Authigenic albite in carbonate rocks - a tracer for deep-burial brine migration?

Euhedral, post-depositional albite from the Eastern and Western Alps, the w estern Carpathians and some Creek islands was examined petrographically and geochemically to gain insights into the nature of feldspar reactions in ca rbonate rocks. This study focuses on coarsely crystalline, homogeneously nu cleated albite in order to avoid problems related to the presence of insepa rable detrital material in fine-grained albite varieties. All albite sample s show a very restricted compositional variability and are typically greate r than or equal to 99 mol% Ab component. Unit-cell parameters determined by Rietveld analysis are slightly more variable than previously accepted, but confirm high Al-Si ordering characteristic of low albite. The oxygen isoto pic composition of albite ranges from + 19.4 parts per thousand to + 28.3 p arts per thousand VSMOW. There is no direct relationship between the delta( 18)O value and the inferred temperature of albite formation, nor is there o ne with stoichiometry. The coarse crystal size (up to several millimetres i n diameter), petrographic evidence showing albite cross-cutting stylolites, greater abundance of albite in carbonate rocks subject to high-grade diage netic or weak metamorphic overprinting and available fluid inclusion data s uggest that albite precipitation is favoured at higher temperatures in carb onates than in sandstones. Pore fluids were invariably brines, as suggested by the inferred high positive delta(18)O(fluid) values, the common associa tion of albite-bearing carbonates and evaporites and reports of saline flui d inclusions in albite. The presence of authigenic albite may thus be a use ful tracer of palaeobrine-carbonate reactions, particularly in deep-burial and incipient metamorphic settings.