Diamond facies metamorphism in the Greek Rhodope

Diamond has been known hitherto to occur either in special volcanic pipes, which carry diamond-bearing magma from as far down as 300 km up towards the Earths surface, or in the sediments produced by the erosion of these pipes. Over the last two decades, however, microdiamonds have been found in a third, very unusual type of geologic setting: in so-called ultrahigh-pressure (UHP) terrains. These terrains occur in continental collision zones, i.e. where the continental parts of two plates collide. Material of the continental crust has always been considered to be too light to be dragged down into the mantle region (below ~60 km) by subduction like the more dense oceanic crust. Instead, it is disrupted and piled up to form mountain belts like the Alps or the Himalayas. UHP terrains are many km-sized rock units which have originated in such collisional settings, but which have experienced pressures that correspond to depths of 100 km and more, where coesite and diamond, the high- pressure modifications of quartz and graphite, are stable. Preliminary data from the Rhodope massif shows that the UHP-rocks there have been down as far as ~200 km. The geological processes capable of subjecting the relatively light continetal rocks to such extreme pressures/depths, and even more of transporting this material back up against the general downward motion, are widely disputed amongst scientists and no conclusive model has yet been developed. Another interesting aspect is that minerals under such extreme pressure conditions change markedly in their structures and crystal chemistry. We know some of this behaviour already from the diamond-bearing kimberlites, but UHP-terrains host a broad range of rock types with a varied mineralogy, which all to some degree preserve relics of their UHP past. In this project we investigate the associations and chemistry of minerals of different rock types in the Rhodope UHP-terrain, we try to decipher the tectonic relationship between UHP and neighbouring terrains and complexes, and we make experiments to parallel the metamorphic conditions of these rocks, so we know more about the actual conditions of formation in nature. From all this we want to find better answers concerning the geological processes operating in continental subduction zones during mountain building and thus also the processes forming metamorphic diamond.

Diamond is one of the rarest and most precious minerals known to man. In 2001 the first diamond occurrence was found in Greece and soon afterwards a working group and a project were started to investigate it. In contrast to the usual, "classical" diamond occurrences in kimberlites and other volcanic rocks that originate from source regions deep in the Earth`s mantle, the new occurrence in the Greek Rhodope belongs to a different type that became known only recently, and was described first in Kazakhstan and then in the German Erzgebirge: Diamond that grows during metamorphism, i.e. during the transformation processes characteristic for orogeny (mountain building). The conditions must have been extraordinary, which means that sedimentary rocks containing organic matter must have been transported to depths sufficient for diamond formation (about 100 kilometers or more) and been transported back to the Earth`s surface. Such unusual processes may happen at the onset of mountain building eras. This was also found for the Greek Rhodope, where ages of 140 to 150 years were determined by radiometric dating, which is very early in the entire sequence of the Alpidic orogeny. Also in contrast to kimberlite diamonds, metamorphic diamonds found to date are generally too small for jewellery. The largest crystals are up to several tenths of a millimeter, those in the Rhodope are particularly small - a few thousandths of a millimeter. Of special interest for the geologist are the mountain building processes leading to diamond formation and the rocks formed during that particular stage, which are expected to be extraordinary, too. Our investigations have revealed, however, that the host rocks of diamonds have remained for a long time at "normal" crustal depths during their ascent to the surface and hence changed their mineralogy back to "normal", with virtually no trace of an extraordinary early mineralogy except diamond. However, by deriving the kinematics of the orogeny from rock structures and by dating growth zones in some adequate minerals we were able to reconstruct the various stages of the orogeny and could thus contribute quite essentially to the general understanding of the formation of the mountain chains of southeastern Europe. Why conditions were so extreme in this part of the orogen and whether more spectacular finds of diamond can be expected are interesting questions for future research.