Origins of compositional diversity in natural zeolites within the Antrim Lava Group, Northern Ireland, UK

The Antrim Lava Group comprises voluminous alkaline basalts, less extensive tholeiitic basalts, and inter-flow sediments formed in the Palaeocene between 63.2 and 60.0 Ma during early rifting of the North Atlantic. Zeolites and associated minerals crystallized within vesicles during post-eruptive hydrothermal overprinting as the lava pile of >80 flows accumulated. Studies in the mid-20th century established geographical patterns in zeolite species distributions which reflect host rock chemistry (alkaline vs. tholeiitic) and coastal vs. inland exposures of the lava pile. Zeolites crystallized from relatively high-salinity hydrothermal fluids, such as phillipsite-Na, gmelinite-Na and gobbinsite, appear to be spatially associated with coastal outcrops of the Antrim Lava Group. Unusual Mg-rich varieties of chabazite and heulandite are geographically restricted to a coastal location on Islandmagee.
Data presented here as bivariate and ternary compositional diagrams, from EMPA obtained in PhD research in the 1980s and more recent EMPA and SEM-EDS analyses, illustrates a wide heterogeneity in alkaline element chemistry and Si/Al ratios in Antrim zeolites. We explore the extent to which this diversity may be due to ionic exchange with modern seawater in specimens from coastal locations.
Inland and coastal localities of Antrim zeolite specimens are distinguished on previously published discrimination diagrams utilizing graphs of Si/Al vs. (Na+K)/Ca in phillipsite and chabazite associated with freshwater and seawater-influenced environments in Antarctica. A marine influence is also clearly evident in erionite and gmelinite given their wide compositional ranges extending to high ratios of Si/Al and (Na+K)/Ca in specimens from the same localities where phillipsite and chabazite show high ratios. Individual crystals commonly show zonation from Ca-dominant cores to Na-dominant rims, for example changing from garronite in the core to gobbinsite in the rim. Chabazite from Rathlin Island and erionite and cowlesite from the north coast location of Dunseverick have exceptionally high Na contents. We infer that seawater interaction is more rapid and extensive with fibrous zeolites such as erionite than with species that form blocky crystals.