Submarine Lava Research Articles

Along-strike variations in rifted margin crustal architecture and lithosphere thinning between northern Vøring and Lofoten margin segments off mid-Norway

An extensive geophysical and geological database is used to integrate observations and structural and stratigraphic modelling along the northern Vøring and Lofoten margins, off mid-Norway. Subsidence and fault analysis demonstrates depth-dependent lithosphere stretching along both margin segments, and the modelling concurs with an emergent or relatively shallow-water emplacement environment for the breakup lavas along the northern Vøring margin. However off Lofoten, subsidence modelling predicts ~ 1500 m of bathymetric relief during breakup in contradiction to the view of subaerial volcanic emplacement environment off mid-Norway. A distinct Early Eocene bathymetric gradient from Vøring to Lofoten margin is suggested exhibiting depths in the order of 500–1000 m on the latter margin, and thus an environment for submarine lava emplacement. The water depth may even have reached ~ 1250 m following breakup due to an initial, rapid margin subsidence of the Lofoten margin along a deep-rooted low-angle detachment fault. The differences between northern Vøring and Lofoten margin segments in pre- and post-breakup evolution are attributed to the Bivrost Lineament, a transfer zone separating the two margin segments, which appears as a major across-margin boundary in terms of margin physiography and paleogeography, differential sediment accommodation space, crustal- and lithosphere-scale structure, and breakup magmatism. Furthermore, gravity inversion results appear to independently predict a significantly greater volcanic addition on the Vøring than on the Lofoten margin.

Construction of the Galápagos platform by large submarine volcanic terraces

New multibeam bathymetric and side‐scan sonar data from the southwestern edge of the Galápagos platform reveal the presence of ∼60 large, stepped submarine terraces between depths of 800 m and 3500 m. These terraces are unique features, as none are known from any other archipelago that share this geomorphic form or size. The terraces slope seaward at <2° and are surrounded by escarpments that average ∼300 m in height with average slopes of 24°. The stepped morphology, fine‐scale features, and sinuous planform continuity of terrace edges indicate that each terrace results from a sequence of major submarine volcanic eruptions, similar in extent to young deep‐water (>3000 m) lava flow fields west of Fernandina and Isabela Islands. The terraces are formed of thick sequences of lava flows that coalesce to form the foundation of the Galápagos platform, on which the subaerial central volcanoes are built. The compositions of basalts dredged from the submarine terraces indicate that most lavas are chemically similar to subaerial lavas erupted from Sierra Negra volcano on southern Isabela Island. There are no regular major element, trace element, or isotopic variations in the submarine lavas as a function of depth, relative stratigraphic position, or geographic location along the southwest margin of the platform. We hypothesize that magma supply at the western edge of the Galápagos hot spot, which is influenced by both plume and mid‐ocean ridge magmatic processes, leads to episodic eruption of large lava flows. These large lava flows coalesce to form the archipelagic apron upon which the island volcanoes are built.

Lava penetrating water: Submarine lava flows around the coasts of Pico Island, Azores

Bathymetry data collected with a multibeam echo sounder around Pico Island, Azores (Portugal), reveal a remarkable series of lava flows on the island's shelf with a variety of pristine structures that suggest how lava behaves on entering water. Many flows are dendritic in plan view, some with channels and tumuli. Dendritic geometries are interpreted to arise from flow fronts repeatedly arrested by enhanced cooling and magma pressure subsequently causing new breakouts. Cascades of elongated flow fingers also occur, with individual fingers of comparable diameters to the largest known megapillows. Some flows have wide transverse clefts, in cases separating flows into segments, which are interpreted as caused by their upper surfaces having solidified, while their still‐fluid cores allowed the surfaces to extend. A number of flows moved onto the shelf as large bodies, stopped, and then sourced smaller lobes forming the dendritic patterns. This two‐stage evolution and the tumuli (which lie on a low gradient immediately below a steep nearshore gradient) suggest that, after initial emplacement and development of a crust by cooling, some flows pressurized. Once movements ceased and viscous stresses dissipated, magma static pressure developed from the weight of flow interiors passing over cliffs and nearshore gradients. One group of flows traverses the island's submarine slope, so direct supply of lava to the slopes is possible, although volumetrically how important it is to the island's internal composition is difficult to tell from these data. On the basis of observed strong surf erosion of historical flows, these delicate structures probably could not have survived passage through a moving sea level unmodified by erosion so they are unlikely to be pre‐Holocene subaerial flows. They are interpreted to have formed in the Holocene from flows penetrating sea level or possibly some from nearshore tube openings or vents. Such flows and abundant clastic deposits are ephemeral features that become remobilized by surf during times of lower sea level. The shelves of active volcanic islands are therefore active geologically and are far from being simple products of erosional truncation as was once envisaged.

Reconstruction of an extensive Archaean dacitic submarine volcanic complex associated with the komatiite-hosted Mt Keith nickel deposit, Agnew-Wiluna Greenstone Belt, Yilgarn Craton, Western Australia

Felsic volcanic rock units characterise a large portion of the 2.7 Ga. Agnew-Wiluna Greenstone Belt (Western Australia) stratigraphy and form the hangingwall and footwall sequences to several komatiite-hosted nickel sulphide deposits in the region. The Mt Keith region of the Agnew-Wiluna Greenstone Belt (AWB) comprises a sequence of felsic, mafic and ultramafic extrusive and intrusive units. The felsic volcanic dominated footwall and hangingwall sequences to the Mt Keith nickel deposit comprise cyclical packages of coherent dacite lavas, with associated monomictic in situ to resedimented breccias. Clasts in the monomictic breccia facies are identical to the coherent dacite and the two facies are often in gradational contact. The dacites are interpreted as submarine lavas and the breccias as associated autoclastic facies (hyaloclastite, autobreccia). These dacite lavas are inferred to have been emplaced into a deep water environment which retarded volatile exsolution suppressing explosive fragmentation and increasing lava mobility, resulting in large tabular flow units. The AWB sequence has undergone multiple periods of deformation and metamorphism subsequent to emplacement resulting in strain and alteration in large sections of the coherent lava facies. A notable textural variant within the lavas is a set of pseudo-textures/structures resembling tuffaceous and brecciated dacitic units. The process of pseudo-clastic texture/structure generation is related to a combination of polyphase alteration, brecciation and development of a secondary structural/metamorphic layering. Understanding the origin of these pseudo-clastic textures/structures has allowed recognition of the lateral and vertical facies variation of the felsic sequence and reconstruction of the original volcanic architecture.

Flood basalts from Mt. Capitole in the central Kerguelen Archipelago: Insights into the growth of the archipelago and source components contributing to plume‐related volcanism

The Kerguelen Archipelago, constructed on the submarine Northern Kerguelen Plateau, is attributed to Cenozoic volcanism arising from the Kerguelen hot spot. Geochemical studies of 325 to 1000 m thick lava sections of the ∼30 to 25 Ma flood basalt forming the bulk of the archipelago show a temporal change from older tholeiitic basalt to younger slightly alkalic basalt. This compositional transition is expressed in a 630 m lava section at Mt. Capitole where the lava sequence is lowermost tholeiitic basalt overlain by slightly alkalic basalt overlain by plagioclase‐rich cumulates that are mixtures of plagioclase‐phyric basalt and more evolved magmas. During growth of the archipelago, magma supply from the hot spot was variable and at times sufficiently low to enable extensive crystal fractionation; e.g., at Mt. Capitole and nearby Mt. Tourmente only 10 of 120 lava flows have >6 wt% MgO. On the basis of this study and previous isotopic data for the ∼34 Ma submarine lavas erupted on the Northern Kerguelen Plateau, other flood basalt sections in the Kerguelen Archipelago, and younger lavas erupted in the archipelago and at Heard Island, there is significant Sr, Nd, Hf, and Pb isotopic heterogeneity that can be explained by two stages of mixing. The first mixing event, best shown by the submarine lavas, is between components that are related to Indian Ocean mid‐ocean ridge basalt (MORB) and the Kerguelen hot spot. From ∼34 Ma to <1 Ma, on average the proportion of the MORB‐related component decreased. Subsequently, a second mixing process involved addition of a component with relatively high 87Sr/86Sr (>0.7060) and low 143Nd/144Nd (<0.5125) and 176Hf/177Hf (<0.2827) and nonradiogenic Pb isotope ratios (<17.9 for 206Pb/204Pb). We infer that this component was lower continental crust.

Submarine lava flow emplacement and faulting in the axial valley of two morphologically distinct spreading segments of the Mariana back‐arc basin from Wadatsumi side‐scan sonar images

High‐resolution, deep‐tow side‐scan sonar data were collected over two distinct spreading segments in the central part of the Mariana back‐arc basin. These data allow mapping of small fissures and faults and the distinguishing of hummocky from smooth lava flows. Using these data, we observe spatial variations in seafloor deformation and volcanic activity within each segment, and also significant differences in the degree of tectonic deformation between the two segments. One segment, characterized by an hourglass shape suggestive of magmatic processes dominating over tectonic processes, is in fact currently dominated by intense deformation rather than volcanism. The other segment, which exhibits morphology (wide and deep linear axial valley) typical of magma‐starved segments, is subjected to very limited deformation and is covered by mostly unfaulted, recent flows. Each segment also displays along‐axis variations in the degree of tectonic deformation and in lava flows freshness. We observe a decrease of lava effusion rate from segment centers toward their ends. We also investigated the apparent asymmetry of the Mariana basin. On the southern segment of the study area, azimuths of tectonic structures are divided into two groups, one segment‐parallel and one ∼15° oblique to strike of the segment. These two trends of linear features developed synchronously with volcanic activity. Currently oblique structures seem to be the most active ones. The tectonic structures that are parallel to the overall valley trend are distributed over the entire valley, whereas the oblique structures are only located proximal to the eastern valley wall. They are likely related to changes in the local stress field related to the obliquely trending eastern axial valley wall. Asymmetric character, such as nonuniform spacing and throw of faults, was not observed. These observations suggest that the asymmetry of the basin is not due to asymmetric spreading, but rather to eastward ridge jumps of several kilometers. Such small‐amplitude ridge jumps likely occurred frequently during basin development because the asymmetric character of the basin is strongly pronounced and no abandoned rift valley is recognizable within the entire basin.

Submarine Fernandina: Magmatism at the leading edge of the Galápagos hot spot

New multibeam and side‐scan sonar surveys of Fernandina volcano and the geochemistry of lavas provide clues to the structural and magmatic development of Galápagos volcanoes. Submarine Fernandina has three well‐developed rift zones, whereas the subaerial edifice has circumferential fissures associated with a large summit caldera and diffuse radial fissures on the lower slopes. Rift zone development is controlled by changes in deviatoric stresses with increasing distance from the caldera. Large lava flows are present on the gently sloping and deep seafloor west of Fernandina. Fernandina's submarine lavas are petrographically more diverse than the subaerial suite and include picrites. Most submarine glasses are similar in composition to aphyric subaerially erupted lavas, however. These rocks are termed the “normal” series and are believed to result from cooling and crystallization in the subcaldera magma system, which buffers the magmas both thermally and chemically. These normal‐series magmas are extruded laterally through the flanks of the volcano, where they scavenge and disaggregate olivine‐gabbro mush to produce picritic lavas. A suite of lavas recovered from the terminus of the SW submarine rift and terraces to the south comprises evolved basalts and icelandites with MgO = 3.1 to 5.0 wt.%. This “evolved series” is believed to form by fractional crystallization at 3 to 5 kb, involving extensive crystallization of clinopyroxene and titanomagnetite in addition to plagioclase. “High‐K” lavas were recovered from the southwest rift and are attributed to hybridization between normal‐series basalt and evolved‐series magma. The geochemical and structural findings are used to develop an evolutionary model for the construction of the Galápagos Platform and better understand the petrogenesis of the erupted lavas. The earliest stage is represented by the deep‐water lava flows, which over time construct a broad submarine platform. The deep‐water lavas originate from the subcaldera plumbing system of the adjacent volcano. After construction of the platform, eruptions focus to a point source, building an island with rift zones extending away from the adjacent, buttressing volcanoes. Most rift zone magmas intrude laterally from the subcaldera magma chamber, although a few evolve by crystallization in the upper mantle and deep crust.

Incorporation of seawater into mid-ocean ridge lava flows during emplacement

Evidence for the interaction between seawater and lava during emplacement on the deep seafloor can be observed in solidified flows at a variety of scales including rapid quenching of their outer crusts and the formation of lava pillars through the body of the flow. Recently, an additional interaction, incorporation of heated seawater (vapor) into the body of a flow, has been proposed. Large voids and vesicles beneath the surface crusts of mid-ocean ridge crest lobate and sheet lava flows and lava drips found within those cavities have been cited as evidence for this interaction. The voids resulting from this interaction contribute to the high porosity of the shallow ocean crust and play an important role in crustal permeability and hydrothermal circulation at mid-ocean ridges, and thus it is important to understand their origin. We analyze lava samples from the fast-spreading East Pacific Rise and intermediate-spreading Galapagos Spreading Center to characterize this process, identify the source of the vapor, and investigate the implications this would have on submarine lava flow dynamics. We find that lava samples that have interacted with a vapor have a zone of increased vesicularity on the underside of the lava crust and a coating of precipitate minerals ( i.e., crystal fringe) that are distinct in form and composition from those crystallized from the melt. We use thermochemical modeling to simulate the reaction between the lava and a vapor and find that only with seawater can we reproduce the phase assemblage we observe within the crystal fringes present in the samples. Model results suggest that large-scale contamination of the lava by mass exchange with the vapor is unlikely, but we observe local enrichment of the lava in Cl resulting from the incorporation of a brine phase separated from the seawater. We suggest that high eruption rates are necessary for seawater incorporation to occur, but the mechanism by which seawater enters the flow has yet to be resolved. A persistent vapor phase may be important in inhibiting the collapse of lava flow roofs during natural waxing and waning of lava levels during emplacement allowing lava pathways to be maintained during long lived eruptions. In addition, we illustrate the potential for a persistent vapor layer to increase local flow rates within submarine flows by up to a factor of three, thereby influencing how lava is distributed across the ridge crest.

A morphometric analysis of the submarine volcanic ridge south-east of Pico Island, Azores

A region of crustal extension, the Azores Plateau contains excellent examples of submarine volcanic edifices constructed over a wide range of ocean depths along the Pico Ridge. Using bathymetric data and Towed Ocean Bottom Instrument (TOBI) side-scan sonar imagery, we measured the dimensions (diameter, height, slopes), shape, and texture of these volcanic edifices to further understanding of the geometric development of a submarine ridge. Our analysis and interpretation of the measurement and texture data suggest the following: (1) the various edifice types do not correlate with depth ranges, suggesting that ambient water pressure is not a controlling factor in configuration of the edifice formed; (2) the cones have a mean diameter of 948 m, height of 152 m, and slope of 15.6°, and are peaked rather than displaying flat-topped summits; and (3) while hummocky-textured cones also occur, smooth-textured cones predominate. The cones seem to develop preferentially outwards, then upwards, with only a weak correlation between diameter and height, suggesting that the cone population does not evolve self-similarly. Although smooth and hummocky cone populations are not statistically different in mean slope angle and eruption depth, the hummocky cones have a significantly greater mean diameter than the smooth cones. We suggest that hummocky-textured cones (probably involving eruption of pillow lavas) are formed after voluminous smooth textured flows. Nearest-neighbour analysis suggests that submarine cones are distributed randomly whereas subaerial cones are not. We interpret this finding to suggest subaerial cones being masked by over-covering flows, which tend to flow further than submarine lava flows. Furthermore, the spatial distribution of cones away from the ridge centre is not easily explained by a magma supply fed via lava tubes from central eruptions, because of a lack of consistent pathway down gradient. Fissure vents seem to be important in determining construction of the ridge and result in linear arrangements of edifices and cone elongation, consistent with the regional tectonic trend of this part of the Azores Plateau. Cones form as each feeding dyke intrusion cools and the eruption becomes localized along point-source vents along the fissure. The ridge seems to be predominantly formed from fissure eruptions along the ridge axis, with subordinate transport of lava down its flanks.

Submarine radial vents on Mauna Loa Volcano, Hawai'i

A 2002 multibeam sonar survey of Mauna Loa's western flank revealed ten submarine radial vents and three submarine lava flows. Only one submarine radial vent was known previously. The ages of these vents are constrained by eyewitness accounts, geologic relationships, Mn‐Fe coatings, and geochemical stratigraphy; they range from 128 years B.P. to possibly 47 ka. Eight of the radial vents produced degassed lavas despite eruption in water depths sufficient to inhibit sulfur degassing. These vents formed truncated cones and short lava flows. Two vents produced undegassed lavas that created “irregular” cones and longer lava flows. Compositionally and isotopically, the submarine radial vent lavas are typical of Mauna Loa lavas, except two cones that erupted alkalic lavas. He‐Sr isotopes for the radial vent lavas follow Mauna Loa's evolutionary trend. The compositional and isotopic heterogeneity of these lavas indicates most had distinct parental magmas. Bathymetry and acoustic backscatter results, along with photography and sampling during four JASON2 dives, are used to produce a detailed geologic map to evaluate Mauna Loa's submarine geologic history. The new map shows that the 1877 submarine eruption was much larger than previously thought, resulting in a 10% increase for recent volcanism. Furthermore, although alkalic lavas were found at two radial vents, there is no systematic increase in alkalinity among these or other Mauna Loa lavas as expected for a dying volcano. These results refute an interpretation that Mauna Loa's volcanism is waning. The submarine radial vents and flows cover 29 km2 of seafloor and comprise a total volume of ∼2 × 109 m3 of lava, reinforcing the idea that submarine lava eruptions are important in the growth of oceanic island volcanoes even after they emerged above sea level.

Formation of submarine lava channel textures: Insights from laboratory simulations

Laboratory simulations using polyethylene glycol (PEG) extruded at a constant rate and temperature into a tank with a uniform basal slope and filled with a cold sucrose solution generate channels that are defined by stationary levees and mobile flow interiors. These laboratory channels consistently display the following surface textures in the channel: smooth, folded, lineated, and chaotic. In the simulations, we can observe specific local conditions including flow rate, position within the channel, and time that combine to develop each texture. The textures in PEG flows form due to relative differences in shear forces between the PEG crust and the underlying liquid wax. Minimal shear forces form smooth crust, whereas folded crust forms when the shear is sufficiently high to cause ductile deformation. Brittle deformation of solid PEG creates a chaotic texture, and lineated crust results from shear forces along the channel‐levee margin. We observe similar textures in submarine lava channels with sources at or near the Axial Summit Trough of the East Pacific Rise between 9° and 10°N. We mapped the surface textures of nine submarine lava channels using high‐resolution digital images collected during camera tows. These textural maps, along with observations of the formation of similar features in analog flows, reveal important information about the mechanisms occurring across the channel during emplacement, including relative flow velocity and shear stress.

Carbon and hydrogen isotope constraints on degassing of CO2 and H2O in submarine lavas from the Pitcairn hotspot (South Pacific)

The analysis of CO2–δ13C–H2O–δD in vesicles of 35 submarine volcanic glasses reveals that open‐system degassing, following closed‐system degassing, is responsible for the observed variations in volatile concentration and isotopic ratios. The least degassed samples allow the assessment of minimum estimates for the flux of CO2 and H2O from the Pitcairn hotspot: 2.24 ± 0.17 104 tons C yr−1 and 1.32 ± 0.82 105 tons H2O yr−1. The results also suggest that the carbon in the source of the Pitcairn hotspot may have a δ13C value around −6‰, 2‰ lower than the source of mid‐ocean ridge basalts. This indicates either a possible contribution from the lower mantle or the possible involvement of decarbonated recycled materials with an unusual carbon isotopic signature.

A submarine perspective of the Honolulu Volcanics, Oahu

Lavas and volcaniclastic deposits were observed and collected from 4 submarine cones that are part of the Honolulu Volcanics on Oahu, Hawaii. The locations of these and a few additional, but unsampled, vents demonstrate that nearly all the vents are located on or very close to the shoreline of Oahu, with the most distal vent just 12 km offshore. The clastic samples and outcrops range from coarse breccias to cross-bedded ash deposits and show that explosive volcanism at depths between about 350 and 590 m depth played a part in forming these volcanic cones. The eruptive styles appear to be dominantly effusive to strombolian at greater depths, but apparently include violent phreatomagmatic explosive activity at the shallower sites along the submarine southwest extension of the Koko Rift. The compositions of the recovered samples are broadly similar to the strongly alkalic subaerial Honolulu Volcanics lavas, but the submarine lavas, erupted further from the Koolau caldera, have slightly more radiogenic Sr isotopic ratios, and trace element patterns that are distinct from either the subaerial Honolulu Volcanics or the submarine North Arch lavas. These patterns are characterized by moderate to strong positive Sr and P anomalies, and moderate to strong negative Cs, Rb, U, Th, Zr, and Hf anomalies. Most samples have strong negative K and moderate negative Ti anomalies, as do all subaerial Honolulu Volcanics and North Arch samples, but one group of samples from the Koko Rift lack this chemical signature. The data are consistent with more garnet in the source region for the off-shore samples than for either the on-shore Honolulu Volcanics lavas. New Ar–Ar ages show that eruptions at the submarine vents and Diamond Head occurred between about 0.5 Ma and 0.1 Ma, with the youngest ages from the Koko Rift. These ages are in general agreement with most published ages for the formation and suggest that some much younger ages reported previously from the Koko Rift are probably erroneously young.

Isotope geochemistry of early Kilauea magmas from the submarine Hilina bench: The nature of the Hilina mantle component

Submarine lavas recovered from the Hilina bench region, offshore Kilauea, Hawaii Island provide information on ancient Kilauea volcano and the geochemical components of the Hawaiian hotspot. Alkalic lavas, including nephelinite, basanite, hawaiite, and alkali basalt, dominate the earliest stage of Kilauea magmatism. Transitional basalt pillow lavas are an intermediate phase, preceding development of the voluminous tholeiitic subaerial shield and submarine Puna Ridge. Most alkalic through transitional lavas are quite uniform in Sr–Nd–Pb isotopes, supporting the interpretation that variable extent partial melting of a relatively homogeneous source was responsible for much of the geochemical diversity of early Kilauea magmas ( Sisson et al., 2002). These samples are among the highest 206Pb/ 204Pb known from Hawaii and may represent melts from a distinct geochemical and isotopic end-member involved in the generation of most Hawaiian tholeiites. This end-member is similar to the postulated literature Kea component, but we propose that it should be renamed Hilina, to avoid confusion with the geographically defined Kea-trend volcanoes. Isotopic compositions of some shield-stage Kilauea tholeiites overlap the Hilina end-member but most deviate far into the interior of the isotopic field defined by magmas from other Hawaiian volcanoes, reflecting the introduction of melt contributions from both “Koolau” (high 87Sr/ 86Sr, low 206Pb/ 204Pb) and depleted (low 87Sr/ 86Sr, intermediate 206Pb/ 204Pb) source materials. This shift in isotopic character from nearly uniform, end-member, and alkalic, to diverse and tholeiitic corresponds with the major increase in Kilauea's magmatic productivity. Two popular geodynamic models can account for these relations: (1) The upwelling mantle source could be concentrically zoned in both chemical/isotopic composition, and in speed/extent of upwelling, with Hilina (and Loihi) components situated in the weakly ascending margins and the Koolau component in the interior. The depleted component could be refractory and spread throughout or scavenged from the overlying lithosphere. (2) The Hilina (and Loihi) components could be a more fertile material (lower melting temperature) spread irregularly throughout the Hawaiian source in a matrix of more refractory depleted and Koolau compositions. Modest upwelling along the leading hotspot margin melts the fertile domains predominantly, while the refractory matrix also partially melts in the more vigorously upwelling hotspot interior, diluting the Hilina and Loihi components and yielding voluminous isotopically diverse tholeiitic magmas.

Trace element abundances in the Rock Canyon Anticline, Pueblo, Colorado, marine sedimentary section and their relationship to Caribbean plateau construction and oxygen anoxic event 2

Rapid eruption of submarine lava flows during formation of the Caribbean plateau correlates closely with ocean anoxic event 2 (OAE2), which bracketed the Cenomanian/Turonian boundary (∼93.5 Ma). These events also correspond with a positive excursion in carbon isotopic composition of seawater. Hydrothermal activity associated with large‐scale submarine volcanism may have been responsible for this abrupt change in ocean chemistry. We determined the distribution of major, minor, and trace element abundances in the Rock Canyon marine sedimentary section (Pueblo, Colorado). After normalizing element concentrations to Zr to remove the variable contribution of terrigenous material to these sediments, we detected an interval of concentrated metal abundance anomalies that coincides with the abrupt beginning of the positive δ13C isotope excursion. The metal abundance anomalies indicate that intermittent hydrothermal activity, in the form of both water/rock exchange and magmatic degassing, introduced large concentrations of trace metals into the Cretaceous ocean at the same time that extinctions of benthic species, turnover in plankton communities, and increases in isotopically light organic carbon burial occurred. The stratigraphic position of this interval of trace metal anomalies matches events associated with OAE2 and indicates that intermittent hydrothermal activity on a massive scale triggered abrupt changes in carbon burial and deep ocean oxygen contents.

Channelized lava flows at the East Pacific Rise crest 9°–10°N: The importance of off‐axis lava transport in developing the architecture of young oceanic crust

Submarine lava flows are the building blocks of young oceanic crust. Lava erupted at the ridge axis is transported across the ridge crest in a manner dictated by the rheology of the lava, the characteristics of the eruption, and the topography it encounters. The resulting lava flows can vary dramatically in form and consequently in their impact on the physical characteristics of the seafloor and the architecture of the upper 50–500 m of the oceanic crust. We have mapped and measured numerous submarine channelized lava flows at the East Pacific Rise (EPR) crest 9°–10°N that reflect the high‐effusion‐rate and high‐flow‐velocity end‐member of lava eruption and transport at mid‐ocean ridges. Channel systems composed of identifiable segments 50–1000 m in length extend up to 3 km from the axial summit trough (AST) and have widths of 10–50 m and depths of 2–3 m. Samples collected within the channels are N‐MORB with Mg# indicating eruption from the AST. We produce detailed maps of lava surface morphology across the channel surface from mosaics of digital images that show lineated or flat sheets at the channel center bounded by brecciated lava at the channel margins. Modeled velocity profiles across the channel surface allow us to determine flux through the channels from 0.4 to 4.7 × 103 m3/s, and modeled shear rates help explain the surface morphology variation. We suggest that channelized lava flows are a primary mechanism by which lava accumulates in the off‐axis region (1–3 km) and produces the layer 2A thickening that is observed at fast and superfast spreading ridges. In addition, the rapid, high‐volume‐flux eruptions necessary to produce channelized flows may act as an indicator of the local magma budget along the EPR. We find that high concentrations of channelized lava flows correlate with local, across‐axis ridge morphology indicative of an elevated magma budget. Additionally, in locations where channelized flows are located dominantly to the east or west of the AST, the ridge crest is asymmetric, and layer 2A appears to thicken over a greater distance from the AST toward the side of the ridge crest where the channels are located.

Degassing of CO 2 and H 2O in submarine lavas from the Society hotspot

We analyzed CO 2– δ 13C–H 2O– δD in vesicles of a suite of 23 vesicular submarine volcanic glasses (4 alkalic basalts and 19 trachytes) from Teahitia, Volcano 4 and Rocard seamounts and Mehetia Island in the active area of the Society hotspot (French Polynesia, South Pacific). Vesicle C and H 2O concentrations ([C vesicle] and [H 2O vesicle]) vary in the range 1–1487 ppm C and ∼0–5431 ppm H 2O, respectively. The δ 13C of CO 2 in vesicles ( δ 13C vesicle) vary from − 3.8‰ to − 9.7‰ and the δD of H 2O in vesicles ( δD vesicle) vary from − 32‰ to − 71‰. The δ 13C vesicles values are correlated with the vesicularity, Log([C vesicle]) and the δD vesicle values. The Log([H 2O vesicle]) is also correlated to the δD vesicle values and Log([C vesicle]). These observations show that late-stage open-system degassing, following a first stage of closed-system degassing, is the chief process controlling the concentration and isotopic ratio variations observed in the vesicles. The highest measured δ 13C vesicle values fall in a typical mantle range (− 4.0 ± 0.5‰). Taking into account the effect of vesicle bursting and crystal fractionation, the reconstructed pre-eruptive carbon and water concentrations in the alkalic basalt parental magma are ∼3810 ppm C and 14,360 ppm H 2O (1.43 wt.%). These concentrations correspond to carbon and water fluxes for the Society hotspot during the last 900 kyr of 2.57 × 10 4 tons C yr − 1 and 97 × 10 3 tons H 2O yr − 1 and volatile concentrations of the source of 190 ppm C and 720 ppm H 2O. These values are similar in magnitude to previous estimates for other hotspots. Our study emphasizes that H 2O degassing must be taken into account before discussing H 2O content and δD value variations in the lavas in terms of source variation in the case of the Society hotspot.

Tehama-Colusa Serpentinite Mélange: A Remnant of Franciscan Jurassic Oceanic Lithosphere, Northern California

The Coast Range ultramafic belt that borders the Sacramento Valley is largely a serpentinitematrix mélange, called the Tehama-Colusa serpentinite mélange (TCSM) after the counties it spans. It is bordered on the west across the Coast Range fault by exhumed high-P/T metamorphic rocks of the Franciscan subduction complex, and on the east, across the steep Stony Creek fault, by the Jura-Cretaceous terriginous clastic strata of the Great Valley Group (GVG). Remnants of the Coast Range ophiolite (CRO) that lie stratigraphically beneath the basal GVG are exposed only at the northern end of the Tehama-Colusa serpentinite mélange belt. The serpentinite protolith was peridotite tectonite (harzburgite > dunite), comprising oceanic upper mantle. Non-native mélange blocks in the serpentinite are chiefly basaltic submarine lava and sparse radiolarian ribbon chert (Middle and early Late Jurassic), plus rare plutonic rocks and slaty argillite. The serpentinite mélange protolith was basaltic oceanic crust above uppermost mantle, overlain by radiolarian chert and thin argillite (deep-sea clay?). An open-ocean setting, far from a continent margin or volcanic arc, is inferred from the absence of terriginous clastic or volcaniclastic sediments. The TCSM wraps around the Stonyford volcanic complex, a basaltic seamount that grew atop the basaltic oceanic crust during the late Middle and Late Jurassic. A widespread deepsea tectonic event in the Late Jurassic disrupted the proto-TCSM upper mantle and crust. Pervasive hydration turned peridotite into serpentinite, which invaded and mixed with the basalt/chert oceanic crust, creating mélange. Possible causes are discussed. The TCSM basaltic lavas and radiolarian chert closely resemble Franciscan oceanic crustal (not seamount) rocks, and are within the Franciscan basalt/chert age span; they are unlike the CRO igneous assemblage and its Late Jurassic tuffaceous chert. The TCSM was a segment of Franciscan oceanic lithosphere that records only Jurassic open-ocean depositional and tectonic history. It escaped latest Jurassic-Cretaceous continent-margin and trench sedimentation, subduction, subduction accretion, high-P/T metamorphism, and exhumation of deeply subducted rocks that characterize the Franciscan Complex farther west in the Coast Ranges. The TCSM and CRO terranes are remnants of two different mid Jurassic paleoequatorial oceanic plates, separated now by the Stony Creek-Beehive Flat fault system. TCSM evolution involves four successive stages of Mesozoic tectonic history: (1) Late Jurassic, when the CRO oceanic plate was being drawn NNE toward the Great Valley subduction zone in front of the oceanic Nevadan arc, trailed by TCSM (eastern Franciscan) lithosphere; (2) the latest Jurassic-earliest Cretaceous, when (a) N-S dextral transform faulting replaced earlier dextral oblique subduction, bringing the Jurassic TCSM/Franciscan oceanic plate northward alongside the unsubducted CRO plate remnant, and (b) terriginous sediments from the new Nevadan orogen spread progressively seaward across the CRO and adjacent TCSM/Franciscan Jurassic ocean floor; (3) resumption of eastward subduction in the Early Cretaceous, from a new trench lying farther west, caused (a) disruption, mélanging, and metamorphism of the Tithonian-Valanginian clastic strata and underlying Jurassic (Franciscan) oceanic crust, and (b) underthrusting, uplift, and eastward tilting of the unbroken TCSM/CRO oceanic basement, forming a new forearc ridge; and (4) a post-Valanginian Cretaceous era when Franciscan subduction operated west of the TCSM/CRO submarine ridge, and GVG forearc basin sedimentation progressed behind the ridge on the east. The Stony Creek-Beehive Flat composite fault sytem between the TCSM and CRO was a dextral transform fault during stage 2 (Tithonian-Valanginian) and a W-vergent reverse fault during stage 3 (Valanginian), during underthrusting and tilting of TCSM/CRO basement. The extensional Coast Range fault, bounding TCSM on the west, brought up deeply subducted Franciscan Cretaceous high-P/T metamorphic rocks in the late Cretaceous (Jayko et al., 1987).

Morphology and growth style of a Miocene submarine dacite lava dome at Atsumi, northeast Japan

A Miocene submarine lava dome at Tate-iwa, Atsumi, Yamagata, Japan, displays well-preserved primary morphological features. The dome currently rises above a wave-cut coastal platform and is 55 m high and 90–180 m across. It comprises a massive core and a lava lobe–hyaloclastite rim, both of which are composed of compositionally uniform, feldspar–phyric dacite (SiO 2=64 wt.%). The boundary between these two zones is distinct but gradational. The massive core consists of homogeneous, coherent dacite and is characterized by flow banding along the margin and by columnar joints radiating from the centre to the margin. The lava lobe–hyaloclastite rim encircles the massive core and consists of a complex of 80–90% dacitic lava lobes and 10–20% hyaloclastite. The lava lobes are 1–6 m thick, 3–12 m wide and more than 5 m long. Each lobe consists of a radially columnar-jointed core and a glassy rim 10–30 cm thick. The hyaloclastite comprises polyhedral dacite clasts 5–100 cm across in a matrix of dacite fragments up to 5 mm across. The lava lobes and hyaloclastite have gradational contacts in places. Paleobathymetric studies based on foraminifera in sediment beneath the dome, and the K–Ar age of the dome, suggest that it was extruded in a middle-bathyal environment (1500–2000 m below sea level) at 12.9±0.6 Ma. The internal structures of the dome suggest that it formed by a combination of exogenous growth involving extrusions of small dacite lobes, and endogenous growth involving a continuous magma supply and simple expansion from the interior. The presence of small lava lobes along the rim suggests that the magma had relatively low viscosity at the time of extrusion, in spite of its high silica content. Magma temperatures calculated by two-pyroxene and Fe–Ti oxide geothermometers were 999–1042 and 957–1005 °C, respectively. The inferred low viscosity may be attributed to the high temperature of the magma and/or a high confining pressure resulting from the deep-sea environment.

Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project

This paper presents new magmatic helium isotopic compositions in a suite of lavas from phase II of the Hawaiian Scientific Drilling Project (HSDP2) core, which sampled Mauna Kea volcano to a maximum depth of 3098 m below sea level. Most of the measurements were performed by in vacuo crushing of olivine phenocrysts, but include submarine pillow glasses from the 2200 to 2500 meter depth interval, and orthopyroxene phenocrysts from an intrusive at 1880 m. The magmatic 3He/4He ratios range from 6 to 24.7 times atmospheric (Ra), which significantly extends the range of values for Mauna Kea volcano. The 3He/4He ratios are lowest (i.e., close to MORB values of ∼8 Ra) near the top of the Mauna Kea section and rise slowly, to 10–12 Ra, at 1000 m below sea level, consistent with results from the HSDP1 core. At depths greater than 1000 m in the core, primarily in the submarine lavas, there are brief periods when the 3He/4He ratios are higher than 14.5 Ra, always returning to a baseline value. Twelve such excursions were identified in the core; all but one are in the submarine section, and most (7) are in the deepest section, at depths of 1950 to 3070 m. The baseline 3He/4He value rises from 10–12 Ra near 1000 m depth to 12–14 Ra at 3000 m. The helium spikes are found only in lavas that are older than 380 Ka in age, based on an age model derived from Ar‐Ar data (W. D. Sharp et al., manuscript in preparation, 2003). Excluding the excursions defined by single lava flows (3) and intrusive units (3), the average spike duration is approximately 15 (±9) Ka (n = 6). The high 3He/4He spikes are interpreted as pulses of magma from the center of the actively upwelling Hawaiian hot spot. The short duration of the high 3He/4He excursions suggests that Mauna Kea was never directly over high the 3He/4He component of the plume (during the HSDP2 eruptive period), presumed to be the plume center. Assuming that the Mauna Kea helium spikes result from melting of heterogeneities within the plume, their short duration implies that the length scales of heterogeneities in the solid upwelling mantle are between 60 m and 12 km (for upwelling rates of 2 to 40 cm/yr). The high 3He/4He are associated with high 208Pb/204Pb, and relatively low 143Nd/144Nd, Zr/Nb, and SiO2. The correlations with major elements, trace elements and isotopes demonstrate that helium is coupled to the other geochemical variations, and that the Mauna Kea isotopic variability is caused by heterogeneities within the upwelling plume.