Showing papers on "Mid-ocean ridge published in 2002"

Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle

Abstract: The analysis of volatiles in magmatic systems can be used to constrain the volatile content of the Earth’s mantle and the influence that magmatic degassing has on the chemistry of the oceans and the atmosphere. But most volatile elements have very low solubilities in magmas at atmospheric pressure, and therefore virtually all erupted lavas are degassed and do not retain their primary volatile signatures. Here we report the undersaturated pre-eruptive volatile content for a suite of mid-ocean-ridge basalts from the Siqueiros intra-transform spreading centre. The undersaturation leads to correlations between volatiles and refractory trace elements that provide new constraints on volatile abundances and their behaviour in the upper mantle. Our data generate improved limits on the abundances of carbon dioxide, water, fluorine, sulphur and chlorine in the source of normal mid-oceanridge basalt. The incompatible behaviour of carbon dioxide, together with the CO2/Nb and CO2/Cl ratios, permit estimates of primitive carbon dioxide and chlorine to be made for degassed and chlorine-contaminated mid-ocean-ridge basalt magmas, and hence constrain degassing and contamination histories of mid-ocean ridges.

Noble Gas Isotope Geochemistry of Mid-Ocean Ridge and Ocean Island Basalts: Characterization of Mantle Source Reservoirs

Abstract: The study of noble gases in oceanic basalts is central to understanding chemical heterogeneity of the Earth’s mantle and origin of the atmosphere. In the terrestrial environment the abundances of noble gases are quite low because they were excluded from solid materials during planetary formation in the inner solar system. This low background inventory helps to make the noble gases excellent tracers of mantle reservoirs. In this context, mid-ocean ridge and ocean island basalts provide valuable windows into the Earth’s mantle. These oceanic basalts are not prone to the degree of contamination often observed in continental lavas that results from their passage through thick continental lithosphere and crust. Mid-ocean ridge basalts (MORBs) form by partial melting as the ascending mantle beneath spreading ridges reaches its solidus temperature, and MORBs are generally accepted to represent a broad sampling of the convecting upper mantle. Ocean island basalts (OIBs) represent melting ‘anomalies’ that are generally related to mantle upwelling. The extent to which ocean islands are derived from a thermal boundary layer in the deep mantle (e.g., as a mantle plume) or from chemical heterogeneities embedded within the mantle convective flow (e.g., as a mantle ‘blob’) has been debated for decades, and is not currently resolved. The isotope compositions of noble gases in oceanic basalts bear significantly on such debates over the chemical structure of the mantle. When oceanic basalts erupt as submarine lavas, their quenched rims of glass may contain high volatile abundances (especially when they are deeply erupted under elevated hydrostatic pressure), providing the best available opportunity for precisely characterizing the noble gas composition of the Earth’s mantle. In favorable cases, inclusions of melt or fluids trapped within magmatic phenocrysts and mantle xenoliths can also be precisely analyzed for noble gas composition. Measurable changes in the isotope compositions of noble gases …

Recycled dehydrated lithosphere observed in plume-influenced mid-ocean-ridge basalt

Abstract: A substantial uncertainty in the Earth's global geochemical water cycle is the amount of water that enters the deep mantle through the subduction and recycling of hydrated oceanic lithosphere. Here we address the question of recycling of water into the deep mantle by characterizing the volatile contents of different mantle components as sampled by ocean island basalts and mid-ocean-ridge basalts. Although all mantle plume (ocean island) basalts seem to contain more water than mid-ocean-ridge basalts, we demonstrate that basalts associated with mantle plume components containing subducted lithosphere—‘enriched-mantle’ or ‘EM-type’ basalts—contain less water than those associated with a common mantle source. We interpret this depletion as indicating that water is extracted from the lithosphere during the subduction process, with greater than 92 per cent efficiency.

Volcanoes, Fluids, and Life at Mid-Ocean Ridge Spreading Centers

Abstract: ▪ Abstract The recent recognition of a potentially vast, unexplored hot microbial biosphere associated with active volcanism along the global mid-ocean ridge network has fundamentally shifted concepts of how planets and life coevolve. Many processes intrinsic to the dynamics of the spreading center volcanic system provide partial or complete nutritional fluxes that support diverse microbial communities that thrive under extreme conditions on and beneath the seafloor. Mantle melting, volcanism, and fluid-rock reactions transport volatiles from the asthenosphere to the hydrosphere. Volcanic heat and exothermic reactions drive circulation of nutrient-rich fluids from which chemosynthetic organisms gain metabolic energy. In turn, many of these organisms symbiotically support macrofaunal communities that populate the vents. Long-term seafloor observatories will allow exploration of linkages between volcanism and this newly discovered biosphere. Such approaches may provide essential new information about our ow...

Volcanic eruptions on mid‐ocean ridges: New evidence from the superfast spreading East Pacific Rise, 17°–19°S

Abstract: uniform sediment cover were recovered from lava that buries older faulted terrain. The boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector, consistent with magmas from two separate reservoirs being erupted in the same event. Chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments. A comparison of our results to global data on submarine mid-ocean ridge eruptions suggests consistent dependencies of erupted volume, activated fissure lengths, and chemical heterogeneity with spreading rate, consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 8414 Volcanology: Eruption mechanisms; 8439 Volcanology: Physics and chemistry of magma bodies; 3655 Mineralogy and Petrology: Major element composition; KEYWORDS: lava flow, chemical heterogeneity, erupted volume, lava morphology, side-scan sonar

Direct geological evidence for oceanic detachment faulting: The Mid-Atlantic Ridge, 15°45′N

Abstract: From a detailed survey and sampling study of corrugated massifs north of the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge, we demonstrate that their surfaces are low-angle detachment fault planes, as proposed but not previously verified. Spreading-direction–parallel striations on the massifs occur at wavelengths from kilometers to centimeters. Oriented drill-core samples from the striated surfaces are dominated by fault rocks with low-angle shear planes and highly deformed greenschist facies assemblages that include talc, chlorite, tremolite, and serpentine. Deformation was very localized and occurred in the brittle regime; no evidence is seen for ductile deformation of the footwall. Synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low-angle fault surface at very shallow levels directly beneath the ridge axis. Strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient.

Characteristics of volcanic rifted margins

Abstract: Volcanic rifted margins evolve by a combination of extrusive flood volcanism, intrusive magmatism, extension, uplift, and erosion. The temporal and spatial relationships between these processes are influenced by the plate tectonic regime; the preexisting lithosphere (thickness, composition, geothermal gradient); the upper mantle (temperature and character); the magma production rate; and the prevailing climatic system. Of the Atlantic rifted margins, 75% are believed to be volcanic, the cumulative expression of thermotectonic processes over 200 m.y. Volcanic rifted margins also characterize Ethiopia-Yemen, India-Australia, and Africa-Madagascar. The transition from continental flood volcanism (or formation of a large igneous province) to ocean ridge processes (mid-ocean ridge basalt) is marked by a prerift to synrift transition with formation of a subaerial and/or submarine seaward-dipping reflector series and a significant thickness (to 15 km) of juvenile, high-velocity lower crust seaboard of the continental rifted margin. Herein we outline the similarities and differences between volcanic rifted margins worldwide and list some of their diagnostic features.

Petrogenesis of the Back-arc East Scotia Ridge, South Atlantic Ocean

Abstract: The East Scotia Ridge is an active back-arc spreading centre located to the west of the South Sandwich island arc in the South Atlantic Ocean, consisting of nine main segments, E1 (north) to E9 (south). Major and trace element and Sr–Nd–Pb isotope compositions are presented, together with water contents, for lavas sampled along the active ridge axis. Magmatism along the East Scotia Ridge is chemically heterogeneous, but there is a common mid-ocean ridge basalt (MORB)-type source component for all the magmas. An almost unmodified MORB-source mantle appears to underlie the central part of the back-arc. Subduction components are found at the northern and southern ends of the ridge, and there is a marked sediment melt input of up to 2% in segment E4. Enriched (plume) mantle is present beneath segment E2 at the northern end of the ridge, suggesting that plume mantle is flowing westward around the edges of the subducting slab. The southern part of segment E8 is unique in that its magma source is similar to sub-arc depleted mantle.

Mineralogy of the mid-ocean-ridge basalt source from neodymium isotopic composition of abyssal peridotites

Abstract: Inferring the melting process at mid-ocean ridges, and the physical conditions under which melting takes place, usually relies on the assumption of compositional similarity between all mid-ocean-ridge basalt sources. Models of mantle melting therefore tend to be restricted to those that consider the presence of only one lithology in the mantle, peridotite. Evidence from xenoliths and peridotite massifs show that after peridotite, pyroxenite and eclogite are the most abundant rock types in the mantle. But at mid-ocean ridges, where most of the melting takes place, and in ophiolites, pyroxenite is rarely found. Here we present neodymium isotopic compositions of abyssal peridotites to investigate whether peridotite can indeed be the sole source for mid-ocean-ridge basalts. By comparing the isotopic compositions of basalts and peridotites at two segments of the southwest Indian ridge, we show that a component other than peridotite is required to explain the low end of the (143)Nd/(144)Nd variations of the basalts. This component is likely to have a lower melting temperature than peridotite, such as pyroxenite or eclogite, which could explain why it is not observed at mid-ocean ridges.

Correlated geophysical, geochemical, and volcanological manifestations of plume-ridge interaction along the Galápagos Spreading Center

Abstract: [1] As the Galapagos hot spot is approached from the west along the Galapagos Spreading Center there are systematic increases in crustal thickness and in the K/Ti, Nb/Zr, 3He/4He, H2O, and Na2O content of lavas recovered from the spreading axis. These increases correlate with progressive transitions from rift valley to axial high morphology along with decreases in average swell depth, residual mantle Bouguer gravity anomaly, magma chamber depth, average lava Mg #, Ca/Al ratio, and the frequency of point-fed versus fissure-fed volcanism. Magma chamber depth and axial morphology display a “threshold” effect in which small changes in magma supply result in large changes in these variables. These correlated variations in geophysical, geochemical, and volcanological manifestations of plume-ridge interaction along the western Galapagos Spreading Center reflect the combined effects of changes in mantle temperature and source composition on melt generation processes, and the consequences of these variations on magma supply, axial thermal structure, basalt chemistry, and styles of volcanism.

Geologic implications of seawater circulation through peridotite exposed at slow-spreading mid-ocean ridges

Abstract: Peridotite denuded by tectonic extension and exposed at the seafloor adjacent to slow-spreading centers hosts hydrothermal circulation of seawater. The reaction of seawater with peridotite causes serpentinization, which generates a high-pH, strongly reducing fluid rich in methane and hydrogen, and is accompanied by as much as 40% volume expansion. Complete serpentinization of peridotite requires tectonic activity to open fluid paths sealed by volume expansion. Diffuse venting of serpentinization fluids causes lithification of calcareous ooze on the seafloor to chalk-like limestone. This may be the main mechanism of deposition of ophicarbonates common in ophiolites. The degree of induration is a function of the fluid flux through the sediment column. Calcareous ooze infiltrates faults and fractures and can be deformed following lithification. Focused venting of serpentinization fluids may lead to deposition of large chimneys composed of calcite, aragonite, and brucite, such as those in the recently documented Lost City vent field (30°N, Mid-Atlantic Ridge). Geophysical implications of serpentinization include (1) creation of magnetic anomalies due to growth of magnetite in serpentinite and (2) lowered seismic velocity. Integrated studies of geologic and geophysical effects of serpentinization may aid in a more complete understanding of the structure of oceanic lithosphere and the mechanisms that expose mantle peridotite at the seafloor.

Dunite distribution in the Oman Ophiolite: Implications for melt flux through porous dunite conduits

Abstract: [1] Dunites in the mantle section of the Oman ophiolite represent conduits for chemically isolated melt transport through the shallow mantle beneath oceanic spreading centers. These dunite melt conduits exhibit a scale-invariant power law relationship between width and cumulative abundance, as measured over 4 orders of magnitude. We use this size/frequency distribution to assess several hypotheses for dunite formation and estimate the total melt flux that a dunite network can accommodate beneath an oceanic spreading center. Dunites, measured from one-dimensional lithologic sections and digital image mosaics at a variety of length scales, range in width from ∼3 mm to ∼100 m and follow a power law with a slope of ∼1.1. Extrapolation of the power law predicts that dunites as wide as 3.5 km may exist in the melting region beneath a mid-ocean ridge. Alternatively, perhaps the widest dunites we observe (∼100 m) represent a maximum size. Modeling of dunites as diffusive reaction zones around melt-filled hydrofractures cannot explain the existence of dunites wider than ∼10 m in Oman. Instead, dunites may represent high porosity conduits formed by reactive porous flow. Using the observed size/frequency relationship, the assumption that dunites form a coalescing network and the requirement that flux is conserved where dunites merge, we estimate the total flux through a porous dunite network and the fraction of that flux that remains chemically isolated. Our flux model predicts that the porosity in a dunite scales with the width. For maximum porosities of ∼1–4% in the widest dunites, a network of porous dunite conduits with the abundances observed in Oman can supply a sufficient flux of melt (of which > 95% remains chemically unequilibrated with shallow residual peridotites) to satisfy the observed mid-ocean ridge flux.

Discovery of ancient and active hydrothermal systems along the ultra‐slow spreading Southwest Indian Ridge 10°–16°E

Abstract: [1] We report the discovery of active and fossil hydrothermal systems during R/V Knorr Cruise 162, Legs VII and IX along a 400 km long segment of the ultra-slow Southwest Indian Ridge (SWIR) between 10 and 16°E, where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date (8.4 mm yr−1 full rate). Eight of forty-one optical/temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others. Fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias, four deposits ofsepiolite and silica, as well as Mn-oxide and nontronite cemented breccias. The massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep-seated normal fault bounding the southern rift valley wall. Silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low- to moderate-temperature ultramafic-hosted hydrothermal activity. The abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates, and hence the magmatic heat input, along this section of the SWIR are lower than on any other explored segment of the global mid-ocean ridge system. This observation suggests that high mantle upwelling and magma supply rates are not required to drive mid-ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra-slow end of the ridge spreading spectrum. The frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation, with hydrothermal vent sites being preferentially associated with long-lived faults that provide fluid pathways. Our results suggest the contribution of magma-starved ridge segments to the global ocean-lithosphere geochemical budget is potentially much larger than previously thought.

Seismic anisotropy in the upper mantle 2. Predictions for current plate boundary flow models

Abstract: [1] The anisotropic seismic structure due to flow-induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary. The orientation distributions of olivine:enstatite aggregates evolve along streamlines of each flow model, based on each grains plastic response to the local stress/strain field. The effective elastic tensor for these textured aggregates provides predictions of P wave anisotropy and shear wave splitting throughout the model space. P and S travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model. Comparison of predicted fast P direction for our method versus a finite-strain based estimate shows that agreement is acceptable for much of the model space, but notable differences occur in regions up to several tens of kilometers in size. Two-dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle. The effect of flow in the third dimension is addressed in a few calculations. For one comparison of flow in the mantle wedge at a subduction zone, the introduction of trench parallel flow causes significant changes in the predicted patterns of P wave anisotropy (magnitude, more than orientation) and SKS splitting.

Morphology of Europan bands at high resolution: A mid‐ocean ridge‐type rift mechanism

Abstract: [1] We utilize imaging data from the Galileo spacecraft to investigate band formation on one of Jupiter's moons, Europa. Bands are polygonal features first observed in Voyager data close to Europa's anti-Jovian point and represent areas where preexisting terrain has been pulled apart, allowing new material to move up into the gap. We examine the detailed morphology of several bands imaged at different resolutions and lighting geometries. We identify several distinct morphological characteristics, including central troughs, hummocky textures, and ridge and trough terrains, some of which are common among the bands studied. In many cases, bands have initiated along segments of one or more preexisting double ridges, ubiquitous within Europa's ridged plains. Distinctive morphological features and high standing topography imply that the bands formed from compositionally or thermally buoyant ice, rather than liquid water. Comparisons between Europan band morphologies and features found on terrestrial mid-ocean ridges reveal several similarities, including axial troughs, subcircular hummocks, normal faults, and indications of symmetrical spreading. We conclude that terrestrial mid-ocean ridge rifting is a good analogy for Europan band formation. If a terrestrial seafloor-spreading model is applicable to Europan bands, we speculate that band morphologies might be related to the relative rate of spreading of each band. Bands may have contributed significantly to the resurfacing of Europa. Europan bands we examine predate (but do not postdate) lenticulae and related features, implying that the style of resurfacing on Europa has changed over recent geological time in these regions.

Kistufell: Primitive Melt from the Iceland Mantle Plume

Abstract: This paper presents new geochemical data from Kistufell (64°48′N, INTRODUCTION 17°13′W), a monogenetic table mountain situated directly above Iceland is the surface manifestation of a plume of actively the inferred locus of the Iceland mantle plume. Kistufell is composed upwelling mantle superimposed on a mid-ocean ridge. of the most primitive olivine tholeiitic glasses found in central The Icelandic crust is distinctive for its anomalous thickIceland (MgO 10·56 wt %, olivine Fo89·7). The glasses are ness (Darbyshire et al., 2000) and its significant petrointerpreted as near-primary, high-degree plume melts derived from logical and geochemical variations relative to the adjacent a heterogeneous mantle source. Mineral, glass and bulk-rock (glass sectors of the mid-ocean ridge (e.g. Taylor et al., 1997). + minerals) chemistry indicates a low average melting pressure These features are generally ascribed to active upwelling (15 kbar), high initial crystallization pressures and temperatures and melting of anomalously hot mantle material that is (10–15 kbar and 1270°C), and eruption temperatures (1240°C) compositionally different from that of the surrounding that are among the highest observed in Iceland. The glasses have upper mantle (e.g. Ito et al., 1999; Chauvel & Hemond, trace element signatures (Lan/Ybn <1, Ban/Zrn 0·55–0·58) 2000). The nature of the individual source components indicative of a trace element depleted source, and the Sr–Nd–Pb within the plume remains, however, enigmatic. isotopic ratios ( Sr/Sr 0·70304–0·70308, Nd/Nd The most compelling evidence for involvement of a 0·513058–0·513099, Pb/Pb 18·343–18·361) further lower-mantle plume component are the high He/He suggest a long-term trace element depletion relative to primordial ratios (up to 37 R/Ra, Hilton et al., 1999) (R/Ra is the mantle. High He isotopic ratios (15·3–16·8 R/Ra) combined ratio of He/He in the sample relative to that in the with low Pb/Pb (15·42–15·43) suggest that the mantle atmosphere). This is well outside the variation in the source of the magma is different from that of North Atlantic midglobal ridge system (9·13 ± 3·57 R/Ra, Anderson, 2000) ocean ridge basalt. Negative Pb anomalies, and positive Nb and and a feasible explanation is derivation from relatively Ta anomalies indicate that the source includes a recycled, subducted undegassed mantle material hitherto isolated at depth oceanic crustal or mantle component. Positive Sr anomalies (Srn/ (Kurz et al., 1985; Hilton et al., 1999; Breddam et al., Ndn = 1·39–1·50) further suggest that this recycled source 2000). Incompatible trace element rich, high Sr/Sr component involves lower oceanic crustal gabbros. The O values and Pb/Pb, and low Nd/Nd ‘enriched’ Icelandic (4·2–4·7‰), which are lower than those observed in mantle lavas have also been attributed to an inherent plume peridotites but similar to those observed in ophiolites and in situ component, which could either be (high He/He) lower oceanic gabbros, are consistent with this interpretation. The elevated mantle (e.g. Hanan et al., 2000; Kempton et al., 2000) or He/He ratios are primarily attributed to a primitive, relatively recycled oceanic basalts (Chauvel & Hemond, 2000). undegassed component in the Iceland mantle plume, which dominates However, the relation between the plume and the mathe He isotope signature as a result of long-term depletion of U, terial giving rise to incompatible trace element poor, low Th and He in the recycled gabbroic component. Sr/Sr and Pb/Pb, and high Nd/Nd ‘depleted’

Hydroacoustic monitoring of seismicity at the slow‐spreading Mid‐Atlantic Ridge

Abstract: [1] In February 1999, long-term hydroacoustic monitoring of the northern Mid-Atlantic Ridge (MAR) was initiated. Six autonomous hydrophones were moored between ∼15°N and ∼35°N on the flanks of the MAR. Results from the first year of data reveal that there is significant variability in along-axis event rate. Groups of neighboring segments behave similarly, producing an along-axis pattern with high and low levels of seismic activity at a wavelength of ∼500 km. This broad scale pattern is likely influenced by the axial thermal regime. Several earthquake sequences with variable temporal characteristics were detected, suggesting fundamental differences in the cause of their seismicity. Off-axis, most seismic faulting occurs within a zone < 15 km from the axis center.

Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber

Abstract: A well-preserved contact aureole at the base of the sheeted dyke is likely to be centred at, or very close to, the sheeted complex in the Troodos ophiolite, Cyprus, records the partial melting dyke–gabbro boundary. This region of the crust is a of the roof of a mid-ocean ridge magma chamber. Hydrothermally dynamic environment that is subjected to rapid changes altered dykes within the lowermost 10–30 m of the sheeted dyke in rheology and thermal structure. It is here that melt is complex were recrystallized to pyroxene and hornblende hornfels episodically extracted to build the upper crust (e.g. Deand, closest to the underlying gabbros ( 30%) at water-undersaturated conditions and temperatures if not all, of the lower crust (e.g. Phipps Morgan & Chen, [875°C. Geological considerations indicate that the duration of 1993; Quick & Denlinger, 1993; Boudier et al., 1996), melting events was probably on the time scale of eruptive cycles (i.e. and magmatic heat is rapidly transferred across a thin years to centuries). This resulted in disequilibrium partial melting (<100 m) conductive boundary layer (e.g. Lowell & and highly imperfect melt segregation within the contact aureole. Burnell, 1991) and advectively transported out of the We hypothesize that partial melting is a common process within crust by hydrothermal fluids (e.g. Cann et al., 1985). the roof zones of axial magma chambers at fast-spreading ridges Geophysical surveys along the East Pacific Rise (EPR) and that a component of geophysically imaged axial magma chambers provide indirect evidence that the depth and internal may be partially melted upper crust rather than partially crystallized properties (e.g. melt volumes, proportion of melt and magma. Field relations suggest that where the highest degrees of crystals) of AMCs are not static and change on the time melting occurred, a mixture of anhydrous hornfels and hydrous scale of eruptive cycles (years to centuries) (Wilcock & leucocratic melt disaggregrated, and was assimilated into the magma Delaney, 1996; Singh et al., 1998). chamber. It is well known from ophiolite studies that the roofs of AMCs can be marked by polyphase magmatism and intense interactions between hydrothermal fluids and rocks, particularly within the uppermost few hundred

A late neoproterozoic eastern Laurentian superplume: Location, size, chemical composition, and environmental impact

Abstract: A database consisting of 25 data-sets has been compiled that includes published chemical analyses of most known eastern North American occurrences of late Neoproterozoic (Vendian) flood basalt and dike swarms together with some new geochemical data. Four additional eastern North American basalt occurrences of probable early Paleozoic age were also examined. The Vendian occurrences comprise the Central Iapetus magmatic event of Eastern North America which extends from the Long Range dikes of Labrador and Newfoundland to the Catoctin flood basalts of Virginia and includes the Grenville dike swarm of Ontario and Quebec. The strike of the Vendian dike swarms converge near a major gravity and magnetic anomaly at Sutton Mountain, Quebec. The chemical composition of the Vendian basalts and diabase dikes plotted onto spider diagrams normalized to the composition of the silicate earth can be subdivided into a tight cluster of parallel lines displaying HFSE enrichment and a more diverse group that is less enriched. The tight grouping of the enriched cluster suggests a mutual genetic relationship consistent with mantle plume derivation based largely on chemical similarity to standard Ocean Island Basalt (OIB). However, detailed examination of this eastern Laurentian OIB related cluster (LOIB) reveals regional variations in chemical composition, particularly TiO2 and Zr, that can be contoured to delineate a narrow lens shape with peak concentrations centered close to the Sutton Mountain triple junction. The central portion of the LOIB is geochemically the same as superplume derived basalt. Radiometric evidence indicates that the LOIB superplume magmatic activity peaked at about 550 Ma. The less enriched group geochemically resembles some of Earth’s larger continental flood basalts, such as the Columbia River basalt province, derived from a subcontinental lithospheric mantle source mixed with magma from a mantle plume source. Radiometric and paleomagnetic evidence indicates that this less enriched group is about 615 to 564 Ma and probably extruded out of rifts that preceded the break-up of Pannotia. This mid-Vendian magmatism is consistent with the early stages of deep mantle plume upwelling but may be genetically independent of the superplume magmatism that followed. The four early Paleozoic basalt suites examined are interpreted as post-LOIB, Iapetan Ocean ridge basalt accreted onto eastern Laurentian or perhaps basalt extruded from transform faults that intersected the trailing edge of Laurentia. If LOIB magmatism was generated by a superplume head, there may be important environmental implications. The LOIB event may have been partially responsible for considerable mantle outgassing, particularly carbon dioxide and nutrients, global warming, and major increases in the growth rate and deposition of marine life that occurred during the early Cambrian.

Probing the mantle: The story from carbonatites

Abstract: One of the most exciting advances in Earth science in the last several decades has been our increased understanding of the structure and composition of the mantle. Seismic tomography and isotope geochemistry have been major players in those advances. The isotopic studies used basalts from ocean basins to minimize the problems of possible crustal contamination. Data from mid-ocean ridge basalts (MORBs) and oceanic island basalts (OIBs) revealed a relatively detailed picture of the isotope geochemistry of the sub-oceanic mantle [e.g. Hofmann, 1997] that led to the recognition of four principal magma components that define end-member compositions. These are DMM, HIMU, EMI, and EM2 (see Table 1 for further details). All of these components except DMM have been attributed to subduction of different materials, such as oceanic and continental crust and lithosphere, down into the mantle. Several studies have indicated that the mantle is isotopically heterogeneous with heterogeneities that were probably established at least several billions of years ago.

Asthenospheric flow and asymmetry of the East Pacific Rise, MELT area

Abstract: [1] Although the Pacific and Nazca plates share the East Pacific Rise (EPR) as a boundary, they exhibit many differing characteristics. The Pacific plate subsides more slowly and has more seamounts than the Nazca plate. Both the seismic and magnetotelluric components of the Mantle ELectromagnetic and Tomography Experiment (MELT) found pronounced asymmetry in mantle structure across the spreading axis near 17°S. The Pacific (west) side has lower S-wave velocities, exhibits greater shear wave splitting, and is more electrically conductive than the Nazca (east) side. These results suggest asymmetric mantle flow and melt distribution beneath the EPR. To better understand the causes for these asymmetric properties, we construct numerical models of melting and mantle flow beneath a midocean ridge migrating to the west over a fixed mantle. Although the ridge is migrating to the west, the migration has little effect on the upwelling rates, requiring a separate mechanism to create the asymmetry. Models that produce asymmetric melting with a temperature anomaly require large (>100°C) excess temperatures and may not be consistent with the observed subsidence and crustal thickness. A possible mechanism for creating asymmetry without a temperature anomaly is across-axis asthenospheric flow, possibly driven by pressures created by upwelling beneath the Pacific Superswell to the west. Pressure-driven asthenospheric flow follows the base of the lithosphere, extending the upwelling region to the west as it follows the thinning lithosphere toward the axis, and shutting off melting as it crosses the axis and encounters an increasingly thick lithosphere to the east.

Flow and mixing in the rift valley of the Mid-Atlantic Ridge

Abstract: High levels of diapycnal mixing and geothermal heating near midocean ridges contribute to the buoyancy fluxes that are required to close the global circulation. In topographically confined areas, such as the deep median valleys of slow-spreading ridges, these fluxes strongly influence the local hydrography and dynamics. Data from a segment-scale hydrographic survey of the rift valley of the Mid-Atlantic Ridge and from an array of current meters deployed there during an entire year are analyzed in order to characterize the dominant hydrographic patterns and dynamical processes. Comparison with historic hydrographic data indicates that the temporal variability during the last few decades has been small compared to the observed segment-scale gradients. The rift valley circulation is characterized by inflow from the eastern ridge flank and persistent unidirectional alongsegment flow into a cul-de-sac. Therefore, most of the water flowing along the rift valley upwells within the segment with a mean vertical velocity .1025 ms 21. The observed streamwise hydrographic gradients indicate that diapycnal mixing dominates the rift valley buoyancy fluxes by more than an order of magnitude, in spite of the presence of a large hydrothermal vent field supplying several gigawatts of heat to the water column. Hydrographic budgets in the rift valley yield diffusivity values of order 5 3 1023 m2 s 21, consistent with estimates derived from statically unstable overturns, the largest of which were observed downstream of topographic obstacles in the path of the along-segment flow. This suggests vertical shear associated with cross-sill flows as the dominant contributor to the mechanical mixing in the rift valley.