Seafloor Lavas Research Articles

Boron isotopes identify deep-slab serpentinite in the source of Aleutian arc magma

Seafloor lavas of the Western Aleutian arc have isotopically heavy boron (δ11B to +13.4‰) that is negatively correlated with B content (ppm). Endmember samples are primitive dacites and rhyodacites (δ11B > +10‰, SiO2 = 63%−70%, Mg# > 0.60) with adakitic trace-element and isotopic characteristics that require roles for residual garnet and rutile in their formation. The source of isotopically heavy B is likely serpentinite in the mantle section of the subducting plate, which dewaters into an inverted geothermal gradient and drives melting within the overlying volcanic section at depths where prior effects of seawater alteration were minimal. Most volcanic rocks from the Aleutian Island locations have 10−30 ppm B with an average δ11B of ∼+1.0‰ ± 1.3‰, reflecting a mixed source dominated by subducted sediment. A subset of island samples has B that is isotopically light (δ11B < −2.4‰) and at low concentrations (<11.0 ppm), which is typical of arc lavas globally from rear-arc settings where depth-to-slab is high, and where δ11B may be interpreted to reflect a source in dehydrated (isotopically light) altered oceanic crust. Mass balance modeling indicates that isotopically heavy B from deep-slab serpentinite is present in the Aleutian source arc-wide but is typically masked by sediment-derived B at volcanic centers outside of the westernmost segment of the arc.

A new 1D velocity model and absolute locations image the Mayotte seismo-volcanic region

In May 2018, a seismically quiet region of the Indian Ocean awoke. More than 130 magnitude 4+ earthquakes were recorded in the first month, including a MW 5.9 event on May 15th, 2018. This seismic activity was later identified as being related to an exceptional underwater volcanic eruption offshore Mayotte island, which had emitted more than 6.5 km3 of lava by the time of writing. To better constrain the geodynamic processes responsible for the seismic and volcanic activity, a new network of ocean-bottom seismometers and land stations has been deployed around the seismically active region since February 2019. We present here an improved 1D velocity model for the active area and relocations of manually-picked earthquakes using this new model. The best-constrained events image detailed structures within two clusters of seismic activity east of Mayotte. The westernmost, proximal cluster, close to Mayotte's Petite-Terre island, has a “donut” shape horizontally and an “hourglass” shape in depth. The events distribution suggests the presence of a magma reservoir at around 27 km depth, with earthquakes focused along its sides, and a collapsing system underneath, related to the drainage of another, deeper magma storage zone. The distal cluster, focused 30–50 km offshore of Petite-Terre island, highlights the propagation of a dike between 45 and 25 km depth, aligned towards the new volcanic activity on the seafloor. We interpret this cluster as the fluid pathway towards the new volcano and nearby active seafloor lava fields. The improved velocity model also permits more robust daily monitoring of the seismicity using land stations, allowing local authorities to better assess seismic and volcanic hazards and to communicate them to the island's population.

Alkalinity of ocean island lavas decoupled from enriched source components: A case study from the EM1-PREMA Tasmantid mantle plume

The alkalinity of ocean island basalts (OIB), which form by upwelling thermo-chemical instabilities in the mantle, is often associated with the degree of melting. Yet it remains to be tested if alkalinity and the degree of melting are systematically associated with enriched mantle components. The Tasmantid Seamounts, which are fossil remnants of the Tasmantid mantle plume, comprise a north-south age-progressive submarine volcanic chain in the Coral and Tasman Seas East of Australia. Dredged seafloor lavas from nine seamounts along the seamount chain, ranging in age from 50 to 6.5 Ma, show a dichotomy in alkalinity, similar to those observed in other hotspot areas such as Hawai’i or Pitcairn. In radiogenic Sr-Nd-Hf-Pb isotope systematics, Tasmantid Seamount lavas form a continuum between EM1 and PREMA and are thus part of the prevalent EM1-PREMA association in the Pacific. However, their radiogenic Pb isotope systematics mark the Tasmantid plume as an individuum with no resemblance to any other plume. In stable Fe isotopes (expressed as δ57FePrim, which is the Fe isotopic composition calculated to primitive lavas along a liquid line of descent) alkali basalts are, on average, isotopically heavier than tholeiitic lavas, ranging in δ57FePrim from +0.10 to +0.21‰, unrelated to radiogenic isotope systematics, but with co-variations of δ57FePrim and Ti* (which is the primitive melt Ti content). These systematics point towards residual garnet as a key factor and thus indicate a solely petrologic relation between degrees of melting and Fe isotopes. Tholeiitic lavas exhibit a near-depleted mantle like isotopic composition (ranging in δ57FePrim from −0.01 to +0.22‰, median at δ57FePrim + 0.04‰), yet with values of up to δ57FePrim + 0.22‰ that require an isotopically heavy source of unknown origin.The lack of systematics between alkalinity and radiogenic isotope signatures indicates that temperature is the likely driving force for variable melting degrees in some samples over others. Based on these observations, we surmise that tholeiitic lavas form in the plume centre whereas alkaline lavas form at the cooler rim of the conduit. Mixing between melts and associated enriched components in the Tasmantid mantle plume may occur but, at least for alkaline lavas, only on a small scale.

Petrogenesis of Basaltic Lavas From the West Pacific Seamount Province: Geochemical and Sr‐Nd‐Pb‐Hf Isotopic Constraints

AbstractIn this study, we report new major and trace element, Sr‐Nd‐Pb‐Hf isotopic compositions and Ar‐Ar ages for basaltic lavas from seven poorly studied seamounts/guyots in the Western Pacific Seamount Province (WPSP). A review of new and published Ar‐Ar ages for seamounts/guyots confirm no linear age progression, as would be expected if these seamounts were produced by the Pacific Plate moving over a stationary mantle plume. These samples exhibit an alkalic character and can be produced by small‐degree partial melting of garnet pyroxenite with CO2. Their isotopic data identify four mantle end‐members (DMM, EMI, EMII, and HIMU) in the WPSP source region. These results confirm previous studies that invoke multiple contemporaneous plumelets in the region and a heterogeneous upper mantle reservoir which has been overprinted by multiple discrete hotspots, which create an anomalous and heterogeneous upper mantle because of mixing/overlapping EMI, EMII, and HIMU tracks, and age progressive volcanism is totally obscured. We propose a model where relict fragments of enriched mantle with distinct isotopic compositions residing in the upper mantle were a product of the formation of the adjacent large oceanic plateaus. These events introduce heterogeneity in the depleted mantle. The infiltration and metasomatism of these relict enriched components by carbonatitic fluids from the low‐velocity zone, create a source that then underwent small degrees of decompressional melting. The partial melts exploited pre‐existing lithospheric faults as an upwelling conduit and the resultant seafloor lava flows have obscured evidence to those pre‐existing lithospheric faults.

Characteristics and Deposit Stratigraphy of Submarine-Erupted Silicic Ash, Havre Volcano, Kermadec Arc, New Zealand

Submarine eruptions dominate volcanism on Earth, but few are observed or even identified. Knowledge of how they operate is largely based on inference from ancient deposits, lagging by a decade or more our understanding of subaerial eruptions. In 2012, the largest wholly deep-subaqueous silicic eruption with any observational record occurred 700–1220 m below sea level at Havre volcano, Kermadec Arc, New Zealand. Pre- and post-eruption shipboard bathymetry surveys, acquisition by autonomous underwater vehicle of meter-scale-resolution bathymetry, and sampling by remote-operated vehicle revealed 14 seafloor lavas and three major seafloor clastic deposits. Here we analyze one of these clastic deposits, an Ash with Lapilli (AL) unit, which drapes the Havre caldera, and interpret the fragmentation and dispersal processes that produced it. Seafloor images of the unit reveal multiple subunits, all ash-dominated. Sampling destroyed layering in all but two samples, but by combining seafloor imagery with granulometry and componentry, we were able to determine the subunits’ stratigraphy and spatial extents throughout the study area. Five subunits are distinguished; from the base these are Subunit 1, Subunit 2a, Subunit 3, Subunit 4 (comprising the coeval Subunit 4 west and Subunit 4 east), and Subunit 2b. The stratigraphic relationships of the four AL unit subunits to other seafloor products of the 2012 Havre eruption, coupled with the wealth of remote-operated vehicle observations and detailed AUV bathymetry, allow us to infer the overall order of events through the eruption. Ash formed by explosive fragmentation of a glassy vesicular magma and was dispersed by a buoyant thermal plume and dilute density currents from which Subunits 1 and 2 were deposited. Following a time break (days/weeks?), effusion of lava along the southern caldera rim led to additional ash generation; first by syn-extrusive ash venting, quenching, brecciation, and comminution (S3 and S4e) and then by gravitational collapse of a dome (S4w). Slow deposition of extremely fine ash sustained S2 deposition across the times of S3 and S4 emplacement, so that S2 ash was the last deposited. These thin ash deposits hold information critical for interpretation of the overall eruption, even though they are small in volume and bathymetrically unimpressive. Ash deposits formed during other submarine eruptions are similarly likely to offer new perspectives on associated lavas and coarse pumice beds, both modern and ancient, and on the eruptions that formed them. Submarine ash is widely dispersed prior to deposition, and tuff is likely to be the first product of eruption identified in reconnaissance exploration; it is the start of the trail to vent hydrothermal systems and associated mineralized deposits of submarine volcanoes, as well as a sensitive indicator of submarine eruptive processes.

Sr and O isotopes in western Aleutian seafloor lavas: Implications for the source of fluids and trace element character of arc volcanic rocks

High Mg# andesites and dacites (Mg# = molar Mg/Mg + Fe) from western Aleutian seafloor volcanoes carry high concentrations of Sr (>1000 ppm) that is unradiogenic (87Sr/86Sr < 0.7029) compared to lavas from emergent volcanoes throughout the arc (200–800 ppm Sr, 87Sr/86Sr >0.7030). Data patterns in plots of 87Sr/86Sr vs Y/Sr and Nd/Sr imply the existence of an eclogite-melt source component – formed by partial melting of MORB eclogite in the subducting Pacific Plate – which is most clearly expressed in the compositions of western Aleutian andesites and dacites (Nd/Sr and Y/Sr < 0.02) and which dominates the source budget for Sr in volcanic rocks throughout the arc. When viewed in combination with inversely correlated εNd and 87Sr/86Sr, these patterns rule out aqueous fluids as an important source of Sr because mixtures of fluids from altered oceanic crust with depleted mantle and sediment produce compositions with 87Sr/86Sr higher than in common Aleutian rocks. The unradiogenic nature of Sr in the western Aleutian andesite–dacite end-member may be understood if H2O required to drive melting of the subducting oceanic crust is transported in fluids containing little Sr. Mass balance demonstrates that such fluids may be produced by dewatering of serpentinite in the mantle section of the subducting plate. If the eclogite-melt source component is present throughout the Aleutian arc, melting of the subducting plate must extend into minimally altered parts of the sheeted dike section or upper gabbros, at depths >2 km below the paleo-seafloor. Oxygen isotopes in western Aleutian seafloor lavas, which fall within a narrow range of MORB-like values (δ18O=5.1–5.7), are also consistent with this model. These results indicate that the subducting Pacific lithosphere beneath the Aleutian arc is significantly hotter than indicated my most thermal models.

The Role of Subducted Basalt in the Source of Island Arc Magmas: Evidence from Seafloor Lavas of the Western Aleutians

Discovery of seafloor volcanism west of Buldir Volcano, the westernmost emergent volcano in the Aleutian arc, demonstrates that surface expression of active Aleutian volcanism falls below sea level just west of 175·9°E longitude, but is otherwise continuous from mainland Alaska to Kamchatka. Lavas dredged from newly discovered seafloor volcanoes up to 300 km west of Buldir have end-member geochemical characteristics that provide new insights into the role of subducted basalt as a source component in Aleutian magmas. Western Aleutian seafloor lavas define a highly calc-alkaline series with 50–70% SiO2. Most samples have Mg-numbers [Mg# = Mg/(Mg + Fe)] greater than 0·60, with higher MgO and lower FeO* compared with average Aleutian volcanic rocks at all silica contents. Common basalts and basaltic andesites in the series are primitive, with average Mg# values of 0·67 (±0·02, n = 99, 1SD), and have Sr concentrations (423 ± 29 ppm, n = 99) and La/Yb ratios (4·5 ± 0·4, n = 29) that are typical of island arc basaltic lavas. A smaller group of basaltic samples is more evolved and geochemically more enriched, with higher and more variable Sr and La/Yb (average Mg# = 0·61 ± 0·1, n = 31; Sr = 882 ± 333 ppm, n = 31; La/Yb = 9·1 ± 0·9, n = 16). None of the geochemically enriched basalts or basaltic andesites has low Y ( 1000 ppm) and are adakitic, with strongly fractionated trace element patterns (Sr/Y = 50–350, La/Yb = 8–35, Dy/Yb = 2·0–3·5) with low relative abundances of Nb and Ta (La/Ta > 100), consistent with an enhanced role for residual or cumulate garnet + rutile. All western seafloor lavas have uniformly radiogenic Hf and Nd isotopes, with eNd = 9·1 ± 0·3 (n = 31) and eHf = 14·5 ± 0·6 (n = 27). Lead isotopes are variable and decrease with increasing SiO2 from basalts with 206Pb/204Pb = 18·51 ± 0·05 (n = 11) to dacites and rhyodacites with 206Pb/204Pb = 18·43 ± 0·04 (n = 18). Western seafloor lavas form a steep trend in 207Pb/204Pb–206Pb/204Pb space, and are collinear with lavas from emergent Aleutian volcanoes, which mostly have 206Pb/204Pb > 18·6 and 207Pb/204Pb > 15·52. High MgO and Mg# relative to silica, flat to decreasing abundances of incompatible elements, and decreasing Pb isotope ratios with increasing SiO2 rule out an origin for the dacites and rhyodacites by fractional crystallization. The physical setting of some samples (erupted through Bering Sea oceanic lithosphere) rules out an origin for their garnet + rutile trace element signature by melting in the deep crust. Adakitic trace element patterns in the dacites and rhyodacites are therefore interpreted as the product of melting of mid-ocean ridge basalt (MORB) eclogite in the subducting oceanic crust. Western seafloor andesites, dacites and rhyodacites define a geochemical end-member that is isotopically like MORB, with strongly fractionated Ta/Hf, Ta/Nd, Ce/Pb, Yb/Nd and Sr/Y. This eclogite component appears to be present in lavas throughout the arc. Mass-balance modeling indicates that it may contribute 36–50% of the light rare earth elements and 18% of the Hf that is present in Aleutian volcanic rocks. Close juxtaposition of high-Mg# basalt, andesite and dacite implies widely variable temperatures in the western Aleutian mantle wedge. A conceptual model explaining this shows interaction of hydrous eclogite melts with mantle peridotite to produce buoyant diapirs of pyroxenite and pyroxenite melt. These diapirs reach the base of the crust and feed surface volcanism in the western Aleutians, but are diluted by extensive melting in a hotter mantle wedge in the eastern part of the arc.

A combined TEM and NanoSIMS study of endolithic microfossils in altered seafloor basalt

The incipient alteration of basaltic glass to palagonite in recent seafloor lavas from the arctic Mohns Ridge was studied by complimentary high-spatial-resolution geochemical techniques: TEM-EDS (transmission electron microscopy energy dispersive X-ray spectroscopy) and NanoSIMS (nano-scale secondary ion mass spectrometry). Rounded to elongated pores 0.5–2 μm across were found embedded in compact palagonite that have sizes and shapes comparable to microbial cells. In-situ elemental mapping revealed that the micropore rims are comparable in composition to the bulk palagonite and that some are enriched in manganese. Elevated concentrations of carbon and nitrogen were also found in some of the micropores. Hence these structures are interpreted as fossilised bacteriomorphs of endolithic microorganisms that inhabited fractures in the basaltic glass. The preferential accumulation of Mn in some of the cell encrustations suggests the mineralisation of Mn-oxidising bacteria. These data provide further evidence for the involvement of microorganisms in the colonisation and chemical alteration of recent seafloor volcanic glass and identify micro-scale Mn enrichments associated with micropores as a promising biosignature in such rocks.

Paving the seafloor: Volcanic emplacement processes during the 2005–2006 eruptions at the fast spreading East Pacific Rise, 9°50′N

The 2005–2006 eruptions near 9°50′N at the East Pacific Rise (EPR) marked the first observed repeat eruption at a mid‐ocean ridge and provided a unique opportunity to deduce the emplacement dynamics of submarine lava flows. Since these new flows were documented in April 2006, a total of 40 deep‐towed imaging surveys have been conducted with the Woods Hole Oceanographic Institution's (WHOI) TowCam system. More than 60,000 digital color images and high‐resolution bathymetric profiles of the 2005–2006 flows from the TowCam surveys were analyzed for lava flow morphology and for the presence of kipukas, collapse features, faults and fissures. We use these data to quantify the spatial distributions of lava flow surface morphologies and to investigate how they relate to the physical characteristics of the ridge crest, such as seafloor slope, and inferred dynamics of flow emplacement. We conclude that lava effusion rate was the dominant factor controlling the observed morphological variations in the 2005–2006 flows. We also show that effusion rates were higher than in previously studied eruptions at this site and varied systematically along the length of the eruptive fissure. This is the first well‐documented study in which variations in seafloor lava morphology can be directly related to a well documented ridge‐crest eruption where effusion rate varied significantly.

The diversity and abundance of bacteria inhabiting seafloor lavas positively correlate with rock alteration

Young, basaltic ocean crust exposed near mid-ocean ridge spreading centers present a spatially extensive environment that may be exploited by epi- and endolithic microbes in the deep sea. Geochemical energy released during basalt alteration reactions can theoretically support chemosynthesis, contributing to a trophic base for the ocean crust biome. To examine associations between endolithic microorganisms and basalt alteration processes, we compare the phylogenetic diversity, abundance and community structure of bacteria existing in several young, seafloor lavas from the East Pacific Rise at approximately 9 degrees N that are variably affected by oxidative seawater alteration. The results of 16S rRNA gene analyses and real-time, quantitative polymerase chain reaction measurements show that the abundance of prokaryotic communities, dominated by the bacterial domain, positively correlates with the extent of rock alteration--the oldest, most altered basalt harbours the greatest microbial biomass. The bacterial community overlap, structure and species richness relative to alteration state is less explicit, but broadly corresponds to sample characteristics (type of alteration products and general alteration state). Phylogenetic analyses suggest that the basalt biome may contribute to the geochemical cycling of Fe, S, Mn, C and N in the deep sea.

地球化学的手法による熱水活動変遷の解析

As our understanding of seafloor hydrothermal systems grows, we recognize they are not always stable and sometimes show dramatic changes. In this review, the authors present a compilation of geochemical and geochronological studies that are helpful when investigating the evolving processes of submarine hydrothermal systems. Chapter II describes the systematics and methodology of three dating techniques with discussions on their application to minerals formed by seafloor hydrothermal activities. The K-Ar (Ar-Ar) technique is popular for dating igneous rocks, but it is not appropriate for dating hydrothermal minerals because potassium is a trace component of sulfide/sulfate minerals. Following recent progress, micro-analytical techniques applying laser fusion are applicable for dating fluid inclusions and/or hydrothermal alteration minerals, which could provide important geochronological information. Uranium and thorium series disequilibrium dating have been employed for previous geochronological studies of hydrothermal minerals obtained from submarine ore deposits. To cover a wide time range, it is necessary to use various combinations of parent and daughter nuclides. Applying ESR dating to hydrothermal minerals is a rather new challenge. Although it needs several investigations to establish the methodology, it could be a useful rapid dating technique for a time range of less than one thousand years. Chapter III introduces studies focusing on the evolution of seafloor hydrothermal activities over a short time scale (one week to a few years). Detection of event plumes associated with seafloor lava eruption brought an awareness of episodic hydrothermal activity triggered by magmatic perturbation. Subsequent dive studies revealed evolving geochemical processes, such as major changes of volatiles and elemental species concentrations of venting fluid. With remote real-time monitoring of acoustic T-waves generated by seafloor seismic activities, event detection and response cruises have been conducted successfully to investigate various evolving processes in more detail. Chapter IV introduces studies focusing on the evolution of seafloor hydrothermal activities over a long time scale (tens of thousands of years). Radiometric dating studies of hydrothermal minerals such as sulfide and manganese oxide collected from the TAG mound, which is one of the largest hydrothermal mound structures, reveal an age distribution over at least 15000 years separated by quiescent intervals lasting up to 2000 years. On slow spreading ridges such as the Mid-Atlantic ridge, major fracture systems focus the hydrothermal discharge at one place for more than one thousand years with repeated reactivation. In Chapter V, the authors discuss the direction of future studies. Although hydrothermal systems on mid-oceanic ridges have been well studied, those related to arc-backarc magmatic activities could provide more appropriate fields for studying the evolutionary process of submarine hydrothermal systems. Combining geochronological studies with geochemical and mineralogical studies would be important for reconstructing the evolution process in more detail.

Seafloor alteration of basaltic glass: Textures, geochemistry, and endolithic microorganisms

The incipient low‐temperature alteration of glassy margins of recent seafloor lavas from the Mohns Ridge is characterized by scanning electron microscopy and bulk chemical analyses of major and trace elements. These investigations are designed to evaluate the endolithic microbial biomass, to identify relations between alteration textures and microorganisms, and to identify the chemical processes that take place during alteration. The basaltic glass along intersecting fractures and around vesicles is typically altered to concentrically zoned, yellow‐brown amorphous gel‐palagonite. In most fractures microorganisms are observed at the outer surfaces of palagonite rims, within porous zones of the rims, and frequently also at the glass‐palagonite interfaces. The cells act as nucleation sites for precipitation and become encrusted and embedded in palagonite with time. Zones of porous palagonite containing numerous hollow, fossilized cells alternate with zones of compact palagonite lacking distinct cell structures, which together indicate that the microbial growth is discontinuous. The palagonite has an average organic carbon content of 0.9 wt% (δ13Corg: −22‰), which derive from both living and fossilized biomass. The microbial growth and biomineralization are major controls on the porosity and texture of the palagonite and thus likely on the chemical exchange between glass and seawater. Pit marks in the glass in fractures both with and without microbes indicate that microbial as well as abiotic processes mediate pitting. Elemental and isotope data show that the transformation of glass to gel‐palagonite at seafloor conditions with high water/rock ratios results in near complete loss of Si and alkali elements to seawater; formation of Fe‐ and Al‐oxyhydroxides; complete exchange of alkaline earth elements like Sr with seawater; retention of V, Cu, Y, Pb, Th, U, and trivalent REEs, accompanied by a strong gain of these and other elements from seawater. Higher Fe/Ti ratios in the gel‐palagonite compared to the parental glass suggest that the palagonite represents a mixture of Fe‐Ti oxyhydroxides derived from both the glass and seawater.

Variable morphologic expression of volcanic, tectonic, and hydrothermal processes at six hydrothermal vent fields in the Lau back‐arc basin

Ultrahigh‐resolution bathymetric maps (25 cm grid) are used to quantify the physical dimensions of and spatial relationships between tectonic, volcanic, and hydrothermal features at six hydrothermal vent fields in the Lau back‐arc basin. Supplemented with near‐bottom photos, and nested within regional DSL‐120A side‐scan sonar data, these maps provide insight into the nature of hydrothermal systems along the Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR). Along‐axis transitions evident in localized volcanic morphology and tectonic characteristics include a change from broad low‐relief volcanic domes (hundreds of meters wide, &lt;10 m tall) that are dominated by pillow and lobate lava morphologies and are cut by faults and fissures to higher aspect ratio volcanic domes (tens of meters wide, tens of meters tall) dominated by aa‐type lava morphologies, with finger‐like flows, and few tectonic structures. These along‐axis differences in localized seafloor morphology suggest differences in hydrothermal circulation pathways within the shallow crust and correlate with regional transitions in a variety of ridge properties, including the large‐scale morphology of the ridge axis (shallow axial valley to axial high), seafloor lava compositions, and seismic properties of the upper crust. Differences in morphologic characteristics of individual flows and lava types were also quantified, providing an important first step toward the remote characterization of complex terrains associated with hydrothermal vent fields.

Petrology of Alarcon Rise lavas, Gulf of California: Nascent intracontinental ocean crust

The Alarcon Basin in the southern Gulf of California originated by intracontinental rifting of the southwestern margin of North America and has evolved into an intercontinental ocean basin by 3.7 m.y. of seafloor spreading. Lava samples collected on the axial ridge along Alarcon Rise, two of the many near‐axis seamounts on the northwest flank of the rise, and an abyssal hill on 0.6 Ma crust on the southeast flank were examined petrographically and analyzed for major and trace element and He‐Sr‐Nd‐Pb isotope composition. Most samples are typical mid‐ocean ridge basalt (MORB), but rocks at the axial ridge site closest to continent are more differentiated. Rocks from the axial ridge and abyssal hill are genetically related through separate episodes of fractional crystallization. Seamount samples are relatively more primitive than seafloor samples and may have been formed by smaller degree of melting of the same mantle source as the seafloor lavas. Tamayo transform fault has no apparent effect on the geochemistry of axial ridge lavas erupted close to it; this is contrary to the postulated transform fault effect. Alarcon Basin differs from the Salton Trough and Guaymas Basin in that its youngest seafloor shows no evidence for contamination with continental crust or a subduction component. We propose that Pacific MORB source mantle underlies the Gulf of California because the western North American margin has overridden the Pacific suboceanic mantle. Miocene rifting and Pliocene spreading in Alarcon Basin have created a window allowing the most recent MORB melts to rise uncontaminated to the surface.

Geochemistry of near-EPR seamounts: importance of source vs. process and the origin of enriched mantle component

Niu and Batiza [Earth Planet. Sci. Lett. 148 (1997) 471–483] show that lavas from the seamounts on the flanks of the East Pacific Rise (EPR) between 5° and 15°N vary from extremely depleted tholeiites to highly enriched alkali basalts. The extent of depletion and enrichment exceeds the known range of seafloor lavas in terms of the abundances and ratios of incompatible elements. New Sr–Nd–Pb isotope data for these lavas show variations (87Sr/86Sr=0.702362–0.702951; 206Pb/204Pb=18.080–19.325 and 143Nd/144Nd 0.512956–0.513183) larger than observed in lavas erupted on the nearby EPR axis. These isotopic ratios correlate with each other, with the abundances and ratios of incompatible elements, with the abundances of measured major elements such as MgO, CaO, Na2O and TiO2 contents, and with the abundances and ratios of major elements corrected for crystal fractionation to Mg#=0.72 (Ti72, Al72, Fe72, Ca72, Na72, and Ca72/Al72). These coupled correlations and the spatial distribution of seamounts require an EPR mantle source that has long-term (>1 Ga) lithological heterogeneities on very small scales [Niu and Batiza, Earth Planet. Sci. Lett. 148 (1997) 471–483]. Mid-ocean ridge basalt (MORB) major element systematics are, to a great extent, inherited from their fertile sources, which requires caution when using major element data to infer melting conditions. The significant correlations in elemental and isotopic variability (defined as RSD%=1σ/mean×100) between seamount and axial lavas suggest that both seamount and axial volcanisms share a common heterogeneous mantle source. We confirm previous interpretations [Niu and Batiza, Earth Planet. Sci. Lett. 148 (1997) 471–483; Niu et al., J. Geophys. Res. 104 (1999) 7067–7087] that the geochemical variability of lavas from the broad northern EPR region results from melting-induced mixing of a two-component mantle with the enriched (easily melted) component dispersed as physically distinct domains in a more depleted (refractory) matrix prior to the major melting events. The data also allow the conclusion that recycled oceanic crust cannot explain elevated abundances of elements such as Ba, Rb, Cs, Th, U, K, Pb, Sr etc. in enriched MORB and many ocean island basalts. These elements will be depleted in recycled oceanic crust that has passed through subduction zone dehydration reactions. We illustrate that deep portions of recycled oceanic lithosphere are important geochemical reservoirs hosting these and other incompatible elements as a result of metasomatism taking place at the interface between the low velocity zone and the cooling and thickening oceanic lithosphere.

Helium, heat, and the generation of hydrothermal event plumes at mid-ocean ridges

Hydrothermal event plumes are unique water-column features observed over mid-ocean ridges, presumably generated by the sudden release of large volumes of hot, buoyant fluid. Although the specifics of event plume generation are unknown, event plumes have been attributed to the rapid emptying of a hydrothermal reservoir or to rapid heat extraction from a recently emplaced dike or seafloor lava flows. The chemical and thermal signatures of event plumes as compared to the underlying steady-state plumes offer important clues to the generation of event plumes. Event plumes have low 3He/heat ratios of ∼0.4 × 10−17 mol J−1, similar to vent fluids from mature hydrothermal systems. In contrast, the steady-state plumes found beneath the event plumes have elevated and variable 3He/heat ratios of 2 to 5 × 10−17 mol J−1. Fluids collected directly over fresh lava flows have even higher 3He/heat ratios of 2 to 8 × 10−17 mol J−1, up to 30 times the event plume values. These disparate 3He/heat ratios place strong constraints on models of event plume generation, especially models which rely on heat extraction from seafloor eruptions. Published by Elsevier Science B.V.

Tracing the Indian Ocean Mantle Domain Through Time: Isotopic Results from Old West Indian, East Tethyan, and South Pacific Seafloor

The isotopic difference between modern Indian Ocean and Pacific or North Atlantic Ocean ridge mantle (e.g. variably lower 206Pb/204Pb for a given eNd and 208Pb/204Pb) could reflect processes that occurred within a few tens of millions of years preceding the initial breakup of Gondwana. Alternatively, the Indian Ocean isotopic signature could be a much more ancient upper-mantle feature inherited from the asthenosphere of the eastern Tethyan Ocean, which formerly occupied much of the present Indian Ocean region. Age-corrected Nd, Pb, and Sr isotopic data for 46–150 Ma seafloor lavas from sites in the western Indian Ocean and ocean-ridge-type Tethyan ophiolites (Masirah, Yarlung–Zangpo) reveal the presence of both Indian-Ocean-type compositions and essentially Pacific–North Atlantic-type signatures. In comparison, Jurassic South Pacific ridge basalts from Alexander Island, Antarctica, possess normal Pacific–North Atlantic-type isotopic ratios. Despite the very sparse sampling of old seafloor, the age-corrected eNd(t) values of the old Indian Ocean basalts cover a greater range than seen for the much more thoroughly sampled present-day spreading axes and islands within the Indian Ocean (e.g. 18 eNd units for basalts in the 60–80 Ma range vs 15 eNd units for 0–10 Ma ones). The implications of these results are that the upper mantle in the Indian Ocean region is becoming increasingly well mixed through time, and that the Indian Ocean mantle domain may not greatly pre-date the age of earliest spreading in the Indian Ocean.

Constraining crustal emplacement processes from the variation in seismic layer 2A thickness at the East Pacific Rise

A stochastic model for the emplacement of dikes and lava flows at a fast spreading ridge can generate an upper oceanic crustal structure similar to that observed in seismic data from the East Pacific Rise (EPR), in ocean drill holes, and in ophiolites. In this model the location of successive dike intrusion events relative to the ridge axis is determined by a Gaussian probability function and the cumulative flow lengths of the erupted lavas are chosen to build a Gaussian-shaped lava pile. We interpret wide-angle seismic reflections from the steep velocity gradient at the base of seismic layer 2A to be the extrusive/sheeted dike contact. Seismic data from the northern and southern EPR place constraints on the on-axis extrusive layer thickness (230 ± 50 m), the distance over which the thickening of the extrusive layer occurs (width of the accretion zone = 1–3 km) and its off-axis thickness (300–800 m). Ophiolites and ocean drill holes (DSDP Hole 504B) provide additional estimates of the thickness of the extrusive layer and constrain the thickness of the transition region from extrusives to sheeted dikes (∼ 100–200 m). A simple stochastic emplacement model, where the lavas are described by one mean flow length, fits the thickening of the extrusive layer off-axis inferred from the deepening of seismic layer 2A, but the predicted transition from sheeted dikes to extrusives is too thick. In order to match the dimensions and flat-topped shape of the seismic layer 2A boundary as well as the thickness of the extrusive-sheeted dike transition, a bimodal distribution of lava flows is used. Short flows, confined within the axial summit caldera (ASC), build up approximately half the extrusive volume. Occasional voluminous flows spill out of the ASC, or erupt outside of the ASC, and pond at a considerable distance off-axis to build up the remainder of the extrusive section. The upper part of the final extrusive section will be dominated by the off-axis flows, while the lower portions will be primarily composed of short flows erupted within the ASC. Magnetic transition widths predicted from the overlap of lavas (∼ 2 km) in this model are similar to those measured in deep-tow studies. Assuming a smoothing function which acts over one seismic wavelength, the upper crustal velocity structure predicted by the bimodal lava emplacement model is consistent with the shallow seismic velocity structure measured on the EPR. The ages of seafloor lavas in this model are younger than the tectonic spreading model ages by ∼ 30–70 kyr, in agreement with anomalously young lava ages obtained from radioisotope dating of seafloor basalts near the EPR.

Role of the Kerguelen Plume in generating the eastern Indian Ocean seafloor

Mid‐ocean ridge basalts (MORB) in the Indian Ocean have Sr‐Nd‐Pb isotopic characteristics that distinguish them from seafloor basalts in the Atlantic and Pacific Oceans. These differences have important implications for mantle dynamics. We discuss the isotopic variation with eruption age of seafloor basalts recovered by deep sea drilling at 10 sites in the eastern Indian Ocean ranging in age from Eocene to Late Jurassic. Except for alkalic basalts recovered from near Christmas Island in the northeast Indian Ocean, the basement lavas are tholeiitic basalts that are characterized by a wide range in incompatible element abundance ratios, such as La/Yb and Zr/Nb. Most of the tholeiitic basalts from seven sites are geochemically similar to recent Indian Ocean MORB, but the alkalic basalts and tholeiitic lavas from two other sites have isotopic and incompatible element abundance ratios similar to lavas associated with the Kerguelen Plume. Two of these three sites, however, are not close to the track of this plume. The Dupal isotopic signature (relatively high 87Sr/86Sr and high 208Pb/204Pb at a given 206Pb/204Pb) is characteristic of lavas that have been attributed to the Kerguelen Plume, i.e., the Kerguelen Archipelago, Ninetyeast Ridge, and Kerguelen Plateau. Among eastern Indian Ocean seafloor basalts, a Dupal component is apparent in basement lavas from six of the seven drill sites in the eastern Indian Ocean that range in inferred age from ∼57 to 125 Ma. The oldest (∼155 Ma) seafloor lavas recovered from the Indian Ocean, derived from a spreading center in the Argo Abyssal Plain near northwest Australia, have high 143Nd/144Nd and low 87/86Sr similar to the most depleted recent Indian MORB. Because the oldest volcanism on the Kerguelen Plateau (∼118 Ma) is the first evidence of the activity of the Kerguelen Plume, this plume is inferred to be the source of Dupal isotopic characteristics in Indian Ocean MORBs. Some recent Indian Ocean MORB are also distinctive because many have relatively low 206Pb/204Pb (&lt;17.4). Some of the oldest (110 to 155 Ma) seafloor lavas in the eastern Indian Ocean also have relatively low 206Pb/204Pb ratios. This low 206Pb/204Pb signature predates volcanism associated with the Kerguelen Plume and may reflect a significant role for continental lithosphere as a long‐term source component for Indian Ocean MORB.

Submarine tumuli and inflated tube-fed lava flows on Axial Volcano, Juan de Fuca Ridge

A seafloor lava field was mapped within the summit caldera of Axial Volcano, Juan de Fuca Ridge, using SeaMARC I sidescan sonor and submersible observations. By analogy with similar subaerial features, we infer that several volcanic seafloor features here formed by the process of lava flow inflation. Flow inflation occurs within tube-fed lava flows when lava continues to be supplied to the interior of a flow that has ceased advancing, thus uplifting the flow's rigid surface and creating a suite of characteristic surface structures. Inflated lavas require a feeder lava tube or tube system connected to a remote lava source, and therefore we infer that inflated submarine lava flows contain lava tubes. Inflated flow features identified from sidescan sonar images elsewhere on Axial Volcano and within the axial valley of the southern Juan de Fuca ridge suggest that flow inflation is a widespread submarine volcanic process.