Quantitative field methods for estimating melt production and melt loss

Melt extraction in migmatites occurs via melt channels of increasing width from source areas (often recognizable by the location of incongruent phases) via concordant leucosomes to discordant melt conduits. To test the contribution of migmatites to granite genesis, melt production and melt loss need to be quantified at specimen and outcrop scale.

Melt production during dehydration melting can be modelled by calculating the volume ratio of melt and incongruent phases from balanced melting reactions. Melt loss can be quantified by (i) comparing these predicted volume ratios with ratios derived in outcrop; and (ii) modelling strain patterns near melt loss structures. A field test in SW Finland shows a reasonable correspondence between melt loss estimates from leucosome/garnet volume ratios and from melt loss structures, if the original shape of the melt patch is assumed to have been linear.