Geology topics

Wayne C. Shanks

Publications and source records attributed to Wayne C. Shanks.

At least 37 records · Page 2Linked to original sources

USGS2007

Hydrothermal nontronite formation at Eolo Seamount (Aeolian volcanic arc, Tyrrhenian Sea)

A sediment core containing a yellowish-green clay bed was recovered from an area of extensive hydrothermal deposition at the SE slope of the Eolo Seamount, Tyrrhenian Sea. The clay bed is composed of pure nontronite (described for the first time in the Tyrrhenian Sea), which appears to be the most aluminous nontronite ever found among the seafloor hydrothermal deposits. The high Al content suggests precipitation from Al-containing hydrothermal solutions. The REE distribution of the Eolo nontronite has a V-shape pattern. The heavy REE enrichment is in part due to their preferential partitioning in the nontronite structure. This enrichment was possibly further enhanced by the HREE preferential sorption on bacterial cell walls. The light REE enrichment is the result of scavenging uptake by one of the nontronite precursors, i.e., poorly-ordered Fe-oxyhydroxides, from the hydrothermal fluids. Oxygen isotopic composition of the nontronite yields a formation temperature of 30????C, consistent with a low-temperature hydrothermal origin. The relatively radiogenic Nd isotopic signature of the nontronite compared to the present-day Mediterranean seawater indicates that approximately half of Nd, and presumably the rest of the LREE, are derived from local volcanic sources. On the other hand, 87Sr/86Sr is dominated by present-day seawater Sr. Scanning electron microscopy investigation revealed that the nontronite is composed of aggregates of lepispheres and tube-like filaments, which are indicative of bacteria assisted precipitation. Bacteria inhabiting this hydrothermal site likely acted as reactive geochemical surfaces on which poorly-ordered hydrothermal Fe-oxyhydroxides and silica precipitated. Upon aging, the interactions of these primary hydrothermal precipitates coating bacterial filaments and cell walls likely led to the formation of nontronite. Finally, the well-balanced interlayer and layer charges of the crystal lattice of seafloor hydrothermal nontronite decrease its sorption capacity to zero. Thus the ubiquitous nontronite precipitation along the active plate boundaries and around the hot spots has no significant impact on oceanic trace element chemistry. ?? 2007 Elsevier B.V. All rights reserved.

Chemical Geology↗

USGS2007

S-33 constraints on the seawater sulfate contribution in modern seafloor hydrothermal vent sulfides

Sulfide sulfur in mid-oceanic ridge hydrothermal vents is derived from leaching of basaltic-sulfide and seawater-derived sulfate that is reduced during high temperature water rock interaction. Conventional sulfur isotope studies, however, are inconclusive about the mass-balance between the two sources because 34S/32S ratios of vent fluid H2S and chimney sulfide minerals may reflect not only the mixing ratio but also isotope exchange between sulfate and sulfide. Here, we show that high-precision analysis of S-33 can provide a unique constraint because isotope mixing and isotope exchange result in different ??33S (?????33S-0.515 ??34S) values of up to 0.04??? even if ??34S values are identical. Detection of such small ??33S differences is technically feasible by using the SF6 dual-inlet mass-spectrometry protocol that has been improved to achieve a precision as good as 0.006??? (2??). Sulfide minerals (marcasite, pyrite, chalcopyrite, and sphalerite) and vent H2S collected from four active seafloor hydrothermal vent sites, East Pacific Rise (EPR) 9-10??N, 13??N, and 21??S and Mid-Atlantic Ridge (MAR) 37??N yield ??33S values ranging from -0.002 to 0.033 and ??34S from -0.5??? to 5.3???. The combined ??34S and ??33S systematics reveal that 73 to 89% of vent sulfides are derived from leaching from basaltic sulfide and only 11 to 27% from seawater-derived sulfate. Pyrite from EPR 13??N and marcasite from MAR 37??N are in isotope disequilibrium not only in ??34S but also in ??33S with respect to associated sphalerite and chalcopyrite, suggesting non-equilibrium sulfur isotope exchange between seawater sulfate and sulfide during pyrite precipitation. Seafloor hydrothermal vent sulfides are characterized by low ??33S values compared with biogenic sulfides, suggesting little or no contribution of sulfide from microbial sulfate reduction into hydrothermal sulfides at sediment-free mid-oceanic ridge systems. We conclude that 33S is an effective new tracer for interplay among seawater, oceanic crust and microbes in subseafloor hydrothermal sulfur cycles. ?? 2006 Elsevier Inc. All rights reserved.

Geochimica et Cosmochimica Acta↗

USGS2007

Oxygen and sulfur isotope systematics of sulfate produced by bacterial and abiotic oxidation of pyrite

To better understand reaction pathways of pyrite oxidation and biogeochemical controls on ??18O and ??34S values of the generated sulfate in acid mine drainage (AMD) and other natural environments, we conducted a series of pyrite oxidation experiments in the laboratory. Our biological and abiotic experiments were conducted under aerobic conditions by using O2 as an oxidizing agent and under anaerobic conditions by using dissolved Fe(III)aq as an oxidant with varying ??18OH2O values in the presence and absence of Acidithiobacillus ferrooxidans. In addition, aerobic biological experiments were designed as short- and long-term experiments where the final pH was controlled at ???2.7 and 2.2, respectively. Due to the slower kinetics of abiotic sulfide oxidation, the aerobic abiotic experiments were only conducted as long term with a final pH of ???2.7. The ??34SSO4 values from both the biological and abiotic anaerobic experiments indicated a small but significant sulfur isotope fractionation (???-0.7???) in contrast to no significant fractionation observed from any of the aerobic experiments. Relative percentages of the incorporation of water-derived oxygen and dissolved oxygen (O2) to sulfate were estimated, in addition to the oxygen isotope fractionation between sulfate and water, and dissolved oxygen. As expected, during the biological and abiotic anaerobic experiments all of the sulfate oxygen was derived from water. The percentage incorporation of water-derived oxygen into sulfate during the oxidation experiments by O2 varied with longer incubation and lower pH, but not due to the presence or absence of bacteria. These percentages were estimated as 85%, 92% and 87% from the short-term biological, long-term biological and abiotic control experiments, respectively. An oxygen isotope fractionation effect between sulfate and water (??18 OSO4 s(-) H2 O) of ???3.5??? was determined for the anaerobic (biological and abiotic) experiments. This measured ??18 OSO42 - s(-) H2 O value was then used to estimate the oxygen isotope fractionation effects (??18 OSO42 - s(-) O2) between sulfate and dissolved oxygen in the aerobic experiments which were -10.0???, -10.8???, and -9.8??? for the short-term biological, long-term biological and abiotic control experiments, respectively. Based on the similarity between ??18OSO4 values in the biological and abiotic experiments, it is suggested that ??18OSO4 values cannot be used to distinguish biological and abiotic mechanisms of pyrite oxidation. The results presented here suggest that Fe(III)aq is the primary oxidant for pyrite at pH < 3, even in the presence of dissolved oxygen, and that the main oxygen source of sulfate is water-oxygen under both aerobic and anaerobic conditions. ?? 2007 Elsevier Ltd. All rights reserved.

Geochimica et Cosmochimica Acta↗

USGS2007

Integrated Geologic, Geochemical, and Geophysical Studies of Big Bend National Park, Texas

Introduction Big Bend National Park (BBNP), Texas, covers 801,163 acres (3,242 km2) and was established in 1944 through a transfer of land from the State of Texas to the United States. The park is located along a 118-mi (190-km) stretch of the Rio Grande at the United States border with Mexico. The U.S. Geological Survey (USGS) began a 5-year project in 2003 with the objective of studying a number of broad and diverse geologic, geochemical, and geophysical topics in BBNP. This fact sheet describes results of some of the research by USGS scientists working in BBNP.

Fact Sheet↗

USGS2007

Geochemical and sulfur-isotopic signatures of volcanogenic massive sulfide deposits on Prince of Wales Island and vicinity, southeastern Alaska

Stratabound volcanogenic massive sulfide (VMS) deposits on Prince of Wales Island and vicinity, southeastern Alaska, occur in two volcanosedimentary sequences of Late Proterozoic through Cambrian and of Ordovician through Early Silurian age. This study presents geochemical data on sulfide-rich samples, in situ laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of sulfide minerals, and sulfur-isotopic analyses of sulfides and sulfates (barite) for identifying and distinguishing between primary sea-floor signatures and later regional metamorphic overprints. These datasets are also used here in an attempt to discriminate the VMS deposits in the older Wales Group from those in the younger Moira Sound unit (new informal name). The Wales Group and its contained VMS deposits have been multiply deformed and metamorphosed from greenschist to amphibolite grade, whereas the Moira Sound unit and related VMS deposits are less deformed and generally less metamorphosed (lower to middle greenschist grade). Variations in the sulfide mineral assemblages and textures of the VMS deposits in both sequences reflect a combination of processes, including primary sea-floor mineralization and sub-sea-floor zone refining, followed by metamorphic recrystallization. Very coarse grained (>1 cm diam) sulfide minerals and abundant pyrrhotite are restricted to VMS deposits in a small area of the Wales Group, at Khayyam and Stumble-On, which record high-grade metamorphism of the sulfides. Geochemical and sulfur-isotopic data distinguish the VMS deposits in the Wales Group from those in the Moira Sound unit. Although base- and precious-metal contents vary widely in sulfide-rich samples from both sequences, samples from the Moira Sound generally have proportionately higher Ag contents relative to base metals and Au. In situ LA-ICP-MS analysis of trace elements in the sulfide minerals suggests that primary sea-floor hydrothermal signatures are preserved in some samples (for example, Mn, As, Sb, and Tl in pyrite from the Moira Sound unit), whereas in other samples the signatures are varyingly annealed, owing to metamorphic overprinting. A limited LA-ICP-MS database for sphalerite indicates that low-Fe sphalerite is preferentially associated with the most Au rich deposits, the Niblack and Nutkwa. Sulfur-isotopic values for sulfide minerals in the VMS deposits in the Wales Group range from 5.9 to 17.4 permil (avg 11.5?2.7 permil), about 5 to 6 permil higher than those in the Moira Sound unit, which range from -2.8 to 10.4 permil (avg 6.1?4.0 permil). This difference in 34Ssulfide values reflects a dominantly seawater sulfate source of the sulfides and is linked to the 34S values of contemporaneous seawater sulfate, which were slightly higher during the Late Proterozoic through Cambrian than during the Ordovician through Early Silurian.

Alaska↗

USGS2007

Bathymetry and Geology of the Floor of Yellowstone Lake, Yellowstone National Park, Wyoming, Idaho, and Montana

High-resolution, multi-beam sonar mapping of Yellowstone Lake was conducted by the U.S. Geological Survey in conjunction with the National Park Service from 1999 to 2002. Yellowstone Lake is the largest high-altitude lake in North America, at an altitude of 2,357 m with a surface area of 341 km2. More than 140 rivers and streams flow into Yellowstone Lake. The Yellowstone River, which enters at the southern end of the lake into the Southeast Arm, dominates the inflow of water and sediment (Shanks and others, 2005). The only outlet from the lake is at Fishing Bridge where the Yellowstone River flows northward discharging 375 to 4,600 cubic feet per second. The multi-beam sonar mapping occurred over a four-year period beginning in 1999 with mapping of the northern basin, continued in 2000 in West Thumb basin, in 2001 in the central basin, and in 2002 in the southern part of the lake including the Flat Mountain, South, and Southeast Arms.

Scientific Investigations Map↗

USGS2006

Stable isotope compositions of serpentinite seamounts in the Mariana forearc: Serpentinization processes, fluid sources and sulfur metasomatism

The Mariana and Izu-Bonin arcs in the western Pacific are characterized by serpentinite seamounts in the forearc that provide unique windows into the mantle wedge. We present stable isotope (O, H, S, and C) data for serpentinites from Conical seamount in the Mariana forearc and S isotope data for Torishima seamount in the Izu-Bonin forearc in order to understand the compositions of fluids and temperatures of serpentinization in the mantle wedge, and to investigate the transport of sulfur from the slab to the mantle wedge. Six serpentine mineral separates have a restricted range of ??18O (6.5-8.5???). Antigorite separates have ??D values of -29.5??? to -45.5??? that reflect serpentinization within the mantle wedge whereas chrysotile has low ??D values (-51.8??? to -84.0???) as the result of re-equilibration with fluids at low temperatures. Fractionation of oxygen isotopes between serpentine and magnetite indicate serpentinization temperatures of 300-375 ??C. Two late cross-fiber chrysotile veins have higher ??18O values of 8.9??? to 10.8??? and formed at lower temperatures (as low as ???100 ??C). Aqueous fluids in equilibrium with serpentine at 300-375 ??C had ??18O = 6.5-9??? and ??D = -4??? to -26???, consistent with sediment dehydration reactions at temperatures <200 ??C in the subducting slab rather than a basaltic slab source. Three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have ??18O = 16.7-24.5???, consistent with the serpentinizing fluids at temperatures <250 ??C. ??13C values of 0.1-2.5??? suggest a source in subducting carbonate sediments. The ??34S values of sulfide in serpentinites on Conical Seamount (-6.7??? to 9.8???) result from metasomatism through variable reduction of aqueous sulfate (??34S = 14???) derived from slab sediments. Despite sulfur metasomatism, serpentinites have low sulfur contents (generally < 164 ppm) that reflect the highly depleted nature of the mantle wedge. The serpentinites are mostly enriched in 34S (median ??34Ssulfide = 4.5???), consistent with a 34S-enriched mantle wedge as inferred from arc lavas. ?? 2006 Elsevier B.V. All rights reserved.

Earth and Planetary Science Letters↗

USGS2006

Iron isotope fractionation during microbially stimulated Fe(II) oxidation and Fe(III) precipitation

Interpretation of the origins of iron-bearing minerals preserved in modern and ancient rocks based on measured iron isotope ratios depends on our ability to distinguish between biological and non-biological iron isotope fractionation processes. In this study, we compared 56Fe/54Fe ratios of coexisting aqueous iron (Fe(II)aq, Fe(III)aq) and iron oxyhydroxide precipitates (Fe(III)ppt) resulting from the oxidation of ferrous iron under experimental conditions at low pH (<3). Experiments were carried out using both pure cultures of Acidothiobacillus ferrooxidans and sterile controls to assess possible biological overprinting of non-biological fractionation, and both SO42- and Cl- salts as Fe(II) sources to determine possible ionic/speciation effects that may be associated with oxidation/precipitation reactions. In addition, a series of ferric iron precipitation experiments were performed at pH ranging from 1.9 to 3.5 to determine if different precipitation rates cause differences in the isotopic composition of the iron oxyhydroxides. During microbially stimulated Fe(II) oxidation in both the sulfate and chloride systems, 56Fe/54Fe ratios of residual Fe(II)aq sampled in a time series evolved along an apparent Rayleigh trend characterized by a fractionation factor ??Fe(III)aq-Fe(II)aq???1.0022. This fractionation factor was significantly less than that measured in our sterile control experiments (???1.0034) and that predicted for isotopic equilibrium between Fe(II)aq and Fe(III)aq (???1.0029), and thus might be interpreted to reflect a biological isotope effect. However, in our biological experiments the measured difference in 56Fe/54Fe ratios between Fe(III)aq, isolated as a solid by the addition of NaOH to the final solution at each time point under N2-atmosphere, and Fe(II)aq was in most cases and on average close to 2.9??? (??Fe(III)aq-Fe(II)aq ???1.0029), consistent with isotopic equilibrium between Fe(II)aq and Fe(III)aq. The ferric iron precipitation experiments revealed that 56Fe/54Fe ratios of Fe(III)aq were generally equal to or greater than those of Fe(III)ppt, and isotopic fractionation between these phases decreased with increasing precipitation rate and decreasing grain size. Considered together, the data confirm that the iron isotope variations observed in our microbial experiments are primarily controlled by non-biological equilibrium and kinetic factors, a result that aids our ability to interpret present-day iron cycling processes but further complicates our ability to use iron isotopes alone to identify biological processing in the rock record. ?? 2005 Elsevier Inc. All rights reserved.

Geochimica et Cosmochimica Acta↗

USGS2004

The springs of Lake Pátzcuaro: chemistry, salt-balance, and implications for the water balance of the lake

Lake Pa??tzcuaro, the center of the ancient Tarascan civilization located in the Mexican altiplano west of the city of Morelia, has neither river input nor outflow. The relatively constant lake-salinity over the past centuries indicates the lake is in chemical steady state. Springs of the south shore constitute the primary visible input to the lake, so influx and discharge must be via sub-lacustrine ground water. The authors report on the chemistry and stable isotope composition of the springs, deeming them representative of ground-water input. The springs are dominated by Ca, Mg and Na, whereas the lake is dominated by Na. Combining these results with previously published precipitation/rainfall measurements on the lake, the authors calculate the chemical evolution from spring water to lake water, and also calculate a salt balance of the ground-water-lake system. Comparing Cl and ??18O compositions in the springs and lake water indicates that 75-80% of the spring water is lost evaporatively during evolution toward lake composition. During evaporation Ca and Mg are lost from the water by carbonate precipitation. Each liter of spring water discharging into the lake precipitates about 18.7 mg of CaCO3. Salt balance calculations indicate that ground water input to the lake is 85.9??106 m3/a and ground water discharge from the lake is 23.0??106 m3/a. Thus, the discharge is about 27% of the input, with the rest balanced by evaporation. A calculation of time to reach steady-state ab initio indicates that the Cl concentration of the present day lake would be reached in about 150 a. ?? 2004 Elsevier Ltd. All rights reserved.

Applied Geochemistry↗

USGS2004

Massive sulfide deposition and trace element remobilization in the Middle Valley sediment-hosted hydrothermal system, northern Juan de Fuca Rdge

The Bent Hill massive sulfide deposit and ODP Mound deposit in Middle Valley at the northernmost end of the Juan de Fuca Ridge are two of the largest modern seafloor hydrothermal deposits yet explored. Trace metal concentrations of sulfide minerals, determined by laser-ablation ICP-MS, were used in conjunction with mineral paragenetic studies and thermodynamic calculations to deduce the history of fluid-mineral reactions during sulfide deposition. Detailed analyses of the distribution of metals in sulfides indicate significant shifts in the physical and chemical conditions responsible for the trace element variability observed in these sulfide deposits. Trace elements (Mn, Co, Ni, As, Se, Ag, Cd, Sb, Pb, and Bi) analyzed in a representative suite of 10 thin sections from these deposits suggest differences in conditions and processes of hydrothermal alteration resulting in mass transfer of metals from the center of the deposits to the margins. Enrichments of some trace metals (Pb, Sb, Cd, Ag) in sphalerite at the margins of the deposits are best explained by dissolution/reprecipitation processes consistent with secondary remineralization. Results of reaction-path models clarify mechanisms of mass transfer during remineralization of sulfide deposits due to mixing of hydrothermal fluids with seawater. Model results are consistent with patterns of observed mineral paragenesis and help to identify conditions (pH, redox, temperature) that may be responsible for variations in trace metal concentrations in primary and secondary minerals. Differences in trace metal distributions throughout a single deposit and between nearby deposits at Middle Valley can be linked to the history of metal mobilization within this active hydrothermal system that may have broad implications for sulfide ore formation in other sedimented and unsedimented ridge systems. ?? 2004 Elsevier Ltd.

Geochimica et Cosmochimica Acta↗

USGS2003

Extraordinary phase separation and segregation in vent fluids from the southern East Pacific Rise

The discovery of Brandon vent on the southern East Pacific Rise is providing new insights into the controls on midocean ridge hydrothermal vent fluid chemistry. The physical conditions at the time ofsampling (287 bar and 405??C) place the Brandon fluids very close to the critical point of seawater (298 bar and 407??C). This permits in situ study of the effects of near criticalphenomena, which are interpreted to be the primary cause of enhanced transition metal transport in these fluids. Of the five orifices on Brandon sampled, three were venting fluids with less than seawater chlorinity, and two were venting fluids with greater than seawater chlorinity. The liquid phase orifices contain 1.6-1.9 times the chloride content of the vapors. Most other elements, excluding the gases, have this same ratio demonstrating the conservative nature of phase separation and the lack of subsequent water-rock interaction. The vapor and liquid phases vent at the same time from orifices within meters of each other on the Brandon structure. Variations in fluid compositions occur on a time scale of minutes. Our interpretation is that phase separation and segregation must be occurring 'real time' within the sulfide structure itself. Fluids from Brandon therefore provide an unique opportunity to understand in situ phase separation without the overprinting of continued water-rock interaction with the oceanic crust, as well as critical phenomena. ?? 2002 Elsevier Science B.V. All rights reserved.

Earth and Planetary Science Letters↗

USGS2003

Serpentinization of abyssal peridotites from the MARK area, Mid-Atlantic Ridge: Sulfur geochemistry and reaction modeling

The opaque mineralogy and the contents and isotope compositions of sulfur in serpentinized peridotites from the MARK (Mid-Atlantic Ridge, Kane Fracture Zone) area were examined to understand the conditions of serpentinization and evaluate this process as a sink for seawater sulfur. The serpentinites contain a sulfur-rich secondary mineral assemblage and have high sulfur contents (up to 1 wt.%) and elevated ??34Ssulfide (3.7 to 12.7???). Geochemical reaction modeling indicates that seawater-peridotite interaction at 300 to 400??C alone cannot account for both the high sulfur contents and high ??34Ssulfide. These require a multistage reaction with leaching of sulfide from subjacent gabbro during higher temperature (???400??C) reactions with seawater and subsequent deposition of sulfide during serpentinization of peridotite at ???300??C. Serpentinization produces highly reducing conditions and significant amounts of H2 and results in the partial reduction of seawater carbonate to methane. The latter is documented by formation of carbonate veins enriched in 13C (up to 4.5???) at temperatures above 250??C. Although different processes produce variable sulfur isotope effects in other oceanic serpentinites, sulfur is consistently added to abyssal peridotites during serpentinization. Data for serpentinites drilled and dredged from oceanic crust and from ophiolites indicate that oceanic peridotites are a sink for up to 0.4 to 6.0 ?? 1012 g seawater S yr-1. This is comparable to sulfur exchange that occurs in hydrothermal systems in mafic oceanic crust at midocean ridges and on ridge flanks and amounts to 2 to 30% of the riverine sulfate source and sedimentary sulfide sink in the oceans. The high concentrations and modified isotope compositions of sulfur in serpentinites could be important for mantle metasomatism during subduction of crust generated at slow spreading rates. ?? 2003 Elsevier Science Ltd.

Geochimica et Cosmochimica Acta↗

USGS2003

Exploration and discovery in Yellowstone Lake: Results from high-resolution sonar imaging, seismic reflection profiling, and submersible studies

Discoveries from multi-beam sonar mapping and seismic reflection surveys of the northern, central, and West Thumb basins of Yellowstone Lake provide new insight into the extent of post-collapse volcanism and active hydrothermal processes occurring in a large lake environment above a large magma chamber. Yellowstone Lake has an irregular bottom covered with dozens of features directly related to hydrothermal, tectonic, volcanic, and sedimentary processes. Detailed bathymetric, seismic reflection, and magnetic evidence reveals that rhyolitic lava flows underlie much of Yellowstone Lake and exert fundamental control on lake bathymetry and localization of hydrothermal activity. Many previously unknown features have been identified and include over 250 hydrothermal vents, several very large (>500 m diameter) hydrothermal explosion craters, many small hydrothermal vent craters (∼1–200 m diameter), domed lacustrine sediments related to hydrothermal activity, elongate fissures cutting post-glacial sediments, siliceous hydrothermal spire structures, sublacustrine landslide deposits, submerged former shorelines, and a recently active graben. Sampling and observations with a submersible remotely operated vehicle confirm and extend our understanding of the identified features. Faults, fissures, hydrothermally inflated domal structures, hydrothermal explosion craters, and sublacustrine landslides constitute potentially significant geologic hazards. Toxic elements derived from hydrothermal processes also may significantly affect the Yellowstone ecosystem.

Wyoming↗

USGS2003

Hydrothermal and tectonic activity in northern Yellowstone Lake, Wyoming

Yellowstone National Park is the site of one of the world's largest calderas. The abundance of geothermal and tectonic activity in and around the caldera, including historic uplift and subsidence, makes it necessary to understand active geologic processes and their associated hazards. To that end, we here use an extensive grid of high-resolution seismic reflection profiles (∼450 km) to document hydrothermal and tectonic features and deposits in northern Yellowstone Lake. Sublacustrine geothermal features in northern Yellowstone Lake include two of the largest known hydrothermal explosion craters, Mary Bay and Elliott's. Mary Bay explosion breccia is distributed uniformly around the crater, whereas Elliott's crater breccia has an asymmetric distribution and forms a distinctive, ∼2-km-long, hummocky lobe on the lake floor. Hydrothermal vents and low-relief domes are abundant on the lake floor; their greatest abundance is in and near explosion craters and along linear fissures. Domed areas on the lake floor that are relatively unbreached (by vents) are considered the most likely sites of future large hydrothermal explosions. Four submerged shoreline terraces along the margins of northern Yellowstone Lake add to the Holocene record of postglacial lake-level fluctuations attributed to “heavy breathing” of the Yellowstone magma reservoir and associated geothermal system. The Lake Hotel fault cuts through northwestern Yellowstone Lake and represents part of a 25-km-long distributed extensional deformation zone. Three postglacial ruptures indicate a slip rate of ∼0.27 to 0.34 mm/yr. The largest (3.0 m slip) and most recent event occurred in the past ∼2100 yr. Although high heat flow in the crust limits the rupture area of this fault zone, future earthquakes of magnitude ∼5.3 to 6.5 are possible. Earthquakes and hydrothermal explosions have probably triggered landslides, common features around the lake margins. Few high-resolution seismic reflection surveys have been conducted in lakes in active volcanic areas. Our data reveal active geothermal features with unprecedented resolution and provide important analogues for recognition of comparable features and potential hazards in other subaqueous geothermal environments.

Wyoming↗

USGS2002

Geochemistry of fluid phases and sediments: Relevance to hydrothermal circulation in Middle Valley, ODP Legs 139 and 169

Geochemical and isotopic studies of pore fluids and solid phases recovered from the Dead Dog and Bent Hill hydrothermal sites in Middle Valley (Ocean Drilling Program Leg 169) have been compared with similar data obtained previously from these sites during Ocean Drilling Program Leg 139. Although generally the hydrothermal systems reflect non-steady state conditions, the data allow an assessment of the history of the hydrothermal processes. Sediment K/A1 ratios as well as the distribution of anhydrite in the sediments suggest that the Dead Dog hydrothermal field has been, and still is, active. In contrast, similar data in the Bent Hill hydrothermal field indicate a waning of hydrothermal activity. Pore fluid and hydrothermal vent data in the Dead Dog hydrothermal field are similar in nature to the data collected during ODP Leg 139. In the area of the Bent Hill sulfide deposit, however, the pore water data indicate that recent wholesale flushing of the sediment column with relatively unaltered seawater has obliterated a previous record of hydrothermal activity in the pore fluids. Data from the deepest part of Hole 1035A in the Bent Hill locality show the presence of hydrothermal fluids at greater depths in this area. This suggests the origin of the hydrothermal fluids found to be emanating from Hole 1035F, which constitutes one of the first man made hydrothermal vents in the Middle Valley hydrothermal system. Similarly, CORKed Hole 858G, because of seal failures, has acted as a hydrothermal vent, with sulfide deposits forming inside the CORK. ?? 2002 Elsevier Science Ltd. All rights reserved.

Applied Geochemistry↗

USGS2002

Helium and carbon gas geochemistry of pore fluids from the sediment-rich hydrothermal system in Escanaba Trough

Ocean Drilling Program (ODP) Leg 169, which was conducted in 1996 provided an opportunity to study the gas geochemistry in the deeper part of the sediment-rich hydrothermal system in Escanaba Trough. Gas void samples obtained from the core liner were analyzed and their results were compared with analytical data of vent fluid samples collected by a submersible dive program in 1988. The gas geochemistry of the pore fluids consisted mostly of a hydrothermal component and was basically the same as that of the vent fluids. The He isotope ratios (R/RA = 5.6-6.6) indicated a significant mantle He contribution and the C isotopic compositions of the hydrocarbons [??13C(CH4) = -43???, ??13C(C2H6) = -20???] were characterized as a thermogenic origin caused by hydrothermal activity. On the other hand, the pore fluids in sedimentary layers away from the hydrothermal fields showed profiles which reflected lateral migration of the hydrothermal hydrocarbons and abundant biogenic CH4. Helium and C isotope systematics were shown to represent a hydrothermal component and useful as indicators for their distribution beneath the seafloor. Similarities in He and hydrocarbon signatures to that of the Escanaba Trough hydrothermal system were found in some terrestrial natural gases, which suggested that seafloor hydrothermal activity in sediment-rich environments would be one of the possible petroleum hydrocarbon generation scenarios in unconventional geological settings. ?? 2002 Elsevier Science Ltd. All rights reserved.

Applied Geochemistry↗

USGS2001

Stable isotopes in seafloor hydrothermal systems: Vent fluids, hydrothermal deposits, hydrothermal alteration, and microbial processes

The recognition of abundant and widespread hydrothermal activity and associated unique life-forms on the ocean floor is one of the great scientific discoveries of the latter half of the twentieth century. Studies of seafloor hydrothermal processes have led to revolutions in understanding fluid convection and the cooling of the ocean crust, the chemical and isotopic mass balance of the oceans, the origin of stratiform and statabound massive-sulfide ore-deposits, the origin of greenstones and serpentinites, and the potential importance of the subseafloor biosphere. Stable isotope geochemistry has been a critical and definitive tool from the very beginning of the modern era of seafloor exploration.

Reviews in Mineralogy and Geochemistry↗

USGS2001

Results of chemical and stable isotopic analyses of water samples collected in the Patagonia Mountains, southern Arizona

Water samples were collected in the Patagonia Mountains in February, 1997. Most of the samples were collected from portals of abandoned mines, or from stream drainages immediately downstream from abandoned mines. Most of the samples have low pH (<4) and high total dissolved solids (> 1000 mg/L). Anion composition of the water samples is dominated by sulfate, while cation compositions range from calcium-dominated to mixed calcium-magnesium or calcium-sodium-dominated waters. Metals such as iron, manganese, copper, zinc, and aluminum contribute a significant portion (>10%) of the cation content to the water samples. Because of the low pH?s, protons contribute up to several percent of the cation character of the waters in some of the samples. The data are presented in tabular and graphical formats, with descriptions of data quality and brief descriptions of results.

Open-File Report↗