Electrical conductivity and partial melting of mafic rocks under pressure

We demonstrate the importance of electric conductivity measurements of partially molten mafic rocks by examining of Oman gabbro, Karelia olivinite, Ronda and Spitzbergen peridotites. The electrical conductivities of these rocks were estimated using the impedance spectroscopy at temperatures between 800°C and 1450°C and at pressures between 0.3 and 2 GPa in experiments performed in a piston cylinder apparatus. At temperatures below and above melting, samples were equilibrated during durations on the order of 200 h. Our results show that a jump in electrical conductivity can be correlated with the temperature range slightly above the solidus, due to the delayed formation of an interconnected melt phase. Thin sections of quenched samples were used to estimate volume fractions and chemical compositions of the partial melts. The increase of the electrical conductivity compares well with the connectivity of melt in partially molten samples. Above the solidus, the electrical conductivity increases by ∼1 to 2 orders of magnitude in comparison with the conductivity of non-melted rock below solidus. When a complete melt connectivity is established, the charge transport follows the network of the formed melt films at grain boundaries. Durations of up to ?200 h are required in order to reach a steady state electrical resistance in a partially molten rock sample. The experimental results were compared with the conductivity data obtained from magnetotelluric (MT) and electromagnetic (EM) measurements in the Northern part of the mid-Atlantic ridge where a series of axial magma chambers (AMC) are presumably located. There is good agreement between the measured electric conductivity of gabbroic samples with a melt fraction of 10 to 13 vol.% and the conductivity estimated at AMC, beneath the central part of Reykjanes ridge, as well as between the conductivity of partially molten peridotites and the source zone beneath the mid-Atlantic ridge at ?60 km.     

The effect of the large-scale mantle flow field on the Iceland hotspot track

Fluid dynamical simulations were carried out in order to investigate the effect of the large-scale mantle flow field and the depth of the plume source on the structure of the Iceland plume through time. The time-dependent location and shape of the plume in the Earth's mantle was calculated in a global model and it was refined in the upper mantle using a 3D Cartesian model box. Global flow was computed based on density heterogeneities derived from seismic tomography. Plate motion history served as a velocity boundary condition in both models. Hotspot tracks of the plume conduits and the plume head were calculated and compared to actual bathymetry of the North Atlantic. If a plume source in the lowermost mantle is assumed, the calculated surface position of the plume conduit has a southward component of motion due to southward flow in the lower mantle. Depending on tomography model, assumed plume age and buoyancy the southward component is more or less dominating. Plume models having a source at the 660 km discontinuity are only influenced by flow in the upper mantle and transition zone and hence rather yield westward hotspot motion. Many whole-mantle plume models result in a V-shaped track, which does not match the straight Greenland–Iceland–Faroe ridge. Models without strong southward motion, such as for a plume source at 660 km depth, match actual bathymetry better. Plume tracks were calculated from both plume conduits and plume heads. A plume head of 120 K anomalous temperature gives the best match between plume head track and bathymetry.     

Numerical models of ductile rebound of crustal roots beneath mountain belts

Crustal roots formed beneath mountain belts are gravitationally unstable structures, which rebound when the lateral forces that created them cease or decrease significantly relative to gravity. Crustal roots do not rebound as a rigid body, but undergo intensive internal deformation during their rebound and cause intensive deformation within the ductile lower crust. 2-D numerical models are used to investigate the style and intensity of this deformation and the role that the viscosities of the upper crust and mantle lithosphere play in the process of root rebound. Numerical models of root rebound show three main features which may be of general application: first, with a low-viscosity lower crust, the rheology of the mantle lithosphere governs the rate of root rebound; second, the amount of dynamic uplift caused by root rebound depends strongly on the rheologies of both the upper crust and mantle lithosphere; and third, redistribution of the rebounding root mass causes pure and simple shear within the lower crust and produces subhorizontal planar fabrics which may give the lower crust its reflective character on many seismic images.     

Measuring Concentrations of Dissolved Methane and Ethane and the 13C of Methane in Shale and Till

Baseline characterization of concentrations and isotopic values of dissolved natural gases is needed to identify contamination caused by the leakage of fugitive gases from oil and gas activities. Methods to collect and analyze baseline concentration‐depth profiles of dissolved CH₄ and C₂H₆ and δ¹³C‐CH₄ in shales and Quaternary clayey tills were assessed at two sites in the Williston Basin, Canada. Core and cuttings samples were stored in Isojars^® in a low O₂ headspace prior to analysis. Measurements and multiphase diffusion modeling show that the gas concentrations in core samples yield well‐defined and reproducible depth profiles after 31‐d equilibration. No measurable oxidative loss or production during core sample storage was observed. Concentrations from cuttings and mud gas logging (including IsoTubes^®) were much lower than from cores, but correlated well. Simulations suggest the lower concentrations from cuttings can be attributed to drilling time, and therefore their use to define gas concentration profiles may have inherent limitations. Calculations based on mud gas logging show the method can provide estimates of core concentrations if operational parameters for the mud gas capture cylinder are quantified. The δ¹³C‐CH₄ measured from mud gas, IsoTubes^®, cuttings, and core samples are consistent, exhibiting slight variations that should not alter the implications of the results in identifying the sources of the gases. This study shows core and mud gas techniques and, to a lesser extent, cuttings, can generate high‐resolution depth profiles of dissolved hydrocarbon gas concentrations and their isotopes.     

Crustal accretion and dynamic feedback on mantle melting of a ridge centred plume: The Iceland case

A major consequence of the interaction of a plume with an oceanic ridge is the enhanced melt production and associated crust generation. In the case of Iceland crustal thickness as large as 20 to 40 km has been reported. Crustal seismic velocities are high, and have to be explained by thermal or chemical effects. In the first part of the paper we address the question whether extraction of melt out of the plume beneath a slowly spreading ridge and deposition of extracted basalt volumes at the surface produces a dynamic feedback mechanism on mantle melting. To study this question we solve the convection equations for a ridge centred plume with non-Newtonian rheology including melting, melt extraction associated with deposition of cold crust at the surface of the model, and using a simplified approach for compaction. The assumption of cold crust is justified if the thickness of each deposited basaltic layer is less than roughly 1 km. Depending on the buoyancy flux of the plume, crustal thicknesses between 10 and 40 km are modelled, showing characteristic dipping structures resembling the rift-ward dipping basaltic layers of East- and Western Iceland. Comparing the resulting crustal thickness and magma generation rate with models in which the dynamic effect of crust deposition has been suppressed indicates, that melt generation beneath a slowly spreading ridge is considerably damped by the dynamic feedback mechanism if the plume buoyancy flux exceeds 400 to 600 kg/s. Based on the observed crustal thickness of Iceland our models predict a plume buoyancy flux of 1140 kg/s.In the second part we study the accretion of the Icelandic crust by a thermo-mechanical model in more detail based on the Navier–Stokes-, the heat transport and the mass conservation equations including volumetric sources. Hot (1200 °C) molten crustal material is injected into the newly forming crust with a constant rate at different crustal source regions: a) deep, widespread emplacement of dykes and sills including crustal underplating, b) magma chambers at shallow to mid-crustal level, and c) surface extrusions and intrusions in fissure swarms at shallow depth connected to volcanic centres. We identify the material from the different source regions by a marker approach. Varying the relative dominance of these source regions, characteristic crustal structures evolve, showing shallow dipping upper crustal layers with dip angles between 10 and 15°. The thermal structure of the crust varies between cold crust (shallow-source region dominating) and hot crust (deep-source region dominating). We use observations of maximum depth of seismicity to constrain the depth of the 650 °C isotherm and seismological inferences on the lower crust to constrain temperatures in that region. The best agreement with our models is achieved for crust formation dominated by deep dykes and underplating with a considerable influence of magma chamber accretion.     

Numerical models on the influence of partial melt on elastic,anelastic and electric properties of rocks. Part I: elasticity and anelasticity

With the aim of a simultaneous interpretation of elastic, anelastic and electric in situ data from the asthenosphere a comprehensive set of numerical models is developed for partial melt in different geometrical configurations. For the elastic and anelastic modulus use is made throughout of the melt squirt mechanism. Frequency dependence is not treated in detail but estimated from the limiting cases of the relaxed and unrelaxed modulus. This has the advantage that quantitative values of viscocity and flow path dimensions are not required. In the models melt can be assumed to occur in the form of tubes, films, and triaxial ellipsoidal inclusions of arbitrary aspect ratio. The conditions in which the solutions for triaxial ellipsoidal inclusions can be approximated by simpler ones for spheroidal inclusions are discussed. It is then shown up to which aspect ratio a published model on melt films is applicable. The problem of interconnection of inclusions is treated with a statistical numerical approach. It is found that a reduced degree of interconnection may have a significant influence on anelastic relaxation at melt fractions corresponding to a moderate modulus decrease. A useful representation of the anelastic melt models is introduced by plotting the relaxation strength against the effective modulus, both of which depend on the state of melting. Such diagrams allow a clear distinction between the different melt geometries and may be used for the interpretation of observed data. Finally, different melt geometries are superimposed and it is found that under certain conditions bulk dissipation may reach the order of that for shear.     

Shear modulus and Q of forsterite and dunite near partial melting from forced-oscillation experiments

An apparatus designed to determine the complex shear modulus of rock samples by forced torsion oscillations at high temperature and in the seismic frequency band 0.003–30 Hz is briefly described. Measurements were performed on natural dunite from Åheim (Norway) up to 1400°C and on polycrystalline forsterite up to 1500°C at 1 atm pressure. The two materials were chosen to study, by comparison, the effect of melt on the elasticity and anelasticity of mantle rocks.Between 1000 and 1200°C the absolute values of the shear modulus G are almost equal for both materials. Above 1200°C G for natural dunite decreases progressively with temperature and at 1400°C and 1 Hz reaches $\text{～}\text{1}\text{3}$ of its value at 1100°C. In contrast, G of pure forsterite depends little on temperature. For petrological reasons, supported by simultaneous measurements of the electric resistivity, there is strong evidence that the decrease of G in dunite above 1200°C is due to melt from the lower melting components of the dunite. Based on different models estimates of the melt fraction are made.At high temperature, in both materials Q^?1 is characterized by a monotonic decrease with frequency according to ω^?α, with α ≈ 0.25. An apparent activation energy of 38±5 kcal mol^?1 for forsterite and 48±8 kcal mol^?1 for dunite was found with no significant change in the regime of partial melting. From this it is concluded that Q^?1, even at partial melting, is dominated by solid state high temperature background absorption. There is no indication from these experiments for a constant-Q-band at low seismic frequencies or an increase of Q proportional to frequency as suggested by some seismologists. The present results are in good qualitative agreement with those for Young's modulus obtained previously by strain retardation experiments.