Crystallization conditions and evolution of magmatic fluids in the Harney Peak Granite and associated pegmatites, Black Hills, South Dakota—Evidence from fluid inclusions

A microthermometric study of inclusions in granites and pegmatites in the Proterozoic Harney Peak Granite system identified four types of inclusions. Type 1 inclusions are mixtures of CO₂ and H₂O and have low salinities, on average 3.5 wt.% NaCl_eq; type 2 inclusions are aqueous solutions of variable salinities, from 0 to 40% wt.% NaCl_eq; type 3 inclusions are carbonic, dominated by CO₂, with no detectable water; and type 4 inclusions consist of 20 to 100% solids, with the remaining volume occupied by a CO₂-H₂O fluid. Many inclusions have a secondary character; however, a primary character can be unambiguously established in several occurrences of the type 1 inclusions. These inclusions were trapped above the solidus and represent the exsolved magmatic fluid. The secondary populations of types 1, 2, and 3 probably formed as a result of reequilibration and unmixing of the type 1 fluid that progressively changed composition and density with decreasing temperature and pressure and was finally trapped along healed microfractures under subsolidus conditions. Type 4 inclusions are primary and are interpreted to be trapped, fluid-bearing, complex silicate melts that subsequently solidified or underwent other posttrapping changes.It is demonstrated that primary type 1 fluid inclusions that coexist with crystallized melt inclusions in the complex, Li-bearing Tin Mountain pegmatite 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-, P-, and B-rich pegmatite crystallized at some of the lowest known temperatures for a silicate melt in the crust. An internally consistent, empirical solvus surface in P-T-X_CO2 coordinates was generated for the pseudobinary CO₂-(H₂O-4.3 wt.% NaCl_eq) pegmatite fluid system. Distribution coefficients for the major species CO₂, H₂O, NaCl, and CH₄ between the immiscible CO₂-rich and H₂O-rich fluid phases as a function of pressure and temperature were extracted from data for the two cogenetic fluid inclusions types.