diamond, inclusions, gems

Gübelin examines mineral inclusions in natural diamonds as evidence for the conditions and processes of diamond formation in the upper mantle. He distinguishes two principal inclusion assemblages: an ultramafic suite characterized by chromium-bearing olivine, chrome-pyrope garnet, enstatite, chrome diopside, and chromite, and an eclogitic suite characterized particularly by pyrope-almandine garnet and omphacitic clinopyroxene, with rarer coesite, corundum (ruby), rutile, kyanite, and ilmenite. Minerals of the two suites had not then been observed together in a single diamond, whereas several inclusions belonging to one suite may coexist, indicating distinct paragenetic environments. Their chemistry, morphology, and relationships to the enclosing diamond are used to argue that diamond and its inclusions crystallized at great depth under upper-mantle pressures and temperatures and that diamond did not originate in kimberlite during its subsequent ascent. Gübelin discusses experimental and mineralogical evidence for crystallization from carbon-bearing ultramafic or eclogitic melts or partially molten material, with H₂O- and CO₂-rich fluids and Fe-Ni-Cu-Co sulphides playing potentially important roles. Sulphide inclusions, particularly pyrrhotite, are considered in relation to possible reduction reactions capable of precipitating carbon as diamond, while trapped gases provide additional evidence concerning the diamond-forming environment. The preservation of nickel-rich pyrrhotite is interpreted as evidence of rapid cooling, and the survival of diamond during transport requires rapid ascent in kimberlitic eruptions. Photomicrographs illustrate characteristic inclusions of olivine, garnet, enstatite, chrome diopside, omphacitic clinopyroxene, chromite, coesite, ruby, rutile, biotite, sulphides, and even diamond within diamond. The latter demonstrate that diamond growth could occur in successive episodes rather than as a single continuous event. Gübelin concludes that mineral inclusions provide direct samples of the otherwise inaccessible environment in which diamonds formed and permit important constraints to be placed on their paragenesis, pressure-temperature conditions, growth history, and subsequent transport to the Earth’s surface.