NANOSTRUCTURES IN ULTRAHIGH-PRESSURE METAMORPHIC COESITE AND DIAMOND: A GENETIC FINGERPRINT

Metamorphic coesite and diamond were studied in detail by transmission electron microscopy to test whether the high-pressure minerals preserve a nanostructural memory and report of their metamorphic formation and evolution. Metamorphic coesite from Dora Maira contains few, mostly sessile dislocations and is associated with retrograde quartz that nucleated at the margin and along twin boundaries in coesite. This back transformation occurred during exhumation and liberated water that now decorates Brazil twins in retrograde quartz. Metamorphic diamonds from the Kokchetav and Erzgebirge massifs are absolutely defect-free. The Erzgebirge diamonds are surrounded by a shell of hydrous minerals, suggesting a formation by precipitation from a C-O-H fluid. The lack of retrograde graphite indicates that metamorphic diamond is obviously more resistant to back transformation than coesite. These nanostructural characteristics contrast with those observed in coesite and diamond from kimberlite and impact rocks, substantiating that the defect structure provides a valuable genetic fingerprint.