Origin of negative cerium anomalies in subduction-related volcanic samples: Constraints from Ce and Nd isotopes

Negative Cerium (Ce) anomalies are observed in chondrite-normalized Rare Earth Element patterns from various volcanic arc suites. These anomalies are well defined in volcanic rocks from the Mariana arc and have been interpreted as the result of addition of subducted sediments to the arc magma sources. This study combines ¹⁴³Nd/¹⁴⁴Nd and ¹³⁸Ce/¹⁴²Ce isotope measurements in Mariana volcanic rocks that have Ce anomalies ranging from 0.97 to 0.90. The dataset includes sediments sampled immediately before subduction at the Mariana Trench (Sites 801 and 802 of ODP Leg 129) and primitive basalts from the Southern Mariana Trough (back-arc basin). Binary mixing models between the local depleted mantle and an enriched end-member using both types of sediment (biosiliceous and volcaniclastic) found in the sedimentary column in front of the arc are calculated. Marianas arc lavas have Ce and Nd isotopic compositions that require <2.5% of a sediment component derived from the volcaniclastics. With this proportion of sediment, most of the Ce/Ce* range measured in lavas is reproduced. Thus, this study confirms that the origin of the Ce anomalies in the Mariana arc magmas can be principally attributed to recycling of trench sediments through active subduction. The participation of a component derived from biosiliceous sediments does not explain the Ce-Nd isotope composition of the lavas because the involved proportion is too high (up to 8%) in comparison to results obtained from other geochemical proxys. Using this end-member, the modeled Ce anomalies are also too high (0.91–0.84) in comparison to those measured in lavas. Various processes and conditions are able to generate Ce anomalies: oxygen fugacity, residual mineral phases, partial melting, fractional crystallization and tropical weathering. Their influence in the case of Mariana volcanic arc magmas seems to be very limited but partial melting effect may explain the lowest measured Ce/Ce* values. Magmatic processes cannot be definitely ruled out in producing Ce anomalies in other arc system environments. Additional experimental data, however, are needed for a better understanding of the behavior of cerium relative to its neighboring elements. Also, this study highlights the importance of using local depleted mantle and sediments to model the isotopic compositions of arc lavas

Temporal evolution of the Cabo Verde archipelago: new constraints from 40Ar-39Ar data

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Radiogenic and stable Ce isotope measurements by 1 thermal ionisation mass spectrometry

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Isotope evidences for the origin of Cape Verde oceanic carbonatites

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Mineral-whole rock isotope fidelity? A comparative study of Hf-Nd-O from high Ba Sr granitoids

International audienceIt is well-established that lanthanide rare earth elements (REEs) have the potential to record the nature and source characteristics of their host magmas, in both whole-rock and their minerals. Accessory minerals that concentrate REEs are especially useful in crustal evolution studies, both for their elemental and isotopic information; the classic and unrivalled example being zircon. Approaches using a single radiogenic isotopic system, or one radiogenic and one stable isotope system (e.g., Hf and O) in one REE-bearing mineral (usually zircon) are common, but those involving multiple isotopes in several minerals remain scarce despite offering many advantages. Importantly, the latter approaches also allow comparing different techniques and provide evidence on whether isotopic systems were disturbed by secondary processes. This contribution documents several isotopic systems within the abundant accessory mineral of Caledonian high Ba-Sr granitoids from Northwest Scotland. We present a multi-isotope study of titanite, zircon and apatite from two localities (Strontian and Rogart), which were selected for their contrasting whole-rock isotopic signatures-the former deriving from a depleted mantle source, whereas the latter derived from a strongly enriched mantle source. New in-situ Sm-Nd in titanite and apatite and Hf in zircon isotope data are discussed and compared with in-situ 2 oxygen isotope data previously published for the same samples. An internal consistency is observed for Nd isotopes in apatite and titanite. Nd isotopes values for both minerals strongly correlate with Hf isotopes in zircon. Isotopic data at the mineral scale confirm the Strontian and Rogart source characteristics previously defined from whole-rock isotope data, with the Rogart having a more enriched signature than the Strontian source along the "Caledonian Parental Magma Array"(CPMA). In contrast, a significant discrepancy exists between whole-rock and REE-mineral oxygen isotope data as the latter can be affected by post emplacement alteration and hence can be misinterpreted. Importantly, the contribution of sediments in the CPMA source(s) can be estimated from a diagram combining Sm-Nd isotopic signatures and (La/Sm)N in apatite and titanite. Overall, we demonstrate that detailed petrogenetic records are not only available in zircons but also in magmatic titanite and apatite, and we suggest that integrated multi-mineral approaches have potential to maximise constraints from in-situ mineral isotope geochemistry

Further Helium isotopic evidence for a lower mantle contribution to the Cape Verde plume

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In-situ determination of Nd isotope ratios in apatite

International audienceThe Samarium-Neodymium isotope systematics relies on the radioactive decay of the long-lived 147 Sm isotope to the radiogenic 143 Nd (half-life of 106 ± 0.8 Gyr). Its application as a source tracer and/or a chronometer covers a wide range of geological subjects dealing with the differentiation and history of the Earth and other planets. The Nd isotopic ratios are classically determined on bulk rocks and/or mineral separates by thermal ionization (TIMS) or plasma source mass spectrometry (MC-ICP-MS), after acid digestion of the samples and chemical separation of Nd from the other elements by ion-exchange resin chromatography. However, in-situ measurements are also possible by coupling the MC-ICP-MS instrumentation to a laser ablation system. In this case, the determination of the isotopic compositions is made quicker, being free from the steps of dissolution and chemical separation. Most importantly, it gives the possibility of considering much finer scales of heterogeneity than bulk-rock analyses, at the level of a few tens of micrometers. The in-situ measurement of isotopic compositions of Nd is aimed at minerals rich in this element. Apatite is an ideal candidate as its Nd concentrations are generally greater than several hundred of ppm, and it is an ubiquitous phase in most rocks (magmatic, metamorphic and sedimentary), giving its analysis a wide field of applications. Here, we present new Sm-Nd results acquired on the Thermo Scientific™ Neptune Plus™ instrument at the Laboratoire Magmas et Volcans (LMV), France, on apatite grains from carbonatites. We detail the different corrections needed to obtain accurate and precise Nd isotopic ratios with a focus on the laser (size of the spot, energy and frequency) and MC-ICP-MS parameters (X skimmers vs. H skimmers) and their influence of the measurements. We also show the necessity for external standardisation (here we used the Durango apatite) in order to obtain both accurate Nd isotopic compositions and Sm/Nd chemical ratios. Finally, we demonstrate that our best measurements are obtained when considering raster analysis rather than individual spots. From a more general point of view, this contribution also aims to help geochemical labs to implement an in-situ analysis protocol for Nd isotope ratios

Diversity in earth's early felsic crust: Paleoarchean peraluminous granites of the Barberton Greenstone Belt

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The influence of igneous processes on the chromium isotopic compositions of Ocean Island basalts

International audienceWe present the first stable chromium isotopic data for a suite of ocean island basalts (OIB) in order to investigate the Cr isotope fractionation during major igneous processes such as partial melting and fractional crystallisation. Twenty-one basaltic samples from Fangataufa Island (Tuamotu Archipelago, Pacific Ocean) have been analysed for major- and trace-element concentrations, and Sr, Nd and Cr isotopic compositions. They define two distinct series: medium to high-K calc-alcaline and low to medium-K calc-alcaline. The variations in incompatible elements such as La and Yb mostly result from varying degrees of partial melting of a mixed mantle source composed of two lithologies: garnet bearing peridotite and a recycled “fertile” component. The recycled component is also identified with the Sr and Nd isotopic composition of Fangataufa basalts. In contrast, the variations in compatible element contents such as Cr and Ni are governed by fractional crystallisation of a mixture of olivine, clinopyroxene and spinel. The samples analysed in this study are also characterised by small Cr isotope variations from −0.24 to −0.17‰. The Cr-poor samples have on average lighter Cr isotopic compositions compared to Cr-rich ones. The observed variations in the low-K series can be modelled by a Rayleigh fractionation model with a fractionation factor (Crcrystals-melt) of −0.010 ± 0.005‰. The fractionation is more limited than that observed in lunar basalts and two hypotheses may explain this observation: a change in crystallising phases (cpx + spinel on Earth and spinel on the Moon) and/or the difference in temperature and oxygen fugacity between the crystallisation of lunar and terrestrial basalts. The more primitive basalts from Fangataufa have an average Cr isotopic composition of −0.18 ± 0.01‰, lighter than the Cr isotopic composition of pristine mantle xenoliths. Chromium isotopes are therefore slightly fractionated during partial melting with the melts depleted in heavy Cr isotopes. The difference between silicate melts and mantle xenoliths indicates that partial melting could produce small but resolvable shift in mantle xenoliths Cr isotopic composition of up to 0.05‰. The covariation between Cr and Nd values in Fangataufa basalts can be explained by two processes: (i) lower degrees of partial melting produce lighter Cr isotopic composition in basaltic liquids relative to their sources and (ii) melts produced by low degree of partial melting have also experienced more fractional crystallisation

²³⁴U/²³⁸U Disequilibrium along stylolitic discontinuities in deep Mesozoic limestone formations of the Eastern Paris basin: evidence for discrete uranium mobility over the last 1–2 million years

International audienceThe (234U/238) equilibrium state of borehole core samples from the deep, low-permeability limestone formations surrounding the target argilite layer of the Meuse/Haute-Marne experimental site of the French agency for nuclear waste management -ANDRA- (Agence nationale pour la gestion des déchets radioactifs) was examined to improve understanding of naturally occurring radionuclide behaviour in such geological settings. Highly precise, accurate MC-ICP-MS measurements of the (234U/238U) activity ratio show that limestone samples characterised by pressure dissolution structures (stylolites or dissolution seams) display systematic (234U/238U) disequilibria, while the pristine carbonate samples remain in the secular equilibrium state. The systematic feature is observed throughout the zones marked by pressure dissolution structures: (i) the material within the seams shows a deficit of 234U over 238U ((234U/238U) down to 0.80) and (ii) the surrounding carbonate matrix is characterised by an activity ratio greater than unity (up to 1.05). These results highlight a centimetric-scale uranium remobilisation in the limestone formations along these sub-horizontal seams. Although their nature and modalities are not fully understood, the driving processes responsible for these disequilibria were active during the last 1–2 Ma. Keywords: uranium isotopes, multiple-collector ICP-MS, waste management, remobilisation, migratio