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  • Institute of Physics  (164,711)
  • American Physical Society  (115,485)
  • American Geophysical Union  (36,523)
  • Wiley-Blackwell  (36,104)
  • 2005-2009  (251,786)
  • 1975-1979  (101,037)
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  • 11
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    A general inversion for end-member ratios in binary mixing systems (2005)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 6 (2005): Q11007, doi:10.1029/2005GC000975.
    Description: Binary mixing is one of the most common models used to explain variations in geochemical data. When the data being modeled are ratios of elements or isotopes, the mixtures follow hyperbolic trends with curvatures that depend on a cross-term representing the relative concentrations of the elements or isotopes under consideration in the mixing components. The inverse problem of estimating mixing components is difficult because of the cross-term in the hyperbolic equation, which requires the use of nonlinear methods to estimate the mixing parameters, and because the end-member ratio values are intrinsically underdetermined unless the mixing proportions of the samples are known a priori, which is not generally the case. I use maximum likelihood methods to address these issues and derive a general inversion for binary mixing model parameters from ratio-ratio data. I apply the method to synthetic test data and a global compilation of 230Th/232Th versus 87Sr/86Sr data from oceanic basalts and find that the concentration ratio parameter is well-constrained by the inversion while the end-member ratio estimates are strongly dependent on the initial guesses used to start the iterative solver, reflecting the underdetermined nature of the end-member positions on the mixing hyperbola. Monte Carlo methods that randomly perturb the initial guesses can be used to improve error estimates, and goodness-of-fit statistics can be used to assess the performance of the mixing model for explaining data variance.
    Keywords: Inverse theory ; Isotopes ; Mixing
    Repository Name: Woods Hole Open Access Server
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  • 12
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    Evolution of volcanism and faulting in a segment of the Mid-Atlantic Ridge at 25°N (2006)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 6 (2005): Q09008, doi:10.1029/2005GC000954.
    Description: We reconstruct the volcanic and tectonic evolution over the last 250,000 years of the median valley floor in the spreading segment of the Mid-Atlantic Ridge centered at 25°N. In the center of the segment, multibeam bathymetry and deep-towed side-scan images show a large area of smooth-textured lava flows more like those of the East Pacific Rise than those of the Mid-Atlantic Ridge. Hummocky flows more typical of the Mid-Atlantic Ridge are found toward the southern end of the segment. The presence of the abundant smooth-textured flows allows us to interpret the volcanic and tectonic relationships in the segment. We construct a geological map using (1) multibeam bathymetry to identify the key volcanic structures and fault scarps and (2) high-resolution TOBI side-scan sonar images to interpret age relationships between features on the basis of overall sediment cover as shown by backscatter brightness. Bottom photographs across key features on the median valley floor yield detailed information on stratigraphic relationships between volcanic features and faults and allow us to calibrate backscatter brightness in terms of sediment cover and hence of age. In this way we derive a history of volcanic activity and deformation in a detailed survey area at the segment center, with the most recent flows erupted about 5000 years ago, and the youngest smooth flows about 10,000 years ago, separated by an episode of faulting. Using bathymetry and side-scan surveys, we extrapolate this to the whole of the median valley floor. The volcanic activity giving rise to the smooth flows has been continuous for about a quarter of a million years at the segment center. Over the same period, hummocky flows have been continuously erupted at the southern end of the segment. Electron probe analyses of dredged basalt glasses show that there is a systematic variation in composition with position in the segment. Basalts from the segment center are all more evolved than those at the southern end of the segment. There is, however, no relation of chemistry with lava type. The basalts from the segment center have very nearly the same composition whether they come from hummocky flows or smooth flows. The boundary between the smooth flows and hummocky flows has fluctuated with time and migrated rapidly northward over the last few thousand years, so that shortly the eruption of smooth flows will probably have ceased. The survey shows that flows that are smooth on side-scan images are not necessarily sheet flows. In this study they uniformly show pillow morphology. We conclude that smooth flows were probably erupted at faster eruption rates than hummocky flows.
    Description: This project was funded by an NERC grant that enabled Charles Darwin cruise 65 and by NSF grant OCE-9811575.
    Keywords: Mid-Atlantic Ridge ; Volcanism ; Faulting ; Tectonic history ; Smooth-textured flows ; Hummocky flows
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  • 13
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    Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project (2006)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 5 (2004): Q04G14, doi:10.1029/2002GC000439.
    Description: This paper presents new magmatic helium isotopic compositions in a suite of lavas from phase II of the Hawaiian Scientific Drilling Project (HSDP2) core, which sampled Mauna Kea volcano to a maximum depth of 3098 m below sea level. Most of the measurements were performed by in vacuo crushing of olivine phenocrysts, but include submarine pillow glasses from the 2200 to 2500 meter depth interval, and orthopyroxene phenocrysts from an intrusive at 1880 m. The magmatic 3He/4He ratios range from 6 to 24.7 times atmospheric (Ra), which significantly extends the range of values for Mauna Kea volcano. The 3He/4He ratios are lowest (i.e., close to MORB values of ∼8 Ra) near the top of the Mauna Kea section and rise slowly, to 10–12 Ra, at 1000 m below sea level, consistent with results from the HSDP1 core. At depths greater than 1000 m in the core, primarily in the submarine lavas, there are brief periods when the 3He/4He ratios are higher than 14.5 Ra, always returning to a baseline value. Twelve such excursions were identified in the core; all but one are in the submarine section, and most (7) are in the deepest section, at depths of 1950 to 3070 m. The baseline 3He/4He value rises from 10–12 Ra near 1000 m depth to 12–14 Ra at 3000 m. The helium spikes are found only in lavas that are older than 380 Ka in age, based on an age model derived from Ar-Ar data (W. D. Sharp et al., manuscript in preparation, 2003). Excluding the excursions defined by single lava flows (3) and intrusive units (3), the average spike duration is approximately 15 (±9) Ka (n = 6). The high 3He/4He spikes are interpreted as pulses of magma from the center of the actively upwelling Hawaiian hot spot. The short duration of the high 3He/4He excursions suggests that Mauna Kea was never directly over high the 3He/4He component of the plume (during the HSDP2 eruptive period), presumed to be the plume center. Assuming that the Mauna Kea helium spikes result from melting of heterogeneities within the plume, their short duration implies that the length scales of heterogeneities in the solid upwelling mantle are between 60 m and 12 km (for upwelling rates of 2 to 40 cm/yr). The high 3He/4He are associated with high 208Pb/204Pb, and relatively low 143Nd/144Nd, Zr/Nb, and SiO2. The correlations with major elements, trace elements and isotopes demonstrate that helium is coupled to the other geochemical variations, and that the Mauna Kea isotopic variability is caused by heterogeneities within the upwelling plume.
    Description: This work was supported by EAR/NSF through the Continental Dynamics and Instrumentation and Facilities programs.
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  • 14
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    Seafloor seismic monitoring of an active submarine volcano : local seismicity at Vailulu'u Seamount, Samoa (2006)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 5 (2004): Q06007, doi:10.1029/2004GC000702.
    Description: We deployed five ocean bottom hydrophones (OBHs) for a 1-year seismic monitoring study of Vailulu'u Seamount, the youngest and easternmost volcano in the Samoan Archipelago. Four instruments were placed on the summit crater rim at 600–700 m water depth, and one was placed inside the crater at 1000 m water depth. An analysis of the first 45 days of records shows a very large number of seismic events, 211 of them local. These events define a steady background activity of about four seismic events per day, increasing to about 10 events per day during a week of heightened seismic activity, which peaked at 40 events during 1 day. We identified 107 earthquakes, whose arrivals could be picked on all five stations and that are likely located within the seamount, based on their similar waveforms. Two linear trends are defined by 21 of these events. These are extremely well correlated and located, first downward then upward on a steeply inclined plane that is close to the axial plane of the southeast rift as it emerges from the main summit of Vailulu'u. These events resemble volcanotectonic earthquakes from subaerial volcanoes in displaying very coherent seismic waveforms and by showing systematic, narrowly defined progressions in hypocenter locations. We propose that these events reflect brittle rock failure due to magma redistribution in or near a central magma reservoir.
    Description: The bulk of this work was funded by NSF-OCE, in grants to HS and SRH and the OBSIP facility at Scripps.
    Keywords: Samoa ; Vailulu'u ; Volcano ; Seismic monitoring ; Volcanic activity ; Submarine
    Repository Name: Woods Hole Open Access Server
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  • 15
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    Volcanic glasses at the Izu arc volcanic front : new perspectives on fluid and sediment melt recycling in subduction zones (2006)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 5 (2004): Q01007, doi:10.1029/2002GC000408.
    Description: 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.
    Description: This study was funded by the “Deutsche Forschungsgemeinschaft” (grants Str 441/3 and 441/4). The Northeast National Ion Microprobe Facility at WHOI was supported by grants EAR-9628749 and EAR-990440 from the National Science Foundation.
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  • 16
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    Application of secondary ion mass spectrometry to the determination of Mg/Ca in rare, delicate, or altered planktonic foraminifera : examples from the Holocene, Paleogene, and Cretaceous (2006)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 6 (2005): Q12P07, doi:10.1029/2005GC000974.
    Description: Secondary ion mass spectrometry (SIMS) is useful for measuring Mg/Ca in both primary calcite and diagenetic minerals in planktonic foraminifera. The excellent spatial resolution (〈10 μm) and small amount of material removed (〈2 ng) makes it easy to avoid targets that include obvious embedding material and encrusting or infilling minerals such as secondary calcite and authigenic clays in diagenetically altered samples. Because analyses can be performed on individuals, SIMS is also a viable technique for generating Mg/Ca values from sediment samples in which foraminifera are rare or have low mass. For clean primary calcite samples, Mg/Ca ratios from SIMS compare well to those obtained using inductively coupled plasma mass spectrometry (ICP-MS), while maintaining information regarding the true variability of elemental ratios within individual tests. For samples with secondary calcite or stubbornly adhering clays, SIMS enables us to accurately measure primary calcite compositions and to assess and reconcile contamination problems in bulk samples analyzed by solution-based ICP-MS. We have observed that SIMS is an invaluable and reliable tool for the identification and avoidance of problems of diagenesis and the analysis of rare or delicate planktonic foraminifera. However, because of operator time required to properly target delicate (thin-walled) or contaminated planktonic foraminifera, SIMS may not be feasible for Mg/Ca studies where large numbers (hundreds) of samples must be processed and bulk measurements on multiple individuals will suffice.
    Description: Funding for this research was provided by The Andrew W. Mellon Foundation Endowed Fund for Innovative Research and by the U.S. Science Support Program of the Joint Oceanographic Institutions. This material is also based on work supported by the National Science Foundation under grant OCE-0334598. Partial support for the Northeast National Ion Microprobe Facility was provided by NSF (EAR-0115433). This research used samples and data provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation and participating countries under management of Joint Oceanographic Institutions, Inc.
    Keywords: Cenomanian ; Eocene ; Holocene ; Mg/Ca ; Paleoceanography ; Paleocene
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  • 17
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    Some thoughts on the use of InSAR data to constrain models of surface deformation : noise structure and data downsampling (2006)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 6 (2005): Q01007, doi:10.1029/2004GC000841.
    Description: Repeat-pass Interferometric Synthetic Aperture Radar (InSAR) provides spatially dense maps of surface deformation with potentially tens of millions of data points. Here we estimate the actual covariance structure of noise in InSAR data. We compare the results for several independent interferograms with a large ensemble of GPS observations of tropospheric delay and discuss how the common approaches used during processing of InSAR data affects the inferred covariance structure. Motivated by computational concerns associated with numerical modeling of deformation sources, we then combine the data-covariance information with the inherent resolution of an assumed source model to develop an efficient algorithm for spatially variable data resampling (or averaging). We illustrate these technical developments with two earthquake scenarios at different ends of the earthquake magnitude spectrum. For the larger events, our goal is to invert for the coseismic fault slip distribution. For smaller events, we infer the hypocenter location and moment. We compare the results of inversions using several different resampling algorithms, and we assess the importance of using the full noise covariance matrix.
    Description: R. Lohman is partially supported by a NASA New Investigator Program grant award to M. Simons.
    Keywords: Correlated noise ; InSAR ; Little Skull Mountain
    Repository Name: Woods Hole Open Access Server
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  • 18
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    Hydrothermal venting in magma deserts : the ultraslow-spreading Gakkel and Southwest Indian Ridges (2006)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 5 (2004): Q08002, doi:10.1029/2004GC000712.
    Description: 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.
    Description: This research was partially supported the NOAA VENTS Program. P.J.M. and H.J.B.D. gratefully acknowledge NSF grant OPP 9911795 for support of the AMORE Expedition; P.J.M. and E.T.B. acknowledge NSF grant OPP 0107767 and the VENTS Program for development and construction of MAPRs for use in ice-covered seas. H.J.B.D. acknowledges NSF grant OCE-9907630 for support of SWIR studies. J.E.S. was supported by Deutsche Forschungsgemeinschaft grant SN15/2.
    Keywords: Gakkel Ridge ; Hydrothermal venting ; Magmatic budget ; Southwest Indian Ridge ; Ultraslow ridges
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  • 19
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    Temperature variations at diffuse and focused flow hydrothermal vent sites along the northern East Pacific Rise (2006)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 7 (2006): Q03002, doi:10.1029/2005GC001094.
    Description: In the decade following documented volcanic activity on the East Pacific Rise near 9°50′N, we monitored hydrothermal vent fluid temperature variations in conjunction with approximately yearly vent fluid sampling to better understand the processes and physical conditions that govern the evolution of seafloor hydrothermal systems. The temperature of both diffuse flow (low-temperature) and focused flow (high-temperature) vent fluids decreased significantly within several years of eruptions in 1991 and 1992. After mid-1994, focused flow vents generally exhibited periods of relatively stable, slowly varying temperatures, with occasional high- and low-temperature excursions lasting days to weeks. One such positive temperature excursion was associated with a crustal cracking event. Another with both positive and negative excursions demonstrated a subsurface connection between adjacent focused flow and diffuse flow vents. Diffuse flow vents exhibit much greater temperature variability than adjacent higher-temperature vents. On timescales of a week or less, temperatures at a given position within a diffuse flow field often varied by 5°–10°C, synchronous with near-bottom currents dominated by tidal and inertial forcing. On timescales of a week and longer, diffuse flow temperatures varied slowly and incoherently among different vent fields. At diffuse flow vent sites, the conceptual model of a thermal boundary layer immediately above the seafloor explains many of the temporal and spatial temperature variations observed within a single vent field. The thermal boundary layer is a lens of warm water injected from beneath the seafloor that is mixed and distended by lateral near-bottom currents. The volume of the boundary layer is delineated by the position of mature communities of sessile (e.g., tubeworms) and relatively slow-moving organisms (e.g., mussels). Vertical flow rates of hydrothermal fluids exiting the seafloor at diffuse vents are less than lateral flow rates of near-bottom currents (5–10 cm/s). The presence of a subsurface, shallow reservoir of warm hydrothermal fluids can explain differing temperature behaviors of adjacent diffuse flow and focused flow vents at 9°50′N. Different average temperatures and daily temperature ranges are explained by variable amounts of mixing of hydrothermal fluids with ambient seawater through subsurface conduits that have varying lateral permeability.
    Description: Field and shore-based analyses have been supported by the National Science Foundation (OCE-0096468, OCE-8917311, OCE-9217026, OCE-9302205, OCE-0327261), the Woods Hole Oceanographic Institution's Vetlesen Fund and W. A. Clark Senior Scientist Chair (DJF), and the Devonshire Foundation (TMS).
    Keywords: Hydrothermal systems ; East Pacific Rise ; Vent fluids ; Seafloor eruptions
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  • 20
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    Seasonality and stable isotopes in planktonic foraminifera off Cape Cod, Massachusetts (2005)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 20 (2005): PA4011, doi:10.1029/2005PA001150.
    Description: Monthly samples of stratified plankton tows taken from the slope waters off Cape Cod nearly 25 years ago are used to describe the seasonal succession of planktonic foraminifera and their oxygen isotope ratios. The 15°C seasonal cycle of sea surface temperature (SST) accounts for a diverse mixture of tropical to subpolar species. Summer samples include various Globigerinoides and Neogloboquadrina dutertrei, whereas winter and early spring species include Globigerina bulloides and Neogloboquadrina pachyderma (dextral). Globorotalia inflata lives all year but at varying water depths. Compared with the fauna in 1960–1961 (described by R. Cifelli), our samples seem warmer. Because sea surface salinity varies little during the year, δ18O is mostly a function of SST. Throughout the year, there are always species present with δ18O close to the calculated isotopic equilibrium of carbonate with surface seawater. This raises the possibility that seasonality can be estimated directly from the range of δ18O in a sediment sample provided that the δ18O-salinity relationship is the same as today.
    Description: Funding was provided by NSF grant OCE-0117149.
    Keywords: Plankton tow ; Foraminifera ; Stable isotopes
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