Zeolite parageneses in the north Atlantic igneous province: Implications for geotectonics and groundwater quality of basaltic crust

Zeolites are among the most common products of chemical interaction between groundwaters and the Earth's crust during diagenesis and low-grade metamor phism. The unique crystal structures of zeolites result in large molar volu mes, high cation-exchange capacities, and reversible dehydration. These pro perties influence both the stability and chemistry of zeolites in geologic systems, leading to complex parageneses and compositional relationships tha t provide sensitive indicators of physicochemical conditions in the crust. Observations of zeolite occurrence in Tertiary basaltic lavas in the North Atlantic region indicate that individual zeolite minerals are distributed i n distinct, depth-controlled zones that parallel the paleosurface of the pl ateau basalts and transgress the lava stratigraphy. The zeolite zones are i nterpreted to have formed at the end of burial metamorphism of the lavas. R elative timing relations between various mineral parageneses and crustal-sc ale deformal features indicate that the minerals indicative of the zeolite zones formed within 1 million years after cessation of volcanism. Empirical correlation between the depth distribution of zeolite zones and the temper atures of formation of zeolites in geothermal systems provides estimates of regional thermal gradients and heat flow in flood-basalt provinces. Simila rly, the orientations of zeolite zones can be used to distinguish synvolcan ic and post-volcanic crustal deformation. Because zeolites that characteriz e the individual zones display different ion-exchange selectivities for var ious cations, reactions between groundwaters and zeolites in basaltic aquif ers can result in depth-controlled zones where individual elements are conc entrated in the crust. This is established for Sr, which is concentrated by at least an order of magnitude in heulandite, resulting in an overall Sr e nrichment of lavas in the heulandite-stilbite zeolite zone.