Halogen stable isotope studies in formation waters, a comparison between their chlorine and bromine isotope systematics

Due to the relatively high bromide concentration in formations water Br stable isotope ratios are regularly measured. As the Br isotope geochemistry is remarkably different form the Cl isotope geochemistry (in spite of their similar chemistries) it is necessary to study the origin of these differences. We will discuss three processes that we know of are responsible (or suspect) for significant Cl isotope fractionation in formation waters and from which we study the Br isotope fractionation in order to understand agreements and disagreements between the two isotope systems. We will discuss diffusion, salt precipitation and ion-filtration, three processes from which we either understand how or suspect that they can cause large Cl isotope variations. The Br isotope systematics are not yet fully understood for these three processes. Br isotope effects due to diffusion are understood they have never been tested in natural samples. Br isotope systematics in evaporites are clearly different form the Cl isotope systematics and we start to understand its systematics. Ion-filtration is a still not well understood process that may cause large negative Cl isotope excursions but no or positive Br isotope excursions. We will discuss the effects of these three processes at the workshop.