Dissolution-reprecipitation reaction of dolomite: investigation of self- and tracer-diffusion using SIMS mini-spot and NanoSIMS profiles

Recrystallization of minerals due to fluid infiltration modifies their original chemical and isotopic signature. Such processes are sought after as they give access to timescales of geologic processes, which are commonly quantified using diffusion chronometry. Here, we focus 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. Dolomite crystals at the vein boundary are characterized by up to 100μm thick, crystallographically continuous recrystallization zones. This new generation of dolomite is characterized by lower δ18O (about 16 ‰) and δ13C values (about 6 ‰), and slightly higher Fe (~1000 ppm) and Mn (~100 ppm) concentrations.δ18O diffusion profiles across the recrystallization fronts were measured with SIMS, using “mini-spots” of 3μm. The newly installed RF Hyperion source on the NanoSIMS was used to measure Fe and Mn profiles with a beam size of ~200nm.Diffusion distances of 10-20 μm were obtained for oxygen isotope profiles. Fitting the results to the diffusion equation, results in geologically plausible timescales (1-5 Myrs for temperatures of 500-600°C). Surprisingly, however, the Mn and Fe profiles are extremely sharp, with measured diffusion distances ranging from 150 to 300nm. With the available diffusion coefficients for Fe and Mn in dolomites, such distances result in geologically unrealistic times of less than 1 year. In addition, the profiles are likely even sharper, since the beam size is very similar to the overall diffusion distances. Hence this raises the question whether Fe and Mn diffusion profiles in these dolomites can actually be spatially resolved here or if we are only measuring analytical mixing due to the beam size.Further investigations are required to understand this large discrepancy between results from these two types of diffusers. Could they be the result of distinct diffusion mechanisms between oxygen self-diffusion versus Fe and Mn tracer diffusion?