PEGMATITE MELTS AT 340 °C?

Fluids and fluxing elements, such as B, Li, P, and possibly carbonate anions, in addition to K, Na, and other large-ion lithophile elements play a significant role in the generation and evolution of peraluminous granitic melts. Magmatic fluid and crystallized melt inclusions in rare-element bearing pegmatites associated with the world famous Harney Peak Granite, Black Hills, South Dakota, shed a new light on the petrogenesis of the leucogranite-pegmatite complexes.

We have investigated magmatic fluid inclusions in the zoned, Li-rich, Tin Mountain pegmatite (TM) by microthermometry and combined ICP-AES and IC analysis of crush-leach extracts from quartz. Primary fluid inclusions that coexist with crystallized melt inclusions were trapped along the two-fluid phase boundary in the system CO₂-H₂O-NaCl_eq. Consequently, the temperature and pressure conditions of trapping are identical to the bulk homogenization conditions – on average 340 °C and 2.7 kbar. These conditions indicate that this Li, Cs, Rb, Be, P, and B-rich pegmatite crystallized at some of the lowest known temperatures for a silicate melt in the crust. In addition, fluid inclusion results suggest only small differences between the temperatures at which the TM wall zone and core consolidated, consistent with rapid crystallization of the pegmatite at large undercooling conditions.

Although not obviously reflected in the pegmatite’s mineralogy, the significant role played by carbonate anions in TM petrogenesis is suggested by the omnipresence of CO₂ rich inclusions in the magmatic mineral assemblages and of carbonate minerals in the crystallized melt inclusions. Dawsonite - NaAlCO₃(OH)₂ has been identified using Raman Spectroscopy and SEM. Chemistry of bulk-fluid leachates indicates that the magmatic inclusions also contain significant amounts of soluble Li carbonates and/or Li borates. The presence of these powerful fluxes in the fluid-rich pegmatitic melt may further explain the depression of TM crystallization temperatures.