Crustal melts below 400 °C

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Abstract

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.

Original languageEnglish
Pages (from-to)685-688
JournalGeology
Issue number31
StatePublished - 2003

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