Depth of formation of super-deep diamonds

Diamonds and the mineral inclusions that they trap during growth are among the most studied geological “samples” as they provide unique information about the deepest regions of the Earth, which cannot be directly investigated. Such diamonds can be conveniently subdivided in two main categories: lithospheric (LDs) and sub- lithospheric or “super-deep” (SDDs). In terms of relative abundance, within the global diamond population, 94% are represented by LDs and only 6% by SDDs. LDs form at depths ranging between 120 and 250 km, whilst SDDs are interpreted to crystallize between 300 km and 800 km depth, because some of the inclusions trapped within them are considered to be the products of retrograde transformation from lower- mantle or transition-zone precursors. However, due to the lack of experimental evidence relating composite inclusions directly to high-pressure precursors, the real depth of origin of SDDs has never been proven. My Ph.D. project aims to obtain the depth of formation of SDDs by studying the most common mineral phases enclosed within them by non-destructive methods. At present, we have studied about 40 diamonds with such inclusion phases as CaSiO3- walstromite, jeffbenite, and ferropericlase using in-house single-crystal X-ray diffraction and micro-Raman spectroscopy as well as laser-heated diamond-anvil cells and both synchrotron tomographic microscopy and synchrotron Mössbauer source at outside Institutions. In addition, “single-inclusion elastic barometry” has been completed on CaSiO3-walstromite together with thermodynamic and first-principles calculations. So far, one of our principal results suggest that CaSiO3-walstromite may be considered a sub-lithospheric mineral, but retrograde transformation from a CaSiO3-perovskite precursor is only possible if the diamond around the inclusion expands in volume by ~30%. Moreover, high-pressure and high-temperature experiments indicate that Ti-free jeffbenite, which occurs as single-phase inclusion in diamonds, could be directly incorporated into diamond in the transition zone or lower mantle and therefore this mineral may represent a high-pressure marker to detect SDDs. Currently, further investigations are in progress on ferropericlase inclusions and these results will be also presented.