Chemical transport at a carbonatite-gneiss contact, Alnö, Sweden

We evaluated balanced metasomatic reactions and modelled coupled reactive and isotopic transport at a shear zone contact between carbonatite and gneiss at Alnö, Sweden. We interpreted structurally channelled fluid flow along the shear zone at ~640°C.
This fluid flow caused metasomatism of the adjacent gneiss and carbonatite and isotopic homogenization of the metasomatised region. Metasomatism of the gneiss adjacent to the shear zone occurred by the reaction: biotite + quartz + oligoclase + K₂O + Na₂O ± CaO ± MgO ± FeO = albite + K-feldspar + arfvedsonite + aegirene-augite + H₂O + SiO₂. This reaction is similar to fenitisation, as defined by Brgger (1921), in that alkalis are added and silica is removed, but differs in that addition of CaO, MgO and FeO is also predicted. Balancing this reaction by making the assumption that Al₂O₃ as immobile yielded a volume reduction of 3.9 %. Metasomatism of the carbonatite adjacent to the shear zone occurred by the reaction: calcite + SiO₂ = wollastonite + CO₂. Balancing this reaction yielded a volume increase of 1.6 %. By integrating profiles of reaction progress we estimated respective front displacements of 3.9 m and 2.9 m for metasomatism of the gneiss and carbonatite. The corresponding volume changes per m² contact area are -0.15 m³.m^-2 and +0.05 m³.m^-2, respectively.
Thus volume gain in the carbonatite can easily be accommodated by volume loss in the gneiss. Furthermore, SiO₂ released by metasomatism of the gneiss could be consumed during metasomatism of the carbonatite. We thus conclude that metasomatism of the carbonatite and gneiss are both mechanically and chemically coupled. Spatial separation of reaction and isotope fronts in the carbonatite conforms to a chromatographic model which assumes local calcite-fluid equilibrium, yields a timescale of 100 - 10,000 years for fluid-rock interaction and confirms that chemical transport towards the carbonatite interior was mainly by diffusion. Spatial coincidence of reaction and isotope fronts in the gneiss conforms with a model whereby fluid flow in the gneiss is parallel to the contact and channelled within the metasomatised region.
Hode Vuorinen & Skelton (2004) concluded that most silicate phases present in this carbonatite were acquired by either (1) corrosion and assimilation of ijolite (diopside, nepheline), or (2) as a reactive by-product of this process (biotite). We extend this conclusion, by confirming that wollastonite was acquired by metasomatisms. We can thus infer that the carbonatite was a relatively pure calcite-H₂O-CO₂-salt melt or fluid.