Fluid–rock interaction at a carbonatite-gneiss contact, Alnö, Sweden

• We evaluate balanced metasomatic reactionsand model coupled reactive and isotopic transportat a carbonatite-gneiss contact at Alno, Sweden.We interpret structurally channelled fluid flow alongthe carbonatite-gneiss contact at ~640C. This caused(1) metasomatism of the gneiss, by the reaction:biotite + quartz + oligoclase + K2O + Na2O +/- CaO +/-MgO +/- FeO = albite + K-feldspar + arfvedsonite +aegirene-augite + H2O + SiO2, (2) metasomatism ofcarbonatite by the reaction: calcite + SiO2 = wollastonite+ CO2, and (3) isotopic homogenization of themetasomatised region. We suggest that reactiveweakening caused the metasomatised region to widenand that the metasomatic reactions are chemically(and possibly mechanically) coupled. Spatial separationof reaction and isotope fronts in the carbonatiteconforms to a chromatographic model which assumeslocal calcite–fluid equilibrium, yields a timescale of10^2–10^4 years for fluid–rock interaction and confirmsthat chemical transport towards the carbonatiteinterior was mainly by diffusion. We conclude thatmost silicate phases present in the studied carbonatitewere acquired by corrosion and assimilation of ijolite, as a reactive by-product of this process and bymetasomatism. The carbonatite was thus a relativelypure calcite–H2O–CO2–salt melt or fluid.Keywords Carbonatite, Fluid flow, Metasomatism, Alno

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