PACROFI VI - Electronic Program

Fluid inclusion evidence for the evolution and solubility of fluids in the Harney Peak leucogranite magma, Black Hills, South Dakota

Peter I. Nabelek and Kim Ternes

Department of Geological Sciences, University of Missouri, Columbia, MO 65211, USA

Experimental and theoretical studies have shown that magmatic fluids have strong influence the phase equilibria of granitic systems. A microthermometric study of fluid inclusions in the Proterozoic Harney Peak Granite, Black Hills, South Dakota, was undertaken to obtain direct evidence for the composition and evolution of magmatic fluids in a leucogranitic system. The Proterozoic Harney Peak peraluminous leucogranite (HPG) of the Black Hills, South Dakota, was emplaced at ~4 kbar as multiple sills, dikes and small intrusions. It is also thought to be genetically related to a large pegmatite field in its proximity. The influence of magmatic fluids on crystallization of the granite is demonstrated by prominent local granite-pegmatite layering and chemical heterogeneities. Previous stable isotope study of the granite has shown that the granite has not interacted with non-magmatic fluids since its crystallization [1]. Therefore, fluids found in fluid inclusions are likely of magmatic origin and present an opportunity to directly sample magmatic fluids in a crystallizing granitic magma and during subsolidus conditions.

Fluid inclusions were analyzed in tourmaline and quartz. Primary and secondary inclusions were identified. Primary inclusions were identified by their isolated positions within crystals and negative shapes of the enclosing crystals. They consist of variable H2O-CO2-Salt +/-(CH4) mixtures (Figure 1). Salinity of the inclusions increases with water concentration. Because most primary inclusions decrepitated before homogenization, trapping temperatures were estimated from positions of calculated isochores (following Brown and Lamb, [2]), assuming the ~4 kbar emplacement pressure of HPG. The isochore temperatures decrease with increasing water content from ~750oC to 400oC. This drop in temperature is ascribed to progressive exsolution of the magmatic fluid, which, as observed, should lead to early CO2 enrichment due to the low solubility of CO2 in high-silica magmas. The calculated isochore temperatures of the most water-rich primary inclusions are below water saturated granite solidus of ~650oC and crystallization temperatures estimated from oxygen isotope fractionation between minerals. They could potentially be interpreted to indicate unusually low solidus temperature of the pegmatic magma. However, the temperatures may also be partly erroneous, because in calculation of the isochores it was assumed that the only solute in the inclusions is NaCl. Ice melting temperatures, birefringent daughter crystals, phase-equilibria considerations, and alpha-track mapping all indicate that K, Li, B and other solutes must be additional constituents in the inclusions. Alternatively, the discrepancy between isochore and crystallization temperatures may indicate fluid overpressure in the magma during crystallization.

Using published solubility models for H2O and CO2 in granitic melts [3, 4] and the composition of the most CO2-rich primary inclusions (XH2O = 0.45), the initial concentrations of H2O and CO2 in the HPG magma are deduced to have been ~ 4 wt. % and 1500 ppm CO2 respectively. The relatively low water concentration is consistent with dehydration-melting reactions that are thought to have produced the melts [5] and the estimated crystallization temperatures indicated by oxygen isotope equilibration among minerals [1]. The presence of CO2 in the magma and the evident change in fluid composition with decreasing temperature are incorporated into a model for crystallization of coupled aplite-pegmatite layers, such as those found n much of the Harney Peak Granite. Secondary inclusions in quartz are either CO2 dominated carbonic fluids or H2O-salt mixtures. They fall at the extreme ends the primary inclusion compositional trend (Figure) 1. Their composition is consistent with unmixing of the primary magmatic fluids during subsolidus fracturing of the granite or uplift ot the terrane.