Fluid migration above a subducted slab — Thermodynamic and trace element modelling of fluid–rock interaction in partially overprinted eclogite-facies rocks (Sesia Zone, Western Alps)

Abstract

The amount and composition of subduction zone fluids and the effect of fluid–rock interaction at a slab–mantle interface have been constrained by thermodynamic and trace element modelling of partially overprinted blueschist-facies rocks from the Sesia Zone (Western Alps). Deformation-induced differences in fluid flux led to a partial preservation of pristine mineral cores in weakly deformed samples that were used to quantify Li, B, Sr and Pb distribution during mineral growth, -breakdown and modification induced by fluid–rock interaction. Our results show that Li and B budgets are fluid-controlled, thus acting as tracers for fluid–rock interaction processes, whereas Sr and Pb budgets are mainly controlled by the fluid-induced formation of epidote. Our calculations show that fluid–rock interaction caused significant Li and B depletion in the affected rocks due to leaching effects, which in turn can lead to a drastic enrichment of these elements in the percolating fluid. Depending on available fluid-mineral trace element distribution coefficients modelled fluid rock ratios were up to 0.06 in weakly deformed samples and at least 0.5 to 4 in shear zone mylonites. These amounts lead to time integrated fluid fluxes of up to 1.4 ∙ 10 2 m 3 m − 2 in the weakly deformed rocks and 1–8 ∙ 10 3 m 3 m − 2 in the mylonites. Combined thermodynamic and trace element models can be used to quantify metamorphic fluid fluxes and the associated element transfer in complex, reacting rock systems and help to better understand commonly observed fluid-induced trace element trends in rocks and minerals from different geodynamic environments.