Reactivity of dolomitizing fluids and Mg source evaluation of fault-controlled dolomitization at the Benicassim outcrop analogue (Maestrat basin, E Spain)

The mechanisms responsible for the formation of huge volumes of dolomitized rocks associated with faults are not well understood. We present a case study for high-temperature dolomitization of an Early Cretaceous (Aptian-Albian) ramp in Benicassim (Maestrat basin, E Spain). In this area, seismic-scale fault-controlled stratabound dolostone bodies extend over several kilometres away from large-scale faults. This work aims at evaluating different Mg sources for dolomitization, estimating the reactivity of dolomitizing fluids at variable temperature and quantifying the required versus available fluid volumes to account for the Benicassim dolostones. Field relationships, stable C-13 and O-18 isotopes, as well as radiogenic Sr-87/Sr-86 isotopes, indicate that dolomitization at Benicassim was produced by a high-temperature fluid (>80 degrees C). C-13 and O-18 isotopic compositions for dolomite vary from +0.5 to +2.9 parts per thousand, V-PDB and from +21.1 to +24.3 V-SMOW, respectively. A Mg source analysis reveals that the most likely dolomitizing fluid was seawater-derived brine that interacted with underlying Triassic red beds and the Paleozoic basement. Geochemical models suggest that evolved seawater can be considerably more reactive than high-salinity brines, and the maximum reactivity occurs at about 100 degrees C. Mass-balance calculations indicate that interstitial fluids with high pressure and/or high temperature relative to the normal geothermal gradient cannot account for the volume of dolomite at Benicassim. Instead a pervasive fluid circulation mechanism, like thermal convection, is required to provide a sufficient volume of dolomitizing fluid, which most likely occurred during the Late Cretaceous post-rift stage of the Maestrat basin. This study illustrates the importance of fluid budget quantification to critically evaluate genetic models for dolomitization and other diagenetic processes. (C) 2013 Elsevier Ltd. All rights reserved.