A pantelleritic welded ash-flow tuff from the Ethiopian Rift Valley

Fantale (Lat. 8° 58 N., Long. 39° 54 E) is a typical Quaternary silicic strato-volcano situated on the floor of the northern part of the Main Ethiopian Rift. Following the growth of the main central cone, a voluminous ash-flow tuff was erupted in association with the development of the 4 km summit caldera. On the upper parts of the volcano the tuff is virtually restricted to three major eruptive pathways descending from the caldera. The tuff is more extensive on the lower flanks and forms a continuous sheet surrounding the volcano.The tuff is welded throughout, even when it is less than 1 m thick. It shows typical vitroclastic texture and is markedly eutaxitic in the intensely welded sections. The degree of welding appears to he largely independent of the total thickness of the tuff. Microscopic observations provide confirmatory evidence of postdepositional vesiculation of the tuff and also suggest that recrystallisation has not induced the development of cavities in the tuff.The detailed analytical data presented suggest that the chemical composition of the tuff was modified by recrystallisation which produced relative depletion in SiO₂, total Fe, Cl, and Na₂O and enrichment in Al₂O₃, K₂O and CaO. The trace elements Ba, La, Nb, Rb, and Zr were not affected by this process. Study of serial samples from three sections through the tuff has led to the identification of five of the constituent flow units of the tuff. The units are approximately homogeneous in composition but differ from each other. The earliest unit is more silicic and pantelleritic, whereas the later flow units are more trachytic in composition. This data is interpreted in terms of the progressive emptying of a small but strongly zoned high-level magma chamber.The presence of primary laminar flowage structures, postdepositional vesiculation effects, intense welding in thin units and high initial dips suggests that the Fantale tuff was deposited from a series of dense, perhaps partially fluidized, magma pulses.     

An annual survey ofHermesium adriaticum andEbria tripartita,two ebridian algae in the lower Chesapeake Bay

Two colorless flagellates,Hermesinum adriaticum andEbria tripartita, were found in the upper meter of water in Mobjack Bay and the York River.Hermesinum was abundant in the summer months when water temperatures were maximum for the area.Ebria was abundant at this and other times of the year. There was no correlation of temperature and abundance for the latter flagellate.     

Measurements of turbulence and fossil turbulence near ampere seamount

Measurements of temperature and velocity microstructure near and downstream of a shallow seamount are used to compare fossil turbulence versus non-fossil turbulence models for the evolution of turbulence microstructure patches in the stratified ocean. According to non-fossil oceanic turbulence models, all overturn length scales L_T of the microstructure grow and collapse in constant proportion to each other and to the turbulence energy (Oboukov) scale L_O and the inertial buoyancy (Ozmidov) scale of the patches; that is, with L_Trms ≈1.2L_R and viscous dissipation rate ≈ ₀^*. According to the Gibson fossil turbulence model, all microstructure originates from completely active turbulence with ₀ ≈ 3L_T²N³(≈ 28₀^*) and L_{T/√6 ≈ LTrms}, but this rapidly decays into a more persistent active-fossil state with _0F ≈ 30vN², where N is the buoyancy frequency and v is the kinematic viscosity and, without further energy supply, finally reaches a completely fossil turbulence hydrodynamic state of internal wave motions, with _F. The last turbulence eddies, with ≈ _F, vanish at a buoyant-inertial-viscous (fossil Kolmogorov) scale L_KF that is much smaller than the remnant overturn scales L_T for large ₀/_F ratios. These density, temperature, and salinity overturns with L_T ≈ 0.6 L_R0 0.6 L_R persist as turbulence fossils (by retaining the memory of _o) and collapse very slowly. In the near wake below the summit depth of Ampere seamount, a much larger proportion of completely active turbulence patches was found than is usually found in the ocean interior away from sources. Dissipation rates and turbulence activity coefficients of microstructure patches were found to decrease downstream, suggesting that the active turbulence indicated by the patches with A_T 1 was caused by the presence of the seamount as a turbulence source. Therefore, the turbulence and mixing processes of ocean layers far away from turbulence sources probably have been undersampled by microstructure data sets lacking any A_T 1 patches. This is because large fractions of the mixing and viscous dissipation of the patches occur in short-lived active turbulence regimes that are too brief to be detected. Consequently, large underestimates of the true space-time average turbulence fluxes and turbulence and scalar dissipation rates may result if non-fossil turbulence models are assumed in ocean microstructure data interpretation.     

The use of pump-out facilities by recreational boaters in Maryland

Because of the intense interest in the water quality of the Chesapeake Bay, the State of Maryland sponsored a survey of 500 Maryland boaters to determine their use of marina facilities to discharge sewage from portable toilets or holding tanks. We report results of that survey as well as examine some of the factors that contribute to the use of pump-out facilities. Presently, with under 10% of Maryland’s marinas having facilities available to pump out holding tanks or portable toilets, less than one in twenty boaters has ever used such a facility. Moreover, less than one in three boats equipped with holding tanks or portable toilets has ever used such a facility. Using a discrete choice behavioral model, we found that boaters with portable toilets were more likely to use facilities than ones with holding tanks, especially if the holding tanks were equipped with macerating devices. The price of using the pump-out facility negatively influenced pump-out use. We also found that vessels in transition were less likely to use marina facilities. Finally, the availability of a pump-out facility at a boater’s marina increased the likelihood of pumping by twofold. In Maryland reduction of boat-generated pollution will likely require a policy of both extensive pump-out services and low costs for the services.     

First samples of acoustic basement recovered from the alpha ridge, Arctic ocean: New constraints for the origin of the ridge

The Alpha Ridge is one of three subparallel trending ridges that cut the Arctic Ocean. It is roughly Late Cretaceous to Eocene in age, and seismic refraction records suggest it comprises a thick sequence of oceanic crust. During the 1983 CESAR expedition 20 similar samples of acoustic basement were dredged from the walls of a major graben of the Alpha Ridge, at one site. These are the only basement samples ever recovered from the ridge and provide the first direct evidence for its nature, composition and possible origin.The basement samples are highly altered pyroclastic rocks composed almost entirely of basaltic volcanic clasts with little matrix. Although the rocks are highly altered, most primary textures and structures are preserved. Most clasts are highly amygdaloidal to scoriaceous, fine grained to glassy, and angular to subround with rare vesicle controlled boundaries. Little reworking is suggested because a single clast type predominates, many of the clasts are subangular, and any amount of reworking would result in destruction of the delicate scoriaceous clasts.Rare clinopyroxene phenocrysts comprise the only unaltered portion of the rocks. They are salitic in composition (Wo_49–53, En_32–41, Fs_11–15), with significant amounts of Ca, Al and Ti. Salitic clinopyroxenes are typical of alkalic basalts.Interpretation of the whole rock geochemistry based on relatively immobile elements, (Nb, Zr, Tio₂, and Y), and chondrite-normalized incompatible trace element and REE patterns indicates that the volcanic rock fragments are of alkalic basalt. Geochemical discriminators suggest a within-plate tectonic setting.Textural evidence suggests that the CESAR basement rocks were sampled from a rapidly emplaced submarine fallout deposit that was erupted at a depth at least less than 800 m and likely less than 200 m. High extrusive rates would have been required to build the ridge up to shallow depth prior to the cessation of volcanism. The alkalic affinity of the rocks strongly suggests that the Alpha ridge was not formed by volcanism at an island arc or a mature spreading centre. It is also unlikely that it formed as a “leaky” fracture zone. Alkalic basalts, however, are commonly associated with various types of oceanic aseismic ridges. It is suggested that the Alpha Ridge is an aseismic ridge that formed due to voluminous hotspot volcanism as spreading began in the Canada Basin. Such hotppot activity may have been responsible for initiating the rifting, breakup, and dispersal that eventually formed the Canada Basin.     

The timing of sub-solidus hydrothermal alteration in the Central Zone, Limpopo Belt (South Africa): Constraints from titanite U–Pb geochronology and REE partitioning

In the Central Zone of the Limpopo Belt (South Africa), Palaeoproterozoic granulite-facies metamorphism was superimposed on an earlier Archaean orogenic history. Previously determined ages of  2030–2020 Ma obtained from high-temperature chronometers (zircon, garnet, monazite) are generally thought to provide the best estimate of the peak of Palaeoproterozoic granulite-facies metamorphism in the Central Zone, whereas ages as young as  2006 Ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period. We present a new MC-ICP-MS ²⁰⁷Pb–²⁰⁶Pb age of 2030.9 ± 1.5 Ma for titanite found in amphibolite- to greenschist-facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed Central Zone metabasite. This age could potentially date cooling of rocks at this locality to temperatures below the wet solidus. Alternatively, the titanite could be inherited from the metabasite host, and the age determined from it date the peak of metamorphism. Integration of the geochronology with LA-ICP-MS trace element data for minerals from the metabasite, the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite-facies hydrothermal alteration, not at the metamorphic peak or during the greenschist-facies phase of veining. This suggests that high-grade rocks in the Central Zone have cooled differentially through the wet solidus, and provides timing constraints on when Palaeoproterozoic reworking in the Central Zone began. This study illustrates the potential of combined geochronological and high-resolution geochemical studies to accurately match mineral ages to distinct crustal processes.     

ON MOBILE ELEMENT TRANSPORT IN HEATED ABEE

Seven trace elements (Ag, Co, Cs, Ga, In, Te, Tl) are either completely retained or are lost to the same extent in Abee samples heated at 700 °C for one week at 10^?5-10^?3 atm Ne or in 10^?5 atm H₂. Bi and Se are lost significantly more easily and Zn is better retained in samples heated in Ne than in H₂. Zn retention varies inversely with ambient Ne pressure. Mobile element transport seems unaffected by physical interactions in the gas phase but may reflect solid-state surface effects. During week-long heating at low pressures (initially ～ 10^?5 atm H₂) S is mobilized only at 1000 °C while C contents decrease progressively from 600–1000 °C. Apparent activation energies for C are 60 kcal/mole below 700 °C and 16 kcal/mole above this temperature suggesting diffusive loss from different hosts and/or processes over different temperature intervals. In E4–6 chondrites C and S contents largely reflect nebular fractionation and condensation processes.