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

Abstract

The chemical evolution of fluids in Alpine fissure veins (open cavities with large free‐standing crystals) has been studied by combination of fluid inclusion petrography, microthermometry, LA‐ICPMS microanalysis, and thermodynamic modeling. The quartz vein systems cover a metamorphic cross section through the Central Alps (Switzerland), ranging from subgreenschist‐ to amphibolite‐facies conditions. Fluid compositions change from aqueous inclusions in subgreenschist‐ and greenschist‐facies rocks to aqueous–carbonic inclusions in amphibolite‐facies rocks. The fluid composition is constant for each vein, across several fluid inclusion generations that record the growth history of the quartz crystals. Chemical solute geothermometry, fluid inclusion isochores, and constraints from fluid–mineral equilibria modeling were used to reconstruct the pressure–temperature conditions of the Alpine fissure veins and to compare them with the metamorphic path of their host rocks. The data demonstrate that fluids in the Aar massif were trapped close to the metamorphic peak whereas the fluids in the Penninic nappes record early cooling, consistent with retrograde alteration. The good agreement between the fluid–mineral equilibria modeling and observed fluid compositions and host‐rock mineralogy suggests that the fluid inclusions were entrapped under rock‐buffered conditions. The molar Cl/Br ratios of the fluid inclusions are below the seawater value and would require unrealistically high degrees of evaporation and subsequent dilution if they were derived from seawater. The halogen data may thus be better explained by interaction between metamorphic fluids and organic matter or graphite in metasedimentary rocks. The volatile content (CO2, sulfur) in the fluid inclusions increases systematically as function of the metamorphic grade, suggesting that the fluids have been produced by prograde devolatilization reactions. Only the fluids in the highest grade rocks were partly modified by retrograde fluid–rock interactions, and all major element compositions reflect equilibration with the local host rocks during the earliest stages of postmetamorphic uplift.

Fluid signature of the shear zone–controlled Veio de Quartzo ore body in the world-class BIF-hosted Cuiabá gold deposit, Archaean Rio das Velhas greenstone belt, Brazil: a fluid inclusion study

“…8,9a). According to Lüders et al (2005), Marsala et al (2013) and Rauchenstein-Martinek et al (2016), leaching of organic-rich sedimentary rocks increases Br and lowers the Cl/Br and Na/Br ratios, characterising data above the seawater evaporation line (as Qz2 of V2 veins; Fig. 9a).…”

Apatite as a tracer of the source, chemistry and evolution of ore-forming fluids: The case of the Olserum-Djupedal REE-phosphate mineralisation, SE Sweden

“…However, the involvement of formation waters can be ruled out because of the distinctly different halogen ratios of the granite-hosted fluorapatite. The halogen signatures of metamorphic fluids vary, but they tend to have Br/Cl ratios close to mantle or primitive magmatic values, and can extend to much higher values and elevated I/Cl ratios reflecting interaction with organic-matter rich lithologies (Svensen et al, 2001;Miron et al, 2013;Rauchenstein-Martinek et al 2016;Fusswinkel et al, 2018). The halogen data of the primary fluorapatite from the Olserum-Djupedal REE-phosphate mineralisation would be compositionally compatible with metamorphic fluids.…”

Dating tectonic activity in the Lepontine Dome and Rhone-Simplon Fault regions through hydrothermal monazite-(Ce)

“…Early high-pressure (P ) metamorphism in the western Alpine Sesia-Lanzo Zone during subduction below the Southern Alps is dated to 75-65 Ma (e.g., Ruffet et al, 1997;Rubatto et al, 1998;Regis et al, 2014). This was followed by underthrusting and nappe stacking from ca.…”