Active Fluid Circulation Research Articles

New heat flow data on the South Balearic margin: Evidence of regional fluid circulation system

Crustal heat loss processes in contexts other than mid-ocean ridge flanks are still poorly understood and described in the literature. In this work, we present 54 new surface heat flow measurements recorded along the South Balearic margin and across the Hannibal High area, in the Western Mediterranean Sea, away from spreading centres. The South Balearic margin is a narrow continental margin expressed on the seabed topography by the steep Emile Baudot Escarpment while the Hannibal High area, further south, is a presumed volcanic oceanic crust zone. Recent heat flow studies pointed out the presence of strong local thermal anomalies, imprinted in regional deep basin heat flow increasing from the eastern to the western Algerian basins (70–90 to 100–130 mW/m2). The new data in this study confirm local anomalies around the Emile Baudot seamounts and allow the delineation of some low and high heat zones. We show that (1) a correlation exists between the heat flow variability and the sedimentary cover thickness, and (2) the most pronounced anomaly is a low heat flow zone (30 km wide) observed in the mid-slope South Formentera basin, close to the basaltic basement of the Los Martines volcano. The correlation of heat flow with sedimentary thickness, the lateral extent of the heat flow anomalies, the flux peak magnitudes, and the co-existence of both low and high anomalies suggest the presence of an active fluid circulation system. Based on the arrangement of the anomalies in relation to the basaltic basement seamounts, we consider a syphon-type circulation system in the Formentera basin. Reduced heat flow suggests a recharge zone close to the Los Martines seamount, while local high heat flow located close to a smaller seamount could be a potential zone of fluid discharge. Although different buoyancy-driven circulations could occur in the South Balearic margin, we believe that a syphon-type hydrothermal circulation along permeable basaltic mounds and sealed basement conduits, similar to the off-axis oceanic crust, is best developed in the young marginal basin of the South Balearic margin.

Magmatic and tectonic segmentation of the intermediate-spreading Costa Rica Rift—a fine balance between magma supply rate, faulting and hydrothermal circulation

SUMMARY 3-D tomographic modelling of wide-angle seismic data, recorded at the intermediate-spreading Costa Rica Rift, has revealed a P-wave seismic velocity anomaly low located beneath a small overlapping spreading centre that forms a non-transform discontinuity at the ridge axis. This low velocity zone displays a maximum velocity anomaly relative to the ‘background’ ridge axis crustal structure of ∼0.5 km s−1, has lateral dimensions of ∼10 × 5 km, and extends to depths ≥2.5 km below the seabed, placing it within layer 2 of the oceanic crust. We interpret these observations as representing increased fracturing under enhanced tectonic stress associated with the opening of the overlapping spreading centre, that results in higher upper crustal bulk porosity and permeability. Evidence for ongoing magmatic accretion at the Costa Rica Rift ridge axis takes the form of an axial magma lens beneath the western ridge segment, and observations of hydrothermal plume activity and microearthquakes support the presence of an active fluid circulation system. We propose that fracture pathways associated with the low velocity zone may provide the system through which hydrothermal fluids circulate. These fluids cause rapid cooling of the adjacent ridge axis and any magma accumulations which may be present. The Costa Rica Rift exists at a tipping point between episodic phases of magmatic and tectonically enhanced spreading. The characteristics inherited from each spreading mode have been preserved in the crustal morphology off-axis for the past 7 Myr. Using potential field data, we contextualize our seismic observations of the axial ridge structure at the whole segment scale, and find that the proposed balance between magmatic and tectonically dominated spreading processes observed off-axis may also be apparent along-axis, and that the current larger-scale magma supply system at the Costa Rica Rift may be relatively weak. Based on all available geophysical observations, we suggest a model for the inter-relationships between magmatism, faulting and fluid circulation at the Costa Rica Rift across a range of scales, which may also be influenced by large lithosphere scale structural and/or thermal heterogeneity.

Pinpointing Deep Geothermal Upflow in Zones of Complex Tectono‐Volcanic Degassing: New Insights from Aluto Volcano, Main Ethiopian Rift

AbstractActive rifts release large amounts of gases from deep sources to the atmosphere by advection and diffusion processes along permeable fracture zones. The objective of this study is to develop geothermal exploration concepts for areas with little or no hydrothermal surface expressions suitable for fluid sampling and analyses (e.g., hot springs, geysers, and fumaroles). In such areas, soil gas surveys can complement established geophysical and geochemical exploration. We report CO2, 222Rn (radon), and 220Rn (thoron) emission data and ground temperatures from the Aluto volcanic complex in the Main Ethiopian Rift to improve understanding of tectonic and volcanic controls on the existing geothermal system. This suite of gas emission measurements allows us to identify major, deep‐rooted permeable structures with active fluid circulation and identify suitable drilling targets for geothermal production wells on Aluto. We show that significant differences in gas signatures (i.e., efflux and spatial pattern) can be used to identify predominantly volcanically and/or tectonically influenced compartments. Major gas emissions indicate significant fluid circulation at depth, which is typical for magmatic systems. Such high gas emissions have been observed in areas affected by major tectonic structures interacting with magmatic bodies at depth (tectono‐volcanic). Predominantly fault‐controlled sectors also show hydrothermal fluid circulation, but to a lower extent compared to tectono‐volcanic sectors. Within the Aluto volcanic complex, geothermal production wells mainly target such fault‐controlled domains, whereas results of the study indicate strongest fluid circulation in tectono‐volcanic sectors. This result should be considered for the future exploration and development strategy of the site.

Clumped isotopologue constraints on the origin of methane at seafloor hot springs

Hot-spring fluids emanating from deep-sea vents hosted in unsedimented ultramafic and mafic rock commonly contain high concentrations of methane. Multiple hypotheses have been proposed for the origin(s) of this methane, ranging from synthesis via reduction of aqueous inorganic carbon (∑CO2) during active fluid circulation to leaching of methane-rich fluid inclusions from plutonic rocks of the oceanic crust. To further resolve the process(es) responsible for methane generation in these systems, we determined the relative abundances of several methane isotopologues (including 13CH3D, a “clumped” isotopologue containing two rare isotope substitutions) in hot-spring source fluids sampled from four geochemically-distinct hydrothermal vent fields (Rainbow, Von Damm, Lost City, and Lucky Strike).Apparent equilibrium temperatures retrieved from methane clumped isotopologue analyses average 310-42+53 °C, with no apparent relation to the wide range of fluid temperatures (96–370 °C) and chemical compositions (pH, [H2], [∑CO2], [CH4]) represented. Combined with very similar bulk stable isotope ratios (13C/12C and D/H) of methane across the suite of hydrothermal fluids, all available geochemical and isotopic data suggest a common mechanism of methane generation at depth that is disconnected from active fluid circulation. Attainment of equilibrium amongst methane isotopologues at temperatures of ca. 270–360 °C is compatible with the thermodynamically-favorable reduction of CO2 to CH4 at temperatures at or below ca. 400 °C under redox conditions characterizing intrusive rocks derived from sub-ridge melts. Collectively, the observations support a model where methane-rich aqueous fluids, known to be trapped in rocks of the oceanic lithosphere, are liberated from host rocks during hydrothermal circulation and perhaps represent the major source of methane venting with thermal waters at unsedimented hydrothermal fields. The results also provide further evidence that water-rock reactions occurring at temperatures lower than 200 °C do not contribute significantly to the quantities of methane venting at mid-ocean ridge hot springs.

Deep slab hydration induced by bending-related variations in tectonic pressure

Geophysical data reveal that at subduction zones oceanic plates could be pervasively hydrated for several kilometres below the crust–mantle boundary. Numerical experiments suggest that such deep hydration is facilitated by negative pressure gradients that lead to the downward pumping of water along bending-related normal faults. Bending of oceanic plates at subduction zones results in extension and widespread normal faulting1 in the upper, brittle part of the slab2,3. Detailed seismic surveys at trenches reveal that this part of the oceanic plate could be pervasively hydrated for several kilometres below the crust–mantle boundary4,5,6,7. Similarly, heat-flow surveys indicate active fluid circulation within the slab8. Yet, the mechanisms that enable fluids to percolate to such depths in spite of their natural buoyancy remain unclear. Here we use two-dimensional numerical experiments to show that stress changes induced by the bending oceanic plate produce subhydrostatic or even negative pressure gradients along normal faults, favouring downward pumping of fluids. The fluids then react with the crust and mantle surrounding the faults and are stored in the form of hydrous minerals. We suggest that this process is the dominant mechanism of deep slab hydration, although it may be locally aided by the enhancement in porosity due to prefailure dilatancy9, pre-existing cracks10 and migrating fluid-filled cracks11. Our results have implications for the transport of water into the deeper parts of the mantle12, and for further clarifying the seismic anisotropy of slabs13.

Geochronology and isotope transport systematics in a subsurface granite from the Larderello–Travale geothermal system (Italy)

A monzogranite, here referred to as the Montieri pluton, was encountered along two depth profiles from ca. 2 to 4 km depth in wells of the Larderello–Travale geothermal system, Italy. We obtained high-resolution petrographic and microchemical data on seven fresh samples of the Montieri pluton. These data are required to correctly interpret the combined isotope data for the Rb–Sr and K–Ar systems. Biotite and K-feldspar from all samples were analyzed by Rb–Sr and 39Ar– 40Ar. Rb–Sr analyses gave extremely variable two-point Rb–Sr apparent ages and Sr initials, indicating that biotite did not equilibrate with feldspar. Bulk dissolution of K-feldspars does not define a Rb–Sr alignment that could have proved a common origin of all K-feldspar samples from a single magma batch. The leachable fractions of K-feldspar separates have much more homogeneous 87Sr/ 86Sr ratios around 0.712, which may reflect the isotopic composition of a late-stage circulating fluid. Because of the anatectic origin of the granites from the Larderello–Travale geothermal system, it is likely that every K-feldspar separate contains at least three isotopically distinct feldspar generations: relics of the Hercynian gneiss basement, Pliocene magmatic minerals, and hydrothermal retrogression products. Such heterogeneity can be confirmed and quantified by petrographical observations and electron microprobe analyses. The 39Ar– 40Ar age spectra of the biotite samples show some internal discordance. Because of deviations from stoichiometry, biotite discordance can be attributed both to chlorite intergrowths, as predicted from the active fluid circulation, and to the presence of multiple biotite generations, such as have already been documented from other micas from the Larderello–Travale geothermal system and as indeed confirmed by high-resolution petrography. Total K–Ar biotite ages on cluster around 3 Ma and broadly agree with a Rb–Sr age obtained by regressing only the biotite analyses. This suggests that the Montieri pluton, whose known volume is at least 25 km 3, was emplaced at that time. The K-feldspar spectra are strongly discordant; no step age approaches the zero-age which would be predicted by Ar diffusivity modelling. The extraneous Ar in the feldspars is not excess Ar, but inherited Ar instead. Ar inheritance in the feldspar separates correlates with inheritance of Sr. Arrhenius trajectories of the apparent Ar diffusivities for all seven feldspars are astonishingly identical, despite the heterochemism amongst samples. However, the apparent diffusivity calculated from the step-heating experiments led to irreproducible models for the thermal history along the depth profile, as well as time-scales shorter than the historical record. This indicates that Ar diffusivities calculated from laboratory experiments must not be extrapolated to geological conditions.

Sequence Stratigraphic Context of Syndepositional Diagenesis in Cool-Water Shelf Carbonates: Pliocene Limestones, New Zealand

Abstract Pliocene limestones in eastern North Island, New Zealand, are cool-water deposits composed of barnacle–epifaunal bivalve–bryozoan dominated calcitic biota formed in shoal areas about the margins of a tectonically active forearc basin seaway. Vertical changes in the thickness and depositional facies of the limestones, and fundamentally the position and nature of internal bounding discontinuities within them, have enabled identification of high-order (likely 41 ka) transgressive (deepening-upwards) and transgressive–regressive (deepening-then shallowing-upwards) sequences within the formations. Syndepositional (or sea-floor or pre-compaction) diagenesis in the limestones involves a spectrum of destructive to constructive features, including skeletal abrasion, bioerosion, encrustation, dissolution, and cementation. Upward changes in the type and extent of these pre-compaction diagenetic features within the discontinuity-bounded sequences relate closely to the stacking patterns of depositional facies and their associated skeletal and mineralogical composition. Consequently, it has been possible to develop a series of predictive diagenetic models for cool-water shelf carbonates deposited upon and about upthrust antiforms in the forearc that integrate their patterns of syndepositional diagenesis into a sequence stratigraphy based framework driven ultimately by relative sea-level changes. Marine constructive diagenesis, characterized by bioencrustation, turbid cements, and interstratified internal sediments, occurs most commonly in those facies close to the sequence-bounding discontinuities, within the lower levels of transgressive systems tracts (TSTs) and the uppermost levels of regressive systems tracts (RSTs). These sediments equate to the highest-energy facies upon antiform tops, close to the wave abrasion base, where sedimentation rates were low. In contrast, destructive diagenetic processes typically dominate over constructive ones, often exclusively so, in the upper part of TSTs, in highstand systems tracts (HSTs), and in the lower portion of RST carbonates, where the sediments were not so influenced by active fluid circulation or strong reworking for prolonged periods, but were more rapidly buried within accommodation traps below wave abrasion base about the antiforms. The link between the facies motifs and synsedimentary diagenesis in these cool-water Pliocene limestones helps unlock their otherwise often cryptic, high-order sedimentary architecture. Such an interpretive opportunity is not afforded by the subsequent post-compaction diagenetic features in the limestones because these are no longer closely related to the sedimentation realm.

Fluxes of fluid and heat from the oceanic crustal reservoir

Recent discoveries define a global scale fluid reservoir residing within the uppermost igneous oceanic crust, a region of seafloor that is both warm and may harbor a substantial biosphere. This hydrothermal fluid reservoir formed initially within volcanic rocks newly erupted at mid-ocean ridges, but extends to the vastly larger and older ridge flanks. Upper oceanic crust is porous and permeable due to the presence of lava drainbacks, fissuring, and inter-unit voids, and this porosity and permeability allows active fluid circulation to advect measurable quantities of lithospheric heat from the crust to an average age of 65 Myr. A compilation of crustal porosities shows that this fluid reservoir contains nearly 2% of the total volume of global seawater. Heat flow and sediment thickness data allow calculation of reservoir temperatures, predicting 40°C mean temperatures in Cretaceous crust. Utilizing these temperature estimates, heat flow measurements and models for the thermal structure and evolution of the oceanic lithosphere, we have computed mean hydrothermal fluxes into the deep ocean as a function of plate age. The total hydrothermal volume flux into the oceans approaches 20% of the total riverine input and may contribute to the global seawater mass balance.

The origin of reservoir fluids in the geothermal field of Los Azufres, Mexico — isotopical and hydrological indications

The calculation of hydrological balance resulted in a potential, average annual infiltration rate of 446±206 mm/m 2 for the Los Azufres geothermal area, which corresponds to a total of 82×10 6 m 3 per a. Due to the highly fractured and faulted structure of the volcanic formations, a considerable potential for the infiltration of recent meteoric water into deeper sections of the volcanic formations can be assumed. Isotopic data indicate the minor importance of recent meteoric water for the recharge of the geothermal reservoir. Very negative δ 13C values can be explained by the input of organic C from the surface, but the lack of 14C in the deep fluids reflects a pre-historic age for the infiltration event of fossil meteoric water. The dilution of the meteoric water by 14C-free CO 2 gas from a shallow magma chamber complicates the exact age determination of the infiltration event, which probably occurred during the Late Pleistocene or Early Holocene glacial period. Strong water–rock interaction processes, such as sericitization/chloritization, caused the primary brine composition to be camouflaged. A preliminary hydrological model of the reservoir can be postulated as follows: the fossil hydrodynamic system was characterized by the infiltration of meteoric water and mixing with andesitic and/or magmatic water. Strong water–rock interaction processes in the main part of the production zone prove the existence of former active fluid circulation systems. Due to changes in pressure and temperature, the rising fluids get separated into liquid and vapour phases at a depth of 1500 m. After cooling, the main portions of both phases remain within the convective reservoir cycle. Isotope analyses of hot spring waters indicate the direct communication of the reservoir with the surface at some local outcrops. A recent reactivation of the hydrodynamic system is caused by the geothermal production, as indicated by the detection of lateral communication between some production and reinjection wells.

Freeze Substitution for EM-Studies Time Measuring.

Abstract We conducted physical and mathematical modelling of the process of substituion of ice with organic solvent. The result was a relationship between the time of freeze-substitution and the sample thickness. The model is based on the solution of mass exchange problem between a porous plate (frozen tissue) filled with a solid matter (ice) and an organic solvent (alcohol, acetone) that is contained in an apparatus with an active fluid circulation. The model takes in account porousness of tissue, dissolving of ice with solvent that produces a new medium, dissolving of that medium and its substitutions with solvent after solvent concentration exceeds evtectic concentration for the given temperature. Our results (fig. 1) agree with previously published results [1]. We also conducted experiments using original device [2] that implements freeze-substitution method. The device is made of the desk apparatus, comprising cryogenic and electron parts. Cryochamber ( Fig. 2 ) contains cylindrical unit (1), central channel (2) and located around it cylindrical vessels (3), connected by means of upper (4) and lower (5) channels with the central one. Perforated containers (6) with frozen bioobjects are adjusted on metal rack (7). The flux inducer (8) provides the circulation of substituting medium. The cold-conduct or (9) is submerged into a liquid nitrogen. Temperature control is performed be the thermogauge (10). The range of operating temperatures stabilization is from +20° to −120° C.

Mantle-derived volatiles in continental crust: the Massif Central of France

CO 2-rich gases and groundwaters from springs and boreholes originating within the basement of the Massif Central have variable 3He/ 4He ratios with corresponding R/R a values ranging from 0.8 to 5.5 and 0.3 to 2.8 respectively, indicating the presence of a significant component of mantle helium. Molar concentrations of rare gases in the CO 2-rich gases are approximately 5 orders of magnitude greater than in the waters and suggest that a near-surface Henry's Law fractionation has occurred between exsolving CO 2 and water. δ 13C values of the CO 2-rich gases are in the range −4.2 to −6.1‰, i.e. in that range normally attributed to mantle carbon, but which could also represent an average crustal composition and therefore do not discriminate between mantle and crustal sources. C/ 3He ratios show 4 orders of magnitude variation from 1.4 × 10 12 to 5 × 10 8 and, compared to a mantle C/ 3He ratio of ∼ 10 9, indicate that either a complex fractionation has occurred between mantle helium and mantle CO 2 or more likely that mantle rare gases have been diluted by large quantities of CO 2 with an average crustal carbon isotope composition. The regional distribution of 3He and C does not show any obvious relationship to age or proximity of volcanic centres or major faults, suggesting that mantle-derived C and He components decoupled from their silicate melt sources at some depth. The results from this area of active fluid circulation suggest that C-isotope data derived from metamorphic terrains should be interpreted with great caution, but that input of some mantle-derived carbon is expected to accompany crustal extension.