New insight into polycrystalline diamond genesis from modern nanoanalytical techniques

Technical developments in analytical methods that reach nanometer spatial resolution have enabled the interrogation of smaller, submicron-sized inclusions in diamond that had previously been elusive. This has inspired and enabled studies of non-classical diamond species from different geological settings, resulting in a strongly faceted and dynamic picture of diamond formation. This article reviews the leap of knowledge achieved by employing state-of-the-art analytical methods with high spatial resolution to polycrystalline diamonds from different settings, i.e. from kimberlite, from crustal ultra-high pressure metamorphic terranes and alluvial carbonados. While crustal metamorphic diamonds are generally formed under oxidizing conditions, polycrystalline diamond from the Earth's mantle and carbonado have inclusion suites reflecting variable, and sometimes extreme, redox conditions. Diamond fluid compositions, however, fall in the same compositional field for worldwide diamond fluids, regardless of their geodynamic environment.On the basis of thermodynamic equilibrium data for C-H-O fluids in the mantle we argue that submicron inclusions in diamonds are products of local remobilization connected to fluid-fluxed partial melting and redox freezing. Thus, evidence from these inclusions complements information from classical work on larger inclusions and allows a unique direct insight into the medium in which diamond formed.