the inclusions (red = graphite, blue =diamond). Nabiei et al. (2018).
The vast majority of the inclusions are iron-rich sulphides, particularly troilite, kamacite, and schreibersite. These are found as isolated grains with sizes up to a few hundred nanometres, and small particles ranging from fifty down to a few nanometres in size. These minerals are faceted, which indicates they were solid when they were trapped within the diamond matrix (a liquid melt that had been trapped within the diamond matrix then set would form an amorphous mass), and are always preserved together, with the three minerals forming a sharply defined polyhedral arrangement. This suggests that the minerals were originally formed as single iron-nickel-sulphur-phosphorous crystals that latter decomposed into the different minerals now present. This implies original crystals in excess of 150 nanometres across, which is significant, as such large crystals would need to form in a sustained high pressure environment, rather than a short term one, as might be encountered during the impact of two bodies.
Also present in the diamonds were chromite and phosphate minerals, and while these were much rarer, they formed much larger grains, often reaching a few hundred nanometres across. Interestingly the chromite minerals contained no magnesium or aluminium, something usually associated with iron meteorites.
Iron sulphur minerals were also found within the graphite areas of the meteorite, though in these cases the minerals showed signs of melting at the time of inclusion in the matrix. This is consistent with the formation of graphite from diamond in a shock event, which would cause melting of the original crystals, followed by a drop in pressure then recrystallisation.
Based upon the large size of the diamonds and their inclusions, Nabiei et al. calculate that these mineral crystals formed under a sustained pressure in excess of 20 gigapascals. They further note that at this sort of pressure the eutectic temperature (temperature at which a substance either melts or solidifies) of iron-sulphur minerals is about 1350 Kelvin. These conditions are consistent with formation at the centre of a body the size of Mercury, or at the core-mantle boundary of a body the size of Mars.
See also...
Electron micrograph and compositional maps of diamond inclusions in ureilite. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images (a), (b), (c), and (d) and associated iron and sulphur elemental maps (e), (f), (g), and (h) of inclusions in diamond. All chemical (EDX) maps show iron (light blue) and sulphur (red) distribution. Kamacite and troilite phases appear as light blue and reddish-pink respectively. Nabiei et al. (2018).
Also present in the diamonds were chromite and phosphate minerals, and while these were much rarer, they formed much larger grains, often reaching a few hundred nanometres across. Interestingly the chromite minerals contained no magnesium or aluminium, something usually associated with iron meteorites.
Iron sulphur minerals were also found within the graphite areas of the meteorite, though in these cases the minerals showed signs of melting at the time of inclusion in the matrix. This is consistent with the formation of graphite from diamond in a shock event, which would cause melting of the original crystals, followed by a drop in pressure then recrystallisation.
Electron micrograph and chemical map of an inclusion in a graphitised region. (a) Bright-field scanning transmission electron microscope (BF STEM) image, and (b) chemical (EDX) map from graphite growth in diamond matrix around an inclusion. Blue dashed lines indicate the diamond–graphite boundary. The yellow arrows point out the iron–sulphur rich regions in graphite. Notice the clear rounded form of the inclusion in graphitised part indicating partial melting. Nabiei et al. (2018).
Based upon the large size of the diamonds and their inclusions, Nabiei et al. calculate that these mineral crystals formed under a sustained pressure in excess of 20 gigapascals. They further note that at this sort of pressure the eutectic temperature (temperature at which a substance either melts or solidifies) of iron-sulphur minerals is about 1350 Kelvin. These conditions are consistent with formation at the centre of a body the size of Mercury, or at the core-mantle boundary of a body the size of Mars.
See also...
Follow Sciency Thoughts on Facebook.