ULTRAHIGH PRESSURE METAMORPHIC FLUID: EVIDENCE FROM SUBDUCTION ZONE MICRODIAMONDS

Subduction zone microdiamonds occurring within metasedimentary rocks are well confirmed now within orogenic belts related to Paleozoic continental collisions within Euro-Asian continent. Two of them, Kokchetav massif, Kazakhstan, and Erzgebirge, Germany, provide definitive evidence that diamonds were crystallized from a supercritical COH fluid. The diamonds contain intact fluid inclusions and those that filled with both fluid and crystalline phases. The microdiamonds are testimony to exhumation of crustal rocks from depths >120 km, and the inclusions within them are pristine witnesses to the geochemical environment of the medium from which such diamonds crystallized. Diamond is a perfect container, hence it resists chemical reaction with its host or included minerals both during residence at depth within the diamond stability field and during ascent to the Earth's surface. Dozens of nanometric fluid and solid inclusions in diamond incorporated in garnet and zircon from felsic gneiss of the Kokchetav and Erzgebirge massifs have been examined utilizing transmission electron microscopy assisted with focused ion beam techniques, and synchrotron infrared microspectroscopy. The TEM results show that some solid nanoinclusions are chemically simple and may be classified as SiO2, TiO2 and Al2SiO5 according to their stoichiometry, whereas most fluid inclusions contain Si, Ti, P, K, Fe, Cl and S in nonstoichiometric combinations and proportions. Synchrotron IR spectra of microdiamonds indicate absorptions corresponding to stretching and bending motions of the H2Omolecule and carbonate radical (CO₃)^2-. Nitrogen-related absorptions with a strong peak at 1,282 cm^-1 and small but well pronounced peaks at 1,130 cm^-1 and 1,134 cm^-1 confirm that these diamonds belong to type 1b-1aA, implying a short residence time ~ 5Ma in the Earth's interior. The ranges of delta 13C =-17to -27‰ suggest that Erzgebirge diamonds were crystallized from ‘‘crustal'' carbon, whereas the Kokchetav diamonds are formed from mixed “crustal” and “mantle” carbon reservoirs (delta 13C = -10‰ - 11‰). Microdiamond is a unique mineral which fluid–solid nanometric inclusions and carbon isotope characteristics provide clear evidence for a pathway by which ‘‘crustal''components could be subducted to mantle depths and back to the Earth's surface.