High-temperature Hydrothermal Activity Research Articles

Oxygen isotope compositions of intracaldera rocks: hydrothermal history of the Long Valley Caldera, California

The Bishop Tuff and associated rocks filling Long Valley Caldera, California represent a case where thick and relatively uniform geologic units of known initial compositions have been subjected to a strong geothermal fluid flux within an enclosed basin. Oxygen isotopic compositions within individual components of this ‘flux plate’ are used to characterize a magmatically driven paleohydrothermal system. Oxygen isotope ratios of silicate components analyzed with a laser-probe/mass-spectrometer system clearly illustrate the isotopic heterogeneity in the hydrothermally altered felsic volcanic rocks. The alteration resulted from moderately high-temperature hydrothermal activity. Hydrothermal activity is linked to periods of post-caldera-collapse volcanism and resurgence in the central caldera. In general, 18O depletion in the Bishop Tuff resulting from exchange with hydrothermal fluids proceeds as pumice matrix sanidine quartz due to progressive resistance to alteration. Samples from the Long Valley Exploratory Well (LVEW) centered on the resurgent dome show downward increasing exchange and disequilibrium over the well's 2000-m depth. A flanking well shows the opposite pattern over 1300 m depth. Oxygen isotope isopleths along a W-E vertical cross section of the caldera reveals convective circulation upwards and outwards from the central resurgent dome. A paleotemperature maximum of ~ 350 °C in LVEW and a geothermal gradient of ~ 130 °C/km are deduced from feldspar/water oxygen isotope fractionation equations using the pumice and sanidine components of the intracaldera volcanic rocks. This is consistent with the degree of alteration and the secondary mineralogy. Present-day temperature maximum in the well is ~ 100 °C, and the geothermal gradient is 50 °C/km. Fossil hydrothermal water, trapped in fluid inclusions in hydrothermal vein quartz hosted by intracaldera rocks, has a calculated oxygen isotope composition of − 10.2%. based on chemistry and trapping temperature. This is 4%. heavier than the average value for the present-day hydrothermal water and consistent with higher temperatures and water/rock ratios. Intrusive activity was initiated in the resurgent dome area soon after caldera collapse (760 ka), but migrated toward the western margin of the caldera, the locus of Holocene volcanism. Thus, the central hydrothermal system matured and waned as the heat sources from intrusions shifted westward. Geophysical evidence of renewed intrusive activity located beneath the resurgent dome may presage a future renewal of the cycle of hydrothermal activity in the central caldera region. Although the hydrothermal activity is complex and transitory, this history is normal for a large, relatively active caldera system such as Long Valley Caldera.

Hydrothermal activity and ridge segmentation on the Mid-Atlantic Ridge: a tale of two hot-spots?

Abstract In the past five years we have completed systematic seabed and water column surveys of two key sections of the Reykjanes Ridge and the Mid-Atlantic Ridge, between 36° and 38° North, close to the Iceland and Azores hot-spots, respectively. These surveys have provided a data set which provides a unique opportunity to address the relationship between the incidence and location of hydrothermal venting relative to volcanism/tectonism and ridge segmentation. Along the Reykjanes Ridge, running SW from Iceland, deep-tow (TOBI) sidescan sonar and swath bathymetry data have indicated a pattern dominated by volcanism in the form of axial volcanic ridges with only minor tectonic fracturing. A water column survey along this section of ridge crest, using conventional profiling techniques, indicated no evidence for high-temperature hydrothermal activity except at 63°N where the Reykjanes Ridge intercepts the bathymetric platform (≤500m water depth) which surrounds Iceland. Along the Mid-Atlantic Ridge between 36–38°N, running SW from the Azores archipelago, TOBI and swath bathymetry data have imaged a ridge with spreading rate and obliquity of spreading motion similar to the Reykjanes Ridge. But this ridge-section is dominated by tectonism rather than volcanism, the axis being partitioned into a series of short (20–55 km length) segments of spreading axis, each systematically displaced from its neighbours by right-lateral non-transform offsets of 15–50 km. Hydrothermal activity (as detected by a combination of deep-tow instruments and conventional profiling) is much more common along this latter section of spreading centre: evidence for discrete sources of hydrothermal discharge was obtained on fourteen different occasions, in seven separate segments of ridge crest. Sources of hydrothermal activity, including the major Rainbow hydrothermal field, were observed to coincide consistently with ridge-offset intersections suggesting a strong link between tectonic segmentation and the occurrence of high temperature venting.

Geological setting of high-temperature hydrothermal activity and massive sulphide formation on fast- and slow-spreading ridges

Abstract Geological features which control massive sulphide formation on the fast-spreading East Pacific Rise (EPR) and slow-spreading Mid-Atlantic Ridge (MAR) can be specified based on data from the Sevmorgeologija Association (St Petersburg) research cruises and results of other studies. Wide crestal surfaces of undisturbed axial volcanoes and the presence of axial grabens that indicate voluminous subsurface magma chambers represent the sites most favourable for sulphide formation on the EPR. The elevation of rift segments and distance from major ridge-axis discontinuities are less important for sulphide formation. Sites of localized magma delivery from subcrustal zones, as indicated by Mg anomalies in basalts, may be favourable. However, at least one site near 21°30′S on the EPR shows evidence of along-axial magma penetration from the central part of the rift segment to its tip, resulting in a lateral shift of hydrothermal activity with time. Higher crustal permeability for magma is required for the formation of subsurface chambers which initiate hydrothermal convection on the magmatically less active MAR. Rift valley marginal faults, and especially their intersections with minor transverse dislocations, locally control hydrothermal activity where magma laterally penetrates from the extrusive zones of the adjacent rift segments.

Helium and carbon geochemistry of hydrothermal fluids from the North Fiji Basin spreading ridge (southwest Pacific)

The North Fiji Basin is one of the active marginal basins in the southwestern Pacific. Hydrothermal fluid emanations were located at two sites on the Central ridge of the basin. High-temperature fluids (230–290°C) venting from anhydrite chimneys at a 17°S site have end-member compositions of: 11.0–14.5 mmol/kg of CO 2; 30.4–43.5 μmol/kg of CH 4; and 2.3–4.5 × 10 −5 cm 3STP/g of He. Taking phase separation processes into account, the gas abundances are not enriched compared with the mid-oceanic ridge hydrothermal systems. Isotopic compositions of CO 2 ( δ 13 C = −6.2 to −5.7‰ PDB), CH 4 ( δ 13 C = −20 to −18‰ PDB), and helium ( R R A = 9.0–10.0 ) are comparable to the mid-oceanic ridge signature. Together with basalt helium data, the helium isotopic signature may be attributed to the incorporation of a hotspot-like primitive component. Low-temperature shimmering fluids (the highest measured temperature was 5.2°C) associated with biological communities at a site at 18°50′S show slight chemical anomalies, in some species, in SiO 2, Mn, Li, pH and CH 4, and helium isotope ratios distinct from ambient seawater. Evaluated helium isotopic compositions prior to dilution fall between R R A = 8.4 and 8.9, supporting the suggestion of high-temperature hydrothermal activity at this site, although this was not observed by dive expeditions. The gas geochemistry of these two different types of fluids show several similar characteristics to the mid-oceanic ridge hydrothermal systems. This result is in accordance with previous petrological studies which demonstrated a dominant N-MORB source signature and a co-existing OIB source influence of the North Fiji Basin magmatism.

Interaction of submarine volcanic and high-temperature hydrothermal activity proposed for the formation of the Agrokipia, volcanic massive sulfide deposits of Cyprus

The results of drilling near the spreading-ridge-type, volcanic-hosted, massive sulfide deposits of Agrokipia, Cyprus, are described. Mineralization and associated argillic hydrothermal alteration occur over intervals of 5–130 m and at depths of 80–230 m beneath the original surface of the oceanic crust. Mineralization occurs in massive flows that probably represent a locally ponded sequence up to 300 m thick. Abundant glass–aphanitic basalt transitions are present from about 100 m below the surface of the ponded sequence, with glass abundances locally reaching 60% of the section. A novel hypothesis, involving the presence of active, high-temperature hydrothermal vents beneath the cooling ponded sequence, with the passage of hydrothermal fluids through the still molten lava, is proposed to account for the observations. While this hypothesis is reasonable, the inferred processes have not, as yet, been demonstrated under either laboratory or field conditions. The seafloor expression of this system was probably one of widely distributed, low-temperature, fluid emission over the surface of a lava pond in the axial graben of a spreading ridge.

Laser microprobe study of sulfur isotope variation in a sea-floor hydrothermal spire, Axial Seamount, Juan de Fuca Ridge, eastern Pacific

The Axial Seamount site is located on the Juan de Fuca Ridge near the intersection of the Cobb-Eikenberg-Brown BEar seamount chain and the axial rift of the ridge. Several large (to 12 m) silica+sulfide+sulfate hydrothermal spires reflect high-temperature (to 300°C) hydrothermal activity. Laser microprobe analyses along traverses of five temporally distinct fluid conduits within an inactive high-temperature spire reveal significant temporal variation inδ34S, both within individual conduits and between conduits. The maximum intra-conduitδ34S variation (on wurtzite) is 5‰ and the maximum inter-conduit variation (on sphalerite) is 7.4‰ Inter-conduitδ34S variation occurs primarily due to variable amounts of seawater mixing with hydrothermal fluids within the spire; generallyδ34Smineral decreases through time due to lesser amounts of seawater that invade more mature, heavily mineralized spires. Intra-conduitδ34S variation has not been documented at this scale (5‰ within 1 mm), and the potential mechanisms responsible for this variation include: (1) within-spire seawater-hydrothermal fluid mixing, which also produces the inter-conduit variations, or (2) more deep-seated convection and redox processes in the sea-floor subsurface that alter theδ34Sfluid.Mixing of hydrothermal fluid with seawater cannot produce a range ofδ34Smineral-values of this magnitude (5‰), and deeper subsurface processes are required. Such deep-seated processes may involve the opening of new fluid conduits in the subsurface, exposing fresh basalt which will increase the reduction potential of the rock system. This in turn will promote increased reduction of seawater sulfate in the hydrothermal fluid and attendant increases ofδ34Sfluid. The fluids will ulti precipitate34S-enriched sulfide phases, although this excursion inδ34S-values is ephemeral and will last only until the fluid has exhausted the reducing potential of the new conduit. At this point, sulfate reduction will be sharply reduced andδ34S-values will correspondingly decrease. This process may explain the major (to +5‰) isotope excursions that occur within individual conduits on a very small (<1m) scale.

The morphology and tectonics of the Mark area from Sea Beam and Sea MARC I observations (Mid-Atlantic Ridge 23° N)

High-resolution Sea Beam bathymetry and Sea MARC I side scan sonar data have been obtained in the MARK area, a 100-km-long portion of the Mid-Atlantic Ridge rift valley south of the Kane Fracture Zone. These data reveal a surprisingly complex rift valley structure that is composed of two distinct spreading cells which overlap to create a small, zero-offset transform or discordant zone. The northern spreading cell consists of a magmatically robust, active ridge segment 40–50 km in length that extends from the eastern Kane ridge-transform intersection south to about 23°12′ N. The rift valley in this area is dominated by a large constructional volcanic ridge that creates 200–500 m of relief and is associated with high-temperature hydrothermal activity. The southern spreading cell is characterized by a NNE-trending band of small (50–200 m high), conical volcanos that are built upon relatively old, fissured and sediment-covered lavas, and which in some cases are themselves fissured and faulted. This cell appears to be in a predominantly extensional phase with only small, isolated eruptions. These two spreading cells overlap in an anomalous zone between 23°05′ N and 23°17′ N that lacks a well-developed rift valley or neovolcanic zone, and may represent a slow-spreading ridge analogue to the overlapping spreading centers found at the East Pacific Rise. Despite the complexity of the MARK area, volcanic and tectonic activity appears to be confined to the 10–17 km wide rift valley floor. Block faulting along near-vertical, small-offset normal faults, accompanied by minor amounts of back-tilting (generally less than 5°), begins within a few km of the ridge axis and is largely completed by the time the crust is transported up into the rift valley walls. Features that appear to be constructional volcanic ridges formed in the median valley are preserved largely intact in the rift mountains. Mass-wasting and gullying of scarp faces, and sedimentation which buries low-relief seafloor features, are the major geological processes occurring outside of the rift valley. The morphological and structural heterogeneity within the MARK rift valley and in the flanking rift mountains documented in this study are largely the product of two spreading cells that evolve independently to the interplay between extensional tectonism and episodic variations in magma production rates.

Heat flow measurements on a hydrothermally‐active, slow‐spreading ridge: The Escanaba Trough

Maximum heat flow measurements at three locations in the sediment‐filled Escanaba Trough of the Gorda ridge exceed 1200 mW/m². At other ridge crests with thick sediment cover, heat flow values of this magnitude are accompanied by high temperature hydrothermal vent activity and massive sulfide deposition. A dredge haul from the southernmost high heat flow location recovered pyrrhotite, thereby confirming the presence of recent high temperature venting.