Experimental petrology of alkalic lavas: constraints on cotectics of multiple saturation in natural basic liquids

Experimental study of natural alkalic lava compositions at low pressures (pO₂QFM) reveals that crystallization of primitive lavas often occurs in the sequence olivine, plagioclase, clinopyroxene, nepheline without obvious reaction relation. Pseudoternary liquidus projections of multiply saturated liquids coexisting with plagioclase (±olivine±clinopyroxene±nepheline) have been prepared to facilitate graphical analysis of the evolution of lava compositions during hypabyssal cooling. Use of (TiAl₂)(MgSi₂)_–1 and Fe³⁺ (Al)_–1 exchange components is a key aspect of the projection procedure which is succesful in reducing a wide range of compositions to a systematic graphical representation. These projections, and the experiments on which they are based, show that low pressure fractionation plays a significant role in the petrogenesis of many alkalic lava suites from both continental and oceanic settings. However, the role of polybaric fractionation is more evident in the major element chemistry of these lava suites than in many tholeiitic suites of comparable extent. For example, the lavas of Karisimbi, East Africa, show a range of compositions reflecting a polybaric petrogenesis from primitive picrites at 1360° C/18 kb and leading to advanced low pressure differentiates. Evolved leucite-bearing potassic members of this and other suites may be treated in a nepheline-diopside-kspar (+olivine+leucite) projection. Compositional curvature on the plagioclase+clinopyroxene+olivine+leucite cotectic offers a mechanism to explain resorption of plagioclase in alkalic groundmass assemblages and the incompatibility of albite and leucite. This projection is useful for evaluating the extent of assimilation of the alkalic portions of crustal granulites. Assimilation appears to have played some role in the advanced differentiates from Karisimbi.     

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The suitability of apatite as an age indicator by the uranium-lead isotope method

Experimental data obtained for 14 samples from the granitic terrain surrounding the Barberton Mountain Land in the Republic of South Africa is used to demonstrate that the common accessory mineral, apatite, is suitable for age measurements by the U-Pb isotope method and may in many cases be a more reliable age indicator than the co-existing zircon. The ages calculated for apatite separated from the granitic rocks are in most cases concordant or nearly so (the biggest difference between two uranium-lead ages exhibited being less than 7%); while the ages calculated for co-existing zircon are mostly grossly disconcordant. It is proposed that the theoretical explanation for this observation is found in the crystal chemical stability of the apatite structure as compared to that of zircon. Zircon is probably much more susceptible to loss of members of the U-Pb decay system especially when metamict.     

Revealing the energy sources of alternating intensity regimes of the evolving Alberto tropical cyclone using microwave satellite sensing data

The trajectory characteristics and the intensity evolution of the Alberto tropical cyclone (TC) in the integral water-vapor field are analyzed in detail using a software-algorithmic complex designed for constructing highly detailed global radio-thermal fields of the ocean-atmosphere system based on microwave satellite measurements. This complex was developed by the authors earlier and allows one to obtain global animated radio-thermal fields with a time step of 1.5 h and a spatial resolution of 0.2°. Special attention is paid to the problem of revealing the energy sources of three consecutive intensifications of this TC. The analysis of satellite data with the use of the animation method developed by the authors shows for the first time that these sources are regions with an increased (exceeding the critical level) integral water-vapor content located in different geographical areas lying along the complex trajectory of the TC with a variable intensity. The first region was located in the equatorial intertropical convergence zone, the second was located above the Gulf Stream water area, and the third resided in the area affected by the Azores High.     

Quaternary acid magma in New Zealand

The voluminous Pleistocene—Recent Taupo rhyolites typically contain phenocrysts of plagioclase (oligoclase-labradorite), quartz, titanomagnetite, ilmenite, and ferromagnesian silicates. Ferromagnesian assemblages correlate with well defined Fe-Ti oxide equilibration temperature ranges and allow the rhyolites to be subdivided as follows: (1) Cummingtonite (c)—calcic hornblende (hb)—orthopyroxene (opx); 725–755°C, (2) Hb-opx, 750–825°C, (3) Biotite-hb-(c-opx), 720–765°C, (4) Opx-clinopyroxene (cpx), 860–915°C, (5) Fe olivine-opx-cpx, one sample with temperature of 900°C. Plagioclase and orthopyroxene phenocryst compositions typically exhibit a range of composition up to ～20 mol.%. Calculated average phenocryst equilibration pressures (P _total) range between 0.5–4.9 kb, and average 2.2 kb (～7–8 km depth), indicating upper crustal crystallization. These calculations are very sensitive to variations in phenocryst composition. Calculated \(/_{{\text{H}}_2 {\text{O}}} \) for the amphibole and biotite-bearing rhyolites indicate phenocryst equilibration under \(P_{{\text{H}}_2 {\text{O}}} \simeq P_{{\text{total}}} \) , with \(X_{{\text{H}}_2 {\text{O}}} \) ～0.17–0.24 (5–8 wt. %). The precipitation of cummingtonite is thus temperature dependent, the upper limit being close to 760°C. Eruptive mechanisms of the lavas, pumices, and ash-flow deposits are evidently not primarily controlled by temperature, P _total, \(P_{{\text{H}}_2 {\text{O}}} \) , or crystal content of the magmas, and explanations must lie in kinetic and fluid dynamic behavior of the magmas. For the Taupo rhyolites, it is suggested that the critical size of a magma body (i.e. Rayleigh number) is a controlling factor in that it will influence the convective regime; fully turbulent convection is deduced to have occurred within the larger magma bodies. One consequence is intense vesiculation, prior to eruption, within the uppermost zones of these magma chambers, and the voluminous pumice deposits are believed to emanate from such chambers. Oscillatory compositional zoning within pyroxene phenocrysts is consistent with magma convection.     

Radium uptake during barite recrystallization at 23 ± 2 °C as a function of solution composition: An experimental Ba and Ra tracer study

High-purity synthetic barite powder was added to pure water or aqueous solutions of soluble salts (BaCl₂, Na₂SO₄, NaCl and NaHCO₃) at 23 ± 2 °C and atmospheric pressure. After a short pre-equilibration time (4 h) the suspensions were spiked either with ¹³³Ba or ²²⁶Ra and reacted under constant agitation during 120-406 days. The pH values ranged from 4 to 8 and solid to liquid (S/L) ratios varied from 0.01 to 5 g/l. The uptake of the radiotracers by barite was monitored through repeated sampling of the aqueous solutions and radiometric analysis. For both ¹³³Ba and ²²⁶Ra, our data consistently showed a continuous, slow decrease of radioactivity in the aqueous phase.Mass balance calculations indicated that the removal of ¹³³Ba activity from aqueous solution cannot be explained by surface adsorption only, as it largely exceeded the 100% monolayer coverage limit. This result was a strong argument in favor of recrystallization (driven by a dissolution-precipitation mechanism) as the main uptake mechanism. Because complete isotopic equilibration between aqueous solution and barite was approached or even reached in some experiments, we concluded that during the reaction all or substantial fractions of the initial solid had been replaced by newly formed barite.The ¹³³Ba data could be successfully fitted assuming constant recrystallization rates and homogeneous distribution of the tracer into the newly formed barite. An alternative model based on partial equilibrium of ¹³³Ba with the mineral surface (without internal isotopic equilibration of the solid) could not reproduce the measured activity data, unless multistage recrystallization kinetics was assumed. Calculated recrystallization rates in the salt solutions ranged from 2.8 × 10⁻¹¹ to 1.9 × 10⁻¹⁰ mol m⁻² s⁻¹ (2.4-16 μmol m⁻² d⁻¹), with no specific trend related to solution composition. For the suspensions prepared in pure water, significantly higher rates (∼5.7 × 10⁻¹⁰ mol m⁻² s⁻¹ or ∼49 μmol m⁻² d⁻¹) were determined.Radium uptake by barite was determined by monitoring the decrease of ²²⁶Ra activity in the aqueous solution with alpha spectrometry, after filtration of the suspensions and sintering. The evaluation of the Ra uptake experiments, in conjunction with the recrystallization data, consistently indicated formation of non-ideal solid solutions, with moderately high Margules parameters (W_AB = 3720-6200 J/mol, a₀ = 1.5-2.5). These parameters are significantly larger than an estimated value from the literature (W_AB = 1240 J/mol, a₀ = 0.5).In conclusion, our results confirm that radium forms solid solutions with barite at fast kinetic rates and in complete thermodynamic equilibrium with the aqueous solutions. Moreover, this study provides quantitative thermodynamic data that can be used for the calculation of radium concentration limits in environmentally relevant systems, such as radioactive waste repositories and uranium mill tailings.     

A 44-year high-resolution ocean and atmospheric hindcast for the Mediterranean Basin developed within the HIPOCAS Project

A set of 44-year (1958–2001) homogeneous and high-resolution hindcasts of atmospheric, sea level residuals, and wave states was performed for the Mediterranean Basin within the framework of the HIPOCAS European Project. To this aim, different numerical models were used. As a first step, a Mediterranean high-resolution atmospheric database, suitable to provide realistic and homogeneous forcing for ocean hindcast runs was generated. The HIPOCAS atmospheric database was created by means of dynamical downscaling from the global reanalysis NCEP, using for that the limited area model SN-REMO along with a spectral nudging technique. In a second stage, different Mediterranean oceanic hindcasts were performed. On one hand a long-term database of sea state over the western Mediterranean was generated by means of the wave model WAM and on the other hand a sea level residual database containing storm surge events was obtained from a long-term integration of the HAMSOM model over the entire basin. The three different hindcast runs have been exhaustively validated. On that score, various simulated parameters have been compared to both satellite and in situ measurements. Such comparisons provide a measure of the skills of the different simulated fields to realistically reproduce the observed features. Once these skills are evaluated, a study of the ocean and atmospheric climate trends as well as the interannual variability for the whole 44-year period was carried out with the hindcasted data. The reliability of the data as shown by its comparison to measurements and a proven temporal homogeneity over the 44 years of simulation make the Mediterranean HIPOCAS ocean–atmosphere hindcasted database a useful tool for studies focused on regional climatic variability, as well as for further applications in coastal and environmental decision processes in the Mediterranean area.     

Geological structure of the Curonian Spit (of the Baltic Sea) and its evolution history (revised)

The origin and evolution history of the Curonian Spit in the Baltic Sea are discussed based on geological and geomorphologic data. Evidence of the correlation between the spit formation stages and the Holocene sea level oscillations is presented. The ledges of moraine basement and marine accumulative features formed a single barrier. Its subsequent transformation occurred under the influence of the wave-related lithodynamic and eolian processes.     

Late Mesozoic magmatism and Cenozoic tectonic deformations of the Barents Sea continental margin: Effect on hydrocarbon potential distribution

The paper is focused on the two tectonic-geodynamic factors that made the most appreciable contribution to the transformation of the lithospheric and hydrocarbon potential distribution at the Barents Sea continental margin: Jurassic-Cretaceous basaltic magmatism and the Cenozoic tectonic deformations. The manifestations of Jurassic-Cretaceous basaltic magmatism in the sedimentary cover of the Barents Sea continental margin have been recorded using geological and geophysical techniques. Anomalous seismic units related to basaltic sills hosted in terrigenous sequences are traced in plan view as a tongue from Franz Josef Land Archipelago far to the south along the East Barents Trough System close to its depocentral zone with the transformed thinned Earth’s crust. The Barents Sea igneous province has been contoured. The results of seismic stratigraphy analysis and timing of basaltic rock occurrences indicate with a high probability that the local structures of the hydrocarbon (HC) fields and the Stockman-Lunin Saddle proper were formed and grew almost synchronously with intrusive magmatic activity. The second, no less significant multitectonic stress factor is largely related to the Cenozoic stage of evolution, when the development of oceanic basins was inseparably linked with the Barents Sea margin. The petrophysical properties of rocks from the insular and continental peripheries of the Barents Sea shelf are substantially distinct as evidence for intensification of tectonic processes in the northwestern margin segment. These distinctions are directly reflected in HC potential distribution.