Diamond-olivine host-inclusion system: crystallography and depth of formation

Diamond is probably the most deepest and oldest material able to reach the Earth’s surface. Thanks to its ability to include any type of inclusion during its ascent it provides the unique direct sampling of our planet interior. Indeed, much of what is believed about the genesis and distribution of diamond in the Earth’s mantle has therefore been deduced indirectly from the characterisation of its mineral inclusions. However, there is still an important debate about the temporal growth relationships between diamond and its inclusions and about the real depth at which diamonds can form. In detail, it is often assumed that the inclusions were formed at the same time as the host diamond (i.e. syngenetic), and do not represent pre-existing material that was passively incorporated into the growing diamond (i.e. protogenetic inclusions). Two main arguments have been used in favour of syngenesis. (1) It has been claimed that inclusions normally exhibit specific orientations relative to the diamond hosts. (2) Inclusions often display a morphology of the diamond host (i.e. typical “cubo-octahedral” shape). We have measured the crystallographic orientation of 47 inclusions of olivine in 21 different diamonds by singlecrystal X-ray diffraction. We found no recurrent crystallographic orientations between olivine and diamond (as instead reported in some literature data) ruling out any possible epitaxial relationship. However, multiple olivine inclusions in a single diamond often show similar orientation to one another. Concerning the depth at which the diamonds form it is reported that about 94% of diamonds are subcratonic, with depths of formation ranging between 120 and 250 km, and only 6% of them could be considered “super-deep diamonds” (see recent paper on Nature, Pearson et al., 2014). However, it is very difficult using chemical geothermobarometer approach to provide the real depth of formation of diamonds and recent the method based on inclusion-host elasticity (see Angel et al., 2014) is representing an extremely valid alternative to get the pressure of diamond formation. We have measured the pressure of formation on four of the above 21 diamonds from Udachnaya applying the method based on inclusion-host elasticity to the diamond-olivine pair and found a pressure of formation very similar for all the diamonds, which could suggest that at Udachnaya diamonds form under the same pressure.