Conductive Boundary Layer Research Articles

Characteristics of magma‐driven hydrothermal systems at oceanic spreading centers

AbstractWe use one‐ and two‐limb single‐pass models to de termine vent field characteristics such as mass flow rate Q, bulk permeability in the discharge zone kd, thickness of the conductive boundary layer at the base of the system δ, magma replenishment rate, and residence time in the discharge zone. Data on vent temperature, vent field area, heat output, and the surface area and depth of the subaxial magma chamber (AMC) constrain the models. The results give Q ~ 100 kg/s, kd ~ 10−13 m2, and δ ~ 10 m, essentially independent of spreading rate, and detailed characteristics of the AMC. In addition, we find no correlation between heat output at individual vent fields and spreading rate or depth to the AMC. We conclude that high‐temperature hydrothermal systems are driven by local magma supply rates in excess of that needed for steady state crustal production and that crustal permeability enables hydrothermal circulation to tap magmatic heat regardless of AMC depth. Using data on partitioning of heat flow between focused and diffuse flow, we find that 80–90% of the hydrothermal heat output is derived from high‐temperature fluid, even though much of the heat output discharges as low‐temperature fluid. In some cases, diffuse flow fluids may exhibit considerable conductive cooling or heating. By assuming conservative mixing of diffuse flow fluids at East Pacific Rise 9°50′N, we find that most transport of metals such as Fe and Mn occurs in diffuse flow and that CO2, H2, and CH4 are taken up by microbial activity.

Numerical Study of Magnetoacoustic Signal Generation with Magnetic Induction Based on Inhomogeneous Conductivity Anisotropy

Magnetoacoustic tomography with magnetic induction (MAT-MI) is a noninvasive imaging modality for generating electrical conductivity images of biological tissues with high spatial resolution. In this paper, we create a numerical model, including a permanent magnet, a coil, and a two-layer coaxial cylinder with anisotropic electrical conductivities, for the MAT-MI forward problem. We analyze the MAT-MI sources in two cases, on a thin conductive boundary layer and in a homogeneous medium, and then develop a feasible numerical approach to solve the MAT-MI sound source densities in the anisotropic conductive model based on finite element analysis of electromagnetic field. Using the numerical finite element method, we then investigate the magnetoacoustic effect of anisotropic conductivity under the inhomogeneous static magnetic field and inhomogeneous magnetic field, quantitatively compute the boundary source densities in the conductive model, and calculate the sound pressure. The anisotropic conductivity contributes to the distribution of the eddy current density, Lorentz force density, and acoustic signal. The proposed models and approaches provide a more realistic simulation environment for MAT-MI.

High temperature metamorphism in the conductive boundary layer adjacent to a rhyolite intrusion in the Krafla geothermal system, Iceland

A rhyolite magma body within the Krafla geothermal system that was encountered at a depth of 2.1km during drilling of the IDDP-1 borehole is producing high temperature metamorphism within a conductive boundary layer (CBL) in adjacent host rocks. Cuttings recovered during drilling within a few meters of the intrusive contact in IDDP-1 are mainly comprised of granoblastic hornfelses, the rock type which confirms the presence of the CBL at the base of the IDDP-1 bore hole. The two pyroxenes in these hornfelses record temperatures that are in the range of 800–950°C. The minimum heat flow across the CBL is 23Wm−2. Country rocks at distances beyond 30m of the intrusive contact are essentially unaltered, implying that they have been emplaced very recently and/or as yet unaffected by hydrothermal fluid flow.

Hydrothermal circulation and the dike-gabbro transition in the detachment mode of slow seafloor spreading

Research Article| April 01, 2012 Hydrothermal circulation and the dike-gabbro transition in the detachment mode of slow seafloor spreading Andrew M. McCaig; Andrew M. McCaig 1Institute of Geophysics and Tectonics, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Michelle Harris Michelle Harris 2National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (4): 367–370. https://doi.org/10.1130/G32789.1 Article history received: 31 Aug 2011 rev-recd: 19 Nov 2011 accepted: 25 Nov 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Andrew M. McCaig, Michelle Harris; Hydrothermal circulation and the dike-gabbro transition in the detachment mode of slow seafloor spreading. Geology 2012;; 40 (4): 367–370. doi: https://doi.org/10.1130/G32789.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract One of the most ubiquitous boundaries within our planet is between sheeted dikes and gabbros in fast-spreading ocean crust. This boundary marks the brittle-ductile transition at the ridge crest, and is localized by a decametric conductive boundary layer between hydrothermal circulation in the sheeted dike layer and a shallow quasi-steady-state melt lens. In contrast, at slow-spreading ridges, the crustal structure appears chaotic, with no consistent sheeted dike layer and widespread occurrences of gabbro and serpentinized peridotite on the seafloor. Recent work suggests that as much as 50% of the Atlantic Ocean crust formed by a detachment mode of seafloor spreading, including the formation of oceanic core complexes capped by long-lived, convex-upward detachment faults. These detachment faults are often associated with large hydrothermal systems in which the location of any magmatic heat source is uncertain. Here we show that detachment faults can act as thermal boundaries between gabbroic melt in the fault footwall and hydrothermal circulation in the fault zone and hanging wall, thus explaining the link between faulting and black smoker systems. We suggest that interaction between magmatism and hydrothermal circulation means that detachment faults can act as the dike-gabbro transition in the detachment mode of spreading, inevitably leading to exposure of gabbros on the seafloor through continued faulting. This concept provides a means of unifying apparently contrasting processes and crustal structures at different spreading rates. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dufour and Soret effect on Double-Diffusive Electrically conducting boundary layer flows in a vertical plate considering internal heat and mass generation

The two dimensional steady laminar MHD free convection heat and mass transfer flow over a vertical plate in an incompressible, viscous and electrically conducting fluid is examined considering the wall

High-grade contact metamorphism in the Reykjanes geothermal system: Implications for fluid-rock interactions at mid-oceanic ridge spreading centers

Granoblastic hornfels identified in cuttings from the Reykjanes seawater-dominated hydrothermal system contains secondary pyroxene, anorthite, and hornblendic amphibole in locally equilibrated assemblages. Granoblastic assemblages containing secondary orthopyroxene, olivine, and, locally, cordierite and spinel occur within groups of cuttings that show dominantly greenschist facies hydrothermal alteration. Granoblastic plagioclase ranges continuously in composition from An54 to An96, in contrast with relict igneous plagioclase that ranges from An42 to An80. Typical hydrothermal clinopyroxene compositions range from Wo49En3Fs48 to Wo53En30Fo17; clinopyroxene from the granoblastic grains is less calcic with an average composition of Wo48En27Fs25. The hornfels is interpreted to form during contact metamorphism in response to dike emplacement, resulting in local recrystallization of previously hydrothermally altered basalts. Temperatures of granoblastic recrystallization estimated from the 2-pyroxene geothermometer range from 927°C to 967°C. Redox estimates based on the 2-oxide oxybarometer range from log fO2 of −13.4 to −15.9. Granoblastic hornfels comprised of clinopyroxene, orthopyroxene, and calcic plagioclase have been described in a number of ancient hydrothermal systems from the conductive boundary layer between the hydrothermal system and the underlying magma source, most notably in Integrated Ocean Drilling Program Hole 1256D, Ocean Drilling Program Hole 504B, and in the Troodos and Oman ophiolites. To our knowledge, this is the first evidence of high-grade contact metamorphism from an active geothermal system and the first description of equilibrated amphibole-absent pyroxene hornfels facies contact metamorphism in any mid-ocean ridge (MOR) hydrothermal system. This contribution describes how these assemblages develop through metamorphic reactions and allows us to predict that higher-temperature assemblages may also be present in MOR systems.

Olivine-rich Troctolites from Ligurian Ophiolites (Italy): Evidence for Impregnation of Replacive Mantle Conduits by MORB-type Melts

The gabbroic plutons of the Internal Ligurian ophiolites (Northern Apennines, Italy) include olivine-rich troctolite bodies up to 80 m thick. The rounded to embayed morphology of olivine (Fo ¼ 87^ 88 mol %) and spinel from the olivine-rich troctolites, together with the high Mg-number (88^90) and Cr contents (2400^ 3000 ppm) of associated poikilitic clinopyroxene, indicates their formation through reaction between an olivine^spinel matrix and an infiltrating melt.This interpretation is corroborated by the occurrence of plagioclase-rich veins that exhibit diffuse margins against the host olivine-rich troctolites and centimetre-scale spinel-bearing dunitic clots, which are interpreted as the crystallization products of the infiltrating melts and relics of pre-existing olivine-rich material, respectively. The incompatible element signature of clinopyroxene from the olivine-rich troctolites shows that the infiltrating melts were chemically similar to mid-ocean ridge basalt (MORB). Spinel from the olivine-rich troctolites contains inclusions of Ti-pargasite/ kaersutite, phlogopite/aspidolite and orthopyroxene; inclusions of ilmenite and loveringite are also locally present. These spinels most probably formed from hybrid melts derived from interaction between melts rich in incompatible elements and new injections of primitive melt. We propose that this interaction occurred in the mantle, in melt conduits of replacive origin near the base of the conductive thermal boundary layer, and led to formation of a matrix made up of olivine and minor inclusion-bearing spinel. This matrix was subsequently impregnated by MORB-type melts saturated in plagioclase þ clinopyroxene to produce the olivine-rich troctolites. The amount of melt added to the olivine^spinel matrix to produce the olivine-rich troctolites is estimated to be � 25 wt %.

Radiation effect on chemically reacting magnetohydrodynamics (MHD) boundary layer flow of heat and mass transfer through a porous vertical flat plate

A mathematical model is presented for a two-dimensional, steady, viscous, incompressible, electrically conducting and laminar free convection boundary layer flow with radiation from a flat plate in a chemically reactive medium in the presence of a transverse magnetic field. The basic equations governing the flow are in the form of partial differential equations and have been reduced to a set of non-linear ordinary differential equations by applying suitable similarity transformations. The problem is tackled numerically using shooting techniques with the fourth order Runge-Kutta integration scheme. Pertinent results with respect to embedded parameters are displayed graphically for the velocity, temperature and concentration profiles and discussed quantitatively. Key words: Heat transfer, thermal radiation, magnetic field, chemical species, mixed convection.

THE SURPRISINGLY CONSTANT STRENGTH OF O VI ABSORBERS OVER COSMIC TIME

O VI absorption is observed in a wide range of astrophysical environments, including the Local ISM, the disk and halo of the Milky Way, high-velocity clouds, the Magellanic clouds, starburst galaxies, the intergalactic medium, damped Lyman-alpha systems, and gamma-ray-burst host galaxies. Here a new compilation of 775 O VI absorbers drawn from the literature is presented, all observed at high resolution (instrumental FWHM<20 km/s) and covering the redshift range z=0-3. In galactic environments [log N(H I)>20], the mean O VI column density is shown to be insensitive to metallicity, taking a value log N(O VI)~14.5 for galaxies covering the range -1.6<[O/H]<0. In intergalactic environments [log N(H I)<17], the mean O VI component column density measured in datasets of similar sensitivity shows only weak evolution between z=0.2 and z=2.3, but IGM O VI components are on average almost twice as broad at z=0.2 than at z=2.3. The existence of a characteristic value of log N(O VI) for galactic O VI absorbers, and the lack of evolution in log N(O VI) for intergalactic absorbers, lend support to the ``cooling-flow' model of Heckman et al. (2002), in which all O VI absorbers are created in regions of initially-hot shock-heated plasma that are radiatively cooling through coronal temperatures. These regions could take several forms, including conductive, turbulent, or shocked boundary layers between warm (~10^4 K) clouds and hot (~10^6 K) plasma, although many such regions would have to be intersected by a typical galaxy-halo sightline to build up the characteristic galactic N(O VI). The alternative, widely-used model of single-phase photoionization for intergalactic O VI is ruled out by kinematic evidence in the majority of IGM O VI components at low and high redshift.

Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256

ODP/IODP Hole 1256D penetrates an in situ section of ocean crust formed at the East Pacific Rise, through lavas and sheeted dikes and ∼100 m into plutonic rocks. We use mineralogy, oxygen isotopes, and fluid inclusions to understand hydrothermal processes. The lavas are slightly altered at low temperatures (&lt;150°C) to phyllosilicates and iron oxyhydroxides, with a stepwise increase in grade downward to greenschist minerals in the upper dikes. This resulted from generally upwelling hydrothermal fluids in the dikes mixing with cooler seawater solutions in the lavas, also producing minor metal sulfide mineralization in the upper dikes. Alteration grade increases downward in the dikes, with increasing recrystallization to amphibole and loss of metals at higher temperatures (&gt;350°C up to ∼600°C). Intrusion of gabbro bodies into the lower dikes resulted in contact metamorphism to granoblastic hornfels at 850°C–900°C, representing a thermal boundary layer between the axial melt lens and the overlying hydrothermal system. Downward penetration of hydrothermal fluids led to rehydration of granoblastic dikes and plutonic rocks at ∼800°C down to &lt;300°C. Fluid inclusion and oxygen isotope data show that vein quartz formed at ∼300°C to &gt;450°C from hydrothermal fluids that were affected by supercritical phase separation. Fluids had variable salinities and were enriched in 18O (+0.4‰ to +3.5‰) relative to seawater, similar to seafloor vent fluids. Dike margins are brecciated and mineralized, suggesting hydrothermal activity coeval with magmatism. Anhydrite formed mainly in the upper dikes when partly reacted seawater fluids were heated as they penetrated deeper into the system. Low‐temperature alteration of the volcanic section continued as cold seawater penetrated along fluid pathways, forming minor iron oxyhydroxides in the rocks. Hydrothermal processes at Site 1256 fit with current models whereby greenschist alteration of dikes at low water/rock ratios is overprinted by fracture‐controlled alteration and mineralization by upwelling hydrothermal fluids, a conductive boundary layer above gabbroic intrusions, leaching of metals from dikes and gabbros in the deep “root zone,” and stepped thermal and alteration gradients in the basement. The Site 1256 section, however, is intact and retains recharge effects (anhydrite), allowing an integrated view of processes in the subsurface.

Vertical distribution of ozone and VOCs in the low boundary layer of Mexico City

Abstract. The evolution of ozone (O3) and 13 volatile organic compounds (VOCs) in the boundary layer of Mexico City was investigated during 2000–2004 to improve our understanding of the complex interactions between those trace gases and meteorological variables, and their influence on the air quality of a polluted megacity. A tethered balloon, fitted with electrochemical and meteorological sondes, was used to obtain detailed vertical profiles of O3 and meteorological parameters up to 1000 m above ground during part of the diurnal cycle (02:00–18:00 h). VOCs samples were collected up to 200 m by pumping air to canisters with a Teflon tube attached to the tether line. Overall, features of these profiles were found to be consistent with the formation of an upper residual layer during nighttime carrying over a fraction of the O3 from the previous day that contributes to the background concentration in surrounding regions. At the same time the release of heat stored in the urban surface forms a shallow unstable layer close to the ground, where the nocturnal emissions are trapped. After sunrise an O3 balance is determined by photochemical production, entrainment from the upper residual layer and destruction by titration with nitric oxide, delaying the ground-level O3 rise by 2 h. The subsequent evolution of the conductive boundary layer and vertical distribution of pollutants are discussed in terms of the energy balance, the presence of turbulence and the atmospheric stability.

Reconciling temperatures of metamorphism, fluid fluxes, and heat transport in the upper crust at intermediate to fast spreading mid‐ocean ridges

Current models of the mineralogical and chemical modification of the crust within mid‐ocean ridge hydrothermal systems at intermediate to fast spreading ridges are difficult to reconcile with the temperature distribution predicted by the required fluid fluxes. In particular, the sharp increase in metamorphic grade passing down from the lavas into the sheeted dike complex places important constraints on the flow of fluid through the crust. This distribution of metamorphic temperatures has typically been discussed in terms of a step in permeability in the downwelling limb of a single‐pass convection cell. Simple modeling of this system shows that downwelling fluid cools the crust too efficiently for high‐temperature metamorphism of the upper dikes to occur in the downflow zone. In fact, temperatures throughout most of the sheeted dike complex are too high. Three alternative models are considered. Fluid‐rock reaction during the cooling of each dike from magmatic temperatures is difficult to reconcile with cooling rate estimates for dikes, reaction rates required to grow metamorphic minerals, and the relatively constant composition of many hydrothermal vents. Metamorphism during reheating of the crust after fluid flow slows or stops is difficult to reconcile with the required fluid fluxes during metamorphism, based on the Sr‐isotopic composition of the sheeted dike complex, and cannot produce high enough temperatures in the upper dikes and low temperatures in the lower lavas. In contrast, fluid‐rock reaction in the sheeted dike complex as hot fluids migrate upward after being heated by the underlying magma source appears to be consistent with the data. In this model, downwelling seawater cools the recharge zone, and the fluid only becomes hot enough to significantly alter the crust once it approaches the conductive boundary layer overlying the magma chamber. Metamorphism of the sheeted dike complex then largely occurs in the discharge zone.

The roof of an axial magma chamber: A hornfelsic heat exchanger

The heat flux from an axial magma chamber into the convecting hydrothermal system at mid-ocean ridges is governed by the conductive boundary layer separating them. The nature of this critical interface has been modeled and seismically imaged, but its characteristics have hitherto rarely been documented in ocean crust. Here, hornfelsic rocks from the sheeted dike–gabbro transitions at two tectonic exposures of fast-spreading East Pacific Rise crust at the Pito and Hess Deeps and in the Oman and Troodos ophiolites are described. These rocks record thermal metamorphism to hornblende and pyroxene hornfels at 700–1000 °C. These temperatures, in conjunction with their geological relationships, imply that the hornfels are preserved fragments of conductive boundary layers. Their distribution at the base of sheeted dike complexes in narrow contact aureoles and dikes intruding the uppermost gabbros tracks the vertical migration of axial magma chambers over minimum depth ranges of 200–400 m. The heat flux across the hornfelsic aureoles (11–44 MW/km) is comparable to hydrothermal fluxes released along fast-spreading ridges, confirming that the heat driving hydrothermal convection is transferred across conductive boundary layers.

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. Part II. Impedance spectroscopy characterization

The dielectric behavior of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained using the traditional ceramic method employing three different precursors was investigated by impedance spectroscopy in the temperature range of 200–300°C. This study was carried out by means of the simultaneous analysis of the complex impedance Z̃, electric modulus M̃, and admittance Ỹ functions from the measurements in the frequency range of 20Hz–1MHz. In correspondence to a previous structural, morphological, and temperature response study, appropriate microstructural and equivalent circuit models were established. Based on the brick layer model, three series of interconnected electrically distinct regions are considered: a conductive grain boundary layer, a capacitive grain boundary surface layer, and a resistive-ferroelectric bulk layer. Two conduction mechanisms were identified: a dielectric relaxation process due to localized conduction associated with the presence of oxygen vacancies and the nonlocalized conduction corresponding to long range conductivity associated with extrinsic mechanisms due, fundamentally, to Fe2+ presence. Both mechanisms were discerned to occur inside the grains and where the contributions of the grain boundary are neglected. Three conductivity components were deconvoluted: a longe range dc conductivity at the low frequency region, a capacitive behavior at higher frequencies, and a universal power law behavior in an intermediate-frequency region. Values of the activation energy corresponding to relaxation processes and dc conductivity were determined and excellent correlation with those obtained from the temperature response was found. A comparative analysis between the behaviors of each sample is presented.

Temperature fields generated by the elastodynamic propagation of shear cracks in the Earth

Thermal perturbations associated with seismic slip on faults may significantly affect the dynamic friction and the mechanical energy release during earthquakes. This paper investigates details of the coseismic temperature increases associated with the elastodynamic propagation of shear cracks and effects of fault heating on the dynamic fault strength. Self‐similar solutions are presented for the temperature evolution on a surface of a mode II shear crack and a self‐healing pulse rupturing at a constant velocity. The along‐crack temperature distribution is controlled by a single parameter, the ratio of the crack thickness to the width of the conductive thermal boundary layer, . For “thick” cracks, or at early stages of rupture ( &gt; 1), the local temperature on the crack surface is directly proportional to the amount of slip. For “thin” cracks, or at later times ( &lt; 1), the temperature maximum shifts toward the crack tip. For faults having slip zone thickness of the order of centimeters or less, the onset of thermally induced phenomena (e.g., frictional melting, thermal pressurization, etc.) may occur at any point along the rupture, depending on the degree of slip localization and rupture duration. In the absence of significant increases in the pore fluid pressure, localized fault slip may raise temperature by several hundred degrees, sufficient to cause melting. The onset of frictional melting may give rise to substantial increases in the effective fault strength due to an increase in the effective fault contact area, and high viscosity of silicate melts near solidus. The inferred transient increases in the dynamic friction (“viscous braking”) are consistent with results of high‐speed rock sliding experiments and might explain field observations of the fault wall rip‐out structures associated with pseudotachylites. Possible effects of viscous braking on the earthquake rupture dynamics include (1) delocalization of slip and increases in the effective fracture energy, (2) transition from a crack‐like to a pulse‐like rupture propagation, or (3) ultimate rupture arrest. Assuming that the pulse‐like ruptures heal by incipient fusion, the seismologic observations can be used to place a lower bound on the dynamic fault friction. This bound is found to be of the order of several megapascals, essentially independent of the earthquake size. Further experimental and theoretical studies of melt rheology at high strain rates are needed to quantify the effects of melting on the dynamic fault strength.

Heat diffusion at the boundary of stratified media: Homogenized temperature field and thermal constriction

Heat diffusion in stratified materials with the layers running parallel to the main heat flux direction is analyzed with special emphasis on the temperature field near the boundaries. In a previous work, a semi-analytical general solution was proposed as an extension of the thermal quadrupole method for heat conduction in heterogeneous media. In this paper, the steady solution is separated into an homogenized transfer in series with a constriction term, and a conductive boundary layer is defined. The same decomposition method is implemented for the semi-infinite transient case, and some simplified models are obtained from asymptotic expansions. For long times, the transient averaged signal is found to be superposed to the steady constriction matrix effect. The main application is to better envision experimental temperature field analysis for thermal non-destructive evaluation methods.

Nerve stimulation with an electrode of finite size: differences between constant current and constant voltage stimulation

Differences between constant current and constant voltage nerve stimulation are controversial. To elucidate this controversy, exact solutions are found for the electrical potential and current of a conducting electrode of finite size placed near a boundary of altered conductivity. Substantial differences in the effects of a finite and a point stimulator are predicted. This was strongly dependent on the stimulator–boundary distance, the conductivity of the media, and the curvature of the boundary. The difference between constant voltage and constant current stimulation was smaller than the effects of changes in medium conductivity and electrode distance. A poorly conducting boundary layer surrounding the stimulator minimized these differences.

Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber

A well-preserved contact aureole at the base of the sheeted dyke is likely to be centred at, or very close to, the sheeted complex in the Troodos ophiolite, Cyprus, records the partial melting dyke–gabbro boundary. This region of the crust is a of the roof of a mid-ocean ridge magma chamber. Hydrothermally dynamic environment that is subjected to rapid changes altered dykes within the lowermost 10–30 m of the sheeted dyke in rheology and thermal structure. It is here that melt is complex were recrystallized to pyroxene and hornblende hornfels episodically extracted to build the upper crust (e.g. Deand, closest to the underlying gabbros ( 30%) at water-undersaturated conditions and temperatures if not all, of the lower crust (e.g. Phipps Morgan & Chen, [875°C. Geological considerations indicate that the duration of 1993; Quick & Denlinger, 1993; Boudier et al., 1996), melting events was probably on the time scale of eruptive cycles (i.e. and magmatic heat is rapidly transferred across a thin years to centuries). This resulted in disequilibrium partial melting (<100 m) conductive boundary layer (e.g. Lowell & and highly imperfect melt segregation within the contact aureole. Burnell, 1991) and advectively transported out of the We hypothesize that partial melting is a common process within crust by hydrothermal fluids (e.g. Cann et al., 1985). the roof zones of axial magma chambers at fast-spreading ridges Geophysical surveys along the East Pacific Rise (EPR) and that a component of geophysically imaged axial magma chambers provide indirect evidence that the depth and internal may be partially melted upper crust rather than partially crystallized properties (e.g. melt volumes, proportion of melt and magma. Field relations suggest that where the highest degrees of crystals) of AMCs are not static and change on the time melting occurred, a mixture of anhydrous hornfels and hydrous scale of eruptive cycles (years to centuries) (Wilcock & leucocratic melt disaggregrated, and was assimilated into the magma Delaney, 1996; Singh et al., 1998). chamber. It is well known from ophiolite studies that the roofs of AMCs can be marked by polyphase magmatism and intense interactions between hydrothermal fluids and rocks, particularly within the uppermost few hundred

Diagrammatic method to describe the self‐inductive response of the magnetosphere‐ionosphere‐atmosphere‐Earth electromagnetically coupled system as a quasi‐particle excitation

A diagrammatic method to intuitively describe the inductive response of wave fields in the magnetosphere‐ionosphere‐atmosphere‐Earth (MIAE) electromagnetically coupled system is developed. The coupling process of magnetohydrodynamic (MHD) waves and atmospheric electromagnetic waves, which interact with an anisotropically conducting thin sheet ionosphere, can be understood in terms of a process of redistribution of the induced current of wave modes. On the basis of the current conservation law and Faraday's law of electromagnetic induction, a redistribution process of the source‐induced current of wave modes to the secondary wave is simply illustrated. The purpose of this paper is to propose a methodology to describe the inductive response of a complex electromagnetically coupled system that is divided by a conducting boundary layer. As an example, we derive the reflection coefficient and the mode conversion ratio of MHD waves at an anisotropic, inductive ionosphere by using the concept of induced current redistribution. We also demonstrate that the redistribution process of wave modes under the law of conservation of induced current also satisfies an energy conservation law in the MIAE system. However, the true merit of this method is its applicability to the highly inhomogeneous region, in which many types of waves exist and interact through the arbitrary boundary layers.

The Rootzone of an Ancient Hydrothermal System Exposed in the Troodos Ophiolite, Cyprus

A well‐exposed, oceanic hydrothermal rootzone in the Troodos ophiolite, Cyprus, is composed of the basal sheeted dikes, the upper 50–100 m of the gabbroic sequence, and a contact aureole, interpreted as a fossil conductive boundary layer. The basal sheeted dikes and upper gabbros were pervasively altered at high temperatures (400°–775°C) and cut by a series of broadly distributed mineralized faults that are oriented parallel to the strike of the sheeted dike complex. Fluid inclusion and mineralogical data for fault breccias show that circulating fluids were 350°–400°C, fairly oxidizing, and relatively H2S rich, with a range in salinity (1.5–7.8 wt % NaCl equivalent). Field and petrological evidence suggest that the rootzone migrated from the basal sheeted dikes into the upper gabbros and back again, perhaps for several cycles. These migrations and P‐T oscillations in the conductive boundary layer were likely caused by fluctuations in thermal conditions imposed by the magmatic system at a spreading center. The hydrothermal rootzone did not migrate to significant depths in the plutonic sequence until the waning stages of magmatism.