Major and trace-element composition and pressure–temperature evolution of rock-buffered fluids in low-grade accretionary-wedge metasediments, Central Alps

Abstract

The chemical composition of fluid inclusions in quartz crystals from Alpine fissure veins was determined by combination of microthermometry, Raman spectroscopy, and LA-ICPMS analysis. The veins are hosted in carbonatebearing, organic-rich, low-grade metamorphic metapelites of the Bündnerschiefer of the eastern Central Alps (Switzerland). This strongly deformed tectonic unit is interpreted as a partly subducted accretionary wedge, on the basis of widespread carpholite assemblages that were later overprinted by lower greenschist facies metamorphism. Veins and their host rocks from two locations were studied to compare several indicators for the conditions during metamorphism, including illite crystallinity, graphite thermometry, stability of mineral assemblages, chlorite thermometry, fluid inclusion solute thermometry, and fluid inclusion isochores. Fluid inclusions are aqueous two-phase with 3.7-4.0 wt% equivalent NaCl at Thusis and 1.6-1.7 wt% at Schiers. Reproducible concentrations of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, B, Al, Mn, Cu, Zn, Pb, As, Sb, Cl, Br, and S could be determined for 97 fluid inclusion assemblages. Fluid and mineral geothermometry consistently indicate temperatures of 320 ± 20°C for the host rocks at Thusis and of 250 ± 30°C at Schiers. Combining fluid inclusion isochores with independent geothermometers results in pressure estimates of 2.8-3.8 kbar for Thusis, and of 3.3-3.4 kbar for Schiers. Pressure-temperature estimates are confirmed by pseudosection modeling. Fluid compositions and petrological modeling consistently demonstrate that chemical fluid-rock equilibrium was attained during vein formation, indicating that the fluids originated locally by metamorphic dehydration during near-isothermal decompression in a rock-buffered system.

Contrasting regimes of Cu, Zn and Pb transport in ore-forming hydrothermal fluids

“…The metal concentration data of natural fluids are mainly from the compilation of Yardley (2005), including single fluid inclusion analysis, bulk crush leaching analysis, and direct sampling of hot springs and basinal brines. These data were combined with metal concentrations in single fluid inclusions obtained using LA-ICP-MS and PIXE (Carpenter et al 1974;Bottrell and Yardley 1988;Williams and McKibben, 1989;Heinrich et al 1992;Campbell et al 1995;Munz et al 1995;Smith et al 1996;Meere and Bank, 1997;Audétat et al 2000;Banks et al 2000;McCaig et al 2000;Zaw et al, 2003;Beuchat et al, 2004;Rusk et al, 2004;Bakker et al, 2006Bakker et al, , 2008Stoffell et al, 2008;Bertelli et al, 2009;Appold and Wenz, 2011;Kotzeva et al, 2011;Catchpole et al, 2011;Leisen et al, 2012;Li et al, 2012;Miron et al, 2013). The Cu, Zn and Pb concentrations of these natural fluids are plotted as a function of the estimated fluid temperature in Fig.…”

GEMSFITS: Code package for optimization of geochemical model parameters and inverse modeling

“…a Sample median temperatures reported by Kidder et al (2013) (ref. 'K') and calculated by using TitaniQ thermometer (Thomas et al, 2010) and ( ⁄ ) estimated using mineral geothermometry (Miron et al, 2013). Pressures calculated using a geothermal gradient of 25 K m À1 and the mean temperature values reported by Kidder et al (2013).…”

“…Then three coefficients (constant term and linear terms for temperature and pressure dependence) of the regular interaction parameter were fitted against the experimental data (Wark and Watson, 2006;Thomas et al, 2010). The optimized solid-solution model was then used in a GEMSFITS inverse modeling task aimed at determining the temperature of quartz crystallization using measured Ti concentration data in natural quartz samples from Kidder et al (2013) along with those from low-grade metamorphic quartz veins in accretionary-wedge sediments of the Swiss Alps (Miron et al, 2013).…”

“…For this inverse modeling problem, the Ti concentration data from Kidder et al (2013) and our own data from quartz veins in low-grade metamorphic rocks from the Central Swiss Alps (Miron et al, 2013) were added to the TiQ database file. All samples had 300°C as starting value for the temperature.…”

Chemical evolution of metamorphic fluids in the Central Alps, Switzerland: insight from LA‐ICPMS analysis of fluid inclusions

“…The samples come from a 1‐by‐3‐m‐sized fissure vein above Thusis, which has already been described and analyzed by Miron et al . (). The metapelitic Bündnerschiefer host rocks are rich in carbonates, organic matter, and quartz.…”