Crustal melts below 400 °C

We propose that the internally zoned, Li-bearing Tin Mountain pegmatite in the Harney Peak granite-pegmatite system of the Black Hills, South Dakota, crystallized from fluidrich, compositionally complex melts at ∼400-350 °C. The low crystallization temperatures resulted from the combined fluxing effects of Li, B, P, H²O, and carbonate anions. The presence of hydrous silicate melts at temperatures of ∼350 °C is revealed by microthermometric data on primary fluid inclusions cogenetic with crystallized-melt inclusions. Mineral assemblages of the crystallized-melt inclusions and chemistry of bulk-fluid leachates indicate that the melts and fluids contained significant amounts of Li and Na as carbonates and/or borates that acted as powerful fluxes. The low temperatures give a new perspective on the stability of silicate melts in the crust.