Shock origin of the largest ureilitic microdiamond: structural observations and δ13C value

We report structural evidence for impact formed diaphite or stacking disorder in the largest known extraterrestrial microdiamond (>300 μm) from the highly shocked ureilite Northwest Africa 6871. Shock indicators within the microdiamond indicate a diamond formation model during the catastrophic disruption of the ureilite parent body, rather than deep static processes. After removal of associated graphite, large geometry secondary ion mass spectrometry yielded δ13 C = –2.89 ± 0.06 ‰, an intermediate value in the range of ureilitic δ13 C values. Combined with the Mg# of Northwest Africa 6871, the carbon isotopic signature shows that carbon was not affected by the impact which destroyed the ureilite parent body. Our findings therefore challenge the hypothesis that large ureilitic diamonds formed deep within their parent body and show that the carbon isotopes did not fractionate during diamond formation.