Subduction zones are an important recycling center at which material from the exterior of the Earth is transported to Earth's interior. The processes that occur along subduction zones have important implications for elemental cycles, geodynamics, and material mass transport. The cold, dense subducting lithosphere experiences prograde metamorphism as it transitions from blueschist to eclogite facies resulting in the breakdown of volatile-bearing minerals and producing anhydrous minerals and a free fluid phase. Previous works attempting to understand the evolution of subducted lithologies have provided a firm foundation in which to apply field work, computational thermodynamic modeling, and geochronological techniques in order to better constraint the Pressure-Temperature-time (P-T-t) paths and dehydration of subducted lithologies.
This dissertation; 1.) Explores novel approaches to modeling and predicting fluid/rock interactions during deep (>60km) subduction, and 2.) Questions what the calculated P-T-t path from eclogite lithologies reveals about early exhumation of subducted terrains. The second chapter focuses on how externally-derived hydrous fluids can decarbonate subducted basalt, liberate carbon and transfer it to the overlying mantle wedge, where it can be incorporated into melt that forms volcanic arcs. Here, the thermodynamic response to the infiltration of external fluids assuming open system, pervasive fluid flow, is quantified. It was determined that while hotter subduction zones have more favorable P-T conditions in which to facilitate decarbonation than colder subduction, the extent of decarbonation is largely dependent on the availability of fluid from the dehydration of underlying serpentine. The third chapter constrains the P-T-t paths of subducted lithologies from Syros, Greece using a combination of thermodynamic modeling, 147Sm/144Nd garnet geochronology, and quartz-in-garnet geobarometry. This provides insight into early exhumation of subducted lithologies, and allows for the exploration of assumptions made in thermodynamic modeling and in quartz-in-garnet geobarometry. Results suggest that garnet grew over a 4.31my period from 45.71±0.98Ma to 41.4±1.7Ma, during initial exhumation from maximum subducted depths. Calculated exhumation rates are a relatively rapid, 0.4-1.7 cm/yr. Because field relationships on Syros suggest the width of the subduction channel along the slab/mantle interface is not adequate to facilitate buoyancy-driven ascension of metabasic blocks, initiation of southward retreat of the Hellenic Subduction Zone and subsequent slab rollback is proposed to have played an important role in the exhumation of subducted lithologies. The final chapter investigates the compositional controls on the P-T conditions at which dehydration due to the breakdown of hydrous minerals occur during subduction (blueschist/eclogite boundary), and the implications they have on the rheology, seismicity, and densification of the down going slab. Total Alkali Silica (TAS) diagrams reveal that eclogites are more alkali rich, implying that initial alteration of the seafloor controls the mineral evolution of subducted basalt in many cases. / Ph. D. / Subduction zones are an important ‘recycling center’ at which material from the exterior of the Earth is transported to Earth’s interior. The processes that occur along subduction zones have important implications for elemental cycles, geodynamics, and material mass transport. The cold, dense rock that gets heated and deeply buried (high pressure metamorphism) releases fluid as water-bearing minerals breakdown. Previous works attempting to understand the history of subducted rocks have provided a firm foundation in which to apply field work, modeling, and dating techniques in order to better constraint the depths and temperatures a rock experienced, and over what time interval metamorphism and subsequent fluid loss occurred.
This dissertation; 1.) Explores novel approaches in modeling and predicting fluid/rock interactions during deep (>60km) subduction, and 2.) Asks what does the calculated Pressure-Temperature-time (P-T-t) path from eclogite lithologies reveal about early exhumation of subducted terrains. The second chapter focuses how water-rich fluids can infiltrate a subducted rock and liberate carbon, which migrates upwards and can be incorporated into melt that forms volcanic arcs. It was determined that while the potential to release more carbon is higher in hotter subduction zones, the extent of decarbonation is largely dependent on the availability of fluid from the dehydration of the water bearing mineral., serpentine. The third chapter constrains the P-T-t paths of subducted lithologies from Syros, Greece using a combination of techniques, which not only provides insight into the return of subducted rocks back to the surface, but allows for the exploration of assumptions made in various analytical methodologies. Results suggest that garnet grew over a span from 45.71±0.98Ma to 41.4±1.7Ma, as the rock moved at a rate of 0.4–1.7 cm/yr to travel approximately 18km back to the surface, which suggests that a large-scale tectonic mechanism is needed to facilitate the rock’s return journey. The final chapter investigates the compositional controls on the P-T conditions in which the breakdown of hydrous minerals, and subsequent fluid loss, occur during subduction (blueschist/eclogite boundary), and the implication they have on the physical properties and behavior of the subducting rock. Chemical data reveal that subducted rocks that dehydrate earlier are more enrich in K and Na, implying that initial alteration of the seafloor controls the mineral evolution of subducted basalt.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/83810 |
| Date | 29 June 2018 |
| Creators | Gorce, Jennifer Shannon |
| Contributors | Geosciences, Caddick, Mark J., Baxter, Ethan Finley, Law, Richard D., Bodnar, Robert J. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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