HR: 09:20h
AN: V51B-05 INVITED     [Abstracts]
TI: Traces of H2O in Ultrahigh-Pressure Metamorphic Rocks
AU: * Dobrzhinetskaya, L F
EM: Larissa@ucr.edu
AF: University of California at Riverside, Department of Earth Sciences, Riverside, CA 92521, United States
AB: Ultrahigh-pressure (UHP) metamorphic rocks accommodate a significant amount of H2O at high pressures and temperatures during their deep subduction. Fluid-driven processes are responsible for mineral reactions; they may trigger phase transformations and provide a decisive weakening effect on the rheological behavior of the rocks during deep subduction, or they may lead to brittle failure and earthquakes. Dehydration reactions, producing fluid (i.e., H2O, CO2) during regional metamorphism at low-to-mid crustal levels (P ~ 0.1 - 1 GPa), are reasonably pressure insensitive. At these levels, the maximum dehydration occurs at the greatest temperatures experienced by buried rocks. In the deep subduction zone (> 120 - 150 km), where both high pressures and temperatures operate, the situation is different. There, on the one hand, the water becomes chemically bonded and incorporated into the structure of both nominally hydrous and anhydrous minerals. On the other hand, the aqueous fluid (a supercritical fluid) dissolves a considerable amount of mineral components at high pressures and temperatures, and a solute concentration increases as pressure is increased. Microstructural observations on ultrahigh-pressure minerals from eclogites and metasediments provide convincing evidence of fluid involvement as deep as the upper mantle and possibly, the mantle transition zone. Diamond is one of the minerals of great importance because it unambiguously records the high pressure (minimum 4 GPa and possibly > 4GPa) at which the host rocks were recrystallized. We present here the results of studies of nano-inclusions associated with dislocations of growth and/or with interstitial defects of carbon in diamond structure obtained with transmission electron microscopy, microRaman, and microInfrared synchrotron assisted spectroscopy. A diverse composition of multicomponent fluid and crystalline inclusions and characteristic of nitrogen aggregations, provide evidence that the diamonds were crystallized from a supercritical C-O-H fluid during a UHP metamorphism related to continental collision. These observations are also consistent with diamonds synthesized at high pressure and high temperature from graphite, amorphous carbon, and coal in the presence of H2O. The crustal signature of carbon isotopes (δ 13C) in the diamonds, together with their multiphase fluid- solid inclusions, provides evidence of a pathway by which organic carbon and H2O were subducted to the mantle depths and returned back to the Earth's surface. Microstructural patterns such as healed cracks, microfabrics, etc., observed in other minerals co-existing with diamonds, can therefore be recognized and attributed to UPH metamorphism events and may cast a light on the deformation and rheology of UHP metamorphic rocks.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8033 Rheology: mantle (8162)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly