Benoît Villemant

Trace element distribution coefficients in alkaline series

Mineral/groundmass partition coefficients for U, Th, Zr, Hf, Ta, Rb, REE, Co and Sc have been systematically measured in olivine, clinopyroxene, amphibole, biotite, Ti-magnetites, titanite, zircon and feldspars, in basaltic to trachytic lavas from alkaline series (Velay, Chaine des Puys: Massif Central, France and Fayal: Azores). Average partition coefficients are denned within the experimental uncertainty for limited compositional ranges (basalt-hawaiite, mugearites, benmoreite-trachyte), and are useful for trace element modelling. The new results for U, Th, Ta, Zr and Hf partition coefficients show contrasting behaviour. They can thus be used as “key elements” for identifying fractionating mineral phases in differentiation processes (e.g. Ta and Th for amphibole and mica). Partition coefficient may be calculated using the two-lattice model suggested by Nielsen (1985). Such values show a considerably reduced chemical dependence in natural systems, relative to weight per cent D values. The residual variations may be accounted for by temperature or volatile influence. This calculation greatly enhances modelling possibilities using trace elements for comparing differentiation series as well as for predicting the behaviour of elements during magmatic differentiation.

Evolution of magma mixing in an alkaline suite: the Grande Cascade sequence (Monts-Dore, French Massif Central). Geochemical modelling

Abstract Trachyandesitic lavas from an alkaline suite (Grande Cascade sequence, Monts-Dore, French Massif Central) have been investigated for evidence of mechanical magma mixing. A wide variety of textures are present in the sequence, as pillow-like enclaves, bandings and emulsified facies. The Grande Cascade flows and pyroclasts range from basalt to trachyte (44–67 wt.% SiO 2 ). Pillow-like enclaves, intricate commingling and local diffusive mixing at the mafic-felsic interface in emulsified types, show that mafic inclusions were incorporated in their trachytic host as blobs of magma. Such inclusions were probably dispersed throughout the trachytic magma by mechanical mixing. Different stages of the evolution of a mechanical mixing process were recognized in relation with the stratigraphy (and chronology) of the products: the most heterogeneous features (enclaves, bandings) are restricted to the bottom of the sequence, where basalt and trachyte coexist; at the top, texturally homogeneous trachyandesites occur. Relict phenocrysts are observed in both basaltic enclaves (oligoclase, Fe-rich clinopyroxene, sanidine of the trachyte) and trachytic hosts (Mg-rich olivine, Mg-rich clinopyroxene, Ca-rich plagioclase of basalt). The intermediate rocks (basic trachyandesites) contain xenocrysts of Mg-rich olivine, clinopyroxenes, oligoclase, sanidine, together with reversely zoned plagioclase and pyroxene phenocrysts. This assemblage is presumed to result from the incorporation of xenocrysts related both to mafic and felsic endmembers. All mineral assemblages are out of equilibrium. Reversed compositional zoning profiles in plagioclase and clinopyroxene crystals were produced by the mixing event. We are able to identify the compositional characteristics of the components that were mixed. A primary alkaline basalt (Mg-olivine, labrador, Mg-clinopyroxene, magnetite) and a silicic trachyte (oligoclase, Fe-clinopyroxene, sanidine) are the endmembers of mixing. Observed chemical variations in major and trace elements can be modelled as pure binary mixtures between basaltic and trachytic endmembers. Using the “best mixing line” method (Provost and Allegre, 1979), the calculations emphasize that trachyandesites are the result of complete hybridization between basalt and trachyte. The data on mesostasis show that melt mixing (and not only crystal transfers) has produced the intermediate lavas of the Grande Cascade sequence. Textural criteria, petrography, mineral chemistry and geochemical data (major and trace elements) support the magma mixing model.

Wall rock-magma interactions in Etna, Italy, studied by UTh disequilibrium and rare earth element systematics

Abstract 230 Th 238 U disequilibria have been studied in xenoliths and associated lavas of the 1892 and 1989 eruptions of Etna. Most xenoliths are out of secular equilibrium within 1σ errors and have lower 230 Th 232 Th ratios than their host magmas. Siliceous and peraluminous xenoliths display large ranges of Th U ratios for similar 230 Th 232 Th values, which are interpreted in terms of Th isotopic rehomogenization. The siliceous xenoliths have also suffered thorium and uranium enrichments, which are best explained by chemical diffusion between xenolith melts and differentiated magmas. Estimates of thorium self-diffusivities and 230Th-238U disequilibria give age constraints on these events corresponding to the last major volcanic event of Etna at 14 ka (formation of the elliptic crater caldera). These results suggest that magma storage in superficial and long-lived magma chambers favours the thorium isotopic homogenization of wall rocks by a thermal effect. Chemical diffusion of uranium and thorium and isotopic homogenization between siliceous melts of wall rocks and differentiated magmas may significantly modify the initial thorium isotopic compositions. Such contamination processes could explain the large variations of the 230 Th 232 Th initial ratios of Etna magmas.

A simple and reliable anion exchange resin method for sulfate extraction and purification suitable for multiple O- and S- isotope measurements.

RATIONALE The O- and S-isotope compositions of sulfates can be used as key tracers of the fate and sink of sulfate in both terrestrial and extra-terrestrial environments. However, their application remains limited in those geological systems where sulfate occurs in low concentrations. Here we present a simple and reliable method to extract, purify and concentrate sulfate from natural samples. The method allows us to take into account the separation of nitrate, which is known to be an issue in O-isotope analysis. METHODS The separation and concentration of sulfate from other anions in any aqueous solution are performed within a few hours via anion-exchange resin. The possible O- (δ18 O and Δ17 O) and S- (δ34 S, Δ33 S and Δ36 S) isotope exchanges, fractionations and/or contaminations are for the first time monitored during the whole procedure using initial O- and S-mass-dependent and mass-independent sulfate solutions. RESULTS After elution in HCl, pure sulfate is fully retrieved and precipitated into BaSO4 , which is suitable for O- and S-isotopic measurements using established techniques. The analysis of retrieved barite presents no variation within 2σ uncertainties: ±0.5‰ and ±0.1‰ in O- (δ18 O, Δ17 O) and ±0.2‰, ±0.02‰ and ±0.09‰ in S- (δ34 S, Δ33 S and Δ36 S) isotope ratios, respectively. CONCLUSIONS This study shows that the resin method for sulfate extraction and purification, in addition to being cheap, simple and quick, is applicable for the measurements of all O- and S-isotopic ratios in sulfates (including the Δ17 O, Δ33 S and Δ36 S values). Therefore, this method can be easily used for a high range of natural samples in which sulfate occurs in low concentration including aerosols, ice cores, sediments, volcanic deposits, (paleo)soils and rainwater, and thus it can be a key to our understanding of the sulfur cycle on Earth. Copyright

Magmatic Evolution of Pavin’s Group of Volcanoes: Petrology, Geochemistry and Modeling of Differentiation Processes. A Preliminary Study

The volcanoes of the Pavin group (Montcineyre, Estivadoux, Montchal and Pavin) represent the most recent (~7 ky) volcanic activity in the Chaine des Puys s.l.) and also occupy a particular geographical and structural position in this volcanic chain. Petrology and geochemistry of the volcanic products (lava flows, scoriae, bombs and pumice clasts) of the different edifices show that they define a magma differentiation series from primary basalts (Montcineyre lava flow) to benmoreites (Pavin pumice clasts) with close similarities with that of the Chaine des Puys (stricto sensu). It is however characterized by some significant difference in the relative behavior of K2O and of numerous trace elements such as Sc, Co, Y, REE, Nb, Ta. These specific behaviors, in particular the relatively high bulk partition coefficients of Nb and Ta are characteristic of the fractionation of amphibole which is the only major mineral phase able to significantly fractionate these incompatible elements in basaltic to benmoreitic magmas. A quantitative modeling of the differentiation process using major and trace element partition coefficients is proposed which evidence the very early fractionation of amphibole from the most primary basaltic melts. This result is consistent with petrological observations and confirms the earlier and more efficient fractionation of amphibole at the expense of clinopyroxene in magmas of the Pavin group compared to those of the Chaine des Puys (stricto sensu). The results are used to discuss the influence of these specific differentiation conditions involving large H2O contents in primary melts likely contributing to the particularly high explosivity of the Pavin eruption which produced benmoreitic to trachytic magmas.