Geochronology and petrochemistry of Easter and Sala y Gomez Islands: implications for the origin of the Sala y Gomez Ridge

Easter and Sala y Gomez Islands lie along the Sala y Gomez Ridge, a broad zone of high topography and scattered seamounts extending east-southeast from the East Pacific Rise. K-Ar ages and major element abundances of volcanic rocks from these islands are used to test the fixed melting spot hypothesis for the origin of this feature. Poike volcano, the oldest center on Easter Island, was constructed in two episodes, occurring at 2.5 and 0.9 m.y. ago. Eruptive activity on Sala y Gomez was nearly contemporaneous with the earliest volcanism on Easter Island. No migration of volcanism with time is apparent along the Sala y Gomez Ridge. Basaltic rocks from Easter Island have tholeiitic affinities, while those dredged from the base of Sala y Gomez belong to an alkali olivine basalt series. Differences in basalt chemistry suggest that the volcanic rocks from the two islands formed from magmas that equilibrated at different pressures, and a model is presented which relates the character of the erupted liquids to magma segregation at the base of a progressively thickening lithosphere. The islands and seamounts comprising the Sala y Gomez Ridge do not fall on a small circle about the Nazca-mantle pole of absolute motion. Available evidence, therefore, does not support a fixed melting spot origin for Easter Island, Sala y Gomez, and the Sala y Gomez Ridge. Rather, it appears that the Sala y Gomez Ridge formed along a leaky fracture zone, and in response to a major re-orientation of spreading centers in this area.     

Velocity profiles and bed shear stress over various bed configurations in a river bend

Vertical velocity profiles measured over various bed configurations (plane beds, ripples, and dunes) in. the meandering River South Esk, Glen Clova, Scotland are presented on semilogarithmic paper. Local bed shear stress and roughness height are calculated from the lowermost parts of the profiles using the Karman-Prandtl law of the wall; these parameters, and the geometrical properties of the profiles, are related to the various bed configurations. A graphical model is used to identify profiles developed on specific regions of dune geometry, in order to discriminate those profiles that define bed shear effective in transporting sediment over dunes. An assessment is made of the errors involved in estimating local mean velocity from extrapolating the law of the wall to the water surface. A Darcy-Weisbach friction coefficient is related to bed configuration and local stream power.     

The lower symmetric regime and its transition to waves in rotating annulus convection

Measurements of the temperature and zonal velocity fields which develop in a rotating annulus of fluid with an upper surface, differentially heated from the inner to outer cylinder, are described for the lower symmetric regime (small radial temperature differences). The temperature field is essentially conductive for moderate to large rotation rates, Ω (>1.0 sec⁻¹). The zonal velocity field is poorly approximated by the thermal wind equation.Measurements of the transition to waves from the lower symmetric regime at very large rotation rates are presented for positive and negative radial temperature differences. They suggest that the centrifugal buoyancy force and the free surface curvature may be important factors for the lower symmetric-wave transition at large Ω. By varying the stratification of the fluid over a range of 10³ independently of the radial temperature difference, Δr_wT, it is conclusively shown that several theories are correct in predicting that the lower symmetric transition is independent of the stratification at small Δr_wT > 0 for large enough Ω.     

The use of 210Pb as a heavy metal tracer in the Susquehanna River system

Analysis of soil profiles and shallow ground water in the Susquehanna River basin, northeastern U.S.A., indicates that the atmospheric flux of ²¹⁰Pb is efficiently scavenged by the organic-rich horizons of the soils. This atmospherically supplied ²¹⁰Pb in soil profiles can only be lost from the system by soil erosion. Based on the annual sediment yield of the Susquehanna River and the excess ²¹⁰Pb concentration in particulate matter, a mean residence time of 2000 yr is calculated for metals similar to Pb in soil profiles.The West Branch of the Susquehanna River (WBSR) is strongly affected by acid mine drainage and is low in pH and high in dissolved ( <0.4 μm) ²¹⁰Pb, Fe and Mn. Along its course iron hydroxide is precipitating at a pH of between 4 and 4.5 and the ²¹⁰Pb supplied by the acid mine water is diminished by about 25% as a result of dilution. As the WBSR enters the Valley and Ridge Province of the Appalachians it has a ²¹⁰Pb concentration of ~ 0.2 dpm/l. At this juncture it receives a considerable influx of alkalinity from tributaries draining carbonate terranes, resulting in neutralization of the sulfuric acid and increase of the river pH to around 6.5–7. This pH adjustment is accompanied by the precipitation of Fe and Mn. Due to the slow rate of Mn removal from solution, the Mn precipitation extends a considerable distance down river from the point of acid neutralization. Analyses for ²¹⁰Pb in the river at points in or below the region of Mn precipitation show that ²¹⁰Pb is rapidly scavenged from solution onto suspended particles. From the data it is possible to calculate the removal rate of Pb from water in the presence of Fe and Mn hydroxides and other particles. At a pH of 4–4.5 Pb removal is nonexistent relative to the river flow rate, but at a pH of 6.5–7 the ²¹⁰Pb data indicate a residence time of <0.7 day for dissolved Pb.     

TRANSIENT EM RESPONSE OF A LARGE LOOP ON A LAYERED GROUND *

The voltage induced in a horizontal loop on a layered ground has been calculated for the case where the loop is excited by a step current and measurements are made during the off-cycle. The expressions derived for a uniform ground show that for large time t the induced voltage E(t) is approximately given by E(t)?— (Ibαμ/20t) (σμ²/t)^3/2 where σ is the conductivity of the ground, μ the permeability, b the loop radius, and I the amplitude of the current step. For small times the corresponding result is E(t)?—Ibμ/2t. When the ground is composed of a number of layers a numerical procedure for calculating the induced voltage is described. The calculated responses of various multilayered structures show that at short times the induced voltage is asymptotic to that produced in the case of a uniform ground of conductivity equal to the top layer. Interference effects in the top layer can lead to anomalous decay curves which may result in the underestimation of the conductivity of a buried layer.     

Distribution of gold and rhenium between nickel-iron and silicate melts: implications for the abundance of siderophile elements on the Earth and Moon

The distribution equilibrium of Au and Re between nickel-iron and basaltic melts was studied at 1400–1600°C, using radioactive tracers. Metal/silicate distribution coefficients were 1–3 orders of magnitude higher than earlier estimates, as follows. Mauna Loa basalt—Fe₁₀Ni₉₀: D_Au = 3.3 × 10⁴, D_Re = (2.4?89) × 10⁴. Gorda Ridge basalt—Fe₁₀Ni₉₀: D_Au = (18?75) × 10⁴. Synthetic lunar basalt—Fe₇₀ Ni₃₀: D_Au≥ 2 × 10⁴, D_Re ≥ 2 × 10³. The experimental ΔG₁₈₀₀^° for the distribution of Au between nickel-iron and Mauna Loa basalt is ?40 kcal/mole, compared to a calculated value of about ?110 kcal/mole for a reaction involving simple Au³⁺ ions. Presumably the difference represents stabilization of Au(III) by complex formation with ligands such as Cl^?, H₂O, etc.Gold abundances in lunar basalts are roughly consistent with the measured D_Au, but those in terrestrial basalts are two orders of magnitude too high. This discrepancy may reflect complexing by volatiles in the Earth's upper lithosphere, as well as oxidative destruction of metal in the final stages of accretion. In the absence of a metal phase, siderophile trace elements would remain trapped in the upper mantle and crust.     

Carbon and sulfur in the Swaziland sequence

Sedimentary and igneous rocks of the Swaziland Sequence of South Africa (>3,200 my. old) contain total carbon (0.09–8.28%), organic carbon (0.04–1.95%) and sulfur (9–53,000 ppm). Amounts of organic carbon are generally high in the lower part of the Sequence (based on two samples from the Theespruit Formation). Sulfur content varies widely with no obvious correlations or trends. These rocks may contain clues to the early carbon and sulfur cycles on earth.     

Gravitational microlensing: A parallel,large-data implementation

Gravitational lensing allows us to probe the structure of matter on a broad range of astronomical scales, and as light from a distant source traverses an intervening galaxy, compact matter such as planets, stars, and black holes act as individual lenses. The magnification from such microlensing results in rapid brightness fluctuations which reveal not only the properties of the lensing masses, but also the surface brightness distribution in the source. However, while the combination of deflections due to individual stars is linear, the resulting magnifications are highly non-linear, leading to significant computational challenges which currently limit the range of problems which can be tackled. This paper presents a new and novel implementation of a numerical approach to gravitational microlensing, increasing the scale of the problems that can be tackled by more than two orders of magnitude, opening up a new regime of astrophysically interesting problems.     

A bulk cloud parameterization in a Venus General Circulation Model

A condensing cloud parameterization is included in a super-rotating Venus General Circulation Model. A parameterization including condensation, evaporation and sedimentation of mono-modal sulfuric acid cloud particles is described. Saturation vapor pressure of sulfuric acid vapor is used to determine cloud formation through instantaneous condensation and destruction through evaporation, while pressure dependent viscosity of a carbon dioxide atmosphere is used to determine sedimentation rates assuming particles fall at their terminal Stokes velocity. Modifications are described to account for the large range of the Reynolds number seen in the Venus atmosphere.Two GCM experiments initialized with 10 ppm-equivalent of sulfuric acid are integrated for 30 Earth years and the results are discussed with reference to “Y” shaped cloud structures observed on Venus. The GCM is able to produce an analog of the “Y” shaped cloud structure through dynamical processes alone, with contributions from the mean westward wind, the equatorial Kelvin wave, and the mid-latitude/polar Mixed Rossby/Gravity waves. The cloud top height in the GCM decreases from equator to pole and latitudinal gradients of cloud top height are comparable to those observed by Pioneer Venus and Venus Express, and those produced in more complex microphysical models of the sulfur cycle on Venus. Differences between the modeled cloud structures and observations are described and dynamical explanations are suggested for the most prominent differences.