Hydrothermal Convection Research Articles

Local Seismicity of the Rainbow Massif on the Mid‐Atlantic Ridge

AbstractThe Rainbow massif, an oceanic core complex located in a nontransform discontinuity on the Mid‐Atlantic Ridge (36°N), is notable for hosting high‐temperature hydrothermal discharge through ultramafic rocks. Here we report results from a 9 month microearthquake survey conducted with a network of 13 ocean bottom seismometers deployed on and around the Rainbow massif as part of the MARINER experiment in 2013–2014. High rates (~300 per day) of low‐magnitude (average ML ~ 0.5) microearthquakes were detected beneath the massif. The hypocenters do not cluster along deeply penetrating fault surfaces and do not exhibit mainshock/aftershock sequences, supporting the hypothesis that the faulting associated with the exhumation of the massif is currently inactive. Instead, the hypocenters demarcate a diffuse zone of continuous, low‐magnitude deformation at relatively shallow (< ~3 km) depths beneath the massif, sandwiched in between the seafloor and seismic reflectors interpreted to be magmatic sills driving hydrothermal convection. Most of the seismicity is located in regions where seismic refraction data indicate serpentinized ultramafic host rock, and although the seismic network we deployed was not capable of constraining the focal mechanism of most events, our analysis suggests that serpentinization may play an important role in microearthquake generation at the Rainbow massif.

Is the Lower Crust Convecting Beneath Mid-Ocean Ridges?

The first attempt to couple models of hydrothermal circulation and magmatic convection along fast-spreading ridges may explain the spacing of hydrothermal vent fields along the East Pacific Rise.

Biologically mediated crystallization of buddingtonite in the Paleoproterozoic: Organic-igneous interactions from the Volyn pegmatite, Ukraine

The Volyn pegmatites from Volodarsk-Volynskyi in the Zhytomyr Oblast, NW Ukraine, are associated with granites genetically related to the Paleoproterozoic Korosten pluton. Their late-stage evolution is characterized by the formation of opal-cemented breccia. A polymineralic pseudomorph after beryl within the breccia includes bertrandite (±euclase) + F-muscovite (with tobelite component) + buddingtonite + organic matter (OM) + opal (+ traces of K-feldspar, albite, columbite, FeS 2 , barite, REE-minerals). Sector-zoned and platy to fibrous buddingtonite has variable (K+Na)- vs. NH 4 -contents (electron microprobe analyses) and some H 2 O or H 3 O + , as indicated by microscope infrared spectroscopy. We suggest that ammonium was produced by decay of OM, which is partly preserved in the pseudomorph. Energy-dispersive electron microprobe data of the OM show with increasing O–decreasing C-N-content due to degassing; the OM contains the high field strength elements Zr (≤7 at%), Y (≤3 at%), Sc (≤0.8 at%), REE (≤0.3 at%), Th (≤0.2 at%), and U (≤1.25 at%), which increase with increasing O-content. Transmission electron microscopy of the OM confirms the presence of N; Zr, Si, and O (with other HFSE) are concentrated in nanometer-sized areas and at the transition from OM to opal in nanometer-sized platy Zr-Si-O crystals. C-rich areas are amorphous but show poorly developed lattice fringes. OM is present in the pseudomorph also as brown pigmentation of opal and in pegmatitic beryl from Volyn as a component in late stage fluid inclusions, identified by C-H vibrational bands in infrared spectra. Stable isotope investigations of C and N of buddingtonite, black opal and kerite (fibrous OM known from the literature to occur in the Volyn pegmatites and interpreted as microfossils) indicate a biogenic origin of the OM. We propose that OM in the pseudomorph is condensed kerite, which achieved the high concentrations of high field strength elements via fluid-pegmatite interaction. Although no age determination of minerals in the pseudomorph is available, textural arguments and phase equilibria indicate its formation in a late stage of the pegmatite evolution, at P-T conditions below ~100 MPa/150 °C. We favor a conceptual model for the formation of the Volyn buddingtonite in analogy to Phanerozoic occurrences of buddingtonite, where over and around the shallow anorthosite-granite Korosten pluton hydrothermal convection cells introduced N-bearing hydrocarbons and its precursors into the cooling igneous rocks. Due to the elevated temperature, the OM disintegrated into degassing volatile and non-volatile residual components analogous to petroleum maturation. Organic N, released as NH 4 , was then incorporated into buddingtonite.

The role of the Devonian Weardale Granite in the emplacement of the North Pennine mineralization

After initial mapping of the fluorite/barite zones (1926) then discovery of the underlying unexposed Weardale Granite (1953), and proving its early Devonian age by the Rookhope borehole, extensive geochemical/mineralogical investigations indicate a likely alkali magmatic contribution to the post-Whin Sill orefield, contrasting with most other Hercynian Mississippi-type orefields in UK. Creaney's mapping of rank of early Namurian coals above the two central cupolas of the granite implied high temperatures of over 200°C just prior to the Whin Sill emplacement (297.4 Ma) in concord with an underlying magmatic source lasting 1 – 5 Ma. Their sharp boundaries unequivocally indicate that heat rose through the granite by hydrothermal convection, not conduction. A Stephanian source of quartz-tholeiitic magma is consistent with the Scottish Midland Valley succession (302 – 295 Ma) and contemporaneous Whin magmatism. The magma underplated the granite because of its higher density. It prepared conditions for the subsequent post-Whin Sill emplacement of the mineral deposits, producing the hot convection cell which drew in saline fluids from adjacent deep Carboniferous troughs. Following Whin Sill emplacement, highly pressurized early Permian alkali basaltic magma also underplated the granite. This was contemporaneous with the succession in the Scottish Midland Valley (age 298 – 292 Ma). Hot mineralizing fluids were forced upwards into the established convection cell in the granite and then into the Carboniferous of the Fluorite Zone, forming the mineral deposits, and then outwards beyond the granite cupola regions to deposit barite at c . 50°C.

Entropy production in a box: Analysis of instabilities in confined hydrothermal systems

AbstractWe evaluate if the concept of thermal entropy production can be used as a measure to characterize hydrothermal convection in a confined porous medium as a valuable, thermodynamically motivated addition to the standard Rayleigh number analysis. Entropy production has been used widely in the field of mechanical and chemical engineering as a way to characterize the thermodynamic state and irreversibility of an investigated system. Pioneering studies have since adapted these concepts to natural systems, and we apply this measure here to investigate the specific case of hydrothermal convection in a “box‐shaped” confined porous medium, as a simplified analog for, e.g., hydrothermal convection in deep geothermal aquifers. We perform various detailed numerical experiments to assess the response of the convective system to changing boundary conditions or domain aspect ratios, and then determine the resulting entropy production for each experiment. In systems close to the critical Rayleigh number, we derive results that are in accordance to the analytically derived predictions. At higher Rayleigh numbers, however, we observe multiple possible convection modes, and the analysis of the integrated entropy production reveals distinct curves of entropy production that provide an insight into the hydrothermal behavior in the system, both for cases of homogeneous materials, as well as for heterogeneous spatial material distributions. We conclude that the average thermal entropy production characterizes the internal behavior of hydrothermal systems with a meaningful thermodynamic measure, and we expect that it can be useful for the investigation of convection systems in many similar hydrogeological and geophysical settings.

Progress in Deciphering the Controls on the Geochemistry of Fluids in Seafloor Hydrothermal Systems.

Over the last four decades, more than 500 sites of seafloor hydrothermal venting have been identified in a range of tectonic environments. These vents represent the seafloor manifestation of hydrothermal convection of seawater through the permeable oceanic basement that is driven by a subsurface heat source. Hydrothermal circulation has fundamental effects on the transfer of heat and mass from the lithosphere to the hydrosphere, the composition of seawater, the physical and chemical properties of the oceanic basement, and vent ecosystems at and below the seafloor. In this review, we compare and contrast the vent fluid chemistry from hydrothermal fields in a range of tectonic settings to assess the relative roles of fluid-mineral equilibria, phase separation, magmatic input, seawater entrainment, and sediment cover in producing the observed range of fluid compositions. We focus particularly on hydrothermal activity in those tectonic environments (e.g., mid-ocean ridge detachment faults, back-arc basins, and island arc volcanoes) where significant progress has been made in the last decade in documenting the variations in vent fluid composition.

Can high‐temperature, high‐heat flux hydrothermal vent fields be explained by thermal convection in the lower crust along fast‐spreading Mid‐Ocean Ridges?

AbstractWe present numerical models to explore possible couplings along the axis of fast‐spreading ridges, between hydrothermal convection in the upper crust and magmatic flow in the lower crust. In an end‐member category of models corresponding to effective viscosities μM lower than 1013 Pa.s in a melt‐rich lower crustal along‐axis corridor and permeability k not exceeding ∼10−16 m2 in the upper crust, the hot, melt‐rich, gabbroic lower crust convects as a viscous fluid, with convection rolls parallel to the ridge axis. In these models, we show that the magmatic‐hydrothermal interface settles at realistic depths for fast ridges, i.e., 1–2 km below seafloor. Convection cells in both horizons are strongly coupled and kilometer‐wide hydrothermal upflows/plumes, spaced by 8–10 km, arise on top of the magmatic upflows. Such magmatic‐hydrothermal convective couplings may explain the distribution of vent fields along the East (EPR) and South‐East Pacific Rise (SEPR). The lower crustal plumes deliver melt locally at the top of the magmatic horizon possibly explaining the observed distribution of melt‐rich regions/pockets in the axial melt lenses of EPR and SEPR. Crystallization of this melt provides the necessary latent heat to sustain permanent ∼100 MW vents fields. Our models also contribute to current discussions on how the lower crust forms at fast ridges: they provide a possible mechanism for focused transport of melt‐rich crystal mushes from moho level to the axial melt lens where they further crystallize, feed eruptions, and are transported both along and off‐axis to produce the lower crust.

The role of hydrothermal cooling of the oceanic lithosphere for ocean floor bathymetry and heat flow

AbstractWe investigate the influence of hydrothermal circulation on cooling of oceanic lithosphere. We include hydrothermal convection in a 1‐D lithosphere cooling model by using a scaling law which relates the Rayleigh to the Nusselt number. This law allows calculating an effective thermal conductivity accounting for the extra heat transport. Based on a global data set for bathymetry and surface heat flow for ocean floor ages between 0.7 and 160 Ma, we perform a joint inversion based on a downhill simplex algorithm to constrain characteristic parameters for hydrothermal cooling (beside the classical parameter mantle temperature, thickness of the lithosphere, and thermal expansivity of lithosphere rocks). Hydrothermal cooling parameters are crack aspect ratio which controls the hydrothermal penetrations depth, characteristic cementation time at which cracks become closed and hydrothermal circulation ceases, and sealing time when enough sedimentary cover on top of the lithosphere has accumulated to prevent hydrothermal fluid escaping into the ocean. Best fitting parameter sets predict mantle temperatures between 1350 and 1450°C and lithosphere thickness between 70 and 90 km and further suggest that (1) the fit to the data is strongly improved if hydrothermal cooling effects are considered, (2) hydrothermal cooling is important for up to 10 Ma old lithosphere and leads to a significant deviation from the 1/square root—law with an exponent closer to 1/3, (3) sedimentary sealing is completed for ~1.5 Ma old lithosphere, and (4) fitting of the data is improved for an apparently 1.4 times higher value of thermal expansivity which may account for effects of melt solidification.

A Hybrid Model Using Time Series and Markov Chain for the Detection of the Main Periodicities on the Temperature Data in Nisyros Volcano, South Aegean

Nisyros volcano has shown a series of volcanic and seismo-tectonic events including recent volcanic phreatic eruptions intruding an impermeable layer near the surface. Geological and atmospheric phenomena contribute to the heat output from the hydrothermal system. To monitor the temperature fluctuations from a vent gas emission in Nisyros, a temperature sensor has been installed in a fumarole near the Lofos and Laki sites. Raw and decomposed temperature data were analyzed to determine the cycles of heat contribution at the surface from the endogenic volcanic activity and from the interaction with the atmospheric temperature. The time series decomposition using Kolmogorov-Zurbenko filter and the Markov chain approach have been utilized as complementary tools for the determination of the main periodicities on the temperature data. The Kolmogorov-Zurbenko filter is used for the time series decomposition of the temperature data into the longandshort term component of the time series. Markov chain analysis was used to determine probabilistically the periodicity in the temperature data and aggregate load impact in forecasting. Using spectral analysis and cluster analysis we determine the main periodicities of the longand short term cycles of heat contribution. Raw temperature data have shown an accelerated frequency of hourly temperature decreases at the surface. Small temperature decreases occur within the warmer atmospheric temperatures and dramatic temperature decreases occur during colder atmospheric temperatures. A physical or a mechanical phenomenon may prevent heat from reaching the surface. The time series decomposition performs well in long term cycles while the Markov chain performs better in short term cycles.Both periods are related to cyclones that occur in the Aegean Sea. Since Nisyros is an island, the temperature measurements are probably affected by various phenomena (cyclones, sea tides) that occur in the sea and affect the hydrothermal convection.

On the impact of spatially heterogenous permeability on free convection in the Perth Basin, Australia

We study the impact of spatially heterogeneous permeability on the formation and shape of hydrothermal porous flow convection in the Yarragadee Aquifer by modelling three simulation scenarios, each with differing permeability distributions.In all scenarios, the southern part of the model is characterised by convection rolls, while the north is dominated by a stable region of decreased temperatures at depth due to hydraulic interaction with shallower aquifers.This suggests that reservoir structure is a first-order controlling factor for the formation of the free convective system. The convective system adjusts to the spatially heterogeneous permeability distribution, yielding locally different convection patterns.

The thermal regime of the Campi Flegrei magmatic system reconstructed through 3D numerical simulations

We illustrate a quantitative conductive/convective thermal model incorporating a wide range of geophysical, petrological, geological, geochemical and isotopical observations that constrain the thermal evolution and present state of the Campi Flegrei caldera (CFc) magmatic system. The proposed model has been computed on the basis of the current knowledge of: (1) the volcanic and magmatic history of the volcano over the last 44ka, (2) its underlying crustal structure, and (3) the physical properties of the erupted magmas. 3D numerical simulations of heat conduction and convection within heterogeneous rock/magma materials with evolving heat sources and boundary conditions that simulate magma rise from a deep (≥8km depth) to shallow (2–6km) reservoirs, magma chamber formation, magma extrusion, caldera collapse, and intra-caldera hydrothermal convection, have been carried out. The evolution of the CFc magmatic system through time has been simulated through different steps related to its changes in terms of depth, location and size of magma reservoirs and their replenishment. The thermal modeling results show that both heat conduction and convection have played an important role in the CFc thermal evolution, although with different timing. The simulated present heat distribution is in agreement with the measured geothermal profiles (Agip, 1987), reproduces the thermal gradient peaks at the CFc margins in correspondence to the anomalies in surface gradients (Corrado et al., 1998), and suggests temperatures of 700°C at depth of 4km in the central portion of the caldera, in agreement with the estimated temperature for the brittle-ductile transition (Hill, 1992).

Using array MT data to image the crustal resistivity structure of the southeastern Taupo Volcanic Zone, New Zealand

Magnetotelluric (MT) data from 169 locations covering a 700km2 region in the southeastern part of the Taupo Volcanic Zone, New Zealand, are used to generate a 3-D resistivity inversion model to a depth of 10km. Isolated zones of low-resistivity (<30Ωm) below 3km are imaged in the basement that appear to be associated with the geothermal fields at Rotokawa, Ngatamariki and Ohaaki, but not with the geothermal fields at Orakei-Korako and Te Kopia. These low-resistivity zones are interpreted to be recent intrusions that feed heat and magmatic fluid to the overlying system of hydrothermal convection supplying the geothermal fields. At Rotokawa, hypocentres of thermally induced seismicity coincide with the top of the deep low-resistivity zone and suggest that temperatures sufficient for melting are reached at ~4.5km depth. In most cases these deep low-resistivity zones are located at the margins of areas of low-gravity, which suggests that the intrusions lie at the margins of old caldera collapse structures. The 3-D resistivity model indicates that geological structure and magmatic intrusions play a more direct role than previously envisaged, and significantly influence the overarching system of convective heat and mass transport in the Taupo Volcanic Zone.

Ore geology and fluid evolution of the giant Caixiashan carbonate-hosted Zn–Pb deposit in the Eastern Tianshan, NW China

The Caixiashan giant carbonate-hosted Zn–Pb deposit (~131Mt@ 3.95% Zn+Pb) formed by replacement of dolomitized marble, with stratiform massive and breccia bodies is located near the base of the Proterozoic Kawabulake Group limestone and marble. It is one of the largest carbonated-hosted massive sulfides Zn–Pb ore deposits in Northwest China to have been discovered in recent years.Abundant pyrite occurs in dolomitized marble, along fractures in dolomitized clasts in the host rocks and filling cracks in the host rock. Locally, colloform or framboidal pyrites are observed in the early period and sometimes replaced by the later sphalerite. The sulfide assemblage of the main ore stage is characterized by massive or disseminated sphalerite and galena, with less pyrite than the earlier stage, and minor pyrrhotite. Galena occurs as small veins cutting the early-formed sphalerite. Dolomite and calcite are the main gangue minerals that co-precipitated with these sulfides. Tremolite and quartz alteration commonly overprints the orebodies. According to the crosscutting relationships and the different mineral associations within the host rocks and ore bodies, three stages are recognized at Caixiashan, i.e., syn-sedimentary pyrite (stage I), pyrite alteration, sphalerite–carbonate and galena–pyrite–carbonate (stage II-1, stage II-2 and stage II-3, respectively) and magmatic/metamorphic reworking (stage III).Calcite and quartz crystals are important host minerals among the three hypogene stages (stages I–III, although quartz mainly occurred in stage III). Stage I contains only aqueous inclusions (W-type), which were homogenized from 110 to 236°C (main range of 138–198°C and average at 168°C; main range=average±σ) and the salinities are from 0.5 to 16.5 (main range of 5.1–15.1 with average of 10.1) wt.% NaCl eqv. In the pyrite alteration of stage II-1 the W-type fluid inclusions homogenized from 175 to 260°C (main range of 210–260 with average of 235) and the salinities range from 8.5 to 22.4 (main range of 16.7–20.1 with average of 18.4) wt.% NaCl eqv. In the main Zn–Pb mineralization stage (stage II-2–3), four types of fluid inclusions were identified an aqueous phase (W-type), a pure carbon phase (PC-type), a carbon phase containing (C-type) and mineral bearing inclusions (S-type). The W-type fluid inclusions of stage II-2–3 homogenized at 210 to 370°C (main range of 253–323 and average at 270) and the salinities range from 5.9 to 23.1 (main range of 13.3–20.3 with average at 16.8) wt.% NaCl eqv.; C-type homogenized at 237°C to 371°C and the salinities range from 6.4–19.7wt.% NaCl eqv.; S-type fluid inclusions homogenized at 211 to 350°C and daughter minerals melted between 340 and 374°C during heating, indicating a salinity range of 42 to 44wt.% NaCl eqv. PC-type fluid inclusions with homogenization temperatures of CO2 phase show large variation from 7.4°C to 21.2°C. Laser Raman analyses show that CH4, CO2 and SO42− coexist in the main mineralization stage fluids. The magmatic/metamorphic reworking stage only contains W-type fluid inclusions which yield homogenized between 220 and 360°C (main range of 251–325 and average at 288), with salinities ranging from 1.7 to 23.0 (main range of 14.3–20.0 and average at 18.8) wt.% NaCl eqv.The textural features, mineral assemblages and fluid geochemistry suggest that the Zn–Pb ores were formed through hydrothermal convection of hot marine waters along the faults and fractures resulting in metal (Zn, Pb and Fe) enriched stratiform orebodies. Subsequent rapid precipitation of sulfides was triggered by sulfate (SO42−) thermal reduction with the CH4 preserved in sedimentary rocks and early stage I pyrite bodies. This process occurred at moderate temperatures (ca. 270°C). Higher-temperature magmatic hydrothermal alteration overprinted the orebodies, but only provided a minor contribution to the mineralization.

Organic matter maturation in the contact aureole of an igneous sill as a tracer of hydrothermal convection

AbstractThe intrusion of magmas can induce hydrothermal convection which in turn enhances the transport of heat and solutes. We use a heat convection model to interpret the temperature evolution documented by organic matter maturation, as recorded by the vitrinite reflectance Rr, in a contact aureole of a 15 m thick basaltic sill in the Deep Sea Drilling Project (DSDP) 41–368 hole near Cape Verde Rise, eastern Atlantic. Here there is a pronounced asymmetry of variations of Rr with distance above and below the sill that cannot be explained by heat conduction alone. Neglecting the effects of possible two‐phase flow and thermal pressurization from the production of fluids, convection begins to enhance the observed asymmetry for permeabilities greater than about 1 md, and the observations can be matched with permeabilities of at least several tens of millidarcies.

From the lavas to the gabbros: 1.25km of geochemical characterization of upper oceanic crust at ODP/IODP Site 1256, eastern equatorial Pacific

Here we present trace element and Sr-Nd-Hf-Pb (double spike) isotopic data covering the entire igneous section of oceanic crust drilled at Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program (IODP) Site 1256 on the Cocos Plate. The penetrated interval extends from the upper lavas through the sheeted dike complex to the gabbroic plutonic rocks, formed during superfast spreading at the mid-Miocene equatorial East Pacific Rise. The data are used to characterize the effects of chemical alteration, resulting from convection of seawater and hydrothermal fluids, on the trace element and isotopic composition of oceanic crust. Compared to normal mid-ocean-ridge basalt, the igneous basement of Site 1256 (Holes 1256C/D) is isotopically slightly enriched but shows only narrow downhole variations in Nd-Hf-Pb isotope ratios: 143Nd/144Nd = 0.513089 ± 0.000028 (2σ), 176Hf/177Hf = 0.283194 ± 0.000033 (2σ), 206Pb/204Pb = 18.61 ± 0.11 (2σ), 207Pb/204Pb = 15.521 ± 0.014 (2σ), 208Pb/204Pb = 38.24 ± 0.15 (2σ). We believe that this minor variability is mainly of primary (magmatic) origin. The Sr isotopic composition shows considerably larger variation and, as expected, serves as sensitive tracer of seawater influence, which is particularly pronounced in the lava-dike transition zone and the sheeted dikes. The seawater influence is most prominent in a highly metal sulfide-enriched breccia layer encountered in the transition zone with 87Sr/86Sr of ~ 0.706, indicating a maximum water-rock mixing ratio of ~ 12. However, compared to the igneous section drilled at Site 504 (Hole 504B), which formed at intermediate, i.e., slower spreading rates at the Galapagos Spreading Center and hosting a much thicker sulfide-rich stockwork zone, the average intensity of water-rock interaction is lower. This is expressed by lesser mobility of base metals, narrower variability of alteration-sensitive incompatible elements, and less radiogenic Sr isotopic compositions on average at Site 1256. The amount of metal sulfide precipitation seems to be positively correlated with the degree of hydrothermal overprint. The less intense alteration of the Site 1256 transition zone, compared to Site 504, most likely reflects the higher rate of spreading, eventually resulting in a shorter period of time of continuous exposure to hydrothermal convection at the ridge crest. The observed seafloor alteration, leading to modified radiogenic parent/daughter ratios in the Site 1256 rocks, is ultimately not sufficient to develop time-integrative high 206Pb/204Pb and moderate 87Sr/86Sr ratios, as being characteristic of the HIMU (high μ = high 238U/204Pb) mantle signature proposed to originate from hydrothermally altered, subducted oceanic crust. Therefore, additional modification during the subduction process must be taken into account.

A comparative study between present and palaeo-heat flow in the Qiangtang Basin, northern Tibet, China

The Qiangtang Basin is a significant prospective area for hydrocarbon and gas hydrate resources in the Tibetan Plateau, China. However, relatively little work has been performed to characterise heat flow in this basin, which has restricted petroleum and gas hydrate exploration. In this study, we compare present and palaeo-heat flow in the Qiangtang Basin to provide information on geothermal regime, hydrocarbon generation and permafrost that is necessary for further petroleum and gas hydrate exploration. We base our study on temperature data from a thermometer well, thermal conductivity tests, vitrinite reflectance data, homogenisation temperature data from fluid inclusions, stratigraphic information and a time-independent modelling approach. Our results indicate that in the central Qiangtang Basin, the present thermal gradient is approximately 15.5 °C/km, and heat flow is approximately 46.69 mW/m2. Heat flow in the Qiangtang Basin is not relatively stable since the Early Jurassic, as previous research has suggested, and it is generally decreasing with time. Additionally, there is a clear difference between the hottest thermal regime of the southern and northern Qiangtang Depressions during Cretaceous to Pleistocene time. In the southern Qiangtang Depression, the palaeogeothermal gradient is approximately 32.0 °C/km, and palaeo-heat flow is approximately 70 mW/m2. However, in the northern Qiangtang Depression, the palaeogeothermal gradient exceeds 81.8 °C/km, and palaeo-heat flow is greater than 172.09 mW/m2. The high thermal regime in the northern Qiangtang Depression is driven mainly by hydrothermal convection. Gas reservoirs are possible targets for hydrocarbon exploration in this depression. Currently, the northwestern part of the northern Qiangtang Depression is the most favourable area for gas hydrate exploration in the Qiangtang Basin.

Determining the Maximum Depth of Hydrothermal Circulation Using Geothermal Mapping and Seismicity to Delineate the Depth to Brittle-Plastic Transition in Northern Honshu, Japan

This paper defines the maximum possible vertical extent of hydrothermal circulation in granitic crust, and thus the maximum depth within which geothermal reservoirs can be encountered. To evaluate prospective geothermal fields we constructed a geothermal database in northern Honshu, Japan that includes 571 points of thermal data of existing wells and hot springs. Depth-temperature curves were normalized by the Activity Index for three-dimensional extrapolation and a depth contour map of the 380 °C isotherm was plotted as an assumed brittle-plastic transition for granitic crust. Shallower-depth anomalies of the brittle-plastic transition on this map are closely coincident with the Quaternary volcanoes and their prospective geothermal fields. It should be noted that the bottom of the spatial distribution of seismicity in the volcanic fields shows strong correlation to the 380 °C isotherm. This result indicates reliability of the subsurface three-dimensional thermal map and suggests that the 380 °C isotherm strongly constrains the bottom surface of seismicity, fracturing and hydrothermal convection in granitic crust.

A scaling law for approximating porous hydrothermal convection by an equivalent thermal conductivity: theory and application to the cooling oceanic lithosphere

A scaling law for approximating porous hydrothermal convection by an equivalent thermal conductivity: theory and application to the cooling oceanic lithosphere

The significance of silica precipitation on the formation of the permeable–impermeable boundary within Earth's crust

AbstractThe hydrological system within Earth's crust is divided into the permeable zone and the underlying, much less permeable zone. In this study, we investigated the solubility and precipitation kinetics of silica in water under the conditions of the Kakkonda geothermal field, Japan, where well WD‐1a penetrates the boundary between the hydrothermal convection zone and the heat conduction zone. We found that quartz solubility has a local minimum at the depth of the hydrological boundary, at ~3100‐m depth (380 °C and 24 MPa), in the case of either hydrostatic conditions or fluid pressure increase above hydrostatic at deeper levels. The hydrothermal experiments reveal that rapid quartz precipitation occurs via nucleation when fluids are brought from the liquid region to the supercritical region. The preferential precipitation of quartz at a specific depth plays a significant role in forming and sustaining the permeable–impermeable boundary in the crust.

New evidence for persistent impact‐generated hydrothermal activity in the Miocene Ries impact structure, Germany

AbstractThe extent of impact‐generated hydrothermal activity in the 24 km sized Ries impact structure has been controversially discussed. To date, mineralogical and isotopic investigations point to a restriction of hydrothermal activity to the impact‐melt bearing breccias, specifically the crater‐fill suevite. Here, we present new petrographic, geochemical, and isotopic data of postimpact carbonate deposits, which indicate a hydrothermal activity more extended than previously assumed. Specifically, carbonates of the Erbisberg, a spring mound located upon the inner crystalline ring of the crater, show travertine facies types not seen in any of the previously investigated sublacustrine soda lake spring mounds of the Ries basin. In particular, the streamer carbonates, which result from the encrustation of microbial filaments in subaerial spring effluents between 60 and 70 °C, are characteristic of a hydrothermal origin. While much of the primary geochemical and isotopic signatures in the mound carbonates have been obliterated by diagenesis, a postimpact calcite vein from brecciated gneiss of the subsurface crater floor revealed a flat rare earth element pattern with a clear positive Eu anomaly, indicating a hydrothermal fluid convection in the crater basement. Finally, the strontium isotope stratigraphic correlation of the travertine mound with the crater basin succession suggests a hydrothermal activity for about 250,000 yr after the impact, which would be much longer than previously assumed.