Hydrothermal Features Research Articles

Sulfur and oxygen isotope insights into sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece.

Shallow-sea (5 m depth) hydrothermal venting off Milos Island provides an ideal opportunity to target transitions between igneous abiogenic sulfide inputs and biogenic sulfide production during microbial sulfate reduction. Seafloor vent features include large (>1 m2) white patches containing hydrothermal minerals (elemental sulfur and orange/yellow patches of arsenic-sulfides) and cells of sulfur oxidizing and reducing microorganisms. Sulfide-sensitive film deployed in the vent and non-vent sediments captured strong geochemical spatial patterns that varied from advective to diffusive sulfide transport from the subsurface. Despite clear visual evidence for the close association of vent organisms and hydrothermalism, the sulfur and oxygen isotope composition of pore fluids did not permit delineation of a biotic signal separate from an abiotic signal. Hydrogen sulfide (H2S) in the free gas had uniform δ34S values (2.5 ± 0.28‰, n = 4) that were nearly identical to pore water H2S (2.7 ± 0.36‰, n = 21). In pore water sulfate, there were no paired increases in δ34SSO4 and δ18OSO4 as expected of microbial sulfate reduction. Instead, pore water δ34SSO4 values decreased (from approximately 21‰ to 17‰) as temperature increased (up to 97.4°C) across each hydrothermal feature. We interpret the inverse relationship between temperature and δ34SSO4 as a mixing process between oxic seawater and 34S-depleted hydrothermal inputs that are oxidized during seawater entrainment. An isotope mass balance model suggests secondary sulfate from sulfide oxidation provides at least 15% of the bulk sulfate pool. Coincident with this trend in δ34SSO4, the oxygen isotope composition of sulfate tended to be 18O-enriched in low pH (<5), high temperature (>75°C) pore waters. The shift toward high δ18OSO4 is consistent with equilibrium isotope exchange under acidic and high temperature conditions. The source of H2S contained in hydrothermal fluids could not be determined with the present dataset; however, the end-member δ34S value of H2S discharged to the seafloor is consistent with equilibrium isotope exchange with subsurface anhydrite veins at a temperature of ~300°C. Any biological sulfur cycling within these hydrothermal systems is masked by abiotic chemical reactions driven by mixing between low-sulfate, H2S-rich hydrothermal fluids and oxic, sulfate-rich seawater.

Distribution of buried hydrothermal alteration deduced from high‐resolution magnetic surveys in Yellowstone National Park

AbstractYellowstone National Park (YNP) displays numerous and extensive hydrothermal features. Although hydrothermal alteration in YNP has been extensively studied, the volume, geometry, and type of rock alteration at depth remain poorly constrained. In this study, we use high‐resolution airborne and ground magnetic surveys and measurements of remanent and induced magnetization of field and drill core samples to provide constraints on the geometry of hydrothermal alteration within the subsurface of three thermal areas in YNP (Firehole River, Smoke Jumper Hot Springs, and Norris Geyser Basin). We observe that hydrothermal zones from both liquid‐ and vapor‐dominated systems coincide with magnetic lows observed in aeromagnetic surveys and with a decrease of the amplitude of short‐wavelength anomalies seen in ground magnetic surveys. This suggests a strong demagnetization of both the shallow and deep substratum within these areas associated with the removal of magnetic minerals by hydrothermal alteration processes. Such demagnetization is confirmed by measurements of rock samples from hydrothermal areas which display significantly decreased total magnetization. A pronounced negative anomaly is observed over the Lone Star Geyser and suggests a significant demagnetization of the substratum associated with areas displaying large‐scale fluid flow. The ground and airborne magnetic surveys are used to evaluate the distribution of magnetization in the subsurface. This study shows that significant demagnetization occurs over a thickness of at least a few hundred meters in hydrothermal areas at YNP and that the maximum degree or maximum thickness of demagnetization correlates closely with the location of hydrothermal activity and mapped alteration.

U-Pb and 40Ar-39Ar Geochronology and Isotopic Constraints on the Genesis of Copper-Gold-Bearing Iron Oxide Deposits in the Hasancelebi District, Eastern Turkey

The Hasancelebi deposit in eastern Turkey, with proven reserves of 95 million metric tons, is a copper-gold-bearing iron oxide deposit. It is chiefly hosted by rocks that underwent widespread sodic-calcic and potassic styles of alteration. In the Hasancelebi district, pervasive Na-Ca and K-Fe alteration types are overprinted by sericitization and by late alteration that occur in multiple, overprinting systems. The age of hydrothermal alteration and mineralization (ca. between 74-68 Ma) overlaps with the age of alkaline magmatism in the Hasancelebi district. Crystallization and cooling of alkaline magmatism is associated with hydrothermal features that spanned the duration of the district’s igneous history. The 40Ar/39Ar and U-Pb geochronology of magmatism indicate that the hydrothermal system was synchronous with diabase and syenite and/or microsyenite porphyry intrusions and consisted of several discrete phases of Na-Ca and K-Fe alteration. The oldest alteration (phase 1) formed at ca 74.4 to 74.3 Ma; it is spatially and temporally associated with the emplacement of diabase dikes and contains scapolite and phlogopite. The next younger alteration (phase 2) formed at about 71.3 Ma and is spatially and temporally associated with syenite porphyry and microsyenite porphyry intrusions. It consists of scapolite, garnet, pyroxene, and actinolite that are superimposed on the first alteration phase. The next alteration assemblage (phase 3) formed at about 68.6 Ma and consists of phlogopite 2 and magnetite mineralization. Still later alteration (phase 4) overprints phases 1, 2, and 3 and consists of sericite-quartz, chalcopyrite, hematite, calcite, fluorite, and barite. Oxygen ( δ 18O) and hydrogen ( δ D) isotope analyses were carried out on phlogopite, sericite, barite, calcite, fluorite, and quartz representing the mineral assemblage formed during phases 1 and 4 of the hydrothermal alteration. The calculated δ 18O (H2O) and δ D (H2O) composition of the fluids that formed preore K-Fe alteration and postore alteration of phase 4 range between 15.1 to 8.5 and −124 to −85.6 per mil, respectively, and partially overlap the range for traditional magmatic waters. Oxygen and H isotope compositions from alteration minerals highlight two major points: Na-Ca and K-Fe alteration assemblages are likely derived from high-temperature primary magmatic fluids, and late sericitization is likely derived from fluids exsolved from cooling alkaline plutons during hydrothermal fracturing and consequent degassing at moderate temperatures. The initial ɛ Ndi values of the alteration zones range from −2.02 to −2.08, close to the calculated ɛ Nd value of associated syenite porphyry at the time of alteration ( ɛ Nd (68.64Ma; −1.9). Nd-Sr isotope compositions combined with 40Ar-39Ar and U-Pb geochronology of the associated alteration suggest that the fluids were sourced from coeval diabase dikes and syenite porphyry and/or microsyenite porphyry intrusions. The rare earth element characteristics, 40Ar-39Ar and U-Pb geochronology of alteration and fresh magmatic rocks, δ 18O, δ D, Nd, and Sr isotope systematics together suggest that the fluids responsible for the high- to moderate-temperature assemblages are largely derived from the coeval alkaline igneous rocks (diabase and microsyenite porphyry dikes) that intrudes the sedimentary-volcanosedimentary basin.

Geochronological and geochemical characterization of magmatic-hydrothermal events within the Southern Variscan external domain (Cévennes area, France)

Geochronological, mineralogical, and geochemical analyses have been focussed on the Mont-Loze're- Borne plutonic complex and surrounding rocks (Ce'vennes, French Massif Central) in which B-W-Sn and As-Au-Sb mineralization is encountered. Two main results are highlighted: (1) the existence of a 301-306 Ma magmatohydrothermal event unrelated to the emplacement of the Pont-de-Montvert-Borne plutonic body at 316 Ma; (2) the magmatic and hydrothermal features are strongly associated, both in time and in space, thus demonstrating an intimate connection between mineralizing processes and magmatism in this part of the French Massif Central. We also show that mineralization and associated hydrothermal occurrences do not correspond to a simple and single geochemical signature and that a contamination model must be invoked in order to account for the complexity of isotopic results. This study demonstrates that the application of the O and H isotopic signatures as tracers of the source and nature of fluids in an orogenic context requires some specific care. Finally, a model of the tectonic-magmatic- hydrothermal evolution of the study area is suggested in which we discuss two alternative scenarios. The first one implies the existence of two different hydrothermal/ mineralizing events (Bo-W-Sn and As-Au-Sb ones). The second one suggests the same source for all hydrothermal and mineralized structures.

Environmental review of geyser basins: resources, scarcity, threats, and benefits

The world’s geysers are known for providing one of nature’s most unusual water spectacles, with most sending water upwards of about 3 m, but a few “grand” or tall geysers erupt to heights of 60 m or more. A geyser basin is a composite resource made up of geysers and hot springs that cluster around a common hydrothermal reservoir, and might also contain other hydrothermal features like fumaroles and mud pots. The world’s remaining geyser basins are exceptionally rare, and are increasingly important recreation, economic, scientific, cultural, spiritual, therapeutic, and national heritage assets. Geyser basins harbor thermophilic microorganisms that contain heat-adapted biomolecules that can be harvested to provide vital components for DNA testing, and other high-temperature industrial processes. However, geysers and other surface hydrothermal features are increasingly vulnerable, and can be quickly and irreversibly damaged. Energy development projects quenched about 249 geysers, or about half of all geysers that were not protected in a national park or reserve. About 100 geysers were driven to extinction in New Zealand, about 46 in Iceland, and about 48 in the USA. Sustaining the world’s remaining geyser basins requires protecting the hydrothermal reservoirs that support them based on an integrated management approach that takes into account the distinctive characteristics of surface hydrothermal features and the benefits that society derives from them. A framework is provided that defines geyser basin resources, the scarcity of the remaining geysers, the environmental threats, and preservation benefits. Multiple case studies of mass geyser extinction are highlighted, including a summary of the landscape changes that can accompany geothermal energy development.

Protecting the Geyser Basins of Yellowstone National Park: Toward a New National Policy for a Vulnerable Environmental Resource

Geyser basins provide high value recreation, scientific, economic and national heritage benefits. Geysers are globally rare, in part, because development activities have quenched about 260 of the natural endowment. Today, more than half of the world's remaining geysers are located in Yellowstone National Park, northwest Wyoming, USA. However, the hydrothermal reservoirs that supply Yellowstone's geysers extend well beyond the Park borders, and onto two "Known Geothermal Resource Areas"-Island Park to the west and Corwin Springs on the north. Geysers are sensitive geologic features that are easily quenched by nearby geothermal wells. Therefore, the potential for geothermal energy development adjacent to Yellowstone poses a threat to the sustainability of about 500 geysers and 10,000 hydrothermal features. The purpose here is to propose that Yellowstone be protected by a "Geyser Protection Area" (GPA) extending in a 120-km radius from Old Faithful Geyser. The GPA concept would prohibit geothermal and large-scale groundwater wells, and thereby protect the water and heat supply of the hydrothermal reservoirs that support Yellowstone's geyser basins and important hot springs. Proactive federal leadership, including buyouts of private groundwater development rights, can assist in navigating the GPA through the greater Yellowstone area's "wicked" public policy environment. Moreover, the potential impacts on geyser basins from intrusive research sampling techniques are considered in order to facilitate the updating of national park research regulations to a precautionary standard. The GPA model can provide the basis for protecting the world's few remaining geyser basins.

Iron Skarns of the Vegas Peladas District, Mendoza, Argentina

The Andean belt southwest of Mendoza, Argentina, hosts 23 Fe, Fe-Cu, and Cu (Ag) deposits classified variously in the literature as skarn, iron oxide-copper-gold (IOCG), and manto-type Cu deposits. The Vegas Peladas deposit is one of the best exposed Fe skarns with mineral assemblages and hydrothermal features similar to many other calcic Fe skarns of the world. The plutonic rocks of Vegas Peladas consist of a series of diorite to granite stocks, dikes, and sills. The major-, trace-, and rare earth-element geochemistry analyses of these igneous rocks indicate they were derived from subarc mantle sources. The Vegas Peladas deposit formed by the overprinting of two different metamorphic and metasomatic events associated with early diorite and later granite intrusions. Alteration associated with the early diorite intrusion consists of a metamorphic halo (800 m wide) and a zoned calcic skarn with inner garnet (Ad31–89 Py0–2) + clinopyroxene + magnetite + quartz, intermediate garnet (Ad38–51 Py1–2) ± clinopyroxene, and distal veins of garnet (Ad92–100) ± clinopyroxene (Hd72–29 Jo1–4). The latest alteration consists of widespread albite (Ab96–98) ± epidote ± quartz ± calcite ± chlorite ± pyrite ± titanite. Magnetite and hematite are the main iron ore minerals and occur as massive orebodies and veins associated with retrograde epidote and amphibole. Alteration of the diorite consists of early orthoclase + quartz followed by later amphibole ± quartz ± magnetite ± epidote ± feldspar. The granite-related skarn overprints the earlier diorite-related skarn and consists of garnet + clinopyroxene + scapolite (Me28–36) ± quartz ± alkali feldspar endoskarn and a zoned exoskarn with proximal garnet ± clinopyroxene ± quartz, intermediate green garnet (Ad30–81 Py0–1) + clinopyroxene (Di82–93 Jo4–2), and distal scapolite (Me25–36) ± ferroactinolite ± pyrite veins. Based on fluid inclusion, stable isotope, and REE data, the prograde skarn formed at depths of ~ 3.5 km under lithostatic pressure of ~1 kbar, from high temperature (670°–400°C), saline and iron-rich (>50 wt % NaCl equiv, NaCl ± KCl ± FeCl2) magmatic fluids (garnet δ18OH2O = 7.2–8.5‰) with intermediate oxygen fugacity. Iron ore and retrograde exoskarn assemblages formed under hydrostatic condition after the fracturing of early skarn. Fluids in this stage had lower temperature (T 550°C) and also generated saline (30.3 to 45.3 wt % NaCl equiv, H2O-NaCl-FeCl2) + vapor fluids by immiscibility that redistributed some of the iron from the previous skarn.

Variable morphologic expression of volcanic, tectonic, and hydrothermal processes at six hydrothermal vent fields in the Lau back‐arc basin

Ultrahigh‐resolution bathymetric maps (25 cm grid) are used to quantify the physical dimensions of and spatial relationships between tectonic, volcanic, and hydrothermal features at six hydrothermal vent fields in the Lau back‐arc basin. Supplemented with near‐bottom photos, and nested within regional DSL‐120A side‐scan sonar data, these maps provide insight into the nature of hydrothermal systems along the Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR). Along‐axis transitions evident in localized volcanic morphology and tectonic characteristics include a change from broad low‐relief volcanic domes (hundreds of meters wide, &lt;10 m tall) that are dominated by pillow and lobate lava morphologies and are cut by faults and fissures to higher aspect ratio volcanic domes (tens of meters wide, tens of meters tall) dominated by aa‐type lava morphologies, with finger‐like flows, and few tectonic structures. These along‐axis differences in localized seafloor morphology suggest differences in hydrothermal circulation pathways within the shallow crust and correlate with regional transitions in a variety of ridge properties, including the large‐scale morphology of the ridge axis (shallow axial valley to axial high), seafloor lava compositions, and seismic properties of the upper crust. Differences in morphologic characteristics of individual flows and lava types were also quantified, providing an important first step toward the remote characterization of complex terrains associated with hydrothermal vent fields.

TERTIARY TILTING AND DISMEMBERMENT OF THE LARAMIDE ARC AND RELATED HYDROTHERMAL SYSTEMS, SIERRITA MOUNTAINS, ARIZONA

Multiple lines of evidence, including new and published geologic mapping and paleomagnetic and geobarometric determinations, demonstrate that the rocks and large porphyry copper systems of the Sierrita Mountains in southern Arizona were dismembered and tilted 50° to 60° to the south by Tertiary normal faulting. Repetition of geologic features and geobarometry indicate that the area is segmented into at least three major structural blocks, and the present surface corresponds to oblique sections through the Laramide plutonic-hydrothermal complex, ranging in paleodepth from ~1 to ~12 km. These results add to an evolving view of a north-south extensional domain at high angles to much extension in the southern Basin and Range, contrast with earlier interpretations that the Laramide systems are largely upright and dismembered by thrust faults, highlight the necessity of restoring Tertiary rotations before interpreting Laramide structural and hydrothermal features, and add to the broader understanding of pluton emplacement and evolution of porphyry copper systems.

Low‐cost aerial photography for high‐resolution mapping of hydrothermal areas in Yellowstone National Park

A site‐specific high‐resolution aerial survey was conducted to map a status quo of two rapidly changing hydrothermal areas in Yellowstone National Park. A 2 m3 helium balloon released to 50–80 m altitude served as a low‐cost platform easy to launch and retract on a highly flexible operating schedule. Pictures were taken with a digital camera with automatic interval shutter release. Owing to the balloon's sensitivity towards wind and the limited possibility of controlling its accurate positioning, the acquired aerial pictures varied significantly in altitude, tilt, and orientation. Automatic stitching was possible in areas where nearly vertical pictures with sufficient overlap existed. Those aerial pictures with little overlap or taken at a highly oblique angle had to be manually post‐processed based on temporary marked ground control points and additional tie points (total time for processing: 10–14 days) to obtain a final mosaic with minimal distortion (0.4 m±0.3 m) and an absolute accuracy of approximately 0.5 m. Ground resolution was better than 2 cm. Vegetation and hydrothermal feature classification, biological and redox zonations, water contents of mud pots, gas activity in hot springs as well as subaquatic sedimentation fans and hydrothermal vents could be outlined, providing a highly detailed thermal inventory for future change detection.

Identification of quartz and carbonate minerals across northern Nevada using ASTER thermal infrared emissivity data—Implications for geologic mapping and mineral resource investigations in well-studied and frontier areas

ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) thermal infrared imagery over a 389 km × 387 km area in northern Nevada (38.5°–42°N, 114°–118.5°W) was analyzed to evaluate its capability for accurate and cost-effective identification and mapping of quartz and carbonate minerals at regional to local scales. The geology of this area has been mapped at a wide range of scales and includes a diversity of rock types and unconsolidated surficial materials, many of which are composed primarily of quartz and carbonate minerals. This area is also endowed with a wide variety of economically and scientifically important ore deposit types that contain an array of commodities (Au, Ag, Pb, Zn, Cu, Mo, W, Sn, Be, F, Mn, Fe, Sb, Hg, and barite). The hydrothermal systems that generated these deposits frequently deposited large amounts of quartz where fluids cool, and generally smaller amounts of calcite or dolomite by other mechanisms. To identify and map quartz and carbonate minerals, band ratioing techniques were developed based on the shapes of laboratory reference spectra and applied to ASTER Level 2 surface emissivity products of 108 overlapping scenes. These mineral maps were mosaicked into a single coverage that was overlain with published, vector-format geologic maps of various scales to determine which geologic terranes, formations, and geomorphic features correspond to identified quartz or carbonate. Where quartz or carbonate minerals were mapped in rocks composed primarily of other minerals, they were inferred to be hydrothermal in origin and compared to known occurrences of hydrothermal alteration and mineralization. The ASTER-based quartz mapping identified thick sequences of quartzite, bedded radiolarian chert, quartz sandstone, conglomerates with clasts of quartzite and chert, silicic and/or altered rhyolites, and silicic welded tuffs. Alluvial fan surfaces, sand dunes, and beach deposits composed of quartz and/or carbonate are prominent, well-mapped features. Quartz was also identified in smaller bodies of jasperoid, quartz-alunite, and quartz-sericite-pyrite alteration, hot spring silica sinter terraces, and several diatomite and perlite mines and prospects. The ASTER-based carbonate mapping identified thick sequences of dolomite, limestone, and marble, as well as small hot spring travertine deposits. Eolian carbonate was identified in several playas. Dolomite exhibited a stronger carbonate response than calcite, as predicted based on their thermal spectral characteristics. Quartz was detected at lower concentrations than carbonates because of the greater strength of the quartz reststrahlen features in the thermal infrared compared to the bending-related spectral features of carbonates. The 90 m ground pixel size of the ASTER thermal imagery prevents the identification of small-scale features. Despite this limitation, numerous bodies of hydrothermal quartz were detected in or near known Carlin-type gold deposits, distal disseminated Au-Ag deposits, high- and low-sulfidation epithermal Au-Ag deposits, and geothermal areas. Detection of hydrothermal carbonate was rare and mainly in geothermal areas. The ASTER-based thermal quartz and carbonate mapping demonstrated here can be used in well-studied or frontier areas to verify the accuracy of existing geologic maps, guide future detailed stratigraphic and structural mapping in lithologically complex terranes and allochthons, and identify hydrothermal features for exploration and resource assessment purposes.

Biogenic mineral production by a novel arsenic-metabolizing thermophilic bacterium from the Alvord Basin, Oregon.

The Alvord Basin in southeast Oregon contains a variety of hydrothermal features which have never been microbiologically characterized. A sampling of Murky Pot (61 degrees C; pH 7.1) led to the isolation of a novel arsenic-metabolizing organism (YeAs) which produces an arsenic sulfide mineral known as beta-realgar, a mineral that has not previously been observed as a product of bacterial arsenic metabolism. YeAs was grown on a freshwater medium and utilized a variety of organic substrates, particularly carbohydrates and organic acids. The temperature range for growth was 37 to 75 degrees C (optimum, 55 degrees C), and the pH range for growth was 6.0 to 8.0 (optimum, pH 7.0 to 7.5). No growth was observed when YeAs was grown under aerobic conditions. The doubling time when the organism was grown with yeast extract and As(V) was 0.71 h. Microscopic examination revealed Gram stain-indeterminate, non-spore-forming, nonmotile, rod-shaped cells, with dimensions ranging from 0.1 to 0.2 microm wide by 3 to 10 microm long. Arsenic sulfide mineralization of cell walls and extracellular arsenic sulfide particulate deposition were observed with electron microscopy and elemental analysis. 16S rRNA gene analysis placed YeAs in the family Clostridiaceae and indicated that the organism is most closely related to the Caloramator and Thermobrachium species. The G+C content was 35%. YeAs showed no detectable respiratory arsenate reductase but did display significant detoxification arsenate reductase activity. The phylogenetic, physiological, and morphological characteristics of YeAs demonstrate that it is an anaerobic, moderately thermophilic, arsenic-reducing bacterium. This organism and its associated metabolism could have major implications in the search for innovative methods for arsenic waste management and in the search for novel biogenic mineral signatures.

Submeter bathymetric mapping of volcanic and hydrothermal features on the East Pacific Rise crest at 9°50′N

Recent advances in underwater vehicle navigation and sonar technology now permit detailed mapping of complex seafloor bathymetry found at mid‐ocean ridge crests. Imagenex 881 (675 kHz) scanning sonar data collected during low‐altitude (∼5 m) surveys conducted with DSV Alvin were used to produce submeter resolution bathymetric maps of five hydrothermal vent areas at the East Pacific Rise (EPR) Ridge2000 Integrated Study Site (9°50′N, “bull's‐eye”). Data were collected during 29 dives in 2004 and 2005 and were merged through a grid rectification technique to create high‐resolution (0.5 m grid) composite maps. These are the first submeter bathymetric maps generated with a scanning sonar mounted on Alvin. The composite maps can be used to quantify the dimensions of meter‐scale volcanic and hydrothermal features within the EPR axial summit trough (AST) including hydrothermal vent structures, lava pillars, collapse areas, the trough walls, and primary volcanic fissures. Existing Autonomous Benthic Explorer (ABE) bathymetry data (675 kHz scanning sonar) collected at this site provide the broader geologic context necessary to interpret the meter‐scale features resolved in the composite maps. The grid rectification technique we employed can be used to optimize vehicle time by permitting the creation of high‐resolution bathymetry maps from data collected during multiple, coordinated, short‐duration surveys after primary dive objectives are met. This method can also be used to colocate future near‐bottom sonar data sets within the high‐resolution composite maps, enabling quantification of bathymetric changes associated with active volcanic, hydrothermal and tectonic processes.

Protist Genetic Diversity in the Acidic Hydrothermal Environments of Lassen Volcanic National Park, USA

We examined eukaryote genetic diversity in the hydrothermal environments of Lassen Volcanic National Park (LVNP), Northern California. We sampled hydrothermal areas of the Bumpass Hell, Sulfur Works, Devil's Kitchen, and Boiling Springs Lake sites, all of which included diverse acidic pools, mud pots, and streams with visible algal mats and biofilms. Temperatures varied from 15 to 85 degrees C and pH from 1.7 to 5.8. DNA extraction methods compared by denaturing gradient gel electrophoresis fingerprinting exhibited similar patterns, and showed limited diversity of eukaryotic small subunit (SSU) rRNA genes compared with prokaryotes. We successfully amplified eukaryotic SSU rRNA genes from most environments up to 68 degrees C. Cloned rDNA sequences reveal acidophilic protists dominate eukaryotes in LVNP hydrothermal environments. Most sites showed phototrophic assemblages dominated by chlorophytes and stramenopiles (diatoms and chrysophytes). Heterotrophic taxa, though less abundant, included diverse alveolates (ciliates), amoebae, and flagellates. Fungi were also found at most sites, and metazoans (hexapods, nematodes, platyhelminths) were sometimes detected in less acidic environments, especially in algal mats. While many cloned rDNA sequences showed 95%-99% identity to known acidophilic isolates or environmental clones from other acidic sites (Rio Tinto), sequence diversity generally declined both with decreasing pH and increasing temperature, and both were controlling physical variables on the abundance and distribution of organisms at our sites. However, a pool at 68 degrees C with pH 1.7 yielded the greatest number of distinct sequences. While some were likely contaminants from nearby cooler sites, we suggest that Lassen's acidic hydrothermal features may harbor novel protists.

Fine-scale segmentation of volcanic/hydrothermal systems along fast-spreading ridge crests

Fine-scale (3rd and 4th order) segments of the mid-ocean ridge (MOR) are morphologically defined units of crustal accretion with enigmatic origins. At fast-spreading centers, 3rd order segments are bounded by discontinuities in the structure of the axial high. Most 3rd order segments are 20±10 km long, and terminate at ridge axis discontinuities (RADs) exhibiting 0.2–3 km lateral offsets and/or ≥20 m axial depth increases. Nested within 3rd order segments are shorter 4th order segments bounded by smaller RADs. Along the fast-spreading East Pacific Rise (EPR) at 10°02′–9°08′N and 17°11′–42′S, spatial distributions of active high-temperature hydrothermal vents, biological communities, and lava flow morphology are known accurately from extensive near-bottom surveys and submersible dives along a total of seven 3rd order segments and twenty-four 4th order segments. The cumulative northern and southern EPR data show that 3rd order segments (but not 4th order segments) consistently exhibit apparently higher rates of lava effusion near segment middles in comparison to segment ends, supporting the hypothesis that each 3rd order segment is a discrete, centrally fed volcanic system, or “volcanic segment”. Spatial analyses of the cumulative hydrothermal data reveal a striking increase in abundance of hydrothermal features within the middle portions of 3rd order segments. Approximately 60% of ongoing high-temperature focused flow is concentrated within the middle 40% of 3rd order segments, and twice as many biological communities are found at mid-segment in comparison to segment ends. The two independent data sets—volcanic and hydrothermal—both indicate that magmatic heat sources are focused beneath the central portions of 3rd order segments, and that magmatic/hydrothermal systems are spatially and temporally segmented at 3rd order length and time scales. We propose that 3rd order segmentation arises from processes controlling melt supply to the middle-to-lower crust; and, that 4th order segmentation largely arises from more rapid/frequent upper crust processes of crack formation and dike intrusion. Future work is needed to test the applicability of our results along portions of the fast-spreading EPR notched by fault-bounded axial troughs, and in areas farther south where spreading rate reaches a maximum.

Palaeoproterozoic, rift-related, 13C-rich, lacustrine carbonates, NW Russia. Part I: Sedimentology and major element geochemistry

ABSTRACTThe 150-m-thick Kuetsjärvi Sedimentary Formation (KSF) from the Pechenga Greenstone Belt, NW Russia, is one of the key formations in the study of a positive δ13Ccarb excursion occurring globally in the Palaeoproterozoic. The KSF formed in an intracratonic rift setting and is sandwiched between two, 2-km-thick subaerially erupted volcanic units. The KSF was previously interpreted as shallow marine, but new data reported here indicate that it is a non-marine unit deposited on a deeply subaerially weathered surface mantling the underlying volcanic rocks. The lowermost part of the KSF represents an alluvial–fluvial plain, followed by a laterally and vertically variable succession of variegated to mottled fine-grained siliciclastic rocks and ‘red beds‘, dolostones containing stromatolite sheets, hydrothermal travertine deposits and abundant desiccation features (e.g.tepees, surfical silicified crusts and dissolution cavities), including probable pseudomorphed evaporites. Measured S and Corg concentrations for the carbonate and siliciclastic rocks are low. Combined, these features indicate that the carbonate rocks of the KSF accumulated in a shallow lacustrine setting. Major types of carbonate facies were formed by: (1) biologically-induced precipitation; (2) evaporitic removal of CO2 in a closed lake environment; and (3) chemical precipitation from thermal springs. Apparently, none of these carbonate facies was in full isotopic equilibrium with atmospheric CO2. This interpretation shows the importance of taking into account the interplay between global and local depositional factors when interpreting the isotopic signature of the KSF dolostones and its implication for the Palaeoproterozoic carbon isotope excursion

Modelling temperature distributions and radon emission at Stromboli Volcano using a non-extensive statistical approach

Studying the hydrothermal features in a volcanic field is an important step in understanding eruption mechanisms as well as volcanically induced earthquakes. These processes can occur cyclically in a volcanic systems and could be explained by modern thermo-fluidodynamics theory. A computer model has been performed on the basis of field measurements. Radon emission are strictly correlated with magma degassing, the local temperature gradient, rock porosities and permeabilities. Anomalies in radon emissions at the surface can provide critical informations about the geothermal system and about the distribution of fracture networks in the volcanic structure. We therefore describe radon migration from the magma column and from the magma chamber in terms of a bi-dimensional model, in order to explain relative radon peak emissions by using a non-extensive statistical approach. The results of the model, compared with the collected field data, are hereby presented and discussed.

Structural, seismic and hydrothermal features at the border of an active intermittent resurgent block: Ischia Island (Italy)

Resurgent blocks are uplifted parts of volcanoes where most of the deformation (uplift) is accommodated along peripheral faults. The island of Ischia, Italy, has an active resurgent block uplifted ∼1000 m in the last ∼30 ka, which allows us to investigate the deformation associated with a strong uplift in volcanic areas. Air-photo interpretation was matched with field work (1:5000 scale mapping and structural analysis) throughout the resurgent block. These results have been matched with a review of the seismic and hydrothermal activity at Ischia. The results show that the block has a polygonal shape in map view and is characterized by a trapdoor uplift, which results in a ∼15° tilt towards the SE. The northern, most uplifted part is bordered by E–W- and NE–SW-trending faults, arranged in parallel segments over an area ∼1.5 km wide. I 0≥5 MCS degree shallow (1–1.6 km depth) earthquakes focused in correspondence with the outermost set of faults on the northernmost part of the block. Here, variations in the hydrothermal activity have been observed during seismic events. Previously performed analogue models show that resurgent blocks are bordered by inward-dipping high-angle reverse faults (outermost fault system) and outward-dipping normal faults (innermost fault system). We propose a model of trapdoor uplift of a block which induces reverse faulting, responsible for seismicity and hydrothermal variations along the most uplifted part. Outward-dipping normal faults, in the innermost part, form as a consequence of the gravitational sliding of the volumes bordered by the reverse faults. The further reactivation of the outermost reverse faults triggers a new cycle of intermittent trapdoor uplift of the block.

GENESIS OF HIGH-GRADE HEMATITE OREBODIES OF THE HAMERSLEY PROVINCE, WESTERN AUSTRALIA—A DISCUSSION

Sir: Taylor et al. (2001) have presented a hypogene-supergene model for the genesis of the premium martite-microplaty hematite ores derived from banded iron formation (BIF), to replace the supergene-metamorphic model (Morris, 1980, 1985, 1998; Harmsworth et al., 1990). The new model is based on hydrothermal features in two of ten Tom Price hematite deposits, recorded previously as carbonate metasomatism of BIF (Ewers and Morris, 1981), and as localized postore metasomatism (Harmsworth et al., 1990; Morris, 1998). From new data, Taylor et al. (2001) proposed that hypogene leaching of free silica with addition of further siderite in BIF, followed by compaction, produced a general hematite ore precursor. During deep weathering episodes, or even within Tertiary weathering profiles, oxidation of magnetite to martite, and siderite to microplaty hematite and ankerite, followed by leaching of residual gangue, enriched this modified BIF to martite-microplaty hematite ore. In contrast, in the supergene-metamorphic model, martite-microplaty hematite ore genesis is linked to metamorphism of precursor ores of the Phanerozoic supergene martite-goethite type. These comprise about 90 percent of the Hamersley BIF-hosted resource, and deposits range to over a billion tonnes (Harmsworth et al., 1990) and >250 m deep, with high phosphorus levels (0.07–0.17% P). Although stratigraphically thinned by leaching (~35%) they maintain primary BIF banding and texture, with gangue minerals altered to phosphorus-bearing goethite pseudomorphs. Their martite-microplaty hematite analogues (< 0.06 % P) are attributed to Proterozoic supergene enrichment of BIF, with burial metamorphism partly converting the goethite pseudomorphs to secondary, largely microplaty, hematite, yet preserving the primary banding. Some 20 known deposits of this type range from small pockets of enrichment to the massive Mount Whaleback (1700 Mt). Later re-exposures of the microplaty hematite deposits produced further stratigraphic thinning (total ~45–50 %), with loss of phosphorus by leaching of remnant goethite. However, significant …

Remnants of an Early Archaean (>3.75 Ga) sea-floor, hydrothermal system in the Isua Greenstone Belt

Localised low-strain domains in the ∼3.75 Ga Isua Greenstone Belt, West Greenland, contain a wealth of well-preserved primary structures. These include pillow lavas with dark, fine-grained cooling rims, pillow breccias, debris flows and conglomerates. Here we describe quartz globules from a relatively undeformed pillow breccia, which contains unstrained quartz. Most quartz globules contain only few quartz grains, but some of the larger ones preserve a subidiomorphic rim reminiscent of former geodic lining. The globules are associated with a complex system of quartz veins. Metamorphic conditions recorded in pelites associated with the pillow breccia represent the early stage of a prograde event at 460–480 °C and ∼4 kb. Both globules and quartz veins pre-date the metamorphic fabric, and are interpreted as remnants of a sea-floor type hydrothermal system, operating at the time of lava eruption and pillow formation at ∼3.75 Ga. The globules, which are interpreted as former gas vesicles filled with quartz (and some carbonate), are embedded in an altered basaltic matrix now comprising biotite, muscovite and quartz±tourmaline and carbonate. Silica-filling in the vesicles was contemporaneous with the formation of an intricate hydrothermal vein system. During deformation, the strain was partitioned into the mica-rich rock matrix, when the vesicles behaved as competent objects, eventually cutting or deforming the thinner veins. Amphibolite-facies metamorphic overprint involved a limited flattening of some vesicles and the development of a very faint, sporadic schistosity, preserving most of the pre-metamorphic hydrothermal features. Major metamorphic structures correspond to quartz annealing, yielding curved grain boundaries and triple point junctions. We report rare primary fluid inclusions from quartz in the vesicles. The inclusions contain remnants of two independent fluid/mineral systems comprising pure methane and highly saline aqueous fluids (about 25 wt.% NaCl equivalent), and co-precipitating carbonates (calcite). Especially for the aqueous system, these fluids bear a strong resemblance to present-day sea-floor hydrothermal fluids. They are interpreted as relics of the sea-floor hydrothermal fluid, responsible for the alteration of the pillow fragments and co-precipitation of quartz and carbonate.