Sorption of Eu(III)/Cm(III) on Ca-montmorillonite and Na-illite. Part 2: Surface complexation modelling

Sorption edges and isotherms for Eu(III) uptake on Ca-montmorillonite and Na-illite in 0.066 mol/L Ca(ClO₄)₂ and 0.1 mol/L NaClO₄ background electrolytes, respectively, were modelled using a quasi-mechanistic sorption model (the two site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) model). For both clay minerals the Eu sorption edges could be quantitatively modelled in the pH range ∼3 to ∼10 using cation exchange reactions for Eu³⁺/Na⁺ and Eu³⁺/Ca²⁺ and three surface complexation reactions on the strong sorption sites forming ≡S^SOEu²⁺, ≡S^SOEuOH⁺ and ≡S^SOEu(OH)₂° inner sphere complexes which appear successively with increasing pH. Time resolved laser fluorescence spectroscopy (TRLFS) measurements of Cm(III) loaded Ca-montmorillonite and Na-illite were available from Part 1 of this work. De-convolution of the normalised fluorescence spectra measured at different pH values indicated three distinct Cm surface complexes, Cm complexes 1, 2 and 3 for both clay minerals, in agreement with model predictions, but with different distribution functions for the individual species. Under the assumption that Eu and Cm exhibit essentially the same hydrolysis and sorption behaviour, the Eu surface complexation constants were used to predict surface species distribution functions for Cm under the same experimental conditions used in the TRLFS measurements. Comparison of modelled and experimentally deduced species distributions indicated that for both clay minerals peak heights and widths of the three peaks did not correspond particularly well. It is shown that the calculated species distribution functions are sensitive to the values of the hydrolysis constants used in the calculations, whereas modelling the sorption edge measurements by applying the 2SPNE SC/CE approach is much less sensitive. By modifying the values of the hydrolysis constants within their uncertainty range and re-modelling the sorption edges, considerably better correspondence between the modelled and TRLFS species distribution functions was found. In particular, peak positions, heights and widths for the model predicted peaks for the ≡S^SOCm²⁺ and ≡S^SOCmOH⁺ species distribution, and those for Cm complexes 1 and 2 derived from TRLFS, were found to be very close for both clay minerals. However, discrepancies were still apparent between the profile for the calculated ≡S^SOEu(OH)₂° surface species and the Cm complex 3 species, especially in the case of Na-illite.     

Evolving metasomatic agent in the Northern Andean subduction zone, deduced from magma composition of the long-lived Pichincha volcanic complex (Ecuador)

Geochemical studies of long-lived volcanic complexes are crucial for the understanding of the nature and composition of the subduction component of arc magmatism. The Pichincha Volcanic Complex (Northern Andean Volcanic Zone) consists of: (1) an old, highly eroded edifice, the Rucu Pichincha, whose lavas are mostly andesites, erupted from 1,100 to 150 ka; and (2) a younger, essentially dacitic, Guagua Pichincha composite edifice, with three main construction phases (Basal Guagua Pichincha, Toaza, and Cristal) which developed over the last 60 ka. This structural evolution was accompanied by a progressive increase of most incompatible trace element abundances and ratios, as well as by a sharp decrease of fluid-mobile to fluid-immobile element ratios. Geochemical data indicate that fractional crystallization of an amphibole-rich cumulate may account for the evolution from the Guagua Pichincha andesites to dacites. However, in order to explain the transition between the Rucu Pichincha andesites and Guagua Pichincha dacites, the mineralogical and geochemical data indicate the predominance of magma mixing processes between a mafic, trace-element depleted, mantle-derived end-member, and a siliceous, trace-element enriched, adakitic end-member. The systematic variation of trace element abundances and ratios in primitive samples leads us to propose that the Rucu Pichincha magmas came from a hydrous-fluid metasomatized mantle wedge, whereas Guagua Pichincha magmas are related to partial melting of a siliceous-melt metasomatized mantle. This temporal evolution implies a change from dehydration to partial melting of the slab, which may be associated with an increase in the geothermal gradient along the slab due to the presence of the subducted Carnegie Ridge at the subduction system. This work emphasizes the importance of studying arc-magma systems over long periods of time (of at least 1 million of years), in order to evaluate the potential variations of the slab contribution into the mantle source of the arc magmatism.     

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Comparison of two methods for seismic hazard assessment in a low-seismicity area

The EGO method, developed by Egozcue et al. and the SRAMSC method, originally developed by Cornell and later programmed by McGuire, to assess the seismic hazard, are compared for the low seismicity area Belgium, The Netherlands, and NW Germany. Using the same input data, the results of the EGO method without the majority criterion and the SRAMSC method with upper bound XII agree very well. The influence of the zoning is investigated for the EGO method. It is not necessary to define the zones for the EGO method so strictly as for the SRAMSC method, but too wide zones can give bad results.     

Genesis of the Nyiragongo lavas

Mt. Nyiragongo is one of the eight major volcanoes of the large Virunga volcanic field in the Lake Kivu area in the Eastern Congo. The lavas of Nyiragongo are rather unique. Starting from the top of the mountain, the rocks are nephelinites with some leucite and melilite. The molten material of the present-day lava lake belongs to this type of lava. Under the nephelinites, there is a thin series of leucite-rich lava beds. The main part of the volcano consists of bergalitic melitite lavas alternating with pyroclastics of similar composition. The nephelinitic material is considered to constitute the main portion of the pre-volcanic magma under the future volcano. It is pointed out that the Nyiragongo represents just the type of volcano with which the African volcanic carbonatites are associated. It is concluded that the Nyiragongo nephelinite must be interpreted in a way accepted for the Central African volcanic nephelinites in general. The bergalitic melilitite material is interpreted as a product of carbonation of the nephelinitic magma.     

Mesozoic rotation of Iberia: Subduction in the Pyrenees?

Following on paleomagnetic studies in the sixties showing ~ 35° counterclockwise rotation of Iberia during the Mesozoic, two classes of scenarios have been proposed for the motion history of Iberia which are currently competing. One class infers convergence in the Pyrenees in response to a scissor-type opening of the Bay of Biscay, described by a pole of rotation for Iberia with respect to Europe located within the Bay. The other class of scenarios assumes extensional or transtensional motions in the Pyrenees, compatible with opening of the Bay of Biscay described by a pole of rotation located in northern France. Although plate-kinematic studies over the last decade increasingly support the scissor-type model, geological studies in the Pyrenees have accumulated arguments in favour of an extensional or transtensional regime in the Pyrenean realm.We perform a detailed plate-kinematic analysis of the Late Jurassic and Cretaceous motion history of Iberia and surrounding plates with respect to Europe. A total of six sea-floor reconstructions in combination with paleomagnetic studies onland allow to recognize four distinct stages. (1) Early rifting and ultraslow spreading since the Kimmeridgean led to the development of an oceanic Neotethys domain north of Iberia. (2) This was followed by ~ 35° CCW rotation of Iberia during the Aptian, kinematically linked to progressive opening of the Bay of Biscay. (3) Motions in the Bay became stagnant during the Albian till Santonian, followed by the latest stages of spreading in the Bay, and (4) onset of largely Tertiary continental collision between Iberia and Europe eventually leading to the present day structure of the belt.Our analysis confirms the results of previous studies indicating that extensional or transtensional motions in the Pyrenean realm during opening of the Bay of Biscay and concurrent rotation of Iberia are incompatible with plate-kinematic reconstructions based on sea-floor anomalies. This invites a reappraisal of the geological data. Convergence in the Pyrenean realm during opening of the Bay and rotation of Iberia was accommodated by up to 300 km of subduction of mantle-dominated ocean floor exhumed during the late Jurassic and early Cretaceous. The stagnant stage in the progressive opening of the Bay indicates that convergence in the Pyrenean realm virtually came to a halt during the Albian. We hypothesize that the lithosphere previously subducted during Aptian convergence became gravitationally unstable, leading to asthenospheric upwelling and consequent magmatism and high temperature metamorphism in the overlying European margin now exposed in the North Pyrenean Zone. Aside from these magmatic and thermal effects, an enhanced gravitational potential energy of the remaining lithosphere column underlain by shallow asthenosphere may have led to a stress state allowing belt-parallel extensional deformation. Such a detachment scenario, inspired by plate-kinematic results, may provide an alternative to explain many of the geological data commonly quoted to infer a transtensional or extensional tectonic regime in the Pyrenees during the rotation of Iberia.     

Rainfall-triggering response patterns of post-seismic debris flows in the Wenchuan earthquake area

Several giant debris flows occurred in southwestern China after the Wenchuan earthquake, causing serious casualties and economic losses. Debris flows were frequently triggered after the earthquake. A relatively accurate prediction of these post-seismic debris flows can help to reduce the consequent damages. Existing debris flow prediction is almost based on the study of the relationship between post-earthquake debris flows and rainfall. The relationship between the occurrence of post-seismic debris flows and characteristic rainfall patterns was studied in this paper. Fourteen rainfall events related to debris flows that occurred in four watersheds in the Wenchuan earthquake area were collected. By analyzing the rainfall data, characteristics of rainfall events that triggered debris flows after the earthquake were obtained. Both the critical maximum rainfall intensity and average rainfall intensity increased with the time. To describe the critical conditions for debris flow initiation, intensity–duration curves were constructed, which shows how the threshold for triggering debris flows increased each year. The time that the critical rainfall intensities of debris flow occurrences return to the value prior to the earthquake could not be estimated due to the absent rainfall data before the earthquake. Rainfall-triggering response patterns could be distinguished for rainfall-induced debris flows. The critical rainfall patterns related to debris flows could be divided on the basis of antecedent rainfall duration and intensity into three categories: (1) a rapid triggering response pattern, (2) an intermediate triggering response pattern, and (3) a slow triggering response pattern. The triggering response patterns are closely related to the initiation mechanisms of post-earthquake debris flows. The main difference in initiation mechanisms and difference in triggering patterns by rainfall is regulated by the infiltration process and determined by a number of parameters, such as hydro-mechanical soil characteristics, the thickness of the soil, and the slope gradient. In case of a rapid triggering response rainfall pattern, the hydraulic conductivity and initial moisture content are the main impact factors. Runoff erosion and rapid loading of solid material is the dominant process. In case of a rainfall pattern with a slow triggering response, the thickness and strength of the soil, high hydraulic conductivity, and rainfall intensity are the impact factors. Probably slope failure is the most dominant process initiating debris flows. In case of an intermediate triggering response pattern, both debris flow initiation mechanisms (runoff erosion and slope failure) can play a role.     

Refining the timing of eclogite metamorphism: a geochemical,petrological, Sm-Nd and U-Pb case study from the Pohorje Mountains,Slovenia (Eastern Alps)

High-pressure metamorphism in the Pohorje Mountains of Slovenia (Austroalpine unit, Eastern Alps) affected N-MORB type metabasic and metapelitic lithologies. Thermodynamic calculations and equilibrium phase diagrams of kyanite–phengite-bearing eclogites reveal PT conditions of >2.1 GPa at T<750°C, but within the stability field of quartz. Metapelitic eclogite country rocks contain the assemblage garnet + phengite + kyanite + quartz, for which calculated peak pressure conditions are in good agreement with results obtained from eclogite samples. The eclogites contain a single population of spherical zircon with a low Th/U ratio. Combined constraints on the age of metamorphism come from U/Pb zircon as well as garnet–whole rock and mineral–mineral Sm-Nd analyses from eclogites. A coherent cluster of single zircon analyses yields a ²⁰⁶Pb/²³⁸U age of 90.7±1.0 Ma that is in good agreement with results from Sm-Nd garnet–whole rock regression of 90.7±3.9 and 90.1±2.0 Ma (εNd: +8) for two eclogite samples. The agreement between U-Pb and Sm-Nd age data strongly suggests an age of approximately 90 Ma for the pressure peak of the eclogites in the Pohorje Mountains. The presence of garnet, omphacite and quartz inclusions in unfractured zircon indicates high-pressure rather than ultrahigh pressure conditions. The analysed metapelite sample yields a Sm-Nd garnet–whole rock scatterchron age of 97±15 Ma. These data probably support a single P-T loop for mafic and pelitic lithologies of the Pohorje area and a late Cretaceous high-pressure event that affected the entire easternmost Austroalpine basement including the Koralpe and Saualpe eclogite type locality in the course of the complex collision of the Apulian microplate and Europe.     

Magnetic Characterization of Amphibolites from the Fomopéa Pluton (West Cameroon): Their Implication in the Pan-African Deformation of the Central African Fold Belt

The Fomopea granitic pluton is emplaced in gnessic and amphibolitic basement.These gneissic and amphibolitic basement rocks are represented in the pluton's body as sub-rounded,elongated or stretched xe...