| This study combines regional- and outcrop-scale investigations of zeolite parageneses with thermodynamic calculations to assess the stability of zeolites in geologic systems. Regional scale mapping of zeolite parageneses in Tertiary basaltic lavas of central East Greenland identified a sequence of depth-controlled isograds delineating regionally extensive metamorphic mineral zones defined by (with increasing depth): no zeolites, chabazite + thomsonite, chabazite + thomsonite + analcime, mesolite + scolecite, heulandite + stilbite, laumontite, and prehnite. On the regional scale, these zones are essentially uniform in thickness throughout the province, parallel paleotopographic surfaces, and transgress lava stratigraphy. Mineral assemblages characterizing the zones formed within one million years after volcanism ceased and provide temporal markers for distinguishing synvolcanic and postvolcanic deformation. The zones are part of a series of prograde mineral parageneses formed during surficial weathering, burial metamorphism, and local hydrothermal alteration. Outcrop and petrographic observations of altered lavas at Teigarhorn, eastern Iceland, indicate that burial metamorphism led to formation of mafic phyllosilicates during burial followed by precipitation of zeolite zone assemblages after burial. Minerals precipitated during weathering (celadonite and silica) and burial metamorphism almost completely occlude primary porosity. Brittle deformation during dike emplacement at Teigarhorn generated secondary pore space that conducted fluid flow and facilitated further zeolite, phyllosilicate, silica, and carbonate alteration of the lavas. In order to evaluate phase relations involving zeolites in the lavas of Iceland and Greenland, a critical summary and critique of the thermodynamic properties of rock-forming zeolites was conducted. Evaluation of calorimetric and phase equilibrium data, along with corresponding states estimation algorithms, were used to generate a database of thermodynamic properties for endmember compositions of common natural zeolites. The calculations leading to this database explicitly account for variations in the thermodynamic properties of zeolitic water between minerals. Thermodynamic calculations indicate that univariant phase equilibria may control the distribution of zeolites at temperatures above ∼100°C. Comparison between calculated equilibrium constants and ion activity products in Icelandic geothermal fluids for zeolite hydrolysis reactions indicate that fluids are typically supersaturated with respect to most zeolites at very low temperatures, but approach saturation in the temperature range where a given zeolite occurs. |
