Primary melt inclusions in andalusite from anatectic graphitic metapelites: Implications for the position of the Al₂SiO₅ triple point

Abstract

Anatectic crustal xenoliths in the Miocene volcanic rocks of Mazarrón (southeast Spain) contain andalusite with melt inclusions, an unprecedented finding. Microstructures indicate that the melt inclusions were trapped during andalusite growth. The volatile content of the peraluminous inclusions is too low (Cl < 0.4 wt%, F < 0.3 wt%, P ≈ 1500 ppm, B < 850 ppm) to have caused a significant reduction of the wet solidus temperature. Moreover, the presence of graphite, as observed, during partial melting is expected to have raised the temperature of the wet solidus. Melting temperatures for the inclusions—obtained from quartz-albite-orthoclase haplogranite system (680–790 °C), Zr (620–705 °C), and light rare earth element (615–725 °C) thermometry—indicate that the stability of andalusite + melt is incompatible with the position of the most commonly used andalusite = sillimanite equilibrium, and that the Al₂SiO₅ triple point must be placed at higher temperatures and pressures.