How deep (and hot) is a diamond? The current state of diamond thermobarometry

A good understanding of the distribution of diamond in Earth’s mantle provides constraints for the evaluation of diamond potential of economic targets and for the assessment of the deep carbon cycle in ancient Earth. Much of what we know about the depth distribution of diamond is derived from the study of rare mineral inclusions that are amenable to conventional two-phase thermobarometry. However, non-touching inclusions were not necessarily in equilibrium at their encapsulation time, whereas touching inclusions could reequilibrate after entrapment. Moreover, since diamonds can form under a wide range of redox conditions, the unknown Fe3+/Fetot ratios in the inclusions may lead to large uncertainties on T estimates based on Fe–Mg exchange thermometry and, in turn, on P estimates. The development of single-mineral thermobarometers applicable to isolated inclusions has in part overcome the equilibrium issue. However, as yet only chromian clinopyroxene allows to estimate both P and T with a precision comparable, if appropriate analytical conditions are used, to that of conventional methods. In fact, no effective barometers exist for some of the most abundant inclusion types, such as olivine and the eclogitic minerals. An additional problem is whether the inclusions are syngenetic with diamond or they represent passively captured pre-existing material. In the latter case, incomplete chemical resetting during rapid diamond growth may prevent determination of the conditions of diamond formation. Recent studies suggest that the imposition of diamond morphology on the inclusion (the most widely used criterion for syngenesis) is not unique to syngenetic inclusions, casting doubts on the real significance of P-T data extracted from many reported “syngenetic” inclusions in diamonds. The determination of the remnant P on the inclusion, e.g., using data from X-ray diffractometry, birefringence analysis or Raman spectroscopy, provides an alternative way to diamond barometry using elasticity theory. Application of this methodology is still at its infancy, because of uncertainties in the thermoelastic behaviour of the minerals, potential non-elastic relaxation and technical limitations. It is a field of on-going development, which may eventually allow evaluation of the depth of provenance of diamonds containing minerals that are chemically insensitive to P, such as the abundant upper-mantle olivine or the super-deep ferropericlase. Although available data suggest that lithospheric diamonds may come from any depth below the graphite–diamond transition, owing to the above limitations recognition of subtle inhomogeneity in the vertical distribution of diamond (or of specific diamond populations) remains challenging. Further development of conventional and non-conventional thermobarometric methods may allow us to increase the statistical significance of the diamond P-T record and may eventually contribute to refine models of diamond formation in the sub-cratonic mantle.