A Sr,Nd, Hf,and Pb isotope perspective on the genesis and long-term evolution of alkaline magmas from Erebus volcano,Antarctica

We report new Nd, Hf, Sr, and high-precision Pb isotopic data for 44 lava and tephra samples from Erebus volcano. The samples cover the entire compositional range from basanite to phonolite and trachyte, and represent all three phases of the volcanic evolution from 1.3 Ma to the present. Isotopic analyses of 7 samples from Mt. Morning and the Dry Valley Drilling Project (DVDP) are given for comparison. The Erebus volcano samples have radiogenic ²⁰⁶Pb/²⁰⁴Pb, unradiogenic ⁸⁷Sr/⁸⁶Sr, and intermediate ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf, and lie along a mixing trajectory between the two end-member mantle components DMM and HIMU. The Erebus time series data show a marked distinction between the early-phase basanites and phonotephrites, whose Nd, Hf, Sr, and Pb isotope compositions are variable (particularly Pb), and the current ‘phase-three’ evolved phonolitic lavas and bombs, whose Nd, Hf, Sr, and Pb isotope compositions are essentially invariant. Magma mixing is inferred to play a fundamental role in establishing the isotopic and compositional uniformity in the evolved phase-three phonolites. In-situ analyses of Pb isotopes in melt inclusions hosted in an anorthoclase crystal from a 1984 Erebus phonolite bomb and in an olivine from a DVDP basanite are uniform and identical to the host lavas within analytical uncertainties. We suggest that, in both cases, the magma was well mixed at the time melt inclusions were incorporated into the different mineral phases.     

Zinc stable isotopes in seafloor hydrothermal vent fluids and chimneys

Many of the heaviest and lightest natural zinc (Zn) isotope ratios have been discovered in hydrothermal ore deposits. However, the processes responsible for fractionating Zn isotopes in hydrothermal systems are poorly understood. In order to better assess the total range of Zn isotopes in hydrothermal systems and to understand the factors which are responsible for this isotopic fractionation, we have measured Zn isotopes in seafloor hydrothermal fluids from numerous vents at 9–10°N and 21°N on the East Pacific Rise (EPR), the TAG hydrothermal field on the Mid-Atlantic Ridge, and in the Guaymas Basin. Fluid δ⁶⁶Zn values measured at these sites range from + 0.00‰ to + 1.04‰. Of the many physical and chemical parameters examined, only temperature was found to correlate with fluid δ⁶⁶Zn values. Lower temperature fluids (< 250 °C) had both heavier and more variable δ⁶⁶Zn values compared to higher temperature fluids from the same hydrothermal fields. We suggest that subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite (Zn sulfide), and that this process is a primary cause of Zn isotope variation in hydrothermal fluids. Thermodynamic calculations carried out to determine saturation state of sphalerite in the vent fluids support this hypothesis with isotopically heaviest Zn found in fluids that were calculated to be saturated with respect to sphalerite. We have also measured Zn isotopes in chimney sulfides recovered from a high-temperature (383 °C) and a low-temperature (203 °C) vent at 9–10°N on the EPR and, in both cases, found that the δ⁶⁶Zn of chimney minerals was lighter or similar to the fluid δ⁶⁶Zn. The first measurements of Zn isotopes in hydrothermal fluids have revealed large variations in hydrothermal fluid δ⁶⁶Zn, and suggest that subsurface Zn sulfide precipitation is a primary factor in causing variations in fluid δ⁶⁶Zn. By understanding how chemical processes that occur beneath the seafloor affect hydrothermal fluid δ⁶⁶Zn, Zn isotopes may be used as a tracer for studying hydrothermal processes.     

Latest Pleistocene and Holocene dune construction at the north-eastern edge of the Rub Al Khali, United Arab Emirates

Optical dating of two dune profiles developed in linear dunes in Ras Al Khaimah, United Arab Emirates, has been used to establish the timing and rate of dune accretion. One section at Awafi was over 17 m high, while that at Idhn was over 40 m high. The Awafi dune appears to have accumulated very rapidly ≈10 000 years ago, with a vertical accumulation rate of about 3·3 m ka⁻¹. The Idhn dune appears to have accumulated over the past 1000 years, with 20 m of sediment accumulating in a time period of about 270 years. The Awafi dune may have accumulated in response to the transgression of the Persian Gulf by rising sea levels in late Pleistocene and Holocene times. The Idhn dune may have accumulated rapidly because of intensified human activity, a short-lived climatic event, or because it may periodically be reactivated after erosion by fluvial action at its base.     

Health monitoring of fluid dampers for vibration control of structures: experimental investigation

This paper presents results from a comprehensive experimental program on medium-size and large-size fluid dampers in an effort to extract their force output during cyclic loading by simply measuring the strain on the damper housing and the end-spacer of the damper. The paper first discusses the stress path within the damper and, subsequently via the use of linear elasticity, shows that the experimental data obtained with commercially available strain gauges yield a force output of the damper that is in good agreement with the readings from the load cell. This comparison is achieved via the use of a position and velocity transducer, which combines good accuracy together with robust performance in a marine environment. The paper then examines the performance of a portable data acquisition system that can be used to collect and transmit data from a damper installed on a bridge to a nearby location (order of a km) where data are collected via either a wired or a wireless Local Area Network (LAN). Alternatively, the data may be transmitted to any remote location via mobile telecommunication networks; however, this requires leased telephone lines. The data show that the proposed arrangement is promising for monitoring in situ the force output of fluid dampers and detecting possible loss of their energy dissipation capability. Copyright © 2012 John Wiley & Sons, Ltd.     

Deep pooling of low degree melts and volatile fluxes at the 85°E segment of the Gakkel Ridge: Evidence from olivine-hosted melt inclusions and glasses

We present new analyses of volatile, major, and trace elements for a suite of glasses and melt inclusions from the 85°E segment of the ultra-slow spreading Gakkel Ridge. Samples from this segment include limu o pele and glass shards, proposed to result from CO₂-driven explosive activity. The major element and volatile compositions of the melt inclusions are more variable and consistently more primitive than the glass data. CO₂ contents in the melt inclusions extend to higher values (167–1596 ppm) than in the co-existing glasses (187–227 ppm), indicating that the melt inclusions were trapped at greater depths. These melt inclusions record the highest CO₂ melt concentrations observed for a ridge environment. Based on a vapor saturation model, we estimate that the melt inclusions were trapped between seafloor depths (～ 4 km) and ～ 9 km below the seafloor. However, the glasses are all in equilibrium with their eruption depths, which is inconsistent with the rapid magma ascent rates expected for explosive activity. Melting conditions inferred from thermobarometry suggest relatively deep (25–40 km) and cold (1240°–1325 °C) melting conditions, consistent with a thermal structure calculated for the Gakkel Ridge. The water contents and trace element compositions of the melt inclusions and glasses are remarkably homogeneous; this is an unexpected result for ultra-slow spreading ridges, where magma mixing is generally thought to be less efficient based on the assumption that steady-state crustal magma chambers are absent in these environments. All melts can be described by a single liquid line of descent originating from a pooled melt composition that is consistent with the aggregate melt calculated from a geodynamic model for the Gakkel Ridge. These data suggest a model in which deep, low degree melts are efficiently pooled in the upper mantle (9–20 km depth), after which crystallization commences and continues during ascent and eruption. Based on our melting model and the assumption that CO₂ is perfectly incompatible, we show that the highest CO₂ concentrations of the melt inclusions (～ 1600 ppm) are consistent with the calculated CO₂ concentrations of primary undegassed melts. The highest measured CO₂/Nb ratio (443) of Gakkel Ridge melt inclusions predicts a mantle CO₂ content of 134 ppm and would result in a global ridge flux of 2.0 × 10¹² mol CO₂/yr.     

Salinity and Soluble Organic Matter on Virus Sorption in Sand and Soil Columns

The objective of this research was to study the sorption and transport of bacteriophage MS-2 (a bacterial virus) in saturated sediments under the effect of salinity and soluble organic matter (SOM). One-dimensional column experiments were conducted on washed high-purity silica sand and sandy soil. In sand column tests, increasing salinity showed distinct effect on enhancing MS-2 sorption. However, SOM decreased MS-2 sorption. Using a two-site reversible-irreversible sorption model and the double layer theory, we explained that pore-water salinity potentially compressed the theoretical thickness of double layers of MS-2 and sand, and thus increased sorption on reversible sorption sites. On irreversible sorption sites, increasing salinity reversed charges of some sand particles from negative to positive, and thus converted reversible sorption sites into irreversible sites and enhanced sorption of MS-2. SOM was able to expand the double layer thickness on reversible sites and competed with MS-2 for the same binding place on irreversible sites. In sandy soil column tests, the bonded and dissolved (natural) soil organic matters suppressed the effects of pore-water salinity and added SOM and significantly reduced MS-2 adsorption. This was explained that the bonded soil organic matter occupied a great portion of sorption sites and significantly reduced sorption sites for MS-2. In addition, the dissolved soil organic matter potentially expanded the double layer thickness of MS-2 and sandy soil on reversible sorption sites and competed with MS-2 for the same binding place.     

Measuring the hydraulic conductivity of shallow submerged sediments

The hydraulic conductivity of submerged sediments influences the interaction between ground water and surface water, but few techniques for measuring K have been described with the conditions of the submerged setting in mind. Two simple, physical methods for measuring the hydraulic conductivity of submerged sediments have been developed, and one of them uses a well and piezometers similar to well tests performed in terrestrial aquifers. This test is based on a theoretical analysis that uses a constant-head boundary condition for the upper surface of the aquifer to represent the effects of the overlying water body. Existing analyses of tests used to measure the hydraulic conductivity of submerged sediments may contain errors from using the same upper boundary conditions applied to simulate terrestrial aquifers. Field implementation of the technique requires detecting minute drawdowns in the vicinity of the pumping well. Low-density oil was used in an inverted U-tube manometer to amplify the head differential so that it could be resolved in the field. Another technique was developed to measure the vertical hydraulic conductivity of sediments at the interface with overlying surface water. This technique uses the pan from a seepage meter with a piezometer fixed along its axis (a piezo-seep meter). Water is pumped from the pan and the head gradient is measured using the axial piezometer. Results from a sandy streambed indicate that both methods provide consistent and reasonable estimates of K. The pumping test allows skin effects to be considered, and the field data show that omitting the skin effect (e.g., by using a single well test) can produce results that underestimate the hydraulic conductivity of streambeds.