Search

Sulfides are the most abundant inclusions in diamonds and a key tool for dating diamond formation via Re-Os isotopic analyses. The manner in which fluids invade the continental lithospheric mantle and the time scale at which they equilibrate with preexisting (protogenetic) sulfides are poorly understood yet essential factors to understanding diamond formation and the validity of isotopic ages. We investigated a suite of sulfide-bearing diamonds from two Canadian cratons to test the robustness of Re-Os in sulfide for dating diamond formation. Single-crystal X-ray diffraction (XRD) allowed determination of the original monosulfide solid-solution (Mss) composition stable in the mantle, indicating subsolidus conditions of encapsulation, and providing crystallographic evidence supporting a protogenetic origin of the inclusions. The results, coupled with a diffusion model, indicate Re-Os isotope equilibration is sufficiently fast in sulfide inclusions with typical grain size, at mantle temperatures, for the system to be reset by the diamond-forming event. This confirms that even if protogenetic, the Re-Os isochrons defined by these minerals likely reflect the ages of diamond formation, and this result highlights the power of this system to date the timing of fluid migration in mantle lithosphere. [ABSTRACT FROM AUTHOR]

Diamonds are the deepest accessible "fragments" of Earth, providing records of deep geological processes. Absolute ages for diamond formation are crucial to place these records in the correct time context. Diamond ages are typically determined by dating inclusions, assuming that they were formed simultaneously with their hosts. One of the most widely used mineral inclusions for dating diamond is garnet, which is amenable to Sm-Nd geochronology and is common in lithospheric diamonds. By investigating worldwide garnet-bearing diamonds, we provide crystallographic evidence that garnet inclusions that were previously considered to be syngenetic may instead be protogenetic, i.e., they were formed before the host diamond, raising doubts about the real significance of many reported diamond "ages." Diffusion modeling at relevant pressures and temperatures, however, demonstrates that isotopic resetting would generally occur over geologically short time scales. Therefore, despite protogenicity, the majority of garnet-based ages should effectively correspond to the time of diamond formation. On the other hand, our results indicate that use of large garnet inclusions (e.g., >100 μm) and diamond hosts formed at temperatures lower than ~1000 °C is not recommended for diamond age determinations. [ABSTRACT FROM AUTHOR]

Diamonds are witnesses of processes that have operated in Earth's mantle over more than 3 b.y. Essential to our understanding of these processes is the determination of diamond crystallization ages. These cannot be directly determined on diamond, but they can be calculated using radiogenic isotopic systematics of suitable minerals included in a diamond. This method relies on the assumption that the mineral inclusions were in isotopic equilibrium with the diamond-forming medium. We evaluated the validity of Sm-Nd ages yielded by clinopyroxene inclusions by combining crystallographic orientation analyses and Nd diffusion modeling at the relevant conditions for Earth's cratonic mantle. We investigated the crystallographic orientation relationships (CORs) for 54 clinopyroxene inclusions within 18 diamonds from South Africa and Siberia. Clinopyroxene inclusions in some diamonds showed specific CORs with their hosts, indicating possible syngenesis. Other samples had clusters of clinopyroxene inclusions sharing the same orientation but no specific orientation relative to their hosts, indicating that the inclusions are older than the diamond (i.e., they are protogenetic). Diffusion modeling in the temperature range typical for lithospheric diamonds (900-1400 °C) showed that resetting of the Sm-Nd isotopic system in clinopyroxene grains larger than 0.05 mm requires geologically long interaction with the diamond-forming fluid/melt (>3.5 m.y. at average temperature of ~1150 °C). Depending on inclusion size and temperature regime, protogenetic clinopyroxene inclusions may not fully reequilibrate during diamond-formation events. We suggest that small clinopyroxene inclusions (<0.2 mm) that equilibrated at temperatures higher than 1050-1080 °C may be the most suitable for age determinations. [ABSTRACT FROM AUTHOR]