Partition coefficients of Nb and Ta between rutile and anhydrous haplogranite melts

Partition coefficients (D) for Nb and Ta between rutile and haplogranite melts in the K₂O-Al₂O₃-SiO₂ system have been measured as functions of the K₂O/Al₂O₃ ratio, the concentrations of Nb₂O₅ and Ta₂O₅, the temperature, in air and at 1 atmosphere pressure. The Ds increase in value as the K^* [K₂O/(K₂O + Al₂O₃)] molar ratio continuously decreases from highly peralkaline [K^* ∼ 0.9] to highly peraluminous [K^* ∼ 0.35] melts. The D values increase more dramatically with a unit decrease in K^* in peraluminous melts than in peralkaline melts. This compositional dependence of Ds can be explained by the high activity of NbAlO₄ species in peraluminous melts and the high activity of KONb species (or low activity of NbAlO₄ species) in peralkaline melts. A coupled substitution, Al⁺³ + Nb⁺⁵ (or Ta⁺⁵) = 2Ti⁺⁴, accounts for the Ds of Nb (Ta) being much greater in peraluminous melts than in peralkaline melts because this substitution allows Nb (Ta) to enter into the rutile structure more easily. The Ds of Ta between rutile and melt are greater than those of Nb at comparable concentrations because the molecular electronic polarizability of Ta is weaker than that of Nb. The Nb⁺⁵ with a large polarizing power forms a stronger covalent bond with oxygen than Ta⁺⁵ with a small polarizing power. The formation of the strong bond, Nb-O, distorts the rutile structure more severely than the weak bond, Ta-O; therefore, it is easier for Ta to partition into rutile than for Nb. These results imply that the utilization of the Nb/Ta ratio in liquid as a petrogenetic indicator in granitic melts must be done with caution if rutile (or other TiO₂-rich phases) is a liquidus phase. The crystallization of rutile will increase the Nb/Ta ratio of the residual liquid because the Ds of Ta between rutile and melts are greater than those of Nb. Received: 28 December 1998 / Accepted 27 September 1999     

Characterization of high molecular weight biomarkers in crude oils

High-temperature gas chromatography (HTGC) has enhanced our ability to characterize hydrocarbons extending to C₁₂₀ in crude oils. As a result, hydrocarbons in waxes (> C₂₀) have been observed to vary significantly between crude oils, even those presumed to originate from the same source. Prior to this development, microcrystalline waxes containing hydrocarbons above C₄₀ were not characterized on a molecular level due to the analytical limitations of conventional gas chromatography. Routine screenings of high pour-point crude oils by high-temperature gas chromatography has revealed that high molecular weight hydrocarbons (> C₄₀) are very common in most oils and may represent 2% of the crude oil. Precise structures, origins, and significance of these high molecular weight compounds remain elusive. As a preliminary step to expand our knowledge of these compounds their general molecular structures and formulas have been investigated in this study. Initial results suggest that the major high molecular weight compounds include a homologous series of n-alkanes, methylbranched alkanes, alkylcyclopentanes, alkylcyclohexanes, alkylbenzenes and alkylcycloalkanes.     

A highstand oolitic sequence and associated facies from a Late Triassic lake basin, south-west England

High energy, lake‐shoreline carbonate sequences are rarely documented in the geological record. However, one example occurs in the Upper Triassic Mercia Mudstone Group (MMG) of southern Britain. The MMG is one of a number of thick, non‐fossiliferous mudstone deposits associated with North Atlantic Mesozoic rift basins. The origin of the MMG mudstones is the subject of current debate, with marine, playa‐lacustrine and alluvial–aeolian models having been proposed. Shoreline features have been documented from the northern margin of the basin, but the rarity of such features elsewhere in the MMG has led many workers to doubt a lacustrine origin for the mudstones. Wave‐dominated, lake‐shoreline deposits have been recognized in several sections from the southern basin margin in the Clevedon area of the Bristol Channel in south‐west England. These deposits provide evidence for the development of a sizeable perennial to semi‐perennial hypersaline lake in which the MMG mudstones accumulated. Shoreline sediments overlie alluvial stream and sheet‐flood deposits, and pass from transgressive gravel–conglomerate beach units with bioclasts, influenced by shore‐normal waves (deposited under semi‐humid conditions), to lower gradient, highstand oolitic sands affected by more varied wave approach (deposited under progressively more arid conditions), which culminated in lowstand, oolitic strand‐plain deposits overlain by a playa‐mudflat unit. Shoreline deposits record a simple shallowing‐upward transgressive–highstand–lowstand sequence. However, a change from a reflective (transgressive) to dissipative (highstand) shoreline is believed to represent a climatically induced change in prevailing wind direction. Shoreline features recognized in the MMG are similar to those of recent playa‐lacustrine basins of the western United States. Ooids display a variety of size, fracture and dissolution features in addition to beachrock fabrics, suggesting that they were originally composed of radial aragonite, similar to modern ooids from the Great Salt Lake, Utah.     

Interaction of Tectonic and Surface Geological Processes

1. Introduction to surface processesThe shape or morphology of Earth's surface is basically the result of the interplay between two competing forces - mountain building and erosion. Tectonic forces, driven by thermal energy from Earth's interior, cause the rocks of the crust to be folded, faulted and uplifted into high plateaus and mountain belts. As soon as uplift begins, the processes of erosion, driven by gravity, start to wear away the rocks. Masses of weathered rock move downhill under t…     

Are Cosmic Rays Influencing Oceanic Cloud Coverage – Or Is It Only El Niño?

The monthly average (C2) cloudcoverage data produced by the International SatelliteCloud Climatology Project (ISCCP) for the period ofJuly 1986–June 1991 show strong global and regionalcloud coverage variations associated with the ElNiño of 1986–1987. The Pacific Ocean, inparticular, shows strong regional variations in cloudcoverage. These agree well with contemporaneoussatellite observations of broadband shortwave infraredcloud forcing measured by the Earth Radiation BudgetExperiment. Svensmark and Friis-Christensen (1997)noted a similarity between the shape of the timeseries curve of average cloud coverage fraction formid- to low-latitude ocean-areas and the time seriescurve of cosmic ray flux intensity. They proposed acausal relationship – a `missing link' for solarcycle influence on Earth climate. Further spatial andtemporal analysis of the same ISCCP C2 data in thispaper indicates that the cloud coverage variationpatterns are those to be expected for the atmosphericcirculation changes characteristic of El Niño,weakening the case for cosmic rays as a climaticforcing factor.     

`Our resistance will be as transnational as capital': Convergence space and strategy in globalising resistance

This paper analyses the emerging globalising networks of communication, solidarity, and information-sharing between social movements and other resistance formations in opposition to neoliberal globalisation. In particular, the paper focuses upon People's Global Action – a network of various social movemnets and grassroots intitiatives from around the world. The paper argues that such a network represents not an organisation, but rather a convergence space – a heterogenous affinity of common ground between resistance formations wherein certain interests, goals, tactics and strategies converge. The paper analyses how space and strategy are negotiated in these globalising terrains of resistance, and argues that place-specific struggles are engaging with alliances and collaborations across diverse boundaries of gender, generation, class, and ethnicity – globalising the common ground between different struggles. Explaining how the internet has been crucial to the development of these networks, the paper argues that the strategies of such networks can be interpreted, in part, through the Taoist text, The Art of War, especially the Taoist notions of terrain, knowing others, and form/lessness. In addition, the paper argues that convergence spaces are fragmented by important issues of language, power, and mobility.     

Determination of the REE in Geological Reference Materials DTS-1 (Dunite) and PCC-1 (Peridotite) by Ultrasonic and Microconcentric Desolvating Nebulisation ICP-MS

Inductively coupled plasma-mass spectrometry is well suited for the precise, accurate and rapid determination of rare earth elements in most geological samples. However, determination of rare earth elements in certain mantle-derived materials, without applying preconcentration techniques, remains problematical due to low natural concentrations (generally < 1 ng g⁻¹). Consequently, USGS reference materials DTS-1 (a dunite) and PCC-1 (a partially serpentinized harzburgite) have only suggested rather than recommended values for the rare earth elements in reference material compilations. We compared results obtained using two ICP-MS instruments: a U-5000AT ultrasonic nebuliser coupled to a PQ2+ quadrupole ICP-MS and an ELEMENT sector field ICP-MS equipped with a MCN-6000 microconcentric desolvating nebuliser, with the suggested literature values for these two reference materials. Precision and accuracy of analytical methods employed by both instruments were demonstrated by excellent relative standard deviations (< 2%) and inter-laboratory agreement (< 5%) for numerous analyses of BHVO-1 and BIR-1, which are well established with rare earth elements contents at the μg g⁻¹ level. Repeat analyses of DTS-1 and PCC-1 at each laboratory indicate that each method is generally precise to better than 5% at sub-g g⁻¹ levels. Furthermore, values from both instruments generally agree to within 10%. Our DTS-1 and PCC-1 values agree reasonably well with selected data reported in the literature (except for Ce and Sm in DTS-1) but exhibit poorer agreement with reported compilation values. With the demonstrated level of precision and accuracy, we contend that these new values for DTS-1 and PCC-1, generated by two different instruments, are the best estimates of the true whole-rock composition of these samples reported to date.