Neovolcanic Zone Research Articles

Microbially Mediated Mobilisation of Redox-Sensitive Metals in Relict Hydrothermal Sulphide Deposits

The diagenetic re-mineralisation of Fe, Mn, Cu, Zn, Cd, Mo and S and the role of microbes in the metalexchange processes were investigated in metalliferous sediments from a relict hydrothermal zone in the TAG area. The TAG hydrothermal field is located within a 40km long, slow-spreading ridge segment of the Mid-Atlantic Ridge (MAR) at 26~ The field comprises the active TAG mound, situated 2.4 km east of the neovolcanic zone, two areas (Mir and Alvin) of former high-temperature venting to the north of the active mound, and an area of lowtemperature Mn-oxide deposits to the east [1]. The core investigated here is from the 'Alvin' zone, which is a 2km long, N-S trending area of discontinuous, highly altered sulphide deposits. Radiometric dating suggests that the Alvin zone deposits formed over a period of at least 100,000 years, and the last hydrothermal event maybe as recent as 6000 years ago [2]. Mass gravitational processes, submarine weathering and re-crystallisation of the original mineral assemblage has led to intensive reworking, modification and alteration of the primary sulphide textures. Since cessation of the active venting, the deposits have been covered by a substantial layer of pelagic sediments. Downcore measurements of the solid phase metals, and the concomitant pore water metal-contents, were made in a 230 cm-long gravity core collected from the central part of the Alvin zone. These detailed measurements were compared with observations from a suite of five cores collected across the Alvin zone. The altered sulphidic sediments are capped with a ~30cm thick layer of carbonate-rich (~80% CaCO3), Fe stained sediments. Two distinct sulphide layers (50-65 cm and 170-230 cm) interbedded with Fe-oxysilicates and overlain by a thin layer of Fe/Mn oxyhydroxides were found in this core. Scanning electron microscopy (SEM) and solid phase data suggest that the dominant mineral-phase in both sulphide layers is FeS, with small amounts of covellite, sphalerite and chalcopyrite (S: 8 mmol/g, Fe: 7 mmol/g, Cu: 0.8 mmol/g, Zn: 50 ~tmol/g in the top layer). The presence of sulphides within hydrothermal sediments has been attributed to mass wasting of active sulphide deposits [3]. The pore-water profiles demonstrates that Mn is released at depth and the upward diffusing Mn 2+ is re-oxidised at the redox-boundary (~30cm), as indicated by a sharp peak in the solid phase Mn. This is well matched by specific activity of Mn reducing bacteria, which are low at the surface but markedly increased below 70 cm, where pore water Mn increases (3.5 ~tmol/1) [4]. Solid phase data indicate re-mobilisation of Zn from primary sulphides within the sulphide layers and re-precipitation directly above and below. SEM analysis has shown that where mobilisation occurs, the sphalerite is marginally corroded, whereas in the zone where it is re-precipitated, grains are rather angular. Sharp peaks of dissolved Cu (12 I.tmol/1) and Cd (1 lamol/l) within the sulphide layer, and Zn (15 Ixmol/1) and Mo (80 nmol/l) below 70 cm demonstrate remobilisation of these metals. Pore water Fe 2+ is elevated below 65cm (30-90 lamol/1), but is low within the sulphide layer (0.5-8 ~tmol/1), indicating upward migration and precipitation as FeS. EDSSEM mapping of individual sulphide grains shows marked zonation of Zn, Cd and Cu across the minerals. This, together with the pore water data, suggests that these metals are sequentially released from the primary sulphides and replaced by Fe. In summary, post-depositional processes leading to re-mineralisation in these metal-rich sediments include oxic diagenesis, driven by penetration of overlying oxygenated seawater, microbially mediated mineralisation, and percolation of reactive pore-fluids

Branching sheeted dikes and seafloor spreading within an Early Proterozoic intra‐arc basin

The Payson ophiolite (∼ 1.73 Ga) is exposed as a shallow‐dipping, pseudostratigraphic sequence of gabbro, sheeted dikes, and submarine volcanic rocks, with screens and roof pendants of older arc crust. The sheeted dike complex is particularly well exposed, and analysis of measured sections reveals a threefold increase in the average dike width with depth in the complex. Magmatic temperatures and slow cooling at the base of the sheeted dike complex, indicated by gabbro‐dike mingling and mutual intrusion and coarse textures, may have facilitated the growth of wide dikes. In contrast, extreme alteration of the top of the sheeted dike complex suggests that hydrothermal circulation dominated and caused rapid cooling as thin dikes. The connection between wide dikes at depth with thinner dikes toward the top requires a new model, where dikes branch upward in response to the strong vertical gradient in the rate of cooling and solidification. The branching dike model has important implications for the timing and spatial distribution of seafloor phenomena in the neovolcanic zone of spreading centers.

Sediments of the active rift zone in the western Woodlark Basin and the development of hydrothermal and volcanic activity

In the western Woodlark Basin within the neovolcanic zone sediments form ponds in fissures and between individual pillows and tubes. A continuous thin sediment cover appears only at the base of the neovolcanic zone. Its thickness increases with the distance from the zone. Data on the lithology, mineralogy and chemistry of sediment cores from the spreading centre in the western Woodlark Basin and adjacent areas show that these calcareous sediments (avg. 44.1 % CaCO 3) contain significant amounts of terrigenous, arc volcanic, hyaloclastic and hydrothermal material, the latter constituting up to 15% of the non-carbonate fraction. Two types of sedimentary sequences occur in the studied area. Those of the first type were formed by particle-by-particle sedimentation; horizons of redeposited sediments occur in sequences of the second type. Redeposition of sediments is widespread and is tentatively correlated with cycles of volcanic activity at swells along the spreading centre. These alternate with hydrothermal cycles, which were determined by the presence of hydrothermal matter in sediments. Radiocarbon dating of sediments showed that both cycles are of similar duration, of the order of 10,000 years. They are not synchronous along the spreading axis, each swell having a specific evolution. Based on these determinations we can say that within the studied spreading centre only the Franklin Seamount swell is prospective for contemporary hydrothermal activity.

Geophysical investigations over a segment of the Central Indian Ridge, Indian Ocean

Swath bathymetric, gravity, and magnetic studies were carried out over a 55 km long segment of the Central Indian Ridge. The ridge is characterized by 12 to 15 km wide rift valley bounded by steep walls and prominent volcanic constructional ridges on either side of the central rift valley. A transform fault at 7°45′S displaces the ridge axis. A mantle Bouguer anomaly low of −14 mGals and shallowing of rift valley over the middle of the ridge segment indicate along axis crustal thickness variations. A poorly developed neovolcanic zone on the inner rift valley floor indicate dominance of tectonic extension. The off-axis volcanic ridgs suggest enhanced magmatic activity during the recent past.

Petrological systematics of the Mid‐Atlantic Ridge south of Kane: Implications for ocean crust formation

Models of ridge segmentation, mantle flow and melt focusing predict how the chemical compositions of mantle melts should vary along a mid‐ocean ridge axis. The compositions of basaltic lavas can be compared to these predictions to test the models. Such tests have been carried out using basalts from the neovolcanic zone south of the Kane fracture zone (the MARK area), where there are both a large transform and nontransform offsets. Before evaluating mantle models, the effects of differentiation must be accounted for. Fractional crystallization at low pressures (constrained by new melting experiments on these samples) does not account for the data. High pressure or in situ crystallization better account for the differentiation trends; however, these two processes imply different relationships between magmatic differentiation and position within a segment. Irrespective of the differentiation model, significant differences exist among parental magmas. Magmas near the transform have much lower levels of highly incompatible trace elements but higher levels of moderately incompatible trace elements, suggesting both lower extents of melting and a more depleted source. These two characteristics may be natural consequences of the truncation of a melting regime by a large‐offset transform: depleted mantle from across the transform may contribute to the melting regime, while the cooler thermal environment produces less melt. Quantitative modeling of these geochemical characteristics produces thin crust near the transform, consistent with seismic and gravity studies. In contrast, thin crust adjacent to nontransform offsets is associated with no reduction in extent of mantle melting. These results, along with data from other regions, suggest that nontransform offsets overlie a continuous melting regime, and melt focusing creates the variations in crustal thickness. Focused flow may also lead to incompatible element enrichment at segment centers, and relative depletion at segment margins. Only offsets that truncate the melting regime, such as large transforms, are associated with diminished extents of melting within the mantle. Petrological evidence obtained thus far is not consistent with active upwelling to explain crustal thickness variations along nontransform offset bounded segments.

Contribution of volcanism and tectonism to axial and flank morphology of the southern East Pacific Rise, 17°10′–17°40′S, from a study of layer 2A geometry

Multichannel seismic images from the East Pacific Rise 17°10′–17°40′S are used to study the geometry of seismic layer 2A in the near‐axis region. Wave‐equation datuming is applied to stacked common midpoint data to remove the distorting effects of seafloor topography on the layer 2A event. Processed stacks are compared with seafloor morphology and tectonic fabric imaged in side‐scan sonar data. Assuming layer 2A corresponds with the extrusive crust, these data are used to study the relationship between volcanism and tectonism in the accumulation of the extrusive layer and to assess the contribution of extrusives to ridge‐crest and ridge‐flank morphology. We find variations in axial thickening of layer 2A which imply twofold variation in the width of the extrusive layer accumulation zone, as well as systematic changes in the pattern of accumulation of this layer. On the ridge flanks the layer 2A horizon mimics abyssal hill relief, consistent with a horst and graben origin for this topography. Short‐wavelength variations in the thickness of layer 2A are superimposed on this relief, which we attribute to volcanic modification of tectonic topography at the edge of the neovolcanic zone. The integrated off‐axis thickness of layer 2A is correlated with ridge cross‐sectional area and indicates persistent (100,000 years) spatial gradients in extrusive layer thickness. Systematic changes in the cross‐axis shape of the ridge, observed over distances of 5–10 km, cannot be attributed to the extrusive layer, and it appears that axial structure beneath the volcanics governs the cross‐axis morphology of the ridge.

Volcanism on the Rockall continental margin

Continental break-up in the northern North Atlantic was accompanied by massive volcanism. This produced sequences of basalt flows erupted close to sea-level, which are imaged as seaward-dipping reflectors on seismic reflection profiles. They comprise the upper 4-5 km of transitional crust in both the Edoras Bank and Hatton Bank areas of the Rockall continental margin. Detailed surveys show that the reflectors are convex-upward on dip lines but are sub-horizontal on strike lines. They are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis. Along the Rockall margin two sequences of seaward-dipping reflectors occur, separated by an 'outer high' near the foot of the continental margin. The landward set of basalt flows are emplaced above stretched and intruded continental crust, while the seaward set span the region from highly distended continental crust to fully oceanic crust. The outer high is capped by basalt flows but exhibits little coherent internal reflectivity. We discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of Iceland.

Färoe‐Iceland Ridge Experiment 1. Crustal structure of northeastern Iceland

Results from the Färoe‐Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s−1). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (<800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

Structure and Stability of Non-Transform Discontinuities on the Mid-Atlantic Ridge between 24° N and 30° N

Observations of the median valley within the 24–30° N area ofthe Mid-Atlantic Ridge (MAR), using the IOSDL high resolutionside-scan sonar instrument TOBI, image four separate areas of themedian valley, containing part or all of nine spreading segments, and fivenon-transform discontinuities between spreading segments (NTDs).These high resolution side scan images were interpreted in parallel withmultibeam bathymetry (Purdy et al., 1990), giving a greater degree ofstructural precision than is possible with the multibeam data alone. Threedistinct types of NTD were identified, corresponding in part to typespreviously identified from the multibeam bathymetric survey of the area.Type 1 NTDs are termed septal offsets, and are marked by a topographic ridgeseparating the two spreading segments. The offset between the spreadingsegments ranges from 9 to 14 km. These can be further subdivided into Type1A in which the septa run parallel to the overall trend of the MAR and Type1B in which the septa lie at a high angle to the bulk ridge trend. Type 1ANTDs are characterised by overlap of the neovolcanic zones of the segmentson each side, and strong offaxis traces, while Type 1B NTDs show no overlapof neovolcanic zones, and weak offaxis traces. Type 2 NTDs arebrittle/ductile extensional shear zones, marked by oblique extensionalfractures, and associated with rotation of tectonic and volcanic structuresaway from the overall trend of the MAR. Type 3 NTDs are associated withoffsets of less than 5 km, and show no sign of any accommodating structure.In this type of NTD, the offset zone is covered with undeformed volcanics.The type of NTD developed at any locality along the ridge axis appears todepend on the amount of segment offset and segment overlap, the overalltrend of the mid-ocean ridge, the width of the zone of discontinuity, themedian valley offset and the longevity of the offset. These factorsinfluence the mechanical properties of the lithosphere across thediscontinuity, and ultimately the tectonic style of the NTD that can besupported. Thus brittle/ductile extensional shear zones are long-livedstructures favoured by large segment offsets, and small or negative segmentoverlaps. Septa can be short or long lived, and are associated with largesegment offsets. Segment overlaps vary from negative (an along axis gap) tozero, for Type 1B septal offsets, or positive to zero for Type 1A septaloffsets. Non-tectonised NTDs are generally short lived structures,characterised by small segment offsets and zero or positive overlaps.

Postrifting anelastic deformation around the spreading plate boundary, north Iceland: 2. Implications of the model derived from the 1987–1992 deformation field

A decade‐long rifting episode that began in the Krafla volcanic system, north Iceland, in 1975 caused substantial, regional, postevent anelastic deformation. This was modeled as viscous relaxation in an elastic/viscoelastic structure by Hofton and Foulger [this issue]. The results from modeling the deformation detected in north Iceland have far‐reaching implications both for local and regional processes and for the fundamental behavior of deformation around spreading plate boundaries in general. Tilt in the vicinity of the Krafla volcano fits the model well after 1988/1989 which suggests that the volcano magma chamber stopped inflating/deflating in 1988. A viscosity of 0.8 × 1018 Pa s was required to match the local tilt data, less than that predicted for north Iceland as a whole. Vertical motion measured using the Global Positioning System (GPS) 1987–1992 around the ice cap Vatnajökull is inconsistent with isostatic uplift. Using the elastic‐viscoelastic model to predict motion in other regions of Iceland suggests that the deformation effects of the Krafla episode are significant in many parts of Iceland and should be taken into account when modeling deformation there. Though not a realistic plate boundary model, interesting complexities of the elastic‐viscoelastic model are highlighted by deformation modeling of an infinitely long dike. This predicts that the amount of horizontal displacement close to the dike may exceed the amount of initial dike opening early in the spreading cycle. A more realistic approximation to the plate boundary in north Iceland, involving five overlapping segments that experience dike emplacements at discrete intervals, suggests that the width of the zone within which transient, time‐dependent deformation occurs may be several hundred kilometers wide, considerably wider than the neovolcanic zone. A kinematic approach to describing plate motions is not appropriate close to spreading plate boundaries and elsewhere where the viscosity of the Earth is low.

Seismic images of crust beneath Iceland contribute to long‐standing debate

Since the 1970s there has been an ongoing debate about whether the crust beneath Iceland is relatively thin (<15 km) and hot, or thicker (>20 km) and cooler. New results from the Faeroe‐Iceland Ridge Experiment (FIRE) conducted in the summer of 1994 suggest that it is thicker and cooler.This major land‐sea study investigated the crust generated where the North Atlantic spreading center intersects the Iceland mantle plume and mapped the transition from thickened oceanic crust to the continental fragment on which the Faeroe Islands sit. Seismic techniques were used to obtain a 600‐km profile of the area (Figure 1). Preliminary results suggest that a 20‐km‐thick crust is being generated beneath the northern Neovolcanic Zone of Iceland, with a high‐level crustal magma chamber beneath Krafla. In northeast Iceland, the igneous crust is considerably thicker, reaching 35 km, while along the Faeroe‐Iceland Ridge it varies between 25–30 km.

The effects of spreading rate, the magma budget, and the geometry of magma emplacement on the axial heat flux at mid‐ocean ridges

We explore two‐dimensional, steady state, flow and thermal models of oceanic spreading center structure. These models include the effects of hydrothermal heat transport and crustal accretion for two basic accretion geometries: (1) a lens model where all crust below the sheeted dike layer passes through a magma lens at the base of the dike layer before subsiding and flowing to deeper levels; and (2) a dike model where crust is emplaced in a vertical slot from the surface to Moho and subsequently moves horizontally (rigidly) away from the axis of accretion. The axial heat flux resulting from these two end‐member accretion geometries is compared, and an assessment is made of the feasibility of using various observations to differentiate between these two accretion geometries. We find that the steady state axial heat flux is predominantly influenced by three factors: the spreading rate, the magmatic budget (crustal thickness) and the efficiency of hydrothermal cooling. The total steady state heat flux from the neovolcanic zone (2 km wide) increases almost linearly with the spreading rate for both the dike and lens accretion geometries. The major difference between the dike and the lens models is nonthermal: they predict different accumulated strain distributions within off‐axis crust. Crustal flow due to crustal accretion within a crustal‐height “dike” leads to little accumulated strain, while intense crustal strain results from crustal subsidence and flow below a steady state magma lens. Ophiolite and marine seismic observations of crustal layering appear to be the strongest observational tests to discriminate between these two accretion geometries; they currently favor a lens‐like model of lower crustal accretion.

Extrusive thickness variability at the East Pacific Rise, 9°–10°N: Constraints from seismic techniques

We calculate synthetic shot gathers and their corresponding common depth point (CDP) profiles over plausible East Pacific Rise (EPR) shallow velocity structures, based on the structures obtained from high‐resolution on‐bottom seismic refraction experiments. We then use these results to analyze the variability in layer 2A thickness at the EPR 9°–10°N region, as measured by CDP, wide‐aperture profile (WAP), on‐bottom seismic refraction experiments, and conventional air gun refraction data. The synthetics indicate that the accuracy of correlating the prominent shallow reflector observed in CDP and wide‐angle data with the layer 2A/2B boundary is strongly dependent on the structure within layer 2A. If layer 2A consists of a surficial low‐velocity layer overlying a steep velocity gradient (our gradient model), then there is an excellent correspondence between the two‐way travel times to the shallow reflector and the base of layer 2A. However, the shallow reflector may originate from a gradient within layer 2A if the upper crust contains more than one high‐gradient region (our step model). This implies that independent estimates of layer 2A velocity structure are needed to properly interpret CDP and wide‐angle data. We also determine that the travel time to the layer 2A reflector, for identical velocity structure, can vary by as much as 50 ms (about 125 m) for differing experimental geometries. This can explain the discrepancy in two‐way travel time to the layer 2A reflector imaged on zero‐age CDP and WAP lines. The depths to a shallow reflector calculated from CDP and wide‐angle data in the 9°–10°N region of the EPR generally correlate with estimated layer 2A thicknesses from on‐bottom refraction profiles and conventional air gun refraction lines, which suggests that the upper crustal structure in this area is similar to the gradient model. WAP and conventional air gun refraction data indicate that there is a 100–200 m decrease in off‐axis layer 2A thickness at 9°35′N on the EPR, the present‐day location of a deviation in axial linearity (deval). There is no bathymetric expression of the 50% decrease in layer 2A thickness. Layer 2A can be interpreted to consist of the extrusive section and transition zone, with the layer 2A/2B boundary corresponding to the top of the sheeted dikes. We suggest that buoyancy forces associated with the axial‐magma chamber (AMC) are supporting the extrusive layer and sheeted dikes at the neovolcanic zone. With distance from the rise axis, the AMC solidifies, the crust cools, the buoyancy forces are reduced, and the sheeted dike complex subsides. Concurrently, the extrusive layer thickens, resulting in significantly less subsidence of the seafloor. We speculate that the 50% decrease in dike subsidence and extrusive thickness at the 9°35′N deval is due to a local reduction in magma supply within the axial magma chamber. The off‐axis pattern of layer 2A thickness suggests that the 9°35′N deval has persisted for 175,000–275,000 years.

Detailed volcanic geology of the MARNOK area, Mid-Atlantic Ridge north of Kane transform

Abstract We present the results of a detailed study of the volcanic geology of the median valley floor of the Mid-Atlantic Ridge in two adjacent but contrasting spreading segments immediately north of the Kane transform fault, based on a high-resolution deep-towed sidescan sonar (TOBI) survey, near-bottom photographic traverses, and geochemical analyses from 28 precisely sited sets of dredge samples. This has allowed us to assess the relative importance of small (intersegment) and large (transform) boundaries on magmatic processes at slow spreading ridges. We find that the petrological effect of the transform (so-called ‘transform fault effect’) is minor compared to the manifestation of the second-order segmentation. The southern, ‘narrowgate’-type segment has a poorly developed neovolcanic zone, which tapers towards its bathymetric centre where continuous faulting causes rapid dismemberment. Flat-topped seamounts are preferentially located at the ends of the segment, particularly near the non-transform offset. There is a striking variation in the degree of fractionation along the length of this segment: primitive lavas were sampled at the centre, while more fractionated basalts, showing a greater range of parental compositions, form discrete volcanic edifices at the ends. In contrast, the northern segment has a wider inner valley, and syn-magmatic faults extend up to 15 km along the crest of its robust axial volcanic ridge. The along-segment trend of increasing MgO towards the bathymetric crest of the axial volcanic ridge is similar to, but less well-defined than, that for the southern segment. There is no variation in the bulk degree of melting along the segments as determined from the major element geochemistry; hence, melt migration is favoured over focused mantle upwelling as the main cause of the crustal thickness variations which define second-order segmentation. Radigenic isotopes show only very small variations with no systematic pattern emerging within or between the ridge segments. The isotopic irregularities are not related to the variations in incompatible elements, implying that the latter result from dynamic melting processes rather than from long-lived source heterogeneity. A model is developed which relates the volcanology, tectonic style and the geochemical trends to the episodicity of magma supply and eruption.

Alteration of basalt hyaloclastite at the off-axis Sea Cliff hydrothermal field, Gorda Ridge

The Sea Cliff hydrothermal field on the northern segment of the Gorda Ridge is situated along a rift-bounding normal fault about 2.6 km east of the neovolcanic zone and approximately 300 m above the spreading axis. The structural setting of this hydrothermal field differs from that of most other active seafloor hydrothermal sites investigated to date, which are typically situated in the neovolcanic zone. Mineralization occurs in basaltic talus covering a large normal-fault scarp. Hydrothermal crusts cover much of the seafloor in the area of the active hydrothermal field. These crusts form by extensive alteration of basaltic hyaloclastite in a zone of mixing between ascending hydrothermal fluid and entrained seawater. The initial stage of alteration is magnesian metasomatism of both crystalline basalt and basaltic glass, converting the rock to Mg-rich smectite and smectite/chlorite. Further alteration removes nearly all cations and ultimately leads to silicification. Preservation of basaltic texture in the silicified rocks provides evidence that even such sparingly soluble elements as A1 and Ti have been removed. Oxygen-isotopic ratios of the altered rocks constrain initial alteration to temperatures near 220°C, close to the maximum measured vent temperatures of 247°C. Silicification proceeded to much lower temperatures, and most amorphous silica deposition occurred at temperatures below 100°C. Sulfur-, strontium-, and lead-isotopic data all indicate a predominantly basaltic source, with important contributions from seawater but no significant contribution from sedimentary sources. Comparison with ophiolite-hosted massive sulfide deposits shows that the structural setting, alteration sequence, and depositional environment are all similar and suggests that mineralization and replacement of basaltic breccia in the Sea Cliff hydrothermal field likely occur in the subsurface beneath a capping layer of silicified hyaloclastite.

The scale of source heterogeneity beneath the Eastern neovolcanic zone, Iceland

Abstract Geochemical characteristics of Icelandic basalts require mixing of several sources: depleted asthenosphere, a plume (presumably heterogeneous), and an enriched component (e.g. hydrothermally altered crust). We use lava compositions and Sr-Nd-Pb isotopic ratios from neighbouring coeval volcanic centres to define spatial variations in source components beneath the neovolcanic zones. Within the Eastern Volcanic Zone, basalt compositions vary systematically from tholeiitic near central Iceland (Veidivotn, Grimsvotn) to Fe-Ti tholeiites at Hekla and Katla, to alkaline in Vestmannaeyjar. Each volcanic centre has distinctive geochemical characteristics that require differences in source components and proportions on the scale of 10–20 km.

Petrographic and geochemical variation along the Reykjanes Ridge, 57°N–59°N

Abstract High-density sampling of the neo-volcanic zone along the Reykjanes Ridge has been used to test the relationship between bathymetric features and geochemistry. The axial region has short wavelength features which correspond to elongate, en-echelon volcanic ridges. These ridges sit atop intermediate wavelength features, termed swells and troughs. A broad correspondence is found between the sections containing more fractionated samples and the intermediate wavelength swells. Conversely, the troughs are dominated by more primitive lavas. This suggests that swells represent areas of greater melt flux, resulting in thicker crust and hence generally more fractionation. Progressively higher concentrations of incompatible elements are found northwards along the ridge, with increases occurring as a series of steps that do not correlate with short or intermediate wavelength bathymetric features. The northward increase in ratios such as Nb/Zr suggests that sections of the ridge are fed by similar aggregate melt fractions, and that melt fractions become slightly smaller northwards. Changes or consistency in ratio, particularly Rb/Nb, cannot be explained by progressively greater northward incorporation of a component from the Icelandic plume mantle.

Deducing patterns of fluid flow and mixing within the TAG active hydrothermal mound using mineralogical and geochemical data

The TAG active hydrothermal mound, located 2.4 km east of the neovolcanic zone at 26°N, Mid‐Atlantic Ridge, is −200 m in diameter, exhibits 50 m of relief, and is covered entirely by hydrothermal precipitates. Eight different types of vent solids were recovered from the mound by the submersibles Alvin and Mir in 1986, 1990, and 1991. Detailed petrographic and geochemical studies of samples and their distribution are used to deduce patterns of fluid flow and seawater/hydrothermal fluid interaction. Geochemical modeling calculations using fluid composition data corroborate these interpretations. Current activity includes highly focused flow of 363°C fluid from a chimney cluster on the top of the mound and deposition of a high ƒS2‐ƒO2 mineral assemblage that reflects low concentrations of H2S in black smoker fluid. Slow percolation of black smoker fluid pooled beneath the black smoker cluster and entrainment of seawater result in formation of massive sulfide crusts and massive anhydrite. These three sample types are enriched in Co and Se. Blocks of sulfide and white smoker chimneys, enriched in Zn, Au, Ag, Sb, Cd, and Pb, are forming on the surface of the mound from black smoker fluid that has been modified by mixing with entrained seawater, precipitation of sulfides and anhydrite, and dissolution of sphalerite within the mound. This is the first time that on‐going remobilization, zone refinement, and significant modification of high‐temperature fluid in the near surface has been documented in a seafloor hydrothermal system. Deposits of ocherous material and massive sulfide with outer oxidized layers that formed during previous hydrothermal episodes are exposed on the steep outer walls of the mound. Studies of the full range of samples demonstrate that highly focused fluid flow, consequent seawater entrainment, and mixing within the mound can result in formation of a large seafloor hydrothermal deposit exhibiting sample types similar to those observed in Cyprus‐type ore bodies.

Uniform accretion of oceanic crust south of the Garrett transform at 14°15′S on the East Pacific Rise

Using migrated common depth point reflection profiles, we find the structural differences along the ultrafast spreading (>150 mm/yr) East Pacific Rise south of the Garrett fracture zone are second‐order, suggesting a remarkably uniform process of crustal accretion. The rise axis south of the Garrett transform is underlain by a narrow (<1.0 km) melt lens which shows great along‐strike continuity. The depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading East Pacific Rise at 9°30′N. This observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent. Melt sill width, however, shows little variation along the East Pacific Rise, suggesting no dependence of magma chamber size on spreading rate. The melt reservoir decreases in width toward/across the 14°27′S ridge axis discontinuity by a modest 250–300 m and appears to be continuous across this feature. Given the small aspect ratio (∼1.0 km by ∼50 m by tens of kilometers) of the axial melt lens, the previously recorded jump in MgO content across the 14°27′S offset is likely the result of a mixing boundary which is sustained through an along‐strike impedance in convection. Wide‐angle reflections originating at the base of seismic layer 2A, assumed to coincide with the extrusive layer, reveal a twofold to threefold increase (200–250 to 500–600 m) in thickness within 1–2 km of the rise axis. The pattern of extrusive thickening imaged south of the Garrett transform is similar to that observed along the slower spreading (110–120 mm/yr) East Pacific Rise at 9°N. Outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile. This implies a degree of temporal stability of the along‐strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone. The inferred uniformity of off‐axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14°27′S discontinuity are caused by long‐term reductions in magma supply. Second‐order differences in the style of extrusive thickening may be related to structural differences within the low‐velocity zone underlying the rise axis and/or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis. Images of Mono on cross‐axis profiles may be traced to within ∼1.0 km of the melt sill edge; this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber.

Petrology and geochemistry of basalts from the southern Juan de Fuca Ridge: Controls on the spatial and temporal evolution of mid‐ocean ridge basalt

Three morphologically distinct regions within the neovolcanic zone of the Cleft segment of the southern Juan de Fuca Ridge were investigated and sampled in detail using the DSRV Alvin. Additional along‐strike and off‐axis samples were recovered by dredge. The southernmost region, the Southern Cleft site, is characterized by a 3‐km‐wide axial valley floored by ponded sheet flows and bisected by a 30‐ to 50‐m‐wide cleft. Farther north at the “Young Sheet Flow” site, the ridge axis is characterized by a distinct 500‐m‐wide inner graben that is largely covered by distinctly younger looking sheet flows. The northernmost of the three regions is defined by a linear series of discontinuous constructional pillow mounds that extend the trend of the Cleft segment well into the zone of overlap with the neighboring Vance segment. The pillowed lavas at the “Young Pillow Mound” site represent the most recent episode of volcanism along the Cleft segment. Strong correlations exist between degrees of fractionation, relative ages of lavas, and latitude; lavas are progressively younger looking and more mafic to the north. The compositional range of mid‐ocean ridge basalts from the neovolcanic zone can generally be accounted for by 35–40% low‐pressure fractional crystallization of relatively primitive, but not primary, depleted (N‐type) melts. Scatter of the geochemical data about calculated liquid lines of descent is probably the result of mixing of magmas with slightly different parental compositions, generated from small‐scale mantle heterogeneities. Furthermore, the chemical variability may be the result of mixing of very depleted and more enriched sources or melts that are present beneath the southern Juan de Fuca Ridge. The more primitive nature of the young pillow mound basalts and their slightly different chemical characteristics indicates they cannot be simply related to the older southernmost lavas by along‐axis flow in a continuous axial magma chamber or conduit. Rather, the data suggest lavas were derived from discrete magma chambers or lenses, each in a different stage of evolution. The youngest events may be associated with a new influx of magma into the northern part of the segment and subsequent northward diking and propagation to form the new pillow mounds. The oldest stage (∼40% additional crystallization of the most mafic composition) is associated with focused hydrothermal activity and tectonic extension, whereas the youngest events are characterized by ridge inflation, diking, and dispersed hydrothermal activity. Geochemical and tectonomagmatic features observed along the Cleft segment are similar to those recently documented along the East Pacific Rise from 9°30′N–10°N suggesting the scales, processes, and stages of magmagenesis are similar along medium to fast spreading ridge segments.