Determination of the 144Ce/238U ratio in spent nuclear fuels using the double isotope dilution technique associated with thermal ionization mass spectrometry measurements.

A broad range of contaminants has been recorded in sediments of the Loire River over the last century. Among a variety of anthropogenic activities of this nuclearized watershed, extraction of uranium and associated activities during more than 50 years as well as operation of several nuclear power plants led to industrial discharges, which could persist for decades in sedimentary archives of the Loire River. Highlighting and identifying the origin of radionuclides that transited during the last decades and were recorded in the sediments is challenging due to i) the low concentrations which are often close or below the detection limits of routine environmental surveys and ii) the mixing of different sources. The determination of the sources of anthropogenic radioactivity was performed using multi-isotopic fingerprints (236U/238U, 206Pb/207Pb and 208Pb/207Pb) and the newly developed 233U/236U tracer. For the first time 233U/236U data in a well-dated river sediment core in the French river Loire are reported here. Results highlight potential sources of contamination among which a clear signature of anthropogenic inputs related to two accidents of a former NUGG NPP that occurred in 1969 and 1980. The 233U and 236U isotopes were measured by recent high performance analytical methods due to their ultra-trace levels in the samples and show a negligible radiological impact on health and on the environment. The determination of mining activities by the use of stable Pb isotopes is still challenging probably owing to the limited dissemination of the Pb-bearing material marked by the U-ore signature downstream to the former U mines.

Uranium milling activities have produced high volumes of long-lived radioactive processed wastes stored worldwide in near surface environment. The aim of this study is to highlight relevant tracers that can be used for environmental impact assessment studies involving U mill tailings. A multi-tracer study involving elemental content, U-238 decay products disequilibria and stable Pb isotopes was performed in different types of U mill tailings (alkaline, acid, neutralized acid) collected from five Tailings Management Facilities in France (Le Bosc, L'Ecarpiere, Le Bernardan, and Bellezane) and Gabon (Mounana). Our results showed that U and Pb concentrations range between 30 and 594 ppm and 66-805 ppm, respectively. These tailings have a strong disequilibrium of (U-234/U-238) and (Th-2(30)/U-238) activity ratios (1.27-1.87 and 6-65, respectively), as well as higher Pb-206/Pb-207 (1.86-7.15) and lower Pb-208/Pb-207 (0.22-2.39) compared to geochemical background ((U-234/U-238) and (Th-230/U-238) equal to unity; Pb-206/Pb-207 = 1.20; Pb-208/Pb-207 = 2.47). In situ analyzes (SEM, SIMS) showed that Pb-bearing phases with high Pb-206/Pb-207 are related to remaining U-rich phases, S-rich phases and potentially clay minerals or oxyhydroxides. We suggest that the combination of the Pb-206/Pb-207 with the (U-234/U-238) ratio is a relevant tool for the fingerprinting of the impact of U milling activities on the environment.

Ambrym Island, located in the Vanuatu Island arc (Southwest Pacific), is a mainly basaltic volcano active since at least 0.1 Ma that host a 12-13 km wide summit caldera since ~2000 years related in part to a paroxysmal eruption recorded by the Ambrym Pyroclastic Series (APS). Remnants of an older volcanic edifice subsists in the northern part of the island. Calc-alkaline lavas with variable potassium enrichments relative to similar silica content (ranging from basalt to dacite) has erupted throughout the Ambrym edification. A new geochemical study including Sr, Nd, Pb, and Hf isotopic compositions of 20 lavas is presented in this paper, with a focus on each K magmatic series recorded during the eruptive history of Ambrym Island. All Ambrym lavas show a typical arc magma signature in terms of trace element content. Compared to the old medium-K series, the more recent high-K series have similar Sr and Pb isotopic ratios, but lower 143 Nd/ 144 Nd (~0.51291 vs. ~0.51295), 176 Hf/ 177 Hf ratios (~0.28312 vs. ~0.28315), Ba/Th (270 vs. 320) and higher Nb/Yb (1.3 vs. 0.8), La/Sm (~2.7 vs. ~1.8), La/Yb (~5 vs. ~3) ratios. Higher Th/Nb (~0.6) and low Nd, Hf isotopic radiogenic compositions compared to N-MORB suggest the contribution (<2%) of subducted sediment melts to the mantle source of Ambrym lavas. Most of the geochemical features of both magma series could be explain by the involvement of two distinct mantle sources (with different pre-existing enrichments) metasomatized with different slab derived fluids and sediment melts. In addition, high-K series seems to be derived from lower extent of partial melting (8-13 vs. 10-22%), based on their Nb, Zr, La, Sm content. The recent regional rifting along fractures oriented N100° in the south of the old edifice allows an easier (medium-K series) and a new (high-K series) access for magmas and led to the edification of the main cone. Both magma series were likely stored in separate shallow depth reservoirs and participate to a giant eruption that resulted to the enlargement of a pre-existing hawaiian-type caldera topping a shield volcano and the deposit of the mainly mafic Ambrym Pyroclastic Series. Since that time, there is no evidences for an activity from the old magma chamber in the north, as only high-K magmas are now emitted all along the N100° rift zone. This highlights the strong tectonic control on the volcanism at Ambrym Island.

Gaua Island (also called Santa Maria), from the central part of the Vanuatu arc, consists of a large volcano marked by a caldera that hosts the active Mount Garet summit cone. In this paper, a geochemical study including Sr, Nd, Pb and Hf isotopic compositions of 25 lavas emitted since 1.8 Ma is presented, with a focus on the volcanic products that preceded (old volcanics, main cone and pyroclastic series) and followed (Mount Garet) the caldera forming event.All lavas show an island arc signature with enrichment in LILE and depletion in HFSE. Post-caldera lavas define a medium-K calc-alkaline trend, whereas lavas from the former main cone have high-K calc-alkaline compositions. Compared to the pre-caldera volcanic suite, the Mount Garet lavas have similar Th/Nb (similar to 1.5), Nd-143/Nd-144 (similar to 0.51295) and Hf-176/Hf-177 (similar to 0.28316) ratios, but higher Ba/La (similar to 42 vs. similar to 27) and Sr-87/Sr-86 (0.70417 vs. 0.70405) ratios and lower Ce/Pb (similar to 2.7 vs. similar to 4.6), La/Sm (similar to 2.5 vs. similar to 4.0) and Pb-206/Pb-204 (18.105 vs. 18.176) ratios. High Th/Nb and low Nd and Hf isotopic ratios compared to N-MORB suggest the contribution of similar to 2% of subducted sediment melt to the mantle source of Gaua magmas. Most of the observed differences between pre- and post-caldera lavas can be accounted for by the involvement of at least two portions of the mantle wedge, metasomatized by different slab-derived aqueous fluids. In addition, the lower La/Sm (at a given Nd-143/Nd-144) ratios of Mount Garet lavas suggest a higher degree of partial melting (similar to 10-15%) compared to the pre-caldera lavas (similar to 5%). The Santa Maria Pyroclastic Series (SMPS) eruption probably triggered the caldera collapse, in response to emptying of the magmatic chamber. This event may have allowed new access to the surface for a geochemically distinct batch of magma issued from a separate magma chamber, resulting in the birth and construction of Mount Garet within the caldera. As both magmatic suites were emitted over a very short time, the storage of their parental magmas beneath the volcano is still possible. (C) 2016 Elsevier B.V. All rights reserved.

Here we present a new geochemical study of Lopevi volcano, one the most active volcanoes in the Vanuatu island arc. We focus on the temporally well-defined sequence of lava flows emitted since 1960, and for the first time, on pre-1960 volcanic products, including high-MgO basalts and felsic andesites, the most evolved lavas sampled so far on this island. This work reports the first Pb and Hf isotopic study of lavas from Lopevi island. These lavas display correlations between differentiation indexes such as SiO2 content and isotopic ratios. The felsic andesites extend the known correlations with both the least (Sr–Pb) and the most (Nd–Hf) radiogenic isotopic compositions on the island. Our results confirm that the rising magma interacted with the sub-arc crust. Assimilation–Fractional Crystallization (AFC) quantitative modeling of trace element ratios and isotopic compositions requires 1% and 10% of assimilated partial melts of a mafic oceanic crust to account for the pre- and post-1960 lavas, respectively. The post-1960 lavas differ from the former lavas emitted ~20years earlier by enrichments in fluid mobile elements (K, Ba, Rb…), Th, and Light Rare Earth Elements (LREE). We ascribe these features to slight variations in the metasomatic agent added to the sub-arc mantle and ultimately derived from the subducted lithosphere. However, the contrasting time scales involved in subducted lithosphere dehydration and magma genesis, relative to the time elapsed between eruptions of the two lava series, suggest that two different portions of mantle which have undergone slightly different metasomatism, gave birth to the Lopevi lavas. These distinct magmas are still present beneath the volcano.