Summary Recently there is an increasing interest in the understanding of the Spectral Induced Polarization (SIP) phenomenon in porous media. The complex electrical conductivity of porous geological materials has a frequency dependent behavior than can be associated to three main mechanisms: Maxwell-Wagner (MW) polarization, Membrane polarization, Electrical double layer (EDL) polarization. This paper aims to develop a mechanistic model for induced polarization in frequency domain, that incorporates both grain and membrane polarization and uses a non-linear mixing rule to combine the contribution of water, air and solid grains. The model is tested on two different lab experimental datasets. Different spectra characteristics can be understood and described by distinguished porous media behaviors.
We have investigated the influence of temperature and salinity upon the spectral induced polarization of 10 samples including rocks with their mineralization (galena, chalcopyrite) plus sand mixed with semiconductors such as magnetite grains, graphite, and pyrite cubes of two different sizes. Measurements are made in a temperature-controlled bath with a high-precision impedance meter and using NaCl solutions. We cover the temperature range 5°C−50°C and the frequency range [Formula: see text] to 45 kHz. For one large pyrite cube, we also investigated six salinities from 0.1 to [Formula: see text] (at 25°C, NaCl) and three salinities for graphite. The spectra are fitted with a Cole-Cole complex parametric conductivity model for which we provide a physical meaning to the four Cole-Cole parameters. As expected, the Cole-Cole exponent and the chargeability are independent of the temperature and salinity. The instantaneous and steady state (direct current [DC]) conductivities depend on the salinity and temperature. This temperature dependence can be fitted with an Arrhenius law (combining the Stokes-Einstein and Vogel-Fulcher-Tammann equations) with an activation energy in the range of [Formula: see text]. This activation energy is the same as for the bulk pore-water conductivity demonstrating the control by the background electrolyte of these quantities, as expected. The instantaneous and DC conductivities depend on the salinity in a predictable way. The Cole-Cole relaxation time decreases with the temperature and decreases with the salinity. This behavior can be modeled with an Arrhenius law with an apparent activation energy of [Formula: see text]. A finite-element model is used further to analyze the mechanisms of polarization, and it can reproduce the temperature and salinity dependencies observed in the laboratory.
Abstract. A multi-method investigation into Lauzière granite, located in the external Belledonne massif of the French Alps, reveals unusually hot hydrothermal conditions in vertical open fractures (Alpine-type clefts). The host-rock granite shows sub-vertical mylonitic microstructures and partial retrogression at temperatures of < 400 ∘C during Alpine tectonometamorphism. Novel zircon fission-track (ZFT) data in the granite give ages at 16.3 ± 1.9 and 14.3 ± 1.6 Ma, confirming that Alpine metamorphism was high enough to reset the pre-Alpine cooling ages and that the Lauzière granite had already cooled below 240–280 ∘C and was exhumed to < 10 km at that time. Novel microthermometric data and chemical compositions of fluid inclusions obtained on millimetric monazite and on quartz crystals from the same cleft indicate early precipitation of monazite from a hot fluid at T > 410 ∘C, followed by a main stage of quartz growth at 300–320 ∘C and 1.5–2.2 kbar. Previous Th-Pb dating of cleft monazite at 12.4 ± 0.1 Ma clearly indicates that this hot fluid infiltration took place significantly later than the peak of the Alpine metamorphism. Advective heating due to the hot fluid flow caused resetting of fission tracks in zircon in the cleft hanging wall, with a ZFT age at 10.3 ± 1.0 Ma. The results attest to the highly dynamic fluid pathways, allowing the circulation of deep mid-crustal fluids, 150–250 ∘C hotter than the host rock, which affect the thermal regime only at the wall rock of the Alpine-type cleft. Such advective heating may impact the ZFT data and represent a pitfall for exhumation rate reconstructions in areas affected by hydrothermal fluid flow.
L’exploitation des ressources minieres dans les Alpes francaises est une histoire an-cienne, de plus de 2 500 ans. Bien que presentant de faibles tonnages, des centaines de gisements ont ete exploites depuis l’Antiquite jusqu’au XXe siecle. Les dernieres exploitations alpines ont cesse dans les annees 1970, si bien que la plupart des mines sont desormais abandonnees, voire parfois tombees dans l’oubli. Si les principaux mi-nerais exploites sont le Fe, le Cu et le Pb-Ag, les gisements metalliferes presentent des mineralisations tres variees (Ag, Au, U, Hg…), souvent de nature polymetallique. L’in-dustrie miniere a ete importante dans les Alpes francaises (plusieurs millions de tonnes de Fe, 200 000 t Pb, 100 000 t Zn et 500 t Ag), en particulier au Moyen Âge et durant l’ere industrielle, contribuant ainsi fortement a l’anthropisation et au developpement socioeconomique des vallees et des versants. Actuellement a l’abandon, les mines et plus particulierement les haldes disseminees sur une grande partie du territoire, sont soumises a l’alteration et se degradent lentement, constituant ainsi une source potentielle de contamination des eaux et des sols. En parallele, les galeries subissent des affaissements localises. L’impact environnemental et les potentiels risques miniers n’ont pas ete evalues pour la plupart des sites alpins. En effet, malgre leur nombre important, seuls quelques sites font actuellement l’objet d’un suivi environnemental : il s’agit d’une gestion de l’urgence. Le passe minier des Alpes francaises n’est toutefois pas totalement oublie : le patrimoine minier est mis en valeur a travers des sentiers thematiques, des musees, et des visites de galeries souterraines.
