Flow and Transport in Contact Aureoles: Petrographic and Stable Isotope Approach

The research within this project was focussed on geological, petrographical and (isotope)geochemical investigations of processes of material transport under conditions of high pressure and temperature which prevail during rock metamorphism in the earth crust. The project was aimed at the documentation of phenomena of syn-metamorphic material transport to obtain a set of data that allows for quantitative modeling of the observed phenomena. In three sub-projects we investigated transport phenomena on different length scales in the Monzoni contact aureole (Dolomite mountains of South Tyrol), Oslo Graben (S-Norway) and in the Ivrea zone (N-Italy). An improved understanding of material transport at high pressures and temperatures has important implications in the context of long term behavior of anthropogenic perturbations of geological systems (e.g. by disposal of radioactive waste). Subprojects In the Monzoni contact aureole we obtained data on the mineralogical, petrographical and stable isotope systematics of the metamorphosed rocks. From these data the thermal evolution of the aureole and the spatial variation of the syn-metamorphic rock permeabilities were inferred. The data suggest a strong lithological control on the syn-metamorphic permeability structure from an aureole-scale to the millimeter scale. Pure carbonates appear to have been impermeable. With increasing contamination of carbonates by siliciclastic components an increase in the syn-metamorphic permeability structure could be identified. This behavior is ascribed to mineral reactions with a negative reaction volume - "reaction enhanced permeability". In the Oslo Graben petrographic and (isotope)geochemical analyses (oxygen-, carbon- and strontium isotopes) of rocks from two contact aureoles (Drammen Granite and Larvic intrusion) were performed. The data indicate two fundamentally different fluid regimes during metamorphism of the two aureoles. Whereas in the Drammen aureole the fluid was relatively cool and water rich, in the Larvik aureole the temperatures were significantly higher and the fluid had a larger concentration of CO2. With respect ot syn-metamorphic rock permeabilities the results from the Monzoni contact aureole could largely be confirmed. In the Ivrea zone metasomatic corona structures were investigated, which were generated in the course of small scale material exchange between mutually incompatible rock types under high grade metamorphic conditions. Mineralogical-petrographical and oxygen isotope data were combined to derive (semi)quantitative information on the mobilities of the major components (Si, Al, Mg, Ca, O). An important result of this study was that oxygen diffusion must have occurred significantly faster than would be indicated from experimental data. The fast diffusion of oxygen may either be due to the effect of grain-boundary diffusion, or, alternatively, it may be due to coupling of oxygen transport to the transport of other chemical components that were re-distributed during corona growth.