Langmuir Charles H.

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Langmuir
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Charles H.
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Now showing 1 - 7 of 7
  • Article
    Links from mantle to microbe at the Lau Integrated Study Site : insights from a back-arc spreading center
    (The Oceanography Society, 2012-03) Tivey, Margaret K. ; Becker, Erin ; Beinart, Roxanne A. ; Fisher, Charles R. ; Girguis, Peter R. ; Langmuir, Charles H. ; Michael, Peter J. ; Reysenbach, Anna-Louise
    The Lau Integrated Study Site (ISS) has provided unique opportunities for study of ridge processes because of its back-arc setting in the southwestern Pacific. Its location allows study of a biogeographical province distinct from those of eastern Pacific and mid-Atlantic ridges, and crustal compositions along the ridge lie outside the range of mid-ocean ridge crustal compositions. The Lau ISS is located above a subduction zone, at an oblique angle. The underlying mantle receives water and other elements derived from the downgoing lithospheric slab, with an increase in slab influence from north to south. Water lowers the mantle melting temperature and leads to greater melt production where the water flux is greater, and to distinctive regional-scale gradients along the ridge. There are deeper faulted axial valleys with basaltic volcanism in the north and inflated axial highs with andesites in the south. Differences in igneous rock composition and release of magmatic volatiles affect compositions of vent fluids and deposits. Differences in vent fluid compositions and small-scale diffuse-flow regimes correlate with regional-scale patterns in microbial and megafaunal distributions. The interdisciplinary research effort at the Lau ISS has successfully identified linkages between subsurface processes and deep-sea biological communities, from mantle to microbe to megafauna.
  • Article
    Hydrothermal venting in magma deserts : the ultraslow-spreading Gakkel and Southwest Indian Ridges
    (American Geophysical Union, 2004-08-18) Baker, Edward T. ; Edmonds, Henrietta N. ; Michael, Peter J. ; Bach, Wolfgang ; Dick, Henry J. B. ; Snow, Jonathan E. ; Walker, Sharon L. ; Banerjee, Neil R. ; Langmuir, Charles H.
    Detailed hydrothermal surveys over ridges with spreading rates of 50–150 mm/yr have found a linear relation between spreading rate and the spatial frequency of hydrothermal venting, but the validity of this relation at slow and ultraslow ridges is unproved. Here we compare hydrothermal plume surveys along three sections of the Gakkel Ridge (Arctic Ocean) and the Southwest Indian Ridge (SWIR) to determine if hydrothermal activity is similarly distributed among these ultraslow ridge sections and if these distributions follow the hypothesized linear trend derived from surveys along fast ridges. Along the Gakkel Ridge, most apparent vent sites occur on volcanic highs, and the extraordinarily weak vertical density gradient of the deep Arctic permits plumes to rise above the axial bathymetry. Individual plumes can thus be extensively dispersed along axis, to distances >200 km, and ∼75% of the total axial length surveyed is overlain by plumes. Detailed mapping of these plumes points to only 9–10 active sites in 850 km, however, yielding a site frequency F s , sites/100 km of ridge length, of 1.1–1.2. Plumes detected along the SWIR are considerably less extensive for two reasons: an apparent paucity of active vent fields on volcanic highs and a normal deep-ocean density gradient that prevents extended plume rise. Along a western SWIR section (10°–23°E) we identify 3–8 sites, so F s = 0.3–0.8; along a previously surveyed 440 km section of the eastern SWIR (58°–66°E), 6 sites yield F s = 1.3. Plotting spreading rate (us) versus F s, the ultraslow ridges and eight other ridge sections, spanning the global range of spreading rate, establish a robust linear trend (F s = 0.98 + 0.015us), implying that the long-term heat supply is the first-order control on the global distribution of hydrothermal activity. Normalizing F s to the delivery rate of basaltic magma suggests that ultraslow ridges are several times more efficient than faster-spreading ridges in supporting active vent fields. This increased efficiency could derive from some combination of three-dimensional magma focusing at volcanic centers, deep mining of heat from gabbroic intrusions and direct cooling of the upper mantle, and nonmagmatic heat supplied by exothermic serpentinization.
  • Preprint
    Central role of detachment faults in accretion of slow-spreading oceanic lithosphere
    ( 2008-08) Escartin, Javier E. ; Smith, Deborah K. ; Cann, Johnson R. ; Schouten, Hans A. ; Langmuir, Charles H. ; Escrig, Stéphane
    The formation of oceanic detachment faults is well established from inactive, corrugated fault planes exposed on seafloor formed along ridges spreading at less than 80 km/My1-4. These faults can accommodate extension for up to 1-3 Myrs5, and are associated with one of two contrasting modes of accretion operating along the northern Mid-Atlantic Ridge (MAR). The first is symmetrical accretion, dominated by magmatic processes with subsidiary high-angle faulting and formation of abyssal hills on both flanks. The second is asymmetrical accretion involving an active detachment fault6 along one ridge flank. An examination of ~2500 km of the MAR between 12.5 and 35°N reveals asymmetrical accretion along almost half of the ridge. Hydrothermal activity identified to date in the study region is closely associated with asymmetrical accretion, which also exhibits high-levels of near continuous hydroacoustically and teleseismically recorded seismicity. Enhanced seismicity is probably generated along detachment faults accommodating a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which concentrates primarily at their ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. Both seismic and geochemical evidence indicate that the axial lithosphere is thicker and colder at asymmetrical sections of the ridge, either because associated hydrothermal circulation efficiently penetrates to greater depths, or because the rising mantle is cooler. We suggest that much of the variability in seafloor morphology, seismicity and basalt chemistry found along slow-spreading ridges can be thus attributed to the frequent involvement of detachments in oceanic lithospheric accretion.
  • Article
    Hydrothermal exploration of the Fonualei Rift and Spreading Center and the Northeast Lau Spreading Center
    (American Geophysical Union, 2006-11-29) German, Christopher R. ; Resing, Joseph A. ; Prien, R. D. ; Walker, Sharon L. ; Edmonds, Henrietta N. ; Langmuir, Charles H.
    We report evidence for active hydrothermal venting along two back-arc spreading centers of the NE Lau Basin: the Fonualei Rift and Spreading Center (FRSC) and the Northeast Lau Spreading Center (NELSC). The ridge segments investigated here are of particular interest as the potential source of a mid-water hydrothermal plume (1500–2000 m depth) which extends more than 2000 km across the SW Pacific Ocean dispersing away from an apparent origin close to the most northeastern limits of the Lau Basin. Our results indicate the presence of at least four new hydrothermal plume sources, three along the FRSC and one on the NELSC, the latter situated within 150 km of the maximum for the previously identified SW Pacific regional-scale plume. However, TDFe and TDMn concentrations in the southernmost FRSC plume that we have identified only reach values of 19 and 13 nmol/L and dissolved 3He anomalies in the same plume are also small, both in relation to the SW Pacific plume and to local background, which shows evidence for extensive 3He enrichment throughout the entire Lau Basin water column. Our results reveal no evidence for a single major point hydrothermal source anywhere in the NE Lau Basin. Instead, we conclude that the regional-scale SW Pacific hydrothermal plume most probably results from the cumulative hydrothermal output of the entire topographically restricted Lau Basin, discharging via its NE-most corner.
  • Article
    Distribution of recycled crust within the upper mantle : insights from the oxygen isotope composition of MORB from the Australian-Antarctic Discordance
    (American Geophysical Union, 2009-12-03) Cooper, Kari M. ; Eiler, John M. ; Sims, Kenneth W. W. ; Langmuir, Charles H.
    Geochemical heterogeneity within the mantle has long been recognized through the diversity of trace element and radiogenic isotopic compositions of mantle-derived rocks, yet the specific origin, abundance, and distribution of enriched material within the mantle have been difficult to quantify. In particular, the origin of the distinctive geochemical characteristics of Indian mantle has been debated for decades. We present new laser fluorination oxygen isotope measurements of mid-ocean ridge basalt from the Australian-Antarctic Discordance (AAD), an area where a particularly abrupt transition occurs between Pacific-type mid-ocean ridge basalts (MORB) and Atlantic-type MORB. These data show no distinction in average δ18O between Pacific- and Atlantic-type MORB, indicating that the origin of Indian-type mantle cannot be attributed to the presence of pelagic sediment. The combined radiogenic isotope, δ18O, and trace element characteristics of Indian-type MORB at the AAD are consistent with contamination of the Indian upper mantle by lower crustal material. We also present a compilation of available laser fluorination δ18O data for MORB and use these data to evaluate the nature and percentage of enriched material within the upper mantle globally. Data for each ocean basin fit a normal distribution, with indistinguishable means and standard deviations, implying that the variation in δ18O of MORB reflects a stochastic process that operates similarly across all ocean basins. Monte Carlo simulations show that the mean and standard deviation of the MORB data are robust indicators of the mean and standard deviation of the parent distribution of data. Further, although some skewness in the data cannot be ruled out, Monte Carlo results are most consistent with a normal parent distribution. This similarity in characteristics of the δ18O data between ocean basins, together with correlations of δ18O with radiogenic isotope and trace element characteristics of subsets of the data, suggest that the upper mantle globally contains an average of ∼5–10% recycled crustal material and that the depleted mantle in the absence of this component would have δ18O of ∼5.25‰. The Monte Carlo simulations also suggest that additional oxygen isotope data may be used in the future to test the ability of geodynamical models to predict the physical distribution of enriched domains within the upper mantle.
  • Article
    Domains of depleted mantle : new evidence from hafnium and neodymium isotopes
    (American Geophysical Union, 2011-08-02) Salters, Vincent J. M. ; Mallick, Soumen ; Hart, Stanley R. ; Langmuir, Charles H. ; Stracke, Andreas
    Isotope systematics of basalts provide information on the distribution of mantle components and the length scale of mantle heterogeneity. To obtain this information, high data and sampling density are crucial. We present hafnium and neodymium isotope data on more than 400 oceanic volcanics. Over length scales of several hundred to over one thousand kilometers hafnium and neodymium isotopes of mid-ocean ridge basalts are correlated and form an array of parallel trends on a global scale. On a larger scale these domains differ in the amount of highly depleted mantle material with radiogenic hafnium and neodymium isotope ratios. Compared to the Atlantic and Indian Ocean basins the asthenosphere of the Pacific basin seems to have a more uniform and a less radiogenic Hf isotopic composition for a given Nd isotopic composition. The parallel arrays of mid-ocean ridge basalts provide strong constraints on the makeup of the MORB mantle and are evidence for the presence of a highly depleted and highly radiogenic neodymium and hafnium component. This component, because of its highly depleted character, is unrecognized in the strontium-neodymium-lead isotope systems alone. Alternatively, the parallel arrays can have an ancient origin of systematic variations in the degree of depletion. Each array then represents the variations in this fossil melting regime. Individual ocean island basalt suites display different slopes in hafnium-neodymium isotope space, which are also best explained by varying amounts of highly residual mantle rather than isotopic differences in enriched mantle components as previously invoked. The ocean island basalt arrays diverge at the depleted end and project to radiogenic compositions that are similar to those of the asthenosphere through which they travel. This is strong evidence that the plume material interacts with its surrounding mantle as it ascends. The isotopic compositions of the ocean island and ridge basalts suggest that their systematics are influenced by a heretofore unrecognized depleted component.
  • Article
    Volcanic glasses at the Izu arc volcanic front : new perspectives on fluid and sediment melt recycling in subduction zones
    (American Geophysical Union, 2004-01-22) Straub, S. M. ; Layne, Graham D. ; Schmidt, A. ; Langmuir, Charles H.
    Volcanic glasses contained in distal fallout tephras from the Izu arc volcanic front (Izu VF) provide unique perspectives on general problems of arc volcanism. Unlike cogenetic lavas, these glasses are liquid compositions where element concentrations as well as ratios have significance. Isotopic evidence and previous work show that there is no sediment melt contribution to the sources of the Izu VF tephras, and hence their trace element characteristics permit determination of the trace element contents of slab fluids. The slab fluid is a composite of metasediment (∼5% of total fluid) and metabasalt (∼95%) component fluids, and carries large ion lithophile elements (LILE) with high LILE/Th and LILE/U, and low Th and U relative to source. Except for Sr and K, the metabasalt fluid is much less enriched than the metasediment fluid, but its large relative proportions make it an important carrier of many trace elements. The metabasalt fluid has the characteristics of the arc trace element signature, obviating the need for ubiquitous involvement of sediment in arc magma genesis. The fluid component in the tephras is remarkably constant in composition over fifteen million years, and hence appears to be a robust composition that may be applicable to other convergent margins. Assuming that the metabasalt fluid is a common component, and that distribution coefficients between sediment and fluid are similar from one arc to another, composite fluid compositions can be estimated for other arcs. Differences from this composition then would likely result from a sediment melt component. Comparison to arcs with sediment melt components in their source (Marianas, eastern Aleutians) shows that partial sediment melts may be so enriched, that they can completely mask the signature of the comingling slab fluids. Hence sediment melts can easily dominate the trace element and isotopic signature of many convergent margins. Since sediment melts inherit the LILE/LILE ratios of the trench sediment, Earth's surface processes must eventually influence the compositional diversity of arcs.