Retrograde mineral and fluid evolution in high-pressure metapelites (Schistes lustrés unit, Western Alps)

Abstract

Fluid inclusions have been analysed in successive generations of syn-metamorphic segregations within low-grade, high-pressure, low-temperature (HP–LT) metapelites from the Western Alps. Fluid composition was then compared to mass transfer deduced from outcrop-scale retrograde mineral reactions. Two types of quartz segregations (veins) occur in the `Schistes lustres' unit: early blueschist-facies carpholite-bearing veins (BS) and retrograde greenschist-facies chlorite-bearing veins (GS). Fluid inclusions in both types of segregations are aqueous (no trace of dissolved gases such as CO2, CH4, N2), with significant differences in density and composition (salinity). BS fluids are moderately saline fluids (average 9.1 wt% eq. NaCl) characterized by a chronological trend towards more dilute composition (from 15 down to 0 wt% eq. NaCl), whereas GS fluids have a very constant salinity of ∼3.7 wt% eq. NaCl. Both types of inclusions were continuously reset to lower densities along the retrograde path, until a temperature of ∼300 °C. Mass-balance calculations, together with fluid inclusion data, suggest that GS fluids result from the mixing between two fluid sources: one initial, early metamorphic, moderately saline HP fluid and a second nearly pure water fluid provided by the breakdown of carpholite. Estimates of the amount of water released by carpholite breakdown result in a dilution of the interstitial fluid phase (from 10 to 2.5–4 wt% eq. NaCl) consistent with the actual shift of the fluid composition. Alkali elements required for the formation of the GS chlorite + phengite assemblage after carpholite could be locally provided by HP phengite. This is taken as an indirect evidence that, during the generation of both BS and GS fluids, mixing with externally derived fluids may have been very limited. The location, amount and constant composition of the less saline GS fluids appear to be related to an interconnected porosity at the time of inclusion formation.