Tiny diamonds from ultrahigh pressure metamorphic rocks for understanding of mantle geodynamics

The discovery of microdiamonds in 1990 in eclogite-bearing metamorphic rocks of crustal affinities in Kazakhstan, became critical to understanding of continental collisions, and the subduction and exhumation of rocks. These microdiamonds triggered a revision of plate tectonic theory by suggesting that buoyant continental slabs can be subducted to a depth of > 150 km and then return back to the surface due to tectonic exhumation. Diamond is one the most exciting Earth materials, because it indicates a depth of subduction and survives subsequent metamorphism after their exhumation to shallow horizons of continental crust.

High resolution electron microscopy, synchrotron X-ray diffraction, FTIR and Raman spectroscopic studies were instrumental in shaping our understanding of the mechanisms of microdiamond formation in natural samples and laboratory experiments. These studies are extraordinary examples of the interaction between 21^st century science and technology which resulted in a paradigm shift about microdiamond formation in geological environments previously believed to be "forbidden" for diamond formation.

The main research areas are: (i) confirmation of microdiamonds crystallization at T= 800-1200^oC and P=7-9 GPa; (ii) diamond crystallization from a supercritical multicomponent C-O-H-rich fluids or melt; (iii) average δ13C = -10 to - 33 ^o% in microdiamonds from different UHPM terranes fall in range of "crustal" characteristics; (iv) the 3He/4He contains evidence of continental crust slab - mantle plume interaction; and (v) successful experimental reproduction of diamonds from C-O-H-rich fluids at geological conditions close to their host rocks.

Studies of microdiamonds from recently discovered UHPM terranes continue to release new observations on deep carbon cycling, mantle convection, geochemical recycling and rejuvenation of the mantle which are critical components for understanding of mantle dynamics.

Acknowledgements: We are grateful to U.S. National Science Foundation - (grants EAR 0229666, EAR 0107118, INT-EAR 0329596); Pacific Rim Program from University of California, Los Alamos National Laboratory (grant 9949 - Science and Technology Base Program); the Consortium for Materials Properties Research in Earth Sciences (COMPRES) and Helmholtz Centre Potsdam, GFZ, Germany.