Crystallographic orientations and timing relationships of clinopyroxene inclusions in diamond

Diamonds are the deepest “fragments” of the primordial Earth that reach the Earth’s surface after traveling a long path through the interior of our planet. This makes diamonds small time capsules, which reveal crucial information about the geological processes that occurred within the inaccessible mantle regions throughout the Earth’s history. Since diamond is almost pure carbon and chemically inert mineral, its age and crystallization environment within the Earth’s mantle have been constrained over the last 50 years from the study of its inclusions. Most of the early studies on diamonds assumed that these inclusions and diamonds are syngenetic, i.e., they crystallized simultaneously and from the same genetic process. However, recent studies have challenged the paradigm of syngenesis, suggesting that the major portion of minerals included in lithospheric diamonds, instead, were formed before diamond, and are called protogenetic. This discovery has implications for all genetic aspects of diamond, particularly the timing of its crystallization. Indeed, syngenesis has been a long-standing prerequisite for dating diamond through the study of radiogenic isotopes contained within its mineral inclusions. One of the most widely used mineral inclusion for dating diamonds is clinopyroxene, which is amenable with Sm-Nd isotope system. Nonetheless, timing relationships(i.e., the syn- versus protogenetic nature) between clinopyroxenes and their host diamonds are still poorly known. In this thesis work, by investigating the crystallographic orientation relationships (CORs) between clinopyroxene inclusions and lithospheric diamonds from the Voorspoed mine (South Africa), it has been provided the clear evidence of the protogenetic origin of clinopyroxene inclusions in diamonds. The results of this thesis work, therefore, raise questions on the real meaning of many “diamond formation ages” obtained from clinopyroxene inclusions. In order to assess the impact of a protogenetic inclusion of clinopyroxene on the validity of diamond age obtained from it, it has been used a diffusion model at consistent pressure and temperatures for diamond formation within the Earth’s mantle. This model demonstrates that chemical re-equilibration of clinopyroxene inclusions with diamond-forming media occurred over a very long geological timescale. Therefore, diamond ages obtained through the dating of clinopyroxene inclusions are likely older than the diamond crystallization event; hence, the use of clinopyroxene is not recommended for the age determination of lithospheric diamonds.