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

Euhedral, post‐depositional albite from the Eastern and Western Alps, the western Carpathians and some Greek islands was examined petrographically and geochemically to gain insights into the nature of feldspar reactions in carbonate rocks. This study focuses on coarsely crystalline, homogeneously nucleated albite in order to avoid problems related to the presence of inseparable detrital material in fine‐grained albite varieties. All albite samples show a very restricted compositional variability and are typically ≥ 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 isotopic composition of albite ranges from + 19·4‰ to + 28·3‰ VSMOW. There is no direct relationship between the δ 18 O value and the inferred temperature of albite formation, nor is there one with stoichiometry. The coarse crystal size (up to several millimetres in diameter), petrographic evidence showing albite cross‐cutting stylolites, greater abundance of albite in carbonate rocks subject to high‐grade diagenetic or weak metamorphic overprinting and available fluid inclusion data suggest that albite precipitation is favoured at higher temperatures in carbonates than in sandstones. Pore fluids were invariably brines, as suggested by the inferred high positive δ 18 O fluid values, the common association of albite‐bearing carbonates and evaporites and reports of saline fluid inclusions in albite. The presence of authigenic albite may thus be a useful tracer of palaeobrine–carbonate reactions, particularly in deep‐burial and incipient metamorphic settings.