A large planetary body inferred from diamond inclusions in a ureilite meteorite - PubMed (original) (raw)

A large planetary body inferred from diamond inclusions in a ureilite meteorite

Farhang Nabiei et al. Nat Commun. 2018.

Abstract

Planetary formation models show that terrestrial planets are formed by the accretion of tens of Moon- to Mars-sized planetary embryos through energetic giant impacts. However, relics of these large proto-planets are yet to be found. Ureilites are one of the main families of achondritic meteorites and their parent body is believed to have been catastrophically disrupted by an impact during the first 10 million years of the solar system. Here we studied a section of the Almahata Sitta ureilite using transmission electron microscopy, where large diamonds were formed at high pressure inside the parent body. We discovered chromite, phosphate, and (Fe,Ni)-sulfide inclusions embedded in diamond. The composition and morphology of the inclusions can only be explained if the formation pressure was higher than 20 GPa. Such pressures suggest that the ureilite parent body was a Mercury- to Mars-sized planetary embryo.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1

Fig. 1

Graphitization of diamond along twinning directions. a The high-angle annular dark-field (HAADF) STEM image shows two twinning regions indicated as twin 1 and twin 2. Twin 1 is intersecting with two inclusions (indicated by orange arrows) and graphitized, while twin 2 is purely diamond. b The graphite-diamond EELS map (from the dashed blue rectangle in panel a) indicates that the graphitization is confined to the twinning region and around the inclusions (red = graphite, blue = diamond)

Fig. 2

Fig. 2

Inclusion trails imaged inside diamond fragments. a HAADF-STEM image from diamond segments with similar crystallographic orientation. Dashed yellow lines show the diamond–graphite boundaries. b High-magnification image corresponding to the green square in a. Diamond and inclusion trails are cut by a graphite band. The dashed orange line shows the direction of the inclusion trails

Fig. 3

Fig. 3

Electron micrograph and compositional maps of diamond inclusions in ureilite. HAADF-STEM images (a, b, c, and d) and associated Fe and S elemental maps (e, f, g, and h) of inclusions in diamond. All chemical (EDX) maps show Fe (light blue) and S (red) distribution. Kamacite and troilite phases appear as light blue and reddish-pink respectively

Fig. 4

Fig. 4

Electron micrograph and chemical map of an inclusion in a graphitized region. a Bright-field (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 Fe–S-rich regions in graphite. Notice the clear rounded form of the inclusion in graphitized part indicating partial melting

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