Using in situ trace-element determinations to monitor partial-melting processes in metabasites

Peak metamorphism (∼800-850° C, 8-10 kbar) in the Harts Range Meta-Igneous Complex (Harts Range, central Australia) was associated with localized partial melting by the reaction hornblende + plagioclase + quartz + H₂O = garnet + clinopyroxene + titanite + melt. In situ trace-element determinations of prograde, peak and retrograde minerals in migmatitic metabasites and associated tonalitic melts using laser-ablation ICP-MS has allowed monitoring of a range of partial-melting processes (melting, melt segregation and back-reaction between crystallizing melt and restitic minerals). Mass balance calculations indicate that titanite is a major carrier of trace elements such as Ti, Nb, Ta, Sm, U and Th, and therefore may be an important accessory phase to control the redistribution of these elements during the partial melting of amphibolites. Titanite preferentially incorporates Ta over Nb and, hence, residual titanite might assist in the formation of melts with high Nb/Ta. The fact that single minerals record different rare earth element (REE) patterns, from prograde to peak to retrograde conditions, demonstrates that REE diffusion is not significant up to ∼800° C. Therefore, trace-element analysis in minerals can be a powerful tool to investigate high-grade metamorphic processes beyond the limits given by major elements.