Vinciane Debaille

Coupled 142Nd-143Nd evidence for a protracted magma ocean in Mars.

Resolving early silicate differentiation timescales is crucial for understanding the chemical evolution and thermal histories of terrestrial planets. Planetary-scale magma oceans are thought to have formed during early stages of differentiation, but the longevity of such magma oceans is poorly constrained. In Mars, the absence of vigorous convection and plate tectonics has limited the scale of compositional mixing within its interior, thus preserving the early stages of planetary differentiation. The SNC (Shergotty–Nakhla–Chassigny) meteorites from Mars retain ‘memory’ of these events. Here we apply the short-lived 146Sm–142Nd and the long-lived 147Sm–143Nd chronometers to a suite of shergottites to unravel the history of early silicate differentiation in Mars. Our data are best explained by progressive crystallization of a magma ocean with a duration of ∼100 million years after core formation. This prolonged solidification requires the existence of a primitive thick atmosphere on Mars that reduces the cooling rate of the interior.

Geochemical and Hf–Pb–Sr–Nd isotopic constraints on the origin of the Amsterdam–St. Paul (Indian Ocean) hotspot basalts

The Amsterdam–St. Paul (ASP) Plateau is a recent (≤5 Ma) volcanic rise constructed along the Southeast Indian Ridge (SEIR) by the combined effects of a relatively small mantle plume and a mid-oceanic ridge. The Amsterdam and St. Paul islands are located 100 km away from each other and formed during the last 0.4 Myr; they are the only subaerial features of the ASP Plateau and the two islands are structurally separated by the presence of a SW–NE transform fault. New geochemical analyses and Hf–Pb–Sr–Nd isotopic compositions of 20 basaltic rocks from Amsterdam and St. Paul Islands constrain the nature and origin of the sources involved in the genesis of the ASP hotspot basalts. Aphyric basalts from St. Paul are mildly alkalic, incompatible element-enriched and highly fractionated; they are distinct from the tholeiitic basalts from Amsterdam, from the recently discovered Boomerang active seamount on the ASP Plateau, and from the Kerguelen Archipelago basalts on the Antarctic Plate. The St. Paul and Amsterdam basalts have very limited isotopic variations with distinct 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, and 176Hf/177Hf isotopic compositions (19.08±0.07, 15.61±0.02, 39.45±0.12, 0.28313±0.00003 for Amsterdam, and 18.70±0.08, 15.56±0.01, 38.87±0.05, 0.28306±0.00002 for St. Paul, respectively) that are not compatible with any direct contribution of the enriched Kerguelen plume end-member. Pb–Nd–Sr isotopic compositions of the St. Paul basalts appear consistent with simple binary mixtures between heterogeneous ambient upper mantle and a highly radiogenic Pb plume component (the ASP plume end-member), particularly expressed in the isotopic compositions of the Amsterdam basalts. However, the Amsterdam basalts have distinctly higher ϵHf than the St. Paul basalts (+13 and +10, respectively) for a given ϵNd (+4) and are inconsistent with such a simple binary mixing scenario. Isotopic systematics in the Amsterdam and St. Paul basalts indicate that the Amsterdam and St. Paul volcanoes were formed by sampling isotopically distinct zones of the ASP plume at a lateral distance of 100 km. Less than 1% variation in the proportion of recycled altered oceanic crust relative to pelagic sediment, combined with minor variations in the proportion of recycled material within the Amsterdam and St. Paul plume sources themselves relative to a peridotitic mantle source, could account for the isotopic differences between the compositions of basalts from these two islands. The particularly high 206Pb/204Pb component recorded in the Amsterdam and St. Paul basalts is also locally recorded at different times and locations within other Indian Ocean basalts (e.g. Ninetyeast Ridge basalts, 38 Ma) and such a component has also contaminated the source of SEIR basalts to varying degrees. The particularly high 206Pb/204Pb component is therefore not exclusive to the Amsterdam–St. Paul plume but is heterogeneously distributed within the Indian Ocean upper mantle. This may reflect the role of the Indian mantle plumes in dispersing recycled material within the Indian upper mantle.

Volcano- and climate-driven changes in atmospheric dust sources and fluxes since the Late Glacial in Central Europe

Atmospheric dusts are an important part of the global climate system, and play an important role in the marine and terrestrial bio- geochemical cycles of major and trace nutrient elements. A peat bog record of atmospheric deposition shows considerable variation in dust deposition during the past 15 k.y., with abrupt changes in fluxes at 12, 9.2, 8.4, 7.2, and 6 cal. kyr B.P. Using Nd isotopes and rare earth elements, it is possible to clearly distinguish between volcanic inputs and those driven by climate change, such as the long-term aridification of the Sahara and regional erosion due to forest clearing and soil cultivation activities. Our results indicate that a major dust event in North Africa and Europe preceded the 8.2 kyr B.P. cold event by 200 yr. This dust event may have played an active role in the following climate cooling of the 8.2 kyr B.P. event. Nd isotope evidence also indicates a relatively slow change in dust regime over Europe from 7 to 5 kyr B.P. due to Sahara expansion. These fi ndings show that the inorganic fraction in high-resolution peat records can provide remarkably sensitive indicators of dust load and sources. Our study supports the priority to better identify the impact of dust loading during the Holocene in terms of direct and indirect impacts on environmental and climate changes.

Deep earth recycling in the Hadean and constraints on surface tectonics

The Hadean mantle was efficiently heated from high internal heat production, high rates of impact bombardment, and primordial heat from accretion. As a result a strong case is made for extremely high internal temperatures, low internal viscosities, and extremely vigorous mantle convection. Previous studies of mixing in such high-Rayleigh number convective environments indicate that chemically heterogeneous mantle anomalies should have efficiently remixed into the mantle on timescales of less than 100 Myr. However, 142Nd and 182W isotope studies indicate that heterogeneous mantle domains survived, without mixing, for over 2 Gyr—at odds with expected mixing rates. Similarly, concentrations of platinum group elements in Archean komatiites, purportedly due to the later veneer of meteoritic addition on the Earth, only achieve current levels at 2.7 Ga—indicating a time lag of almost 1 to 2 Gyr in mixing this material thoroughly into the mantle. Previous studies have sought to explain slow Archean mantle mixing via mantle layering due to endothermic phase changes, or anomalously viscous blobs of material, with limited efficacy. Here we pursue another explanation for inefficient mantle mixing in the Hadean: tectonic regime. A number of lines of evidence suggest that resurfacing in the Archean was episodic, and extending these models to Hadean times implies the Hadean was characterized by long periods of tectonic quiescence. We explore mixing times in 3D spherical-cap models of mantle convection, which incorporate vertically stratified and temperature-dependent viscosities. We show that mixing in stagnant-lid regimes is, at the extreme, over an order of magnitude less efficient than mobile-lid mixing, and for plausible Rayleigh numbers and internal heat production, the lag in Hadean convective recycling can be explained. The attractiveness of this model is that it not only explains the long-lived 142Nd and 182W anomalies, but also posits an explanation for the delay between accretion of the late veneer—between 4.5 to 3.8 Ga on a stagnant surface—and its fully mixed signature apparent in elevated PGEs in 2.7 Ga komatiites. It also provides an explanation for the 400 Myrs of immobility of the mafic protolith from which the Jack Hill zircons were sourced, and retards early heat loss from the mantle, providing a solution to the “Archean thermal catastrophe” of parameterized Earth evolution models.

Nd–Hf Isotope Systematics of Megacrysts from the Mbuji-Mayi Kimberlites, D. R. Congo: Evidence for a Metasomatic Origin Related to Kimberlite Interaction with the Cratonic Lithospheric Mantle

Garnet and clinopyroxene megacrysts from the Cretaceous (70 Ma) Mbuji-Mayi kimberlites and one garnet megacryst from the lower Oligocene (32 Ma) Kundelungu kimberlites in Democratic Republic of Congo have been investigated for combined Nd and Hf isotope compositions. These megacrysts are thought to result from the metasomatic (re)crystallization of lithospheric mantle peridotites during the infiltration of a proto-kimberlitic melt/fluid. In addition, zircon and baddeleyite megacrysts from the Mbuji-Mayi kimberlites have been investigated for Hf isotope composition. Although baddeleyites are uncommon in kimberlite megacryst suites, their origin is most probably related to that of zircon megacrysts from Mbuji-Mayi. Mbuji-Mayi garnet megacrysts display ranges of eNd(t) from −0.6 to +6.1 and eHf(t) from +6.6 to +12.1; the Kundelungu garnet megacryst has overlapping isotopic compositions (+0.8 and +6.0, respectively). By contrast, Mbuji-Mayi clinopyroxene megacrysts display a more restricted range of eNd(t) values (+2.7 to +4.6) and extend toward lower eHf(t) compositions (+3.0 to +9.1). Mbuji-Mayi zircon and baddeleyite megacrysts have similar eHf(t) values (+6.5 to +7.1 and +6.0 to +8.4, respectively). Differences in initial isotopic composition between garnets on the one hand and clinopyroxenes, zircons, and baddeleyites on the other have been confirmed by various Hf and Nd model ages calculations. Clinopyroxene, zircon, and baddeleyite megacrysts plot close to the worldwide kimberlite field in a combined eHf(t)-eNd(t) plot, which favors recent (i.e. at or shortly before the time of kimberlite eruption) crystallization through interaction between the infiltrating proto-kimberlite melt/fluid and peridotite wall rocks. On the other hand, the wide range of eNd(t) values and the higher eHf(t) values for garnet megacrysts suggest that they have been formed through recrystallization of old garnet-bearing peridotitic protoliths in the subcontinental lithospheric mantle. Calculated rare earth element patterns of liquids in equilibrium with clinopyroxene megacrysts confirm the direct relationship to Group I kimberlites, while those in equilibrium with garnet megacrysts show more variability, which could also reflect that their formation results from more complex processes.

The earliest evidence for modern-style plate tectonics recorded by HP–LT metamorphism in the Paleoproterozoic of the Democratic Republic of the Congo

Knowing which geodynamic regimes characterised the early Earth is a fundamental question. This implies to determine when and how modern plate tectonics began. Today, the tectonic regime is dominated by mobile-lid tectonics including deep and cold subduction. However, in the early Earth (4.5 to 2 Ga) stagnant-lid tectonics may also have occurred. The study of high pressure–low temperature (HP–LT) metamorphic rocks is important, because these rocks are only produced in present-day subduction settings. Here, we characterize the oldest known HP–LT eclogite worldwide (2089 ± 13 Ma; 17–23 kbar/500–550 °C), discovered in the Democratic Republic of the Congo. We provide evidence that the mafic protolith of the eclogite formed at 2216 ± 26 Ma in a rift-type basin, and was then subducted to mantle depths (>55 km) before being exhumed during a complete Wilson cycle lasting ca. 130 Ma. Our results indicate the operation of modern mobile-lid plate tectonics at 2.2–2.1 Ga.