Ce isotope fractionation during oxidative reaction

International audienceCerium belongs to the Rare Earth Element (REE) group and unlike any other Light REE can be stable under two oxidation states (Ce(III) and Ce(IV)). The Ce aqueous chemistry is interesting because Ce(III) and Ce(IV) behave differently. While Ce(III) behaves like its REE neighbours and is readily soluble, Ce(IV) is less soluble in natural waters and can therefore be easily decoupled from other LREE. Variations in Ce concentration relative to its neighbours have been extensively used to study changes in redox conditions in natural systems. For example, seawater is characterisedby a large negative Ce anomaly (i.e. Ce is depleted relative to its neighbours) and this has been interpreted as reflecting the insoluble behaviour of Ce(IV) under oxic conditions. Cerium has four stable isotopes (136Ce, 138Ce, 140Ce, 142Ce) and mass dependent variations have been previously reported in Fe-oxides and during chemical precipitation experiments (Nakada et al. 2013, 2016). The main conclusion is that redox reactions play a major role in fractionating Ce isotopes. The study of mass dependent Ce isotope variations can therefore help us constrain the behaviour of Ce and the conditions in which Ce isotopesare fractionated in natural waters. To date, however, there is no laboratory experiments that investigate the behaviour of Ce isotopes during oxidation reactions. We have developed a new triple spike analytical method to measure mass-dependent Ce isotope variations (Bonnand et al. 2019, JAAS). In order to investigate the behaviour of Ce during oxidation reaction, we performed a number of laboratory experiments in which we partially oxidised a Ce(III) solution. We then measured the Ce isotopic composition in the Ce(III) and Ce(IV) fractions. While the oxidation reactions proceeds, the Ce(III) remaining become isotopically heavy(up to 0.6 ‰)relative to the starting isotopic composition. The isotopic composition of the Ce(IV) fraction is lighter than that of the Ce(III) fraction typical of isotopic fractionation during a kinetic reaction. These results confirm the redox control on the Ce isotopes behaviour and suggest that Ce isotope could become a useful tool to study redox reactions and past atmospheric oxygenatio

Stable cerium isotopes as a tracer of oxidation reactions

International audienceRedox conditions in past oceans have attracted significant interest and many proxies have been used to probe redox changes through time. For example, the redox dependent behaviour of Ce, resulting in negative or positive elemental Ce anomalies, has been widely used. More recently, mass dependent Ce isotopic variations have been proposed as a powerful tool to study Ce oxidation in natural environments. In this study, we demonstrate, for the first time, that Ce isotopes are fractionated during oxidation reaction, confirming the utility of Ce isotopes to study redox reactions. This result suggests that seawater Ce isotopic composition should be fractionated toward heavy values relative to the continental crust. Measured natural rock samples (carbonates, banded iron formations and Mn nodules) have variable Ce isotopic compositions, ranging from −0.055 ± 0.045 ‰ to +0.280 ± 0.045 ‰. The relation between Ce elemental anomalies and Ce isotopic composition in carbonate rocks suggest that mass dependent Ce isotopes can be used to distinguish elemental anomalies produced by oxidation reactions from those produced by other processes. Coupled with La-Ce chronology, mass dependent Ce isotope analysis is a very powerful tool to study redox reactions in past oceans

What coupled cerium and neodymium isotopes tell us about the deep source of oceanic carbonatites

International audienceWe present the first Ce isotope compositions together with Nd and Pb isotope ratios measured for whole rocks and mineral separates (apatite, titanite) from oceanic carbonatites and associated silicate samples from the Cape Verde and Canary archipelagos (Atlantic Ocean). We compare them with measurements performed on carbonatites from Tamazert (Morocco) for which a common source has been suggested. We couple the La/Ce and Sm/Nd systems in order to model the rare-earth-element concentrations and the Ce–Nd isotope compositions of marine carbonates and mantle and/or mantle-derived reservoirs, notably the D″ boundary layer. We discuss the two contrasting models that have been previously presented for the origin of the carbon of the oceanic carbonatite source: recycled marine carbonates via subduction vs. primordial carbon. Our measurements favour the recycling of marine carbonates in the source of oceanic carbonatites. We use Pb isotope ratios together with experimental petrology constraints to discuss the age and depth of carbonate recycling in the mantle

Mantle plume heterogeneity versus shallow-level interactions: A case study, the São Nicolau Island, Cape Verde archipelago

We present new Sr–Nd–Pb isotopic data, as well as major- and trace-element concentrations, for 19 basaltic samples from São Nicolau Island, Cape Verde archipelago. Fine-scale study of the island argues in favor of mixing between four endmembers to explain isotopic variations of collected samples: 1) a radiogenic endmember (87Sr/86Sr ~ 0.7034; 143Nd/144Nd ~ 0.51285; 206Pb/204Pb ~ 20.0; 207Pb/204Pb ~ 15.65; 208Pb/204Pb ~ 39.8) representative of the Cape Verde plume deep source; 2) an unradiogenic endmember having isotopic compositions resembling those of Atlantic MORB dredged at the same latitude; 3) a low Sr–high Nd and Pb endmember identified as the Jurassic MORB basement of the archipelago; and 4) São Vicente-like Cape Verde carbonatites. Compositional and isotopic results show that most of the measured variations can be related to mixing of plume-derived melts with shallow-level reservoirs. Therefore the source heterogeneity of the Cape Verde plume is much smaller than the one sampled in basaltic samples. This observation illustrates how caution is required when interpreting global OIB data in terms of mantle topology without filtering from the contribution of shallow-level reservoirs

In-situ determination of Nd isotope ratios in apatite by LA-MC-ICPMS: Challenges and limitations

International audienceIn-situ measurement of Sm-Nd isotopes in rare-earth-element bearing minerals has been successfully used in the recent years to access the history of old Earth materials (e.g. Hammerli et al., 2019; Fisher et al., 2020). However, the analytical protocol of Laser Ablation-Multi Collector-Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS) for Sm-Nd measurements strongly depends on the instruments, the analytical parameters and the applied corrections. In this paper, we intend to summarize some of the difficulties to set up this technique. We present new results by evaluating the influence of laser parameters (spot size, fluence, frequency and He/N2 gas flows) on 143 Nd/ 144 Nd, 145 Nd/ 144 Nd and 147 Sm/ 144 Nd ratios. We also report results on tests of cone geometry and interference and mass discrimination corrections on both the accuracy and precision of Nd isotopic analyzes. We report new Sm-Nd measurements performed by TIMS and LA-MC-ICPMS on Durango reference apatite (Mexico) and apatite crystals from 2 carbonatites (Fogo, Cape Verde and Phalaborwa, South Africa). We conclude from these measurements that the laser parameters have no influence on the 143 Nd/ 144 Nd and 145 Nd/ 144 Nd isotopic ratios. The value of the 147 Sm/ 144 Nd ratio, however, is correlated to the size of the spot (Fisher et al., 2020). More importantly, we show that 147 Sm/ 144 Nd ratio measurements can vary when the He/N2 gas flows are changed even punctually (e.g. sample exchange) during an analytical session and decrease systematically during the day, which we relate to a systematic instrumental drift. We also conclude that our Durango crystal is homogeneous for Nd isotopic ratios, and slightly heterogeneous for Sm/Nd at the level of ±1-1.5% allowing accurate measurements when used as external standard. We retrieve expected isotopic ratios for both recent Fogo (4 Ma old) and ancient Phalaborwa (2060 Ma old) samples and we achieve precisions in the order of 125-150 ppm (1.3-1.5 εNd-units) for the 143 Nd/ 144 Nd ratio for a laser spot of 40 µm, which can be considered as a reasonable size for apatite crystals in most geological samples. Finally, we present raster measurements allowing to improve the precision by a factor of ~2 with 70 ppm (0.7 εNd-units)

New Constraints on the Origin of the EM‐1 Component Revealed by the Measurement of the La‐Ce Isotope Systematics in Gough Island Lavas

International audienceHot spot lavas show a large diversity of isotope compositions resulting from the recycling ofsurface material into the convective mantle. Amongst the mantle end‐members, EM‐1 (enriched mantle)is widely debated and scenarios involving either old pelagic sediments subducted into the deep mantle orsubcontinental lithospheric material incorporated at shallow depths are commonly evoked. We selected 12lavas from Gough Island (south Atlantic Ocean) for the measurement of 138La‐138Ce, 147Sm‐143Nd, and176Lu‐177Hf isotope systems. Results show limited ranges for ε143Nd, ε138Ce, and ε176Hf values, and Ce/Ce*do not correlate with measured isotope ratios. Cerium isotope compositions allow us to exclude thecontribution of old sedimentary material carrying a negative, elemental cerium anomaly in the mantlesource. Pelagic sediments are indeed characterized by strongly negative, elemental cerium anomalies andalso high La/Ce ratios. Modeling a primitive mantle source contaminated by 0.4% to 2.2% of different 2.5Ga‐old pelagic components is able to reproduce the lowest Ce/Ce* values. However, the cerium isotopemeasurements show ε138Ce values between −0.39 and 0.15, too low to give support to the incorporation ofrecycled pelagic sediments in the mantle source of the lavas. Our results suggest that the incorporation ofsubcontinental lithospheric material at shallow depths during the plume ascent is a more suitable modelto explain the formation of the EM‐1 component. Hafnium and Nd isotopes also support this scenario.Subcontinental lithosphere sampled via kimberlites and lamproites has isotopic compositions that plotgenerally below the mantle array, a signature that is also seen in Gough lavas

The La-Ce Isotope Systematics in Gough Island Lavas: New Constraints on the Origin of the EM1 Component

International audienc

Contrasting petrogenesis of Mg–K and Fe–K granitoids and implications for post-collisional magmatism: Case study from the Late-Archean Matok pluton (Pietersburg block, South Africa)

International audienceThis study investigates the origin of the 2.69 Ga-old Matok pluton, emplaced in the Pietersburg block, northern Kaapvaal Craton (South Africa), forthwith after a major tectono-metamorphic event ascribed to continent–continent collision. The Matok pluton consists of diorites, granodiorites and monzogranites. Petrography and whole-rock major- and trace element compositions of the Matok samples are similar to those of post-collisional Fe–K suites, which are very common in Proterozoic terranes. These granitoids are particularly rich in FeOt, TiO2, P2O5, span a wide range of SiO2 contents and display elevated concentrations in K2O, Ba, HFSE and REE, with moderately fractionated REE patterns.All samples of the Matok pluton have unradionegic Nd isotopic compositions (εNd(2.69 Ga) = − 2.7 to − 4.6), but only a few monzogranite samples derive from reworking of older crust. Crustal contamination of basaltic melt cannot explain either the observed compositions. Most of the suite rather fractionated from a common mafic parent, either by partial melting or crystallization. Geochemical modeling shows that this parent magma or source rock is basaltic in composition, intermediate between calc-alkaline and tholeiitic groups, and enriched in incompatible trace elements with respect to the primitive mantle. It ultimately derives from the involvement of two distinct mantle sources: (1) enriched, sub-continental lithospheric mantle, which was metasomatized by sedimentary material derived from local crust of the Pietersburg block, < 0.3 Ga before the pluton emplacement; and (2) asthenospheric mantle.This model accounts for the differences between Fe–K suites, such as the Matok pluton, and Mg–K suites, such as sanukitoids, the origin of which only requires metasomatized mantle. We propose that the most appropriate geodynamic setting for the interaction between enriched lithospheric and asthenospheric mantle sources is an episode of “lithospheric reworking” (through slab breakoff, retreat or sub-continental mantle delamination) in response to continental collision. Depending on the relative contribution of the two mantle sources in this environment, the resulting magmas can show a wide range of compositions from Fe–K to Mg–K end-members. From a regional point of view, this conclusion supports that the Kaapvaal Craton and the Central Zone of the Limpopo Belt were amalgamated between 2.80 and 2.75 Ga owing to continent–continent collision

Pb isotope geochemistry of Piton de la Fournaise historical lavas

International audienceVariations of Pb isotopes in historical lavas (1927–2007) from Piton de la Fournaise are investigated based on new (116 samples) and published (127 samples) data. Lead isotopic signal exhibits smooth fluctuations (18.87 3 years) periods of inactivity of the volcano (1939–1942, 1966–1972, 1992–1998), supporting previous inferences that Pb isotopic variations occur mostly during and not between eruptions. Intermediate compositions (18.904 < 206Pb/204Pb < 18.917) bracket the longest periods of quiescence. In this respect, the highly frequent occurrence of an intermediate composition (18.90 < 206Pb/204Pb < 18.91), which clearly defines an isotopic baseline during the most recent densely sampled period (1975–2007), either suggests direct sampling of plume melts or sampling of a voluminous magma reservoir that buffers Pb isotopic composition. Deviations from this prevalent composition occurred during well-defined time periods, namely 1977–1986 (radiogenic signature), 1986–1990 and 1998–2005 (unradiogenic signatures). The three periods display a progressive isotopic drift ending by a rapid return (mostly during a single eruption) to the isotopic baseline. The isotopic gradients could reflect progressive emptying of small magma reservoirs or magma conduits, which are expected to be more sensitive to wall-rock interactions than the main magma chamber. These gradients provide a lower bound ranging from 0.1 to 0.17 km3 for the size of the shallow magma storage system. The isotopic shifts (March 1986, January 1990 and February 2005) are interpreted as refilling the plumbing system with deep melts that have not interacted with crustal components. The volume of magma erupted between the two major refilling events of March 1986 and February 2005 (0.28 km3) could provide a realistic estimate of the magma reservoir size. Unradiogenic anomalies appear to be linked, more or less directly, to the eruption of olivine-rich lavas. The related samples have low 206Pb/204Pb and 208Pb/204Pb but normal 207Pb/204Pb, suggesting a recent decrease of U/Pb and Th/Pb, for instance through sequestration of Pb into sulfides. Olivine and sulfides, which are both denser than silicate melts, could be entrained with magma pulses, which give rise to high-flux oceanite eruptions

Exploring the links between volcano flank collapse and the magmatic evolution of an ocean island volcano: Fogo, Cape Verde

International audienceAbstract Mass-wasting of ocean island volcanoes is a well-documented phenomenon. Massive flank collapses may imply tens to hundreds of km 3 and generate mega-tsunamis. However, the causal links between this large-scale, low-frequency instability, and the time–space evolution of magma storage, crystal fractionation/accumulation, lithospheric assimilation, and partial melting remains unclear. This paper aims at tracking time variations and links between lithospheric, crustal and surface processes before and after a major flank collapse (Monte Amarelo collapse ca. 70 ka) of Fogo volcano, Cape Verde Islands, by analysing the chemical composition (major, trace elements, and Sr–Nd–Pb isotopes) and age-controlled stratigraphy (K–Ar and Ar–Ar dating) of lavas along vertical sections (Bordeira caldera walls). The high-resolution sampling allows detecting original variations of composition at different time-scales: (1) a 60 kyrs-long period of increase of magma differentiation before the collapse; (2) a 10 kyrs-long episode of reorganization of magma storage and evacuation of residual magmas (enriched in incompatible elements) after the collapse; and (3) a delayed impact at the lithospheric scale ~ 50 kyrs after the collapse (increasing EM1-like materiel assimilation)