Zeolite Parageneses in the North Atlantic Igneous Province: Implications for Geotectonics and Groundwater Quality of Basaltic Crust

Abstract

Zeolites are among the most common products of chemical interaction between groundwaters and the Earth's crust during diagenesis and low-grade metamorphism. The unique crystal structures of zeolites result in large molar volumes, high cation-exchange capacities, and reversible dehydration. These properties influence both the stability and chemistry of zeolites in geologic systems, leading to complex parageneses and compositional relationships that 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 in distinct, depth-controlled zones that parallel the paleosurface of the plateau basalts and transgress the lava stratigraphy. The zeolite zones are interpreted to have formed at the end of burial metamorphism of the lavas. Relative timing relations between various mineral parageneses and crustal-scale 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 temperatures of formation of zeolites in geothermal systems provides estimates of regional thermal gradients and heat flow in flood-basalt provinces. Similarly, the orientations of zeolite zones can be used to distinguish synvolcanic and post-volcanic crustal deformation. Because zeolites that characterize the individual zones display different ion-exchange selectivities for various cations, reactions between groundwaters and zeolites in basaltic aquifers can result in depth-controlled zones where individual elements are concentrated 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 enrichment of lavas in the heulandite-stilbite zeolite zone.