T95 Experimental Investigations into Hydrothermal Systems: Implications for Mass Transfer in the Earth’s Crust

Monday, 23 October 2006
1:30 PM - 5:30 PM

TRACE ELEMENTS IN HYDROTHERMAL FLUID-QUARTZ SYSTEMS: PRELIMINARY EXPERIMENTS

Paper number: T95-8
Program: Technical Sessions
Time: 3:25 PM – 3:40 PM

Authors

Abstract

Trace element concentrations in quartz grown under known pressures and temperatures from fluids of known compositions can be applied to understand conditions of growth of natural quartz veins in hydrothermal and metamorphic environments. To this end, preliminary hydrothermal fluid-mineral experiments are being conducted. Hydrothermal fluids include de-ionized H2O and moderately saline (0.5 M NaHCO3) fluids. Fragments of Minas Gerais quartz are used as seed crystals and fluids are saturated with titanium using titanium metal. Experimental approaches include isothermal experiments (375C, 350 bars, 500 hrs) and isobaric (350 bar) experiments with temperature changes to dissolve seed crystals (300C, 400 hrs) and precipitate (130C, 400 hrs) new solids. Fluid was periodically sampled from ongoing experiments and analyzed for major and trace element chemistry and pH. Solids were analyzed using optical and scanning electron microscopy, cathodoluminescence (CL), and LA-ICP-MS.

At elevated temperature (300C), colloidal (>0.02 microns) titanium (3 uM) and silica (20 mM) are present in addition to dissolved titanium (2-3 uM) and silica (~20 mM). Quartz seed crystals exhibit extensive dissolution textures. With cooling to 130C, both colloidal and dissolved titanium precipitate to form titanium oxide (brookite?), leaving no detectable aqueous titanium. Colloidal silica also disappears with cooling, but no quartz precipitated on the seed crystals. Geochemical modeling is consistent with the high temperature experiments, predicting quartz dissolution (log Q/K -1.0) and no stable titanium solid. Given minimal supersaturation on cooling (log Q/K +0.1), a critical saturation index may be needed to foster quartz growth in experimental systems. Geochemical modeling predicts that no titanium solid will form on cooling whereas bladed rosettes of brookite(?) crystallized.

In addition, natural vein quartz from several hydrothermal ore deposits was subjected to hydrothermal conditions (~600C, ~600 bars) for 1700 hrs to determine how CL textures and trace element concentrations change in response to continued hydrothermal activity. CL textures were annealed in natural samples that formed below ~300C, however quartz formed above ~450C appears unaffected. LA-ICP-MS analyses will reveal whether CL changes were coupled with diffusion of trace elements.