Metasomatic coronas around hornblendite xenoliths in granulite facies marble, Ivrea zone, N Italy, I: constraints on component mobility

Centimeter- to decimeter-thick reaction bands occur at hornblendite/marble interfaces in Val Fiorina in the granulite facies metamorphic Ivrea zone. F rom hornblendite to marble the reaction bands show a consistent succession of sharply bounded mineral layers comprising a monomineralic clinopyroxene layer, a garnet-clinopyroxene layer and a scapolite-clinopyroxene layer. Re action band formation occurred as a response to gradients in the chemical p otentials of calcium and magnesium as defined by the hornblendite assemblag e and the marble matrix. The metasomatic corona primarily replaced the horn blendite, and only minor amounts of marble were consumed. The reaction band behaved as an open system with net transfer of calcium from the marble int o the reaction band, and a net transfer of iron and magnesium in the opposi te direction. Mass balance considerations allow us to constrain a range of feasible mass balance scenarios for which major element fluxes across the b oundaries of the reaction band may be quantified. Modeling of layer growth as a steady diffusion process yields ratios of the phenomenological diffusi on coefficients for Si, Al, Mg, and Ca of L-SiSi/L-CaCa > 2.5, L-AlAl/L-CaC a < 10, L-MgMg/L-CaCa. The relative diffusivities are primarily constrained by the sequence of mineral layers of the reaction band and by the relative thickness of the layers. The results of steady-state diffusion modeling ar e relatively insensitive with respect to variations in the major element bo undary fluxes.