Progressive Hydrothermal Alteration Research Articles

Seafloor Magnetism Under Hydrothermal Alteration: Insights From Magnetomineralogy and Magnetic Properties of the Southwest Indian Ridge Basalts

AbstractTitanomagnetites in mid‐ocean ridge basalt (MORB) experience variable post crystallization alterations associated with seafloor tectonic and environmental processes. Compared to low‐temperature oxidation, seafloor hydrothermal alteration is thought to be more destructive but its magnetic aftermaths are insufficiently documented. Here we present comprehensive rock magnetic and electron microscopic analyses of fresh and hydrothermally‐altered MORBs dredged from the Longqi and Yuhuang hydrothermal fields, Southwest Indian Ridge. We observe large variations in magnetic properties of fresh MORBs, originated from relative proportions of nano‐scale single‐domain to vortex state and micron‐scale vortex to multi‐domain state dendritic titanomagnetites. Progressive hydrothermal alteration produces secondary magnetite through recrystallization of exsolved and dissolved Fe from primary titanomagnetite. Exsolution is evident by a dual Verwey transition signature and coexisting Ti‐poor titanomagnetites and sphenes in partially chloritized basalts. A schematic model is proposed to explain the variations in magnetomineralogy and magnetic properties with progressive hydrothermal alteration. Intermediate hydrothermal alteration products retain a secondary chemical remanent magnetization (CRM) which is related to the long‐term magnetization variations in oceanic basalts. The established framework allows characterizing MORB hydrothermal alteration and ultimately contributes to resolving the complexity of seafloor magnetism.

Alteration of volcanic rocks and changes in physical-mechanical properties on the South-Kambalny thermal field (South Kamchatka)

The aim of this research project was to consider hydrothermal alteration of volcanic rocks and accompanying changes in their physical and mechanical properties on the thermal field of South Kamchatka. Under the influence of thermal water and steam original andesites and basaltic andesites gradually transform to clay-rich soils and then to opalites and secondary quartzites. The changes in the mineral composition, microstructure, and physical-mechanical properties of rocks and soils in the thermal field were studied. Argillic hydrothermal alteration has gradually decomposed andesites to weak clay-rich soils which form a cover above the field. Elevated sites on the thermal field are composed of opalites and monoquartzites. Progressive hydrothermal alteration of andesites and changes in their properties promotes a broad range of geological phenomena including initiation of landslides, migration of thermal manifestations, changes in relief, and surface deformation.

Tracking Water‐Rock Interaction at the Atlantis Massif (MAR, 30°N) Using Sulfur Geochemistry

AbstractHydrothermal alteration and serpentinization of ultramafic rocks at oceanic core complexes involve extensive element exchange between fluid and rock and a wide range of biogeochemical processes. These processes influence the global sulfur cycle due to both biogenic and abiogenic removal of seawater sulfate. Hence, ocean floor serpentinization connects the hydrosphere, biosphere, and lithosphere. This work presents a study of the sulfur geochemistry of highly altered mafic and ultramafic samples from the Atlantis Massif located at 30°N along the Mid‐Atlantic Ridge. The analyzed samples were drilled during International Ocean Discovery Program Expedition 357 and collected during Alvin dives in 2000, 2003, and 2005. Multiple sulfur isotope analyses of sulfide and sulfate phases indicate that several processes took place during progressive hydrothermal alteration: (1) incorporation of seawater sulfate, (2) thermochemical sulfate reduction during interaction with high‐temperature (high‐T; 350–400 °C), low‐pH fluids and input of H2S derived from leaching gabbroic intrusions, (3) microbial sulfate reduction, and (4) oxidation of sulfides at high water‐rock ratios. Petrological examinations show that high‐T fluids mostly postdated the bulk serpentinization and that these fluid pulses were relatively localized (<1 dm scale) resulting in a highly heterogeneous mineralogy. Locally, high‐T fluid influx took place subsequent to microbial sulfate reduction and oxidation as indicated by geochemical modeling. Overall, this study documents the complex interplay of magmatic processes, fluid‐rock interaction, and microbial activity that take place during the formation of oceanic core complexes and where mantle rocks are exposed to seawater.

Shear-wave velocity structure of the Tongariro Volcanic Centre, New Zealand: Fast Rayleigh and slow Love waves indicate strong shallow anisotropy

Models of the velocity structure of volcanoes can help define possible magma pathways and contribute to calculating more accurate earthquake locations, which can help with monitoring volcanic activity. However, shear-wave velocity of volcanoes is difficult to determine from traditional seismic techniques, such as local earthquake tomography (LET) or refraction/reflection surveys. Here we use the recently developed technique of noise cross correlation of continuous seismic data to investigate the subsurface shear-wave velocity structure of the Tongariro Volcanic Centre (TgVC) of New Zealand, focusing on the active Ruapehu and Tongariro Volcanoes. We observe both the fundamental and first higher-order modes of Rayleigh and Love waves within our noise dataset, made from stacks of 15min cross-correlation functions. We manually pick group velocity dispersion curves from over 1900 correlation functions, of which we consider 1373 to be high quality. We subsequently invert a subset of the fundamental mode Rayleigh- and Love-wave dispersion curves both independently and jointly for one dimensional shear-wave velocity (Vs) profiles at Ruapehu and Tongariro Volcanoes. Vs increases very slowly at a rate of approximately 0.2km/s per km depth beneath Ruapehu, suggesting that progressive hydrothermal alteration mitigates the effects of compaction driven velocity increases. At Tongariro, we observe larger Vs increases with depth, which we interpret as different layers within Tongariro's volcanic system above altered basement greywacke. Slow Vs, on the order of 1–2km/s, are compatible with P-wave velocities (using a Vp/Vs ratio of 1.7) from existing velocity profiles of areas within the TgVC, and the observations of worldwide studies of shallow volcanic systems that used ambient noise cross-correlation methods.Most of the measured group velocities of fundamental mode Love-waves across the TgVC are 0.1–0.4km/s slower than those of fundamental mode Rayleigh-waves in the frequency range of 0.25–1Hz. First-higher mode Love-waves are similarly slower than first-higher mode Rayleigh waves. This is incompatible with synthetic dispersion curves we calculate using isotropic, layered velocity models appropriate for Ruapehu and Tongariro, in which Love waves travel more quickly than Rayleigh waves of the same period. The Love-Rayleigh discrepancy is likely due to structures such as dykes or cracks in the vertical plane having increased influence on surface-wave propagation. However, several measurements at Ruapehu have Love-wave group velocities that are faster than Rayleigh-wave group velocities. The differences between the Love- and Rayleigh-wave dispersion curves also vary with the azimuth of the interstation path across Ruapehu and Tongariro Volcanoes. Significant azimuthal dependence of both Love and Rayleigh-wave velocities are also observed. This suggests azimuthal anisotropy within the volcanic structures, which coupled with radial anisotropy, makes the Vs structures of Ruapehu and Tongariro Volcanoes anisotropic with orthorhombic or lower order symmetry. We suggest that further work to determine three-dimensional volcanic structures should include provisions for such anisotropy.

Physical, chemical and mineralogical evolution of the Tolhuaca geothermal system, southern Andes, Chile: Insights into the interplay between hydrothermal alteration and brittle deformation

In this study, we unravel the physical, chemical and mineralogical evolution of the active Tolhuaca geothermal system in the Andes of southern Chile. We used temperature measurements in the deep wells and geochemical analyses of borehole fluid samples to constrain present-day fluid conditions. In addition, we reconstructed the paleo-fluid temperatures and chemistry from microthermometry and LA-ICP-MS analysis of fluid inclusions taken from well-constrained parageneses in vein samples retrieved from a~1000m borehole core. Based on core logging, mineralogical observations and fluid inclusions data we identify four stages (S1–S4) of progressive hydrothermal alteration. An early heating event (S1) was followed by the formation of a clay-rich cap in the upper zone (<670m) and the development of a propylitic alteration assemblage at greater depth (S2). Boiling, flashing and brecciation occurred later (S3), followed by a final phase of fluid mixing and boiling (S4). The evolution of hydrothermal alteration at Tolhuaca has produced a mineralogical, hydrological and structural vertical segmentation of the system through the development of a low-permeability, low-cohesion clay-rich cap at shallow depth. The quantitative chemical analyses of fluid inclusions and borehole fluids reveal a significant change in chemical conditions during the evolution of Tolhuaca. Whereas borehole (present-day) fluids are rich in Au, B and As, but Cu-poor (B/Na~100.5, As/Na~10−1.1, Cu/Na~10−4.2), the paleofluids trapped in fluid inclusions are Cu-rich but poor in B and As (B/Na~10−1, As/Na~10−2.5, Cu/Na~10−2.5 in average). We interpret the fluctuations in fluid chemistry at Tolhuaca as the result of transient supply of metal-rich, magmatically derived fluids where As, Au and Cu are geochemically decoupled. Since these fluctuating physical and chemical conditions at the reservoir produced a mineralogical vertical segmentation of the system that affects the mechanical and hydrological properties of host rock, we explored the effect of the development of a low-cohesion low-permeability clay cap on the conditions of fault rupture and on the long-term thermal structure of the system. These analyses were performed by using rock failure condition calculations and numerical simulations of heat and fluid flows. Calculations of the critical fluid pressures required to produce brittle rupture indicate that within the clay-rich cap, the creation or reactivation of highly permeable extensional fractures is inhibited. In contrast, in the deep upflow zone the less pervasive formation of clay mineral assemblages has allowed retention of rock strength and dilatant behavior during slip, sustaining high permeability conditions. Numerical simulations of heat and fluid flows support our observations and suggest that the presence of a low permeability clay cap has helped increase the duration of high-enthalpy conditions by a factor of three in the deep upflow zone at Tolhuaca geothermal system, when compared with an evolutionary scenario where a clay cap was not developed. Furthermore, our data demonstrate that the dynamic interplay between fluid flow, crack-seal processes and hydrothermal alteration are key factors in the evolution of the hydrothermal system, leading to the development of a high enthalpy reservoir at the flank of the dormant Tolhuaca volcano.

Hydrothermal alteration in the Reykjanes geothermal system: Insights from Iceland deep drilling program well RN-17

The Reykjanes geothermal system is a seawater-recharged hydrothermal system that appears to be analogous to seafloor hydrothermal systems in terms of host rock type and low water/rock alteration. The similarities make the Reykjanes system a useful proxy for seafloor vents. At some time during the Pleistocene, the system was dominated by meteoric water recharge, and fluid composition at Reykjanes has evolved through time as a result of changing proportions of meteoric water influx as well as differing pressure and temperature conditions. The purpose of this study is to characterize secondary mineralization, degree of metasomatic alteration, and bulk composition of cuttings from well RN-17 from the Reykjanes geothermal system. The basaltic host rock includes hyaloclastite, breccia, tuff, extrusive basalt, diabase, as well as a marine sedimentary sequence. The progressive hydrothermal alteration sequence observed with increasing depth results from reaction of geothermal fluids with the basaltic host rock. An assemblage of greenschist facies alteration minerals, including actinolite, prehnite, epidote and garnet, occurs at depths as shallow as 350 m; these minerals are commonly found in Icelandic geothermal systems at temperatures above 250 °C (Bird and Spieler, 2004). This requires hydrostatic pressures that exceed the present-day depth to boiling point curve, and therefore must record alteration at higher fluid pressures, perhaps as a result of Pleistocene glaciation. Major, minor, and trace element profiles of the cuttings indicate transitional MORB to OIB composition with limited metasomatic shifts in easily mobilized elements. Changes in MgO, K 2O and loss on ignition indicate that metasomatism is strongly correlated with protolith properties. The textures of alteration minerals reveal alteration style to be strongly dependent on protolith as well. Hyaloclastites are intensely altered with calc-silicate alteration assemblages comprising calcic hydrothermal plagioclase, grandite garnet, prehnite, epidote, hydrothermal clinopyroxene, and titanite. In contrast, crystalline basalts and intrusive rocks display a range in alteration intensity from essentially unaltered to pervasive and nearly complete albitization of igneous feldspar and uralitization of clinopyroxene. Hydrothermal anorthite (An92–An98) occurs in veins in the most altered basalt cuttings and is significantly more calcic than igneous feldspar (An48–An79). Amphibole compositions change from actinolite to hornblende at depth. Hydrothermal clinopyroxene, which occurs in veins, has greater variation in Fe content and is systematically more calcic than igneous pyroxene and also lacks uralitic textures. Solid solutions of prehnite, epidote, and garnet indicate evolving equilibria with respect to aluminum and ferric iron.

Isotopic fractionation of nitrogen and carbon in Paleoarchean cherts from Pilbara craton, Western Australia: Origin of 15N-depleted nitrogen

Nitrogen and carbon isotopic compositions, together with mineralogy and trace element geochemistry, were studied in a few kerogen-rich Paleoarchean cherts, a barite and a dolomitic stromatolite belonging to the eastern (Dixon Island Formation) and western (Dresser and Strelley Pool Chert Formations; North Pole Dome and Marble Bar) terranes of Pilbara Craton, Western Australia. The aim of the study was to search for 15N-depleted isotopic signatures, often found in kerogens of this period, and explain the origin of these anomalies. Trace elements suggest silica precipitation by hydrothermal fluids as the main process of chert formation with a contamination from volcanoclastic detritus. This is supported by the occurrence of hydrothermal-derived minerals in the studied samples indicating precipitation temperatures up to 350 °C. Only a dolomitic stromatolite from Strelley Pool shows a superchondritic Y/Ho ratio of 72 and a positive Eu/Eu * anomaly of 1.8, characteristic of chemical precipitates from the Archean seawater. The bulk δ 13C vs. δ 15N values measured in the cherts show a roughly positive co-variation, except for one sample from the North Pole (PI-85-00). The progressive enrichment in 15N and 13C from a pristine source having δ 13C ⩽ −36‰ and δ 15N ⩽ −4‰ is correlated with a progressive depletion in N content and to variations in Ba/La and Co/As ratios. These trends have been interpreted as a progressive hydrothermal alteration of the cherts by metamorphic fluids. Isotopic exchange at 350 °C between NH 4 + (rock) and N 2(fluid) may explain the isotopic and elemental composition of N in the studied cherts. However, we need to assume isotopic exchange at 350 °C between carbonate C and graphite to explain the large 13C enrichment recorded. Only sample PI-85-00 shows a large N loss (90%) with a positive δ 15N value (+11‰), while C (up to 120 ppm and δ 13C −38‰) seems to be unaffected. This pattern has been interpreted as the result of devolatilization and alteration (oxidation) of graphite by low-temperature fluids. The 15N- 13C-depleted pristine source has δ 15N values from −7‰ to −4‰ and 40Ar/ 36Ar ratios from 30,000 to 60,000, compatible with an inorganic mantle N source, although the elemental abundance ratios N/C and 40Ar/C are not exactly the same with the mantle source. The component alternatively could be explained by elemental fractionation from metabolic activity of chemolithoautotrophs and methanogens at the proximity to the hydrothermal vents. However, ambiguities between mantle vs organic sources of N subsist and need further experimental work to be fully elucidated.

Development of magnetic fabrics during hydrothermal alteration in the Soultz-sous-Forêts granite from the EPS-1 borehole, Upper Rhine Graben

AbstractThe Variscan, magnetite-bearing Soultz-sous-Forêts granite is found between 1420 and 2230 m of the EPS-1 borehole situated in the Upper Rhine Graben (France). Our study focuses on the changes of magnetic properties that occur during the progressive hydrothermal alteration and fracturing of the Soultz granite after emplacement. The magnetic susceptibility (κ) of the granite is between 10 and 80 × 10−3 SI, and suggests that ferrimagnetic minerals are the primary carrier. During cooling and later tectonic and hydrothermal overprints, including the formation of the Rhine Graben, the granite was deformed under brittle conditions and partially altered by hydrothermal fluids. Along with this fluid activity, oxidation of magnetite to hematite occurred and reduced κ (&lt; 1 × 10−3 SI). AMS analysis on oriented samples documents the history of progressive transformation from primary magmatic fabric to tectonic fabric during hydrothermal alteration and faulting. The fresh granite with multidomain magnetite grains shows sub-horizontal magnetic foliations and randomly oriented magnetic lineations within the foliation plane. This fabric is similar to the magmatic fabric reflected by biotite. Transformation of the magnetic fabric started with localized magnetite oxidation along NW-SE oriented micro-cracks, which are probably associated with a late-magmatic alteration (stage I). Elongated and co-aligned magnetite relics within the newly formed hematite caused a well-defined NW-SE trending magnetic lineation and steeper magnetic foliation. Later alteration associated with intense brittle deformation (stage II) initially adopted this magnetic fabric, but intense cataclasis destroyed it. The geometry and orientation of magnetic fabric clearly indicate a hydrothermal alteration, which relates to the acting tectonic stresses in the post-emplacement history of the Soultz granite.

Complex compositional zoning in epidote from rhyodacitic tuff, Bobae sericite deposit, southeastern Korea

Epidote in the propylitic alteration zone of the Bobae sericite deposit, southeastern Korea, resulted from progressive hydrothermal alteration of rhyodacitic tuff. It shows complex compositional zoning patterns that include patchy zoning, multiple growth zoning, sector zoning, and oscillatory zoning. On the whole, the zoned epidote has Al-rich cores and Fe-rich rims. Remnant textures in zoned epidote indicate that the earlier zone was partially resorbed prior to growth of a later one. Al and Fe in the M-site are the most important components that control patterns of oscillatory zoning. Mn and Sr substituting Ca in the A-site show some fluctuations in the concentration amplitude, although their contribution to the oscillatory zoning is still insignificant relative to Al and Fe. A variety of zoning patterns that occur in the Bobae epidote suggest that there are disequilibrium crystallization and chemical fluctuations in the local system during multiple hydrothermal alteration processes.

Kinetically limited isotope exchange in a shallow level normal fault, Mineral Mountains, Utah

We have conducted a stable isotope and geochemical study of the Corral Canyon fault zone, a shallow‐level, low‐angle normal fault on the west flank of the Mineral Mountains in southwest Utah. Our stable isotope and geochemical data document the infiltration of local meteoric water during brittle deformation and hydrothermal alteration of the shear zone during Tertiary Basin and Range extensional faulting. Within the fault zone, 18O depletion correlates with progressive hydrothermal alteration and cataclasis, demonstrating that the processes of fluid infiltration, hydrothermal alteration, fracturing, comminution of grains, and 18O exchange took place concurrently. Interpretation of our data using a model of combined fluid flow and kinetically limited isotope exchange leads us to conclude that the exposed fault zone was infiltrated by isotopically unevolved fluids from the wall rock adjacent to the fault zone and that an important component of fluid flow in this fault was between the wall rock and the fault zone rather than strictly within the fault plane. We propose that the rate of isotopic alteration in shallow level fault zones is enhanced by comminution and recrystallization of grains during deformation, so that fluids in the wall rock, because of the relatively coarser grain size, remain relatively unevolved until they infiltrate the fault zone where they rapidly become isotopically evolved due to exchange with cataclasized grains. We present a general model for calculating the duration of isotopic alteration in flow systems characterized by kinetically‐limited isotope exchange. Calculated exchange durations for the Corral Canyon fault zone are short, on the order of thousands to a few hundreds of thousands of years, compared to the likely duration of deformation of millions of years. The geologically short duration of isotope exchange leads us to suggest that the fluid flow responsible for the isotopic alteration was episodic, of short duration, and possibly related to discrete seismic events. This result corroborates diverse lines of evidence that support a model of alternating fluid flow and permeability sealing by hydrothermal precipitation within fault zones.

Mechanisms of Mg-phyllosilicate formation in a hydrothermal system at a sedimented ridge (Middle Valley, Juan de Fuca)

We present results of a detailed mineralogical and geochemical study of the progressive hydrothermal alteration of clastic sediments recovered at ODP Site 858 in an area of active hydrothermal venting at the sedimented, axial rift valley of Middle Valley (northern Juan de Fuca Ridge). These results allow a characterization of newly formed phyllosilicates and provide constraints on the mechanisms of clay formation and controls of mineral reactions on the chemical and isotopic composition of hydrothermal fluids. Hydrothermal alteration at Site 858 is characterized by a progressive change in phyllosilicate assemblages with depth. In the immediate vent area, at Hole 858B, detrital layers are intercalated with pure hydrothermal precipitates at the top of the section, with a predominance of hydrothermal phases at depth. Sequentially downhole in Hole 858B, the clay fraction of the pure hydrothermal layers changes from smectite to corrensite to swelling chlorite and finally to chlorite. In three pure hydrothermal layers in the deepest part of Hole 858B, the clay minerals coexist with neoformed quartz. Neoformed and detrital components are clearly distinguished on the basis of morphology, as seen by SEM and TEM, and by their chemical and stable isotope compositions. Corrensite is characterized by a 24 A stacking sequence and high Si- and Mg-contents, with Fe/(Fe+Mg) ratio of ≈0.08. We propose that corrensite is a unique, possibly metastable, mineralogical phase and was precipitated directly from seawater-dominated hydrothermal fluids. Hydrothermal chlorite in Hole 858B has a stacking sequence of 14 A with Fe/(Fe+Mg) ratios of ≈0.35. The chemistry and structure of swelling chlorite suggest that it is a corrensite/chlorite mixed-layer phase. The mineralogical zonation in Hole 858B is accompanied by a systematic decrease in δ18O, reflecting both the high thermal gradients that prevail at Site 858 and extensive sediment-fluid interaction. Precipitation of the Mg-phyllosilicates in the vent region directly controls the chemical and isotopic compositions of the pore fluids. This is particularly evident by decreases in Mg and enrichments in deuterium and salinity in the pore fluids at depths at which corrensite and chlorite are formed. Structural formulae calculated from TEM-EDX analyses were used to construct clay-H2O oxygen isotope fractionation curves based on oxygen bond models. Our results suggest isotopic disequilibrium conditions for corrensite-quartz and swelling chlorite-quartz precipitation, but yield an equilibrium temperature of 300° C±30° for chlorite-quartz at 32 m below the surface. This estimate is consistent with independent estimates and indicates steep thermal gradients of 10–11°/m in the vent region.

Progressive hydrothermal alteration associated with gold mineralization of the Zone 1 intrusion of the Callahan property, Val-d'Or region, Quebec

The Zone 1 intrusion of the Callahan property, Val-d'Or, Quebec, is a relatively small (0.045 km2), pretectonic to syntectonic trondhjemitic mass that has been altered to varying degrees by progressive hydrothermal alteration. The entire intrusion is gold enriched, and the degree of mineralization is proportional to the degree of alteration. Examination of drill core reveals that visible colour variations are related to mineralogic and chemical alteration. Three degrees of alteration are identified: least, medium, and most-altered. The least-altered facies has a green–grey colour, whereas the medium-altered facies has a pale grey colour and the most-altered facies has a cream–white colour. As the intensity of alteration increases, the amount of albite, carbonate, and pyrite increases and the amount of phengite decreases. These progressive, mineralogic changes are correlated chemically to increases in Na2O, CO2, and S and to a decrease in K2O. The median gold values for least-, medium-, and most-altered trondhjemite are 42, 148, and 158 ppb, respectively, and the average gold values (excluding highly anomalous values) are 85, 172, and 188 ppb, respectively, indicating that gold enrichment is already advanced at the medium intensity of alteration. Despite its weak enrichment, tungsten is the most anomalous trace element of the typically gold-associated elements in lode deposits and is, therefore, of special interest. Gold, found principally in the medium- and most-altered facies, is most commonly observed as free grains in the gangue. It may also be associated with pyrite and, to a much lesser extent, with tellurides. The majority of the free gold grains have an estimated diameter between 20 and 60 μm, and their average composition is 95 wt.% Au and 5 wt.% Ag. This study and comparisons of the Zone 1 trondhjemitic intrusion with other gold-enriched, felsic intrusions in the Abitibi greenstone belt suggest that elevated sodium (i.e., albitization) and anomalous gold values, relative to barren felsic rocks, should be used to target ore-grade zones within pretectonic to syntectonic felsic intrusions.

Reasons for crack closure and possibilities for detection and sizing by NDE : 39928 Deuster, G. International Journal of Pressure Vessels and Piping, Vol. 35, pp. 173–188 (1988). (Proceedings of 5th International Seminar on Nondestructive Examination in Relation to Structural Integrity, Davos, Switzerland, 26–27 August 1987)

Models of the velocity structure of volcanoes can help define possible magma pathways and contribute to calculating more accurate earthquake locations, which can help with monitoring volcanic activity. However, shear-wave velocity of volcanoes is difficult to determine from traditional seismic techniques, such as local earthquake tomography (LET) or refraction/reflection surveys. Here we use the recently developed technique of noise cross correlation of continuous seismic data to investigate the subsurface shear-wave velocity structure of the Tongariro Volcanic Centre (TgVC) of New Zealand, focusing on the active Ruapehu and Tongariro Volcanoes. We observe both the fundamental and first higher-order modes of Rayleigh and Love waves within our noise dataset, made from stacks of 15 min cross-correlation functions. We manually pick group velocity dispersion curves from over 1900 correlation functions, of which we consider 1373 to be high quality. We subsequently invert a subset of the fundamental mode Rayleigh- and Love-wave dispersion curves both independently and jointly for one dimensional shear-wave velocity (Vs) profiles at Ruapehu and Tongariro Volcanoes. Vs increases very slowly at a rate of approximately 0.2 km/s per km depth beneath Ruapehu, suggesting that progressive hydrothermal alteration mitigates the effects of compaction driven velocity increases. At Tongariro, we observe larger Vs increases with depth, which we interpret as different layers within Tongariro's volcanic system above altered basement greywacke. Slow Vs, on the order of 1–2 km/s, are compatible with P-wave velocities (using a Vp/Vs ratio of 1.7) from existing velocity profiles of areas within the TgVC, and the observations of worldwide studies of shallow volcanic systems that used ambient noise cross-correlation methods.Most of the measured group velocities of fundamental mode Love-waves across the TgVC are 0.1–0.4 km/s slower than those of fundamental mode Rayleigh-waves in the frequency range of 0.25–1 Hz. First-higher mode Love-waves are similarly slower than first-higher mode Rayleigh waves. This is incompatible with synthetic dispersion curves we calculate using isotropic, layered velocity models appropriate for Ruapehu and Tongariro, in which Love waves travel more quickly than Rayleigh waves of the same period. The Love-Rayleigh discrepancy is likely due to structures such as dykes or cracks in the vertical plane having increased influence on surface-wave propagation. However, several measurements at Ruapehu have Love-wave group velocities that are faster than Rayleigh-wave group velocities. The differences between the Love- and Rayleigh-wave dispersion curves also vary with the azimuth of the interstation path across Ruapehu and Tongariro Volcanoes. Significant azimuthal dependence of both Love and Rayleigh-wave velocities are also observed. This suggests azimuthal anisotropy within the volcanic structures, which coupled with radial anisotropy, makes the Vs structures of Ruapehu and Tongariro Volcanoes anisotropic with orthorhombic or lower order symmetry. We suggest that further work to determine three-dimensional volcanic structures should include provisions for such anisotropy.

Detection sensitivity in thick assemblies (in French) : 39927 Odorico, J.; Lecuru, D. Societe Nationale Industrielle Aerospatiale, Suresnes (France), N88-23221/0/GAR, 60 pp. (Aug. 1987)

Models of the velocity structure of volcanoes can help define possible magma pathways and contribute to calculating more accurate earthquake locations, which can help with monitoring volcanic activity. However, shear-wave velocity of volcanoes is difficult to determine from traditional seismic techniques, such as local earthquake tomography (LET) or refraction/reflection surveys. Here we use the recently developed technique of noise cross correlation of continuous seismic data to investigate the subsurface shear-wave velocity structure of the Tongariro Volcanic Centre (TgVC) of New Zealand, focusing on the active Ruapehu and Tongariro Volcanoes. We observe both the fundamental and first higher-order modes of Rayleigh and Love waves within our noise dataset, made from stacks of 15 min cross-correlation functions. We manually pick group velocity dispersion curves from over 1900 correlation functions, of which we consider 1373 to be high quality. We subsequently invert a subset of the fundamental mode Rayleigh- and Love-wave dispersion curves both independently and jointly for one dimensional shear-wave velocity (Vs) profiles at Ruapehu and Tongariro Volcanoes. Vs increases very slowly at a rate of approximately 0.2 km/s per km depth beneath Ruapehu, suggesting that progressive hydrothermal alteration mitigates the effects of compaction driven velocity increases. At Tongariro, we observe larger Vs increases with depth, which we interpret as different layers within Tongariro's volcanic system above altered basement greywacke. Slow Vs, on the order of 1–2 km/s, are compatible with P-wave velocities (using a Vp/Vs ratio of 1.7) from existing velocity profiles of areas within the TgVC, and the observations of worldwide studies of shallow volcanic systems that used ambient noise cross-correlation methods.Most of the measured group velocities of fundamental mode Love-waves across the TgVC are 0.1–0.4 km/s slower than those of fundamental mode Rayleigh-waves in the frequency range of 0.25–1 Hz. First-higher mode Love-waves are similarly slower than first-higher mode Rayleigh waves. This is incompatible with synthetic dispersion curves we calculate using isotropic, layered velocity models appropriate for Ruapehu and Tongariro, in which Love waves travel more quickly than Rayleigh waves of the same period. The Love-Rayleigh discrepancy is likely due to structures such as dykes or cracks in the vertical plane having increased influence on surface-wave propagation. However, several measurements at Ruapehu have Love-wave group velocities that are faster than Rayleigh-wave group velocities. The differences between the Love- and Rayleigh-wave dispersion curves also vary with the azimuth of the interstation path across Ruapehu and Tongariro Volcanoes. Significant azimuthal dependence of both Love and Rayleigh-wave velocities are also observed. This suggests azimuthal anisotropy within the volcanic structures, which coupled with radial anisotropy, makes the Vs structures of Ruapehu and Tongariro Volcanoes anisotropic with orthorhombic or lower order symmetry. We suggest that further work to determine three-dimensional volcanic structures should include provisions for such anisotropy.

Geothermal study of reunion island: Audiomagnetotelluric survey

Abstract In 1979 and 1980, 535 magnetotelluric soundings using a frequency range of 1700 Hz-8 Hz were performed on the island of Reunion for geothermal exploration. Few direct hot-water discharges were observed. Consequently, geophysical methods, particularly the audiomagnetotelluric method, were used extensively. Favourable geological conditions for this method were encountered and the results, which were controlled using classical electrical methods on test areas, suggest an unusual distribution of resistivities for lava flows situated above suspected geothermal areas. These layers progressively decrease in resistivity down to a very conductive layer. In areas where these conductive layers were nearest the surface, detailed studies were carried out showing a close correlation between decreasing resistivity and increasing hydrothermal alteration. In addition, gradient wells reveal high geothermal gradients in such areas. The conductive layers revealed by audiomagnetotelluric soundings seem to correlate with thermal effects creating progressive hydrothermal alteration from an inferred hot-water reservoir up toward the surface.

Distribution of hydrothermal mineral zones in the Cerro Prieto geothermal field of Baja California, Mexico

Our ongoing studies of water-rock interaction at Cerro Prieto have now been extended to include samples from 40 wells. We have confirmed the regular sequence of progressive hydrothermal alteration zones previously described, and have mapped these alteration zones across the geothermal field. Our earlier work showed the relationships between hydrothermal mineralogy, temperature and permeability, in that alteration occurs at lower temperature in sandstone than in the less permeable shales. The effects of chemical parameters such as silica activity and differences in CO 2 fugacity have also become apparent when mineral assemblages are compared in sandstones from different wells at the same temperature. A rather complete picture of the shape of the reservoir and the nature of its boundaries is developing, and we have begun to identify patterns in the observed hydrothermal mineral zonations which are characteristic of different temperature gradients. We infer such different gradients to be indicative of different parts of the hydrothermal flow regime. In certain wells mineral zones are closely spaced, indicating steep thermal gradients, while in others they are much farther apart. We believe that patterns characteristic of recharge, discharge and upwelling zones as well as areas of primarily horizontal flow can be recognized. The geothermal circulation system at Cerro Prieto appears to be rather young and shows no indication of retrograde reactions due to cooling. The pattern of fluid flow does not seem to be significantly affected by faults, stratigraphic horizons or by the presence of a cap-rock. The mineral zones define a thermal dome which is apparently fed from the east and spreads westward.