Joël Lancelot

Dissolution kinetics of basaltic glasses: control by solution chemistry and protective effect of the alteration film

Basaltic glasses are considered as natural analogs for industrial nuclear aluminoborosilicate glasses. Alteration experiments were conducted in closed and open systems at 90°C with a synthetic basalt glass doped with 1% lithium (dissolution tracer). The evolution of the alteration kinetics over time was assessed by comparison of reaction progress at different degrees in closed system experiments. The maximum dissolution rate (initial rate, r0) was comparable to the value observed for an SON68-type nuclear glass; the basaltic glass alteration rate subsequently dropped by four orders of magnitude. The kinetic models currently proposed in the literature to account for the alteration kinetics of basaltic glasses, nuclear glasses or aluminosilicate minerals are based on the concept of chemical affinity: the chemical affinity alone is assumed to control the dissolution kinetics. When applied to the experimental data for the closed system tests with basaltic glass, these models failed to account for the low measured rates. An inhibiting effect of dissolved silica was then investigated through open system basaltic glass alteration experiments with silicon-enriched solutions. The basaltic glass dissolution rate dropped by a factor not exceeding 200 (or about two orders of magnitude) compared with r0 at the high imposed silicon concentrations (130 ppm). A protective effect of the alteration film was advanced to account for the four-orders-of-magnitude rate drop observed in closed system experiments, based notably on an examination of natural basaltic glasses ranging in age from several thousand to a few million years. The mean alteration rates can be estimated from the measured palagonite thicknesses, taking into account the age of the natural glasses; the rates were comparable to those measured in the laboratory for high reaction progress.

Outline of Upper Precambrian and Lower Paleozoic evolution of the Iberian Peninsula according to U−Pb dating of zircons

In the northwestern part of the Iberian Peninsula, recent geological studies have defined the following stratigraphic sequence from bottom to top: basal orthogneiss of Miranda do Douro is overlain by a complex of schist and greywacke which have been assigned to the Cambrian or late Precambrian; these are followed by the Ollo de Sapo gneisses of volcanic-sedimentary origin which in turn are overlain by the fossiliferous layers of the lower Ordovician. The schist and greywacke complex and the lower part of the Ollo de Sapo Formation are transected by granitic dikes (Viana del Bollo orthogneiss). All these rocks were folded, faulted, metamorphosed, and intruded by granite plutons during Variscan times. The emplacement of the Miranda do Douro and Viana del Bollo orthogneisses have been respectively dated at 618±9 Ma and 465±10 Ma by the U−Pb method on zircons. These data yield lower and upper limits for the deposition of the Ollo de Sapo Formation and demonstrate the occurrence of a Precambrian basement (“Pan-African” or “Cadomian” orogeny) in the Variscan belt in the northwestern part of the Iberian Peninsula. The emplacement of Viana del Bollo orthogneiss could be genetically related to the main phase of crustal thinning (470–520 Ma) characteristic of the Prevariscan evolution of western Europe. Typology and U−Pb data of the Ollo de Sapo zircons indicate the occurrence of inherited old Precambrian zircons and yield an age of 325±3 Ma for the Variscan main thermal event in the area considered. Four possible evolutionary models for the U−Pb system of the Ollo de Sapo zircons are discussed.

Early Archaean component ( > 3.5 Ga) within a 3.05 Ga orthogneiss from northern Nigeria: UPb zircon evidence

Abstract Conventional UPb analyses on selected single zircon grains and on zircon fractions from a granodioritic orthogneiss at Kaduna (northern Nigeria) show that the Nigerian basement contains Archaean components that have not been recognized until now. A distinct U-rich zircon group records an age of 3040 ± 60 Ma, which is interpreted as the age of emplacement and crystallization of the granodioritic magma. The occurrence of a magmatic event at ca. 3000 Ma is confirmed by the addition of core-free abraded grains (3050 ± 23 Ma). These ages are identical to the less precise value (2980 ± 90 Ma) given by zircon fractions. These results also demonstrate that zircons underwent Pb loss during the Pan-African orogeny at 612 ± 23 Ma, which is well supported by an identical age on a single monazite grain (618 ± 8 Ma). Moreover, UPb analyses of abraded U-poor core-bearing crystals clearly identify the presence of source material of early Archaean age ( > 3500 Ma), the oldest age for crust initiating processes ever found in this area. In contrast to earlier RbSr geochronology no evidence for an Eburnean (2000 ± 200 Ma) or Kibaran (1100 ± 200 Ma) orogeny is present in the UPb systems of these zircons.

Basaltic glass: alteration mechanisms and analogy with nuclear waste glasses

A synthetic basaltic glass was dissolved experimentally at 90°C under static conditions in initially pure water. The basaltic glass dissolution rates measured near and far from equilibrium were compared with those of SON 68 nuclear waste glass. Experimental and literature data notably suggested that the alteration mechanisms for the two glasses are initially similar. Under steady-state concentration conditions, the alteration rate decreased of four orders of magnitude below the initial rate (r0). The same alteration rate decrease was observed for basaltic and nuclear glass. These findings tend to corroborate the analogy of the two glasses alteration kinetics. The effect of dissolved silica in solution, observed through dynamic leach tests with silicon-rich solutions, cannot account for the significant drop in the basaltic glass kinetics. Hence, a protective effect of the glass alteration film was assumed and experimentally investigated. Moreover, modeling with lixiver argue for a significant effect of diffusion in the alteration gel.

Charnockitic and monzonitic Pan-African series from north-central Nigeria: Trace-element and Nd, Sr, Pb isotope constraints on their petrogenesis

Abstract Major- and trace-element studies indicate that the evolution of the calc-alkaline (charnockitic) and sub-alkaline (monzonitic) Pan-African rocks in north-central Nigeria cannot be explained in terms of a unique or classical process of fractional crystallisation. At Toro, the crystallisation history of the more differentiated members appears more complex, with the participation of accessory phases. The quartz-fayalite monzonites show evidence of a cumulative phase during their differentiation. The fayalite-bearing quartzmonzonites are not common rocks and their occurrence in the Nigerian basement remains the only reported exposure in the Pan-African belt of West Africa. Like the charnockitic suite at Toro, they were emplaced during the Pan-African and at deep structural levels unlike the adjacent high-level Jurassic peralkaline granites of Jos Plateau. The application of multiple radiogenic tracers (Sr, Nd and Pb) indicates the involvement of pre-existing crust in the formation of these rocks (ϵSr(i) = + 30 to + 86, ϵNd(i) = − 15.5 to − 5.5, 206 Pb 204 Pb i = 16.34–17.71 and 207 Pb 204 Pb i = 15.48–15.58 for Toro Charnockitic Complex; and ϵSr(i) = + 19 to + 51 and ϵNd(i) = − 8.0 to − 4.0 for the quartz-fayalite monzonites of Bauchi). Long crustal residence ages (TNd) of between 2.0 and 1.5 Ga preclude their complete derivation from the mantle. The bulk of evidence is consistent with the partial melting of a U-poor source typical of the lower crust during the Pan-African orogeny and in particular the involvement of older Precambrian crust.

Alteration of a basaltic glass in an argillaceous medium:: The Salagou dike of the Lodève Permian Basin (France). Analogy with an underground nuclear waste repository

Abstract Volcanic basaltic glasses are commonly considered to be very suitable natural analogs of vitrified waste packages intended for geological disposal. The studied basaltic glass of the Salagou dike (Lodeve Basin, France) intruded into a Permian argillaceous formation about 1.4 Ma ago, provides a means of assessing the long-term behavior of nuclear waste glass in an argillaceous repository concept. This study is based on combining chemical, mineralogical and isotopic investigations. The occurrence of a palagonite film no more than 1.2 mm thick characterizes the basaltic glass alteration in contact with the argillaceous host rock. The chemical and structural properties of the palagonite suggest constant volume alteration. The alteration rates estimated from palagonite thickness and age of the glass are comparable to those measured on natural glasses altered in nonargillaceous media. The occurrence of the studied argillaceous material in contact with the basaltic glass did not result in long-term alteration kinetics significantly different from those measured in simple glass/water systems. Mineralogical, chemical, and isotopic data obtained on the basaltic and argillaceous components suggest that an aqueous fluid flowed primarily at the glass/argillite interface and did not propagate in the argillaceous host rock beyond 5 cm from the basaltic dike. The elements released by alteration of the basaltic glass—notably strontium—did not diffuse into the surrounding clay. This conclusion is important from the perspective of a natural analog for a deep geological repository for nuclear waste, and highlights the major role of the structural properties of the clayey barrier.

Evidence of bacterial activity from micrometer-scale layer analyses of black-smoker sulfide structures (Pito Seamount Site, Easter microplate)

Abstract We report the observation of an external layer, less than 20 μm thick, attached to hydrothermal chimney fragments of black smokers collected from the Pito Seamount site on the Easter microplate. Scanning electron microscope observations revealed bacterial imprints located on and within the layer, which was associated with underlying layers of thin jarosite deposits and iron sulfides displaying corrosion features. X-ray, electron microprobe, DRIFT and HPLC measurements were used to determine the precise chemical composition and nature of compounds in the layer. The external layer was composed mainly of a combination of hydrated ferric sulfate, ferrous sulfate, iron oxi-hydroxide and elemental sulfur species. A jarosite-like mineral was found deeper in the layer. We suggest that this layer may not be the result of an abiotic thermochemical precipitation, but instead, was formed chemically by bacterial processes. These processes were (1) elemental sulfur oxidation which represents major energetic exchanges within the external layer and (2) bioleaching processes inducing iron oxidation. Originally, elemental sulfur probably resulted from thermochemical precipitation. The results of this study give new insight on the possible role and impact of related acidophilic microorganisms living in hot vent environments of the deep sea.

The Bertholène uranium deposit — Mineralogical characteristics and UPb dating of the primary U mineralization and its subsequent remobilization: Consequences upon the evolution of the U deposits of the Massif Central, France☆

Abstract In the southern part of the Massif Central, France, U ores of the Rouergue area are spatially related to the unconformity of the Permian-Carboniferous sedimentary terranes overlying the Variscan basement. The Bertholene uraniferous orebody is located in the Palanges Precambrian orthogneiss, close to the unconformity and in the vicinity of a highly deformed part of the orthogneiss massif marked by the occurrence of mylonitic bands for which a Variscan age is assumed. The U ore is mainly expressed as coffinite, but primary U-oxide was also identified. Hydrothermal alteration stages have been defined: a first albitization stage is expressed on a regional scale and successive stages of argillization, silicification, and lastly carbonate crystallization have been described in the mining district. The U-oxide formation followed the silicification stage. UPb systematics were used on coffinite and U-oxide to define the evolution of the Bertholene deposit. 1. (1) Primary U-oxide was found to be 173±9 Ma old. This phase of U concentration (or remobilization) has also been dated in the two other U deposits of the southern part of the Massif Central (Lodeve, Pierres Plantees) as well as in deposits located in Limousin and Morvan (France). The event which mobilized the U in these deposits is related to the consequences of the Liassic pre-rifting phase that preceded the opening of Ligurian ocean during Malm times. 2. (2) Complex phenomena have affected the coffinite UPb system; UPb and PbPb data show that: 2.1. (a) Unsupported isotopically heterogeneous radiogenic Pb was in the coffinites during their genesis and may reflect the role of primary U-oxide as their source. 2.2. (b) In some coffinites, Rn loss phenomena controlled mainly the evolution of the UPb system. Two main types of coffinites were identified and the UPb data indicate that the coffinitization event occurred during Oligocene times (40-30 Ma) or from the Oligocene to actual times.

Métamorphisme miocène de granites panafricains dans le massif de l'Edough (Nord-Est de l'Algérie)

Abstract The Edough Massif is the easternmost crystalline core of the Maghrebides that represents the African segment of the west Mediterranean Alpine belt. U-Pb zircon dating provides upper intercept ages of 595 ± 51 My and 606 ± 55 My for an orthogneiss of the lower unit and a deformed leucogranite of the upper pelitic unit, respectively. These ages suggest emplacement of the two granitoids during the Pan-African orogeny. Monazites from a paragneiss sample gave a 18 ± 5 My U-Pb age that points to a Miocene age of the high-temperature metamorphism.

PB ISOTOPE STUDY OF BLACK-SMOKERS AND BASALTS FROM PITO SEAMOUNT SITE (EASTER MICROPLATE)

Abstract Previous studies of hydrothermal deposits on active sediment-starved spreading centers have shown that the lead isotope compositions of the oceanic crust are homogenized by the circulation of high temperature fluids. This averaging effect is confirmed by the homogeneity of the sulphide isotopic values at numerous sites in the Pacific ocean. Our study was undertaken at an active site located on a seamount (Pito Seamount) formed at the tip of a propagator on the northeast boundary of the Easter microplate near 23°19′S. Lead isotope analyses, combined with a mineral paragenesis study, were performed on hydrothermal phases of black-smokers as well as on the adjacent basalts. The field defined by hydrothermal samples departs unexpectedly from the pillow lava field. Furthermore, the hydrothermal deposits display unexpected heterogeneous isotopic values with respect to 207 Pb / 204 Pb and 208 Pb / 204 Pb ratios. Such a large lead isotope variability has not been encountered elsewhere on the Easter microplate. Hydrothermal sample values plot between an evolved EPR basalt and sediment and/or seawater, suggesting that the lead was not derived from a homogeneous source such as end-member hydrothermal fluids generated in a homogeneous reaction zone in the oceanic crust. The origin of high 207 Pb / 204 Pb and 208 Pb / 204 Pb ratios were tested for various mixing models. We made two major assumptions. (1) It is likely that hydrothermal fluids percolated through very heterogeneous volcanic sequences formed of both enriched and depleted MORBs. This suggests cyclic magmatism and a minor involvement of enriched mantle components with time, as the last magmatic events gave N-MORB-type magmas. (2) Near the seafloor, it is likely that other sources of radiogenic Pb are sediments and/or Mn-crusts in the underlying basaltic sequences that subsequently reacted with hydrothermal fluids.