The role of water in the evolution of the continental crust

1. The role of water in the evolution of the continental crust Bruce W.D. Yardley 2009

2. Primary concerns of paper • Synthesize and evaluate past research to create an overview of water-rock interactions in various crustal environments • Examine role of fluids through crustal cycle of burial, metamorphism, and uplift • Reconcile information from a variety of methods

3. Major Assertions of Paper • Role of water is very different in diagenesis vs. metamorphism and in prograde metamorphism vs. retrograde metamorphism • Mineral reactions are limited or impossible in a truly dry system over the normal range of crustal temperatures • It is unlikely that widespread large fluid fluxes accompany metamorphism • Water is as important in controlling crustal strength as temperature

4. Water and Diagenesis • pressure increasing with depth due to compaction • High fluid pressure at depth contributes to anomalously high porosity of deep buried sediments • Salinity increases with depth due to density of brines and permeability of sedimentary basins • Formation waters can evolve to chemical equilibrium with diagenetic minerals even at low temperatures

5. Water and Geothermal Activity • Isotopic studies show that geothermal waters are predominantly meteoric rather than magmatic in origin • Convection systems develop in shallow, permeable crust • Large volumes of circulating water result in extensive metasomatic alteration • Little exchange of fluids between shallow geothermal system and underlying metamorphic aureole due to high fluid pressure in aureole

6. Geophysical evidence • Magnetotelluric/resistivity studies show evidence of deep crustal fluids • Fluid saturation in reverse fault systems and convergent zones • Transient behavior suggests fluid flow activity • Fluid flow measurements show correlation between flow and seismic tremors

7. Geochemical evidence • Trace fluids indicate mantle sources in areas of active magmatism and deep structures • Combined gophysical and isotopic data indicates that seismic activity can dislodge gases from rock—mixed mantle and continent derived signatures follow tremors • Danger in equating gas behavior with water behavior

8. Water and prograde metamorphism • Overpressures developed from compaction and from water released by minerals—anomolously high porosity with depth • T<250C, ductile deformation, irreversible loss of porosity • Main source of water during subsequent heating is release of chemically bound water from lattice • Permeability evolves to match reaction rate • Concentration of dissolved gases evolves in metamorphic waters • Increased miscibility between gases and water

9. Water and Retrograde Metamorphism • Cooling of metamorphic rock allows reactions that consume water to replace those that release it • Results in low fluid pressure • Fluid can only be contained in fractures insulated from reactive minerals • Short lifetime of free water in cooled crystalline rock

10. Water and deformation • Water facilitates deformation by aiding in mass transfer • Weakens rock • Pervasive ductile deformation occurs in rock that contains pervasive fluid • Drying effects of retrograde metamorphism result in more brittle deformation behavior