Hot Spring Activity Research Articles

Evolution of a dynamic paleo-hydrothermal system at Mangatete, Taupo Volcanic Zone, New Zealand

Recent quarrying and active faulting at Mangatete, Taupo Volcanic Zone (TVZ), New Zealand, illuminate a rare spatial and temporal window on a dynamic Late Quaternary geothermal system. Detailed geological mapping, stratigraphic logging, AMS 14C dating, and textural and mineralogical analyses were used to construct a complex history of hydrothermal, volcanological and tectonic activity from ~36 to 2ka. Extinct, surface hydrothermal manifestations occur over a ~2km2 area, and include in situ siliceous sinters distributed on normal fault terraces, an inferred hydrothermal eruption breccia (HEB) containing acid-etched sinter blocks, another probable HEB that was bathed in silicifying thermal fluids, and sinter clasts that were entrained in a debris flow associated with a volcanic ash event. Preserved sinter textures typical of near-neutral pH, alkali chloride spring discharge channels, aprons, terraces and affiliated marshes comprise plant-rich, palisade, tufted bubble mat, and domal stromatolitic fabrics. In addition, a packed fragmental sinter facies is shown herein to constitute silicified microbial mats that were broken, transported and deposited as point bar deposits in thermal spring-fed streams. Moreover, four unusual siliceous sinter fabrics—vuggy, globular spongy, scalloped, and arcuate wavy layered—are interpreted to have formed from local acid-sulfate-chloride thermal springs, possibly associated with paleo-fumaroles. The reconstructed history of paleo-hydrothermal activity indicates that the oldest sinters (~36ka) at Mangatete developed in alkali chloride hot springs, but then underwent post-depositional alterion/overprinting by acid-sulfate steam condensate and were dismembered, possibly by a hydrothermal eruption. Low pH hot-spring discharges forming the unusual, inferred acid sinter fabrics were localized in the same area. A shift in paleo-hydrology is evidenced by unaltered, alkali chloride sinters dated between ~22 and 3ka. A cluster of sinter dates between 22.7 and 20.7ka denote thermal fluid emissions around the time of the Okareka eruption (~22.1ka). An ~11kyr gap in sinter ages followed, which overlaps with a time of relative quiescence in local fault movement in between the Rotorua and Rotoma eruptions, as determined from previous paleo-seismicity studies on the Whirinaki Fault in the Mangatete area. At ~9.4ka, hot spring activity in thermally fed streams slightly post-dated the Rotoma eruption (~9.5ka). The locations and ages of the youngest (post-9.4ka) alkali chloride sinters and silicified paleosols in relation to Whirinaki Fault movements indicate an overall westward migration of siliceous spring discharge. Dated charcoal (~1.8kyr) in ash at the base of a locally extensive, volcaniclastic debris flow, containing sinter blocks of a range of ages and mineralogical maturity, suggests its affiliation with the Taupo eruption. Mangatete sinters occur along SW–NE striking faults parallel to the regional trend of the TVZ, which dissect the topography into stepped terraces. Inferred basement cross-faults in the study area, i.e. structural lineaments orthogonal to this trend, may have served as fluid pathways at depth, initiating and maintaining hydrothermal activity at Mangatete for more than 30kyr. Within the larger Ngakuru Graben (~150km2), the ages and distributions of sinters and hydrothermal eruption breccias, including those at Mangatete, show that significant hydrothermal activity occurred for more than 60kyr, in an area lacking such activity today. Thus, Mangatete retains a rare stratigraphic record within the TVZ sinter archive of changing geothermal fluids, resulting from shifts in paleo-phreatic water levels. Moreover, although limited, the presented age data imply a relationship among seismic activity, volcanic ash fall events, and the occurrence of permeable pathways for upward migration of thermal fluids. Finally, whereas currently active TVZ geothermal fields appear to be stable over tens of thousands of years or more, the fossil hydrothermal systems in the wider Mangatete area demonstrate the possibility of much greater temporal variability in the lateral expression of local surface manifestations of thermal fluid discharge through time.

Aqueous geochemistry of the Thermopolis hydrothermal system, southern Bighorn Basin, Wyoming, U.S.A.

The Thermopolis hydrothermal system is located in the southern portion of the Bighorn Basin, in and around the town of Thermopolis, in northwest Wyoming. It is the largest hydrothermal system in Wyoming outside of Yellowstone National Park. The system includes hot springs, travertine deposits, and thermal wells. Published models for the hydrothermal system propose the Owl Creek Mountains as the recharge zone, simple conductive heating at depth, and resurfacing of thermal waters up the Thermopolis Anticline. The geochemistry of the thermal waters of three active hot springs—Big Spring, White Sulphur Spring, and Teepee Fountain—is similar in composition and characteristic of carbonate or carbonate-bearing siliciclastic aquifers. Previous studies of the Thermopolis hydrothermal system postulate that the thermal waters are a mixture of waters from Paleozoic formations. Major element geochemical analyses available for waters from these formations are not of sufficient quality to determine whether the thermal waters are a mixture of the Paleozoic aquifers. In the time frame of this study (one year), the geochemistry of all three springs was constant through all four seasons, spanning spring snowmelt and recharge as well as late-summer and fall dryness. This relationship is consistent with a deep source not influenced by shallow, local hydrogeology. Anomalies are evident in the historic data set for the geochemistry of Big Spring. We speculate that anomalies occurring between 1906 and 1926 suggest mixing of source waters of Big Spring with waters from a siliciclastic formation, and that anomalies occurring between 1926 and 1933 suggest mixing with waters from a formation containing gypsum or anhydrite. Decreased concentrations measured in our study—relative to concentrations measured between 1933 and 1976—may reflect mixing of thermal waters with more dilute waters. Current data are not sufficient to rigorously test these suggestions, and events of sufficient scale taking place in these timeframes have not been identified.

Hot spring deposits on a cliff face: A case study from Jifei, Yunnan Province, China

A cliff face in the Jifei karst area, southwest China, is covered by a spectacular succession of precipitates that formed from the hot spring water that once flowed down its surface. This layered succession is formed of aragonite layers that are formed largely of “fountain dendrites”, calcite layers that are formed mostly of “cone dendrites”, and microlaminated layers that contain numerous microbes and extracellular polymeric substances (EPS). Many of the aragonite crystals are hollow due to preferential dissolution of their cores. The calcite cone dendrites are commonly covered with biofilms, reticulate Si–Mg coatings, and other precipitates. The microbial layers include dodecahedral calcite crystals and accessory minerals that include opal-A, amorphous Si–Mg coatings, trona, barite, potassium sulfate crystals, mirabillite, and gaylussite. Interpretation of the δ18O(calcite) and δ18O(aragonite) indicates precipitation from water with a temperature of 54 to 66°C. The active hot spring at the top of the cliff presently ejects water at a temperature of 65°C.Layers, 1mm to 6cm thick, record temporal changes in the fluids from which the precipitates formed. This succession is not, however, formed of recurring cycles that can be linked to diurnal or seasonal changes in the local climate. Indeed, it appears that the climatic contrast between the wet season and the dry season had little impact on precipitation from the spring waters that flowed down the cliff face. Integration of currently available evidence suggests that the primary driving force was aperiodic changes in the CO2 content of the spring waters because that seems to be the prime control on the saturation levels that underpinned precipitation of the calcite and aragonite as well as the dissolution of the aragonite. Such variations in the CO2 content of the spring water were probably due to changes that took place in the subterranean plumbing system of the spring.

Soil baseline geochemistry and plant response in areas of complex geology. Application to NW Euboea, Greece

A soil and vegetation survey was undertaken in NW Euboea Island, Greece. The objectives of the study were to establish the geochemical baseline of soil and identify the impact of local geology on threshold values of potentially harmful elements. The studied area is characterized by complex geology comprising metamorphic and ultramafic rocks as well as active hot springs. A total of 117 soil samples were collected from 89 sites at depths of 0–25cm and 25–50cm. Eighteen vegetation samples were also collected representing prevalent indigenous perennial species in the region. Soil samples from the present study were enriched in As, Ca, Cu, Mg, Ni with concentrations reaching 233mg/kg, 38%, 336mg/kg, 10.8%, 1560mg/kg respectively. Factor analysis revealed three main factors controlling the chemical composition of soil reflecting the influence of ultramafic rocks (Cr, Ca, Mg, Ni), hot spring deposits (Ca, S, Sr, As) and paedogenesis processes (Fe, Co, V, Mn, Al). The first two of these factors showed significant spatial correlation with the geological features within the study area. Subsequently, baseline concentrations based on statistical and spatial data were estimated within sub-areas reflecting the influence of local geology in soil composition. Concentrations of potentially harmful elements in the plant tissues of indigenous perennial vegetation species showed a wide range of variation from below the detection limit up to 1700mg/kg for Ni in the hyperaccumulator Alyssum chalcidicum demonstrating that plant species have adapted to the stressful conditions caused by high elemental concentrations in soil. The results of this study can be utilized in future studies at areas of similar geology by providing an objective basis for setting realistic threshold values for pollution assessment and remediation.

Discussion on ‘A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications’

We welcome the contribution of Parry et al . (2011) concerning the age of the globally important Rhynie hot-spring system, Aberdeenshire, Scotland. However, although we accept that they have provided a valuable and robust U–Pb age constraint for the Milton of Noth Andesite and the Rhynie outlier stratigraphic succession, we do not accept that they have constrained the timing of hot-spring activity, as suggested. The geophysical trace of the unexposed Milton of Noth Andesite indicates that it lies immediately adjacent to the Rhynie Fault Zone and runs parallel to it for some 2 km. It may even lie within the Rhynie Fault Zone (see discussion by Rice & Ashcroft 2004). Its stratigraphic position is therefore uncertain. Parry et al . (2011) stated that the exact stratigraphic position is not important because their preferred stratigraphic locations lie within the Tillybrachty Sandstone and Dryden Flags Formations, both of which have consistently yielded spores belonging to the polygonalis–emsiensis Spore Assemblage Biozone. This assertion is not valid. The polygonalis–emsiensis Spore Assemblage Biozone may have lasted several million years and thus, potentially, there could be a significant hiatus between eruption of the andesite and the onset of hot-spring activity. They further assume that the andesite was linked to an andesitic magma chamber at depth, which powered the hydrothermal system. This remains speculation. Accepting the uncertainty in the stratigraphic position and by asserting a maximum thickness of sediment between the andesite and the Rhynie Cherts and a conservative deposition rate, Parry et al . (2011) attempted to temporally link the andesite to the cherts by calculating the time interval represented by the intervening sediment. The estimation of time depends on the absence of unconformities, disconformities and faulting, with such assumptions and uncertainties casting doubt on the thickness of the intervening sediments …

Reply to Discussion on ‘A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications’

We welcome the opportunity to address the points raised by Mark et al . in their discussion of the chemical abrasion isotope dilution thermal ionization mass spectrometry (CA–ID–TIMS) U–Pb age constraint on the Rhynie Chert Konservat-Lagerstatte presented by Parry et al . (2011) and also to make some further observations of our own. We begin by briefly providing some context for the benefit of the wider readership. Two radio-isotopic age constraints on the Rhynie Chert Konservat-Lagerstatte and, by corollary, its parental hydrothermal (hot-spring) system have recently been published. The first of these is a weighted mean 40Ar/39Ar plateau age of 403.9 ± 2.1 Ma (2σ) derived from the analysis of two samples of vein-hosted hydrothermal K-feldspar and a single sample of hydrothermally altered andesite (Mark et al . 2011). In order to account for systematic uncertainties associated with the 40Ar/39Ar geochronometer, Mark et al . (2011) recalculated their individual sample ages with reference to the Fish Canyon Tuff sanidine (FCs) age of 28.201 Ma (Kuiper et al . 2008), thereby producing a ‘U–Pb comparable’ mean age of 407.1 ± 2.2 Ma (2σ). An alternative ‘preferred age’ for the Rhynie hot-spring activity (407.6 ± 2.2 Ma (2σ)) has now been produced from the ‘raw’ data using the optimization model of Renne et al . (2010, 2011) (this discussion). The 40Ar/39Ar system calibrations on which these various ages are based are summarized in Table 1. The second radio-isotopic age constraint in question is a weighted mean 206Pb/238U zircon age of 411.5 ± 1.3 Ma (2σ, including decay constant- and tracer calibration-related uncertainties; MSWD = 0.12, n = 4) yielded by the Milton of Noth Andesite, a moderately altered basaltic andesite lava flow (cum …

Wetland megabias: ecological and ecophysiological filtering dominates the fossil record of hot spring floras

AbstractSiliceous hot spring deposits form at Earth's surface above terrestrial hydrothermal systems, which create low‐sulphidation epithermal mineral deposits deeper in the crust. Eruption of hot spring waters and precipitation of opal‐A create sinter apron complexes and areas of geothermally influenced wetland. These provide habitat for higher plants that may be preserved in situ, by encrustation of their surfaces and permineralization of tissues, creating T0 plant assemblages. In this study, we review the fossil record of hot spring floras from subfossil examples forming in active hot spring areas, via fossil examples from the Cenozoic, Mesozoic and Palaeozoic to the oldest known hot spring flora, the Lower Devonian Rhynie chert. We demonstrate that the well‐known megabias towards wetland plant preservation extends to hot spring floras. We highlight that the record of hot spring floras is dominated by plants preserved in situ by permineralization on geothermally influenced wetlands. Angiosperms (members of the Cyperaceae and Restionaceae) dominate Cenozoic floras. Equisetum and gleicheniaceous ferns colonized Mesozoic (Jurassic) geothermal wetlands and sphenophytes and herbaceous lycophytes late Palaeozoic examples. Evidence of the partitioning of wetland hydrophytic and dryland mesophytic communities, a feature of active geothermal areas, is provided by well‐preserved and well‐exposed fossil sinter apron complexes, which record flooding of dryland environments by thermal waters and decline of local forest ecosystems. Such observations from the fossil record back‐up hypotheses based on active hot springs and vegetation that suggest removal of taphonomic filtering in hot spring environments is accompanied by an increase in ecological and ecophysiological filtering. To this end we also demonstrate that in the hot spring environment, the wetland bias extends beyond broad ecology. We show that ecosystems preserved from the Cenozoic to the Mesozoic provide clear evidence that the dominant plants preserved in situ by hot spring activity are also halophytic, tolerant of high pH and high concentrations of heavy metals. By extension, we hypothesize that this is also the case in Palaeozoic hot spring settings and extended to the early land plant flora of the Rhynie chert.

The sources and fluxes of dissolved chemistry in a semi-confined, sandy coastal aquifer: The Pingtung Plain, Taiwan

Groundwater chemical fluxes from the Pingtung Plain in SW Taiwan to the ocean were determined by analysing waters from 43 wells at varying depths through a 237m deep window across the Pingtung Plain, for major dissolved cations, anions, dissolved SiO2, and stable isotopic composition of O and H, and computing their subsurface water fluxes from measurements of hydraulic heads and formation permeabilities. The results show that between 1.5% (SO42-) and 12.3% (Ba2+) of the total chemical weathering flux discharged to the ocean (Kaoping River combined with groundwater) can be attributed to the groundwater. Estimated propagated errors at 1σ on subsurface fluxes are ±20%. Multi-year daily hydraulic head data give the direction of groundwater flow through the plain, and indicate that pumping has led to episodic reversals of flow, facilitating seawater intrusion in the near-coast aquifer. Tracing end-member proportions using mixing relationships shows that, in addition to seawater and meteoric water, hot-spring activity contributes to the dissolved chemistry of the groundwater. In addition to these three end-members, the weathering of carbonate and silicate minerals in the plain accounts for the remainder of the chemical budget. Hydrological connectivity exists throughout the drilled depth of the basin, but chemical gradients show that flow is stratified, with up to a twofold increase in silicate-derived Na+ seen in deeper horizons as compared to the near surface. For all ions except SO42-, the average concentrations of dissolved species in the coastal groundwaters exceed those of the river, ranging from a factor of 1.33 in the case of Li+ to 27.29 in the case of Cl−. The results suggest that submarine groundwater chemical fluxes through the drilled depth of the Pingtung Plain into the Taiwan Strait are modest in comparison to those related to surface runoff.

Origin of the Düvertepe kaolin–alunite deposits in Simav Graben, Turkey: Timing and styles of hydrothermal mineralization

The Düvertepe district located on the western end of the Simav Graben is the largest known fossil hydrothermal kaolin deposit in Turkey. Active hot springs occur to the north of Simav city (Eynal, Çitgöl, Naşa) and geothermal fields occur to the west of the graben (Hisaralan). Kaolin in the Düvertepe district formed at the expense of Miocene rhyolites–rhyodacites and tuffs emplaced in a tectonically active system undergoing N–S extension. Upward fan-shaped deposits of silicification and kaolin are found along fault zones and define areas of enhanced kaolinization. Silica sinters located above the kaolin zones suggest that hydrothermal fluids followed the fracture systems and mark outflow of geothermal waters. Fractured volcanic rocks rich in glass shards bear sufficient porosity and permeability for hydrothermal circulation. Two mineral facies, “kaolinite and alunite” dominate the deposits, which respectively include: (1) kaolinite–alunite–quartz and (2) alunite–opal-CT–quartz–halloysite. Kaolinite crystals are well-formed hexagonal vermiform habits and alunite crystals are idiomorphic rhombohedral forms. Needle- and tubular-shaped halloysites are common in the alunite facies. The δ34S values for alunite range from −1.55 to +6.18‰. Kaolinites have δD ranging from −49.0 to −94.3‰ and the δ18O values range from +5.8 to +14.8‰. Calculated formation temperatures of kaolin mineralization (using δ18O values) suggest that hydrothermal alteration occurred in the range of 38° to 129°C. Independent K/Ar and 40Ar/39Ar ages of alunite are concordant and indicate that alteration occurred from 20.1 to 20.6Ma in the southern part of the graben and 17.3 to 19.2Ma in the northern part. These results suggest that metasomatism by steam-heated hydrothermal fluids sourced by rhyolitic magma is responsible for the Düvertepe kaolin and alunite deposit origins.

Analytical and numerical models of hydrothermal fluid flow at fault intersections

AbstractFault intersections are the locus of hot spring activity and Carlin‐type gold mineralization within the Basin and Range, USA. Analytical and numerical solutions to Stokes equation suggest that peak fluid velocities at fault intersections increase between 20% and 47% when fracture apertures have identical widths but increase by only about 1% and 8% when aperture widths vary by a factor of 2. This suggests that fault zone intersections must have enlarged apertures. Three‐dimensional finite element models that consider intersecting 10‐ to 20‐m wide fault planes resulted in hot spring activity being preferentially located at fault zone intersections when fault zones were assigned identical permeabilities. We found that the onset of convection at the intersections of the fault zones occurred in our hydrothermal model over a narrow permeability range between 5 × 10−13 and 7 × 10−13 m2. Relatively high vertical fluid velocities (0.3–3 m year−1) extended away from the fault intersections for about 0.5–1.5 km. For the boundary conditions and fault plane dimensions used, peak discharge temperatures of 112°C at the water table occurred with an intermediate fault zone permeability of 5 × 10−13 m2. When fault plane permeability differed by a factor of 2 or more, the locus of hot spring activity shifted away from the intersections. However, increasing the permeability at the core of the fault plane intersection by 40% shifted the discharge back to the intersections. When aquifer units were assigned a permeability value equal to those of the fault planes, convective rolls developed that extend about 3 km laterally along the fault plane and into the adjacent aquifer.

Radiocarbon dating of residual organic matter in travertine formed along the Yumoto Fault in Oga Peninsula, northeast Japan: Implications for long-term hot spring activity under the influence of earthquakes

Radiocarbon dating was performed on trace amounts of organic matter included in travertines formed along the Yumoto fault in Oga Peninsula, NE Japan. The reliability of the dating method was confirmed by the consistency of ages obtained with the stratigraphy of the travertine mound and the ratio of carbon stable isotopes. The radiocarbon chronology of the travertines shows that (1) the hot springs at Oga have been active for approximately 29,000 years, with average depositional rates ranging from 0.24 to 1.75 mm/yr, and (2) the principal hot spring activity has migrated southward with time. Variations in the accumulation rates and the progressive migration of the main focus of the hot spring activity imply irregular spring behaviour. This behaviour has not been influenced by paleoclimate, but may have been controlled by the earthquakes that occur frequently in the region around Oga Peninsula.

Genesis and Preservation of a Uranium-Rich Paleozoic Epithermal System with a Surface Expression (Northern Flinders Ranges, South Australia): Radiogenic Heat Driving Regional Hydrothermal Circulation over Geological Timescales

The surface expressions of hydrothermal systems are prime targets for astrobiological exploration, and fossil systems on Earth provide an analogue to guide this endeavor. The Paleozoic Mt. Gee-Mt. Painter system (MGPS) in the Northern Flinders Ranges of South Australia is exceptionally well preserved and displays both a subsurface quartz sinter (boiling horizon) and remnants of aerial sinter pools that lie in near-original position. The energy source for the MGPS is not related to volcanism but to radiogenic heat produced by U-Th-K-rich host rocks. This radiogenic heat source drove hydrothermal circulation over a long period of time (hundreds of millions of years, from Permian to present), with peaks in hydrothermal activity during periods of uplift and high water supply. This process is reflected by ongoing hot spring activity along a nearby fault. The exceptional preservation of the MGPS resulted from the lack of proximal volcanism, coupled with tectonics driven by an oscillating far-field stress that resulted in episodic basement uplift. Hydrothermal activity caused the remobilization of U and rare earth elements (REE) in host rocks into (sub)economic concentrations. Radiogenic-heat-driven systems are attractive analogues for environments that can sustain life over geological times; the MGPS preserves evidence of episodic fluid flow for the past ∼300 million years. During periods of reduced hydrothermal activity (e.g., limited water supply, quiet tectonics), radiolytic H(2) production has the potential to support an ecosystem indefinitely. Remote exploration for deposits similar to those at the MGPS systems can be achieved by combining hyperspectral and gamma-ray spectroscopy.

Influence of the Chuxiong Yao'an Earthquake on the Mineralization of Hot Springs in the Tengchong Geothermal Area, Southwestern China

Abstract:The Tengchong geothermal area, an active tectonic region with frequent earthquakes, is located in Yunnan Province of southwestern China. This area contains abundant active hot springs, which often display high metal concentrations and obvious mineralization phenomena. At 19:19 on 9 July 2009, an earthquake occurred in Yao'an, Chuxiong, Yunnan Province, which is 300 km to the northeast of the Tengchong geothermal area. We sampled water in the hot springs in the Tengchong area from 4 July to 9 July 2009 and from 10 July to 15 July 2009 to study the changes of elemental concentrations before and after the earthquake and discuss the influence of the earthquake on the mineralization of the hot springs. The concentrations of most trace elements increased slightly, but the concentration of REE (rare earth elements) decreased by 50% after the earthquake in the hot springs around a NS‐trending fault. The elemental concentrations remained unchanged in Longtan and Suanshuigou, which are related to an inactive crater. The elemental concentrations in other springs controlled by superficial and small‐scale faults decreased after the earthquake. The earthquake can stimulate the activity of deep faults and magma chambers, as is responsible for the increase of metal concentrations in the hot springs along the NS‐trending deep fault; whereas it can decrease the porosity of permeable rocks, resulting in the decline of the flux of ore‐bearing fluids and the corresponding mineralization in the hot springs related to superficial faults.

Active tectonics of the Alaotra–Ankay Graben System, Madagascar: Possible extension of Somalian–African diffusive plate boundary?

The Lake Alaotra–Ankay rift valley of Central Madagascar forms a NE–SW oriented depression filled with Neogene to recent sediments and is part of a more regional post-Miocene graben system that strikes N–S across much of the central part of the island. The region is characterized by a number of small earthquakes, active hot springs, steep fault-scarp bounded valleys, and several levels of terraces. Young, deeply incised topography related to intense tropical weathering, is largely concentrated near active fault systems. The origin and evolution of this extensional structure and its morphological expressions, however, have not been clearly documented. Mountain front sinuosity values indicate that the eastern side of the rift valley is tectonically more active than the western side, and that rapid erosion under subtropical conditions is causing rapid weathering of fault scarps. The active river network within the Alaotra–Ankay rift shows clear signs of surface re-adjustment to recent tectonic activity. The drainage network is asymmetric, with most channels on the east sides of valleys and controlled by regional eastward tilting. First-order rivers flow parallel to the basin axis. Zones of intense lavaka (erosional gully) formation are correlated with active fault systems and seismicity. Extension in the Ankay–Alaotra rift is oriented roughly the same as in the morphologically similar East African rift system, located only 500 km to the west. The eastern edge of the African continent is moving somewhat independently from the rest of Africa and the portion of the African plate east of the East African rift is regarded as a separate plate, the Somali plate. However, few plate configurations have clearly defined the southern extension of the Africa-Somali plate boundary to where it must join the Southwest Indian Ocean ridge, a necessary requirement of any true microplate bounded by plate boundaries. Instead, the southeast part of the African plate, including the Somali microplate, seems to be dominated by a number of individual rift systems, overall defining a broad diffusive extensional plate boundary. We suggest that this diffusive plate boundary includes one segment that extends off the coast of Africa through the Comores, then cuts through northern Madagascar and extends down the active Ankay–Alaotra rift to the fault-block dominated southeastern coast of Madagascar, then extends southeastward to the Southwest Indian Ocean Ridge. Thus, the southeastern part of the African plate is considerably more fragmented by plume-related uplift and diffuse extensional boundaries than previously envisaged.

A preliminary study of older hot spring alteration in Sevenmile Hole, Grand Canyon of the Yellowstone River, Yellowstone Caldera, Wyoming

Erosion in the Grand Canyon of the Yellowstone River, Yellowstone Caldera (640 ka), Wyoming, has exposed a cross section of older hydrothermal alteration in the canyon walls. The altered outcrops of the post-collapse tuff of Sulphur Creek (480 ka) extend from the canyon rim to more than 300 m beneath it. The hydrothermal minerals are zoned, with an advanced argillic alteration consisting of an association of quartz (opal) + kaolinite ± alunite ± dickite, and an argillic or potassic alteration association with quartz + illite ± adularia. Disseminated fine-grained pyrite or marcasite is ubiquitous in both alteration types. These alteration associations are characteristic products of shallow volcanic epithermal environments. The contact between the two alteration types is about 100 m beneath the rim. By analogy to other active geothermal systems including active hydrothermal springs in the Yellowstone Caldera, the transition from kaolinite to illite occurred at temperatures in the range 150 to 170 °C. An 40Ar/ 39Ar age on alunite of 154,000 ± 16,000 years suggests that hydrothermal activity has been ongoing since at least that time. A northwest-trending linear array of extinct and active hot spring centers in the Sevenmile Hole area implies a deeper structural control for the upflowing hydrothermal fluids. We interpret this deeper structure to be the Yellowstone Caldera ring fault that is covered by the younger tuff of Sulphur Creek. The Sevenmile Hole altered area lies at the eastern end of a band of hydrothermal centers that may mark the buried extension of the Yellowstone Caldera ring fault across the northern part of the Caldera.

Jurassic hot-spring activity in a fluvial setting at La Marciana, Patagonia, Argentina

AbstractSilicified rocks at La Marciana farm, Deseado Massif, Argentinean Patagonia, represent an ancient hot-spring discharging into an active fluvial setting. Their fortuitous burial–erosion history and minimal post-depositional structural modification provide an unparalleled view of a complete, exhumed, late Jurassic geothermal landscape, and thus an opportunity to illuminate hot-spring geological context and palaeoenvironmental gradients. Geological mapping, stratigraphy and petrography revealed hydrothermal eruption events, the spring vent source, structural relationships, dimensions of the discharge apron, and hot-spring facies distributions. Remarkable similarities to Quaternary analogues from the Taupo Volcanic Zone, New Zealand, and Yellowstone National Park, USA, are apparent with respect to scale, spatial distribution of facies, and types of microbial and other palaeoenvironmentally significant fabrics.

A new Middle–Late Jurassic flora and hot spring chert deposit from the Deseado Massif, Santa Cruz province, Argentina

We present an initial report of a well-preserved and relatively diverse Gondwanan plant assemblage from Bahía Laura Group, Chon Aike Formation strata of the Estancia Flecha Negra area, central-western region of the Deseado Massif, Santa Cruz province, Patagonia, Argentina. The locality contains the first richly fossiliferous chert with a diverse and well-preserved plant assemblage reported from the Mesozoic which is demonstrably associated with hot spring activity. A compression flora and petrified forest contained in associated clastic and volcaniclastic environments provide an indication of regional plant diversity during this as yet poorly represented stratigraphic interval.

Influence of subduction zone settings on the origin of forearc fluids: Halogen concentrations and 129I/I ratios in waters from Kyushu, Japan

Fluid migration in subduction zones is one of the key phenomena to understand the global mass transfer system. While active volcanoes provide the most recognizable conduits for fluid flow in active margins, the existence of a large number of active fluid seepages demonstrates that other forms of fluid release are also important in subduction zone settings. The authors collected fluid samples from springs and wells across the forearc area in Kyushu, a southwestern island of Japan, covering hot spring activities associated with active volcanism and the Median Tectonic Line (MTL), a major fault system present in the southwestern part of Japan. In order to determine sources of these fluids, halogen concentrations as well as 129I/I and 36Cl/Cl ratios were measured in samples from several locations. While Cl concentrations of the forearc fluids in Kyushu range between seawater and meteoric water value, I concentrations are considerably higher than seawater value. Fluids in the Miyazaki area are much higher in I, and somewhat higher in Br, than waters in the Oita area, which is closely associated with the MTL. The differences between those two areas are also pronounced in 129I/I ratios, which range between 800 and 900 × 10 −15 in the Oita area and between 100 and 360 × 10 −15 in the Miyazaki area. The 129I/I ratios obtained from the Oita area are compatible with an I derivation from subducting marine sediments, similar to findings from an earlier investigation of fluids collected from Satsuma-Iwojima, an active volcano south of Kyushu Island. In the Miyazaki area, on the other hand, I ages are too old to be derived from currently subducting marine sediments and point to a derivation from old organic-rich materials in the upper plate of the forearc region. The results demonstrate the presence of very different fluid systems in the forearc area of Kyushu: old CH 4-rich fluids dominate in the seaward side of the forearc, while fluids close to the MTL and the Quaternary Volcanic Front demonstrate derivations from subducting marine sediments. The latter fluids in the MTL area probably are transported through the fractures associated with the fault activities, suggesting that this fault system reaches the transition zone between upper and lower plates in this region.

Surface mineral mapping at Steamboat Springs, Nevada, USA, with multi-wavelength thermal infrared images

The purpose of this study was to evaluate the use of airborne multispectral and hyperspectral thermal infrared (TIR) image data for mapping surface minerals characteristic of active geothermal systems. TIR image data from the MASTER multispectral spectrometer and the SEBASS hyperspectral spectrometer were acquired over Steamboat Springs, Nevada, USA, in September 1999. Spectral emissivity information was extracted from the data and used to generate mineral maps. Using the MASTER data it was possible to map the extent of the active geothermal area as well as silica-rich vs. clay-rich areas. Using the SEBASS data the same areas could be mapped and additionally the presence of several minerals, including opal, quartz, alunite, anorthite, albite, and kaolinite could be identified. The SEBASS data also detected previously unidentified hydrous sulfate minerals (tamarugite and alunogen) forming around active fumaroles. Laboratory spectral data and XRD analyses of field samples confirmed the dominant mineral phases identified remotely. The airborne, field and laboratory data together with temperature anomaly and fumarole location data from other studies were synthesized and linked to the surface geology to determine the diagnostic surface mineral expression that could be mapped with TIR remotely sensed data for future exploration for similar geothermal systems. Opaline sinter deposits are the most diagnostic surface expression of hot spring activity at Steamboat Springs because they represent areas of the recent geyser activity. The differentiation between opal and chalcedony deposits and the identification of hydrous sulfate minerals forming around active fumaroles are also important because chalcedony represents ancient sinter deposits and hydrous sulfate minerals indicate locations of fumaroles that may be venting diffusely with no geyser activity. The surface expression of the geothermal system at Steamboat Springs is small, which emphasizes the need for high spatial resolution airborne imagers. The 90-m pixels of ASTER TIR data over the same area were only capable of resolving the area of past and recent sinter deposits together. MASTER multispectral TIR data covered the entire area (∼12 km 2) at 5-m spatial resolution and were capable of defining the extent of the sinter deposits and zones of hydrothermally altered rocks. The SEBASS image swath was too narrow (256 m) to add significant spatial information to the overall map, but the high spectral content identified several important minerals related to the local geology and the mineralogic surface expression of the geothermal system.

Geochemistry and petrogenesis of late Pleistocene to Recent volcanism in Southern Dominica, Lesser Antilles

Dominica is the most productive volcanic island in the Lesser Antilles arc and contains the largest number of potentially active volcanoes, but is not well studied petrologically. Quaternary volcanic activity on Dominica has been dominated by intermediate to felsic magmas, erupted as large-volume ignimbrites (Roseau, Grand Savanne, Grand Bay) and dome complexes. One of the largest concentrations of these deposits, and one of the currently most active areas in terms of shallow seismicity and hotspring activity, is the Plat Pays Volcanic Complex (PPVC) at the southern tip of the island. The PPVC is made up of medium-K calc-alkaline silicic andesites and rarer dacites. The major and trace element compositions define regular variation trends that can be explained by fractionation of observed phenocryst phases (plagioclase, clinopyroxene, orthopyroxene, Fe–Ti oxides). The contemporary Morne Anglais center erupted a wider range of magmas, extending from silicic andesites (identical in composition to the PPVC) to basalt. The new geochemical and Sr–Nd–Pb isotope data presented here cover all main units of the PPVC and Morne Anglais, and give a reconnaisance view of three other important Quaternary volcanic centers on Dominica that are associated with large-volume ignimbrite eruptions: Diablotins, Wotten Waven/Micotrin and Trios Pitons. Geochemical modelling of variation trends from the PPVC and Anglais centers indicates these are consistent with derivation from a basaltic parent magma like that of Morne Anglais. With a few exceptions at the mafic end of the spectrum, Sr, Nd and Pb isotope ratios remain constant with fractionation from 52 to 64 wt.% SiO 2. The ɛNd values range from + 2 and + 4, and 87Sr / 86Sr between 0.7041 and 0.7047. The range of values for 208Pb / 204Pb, 207Pb / 204Pb and 206Pb / 204Pb are 39.2–39.4, 15.71–15.75, and 19.4–19.5, respectively. Exceptions to this are basalt scoria clasts from Anglais and an andesite dike cutting PPVC domes. These samples have higher ɛNd and less radiogenic Pb isotope ratios. This, together with other geochemical anomalies observed in the Anglais scoria samples and field evidence for magma mixing from their sample locations, indicates a more open-system behavior of the Anglais system, perhaps involving magma recharge. Application to the PPVC and Morne Anglais samples of magnetite–ilmenite, hornblende–plagioclase and two-pyroxene geothermometry yielded temperature estimates in the range of 800 to 890 °C for silicic andesite and dacite, and 970–1100 °C for basaltic andesites from Morne Anglais. Pressure estimates from magnetite–ilmenite–orthopyroxene equilibria are 200–300 MPa, similar to an estimate of 200 MPa derived from dissolved Cl and H 2O contents in melt inclusions, and consistent with the 5–6 km depth of volcanic seismicity observed in the PPVC area between 1998 and 2000. The similarity of chemical and isotopic compositions, regular differentiation trends and P– T conditions for the different units of the PPVC suggest that recurrent eruptions in the last 40 ka have tapped a common, rather homogeneous magma source. Variations are due to local differences in the degree of differentiation before eruption. The long eruptive history at the PPVC, including episodic dome growth over the last 40 ka, may indicate rejuvination of the system by mafic magma recharge and this is supported by the proximity to the contemporaneous mafic Anglais center which itself shows some evidence of basaltic magma recharge. Despite this, we find no convincing petrographic or geochemical evidence in its eruptive products for mafic recharge into the PPVC magma system.