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51 records · Page 3Linked to original sources
USGS2009

Monitoring and modeling shoreline response due to shoreface nourishment on a high-energy coast

Shoreface nourishment can be an efficient technique to feed sediment into the littoral zone without the order of magnitude cost increase incurred by directly nourishing the beach. An erosion hot spot at Ocean Beach in San Francisco, California, USA, threatens valuable public infrastructure as well as safe recreational use of the beach. In an effort to reduce the erosion at this location, a new beneficial reuse plan was implemented in May 2005 for the sediment dredged annually from the main shipping channel at the mouth of San Francisco Bay. From 2005 to 2007, approximately 230,000 m of sand was placed annually at depths between 9 and 14 m, in a location where strong tidal currents and open-ocean waves could potentially feed sediment onto the section of beach experiencing critical erosion. The evolution of the disposal mound and adjacent beach were monitored with 12 multibeam bathymetric surveys, and over 40 high-resolution beach topographic surveys. In addition, sediment transport processes were investigated using sediment grab samples, acoustic Doppler profilers, and two separate models: a cross-shore profile model (UNIBEST-TC) and a coastal area model (Delft3D). The results of the monitoring and modeling demonstrate that the disposal mound may be effective in dissipating wave energy striking this vulnerable stretch of coast with negligible shadowing effects, but a positive shoreline response can only be achieved by placing the sediment in water depths less than 5 m.

California↗
USGS2017

Can beaches survive climate change?

Anthropogenic climate change is driving sea level rise, leading to numerous impacts on the coastal zone, such as increased coastal flooding, beach erosion, cliff failure, saltwater intrusion in aquifers, and groundwater inundation. Many beaches around the world are currently experiencing chronic erosion as a result of gradual, present-day rates of sea level rise (about 3 mm/year) and human-driven restrictions in sand supply (e.g., harbor dredging and river damming). Accelerated sea level rise threatens to worsen coastal erosion and challenge the very existence of natural beaches throughout the world. Understanding and predicting the rates of sea level rise and coastal erosion depends on integrating data on natural systems with computer simulations. Although many computer modeling approaches are available to simulate shoreline change, few are capable of making reliable long-term predictions needed for full adaption or to enhance resilience. Recent advancements have allowed convincing decadal to centennial-scale predictions of shoreline evolution. For example, along 500 km of the Southern California coast, a new model featuring data assimilation predicts that up to 67% of beaches may completely erode by 2100 without large-scale human interventions. In spite of recent advancements, coastal evolution models must continue to improve in their theoretical framework, quantification of accuracy and uncertainty, computational efficiency, predictive capability, and integration with observed data, in order to meet the scientific and engineering challenges produced by a changing climate.

Journal of Geophysical Research F: Earth Surface↗
USGS2007

Concerns regarding a call for pluralism of information theory and hypothesis testing

1. Stephens et al. (2005) argue for 'pluralism' in statistical analysis, combining null hypothesis testing and information-theoretic (I-T) methods. We show that I-T methods are more informative even in single variable problems and we provide an ecological example. 2. I-T methods allow inferences to be made from multiple models simultaneously. We believe multimodel inference is the future of data analysis, which cannot be achieved with null hypothesis-testing approaches. 3. We argue for a stronger emphasis on critical thinking in science in general and less reliance on exploratory data analysis and data dredging. Deriving alternative hypotheses is central to science; deriving a single interesting science hypothesis and then comparing it to a default null hypothesis (e.g. 'no difference') is not an efficient strategy for gaining knowledge. We think this single-hypothesis strategy has been relied upon too often in the past. 4. We clarify misconceptions presented by Stephens et al. (2005) . 5. We think inference should be made about models, directly linked to scientific hypotheses, and their parameters conditioned on data, Prob(Hj| data). I-T methods provide a basis for this inference. Null hypothesis testing merely provides a probability statement about the data conditioned on a null model, Prob(data |H0). 6. Synthesis and applications . I-T methods provide a more informative approach to inference. I-T methods provide a direct measure of evidence for or against hypotheses and a means to consider simultaneously multiple hypotheses as a basis for rigorous inference. Progress in our science can be accelerated if modern methods can be used intelligently; this includes various I-T and Bayesian methods.

Journal of Applied Ecology↗
USGS2017

Rip currents and alongshore flows in single channels dredged in the surf zone

To investigate the dynamics of flows near nonuniform bathymetry, single channels (on average 30 m wide and 1.5 m deep) were dredged across the surf zone at five different times, and the subsequent evolution of currents and morphology was observed for a range of wave and tidal conditions. In addition, circulation was simulated with the numerical modeling system COAWST, initialized with the observed incident waves and channel bathymetry, and with an extended set of wave conditions and channel geometries. The simulated flows are consistent with alongshore flows and rip-current circulation patterns observed in the surf zone. Near the offshore-directed flows that develop in the channel, the dominant terms in modeled momentum balances are wave-breaking accelerations, pressure gradients, advection, and the vortex force. The balances vary spatially, and are sensitive to wave conditions and the channel geometry. The observed and modeled maximum offshore-directed flow speeds are correlated with a parameter based on the alongshore gradient in breaking-wave-driven-setup across the nonuniform bathymetry (a function of wave height and angle, water depths in the channel and on the sandbar, and a breaking threshold) and the breaking-wave-driven alongshore flow speed. The offshore-directed flow speed increases with dissipation on the bar and reaches a maximum (when the surf zone is saturated) set by the vertical scale of the bathymetric variability.

Journal of Geophysical Research: Oceans↗
USGS2012

Mineralogy and geochemistry of a sediment‐hosted hydrothermal sulfide deposit from the Southern Trough of Guaymas Basin, Gulf of California

Samples dredged from a 15‐m‐high hydrothermal mound atop the flat turbidite pond in the Southern Trough of Guaymas Basin consist of pyrrhotite‐rich massive sulfide, barite, barite + calcite, talc, and opaline silica as well as substrate material composed of fossiliferous, clay‐rich ooze. An 11‐m‐long sediment core taken near the dredge site shows increasing hydrothermal alteration with depth; anhydrite‐filled fractures near the base of the core appear to be channels for hydrothermal discharge. Oxidation of the sulfide‐rich samples to an assemblage of geothite, lepidocrocite, and amorphous Fe oxyhydroxide is ubiquitous. Compared to other massive sulfide deposits on sediment‐starved oceanic ridges, the hydrothermal deposit dredged in Guaymas Basin has a high pyrrhotite/pyrite ratio, a low Zn sulfide and combined ore metal (Cu + Zn + Pb + Ag + Cd) content, and a greater abundance of sulfate, carbonate, and silicate phases. Venting hydrothermal solutions are alkaline with moderately high p H; high Ca, Ba, and SiO 2 content; low ƒS 2 and ƒo 2 ; and very low transition metal content. Disequilibrium assemblages of pyrrhotite and sulfate minerals form during rapid mixing of this evolved vent fluid with ambient bottom waters at the discharge site. Talc is formed at a temperature near 270°C by mixing or entrainment of Mg‐rich bottom water or pore fluid with upwelling hydrothermal fluid that is saturated with silica. Calcite may precipitate from the alkaline, Ca‐rich fluid during degassing of CO 2 . The minimum temperature range for sulfide and nonsulfide deposition is approximately 190°–326°C. The composition of hydrothermal deposits, vent solutions, and altered sediment requires that circulating fluids evolve during deep penetration into the basaltic basement complex, further interaction with the organic‐and carbonate‐rich sediment pile, and near‐surface mixing with ambient seawater. Although the stable assemblage albite‐epidote‐clinochlore present at depth in the sediment pile requires very low dissolved Mg and Fe in the altering fluid, the addition of Mg to deeply buried sediment indicates significant recharge of the system by Guaymas Basin bottom water.

Southern Trough of Guaymas Basin↗
USGS2011

Habitat associations of small fishes around islands in the upper Mississippi River

In large rivers, islands provide a variety of habitat types and increase habitat heterogeneity. Creating or modifying islands with dredged sediments from channel maintenance operations provides an opportunity to enhance habitat features that might promote certain fish communities or general fish abundance. To determine associations between fish species and habitat features of islands, we sampled fish by seining at 62 sites around 20 islands in the upper Mississippi River from Winona, Minnesota, to Prairie du Chien, Wisconsin (180 km). Habitat characteristics were divided into macrohabitat features associated with islands, such as island shape, location, or maximum depth around the island, and mesohabitat features of sites, such as depth, sediment type, and vegetation abundance. Cluster analysis of islands based on macrohabitat features identified four clusters distinguished primarily by water depth and distance from the main channel. Mean fish density did not differ among island clusters. Cluster analysis of sites based on mesohabitat features produced four clusters distinguished primarily by vegetation abundance. Mean densities of most fish taxa were highest in clusters with moderate or dense vegetation and lowest in the cluster with no vegetation. For the eight most abundant fish species, multiple-regression analysis of density on mesohabitat features across all sites indicated that all species were positively correlated with vegetation abundance, which explained 7-49% of variation in density. Our results suggest that mesohabitat features of sites were more important than macrohabitat features of islands in determining density of small fishes and that modifications that increase the abundance of vegetation around islands are most likely to increase fish density.

Minnesota, Wisconsin↗
USGS2012

The collision zone between the North d'Entrecasteaux Ridge and the New Hebrides island arc. 2. Structure from multichannel seismic data

The d'Entrecasteaux zone (DEZ) collides with the central New Hebrides island arc and consists of two subparallel ridges that strike east-west, stand 1–2 km above the surrounding oceanic plate, and subduct obliquely (15°) northward beneath the arc. Rocks dredged from the north ridge as well as reflections evident in multichannel seismic reflection data indicate that this ridge has a volcanic origin. Crystalline volcanic rocks are common along the lower flank of the ridge, but sedimentary, probably volcaniclastic, rock caps the ridge. Seismic reflection data collected over the lower arc slope reveal that mass wasting deposits locally make up most of the accretionary wedge. These deposits appear to form discrete bodies, suggesting that mass wasting occurred episodically. Large anticlines and thrust faults having large vertical separation are not readily evident where the colliding ridge intersects the arc slope; apparently, slope rocks have low strength so that mass wasting deposits formed instead of large-relief structures. Mass wasting is thought to occur as the accretionary wedge is uplifted in response to the northward oblique subduction of the north ridge. The toe of the north ridge flank marks an abrupt transition in the lithologies that make up the footwall of the interplate decollement. Footwall lithologies change from ocean basin to volcaniclastic ridge material, and this transition probably marks a discontinuity in friction along the decollement or in rock mechanical properties because north of the transition, thrust faults deform the accretionary wedge whereas south of the transition, steep reverse faults crosscut the wedge and pierce the north flank of the ridge. This piercement means that the decollement at least locally lies within the ridge and that ridge material exotic to the New Hebrides arc may be incorporated into the accretionary wedge.

Journal of Geophysical Research↗
USGS2023

Monitoring sediment transport pathways from an artificial nearshore berm, South Padre Island, Texas, USA, August 2018 to November 2019: Implications for coastal management

During August 2018 – November 2019, the transport pathways of dredge material from a specially constructed nearshore feeder berm were investigated as part of a collaborative study by the City of South Padre Island, U.S. Army Corps of Engineers–Galveston District, U.S. Geological Survey, Partrac GeoMarine Inc., and Texas A&M University, into the efficacy of beneficial use dredge material (BUDM) as a method of replenishing the beach profile and shoreface at South Padre Island, Texas with sediment. Dual-signature (fluorescent and ferrimagnetic) tracer particles, designed to be hydraulically equivalent to the dredge material, were placed on the berm, and a sampling program was initiated to monitor the spatiotemporal movement of tracer particles under the influence of the prevailing hydrodynamic regime. Wave and current data were collected and were used together with available metocean data to identify the forcing mechanisms of sediment transport; improved understanding garnered from the consideration of multiple datasets can be used to inform future coastal management decisions. Tracer analysis results indicated low magnitude, shoreward transport of dredge material from the berm and along shore transport in both northerly and southerly directions. Small amounts of tracer detected in beach face samples demonstrated connectivity between the constructed feeder berm and the shoreface. However, the generally low tracer concentration within beach face samples indicated low rates of sediment transport for berm sediments, and a low magnitude sediment transport pathway from the berm directly to the beach face, with possible storage of sediment in the nearshore bar system. Given that the berm was at or beyond the closure depth and considering the relatively weak prevailing near bottom hydrodynamic conditions, the potential for sediment to be mobilized and transported during the study period was generally low. Periods of higher energy waves, driven by north-northwest winds, were identified as a key driver of sediment transport but due to the infrequent nature of these events the sediment transport regime across the area of interest was temporally limited. Longshore movement of sediment was both north and south, mainly dependent on prevailing wind directions and resulting longshore currents. We discuss implications for coastal zone management and the use of feeder berms as an artificial beach nourishment mechanism to replenish sediments lost by coastal erosional processes. The insight into nearshore emplacement of material can be used by local governments and coastal managers to optimize the efficacy of berm emplacement and implement such schemes as a cost-effective nourishment option, or when onshore placement of material is not feasible.

Texas↗
USGS1988

Petrology of ultramafic xenoliths from Loihi Seamount, Hawaii

Ultramafic xenoliths were recovered in four alkalic lava flows from Loihi Seamount at depths between 2200 and 1400m. No xenolith bearing flows were sampled near the summit despite a concentrated dredge program. The flows, three of alkalic basalt and one of basanite, contain common olivine megacrysts and small xenoliths of dunite, rarer harzburgite, and a single wehrlite. Olivine megacrysts as large as 8 mm are Fo 84–88 6 and contain magnesiochromite inclusions with 1·1–3·5 wt.% TiO 2 Dunite contains Fo 83 5–88·5 olivine, magnesiochromite with l·5–6·9 wt.% TiO 2 (avg. 3·2 wt.%), and extremely rare chrome-rich diopside. The wehrlite contains euhedral Fo 85 9 olivine and magnesiochromite with 1·9–4·7 wt.% TiO 2 poikilitically enclosed in chrome-rich diopside (Wo 45 4 En 48 0 Fs 6·6 ).Most of the olivine megacrysts, dunite, and the wehrlite are cumulates of Loihi alkalic lavas that accumulated in a magma storage zone located at least 16km below sea level. The rarity of dunite related to tholeiitic magmas supports the interpretation that the alkalic lavas at Loihi generally predate the tholeiitic lavas. The harzburgite xenoliths have cataclastic textures and contain Fo 89 5–92 6 olivine, enstatite (Wo 2 0–2·7 En 90·0–88 7 Fe 8·0–8·6 ), Cr-rich endiopside (Wo 43 4–44 5 En 52 0–50 0 Fs 4 6–4 5 ), and translucent red-brown magnesiochromite. The harzburgite xenoliths, which have 2-pyroxene temperatures of 1066 ± 35°C, originated in the uppermost mantle in a region of high strain rate, probably near the boundary between the mantle and the overlying ocean crust. The presence of upper mantle xenoliths indicates that the magma storage zone is located below the base of the ocean crust within the uppermost mantle.

Journal of Petrology↗
USGS2009

Sediment characterization and dynamics in Lake Pontchartrain, Louisiana

Lake Pontchartrain in southeastern Louisiana is the largest of several shallow estuaries that together cover over 15,000 km 2 . Wetlands, forests, and large urban areas surround the lake. Primary transport mechanisms of sediments to Lake Pontchartrain include urban runoff, major diversions of the Mississippi River, discharge from streams along the north and west shores, and tidal circulation. Sediments deposited in Lake Pontchartrain are subjected to resuspension and mixing by natural and human activities. Bioturbation and water turbulence throughout the lake are the major mixing agents, and mechanical shell dredging has reworked much of the lake bottom over the last century. Sediment characterization through direct sampling and geophysical surveys indicates that these processes continually rework the top meter of sediment. The lake receives discharge from roadways and industrial and agricultural sources. Contaminants from these sources accumulate in the lake sediments and are an important contributor to the degradation of the estuary. Decline in populations of various benthic organisms, such as shrimp and clams, has been documented in the lake. To characterize the health of this important estuary, the U.S. Geological Survey (USGS) conducted a comprehensive evaluation of the geology, geomorphology, coastal processes, and environmental condition of the Pontchartrain Basin from 1994 to 1997. This report presents an assessment of sediment distribution and quality using a multidisciplinary approach to characterize the influence of various physical and chemical parameters: nearsurface stratigraphy, major trace metal concentrations (Cu, Pb, Zn, and Ni), and short-lived radionuclides ( 210 Pb, 7 Be, and 137 Cs). The results are compared with water-circulation patterns to determine high-resolution sedimentation patterns in the lake. The data show a significant increase in trace metals in the top 1 m of lake sediments. Above this horizon, pollen analysis indicates a correlation with land clearing in the area, a proxy for increasing human development of the surrounding landscape and an increase in surface run-off. The data also show that the top meter of sediment undergoes frequent resuspension during high-energy circulation events and via circulation gyres in the lake. This regular turnover does not allow stratification of recently deposited sediments, restricting the sequestration of contaminated material that enters the lake.

Louisiana↗
USGS1993

Petrology and isotopic composition of Quaternary basanites dredged from the Bering Sea continental margin near Navarin Basin

Quaternary basanites were recovered from shallow water depth from the continental margin of the Bering Sea (58°39.0′N, 177°12.9′W) near Navarin Basin. The basanites are highly vesicular flow rock and hyaloclastites similar to other alkalic volcanic rocks erupted repeatedly during the late Cenozoic on islands in the Bering Sea region and in mainland Alaska. K–Ar ages for the basanites indicate at least two episodes of volcanism at about 1.1 and 0.4 Ma. Similar alkalic volcanism occurred sporadically at geographically widely separated centers in the Bering Sea region for at least the past 6 Ma. Chemically, these alkalic lavas are intraplate basalts similar to those erupted from oceanic islands and in some continental settings. Trace-element data indicate these alkalic lavas have been generated by small, but variable, amounts of partial melting of a meta-somatized lherzolite source. The relatively primitive compositions (MgO > 9%), presence of mantle-derived xenoliths in some alkalic lavas, and presence of forsteritic olivine with low CaO and high NiO suggest that magma rose rapidly from greath depth without spending time in large, long-lived magma chambers. Although lavas from different volcanic centers in the Bering Sea region are similar with respect to major elements and many trace-element ratios, isotopic compositions indicate heterogeneities in the source. The Navarin basanites have higher 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd values than any other lavas so far reported from this region. The 207 Pb/ 204 Pb isotopic ratios indicate involvement of a crustal component, which may have resulted from metasomatism associated with subduction-related magmatic activity during the Early Eocene in this region. Although some volcanic episodes appear to have occurred roughly synchronously at geographically widely separated centers, no large-scale regional extension nor presence of large mantle plumes are indicated. Instead, alkalic volcanism apparently resulted from upwelling and decompressional melting of small isolated mantle diapirs in response to local lithospheric attenuation associated with jostling of blocks during adjustment to regional stresses.

Alaska↗
USGS2012

Minor and trace element geochemistry of volcanic rocks dredged from the Galapagos spreading center: Role of crystal fractionation and mantle heterogeneity

A wide range of rock types (abyssal tholeiite, Fe-Ti-rich basalt, andesite, and rhyodacite) were dredged from near 95°W and 85°W on the Galapagos spreading center. Computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional crystallization. However, the P 2 O 5 /K 2 O ratio averages 0.83 at 95°W and 1.66 at 85°W and implies distinct mantle source compositions for the two areas. These source regions also have different rare earth element (REE) abundance patterns, with [La/Sm] EF = 0.67 at 95°W and 0.46 at 85°W. The sequence of fractionated lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95°W. The mantle source regions for these two areas are interpreted to be depleted in incompatible (and volatile?) elements, although the source region beneath 95°W is less severely depleted in La and K. Incompatible trace element abundances in 26 samples are used to infer that the range of Fe-Ti-rich basalt from 85°W represents 19 to 35% residual liquid following crystal fractionation of a mineral assemblage of plagioclase, clinopyroxene, and lesser olivine. The most highly differentiated samples have also had less than 1% titanomagnetite removed. Most samples from 85°W can be related to a common parental magma that contained approximately 9 wt % FeO*, 1 wt % TiO 2 , and had an Mg number (Mg# = 100 Mg/(Mg + Fe 2+ )) of about 65. Although the samples from 95°W cannot all be derived from a common parental magma, the inferred parental magmas may have been derived by varying degrees of partial melting of a common source. The fractionation sequence consists of two parts: an initial iron enrichment trend followed by a silica enrichment trend. We interpret the trace element data to indicate that the most iron rich lavas represent about 32% residual liquid derived by crystal fractionation of plagioclase, clinopyroxene, and lesser olivine from a parental magma with an Mg number of about 66. The silica enrichment trend results from crystallization of titanomagnetite and some apatite. Fractionation of pigeonite, which is a minor phase in the major element models, cannot be distinguished from clinopyroxene fractionation by using trace elements.

Journal of Geophysical Research Solid Earth↗
USGS2012

Geochemistry, mineralogy, and petrogenesis of basalt from the Gorda Ridge

Basalt pillow lava with glassy rims was dredged from 17 sites along the Gorda Ridge between latitudes 43°N and 40.8°N. All samples are low-K 2 O mid-ocean ridge tholeiitic basalt having a narrow compositional range with Mg numbers clustered around 60–62; more primitive and evolved compositions are present but rare. None of the more fractionated samples is as evolved as ferrobasalt from the Juan de Fuca Ridge. Incompatible trace element ratios are typical of normal mid-ocean ridge basalt but show considerable scatter. The 87 Sr/ 86 Sr ratios, ranging from 0.70233 to 0.70267, are near the low end of the range for normal mid-ocean ridge basalt. The phenocrysts present are abundant plagioclase, less abundant olivine ± spinel, and traces of sulfide. Clinopyroxene is absent in the glasses. The mineral chemistry is typical of ocean floor basalt. However, unusually anorthitic plagioclase in some highly plagioclasephyric samples, complex zoning in plagioclase and olivine, and other disequilibrium features indicate complex petrogenetic processes involving magma mixing. Glass inclusions in olivine and plagioclase indicate primitive trapped melts. The chemical diversity of Gorda Ridge lava appears to have been primarily caused by variable degrees of partial melting, but crystal fractionation and magma mixing were also important processes. Compositional variations along the strike of the ridge indicate that lava from the segment south of the offset at latitude 41.6°N was generated by the smallest percentage of partial melting of a source less depleted than that of the two northern segments. Lava from the central and northern segments was generated by variable percentages of partial melting of a uniformly depleted source. Lava from the northern segment shows the greatest diversity with a large range in percentage of melting and both relatively primitive and evolved compositions in spatially and temporally closely associated lava. The diversity of lava from the northern segment may result from thermal perturbations caused by proximity to the Blanco Fracture Zone. Gorda Ridge lava is more similar to that from the Mid-Atlantic Ridge at 22°–25°N than to that from other spreading centers in the Pacific Ocean. Magma chambers under the Gorda Ridge appear to be small, discontinuous in time and space and possibly at greater depth than those beneath other Pacific spreading centers.

Journal of Geophysical Research Solid Earth↗
USGS1988

Ferromanganese deposits from the Gulf of Alaska Seamount Province: Mineralogy, chemistry, and origin

Ferromanganese-oxide deposits dredged from four seamounts (Welker, Miller, Murray, and Patton) in the Gulf of Alaska Seamount Province include poorly crystallized microlaminated crusts on basalt substrate, well-crystallized Mn-oxide veins in epiclastic sedimentary rocks, and crystalline Mn-oxide layers and micronodules in phosphorite. The principal rock types dredged are alkali-basalt pillow fragments and tuffaceous conglomerate and sandstone. The glassy rims of pillow fragments, the glassy goundmass of large volcanic clasts, and the tuffaceous component of the sediment are altered to palagonite. Other low-temperature alteration products include phillipsite, smectite, and carbonate-apatite.Thick (10–50 mm) Fe–Mn crusts consist mainly of δ-MnO 2 ; straight and cuspate growth laminae indicate variable growth rates and periods of nondeposition. A larger number of detrital particles toward the top of thick crusts record the increasing influence of active volcanoes of the Aleutian arc during northwestward movement of the Pacific plate. Thick crusts on basalt substrate have higher Mn/Fe ratios and lower Co content than Fe–Mn crusts from low-latitude seamounts of the central Pacific region. Thin (< 10 mm) crusts on volcaniclastic substrate contain todorokite and birnessite and have higher Mn/Fe ratios, Ni, and Cu and lower Fe and Co than thick Gulf of Alaska crusts.Veins of todorokite and cryptomelane with complex internal structure occur in altered tuffaceous sandstone and conglomerate from Miller Seamount. Fibrous todorokite has a composition similar to those of other marine examples but may contain up to 7% Mn 2+ in M2 sites. Microprobe analysis of the marine cryptomelane indicates a composition that is approximately (K,Ba) 1–2 (Mn 4+ ,Co) 7–8 O 16 ∙ x (H 2 O).A third type of Fe–Mn deposit in phosphorite is an intimate mixture of todorokite (and minor δ-MnO 2 )-bearing layers and micronodules, carbonate-apatite, and phillipsite that encloses grains of altered volcanic glass and lithic fragments.The microlaminated structure, mineralogy (predominantly δ-MnO 2 ), and composition (Mn/Fe ratio and transition metal, rare earth element, U, and Th contents) of the thick crusts are characteristic of hydrogenetic Fe–Mn crusts elsewhere in the Pacific. Conversely, the crystallinity and chemical composition of the Mn-oxide veins and thin crusts indicate formation during diagenesis of the volcanogenic sediment substrate. Mn and other transition metals are mobilized during low-temperature oxidative alteration (palagonitization) of basaltic volcanic glass; the oxidation of Fe 2+ to Fe 3+ during palagonitization and the dissolution of the dilute biogenic fraction of the sediment combine to lower the Eh of ambient pore fluid and enhance the mobility of Mn 2+ . Diagenesis in the phosphatic sandstone from Patton Seamount involves organic-rich sediment and pore waters elevated in phosphorus owing to upwelling above a large volcanic edifice.

Alaska↗
USGS1995

Petrology of Submarine Lavas from Kilauea's Puna Ridge, Hawaii

We have studied 30 quenched tholeiitic lava flows recovered by 20 dredge hauls and one submersible dive along Puna Ridge, the submarine part of the East Rift Zone of Kilauea Volcano, Hawaii Glass grains from numerous additional flows were recovered in turbidite sands cored in the Hawaiian Trough. These quenched lavas document variable primary magma compositions; olivine and multiphase crystallization and fractionation; degassing; wall-rock stoping and assimilation; mixing in the crustal reservoir and the rift zone; entrainment of olivine xenocrysts from a hot, ductile, olivine cumulate body; and disruption of gabbro wallrocks in the rift zone. Glass grains in turbidite sands contain up to 15⋅0wt% MgO, in contrast to < 7⋅0wt% MgO for the sampled glass rinds on lavas. The most forsteritic olivine phenocryst (F0 90·7 ) is in equilibrium with primary Kilauea liquid containing an average 16⋅5 wt% MgO, but ranging from 13⋅4 to 18⋅4%. Lavas and glass grains have more restricted P 2 O 5 /K 2 O and TiO 2 /K 2 O than glass inclusions in olivine, because more diverse liquids trapped as glass inclusions are mixed and homogenized before eruption. Variable trace element compositions in glass grains and whole rocks indicate that the primary liquids form by partial melting of mantle sources retaining clinopyroxene and garnet. Orthopyroxene xenocrysts formed at moderate pressures provide evidence for a sub-crustal staging zone. Chromite and olivine crystallize in the crustal magma reservoir as the liquid cools from an average 1346°C to ∼1170°C. Low viscosities of the primary liquids (0·4 Pas) facilitate olivine settling, and the crystallized olivine forms an olivine cumulate body at the base of the reservoir. Olivine is deformed as the hot ductile dunite body flows down and away from the summit. This flow drives instability of the Hilina landslide on Kilauea. Dikes intrude the dunite, and magma flowing through the dikes disaggregates and entrains olivine xenocrysts in Puna Ridge magmas. Primary liquids pond at or near the base of Kilauea's crustal reservoir because they are denser than more fractionated liquids that occupy the upper parts of the reservoir. The sulfur and water contents of glass rinds indicate that fractionated liquids near the top of the reservoir degas at low pressure, a process that increases their density and causes them to sink to levels where they mix with resident undegassed, near-primary liquid. The fractionated liquids near the top of the magma reservoir acquire excess Cl, owing to assimilation of hydrothermally altered roofrocks. Magma flowing into the rift zone encounters and mixes with low-temperature, multiphase-fractionated melt. The mixed magmas typically contain rare orthopyroxene, plagioclase as sodic as andesine, olivine as fayalitic as F0 75 and Fe-rich augite derived from the fractionated magma. Magma flowing through dikes also dislodged fragments of gabbroic wallrocks that occur as xenoliths. The interrelations in the Kilauean submarine lavas between host glass and glass inclusion compositions, volatile contents and mineral chemistry reveal an extraordinarily complex sequence of petrogenetic processes and events that are difficult or impossible to determine in subaerial Kilauea lavas because of crystallization, reequilibration and degassing during or after their eruption.

Hawaii↗