Geological Sciences Seminar: Florence Begue, University of Geneva

Reactive fluid flow in hydrothermal veins inferred from detailed textural and in-situ stable isotope study

Stable isotopes have proven to be an efficient tool in tracing fluid-rock interaction. This research focuses on hydrothermal veins forming in dolomitic marbles in contact metamorphic environments. The large chemical and isotopic contrast in composition between the magmatic fluids and sedimentary protolith greatly facilitates tracing fluid pathways and reaction mechanism. In addition, they represent a good natural laboratory to study diffusion along recrystallization fronts in minerals

I will present detailed petrologic, textural and Secondary Ion Mass Spectrometry (SIMS) analyses of reactions driven by infiltration of magmatic fluids in carbonate xenoliths and wall pendants of the Bergell and Adamello intrusions (Central Alps, Italy). Fluid infiltration occurred along an open fracture, now filled with olivine (fo) and calcite (cc) forming a central zone, which is symmetrically framed by a replacement zone, where fo+cc replaced the original dolomite (do). Results show that stable isotope exchange occurs only through mineral reaction and recrystallization.

We also observe that dolomite crystals at the vein boundary are characterized by up to 100µm thick, recrystallization zones, where a new generation of dolomite (orange CL signal on the figure) having lower oxygen and carbon isotope values, and higher Fe and Mn concentrations precipitated along grain boundaries. Oxygen isotope, Fe and Mn diffusion profiles across the recrystallization interface were measured with SIMS and NanoSIMS, yielding very contradictory results, which will be addressed during this presentation.

After working a few years as an engineering geologist in Switzerland, Florence Begue started a PhD at the University of Canterbury (UC) in New Zealand in 2011. Her research was part of a multidisciplinary project “From Source to Surface” funded by the geothermal industry, which aimed to better understand magmatism fueling hydrothermal systems and develop exploration tools for blind geothermal systems. After a couple of postdocs at UC and at the University of Lausanne, where she mainly focused on in-situ stable isotope analysis, she is now a lecturer at the University of Geneva (since 2020), where she teachs optical mineralogy and manage our electron microprobe facility.

Email Kelly Wells, kcwells7@stanford.edu, for Zoom link and password.