Enrichment of the continental lithosphere by OIB melts: Isotopic evidence from the volcanic province of northern Tanzania

Alkali basalts and nephelinites from the southern end of the East African Rift (EAR) in northern Tanzania have incompatible trace element compositions that are similar to those of ocean island basalts (OIB). They define a considerable range of Sr, Nd and Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr= 0.7035−0.7058,ε_Nd = −5to+3, and²⁰⁶Pb/²⁰⁴Pb= 17.5−21.3), each of which partially overlaps the range found in OIB. However, they occupy a unique position in combined Nd, Sr and Pb isotopic compositional space. Nearly all of the lavas have radiogenic Pb, similar to HIMU with high time-integrated²³⁸U/²⁰⁴Pb coupled with unradiogenic Nd (+2 to −5) and radiogenic Sr (>0.704), similar to EMI. This combination has not been observed in OIB and provides evidence that these magmas predominantly acquired their Sr, Nd and Pb in the subcontinental lithospheric mantle rather than in the convecting asthenosphere. These data contrast with compositions for lavas from farther north in the EAR. The Pb isotopic compositions of basalts along the EAR are increasingly radiogenic from north to south, indicating a fundamental change to sources with higher time-integratedU/Pb, closer to the older cratons in the south. An ancient underplated OIB melt component, isolated for about 2 Ga as enriched lithospheric mantle and then remelted, could generate both the trace element and isotopic data measured in the Tanzanian samples. Whereas the radiogenic Pb in Tanzanian lavas requires a source with high time-integratedU/Pb, most continental basalts that are thought to have interacted with the continental lithospheric mantle have unradiogenic Pb, requiring a source with a history of lowU/Pb. Such lowU/Pb is readily accomplished with the addition of subduction-derived components, since the lower averageU/Pb of arc basalts (0.15) relative to OIB (0.36) probably reflects addition of Pb from subducted oceanic crust. If the subcontinental lithosphere is normally characterized by low time-integratedU/Pb it would appear that subduction magmatism is more important than OIB additions in supplying the Pb inventory of the lithospheric mantle. However,U/Pb ratios of xenoliths derived from the continental lithospheric mantle suggest that both processes may be important. This apparent discrepancy could be because xenoliths are not volumetrically representative of the subcontinental lithospheric mantle, or, more likely, that continental lithospheric mantle components in basalts are normally only identified as such when the isotopic ratios are dissimilar from MORB or OIB. Lithospheric enrichment from subaccreted OIB components appears to be more significant than generally recognized.     

The relationship between trends in a condition index of the American oyster,Crassostrea virginica,and environmental parameters in three Virginia estuaries

Oyster Condition Index (CI) was partitioned, using a moving average filter, into seasonal cycles and long-term trends in the James, York, and Rappahannock rivers for the period 1970–1983. Seasonal cyclic fluctuations in CI could be explained partially by changes in salinity and number of days within various temperature regimes. Long-term trends in the James River show a steady increase in CI over the study period, while a concurrent decline was noted in the Rappahannock River. Superimposed on these trends is a 4 to 5 yr cycle that is in synchrony with river discharge (salinity). In the York River, CI peaked in 1975–1976 at all stations. Measured environmental parameters do not sufficiently explain the trends. We speculate that the differences in the Rappahannock and James rivers may be due to a decline in bottom oxygen as a result of gravitational circulation differences.     

Therm odynamics of Diagenetic Fluid and Fluid／Mineral Reactions in the Eogene Xingouzui Formation,Oil Field T,Jianghan Basin

This study focuses on the thermodynamics of diagenetic fluid from the Eogene Xingouzui Formation which represents the most important reservoir in Field Oil T in the Jianghan Basin. The measured homogenization temperatures (110–139 °C) of fluid inclusions in diagenetic minerals fell within the range of 67 –155 °C at the middle diagenetic stage. The pressure of diagenetic fluid is estimated at 10.2 –56 MPa. The activity of ions in the fluid shows a tendency of Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ > Fe³⁺ > Fe²⁺ for cations, and HCO ₃ ⁻ > SO ₄ ²⁻ > F⁻ > Cl⁻ > CO ₃ ²⁻ for anions. For the gaseous facies, there is a tendency of CO₂> CO> H₂S> CH₄> H₂. According to the thermodynamic calculations, the pH and Eh of the fluid are 5.86–6.47 and −0.73–−0.64V, respectively. As a result of the interaction between such a diagenetic fluid and minerals in the sediments, feldspars were dissolved or alterated by other minerals. The clay mineral kaolinite was instable and hence was replaced by illite and chloritoid. This project was jointly funded by the National Natural Science Foundation of China (49133080) and the Open Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences.     

Sediment size variation in a braided reach of the Sunwapta River,Alberta, Canada

The downstream diminution in sediment size in a braided reach of the proglacial Sunwapta River, Alberta, Canada, was examined statistically to identify the sources of the observed variation about an expected exponential relationship between clast size and distance. Major deviations from this hypothetical relationship, such as a relative increase in grainsize, may be attributed to the effects of tributary sediment inputs and downstream changes in channel behaviour, whilst local variation is associated with complex patterns of sediment deposition observed at a bar scale. A comparison of diminution coefficients, calculated for separate lithologies and for subreaches along the river, with those obtained from previous studies, is used as an indicator of river behaviour and sediment transport processes. It is shown that rates of diminution vary within the reach in response to differing rates of aggradation and to the backwater effects created by tributary alluvial fans. The relatively high values for the calculated diminution coefficients indicate that processes of differential transport are the main cause of the grain size decrease.     

Metasomatised and veined upper-mantle xenoliths from Pello Hill,Tanzania: evidence for anomalously-light mantle beneath the Tanzanian sector of the East African Rift Valley

Peridotite xenoliths from the Pello tuff cone in the Rift Valley of northern Tanzania, bear witness to upper mantle veining and metasomatism. Veins of katungite composition, with an asthenospheric signature, have imposed K, Fe, Ti, OH and REE metasomatism upon previously depleted peridotite. Chemical and mineralogical gradients are present in the peridotite wall rocks, and hydrous phases developed in the peridotite are generally lower in Ti and Fe, but higher in Mg and Cr, than those in the veins. The metasomatism has reduced the density of affected peridotite by up to 4.5%, supporting earlier geophysical models for low-density mantle beneath the Rift Valley. Age constraints for the metasomatically-induced density decrease permit correlation with Recent faulting in the Rift Valley, but not with the major upwarp of the Kenya Dome in the late Tertiary.     

Evaluation of Urban Local-Scale Aerodynamic Parameters: Implications for the Vertical Profile of Wind Speed and for Source Areas

Nine methods to determine local-scale aerodynamic roughness length \((z_{0})\) and zero-plane displacement \((z_{d})\) are compared at three sites (within 60 m of each other) in London, UK. Methods include three anemometric (single-level high frequency observations), six morphometric (surface geometry) and one reference-based approach (look-up tables). A footprint model is used with the morphometric methods in an iterative procedure. The results are insensitive to the initial \(z_{d}\) and \(z_{0}\) estimates. Across the three sites, \(z_{d}\) varies between 5 and 45 m depending upon the method used. Morphometric methods that incorporate roughness-element height variability agree better with anemometric methods, indicating \(z_{d}\) is consistently greater than the local mean building height. Depending upon method and wind direction, \(z_{0}\) varies between 0.1 and 5 m with morphometric \(z_{0}\) consistently being 2–3 m larger than the anemometric \(z_{0}\). No morphometric method consistently resembles the anemometric methods. Wind-speed profiles observed with Doppler lidar provide additional data with which to assess the methods. Locally determined roughness parameters are used to extrapolate wind-speed profiles to a height roughly 200 m above the canopy. Wind-speed profiles extrapolated based on morphometric methods that account for roughness-element height variability are most similar to observations. The extent of the modelled source area for measurements varies by up to a factor of three, depending upon the morphometric method used to determine \(z_{d}\) and \(z_{0}\).     

Amplifier arrays for CMB polarization

Cryogenic low noise amplifier technology has been successfully used in the study of the cosmic microwave background (CMB). Monolithic millimeter wave integrated circuit (MMIC) technology makes the mass production of coherent detection receivers feasible. We have produced large numbers of MMIC amplifiers for CMB measurements. We have also demonstrated the viability of multi-function multi-chip modules as sensitive receiver front-ends. MMIC integration makes it possible to realize massive arrays of receivers suitable for measurements of the polarization of the CMB. We describe the development of the unit cell of such an array and the development plans for implementation.     

Conservative, shock-capturing transport methods with nonconservative velocity approximations

Conservative high-resolution, or shock-capturing, methods have become widely used for modeling transport equations described by conservation laws. In many geoscience applications, the transport equation is coupled to a conservation or continuity equation for a velocity field. Depending on how the velocity is approximated, the continuity equation may or may not be satisfied, either locally or globally. In this paper, we discuss the effect this has on a typical high resolution scheme, and propose a correction which accounts for the fact that the velocity may be nonconservative. We present several numerical examples and prove stability bounds and an a priori error estimate for the corrected method.     

Long-range transport of Asian emissions to the West Pacific tropical tropopause layer

Journal of Atmospheric Chemistry - Rapid transport by deep convection is an important mechanism for delivering surface emissions of reactive halocarbons and other trace species to the tropical...