Early MagnetoTelluric (MT) resistivity soundings in Iceland revealed a Deep Conductive Layer (DCL), a layer with anomalously high conductivity, at the depth of 5-20 km (upper seismic layer 3) in the crust under most of the country. When first observed in the 1970s, the DCL was interpreted as partial melt below a thin and hot crust. Later seismic and gravity studies have showed that the crust is 20-40 km thick, dense and relatively cold. The DCL is thus an intra-crustal layer below the brittle/ductile transition and does not correlate with any currently known seismic feature. In this paper we try to narrow down the possible origin(s) and nature of the enigmatic DCL and how its presence can be understood in the framework of the thick and cold crustal model. We review existing electrical conductivity data from early MT measurements done in the 1970s and 1980s and from recent surveys mostly done in the context of geothermal exploration. The DCL domes up beneath active central volcanoes and high temperature geothermal areas. It is found to be electrically anisotropic, being more conductive in the spreading direction than in the tectonic/fracture direction. We review several possible conduction mechanisms that could explain the high conductivity in the light of recent experimental work on electrical conductivity of minerals and rocks and discuss the possible prevailing conduction mechanisms in the light of existing geophysical and petrological observations in Iceland. These investigations show that seismic and electrical surveys are both compatible with the presence of melt in the DCL in the volcanic plumbing system associated to volcanoes. Outside active volcanoes, seismic properties of the DCL, inferred from local tomography, are not compatible with the presence of melt or even aqueous fluid in the DCL but confirm thick, cold and fluid-free lower crust, leaving only solid-state conduction as a viable conduction mechanism. Review of laboratory measurements on various type of minerals leads to the conclusion that the presence of Fe-Ti oxides or hydrated minerals, such as amphibole, could explain the DCL. In Eyjafjor & eth;ur, North Iceland, the only low-temperature geothermal area studied using MT to date, the DCL locally domes up to shallow depths under the geothermal area. This suggests possible correlation between low-temperature geothermal activity and relatively shallow DCL. Detailed mapping of the DCL might therefore be an important tool in prospection for low-temperature geothermal resources.

The Valgarður database is a compilation of data describing the physical and geochemical properties of Icelandic rocks. The dataset comprises 1072 samples obtained from fossil and active geothermal systems, as well as relatively fresh volcanic rocks erupted in sub-aerial or sub-aqueous environments. The database includes petrophysical properties (effective and total porosity, grain density, permeability, electrical resistivity, acoustic velocities), as well as mineralogical and geochemical data obtained by point-counting, X-ray Fluorescence (XRF), quantitative X-ray Diffraction (XRD), and Cation Exchange Capacity (CEC) analyses. The motivation behind this database is threefold: (i) aid in the interpretation of geophysical data including uncertainty estimations, (ii) facilitate the parameterization of numerical reservoir models, and (iii) improve our understanding of the relationship between rock type, hydrothermal alteration and petrophysical properties.

This paper revisits the clay mineralogy of the “smectite” alteration zone in the Krafla geo-thermal field via the study of an exploratory well in which temperatures range from 40 °C to 215 °C. The clay alteration consists of several mineral assemblages superimposed in time and space, resulting from different stages of water-rock interaction. Trioctahedral clay minerals (chlorite, corrensite and smectite) are observed throughout the studied section. These minerals can form in nearly closed systems as replacements of groundmass minerals/glass after interactions with resident and nearly stagnant fluids not far from chemical equilibrium (neutral to basic pH conditions) or from direct precipitation from geothermal fluids. They are locally superimposed by Al clay phases (smectite, illite/smectite and kaolinite), which result from intense leaching of the host rocks due to their interaction with low pH fluids under strong W/R ratios. The absence of mineralogical zoning is explained by the fact that hydrothermal alteration is strongly dependent on very recent hydrodynamics. The current fluid circulation generates trioctahedral clays at depth that cannot be distinguished from pervasive earlier alteration. The only easily detectable signature of current activity and the most relevant signature for geothermal exploration is the presence of Al dioctahedral phases since it indicates leaching and intense hydrothermal activity.

The North Tanzanian Divergence (NTD) is a zone of rift initiation. Its surface expression results from interactions between deep-mantle (mantle plume), lithospheric (inherited rheology and stratification, melting...) and crustal (dyke propagation, fault activation...) processes. However, the role of each process on the observed surface activity is still debated, because highly difficult to decorrelate. We recently carried out a study to obtain enhanced P and S-wave tomography, from the surface down to 150-200 km depth. The particularity of our method consists in its initial velocity model. It is composed of a 1D IASP91 regional velocity model in which we inserted an a priori 3D crustal velocity model with a fine grid. This crustal model was deduced from an independent local tomography inversion. The P and S images obtained, resulting from the teleseismic inversion of this hybrid method, show strong contrasted velocity anomalies: from 10 % of P (Vp) and S velocity (Vs) variation on the craton, to -17 % below the rift axis. The anomalies locations are consistent with the surface geology (rifting basin, border faults, volcanoes). At a regional scale, the strongest velocity contrasts correspond to the lithospheric inherited structure (Tanzanian craton and Proterozoic belts) boundaries, which control the propagation of the rift. In particular, the Masai cratonic block, south of the NTD, is inferred to have a strong influence in the rift evolution. The transition from the North-South axial valley into three diverging rift arms (Eyasi, Natron-Manyara and Pangani) is likely due to the change in rheology and to the presence of magma along inherited sutures between the craton and the mobile belts. However, interrogations about the role of the thermal changes, the melt/fluid presence and the mantle composition in the NTD on these velocity anomalies still remain. To distinguish which parameters are acting in the rift, we realize a Vp/Vs ratio map. With this new data, and in the light of parallel petrological studies, we interpret the Vp/Vs anomalies in term of gas and/or melt concentration zones.

The North Tanzanian Divergence (NTD) is the prolongation of the eastern branch of the East African Rift and is a place of intense volcanism. Numerous volcanoes erupted deep subalkaline to highly alkaline magmas, including the particular active natrocarbonatite Oldoinyo Lengai. On the North-South axis (Natron to Manyara basins), three highly alkaline volcanoes, Pello Hills, Lashaine and Labait, erupted melilite magmas that originated from low degree of partial melting of asthenospheric mantle (depth > 120 km). The particularity of these volcanoes is that they sampled numerous mantle xenoliths during ascent. This represents a unique opportunity to study the composition and the rheology of lithospheric mantle. Mantle xenoliths are deep garnet-bearing peridotites (120 km depth), amphibole and phlogopite peridotites and phlogopitites. They contain abundant hydrous minerals as isolated crystals or veins that attest to an important metasomatism beneath the NTD. Previous geochemical and petrological studies have highlighted interactions of alkaline magmas and the thick cratonic lithosphere as metasomatic agent. However, the presence and composition of magmas, the degree of metasomatism, and the role of metasomatism on mantle rheology below the NTD is still debated. To characterize these previous parameters, in this study we performed geochemical and petrophysical analyses on metasomatized, fertile and refractory mantle xenoliths from Labait (on-craton volcano) and Pello Hills (in-rift volcano). Using mineral compositions and thermobarometer calibrations, we estimated the depth of mantle xenoliths between 40 and 140 km (14 to 47 kbar) and temperatures from 930 to 1340°C. EBSD analysis on thin sections indicate that peridotites and amphibole/phlogopite-bearing mantle xenoliths display a moderate to strong deformation induced crystal preferred orientation. In contrast, weak mineral orientations have been observed in phlogopite-amphibole-clinopyroxene-bearing veins. Calculation of seismic properties using MTEX program show that peridotites are seismically anisotropic, up to 12.4% for P-wave velocity (Vp) and 6.8% for S-wave velocity (Vs). The Vp and Vs in hydrous veins are lower than in peridotites (Vp: 7.5-7.9 and 8.3-9.6 km/s; Vs: 4.4-4.6 and 5.0-5.3 km/s respectively) and therefore the Vp and Vs velocities decrease with the increasing proportion of metasomatic minerals. We estimate that a peridotite with 20 vol.% metasomatic vein has a velocity decrease of 3.5% for Vp and 2.9% for Vs, compared to a fertile peridotite. These geochemical and petrophysical approaches are important to understand P- and S-wave propagation in the lithospheric mantle beneath the NTD and more specifically in metasomatized lithospheric mantle. The new in situ data and models from mantle xenoliths will be compared to tomographic acquisition and discussed in term of temperature, presence of melt or metasomatism processes. Both petrophysical and geophysical data will help to precisely determine the structure and rheology of the lithospheric mantle, which may control the propagation of the rift at early stage rifting between the Tanzanian craton and the mobile Proterozoic belts.

In volcanic environments, the presence of smectite may indicate recent hydrothermal circulations. Smectite is also responsible for enhanced rock electrical conductivity, as well as mechanical weakening. Therefore, quantifying smectite is important in geothermal exploration. Smectite identification requires X-ray diffraction (XRD) but quantification based on XRD is time-consuming and not always accurate. In the present study, we investigate the use of an optimized unbuffered Cation Exchange Capacity (CEC) determination, by back-titration of the Copper-triethylenetetramine(II) "Cu-trien" molecule, to quantify the smectite content of altered volcanic rock samples. We establish that a satisfying trade-off between the instrument uncertainty and an independant systematic error is theoretically reached for a fraction of reactants consumed of about 30% at the end of the exchange reaction. We suggest a modification to classical protocols to fall in that range. Finally, we show that optimized CEC determination by Cu-trien are a direct measure of the smectite weight fraction in altered volcanic samples, with an average CEC of pure smectite of 90 +/- 5 meq/100 g.

The aim of the Iceland Deep Drilling Project is to drill into supercritical geothermal systems and examine their economic potential. The exploratory well IDDP-2 was drilled in the Reykjanes geothermal field in SW Iceland, on the landward extension of the Mid-Atlantic Ridge. The Reykjanes geothermal field produces from a <300 degrees C reservoir at 1 to 2.5 km depth and is unusual because it is recharged by seawater. The well was cased to 3000 m depth, and then angled towards the main up-flow zone of the system, to a total slant depth of 4659 m (similar to 4500 m vertical depth). Based on alteration mineral assemblages, joint inversion of wireline logging, and rate of heating measurements, the bottom hole temperature is estimated to be about 535 degrees C. The major problem encountered during drilling was the total loss of circulation below 3 km depth and continuing to the final depth. Drilling continued without recovering drill cuttings, consequently spot coring provided the only deep rock samples from the well. These cores are characteristic of a basaltic sheeted dike complex, with hydrothermal alteration mineral assemblages that range from greenschist to amphibolite facies, hornblende hornfels, and pyroxene hornfels, allowing the opportunity to investigate water-rock interaction in the active roots of an analog of a submarine hydrothermal system. As they have not yet been sampled, the composition of the deep fluids at Reykjanes is unknown at present. Cold water is currently being injected with the aim of enhancing permeability at depth, before allowing the well to heat up prior to flow tests planned for early 2019. The well has at least two fluid feed zones, a dominant one at 3.4 km depth and a second smaller one at 4.5 km. Extensive geophysical surveys of the Reykjanes Peninsula completed recently allow correlation of geophysical signals with rocks properties and in-situ conditions in the subsurface. Earthquake activity monitored with a local seismic network during drilling the IDDP-2 drilling detected abundant small earthquakes (M-L <= 2) within the depth range of 3-5 km. A zone at 3-5 km depth below the producing geothermal field that was generally aseismic prior to drilling, but became seismically active during the drilling. The drilling of the IDDP-2 has achieved number of scientific and engineering firsts. It is the deepest and hottest drill hole so far sited on an active mid-ocean spreading center. It penetrated an active supercritical hydrothermal environment at depths analogous to those postulated as the high temperature reaction zones feeding black smoker systems. (C) 2018 Elsevier B.V. All rights reserved.

Electrical resistivity is extensively used in geothermal systems to accurately determine the existing conditions of the reservoirs at depth. Up to this point, technical challenges related to pore fluid confinement made difficult to measure electrical conductivity at temperatures and pressures representative of very deep geothermal reservoirs. In this study, we are overcoming these limitations thanks to a new electrical resistivity cell designed to fit into a high temperature gas medium apparatus. This allows us to perform resistivity measurement at temperatures up to 700 degrees C and at effective pressures up to 100 MPa ( i.e. a confining pressure of 130 MPa and an equilibrium pore pressure of 30 MPa) using cm-scale plugs. Rock samples originate from five boreholes located in the Icelandic geothermal fields of Reykjanes (RN-17B/Hyaloclastite, RN-19/RN-30/dolerites) and Hengill (NJ-17/basalt and NJ-17B/Hyaloclastite). These samples were selected for their high degree of hydrothermal alteration in the epidote and amphibole fades (i.e. temperature of 250 degrees C and 400 degrees C respectively), and their wide range of porosities (from 3% to 20%). To determine the effects of surface, mineral and electrolytic conductions on bulk electrical conduction, experiments were performed under dry and saturated conditions using three different fluid salinities. At temperatures ranging from 25 to similar to 350 degrees C, electrical conductivity in all our experiments increases as a result of both increasing surface and electrolytic conduction. Then, under supercritical conditions, i.e. temperature from 374 degrees C to 600 degrees C, electrical conductivity strongly decreases due to the evolution of water density and dielectric constant that affect both surface and electrolyte conduction. At higher temperatures (500 degrees C-700 degrees C), the rock conductivities lie within the range of dry rock electrical conductivity values, suggesting that mineral conduction controls the bulk conductivity with ferromagnesian minerals acting as principal contributors of mineral conduction. Amphibole-rich samples show an irreversible increase in conductivity at temperature above 500 degrees C-600 degrees C, which can be attributed to amphibole dehydration. Comparison of these laboratory data to magnetotelluric soundings and downhole temperatures obtained beneath several geothermal areas indicate a good agreement between laboratory and large-scale surveys. Our results provide a general trend that helps interpreting electrical conductivity surveys in the Icelandic crust. (C) 2018 Elsevier B.V. All rights reserved.

ABSTRACT We investigate the complex conductivity behaviour of natural volcanic rocks containing variable amounts of smectite in multi‐salinity experiments. We compare the results with relationships established for sandstones. Considering only samples with little volume of metallic particles, we observe similar and small phase‐angles at low frequency for all samples at all salinities (less than 25 mrad at 1 Hz). Yet, a wide range of cation exchange capacity, porosity and formation factor is covered by the sample set: 0.5–50 meq/100 g, 4–40% and 18–780, respectively. Our results show that, in the absence of metallic particles, the ratio between imaginary conductivity and surface conductivity is significantly lower for altered volcanic rocks than for sandstones and decreases with the smectite content. These observations indicate that an increased smectite content causes more conduction and less polarization, which could be explained by the onset of a continuous conduction pathway throughout connected interfoliar spaces of smectite. Due to this pathway, cations from the pore fluid may penetrate the solid lattice, for example through connected smectite aggregates clogging the fracture network, thus preventing polarization. We also observe that the relationship between imaginary conductivity and surface conductivity, at one salinity or over the whole salinity range, is not more significant than the relationship between the imaginary conductivity and the total real conductivity. Therefore, we suggest that the imaginary conductivity cannot be used to discriminate the contributions from smectite and pore water to the total conductivity of altered volcanic rocks.

During the early years of the Iceland Deep Drilling Project (IDDP), development of three distinctive technological and scientific approaches were formalised and then carried out until 2010 within a European funded project called HiTI (high temperature instruments for supercritical geothermal reservoir characterisation and exploitation). These approaches were: (1) development of several downhole instruments allowing them to function up to 300 degrees C and 400 degrees C, (2) identification of two new Na/Li cation ratio geothermometric relationships valid at very high temperature, (3) tracer testing with high temperature tolerant organic isomers and finally and (4) basalt rock deformation and petrophysical properties laboratory investigations at high temperature and pressure conditions. (C) 2013 Elsevier Ltd. All rights reserved.

Polarized emittance measurements were acquired for synthetic forsterite, the pure magnesium end member of the olivines group, on the whole infrared spectral range and up to the melting point by using CO2 laser heating. The experimental data, fitted with a semi-quantum dielectric function model, allowed the retrieval of the temperature dependence of the absorption coefficient of forsterite both in the opaque and semi-transparent regions. The analysis of the phonon parameters indicates that the lattice dynamics evolve drastically with increasing temperature. The normal modes involving motions of the magnesium cations located in site 1 are the more impacted, and some of them vanish around 1,200 K. The results confirm that the enhancement of the lattice anharmonicity and the increasing mobility of the magnesium cations are closely linked and are at the origin of the anomalies observed in the evolution of the thermophysical properties. This complete set of spectroscopic data may be a step toward a more precise evaluation of the impact of thermal radiation heat transfer inside systems involving forsterite and quantification of their heat budget.

Electrical resistivity soundings are used by geophysicists to determine the structure and composition of the Earth’s crust and mantle and to explore natural resources (ore, oil, gas, water). Their interpretations in terms of composition and in-situ physical conditions depend mainly on laboratory measurements of electrical conductivity of rocks at simulated crustal conditions of temperature, pressure, saturation and pore pressures. These measurements present a numbers of limitations, in particular, in the case where conductive pore fluids are present, as in the case of deep reservoir conditions, where temperature exceeds 250 °C. Here, we present a new cell capable of measuring electrical conductivity of large saturated samples at confining pressure up to 200 MPa, pore pressure up to 50 MPa, and temperature up to 500 °C. The measurement cell has been developed in a commercial, internally heated, gas pressure apparatus (Paterson press). It is based on the concept of “guard ring” electrode, which is adapted to samples that are jacketed by a very conductive, metallic material. Numerical modeling of the current flow in the electrical cell allowed defining the optimal cell geometry. Calibration tests have been performed on Fontainebleau sandstones saturated with electrolytes of different conductivities, up to 350 °C. The resulting electrical formation factor and temperature dependence of electrical conductivity are in very good agreement with previous studies. This new cell will improve the exploration and exploitation of deep fluid reservoirs, as in unconventional, high enthalpy geothermal fields. In particular, the investigations address possible effects of fluid-rock interactions on electrical resistivity of a reservoir host rock.

The complex electrical impedance of a garnet-biotite-sillimanite residual enclave in the Neogene dacite of El Hoyazo (SE Spain) has been determined up to 978 degrees C at 200-300 MPa. This well studied material represents a direct sampling of the Alboran Domain thinned lower crust undergoing partial melting. The paragenesis is garnet + biotite + sillimanite + plagioclase + graphite +/- cordierite coexisting with widespread (similar to 10 wt.%) rhyolitic melt occurring as inclusions and interstitial glass and developed during regional anatexis at 850 +/- 50 degrees C and 500-700 MPa.The samples were used in cyclic measurements consisting in heating-cooling ramps at progressively higher maximum temperature to observe the effect of reactions on conductivity. In the first cycle up to 850 degrees C at 2-3 kbar, re-melting of the interstitial glass is achieved with no additional reactions and logarithmic conductivity (S/m) is up to -1.5. At T>950 degrees C new melt with orthopyroxene + ilmenite is produced from biotite partial breakdown and the logarithmic conductivity (S/m) is up to -0.7. Application of mixing models as Hashin-Shtrikman bounds or Archie's Law, shows that once interconnected, melt controls the electrical conductivity of the enclave. The electrical conductivity of the rock can be simulated with the electrical conductivity data obtained on the starting Matrix glass of the enclave and with the new melt.Our experimental results obtained on a graphitic metasedimentary material evidence show that graphite does not contribute to the conductivity enhancement and that the electrical anomaly observed below the Betics can be explained as effect of partial melting of a residual crust. Comparison with previous works on mafic granulites shows that similar conductivity values can be achieved both by melting of a residual felsic crust or by melting of mafic materials and suggests that pelitic rocks can be more common at deep levels than expected. (C) 2012 Elsevier B.V. All rights reserved.

The brittle to ductile transition (BDT) in rocks may strongly influence their transport properties (i.e., permeability, porosity topology…) and the maximum depth and temperature where hydrothermal fluids may circulate. To examine this transition in the context of Icelandic crust, we conducted deformation experiments on a glassy basalt (GB) and a glass‐free basalt (GFB) under oceanic crust conditions. Mechanical and micro‐structural observations at a constant strain rate of 10−5 s−1 and at confining pressure of 100–300 MPa indicate that the rocks are brittle and dilatant up to 700–800°C. At higher temperatures and effective pressures the deformation mode becomes macroscopically ductile, i.e., deformation is distributed throughout the sample and no localized shear rupture plane develops. The presence of glass is a key component reducing the sample strength and lowering the pressure of the BDT. In the brittle field, strength is consistent with a Mohr‐Coulomb failure criterion with an internal coefficient of friction of 0.42 for both samples. In the ductile field, strength is strain rate‐ and temperature‐dependent and both samples were characterized by the same stress exponent in the range 3 < n < 4.2 but by very different activation energy QGB = 59 ± 15 KJ/mol and QGFB = 456 ± 4 KJ/mol. Extrapolation of these results to the Iceland oceanic crust conditions predicts a BDT at ∼100°C for a glassy basalt, whereas the BDT might occur in non‐glassy basalts at deeper conditions, i.e., temperatures higher than 550 ± 100°C, in agreement with the Icelandic seismogenic zone.

Un pour cent de l'energie thermique contenue dans la croute terrestre assurerait les besoins energetiques de l'humanite pendant 100 000 ans ! Pour recuperer cette energie aujourd'hui sous-exploitee, on fore dans des conditions toujours plus extremes.