Chemical Characterisation of Chromites by XRF Spectrometry

The present study was undertaken to establish and validate a methodology for determining the chemical composition of chromite ores, as no detailed information could be found in the literature. Chromite ores from South Africa, Kazajistan and Mongolia were analysed. X-ray fluorescence (XRF) spectrometry was used for chemical characterisation. Sample preparation in the form of fused beads was optimised in terms of sample:flux ratio, type of flux, type of release agent, fusion temperature and time. Calibration and validation samples were prepared from mixtures of reference materials. The proposed methodology was then used in an interlaboratory test for preparing a new chromite-rich reference material.
L'analyse présentée id a été entreprise afin d'établir et de valider une méthodologie pour déterminer la composition chimique de minerals de chromite, car aucune information détaillée là dessus ne pouvait etre trouvée dans la littérature. Des minerals de chromite d'Afrique du Sud, du Kazakhstan et de Mongolie ont été analysés par spectrométrie de Fluorescence de Rayons X (XRF) afin d'être caractérisés chimiquement. La préparation des échantillons afin d'obtenir des perles fondues a été optimisée en termes du rapport "échantillon/fondant", type de fondant, d'agent mouillant, température et durée de fusion. La calibration et la validation ont été faites à partir de mixtures de matériau de référence. La méthodologie proposée a été ensuite appliquée à un test d'inter-comparaison en vue de préparer un nouveau matériau de référence riche en chromite .     

High-Ti amphibole as a petrogenetic indicator of magma chemistry: evidence for mildly alkalic-hybrid melts during evolution of Variscan basic–ultrabasic magmatism of Central Iberia

Central Iberian Variscan granite batholiths and anatectic complexes are punctuated by coeval stocks of hydrous, high-K calc-alkaline, ultrabasic to intermediate rock series. Despite their overall calc-alkaline affinity, the mafic–ultramafic members contain high-Ti amphibole oikocrysts rimmed by lower-Ti amphibole ± cummingtonite and high-Ti amphibole replacing early phlogopite. To understand the factors controlling the saturation of high-Ti amphibole in the parental magmas, clinopyroxene-melt, phlogopite-melt and amphibole-melt relationships are reviewed. This analysis reveals that for melts with intermediate compositions, the affinity of TiO₂ for amphibole rises in alkalic magmas. Accordingly, mildly alkalic trachytoid to subalkaline medium- to high-K andesite and dacite compositions are estimated for interstitial high-Ti amphibole-saturated melts. Amphibole Ce/Pb ratios reveal a mantle–crust hybrid nature for interstitial melts with subalkaline trachytoid compositions. The hydrous character of the Variscan basic magmas favoured an overall magmatic evolutionary trend with a low rate of variation of Na₂O with respect to silica during amphibole crystallization. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Feasibility of Monitoring the Hontomín (Burgos, Spain) CO2 Storage Site Using a Deep EM Source

Geophysical methods have been used experimentally during the last decade, a period of strong development, being adopted as complementary techniques for characterizing and monitoring hydrocarbon and gas reservoirs. In this study, we evaluated the ability of the controlled source electromagnetic (CSEM) method to monitor the storage of CO₂ at the Research Laboratory on Geological Storage of CO₂ at Hontomín (Burgos, Spain). Two aspects of the CSEM monitoring were examined considering the geoelectrical structure at the site, the technological constraints and the noise conditions of the Hontomín area. Borehole-to-surface simulations were performed to evaluate the detectability of the resistivity changes in the reservoir and the capacity to determine the location of the CO₂ plume. The synthetic time-lapse study explores the possibilities of CSEM monitoring with a deep electric source. Three depths of the source are analyzed: above the plume, inside the plume, and beneath the stored CO₂. In terms of the Hontomín storage site, the study confirmed that a deep electric source located beneath the injection depth can provide valuable information on the behavior of the stored CO_2.     

Seasonal and storm flow dynamics of dissolved organic carbon in a Mediterranean mountain catchment (Vallcebre,eastern Pyrenees)

To improve understanding of DOC dynamics in seasonal Mediterranean environments, rainfall, soil water, groundwater and stream water samples were taken during a 27-month period in the Can Vila catchment (northeast Spain). Using these data, we characterized DOC dynamics in the different hydrological compartments and analysed the factors affecting them. We also analysed DOC dynamics during storm events and the factors that control DOC delivery to the stream. The results show some seasonality in rainwater and soil water DOC concentrations, while no clear seasonality was observed in stream water and groundwater, where DOC dynamics were strongly related to discharge and water table variations. For storm events with several discharge peaks, the slope of the discharge–DOC concentration relationship was higher for the first peak. The rather similar dynamics of stream water DOC concentration in all floods contrast with the observed diversity of hydrological processes. This raises the question of the origin of the observed rapid DOC increase.

EDITOR M.C. Acreman

ASSOCIATE EDITOR K. Heal     

Marbles from Roman Hispania: stable isotope and cathodoluminescence characterization

Pure white marble has been considered a valuable ornamental and architectural material since ancient times. Many scientific techniques have been used to create an extensive data base of “finger-prints” characterizing white marbles from the major classical quarries. However, determining the provenance of white marbles is a difficult task due to their similarity in physical and chemical parameters. Three techniques (petrography, cathodoluminescence and stable C and O isotopes) have been used to characterize white marbles from the ancient quarries of the Iberian Peninsula. Maximum grain size, texture and isotopic composition can be used to identify the different quarries. Each area is generally represented by several cathodomicrofacies, but quantitative CL analysis is also helpful in distinguishing those quarries for which the data provided by other techniques are not sufficiently diagnostic. The database and the discriminating criteria presented in this study have been tentatively applied to some ancient sculptures from the National Museum of Roman Art in Merida (Spain).     

Evolution of Heterogeneous Mantle in the Acampamento Velho and Rodeio Velho Volcanic Events, Camaquã Basin, Southern Brazil

The Camaquã Basin, developed during the last phases of the Brazilian/Pan-African Orogeny and was filled with a thick volcano-sedimentary succesion, in which two volcanic events of alkaline affinity are represented by the Acampamento Velho Alloformation and the Rodeio Velho Member. The Acampamento Velho Alloformation records a bimodal event with a lower association of mafic flows and an upper association of felsic pyroclastic rocks and flows. It was formed during extension, after the subduction of the Adamastor oceanic plate beneath the Rio de La Plata continental plate at the end of the Neoproterozoic III. The second event, the Rodeio Velho Member, represented by mafic flows, intrusions and piroclasts, took place during overall extensional tectonism, probably in the middle Ordovician. Rb, Sr, Sm, and Nd isotopic measurements were carried out on samples from both units. Regardless the event they represent, all the samples display negative values for epsilon Nd, ranging from 2.97 to 10.31 for the Acampamento Velho Alloformation and from 8.39 to 13.92 for the Rodeio Velho Member. The initial ⁸⁷Sr/⁸⁶Sr ratios vary from 0.706 to 0.707 and from 0.704 to 0.707 for the Acampamento Velho Alloformation and Rodeio Velho Member, respectively. Mafic flow deposits in both units show a preferential enrichment in Ba relative to Th. Flow samples from the Rodeio Velho Member also display a distinctive enrichment in the Ba/Th ratio, without a change in the initial Sr, compared to the mafic flow deposits from the Acampamento Velho Alloformation, which show a slight enrichment in those ratios. As for the Acampamento Velho Alloformation, the mafic lavas could be a mixture of depleted mantle-derived basalts plus 20% to 30% of crustal contamination by sediment (probably Neoproterozoic arkosic quartzites). The formation of a magmatic chamber and the separation of the magma into two fractions gave rise initially to the mafic rocks at the base of the Acampamento Velho Alloformation The other magma fraction underwent a significant enrichment in crustal component before the felsic rocks of this Alloformation were formed. The flows from the Rodeio Velho Member originated in a distinct magma chamber, with EM I characteristics that was much more enriched in incompatible elements and depleted in radiogenic Sr.     

The North Maladeta Fault (Spanish Central Pyrenees) as the Vielha 1923 earthquake seismic source: Recent activity revealed by geomorphological and geophysical research

The Spanish Central Pyrenees have been the scenario of at least two damaging earthquakes in the last 800 years. Analysis of macroseismic data of the most recent one, the Vielha earthquake (19 November 1923), has led to the identification of the North Maladeta Fault (NMF) as the seismic source of the event. This E–W trending fault defines the northern boundary of the Maladeta Batholith and corresponds to a segment of the Alpine Gavarnie thrust fault. Our study shows that the NMF offsets a reference Neogene peneplain. The maximum observed vertical displacement is  730 m, with the northern downthrown sector slightly tilting towards the South. This offset provides evidence of normal faulting and together with the presence of tectonic faceted spurs allowed us to geomorphically identify a fault trace of 17.5 km. This length suggests that a maximum earthquake of M_w = 6.5 ± 0.66 could occur in the area. The geomorphological study was improved with a resistivity model obtained at Prüedo, where a unique detritic Late Miocene sequence crops out adjacent to the NMF. The section is made up of 13 audiomagnetotelluric soundings along a 1.5 km transect perpendicular to the fault trace at Prüedo and reveals the structure in depth, allowing us to interpret the Late Miocene deposits as tectonically trapped basin deposits associated with normal faulting of the NMF. The indirect age of these deposits has been constrained between 11.1 and 8.7 Ma, which represents a minimum age for the elevated Pyrenean peneplain in this part of the Pyrenees. Therefore, we propose the maximum vertical dip-slip rate for the NMF to be between 0.06 and 0.08 mm/a. Normal faulting in this area is attributed to the vertical lithospheric stress associated with the thickened Pyrenean crust.     

Spatial variability of soil quality in the surroundings of a saline lake environment

The spatial variability of the quality of the soil on the shores of a lagoon affected by hydromorphy and/or salinity can be identified (Laguna de Villacañas, Castilla La Mancha) by the use of multitemporal Landsat images in order to analyse these changes. For this purpose, TM and ETM images along with field observations and certain edaphic laboratory parameters are used. In order to identify saline-hydromorphic soils, the spatial variability of chemical and physical properties of a transect, which includes from Solonchaks to Regosols and Cambisols, have been correlated with the Normalised Difference Vegetation Index (NDVI). This index, chosen for specific dates, has proven to be very useful in detecting halophytic vegetation and relating it to the variability of the quality of these soils.