Seawater-derived Fluids Research Articles

Oceanic detachment faults focus very large volumes of black smoker fluids

It is generally assumed that the seawater-derived fluids that feed black smoker vent fields on the seafloor are discharged vertically from depths of 1–3 km. We present new oxygen and strontium isotope data that show that fluids at black smoker temperatures of 300–400 °C were focused along a low-angle detachment fault at 15°45'N near the Mid-Atlantic Ridge. Isotopic alteration is the most extreme ever reported from oceanic rocks altered at similar temperatures, indicating intensely focused fluid flow both in recharge and discharge parts of the hydrothermal system. Rare earth element mobility in the fault rocks demonstrates isotopic alteration by evolved hydrothermal fluids, not conductively heated seawater. The fault zone protolith was predominantly ultramafic, but also included mafic rocks, with metasomatic alteration to talc-tremolite-chlorite schists resulting mainly from chemical exchange between these lithologies during fluid flow. Fluids in equilibrium with this assemblage would be similar to ultramafic-hosted black smoker fluids. We present a new model in which hydrothermal circulation around detachment faults evolves from basalt hosted (TAG type), to footwall ultramafic hosted (Rainbow type), to low-temperature ultramafic hosted (Lost City type). Key features of our model are the intrusion of gabbro bodies immediately below the detachment to provide a heat source for circulation, and focusing of fluid flow into the detachment fault to allow venting away from the neovolcanic axis.

Zinc Deposits and Related Mineralization of the Burketown Mineral Field, Including the World-Class Century Deposit, Northern Australia: Fluid Inclusion and Stable Isotope Evidence for Basin Fluid Sources

The stratiform Century Zn-Pb deposit and the discordant Zn-Pb lode deposits of the Burketown mineral field, northern Australia, host ore and gangue minerals with primary fluid inclusions that have not been affected by the Isan orogeny, thus providing a unique opportunity to investigate the nature of the ore-forming brines. All of the deposits are hosted in shales and siltstones belonging to the Isa superbasin and comprise sphalerite, pyrite, carbonate, quartz, galena, minor chalcopyrite, and minor illite. According to Pb model ages, the main ore stage of mineralization at Century formed at 1575 Ma, some 20 m.y. after deposition of the host shale sequence. Microthermometry on undeformed, primary fluid inclusions hosted in porous sphalerite shows that the Zn at Century was transported to the deposit by a homogeneous, Ca 2+ - and Na + -bearing brine with a salinity of 21.6 wt percent NaCl equiv. δD fluid of the fluid inclusion water ranges from −89 to −83 per mil, consistent with a basinal brine that evolved from meteoric water. Fluid inclusion homogenization temperatures range between 74° and 125°C, which are lower than the 120° to 160°C range calculated from vitrinite reflectance and illite crystallinity data from the deposit. This discrepancy indicates that mineralization likely formed at 50 to 85 Mpa, corresponding to a depth of 1,900 to 3,100 m. Transgressive galena-sphalerite veins that cut stratiform mineralization at Century and breccia-filled quartz-dolomite-sphalerite-galena veins in the discordant Zn-Pb lodes have Pb model ages between 1575 and 1485 Ma. Raman spectroscopy and microthermometry reveal that the primary fluid inclusions in these veins contain Ca 2+ , Na + , but they have lower salinities between 23 and 10 wt percent NaCl equiv and higher δD fluid values ranging from −89 to −61 per mil than fluid inclusions in porous sphalerite from Century. Fluid inclusion water from sphalerite in one of the lode deposits has δ 18 O fluid values of 1.6 and 2.4 per mil, indistinguishable from δ 18 O fluid values between −0.3 to +7.4 per mil calculated from the isotopic composition of co-existing quartz, dolomite, and illite. The trend toward lower salinities and higher δD fluid values relative to the earlier mineralizing fluids is attributed to mixing between the fluid that formed Century and a seawater-derived fluid from a different source. Based on seismic data from the Lawn Hill platform and paragenetic and geochemical results from the Leichhardt River fault trough to the south, diagenetic aquifers in the underlying Calvert superbasin appear to have been the most likely sources for the fluids that formed Century and the discordant lode deposits. Paragenetically late sphalerite and calcite cut sphalerite, quartz, and dolomite in the lode deposits and contain Na + -dominated fluid inclusions with much lower salinities than their older counterparts. The isotopic composition of calcite also indicates δ 18 O fluid from 3.3 to 10.7 per mil, which is larger than the range obtained from synmineralization minerals, supporting the idea that a unique fluid source was involved. The absolute timing of this event is unclear, but a plethora of Pb model, K-Ar, and 40 Ar/ 39 Ar ages between 1440 and 1300 Ma indicate that a significant volume of fluid was mobilized at this time. The deposition of the Roper superbasin from ca. 1492 ± 4 Ma suggests that these late veins formed from fluids that may have been derived from aquifers in overlying sediments of the Roper superbasin. Clear, buck, and drusy quartz in veins unrelated to any form of Pb-Zn mineralization record the last major fluid event in the Burketown mineral field and form distinct outcrops and ridges in the district. Fluid inclusions in these veins indicate formation from a low-salinity, 300° ± 80°C fluid. Temperatures approaching 300°C recorded in organic matter adjacent to faults and at sequence boundaries correspond to K-Ar ages spanning 1300 to 1100 Ma, which coincides with regional hydrothermal activity in the northern Lawn Hill platform and the emplacement of the Lakeview Dolerite at the time of assemblage of the Rodinia supercontinent.

Development of secondary porosity in the Fairholme carbonate complex (southwest Alberta, Canada)

Because of their economic importance as hydrocarbon reservoirs, the Upper Devonian dolomitized carbonate reefs in southwest Alberta have been the subject of several studies. Still, there is no consensus on the process of matrix dolomitization and furthermore, the process of vug development is not often addressed. The studied outcrops show features of an early diagenetic matrix-selective dolomitization by a Late Devonian seawater-derived fluid. Seepage reflux dolomitization combined with latent reflux is proposed, which best explains most chemical characteristics. The cements in the vugs are precipitated from warm saline, 87Sr-enriched fluids and testify to thermogenic sulphate reduction based on the presence of sulphur, CO 2 and H 2S in inclusions, relatively high homogenization temperatures and depleted δ 13C values, which sets constraints on the timing of vug formation. Secondary porosity may be created by the mixing of formation water with a tectonically and topographically driven fluid and by the dissolution of anhydrite nodules.

Constraints of stable isotopes on the origin of alunite from advanced argillic alteration systems in Bulgaria

Voluminous areas of advanced argillic alteration (AAA) constitute major exploration targets for surficial Cu–Au epithermal and potentially underlying porphyry-type deposits. In Bulgaria, more than 30 alunite occurrences are recognised, few of them being associated with a mineralised system. A mineralogical study combined with a stable isotopic (O, H, S) study has been carried out on nine alunite occurrences of advanced argillic zones hosted by volcanic rocks of Late Cretaceous age in the Srednogorie belt and of Oligocene age in the Rhodopes belt. This work was realised in order to constrain the origin of alunite and to define criteria to discriminate alunite from ore deposits and alunite from large barren alteration systems. Mineralogy of the nine occurrences consists of alunite + quartz + minor alumino-phospho-sulphates, associated with more or less kaolinite, dickite, pyrophyllite, diaspore and zunyite, depending on formation temperature. Alunite generally occurs as tabular crystals but is also present as fine-crystalline pseudocubic phases at Boukovo and Sarnitsa, in Eastern Rhodopes. In the advanced argillic alterations associated with economic ore, the presence of zunyite in the deeper parts indicates acid–fluorine–sulphate hydrothermal systems, whereas it is absent in uneconomic and barren advanced argillic alteration. All occurrences are formed at temperatures between 200 and 300 °C. (H, O, S) isotopic signatures of alunite combined with mineralogical features from all the studied occurrences, whatever their type, show characteristics of magmatic-hydrothermal systems. Sulphur data indicate essentially a magmatic origin for sulphur. Oxygen and hydrogen data suggest that hydrothermal fluids result from a mixing between magmatic fluids and an external component, which is identified as seawater-derived fluids or meteoric water in the vicinity of a sea. In most of the alunite occurrences, magmatic fluids are dominant and H 2S/SO 4 ratios are estimated to be higher than 2. Two exceptions exist in the Rhodopes. At Boukovo and Sarnitsa, where the estimated formation temperatures of alunite are the lowest, the external fluids are dominant and H 2S/SO 4ratios are estimated to be lower than or close to 1. At this stage of the work, the mineralogical and isotopic criteria do not enable a clear distinction between economic and uneconomic systems. However, some features are common in the economic ore deposits: the presence of zunyite in the deeper part of the system, the relatively high temperatures suggested by the zunyite + pyrophyllite + alunite + diaspore assemblages, the (O, H, S) signature of alunite, which is characteristic of dominant magmatic–hydrothermal acid–sulphate–fluorine systems.

Stable isotopic constraints on Kuroko-type paleohydrothermal systems in the Mesoproterozoic Serra do Itaberaba group, São Paulo State, Brazil

Mesoproterozoic oceanic paleohydrothermal systems developed in the volcanosedimentary Serra do Itaberaba Group, which comprises part of the Ribeira fold belt. Hydrothermal alteration associated with these systems was responsible for large premetamorphic chloritic alteration halos (CZ1 rocks), overprinted by restricted premetamorphic chloritic (CZ2 rocks), argillic, and advanced argillic alterations that correspond to intensely leached rocks within feeder zones. Well-defined trends of increasing δ 18O values with the progressive intensity of the alteration process are observed for igneous metabasites, metabasic hydroclastic rocks, and intermediate metamorphosed igneous and volcaniclastic rocks from CZ1. Systematic stable isotope variations evince that, in the Serra do Itaberaba metamorphosed hydrothermalized rocks, the preexisting isotope signatures of the hydrothermal systems were at least partially preserved. Highly evolved hot seawater is suggested for the genesis of the CZ1 rocks, whereas for the CZ2 rocks and marundites, the 18O fluid enrichments are interpreted as due to the major contribution of evolved seawater-derived fluids with a subordinate magmatic water component. An early near-seafloor, low-temperature alteration in a mid-ocean ridge environment was responsible for heterogeneous 18O whole-rock enrichments and followed by steady hydrothermal circulation with discharge of hot fluids, which previously underwent isotopic exchange with the 18O enriched volcanic rocks in the deeper part of the system with high temperatures and low water: rock ratios in a backarc environment. The subordinate magmatic water component derived from andesitic and rhyodacitic intrusions. The extremely high δ 18O anomalies from the CZ1 rocks suggest an associated base metal massive sulfide ore body. The lower δ 18O values related to the CZ2 rocks represent alteration by a higher temperature fluid, which might indicate the proximity of possible ore zones. The identification of several premetamorphic hydrothermally altered zones, similar to those of Kuroko-type base metal mineralizations, expands the mineral potential of base metal deposits in the Serra do Itaberaba Group and the volcanosedimentary sequences from the Ribeira fold belt.

Does Anisotropy of Thermal Contraction Control Hydrothermal Circulation at the Moho Level below Fast Spreading Oceanic Ridges?

It has been proposed that deep, high-T (up to 1000°C) hydrothermal alteration occurred within the gabbro section, near the paleo-ridge axis in the Oman ophiolite. In the deep, hot gabbros, the main water channels are submillimetric microcracks with a dominantly vertical orientation (Nicolas et al., 2003). Sr and O isotopic investigations point to seawater as the most likely hydrothermal contaminant (Bosch et al., 2004). It was proposed that the mechanism for seawater ingression at temperatures above 700°C was anisotropy of thermal contraction in gabbros, opening microcracks that are controlled by crystallographic fabrics. The exceptionally large anisotropy of thermal contraction of single-crystal calcic plagioclase, when combined with the lattice fabrics of the gabbros, results in the largest thermal contraction being parallel to the mineral lineation, which is itself normal to the ridge, thus inducing vertical fractures parallel to the ridge plane. High-T hydrothermal alteration in gabbros extends to the Moho. Interestingly, in the underlying peridotites, with a dominantly horizontal foliation, the direction of largest thermal contraction, calculated as above from fabrics and thermal expansion coefficients of olivine, is vertical, potentially responsible for subhorizontal cracking. Thus, during the off-axis drift of newly accreted lithosphere, at the limits of the axial magma chamber, thermal contraction opens vertical cracks in the crustal gabbros, favoring seawater ingression down to the Moho. Just below the Moho, the horizontal cracking preferred orientation could be closing the high-temperature hydrothermal circuit at this level. At temperatures below 700°C, oceanic ridge spreading—related tensile stress is dominant, inducing ridge-parallel cracking in both the crust and lithospheric mantle. Circulation of seawater-derived fluids in the Moho transition zone at gabbro solidus conditions should have important consequences for magmatic processes.

Chemical, biological and hydrological controls on the 14C content of cold seep carbonate crusts: numerical modeling and implications for convection at cold seeps

Understanding the hydrology of cold seep environments is crucial to perform accurate estimates of fluid and chemical fluxes at sedimentary wedges. Shallow convection processes may affect fluid flux estimates and could favor the destabilization of gas hydrate accumulations, increasing the sediment-ocean methane flux. Evidence for the occurrence of convection at cold seeps, however, is still limited. We use the concentration of 14C (D 14C) in carbonate crusts formed at cold seeps of the eastern Mediterranean Sea as a tracer for convective recirculation of seawater-derived fluids. A numerical model is applied to investigate the controls on 14C incorporation in cold seep carbonates. Our simulations show that increased amounts of CH 4 in the expelled fluids result in elevated crust D 14C, while high Ca 2+ and HCO 3 − concentrations produce the opposite effect. Convection is the only transport process that can significantly increase crust D 14C. Advection, bioirrigation, eddy diffusion and bioturbation instead, have little effect on, or produce a decrease of, crust D 14C. In addition, the presence of old or modern carbon (MC) in host sediments prior to cementation and the 14C-decay associated to the time needed to form the crust contribute in defining the D 14C of carbonate crusts. We then use the model to reproduce the 14C content of the eastern Mediterranean Sea crusts to constrain the chemical and hydrological conditions that led to their formation. Some crusts contain relatively low amounts of 14C (−945.0<D 14C ‰<−930.2) which, assuming no ageing after crust formation, can be reproduced without considering convection. Other crusts from two sites (the Amsterdam and Napoli mud volcanoes), instead, have a very high 14C-content (−899.0<D 14C ‰<−838.4) which can only be reproduced by the model if convection mixes deep fluids with seawater. Order-of-magnitude calculations using the Rayleigh criterion for convection suggest that the slow seepage (about 10 cm year −1) of low salinity (20‰) fluids at the Amsterdam sites could trigger haline convection there. On the Napoli mud volcano, where high-density brines are expelled, density-driven convection cannot take place and other processes, possibly involving the rapid movement of free gas in the sediment, could be important.

Nature and origin of the fluids responsible for forming the Hellyer Zn–Pb–Cu, volcanic-hosted massive sulphide deposit, Tasmania, using fluid inclusions, and stable and radiogenic isotopes

The Hellyer massive sulphide deposit lies within the Mount Read Volcanics province of western Tasmania. Before mining, it consisted largely of pyrite, sphalerite, galena, arsenopyrite and chalcopyrite, and was overlain by discontinuous barite–sulphide and/or silica–sulphide assemblages. It overlay a downward-tapering cone of hydrothermally altered rocks that shows concentric mineral zonation and contains steeply inclined veins containing sulphide and/or barite. New laser ablation and existing conventional sulphur isotopic compositions of sulphides from the massive sulphide ore range from −5.0‰ to 12.2‰. There is no significant spatial variation in sulphur isotopic composition in the sulphide ore, or evidence of significant change in the textural paragenesis, but there is considerable variation at millimetre scale within and between minerals. Apart from the few negative values, which may be of biogenic origin, the sulphur data can be explained by mixtures of sulphur reduced from seawater sulphate and that derived directly or indirectly from magma. The sulphur of the barite cap and the barite veins in the footwall (mostly 33.5–46.0‰) is probably also derived from seawater sulphate, and radiogenic 87Sr/ 86Sr values in the barite (0.70989–0.71144) suggest fluid circulation deep into the basement. δ 34S values of disseminated sulphides in the footwall alteration cone are like those of overlying ore, the aqueous sulphur being totally reduced due to low fluid velocities and protracted rock interaction. However, unusually high δ 34S sulphide values (up to 41.4‰) are found in some of the footwall vein sulphides, probably because in these veins the fluid velocities are enhanced and rock interaction limited, so that pyrite supersaturation may occur before reduction of entrained seawater sulphate is complete. High δ 34S sulphide values (up to 45.6‰) also occur in “unaltered” volcanic rock outside the alteration cone and may be the product of local convection of seawater prior to, during, or (most probably) after massive sulphide mineralization. Fluid δ 18O values calculated from isotopic analyses of quartz in footwall veins (8.5–11.8‰) and T h data range between −4.4‰ and 3.1‰. Dolomite occurs with chlorite in the core of the footwall alteration cone immediately below the massive sulphide. Its isotopic composition ( δ 13C=−1.5 to 2.8, δ 18O=8.2–18.3‰) may reflect precipitation from an acid, seawater-derived, fluid having δ 13C=1‰ (the ambient seawater value), and δ 18O ranging from 0‰ to 6‰, or, alternatively, its composition is the result of mixing between modified seawater at 150 °C ( δ 18O=6‰) with 1% of fluid at 350 °C having δ 13C=−6 and δ 18O=0‰. Dolomites in the footwall veins ( δ 13C=−1.8 to 1.7, δ 18O=9.3–14.2‰) could be derived from an acid, seawater-derived fluid with negative δ 18O values, or a fluid that has interacted with 600–700 Ma old, 13C-enriched carbonates in the deep footwall during convective circulation. Both the chlorite-associated and the vein carbonates contain highly radiogenic Sr, possibly derived from Devonian metamorphic fluids. A re-interpretation of the fluid inclusion data of Khin Zaw et al. [Ore Geol. Rev. 10 (1996) 251] shows that there are three groups of fluids in the quartz of veins in the altered footwall, viz. (a) saline (6.6–14.8 wt.%) with T h=170–246 °C, high K/Na, K/Ca and K/Fe values (fluid 1); (b) of similar salinity and temperature but with low K/Na, K/Ca and K/Fe values (fluid 2); and (c) an additional fluid of low salinity (2.9–7.0 wt.%) with T h=289–322 °C, element ratios unknown (fluid 3). Fluid 1 has cation ratios like those of magmatic fluids in the K-silicate and phyllosilicate zones of the Panguna and Endeavour 26N porphyry copper deposits. Combined with the lack of alternative source of salts in the pre-ore rock sequences at Hellyer, fluid 1 is thought to be at least partly magmatic. Fluid 2 has cation ratios like those of modern black smoker and Kuroko ore-forming fluids, and was probably derived from both magmatic fluid and seawater; fluid 3 may be evolved seawater like that forming the deposits of the Hokuroku Basin. It is suggested that a pluton or plutonic complex of mixed crustal and lithospheric mantle parentage, like that of the volcanic rocks hosting the Hellyer orebody, was emplaced at several km depth below the deposit during faulting related to crustal extension. The heated zone over and around the pluton displaying plastic behaviour was sealed off from the overlying brittle zone in which groundwater (modified seawater) underwent convection due to heat transfer across the brittle-plastic boundary. The seal was broken intermittently due to tectonic extension or increased fluid pressure in the pluton, and magmatic fluid joined with convecting groundwater in the rising plume, leading to fluid mixing. Upward flow was focused on the Eastern Fault, the fracture system responsible for forming the basin in which the ore sulphides were sedimented.

Water speciation and Sr isotopic exchange during water–melt interaction: a combined NMR–TIMS study on the Cretaio Tephra (Ischia Island, south Italy)

In order to improve the microscopic understanding of the water–magma interaction process during explosive volcanism, volcanic glasses representative of deposits with sedimentological characteristics suggesting different water/melt ratios were studied by a combination of the nuclear magnetic resonance (NMR) and TIMS methods. The glasses were separated from pumices of two surge layers and one fallout bed of the Cretaio Tephra (Ischia Island, Italy), which is the product of an explosive eruption that occurred at Ischia in the second century BC. The 29Si CP–MAS NMR experiments indicate the occurrence of 1H– 29Si dipolar couplings in glasses from the phreatomagmatic activity, suggesting the presence of hydrogen atoms in proximity of silicon atoms. This feature is not detected in the glass from the deposit of the magmatic explosion. 1H MAS NMR spectra reveal different peaks attributed to different hydrous species characterized by different motional properties. These include ‘rigid’ H 2O groups isolated in the glass structure, more mobile water species and possibly structural hydroxyl groups. 1H MAS NMR spectra recorded after deuteration experiments of the glass at a temperature up to 300°C revealed that the exchange reactions of the D 2O vapor with hydrogen were limited to the most mobile water species, possibly on vesicle surfaces or in channels. The hydrogen concentration linearly correlates with the 87Sr/ 86Sr isotope ratio in glasses, suggesting isotopic tracer exchanges between the Sr dissolved in the water vapor and the Sr in the silicon–oxygen network during hydration. It is proposed that the uprising melt interacted with a hydrothermal system of seawater-derived fluids, characterized by relatively high Sr isotopic composition.

Near-solidus evolution of oceanic gabbros: insights from amphibole geochemistry

The near-solidus evolution of plutonic rocks formed at slow-spreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the Mid-Atlantic Ridge. These new data allow unambiguous geochemical discrimination between amphiboles of magmatic and hydrothermal origin. In turn, this allows the gabbro solidus to be constrained to 860 ± 30°C, using amphibole-plagioclase thermometry. This is consistent with temperatures from associated secondary clinopyroxene. Magmatic amphibole, which can be identified in almost all samples, formed during metasomatism of a low-porosity crystal mush by an evolved hydrous silicate melt. These amphiboles are characterised by high F, Nb, and F/Cl and low Cl contents. The amphibole-forming reaction involved melt, plagioclase, and clinopyroxene. Amphibole blebs with a geochemically magmatic signature are found enclosed in the cores of some primitive clinopyroxene crystals. There is no evidence for a seawater component in the magmatic amphibole, as would be expected if high-temperature seawater ingress leads to flux melting, as has recently been suggested. However, the ingress of seawater-derived fluids did occur at temperatures within error of the gabbro solidus forming amphibole in veins and replacing igneous phases. These amphiboles are characterised by high Cl, B, and Cl/F and low Nb, F, and Nb/La. The fluids involved in the formation of these amphiboles had compositions unlike seawater or hydrothermal vent fluids.

Origin of tourmaline in the metamorphosed Sikait pelitic belt, south Eastern Desert, Egypt

A Neoproterozoic metapelitic schist belt at the Sikait area of the south Eastern Desert is a favourable environment for localisation of tourmaline mineralisation in the Pan-African Belt in Egypt. Local concentrations of tourmaline in the Sikait area are closely associated with stratiform metapelitic schists. They are confined to the Nugrus shear zone, along which various leucocratic rocks (leucogranite, pegmatite, aplite) are syntectonically emplaced with various stages of hydrothermal quartz veins. Tourmaline occurs either as disseminated isolated clusters of crystals or as discontinuous tourmalinite bands within the metapelitic rocks, pegmatites and quartz veins. Four types of tourmaline-rich rocks were distinguished: (i) fine-grained tourmaline-rich rocks, which are associated with the biotite schist along contacts with gneissose granites (TR1); (ii) tourmaline-rich rocks, associated with the metapelitic and amphibolitic closely to Nugrus thrust zones (TR2); (iii) tourmaline-rich rocks, associated with the pelitic hornfels at the contact between metapelitic schist and leucogranite (TR3); and (iv) tourmaline-rich rocks and quartz veins, associated with chlorite-graphite schist in contact aureoles with leucogranite and pegmatite veins (TR4). Microprobe analyses revealed that tourmalines are Al saturated for the given Fe/Mg alkali-deficient group tourmaline with minor X-site vacancy and substitution of Ca for Na. Tourmalines belong to the schorl-dravite solid solution series and have a wide compositional range, from nearly end member dravite for TR3 tourmalines to schorl for TR4 tourmalines; TR1 and TR2 tourmalines have intermediate compositions. The Fe/(Fe + Mg) varies between 0.02 and 0.89. Variation in composition of Al-rich tourmalines is essentially caused by variations in Mg, Fe, Na, Ca and Ti. The whole rock chemical analyses of tourmaline-rich rocks closely resemble the trends observed for metapelitic schist and leucocratic rock and reflect mixing between phyllosilicate-rich and quartz-rich end members, which indicates that tourmaline-rich rocks do not contain a significant detrital component. Chondrite-normalised patterns of rare earth elements (REE) in tourmaline-rich rocks and quartz-rich tourmalines are similar to those of the surrounding unaltered metapelitic schists and leucocratic rocks, respectively. Minor depletions of LREE and local negative Ce anomalies characterise the chondrite-normalised REE pattern of TR1 tourmaline-rich rocks, suggesting its formation in the presence of seawater-derived fluids. However, TR4 types are characterised by low content ΣREE, such as that of leucocratic rocks. Thus, the geochemical data imply relative immobility of Al, Ti, Cr and HREE during hydrothermal alteration and later metamorphism. The different tourmaline varieties and their respective compositions are interpreted in terms of multistage evolution. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation from B-rich hydrothermal fluids or colloids, early diagenetic reactions of biotite-pelitic sediments with these fluids and subsequent recrystallisation during late regional deformation and metamorphism to give TR2 tourmaline-rich rocks. The TR3 tourmaline-rich rocks mainly developed by the thermal metamorphic recrystallisation of TR1. Tourmaline-rich rocks and veins adjacent to leucogranite and pegmatite veins (TR4) are the result of B-metasomatism; the primary B having been recycled from tourmalinites during regional metamorphism and magmatism.

Pre-accretion metamorphism of the Teloloapan Terrane (southern Mexico): example of burial metamorphism in an island-arc setting

Volcanic and interbedded volcaniclastic rocks of the lower Cretaceous island-arc series of the Teloloapan Terrane in southern Mexico contain metamorphic assemblages characteristic of the zeolite, prehnite–pumpellyite and lower greenschist facies produced by burial metamorphism prior to its accretion to nuclear Mexico. Distribution of secondary assemblages throughout the stratigraphic succession, together with the chemical evolution of metamorphic minerals, reveals a depth-controlled metamorphic zonation characterized by the presence of the diagnostic assemblages laumontite+pumpellyite+epidote and laumontite+celadonite+pumpellyite±epidote (zeolite facies) followed downward by assemblages containing prehnite+pumpellyite±white mica (prehnite–pumpellyite facies) and finally by the presence of the assemblages pumpellyite+actinolite+epidote and epidote+actinolite (greenschist facies). Analysis of assemblages in the Al–Fe 3+–FM–K system, reveals that facies boundaries are discontinuous, involving the disappearance of at least one phase and the appearance and/or extension of the field of equilibrium of other diagnostic minerals and assemblages. Application of empirically based thermobarometers, phase equilibria, mineral chemistry, and petrogenetic grids indicates that the P– T conditions of metamorphism ranged from 175 to 342°C and P<3 kbar. The data further indicate high geothermal gradients of about ∼55°C km −1. Seawater-derived fluids were characterized by high a K, high f O 2 and low X CO 2 .

Rare earth and trace elements in igneous and high-temperature metamorphic minerals of oceanic gabbros (MARK area, Mid-Atlantic Ridge)

SIMS analyses have been carried out on clinopyroxenes, plagioclases and amphiboles of six gabbroic samples from Holes 921–924 of the Ocean Drilling Program Leg 153 sited in the MARK area of the Mid-Atlantic Ridge at the ridge–transform intersection, to investigate the rare earth, trace and volatile element distribution in the lower ocean crust during igneous crystallization and higher grade metamorphic conditions. The metagabbros underwent granulite to subgreenschist facies conditions through three main tectono-metamorphic phases: (1) ductile regime (750 < T < 1000 °C and P ≈ 0.3 GPa); (2) transitional regime (600 < T < 700 °C and P ≈ 0.2 GPa); (3) brittle regime (350 ≤ T < 600 °C and P < 0.2 GPa). Igneous clinopyroxenes show C1-chondrite normalized patterns depleted in LREE, and nearly flat for HREE. The rare earth and trace element distributions in igneous clinopyroxenes and plagioclases indicate that these minerals act as REE reservoirs, and comprise the main contribution to the overall rock content. The abundances in igneous minerals reflect the degree of fractionation of the parent liquids. In metamorphic clinopyroxenes recrystallized in anhydrous assemblages, the REE and trace elements patterns mimic those of the primary ones. Conversely, clinopyroxene re-equilibrated in amphibole-bearing assemblages shows a significant increase in REE, Ti, Zr, Y and V, a negative Eu anomaly, and slight decreases in Sr and Ba. An overall increase of REE and some trace elements is evident in hydrous assemblages, with preferential partitioning in the amphibole. It shows high Ti (18196–22844 ppm), LREE depleted patterns and LaN/SmN=0.10–0.33, LaN/YbN=0.10–0.30. Amphiboles from granoblastic assemblages show homogeneous patterns with no or a positive anomaly for TiN and negative anomalies for SrN and ZrN. Volatiles in amphibole are low, with Cl/F < 1; H2O% is significantly lower than the stoichiometric ratio (1.33–1.53%). The composition of the clinopyroxene and amphibole recrystallized in low-strain domains records evidence of incomplete re-equilibration, and element diffusion and partitioning is in part controlled by the textural site. The possible origins of the fluids involved in the metamorphic recrystallization are discussed: (1) remobilization from igneous amphibole; (2) exsolution from evolved melts; (3) introduction of seawater-derived fluids modified in rock-dominated systems; (4) injection of highly evolved hydrous melts during the metamorphic process.

Petrology, isotope geochemistry and chemical budgets of oceanic gabbros-seawater interactions in the Equatorial Atlantic

Petrological and chemical variations, as well as oxygen and strontium isotopic data are presented for metagabbros from the Romanche and Vema fracture zones. These rocks were affected by several types and degrees of alterations ranging from slight hydrothermal alteration to complete amphibolitization. Five major kinds of alteration processes ranging from late-magmatic deuteric alteration (stage I) to low temperature (<150 °C) alteration (stage V) were identified. Water-rock interactions between 300 and 650 °C are the most dominant interactions resulting in the most prevailing secondary mineralogical assemblages which characterize the amphibolite and/or greenschist facies (amphibole ± plagioclase ± epidote ± titanite ± chlorite ± prehnite). Hydrothermal alteration of these gabbroic rocks results in isotopic exchanges between rocks and seawater-derived fluids. These exchanges lead to decrease of gabbroic δ18O toward values as low as +3.9‰, and larger Sr isotopic variations than other oceanic gabbroic rocks (87Sr/86Sr ratios shift to 0.7029–0.7051). Calculation of a chemical budget indicates that metagabbros are hydrated and enriched in Fe and probably in Mg and Cl, while Si, Ca and Ti are released to the hydrothermal fluids. In addition to metamorphic recrystallization and geochemical transformation, hydrothermal alteration of oceanic gabbros contributes to the control of the global ocean geochemistry.

Nuclear microprobe analysis of serpentine from the mid-Atlantic ridge

At mid-ocean ridges, ultramafic rocks are serpentinized by interaction with seawater-derived fluids. Elements, dissolved in large quantities in seawater, e.g., Na, K, Cl, Br, Ca and Sr, can be, in small amounts, incorporated as traces into the crystal structure of the various serpentine minerals (Mg3Si2O5(OH)4). These trace elements can be used to track the composition of the reacting fluids and to constrain physico–chemical conditions. This paper represents the first application of particle-induced X- and γ-ray emission (PIXE/PIGE) analysis to serpentine using the nuclear microprobe at the Laboratoire Pierre Süe (CEA-CNRS). Three types of serpentine, belonging to two different serpentinization generations, have been analysed in samples collected from the Mid-Atlantic Ridge (14°45′N/45°W) that exposes serpentinized peridotites on which the Logachev black smoker is placed. The trace elements Cl, F, S, Cu, Zn, Ca, K, Ni, Cr and Mn were detected from several tens to several thousands of ppm. Bromine, As and Sr are close to the detection limit of about 5 ppm. The trace element concentrations and interelement relationships in serpentines vary (a) with the serpentine type and (b) with the geographic location to the black smoker. Chlorine and in part S originated from seawater, whereas Cu, Zn, Ca, K, Ni, Cr and Fe and the major amount of S were mobilized from the unaltered host rock and partitioned between the serpentine and the aqueous solution.

Low-temperature fluid–rock interaction—an isotopic and mineralogical perspective of upper crustal evolution, eastern flank of the Juan de Fuca Ridge (JdFR), ODP Leg 168

During ODP Leg 168, 10 sites were drilled across the eastern flank of the Juan de Fuca Ridge (JdFR), to examine the conditions of fluid–rock interaction in three distinct hydrothermal regimes (referred to as the Hydrothermal Transition (HT), Buried Basement (BB) and Rough Basement (RB) transects), extending over a ∼120 km linear transect perpendicular to the spreading ridge. This was carried out in an attempt to constrain the conditions and processes that control the location, style and magnitude of low temperature (<150°C) fluid–rock interaction within this setting. This paper presents new data on the petrology, mineral chemistry and whole rock strontium and oxygen isotopic compositions of basalts from the eastern flank of the JdFR, in order to investigate the extent, style and sequence of low-temperature hydrothermal alteration and to establish how the hydrothermal regime evolved with time. Throughout the flank, a progressive sequence of low-temperature hydrothermal alteration has been identified, marked by changes in the dominant secondary mineral assemblage, changing from: chlorite+chlorite/smectite; to iron oyxhydroxide+celadonite; to saponite±pyrite; culminating at present with Ca- to CaMg(±Fe,Mn)-carbonate. The changes in secondary mineralogy have been used to infer a series of systematic shifts in the conditions of alteration that occurred as the basement moved off-axis and was progressively buried by sediment. In general, hydrothermal alteration of the uppermost oceanic crust commenced under open, oxidative conditions, with interaction between unmodified to slightly modified seawater and basaltic crust, to a regime in which circulation of a strongly modified seawater-derived fluid was more restricted, and alteration occurred under non-oxidative conditions. Across the flank, petrological observations and microprobe analyses indicate that the observed ranges in secondary mineral composition are directly related to changes in the geochemical and textural characteristics of the basement, as well as to interaction between fluids and phases from the four stages of alteration. This is suggestive of an increase in fluid–rock increased with time. Whole rock 87 Sr/ 86 Sr and δ 18 O analyses of basalts from across the eastern flank of the JdFR reinforce petrological observations, with 87 Sr/ 86 Sr and δ 18 O values slightly elevated above accepted pristine MORB values for this region. These results are consistent with an increase in the amount of fluid–rock interaction with time. Across the flank, enrichment in the 87 Sr/ 86 Sr and δ 18 O relative to MORB, is influenced by a number of factors, including: local and regional variations in the crustal lithology and structure; the age of the crust; the extent of bulk rock alteration; and theoretically, the relative abundance of different isotopically-enriched secondary mineral phases in the crust.

Origin of Differentiated Volcanic and Plutonic Rocks from Ascension Island,South Atlantic Ocean

The first phase of felsic magmatism on Ascension Island, in the form of trachyte and rhyolite domes, coulées, lava flows, and pyroclastic deposits, created the central and eastern parts of the island between about 1.0 and 0.56 my ago. The geochemical characteristics of the felsic rocks are largely consistent with an origin by fractional crystallization of high Zr/Nb mafic magmas as evidenced by identical 143Nd/144Nd and similar Pb isotopic ratios. The high Zr/Nb basalt flows constitute one of the four distinct basalt and hawaiite suites identified from Ascension based on trace element characteristics. Syenite, monzonite, and granite xenoliths associated with the felsic magmatism are interpreted as cumulate rocks from, and intrusive equivalents of, fractionating felsic magmas. Many of the felsic rocks are characterized by high 87Sr/86Sr (>0.704) compared with mafic rocks (87Sr/86Sr <0.703), even when corrected for in situ decay of 87Rb since eruption. Such high 87Sr/86Sr coupled with high 143Nd/144Nd signatures do not correspond to known suboceanic mantle reservoirs and in the most part appear to reflect sub-solidus addition of a high 87Sr/86Sr component. This component is probably a seawater-derived fluid that might be added at the surface from wind-blown spray, or, more likely, at depth through hydrothermal circulation (fluids with high Sr contents have been recovered from fractures in a 3126-m-deep geothermal well). In either case, the extremely low Sr contents of the felsic rocks make them particularly susceptibleto Sr-isotope modification. Internal (mineral) isochrons for two granite xenoliths give ages of ∼0.9 and ∼1.2 Ma, with initial 87Sr/86Sr >0.705. Even though the high 87Sr/86Sr signature of most of the volcanic rocks is demonstrably introduced after solidification, the high initial 87Sr/86Sr values of the granite xenoliths suggest that hydrothermally altered pre-existing volcanic basement may have been melted or assimilated during differentiation of some of the felsic magmas.

Ti-rich Amphibole of Igneous Origin in MOR-Type Gabbroic Rocks (Northern Apennine Ophiolites, Italy): An Ion Microprobe Study

Two major hypotheses have been formulated for the origin of Ti-rich amphibole in MOR-type gabbroic rocks (e.g. Otten, 1984). In the first, amphibole derives from an igneous trapped liquid. This hypothesis implies that the parental magma had a relatively high volatile content, thus contrasting with the nearly anhydrous nature of MORB. The second hypothesis assumes a metamorphic origin, i.e. amphibole was originated by solid state reactions of igneous minerals with volatile-rich seawater-derived fluids. In the gabbroic rocks from Northern Apennine ophiolites, Ti-rich amphibole occurs both with igneous and metamorphic texture. The gabbroic rocks were formed by low-pressure fractional crystallization of N-MORB type liquids in a slowspreading ridge system (Tiepolo et al., 1997). Accessory Ti-rich amphibole with igneous texture is found as the last crystallizing mineral, in interstices between plagioclase and clinopyroxene and as rim around Fe-Ti-oxides. Morever, Ti-rich amphibole occurs as a major mineral in the most evolved, volumetrically subordinate, intrusive lithologies (albitites). These rocks consist of albitic plagioclase and Ti-rich amphibole (up to 30 % by volume), both showing euhedral to subhedral habitus, plus accessory Fe-Ti-oxides, apatite and zircon. This work aims to unravel the origin of Ti-rich amphibole with igneous texture. Amphibole from Mgand Fe-rich gabbroic rocks, as well as from albitite, has been analysed for major elements by electron microprobe, and for halogen (F, CI and H) and trace elements (REE, Y, Sr, Zr, Cr, V, Sc, Ti, Ba, K, Nb) by ion microprobe.

Sediment dewatering and pore fluid migration along thrust faults in a foreland basin inferred from isotopic and elemental geochemical analyses (Eocene southern Pyrenees, Spain)

The lower Eocene Ainsa basin was formed during the first stages of the south-Pyrenean foreland basin evolution due to southwestward migration of imbricated thrust-folds. Isotopic and elemental geochemistry of syn-kinematic veins (calcite and celestite) and their marly host-rock, sampled in three thrust-fault zones and one footwall syncline, allows us to characterize the origin of pore fluids and the early stages of their evolution and circulation during the early deformation of the basin-fill. The isotopic composition of sulfur and the 87Sr 86Sr ratios of calcites and celestites from the veins in the footwall syncline show that the original fluid had the isotopic composition of Eocene seawater. The different 87Sr 86Sr ratio in veins from the thrust-fault zones compared with the same ratio in the marly host-rock of the footwall syncline indicates that the thrust-fault zones acted as conduits for advective fluids. The relatively high 87Sr 86Sr ratio in the veins related to the thrust-fault zones indicates that the fluid originated from the interaction of seawater with an external fluid coming from deeper sources or from the meteoric weathering of the emerged part of the belt. δ 18O and δ 13C values of calcites show that the isotopic composition of the calcite-cements in veins was controlled by the isotopic composition of the marly host sediment. Depletion of both δ 18O and δ 13C with respect to Eocene seawater composition, together with elemental geochemistry of calcite cements in the veins, points to burial transformations of a seawater-derived fluid to a formation water composition. The distribution of δ 18O and δ 13C values of the marly host-rock and calcite cements in veins of the four outcrops probably resulted from differences in the meteoric water influences. The hydrogeological regime at the toe of the submarine thrust system was dominated by tectonically-induced dewatering of the foreland basin sediments. The thrust-fault zones were the channelizing paths for migration of fluids expelled from the surrounding sediments, as well as fluids derived from more internal parts of the belt.

The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone), II. Geochronology and isotope geochemistry

Whole-rock Nd and Sr isotopic compositions of the mafic-ultramafic complex near Finero demonstrate that the magma was derived from a depleted, perhaps MORB-type mantle reservoir. The Sm-Nd data for the Amphibole Peridotite unit can be interpreted as an isochron with an apparent age of 533 ± 20 Ma, which is consistent with a 207Pb/ 206Pb evaporation age of 549 ± 12 Ma of a single zircon grain from the Internal Gabbro unit. However, the interpretation of these apparent ages remains open to question. We therefore retain the alternative hypotheses that the intrusion occurred either about 533 or 270 Ma ago, the latter being the most likely age of emplacement of the much larger magma body near Balmuccia (Val Sesia). The implication of the older emplacement age (if correct) would be that the igneous complex may be related to the numerous amphibolite units, which are intercalated with the metapelites of the overlying Kinzigite Formation, and together with them may constitute an accretionary complex. In this case, the mafic-ultramafic complex itself might also be part of such an accretionary complex (as has been proposed for the Balmuccia peridotite). Internal Sm-Nd isochrons involving grt, cpx, plag and amph from the Internal Gabbro unit yield concordant ages of 231 ± 23, 226 ± 7, 223 ± 10, 214 ± 17, and 203 ± 13 Ma. These results confirm published evidence for a separate, regional heating event about 215 ± 15 Ma ago. Initial ε Nd(533) values average +6.3 ± 0.4 for six samples of the Amphibole Peridotite unit and +6.0 ± 1.2 for ten samples of the External Gabbro unit. 87Sr/ 86Sr ratios require little or no age correction and range from 0.7026 to 0.7047 (with two outliers at 0.7053 and 0.7071). Strong correlations between 87Sr/ 86Sr and K 2O and weaker correlations between initial ε Nd and K 2O imply a comparatively minor (≤ 10%) contamination of the External Gabbro magma by crustal material and a later alteration by a crustal or seawater-derived fluid. These results contrast sharply with the isotopic composition (negative ε Nd and high 87Sr/ 86Sr values) of the associated mantle rocks, the Phlogopite Peridotite unit, which has been pervasively metasomatized by crustal fluids. This type of metasomatism and its isotopic signature are never seen in the magmatic complex. This evidence rules out any direct genetic relationship between the igneous complex and the mantle peridotite. The crust-mantle interaction is the opposite of that seen at Balmuccia, where the mantle peridotite is essentially ‘pristine’ and the magmatic body has been extensively contaminated by assimilation of crustal rocks.