<p>Using the example of dolomitization where calcite is replaced by dolomite, estimation of the fluid composition in equilibrium with dolomite for major and trace elements was estimated based on EPMA and LA-ICP-MS data using a mass balance approach. The method consists in an analytical quantification of the mass transfer between the original calcite and the newly formed dolomite giving us which elements are coming in and out of the system. Chemical composition of the aqueous fluid in equilibrium with dolomite can be estimated such as the partition coefficient for each element involved in the reaction. This approach was tested using three existing datasets obtained from natural dolomite and original limestone in both Jurassic limestones of the Layens anticline in the Pyrenees (France), and two from the Middle Devonian Presqu&#8217;ile barrier from Pine Point (Canada). These are completed with data acquired in Cretaceous limestones of the Benicassim area of the Maestrat Basin (Spain). Using the result obtained with the mass balance calculation, the amount of fluid required to dolomitized a fixed amount of limestone can be obtained for different fluid source (brine and seawater). Results show that the four dolomitization reactions have similar solid volume variation (-14 to -10 vol.%) and the fluid in equilibrium with the dolomite have also similar concentration in trace element. Estimation of the partition coefficients for all trace elements for the three regions were determined and compared.</p>
<p>Faults, joints and stylolites are ubiquitous features in fold-and-thrust belts commonly used to reconstruct the past fluid flow (or plumbing system) at the scale of folded reservoir/basins. Through the textural and geochemical study of the minerals that fills the fractures, it is possible to understand the history of fluid flow in an orogen, requiring a good knowledge of the burial history and/or of the past thermal gradient. In most of the case, the latter derives from the former, itself often argued over, limiting the interpretations of past fluid temperatures. We present the results of a multi-proxy study that combines novel development in both structural analysis of a fracture-stylolite network and isotopic characterization of calcite vein cements/fault coating. Together with new paleopiezometric and radiometric constraints on burial evolution and deformation timing, these results provide a first-order picture of the regional fluid systems and pathways that were present during the main stages of contraction in the Tuscan Nappe and Umbria-Marche Apennine Ridge (Northern Apennines). We reconstruct four steps of deformation at the scale of the belt: burial-related stylolitization, Apenninic-related layer-parallel shortening with a contraction trending NE-SW, local extension related to folding and late stage fold tightening under a contraction still striking NE-SW. We combine the paleopiezometric inversion of the roughness of sedimentary stylolites - that provides a temperature-free constraint on the range of burial depth of strata prior to layer-parallel shortening -, with burial models and U-Pb absolute dating of fault coatings in order to determine the timing of development of mesostructures. In the western part of the ridge, layer-parallel shortening started in Langhian time (~15 Ma), then folding started at Tortonian time (~8 Ma), late stage fold tightening started by the early Pliocene (~5 Ma) and likely lasted until recent/modern extension occurred (~3 Ma onward). The textural and geochemical (&#948;<sup>18</sup>O, &#948;<sup>13</sup>C, &#8710;<sub>47</sub>CO<sub>2</sub> and <sup>87</sup>Sr/<sup>86</sup>Sr) study of calcite vein cements and fault coatings reveals that most of the fluids involved in the belt during deformation are basinal brines evolved from various degree of fluid rock interactions between pristine marine fluids (&#948;<sup>18</sup>O<sub>fluids</sub> = 0&#8240; SMOW) and surrounding limestones (&#948;<sup>18</sup>O<sub>fluids</sub> = 10&#8240; SMOW). The precipitation temperatures (35&#176;C to 75&#176;C) appear consistent with the burial history unraveled by sedimentary stylolite roughness paleopiezometry (600 m to 1500m in the range) and geothermal gradient (23&#176;C/km). However, the western edge of the ridge recorded isotopically depleted past fluids of which corresponding precipitation temperature (100&#176;C to 130&#176;C) are inconsistent with local burial history (1500m). We interpret then pulses of eastward migration of hydrothermal fluids (>140&#176;C), driven by the tectonic contraction and by the difference in structural style of the subsurface between the eastern Tuscan Nappe and the Umbria-Marche Apennine Ridge. Allowed by an unprecedented combination of paleopiezometry and isotopic geochemistry, this fluid flow model illustrates how the larger scale structures control the fluid system at the scale of the range.</p>
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