REACTIVE FLUID-FLOW AND STABLE ISOTOPE EXCHANGE RELATED TO HYDROTHERMAL VEINS IN CARBONATE XENOLITHS: A COUPLED CL IMAGING AND SIMS STUDY

Stable isotopes have proven to be an efficient tool in tracing fluid-rock interaction. With high-resolution Secondary Ion Mass Spectrometry (SIMS) analysis, we can trace even small amounts of fluids allowing for the assessment of exchange mechanisms. Carbonates in contact aureoles are ideally suited for this kind of study, since large (>10‰) differences in oxygen isotope compositions exist between magmatic fluids and meta-sedimentary carbonates.

We focus on olivine veins in carbonate xenoliths (10-100 m) in the Bergell intrusion (Central Alps, Italy)¹. Fluid infiltration along an open crack - now filled with olivine (Ol), calcite (Cc) and retrograde tremolite and talc – forms a central zone, which is symmetrically framed by a replacement zone, with Ol+Cc replacing the original dolomite (Do). Traditional stable isotope analyses across the veins show overlapping and steep δ¹⁸O and δ¹³C fronts, which also coincide with a mineralogical front. A formation temperature of ~550°C for the Ol+Cc pair has been established, which is in agreement with phase petrology².

Numerous δ¹⁸O profiles in Cc and Do across the veins acquired with SIMS, combined with cathodoluminescence imaging (CL), show that stable isotope exchange occurs only through mineral reaction and recrystallization. In addition, the following observations are made:

1) partial dissolution and recrystallization of Do occurs a few mm to cm further into the dolomitic protolith and beyond the Ol+Cc mineralogical front;

2) sharp δ¹⁸O fronts at the core-rim interface of Do are mirrored by different CL intensities, with brighter rims (low δ¹⁸O) around darker cores (high δ¹⁸O);

3) the lack of Ol further into the replacement zone suggests that the silica front lagged behind the Do recrystallization front;

4) a progressive re-equilibration between Cc and Do at the interface between the replacement zone and the original Do supports the presence of only small amounts of fluid during the dissolution/recrystallization reactions.

Since there is no new mineral phase that precipitates in the Do dissolution/recrystallization zone, the reaction is driven by small chemical changes or optimization of the crystal lattice only. We are currently exploring several possible avenues to estimate the necessary driving forces.

¹Bucher, 1998, Min. Pet. 63, 151-171; ²Bégué, 2008, MSc Thesis.