Hidden erosion on volcanic islands

International audienceWe have studied rivers from 3 volcanic tropical Islands (Guadeloupe and Martinique in Lesser Antilles and Reunion) in order to evaluate chemical weathering rates. These islands are constituted of very porous pyroclastic deposits, andesitic or basaltic lava flows, allowing significant water infiltration. From hydrological budgets, we estimate that infiltration represents 10% in Guadeloupe and 40% in Martinique. Element fluxes transported by subsurface water to the ocean have been quantified by coupling the hydrological budget with the chemical composition of the sampled subsurface waters. This is the first estimate of chemical weathering rates from volcanic islands, which takes into account the high temperature weathering contribution to the chemical weathering rates. Subsurface waters are impacted by high-temperature water–rock interactions and present therefore higher concentrations (4 to 100 times) of major and trace elements compare to surface waters, representing thus respectively 90%, 60% and 75% of the total (surface + subsurface) weathering dissolved flux to the ocean for Martinique, Guadeloupe and Reunion. This shows the importance of these fluxes, which until now were not estimated in the basalt weathering geochemical budgets. Chemical weathering rates from subsurface water are 2 to 5 time higher than the rates from surface waters. The total average chemical rates calculated are 290 t/km2/yr in Guadeloupe, 1080 t/km2/yr in Martinique and 270 t/km2/yr in Réunion. These values are among the highest worldwide. Extrapolating this budget to the surface of all volcanic islands world-wide, we estimate that these could contribute between 23% and 31% of the global flux of dissolved load transported to the oceans by major rivers draining silicate rocks whereas their surface area represent only 9% of the silicate rock total surface are

The global control of silicate weathering rates and the coupling with physical erosion: new insights from rivers of the Canadian Shield

International audienceThe chemical evolution of the surface of the Earth is controlled by the interaction of rainwaters, the atmosphere and the continental crust. That is the main reason why the knowledge of the parameters that control chemical denudation on Earth is of crucial importance. We report chemical and isotopic analyses for river waters from the Canadian Shield in order to estimate chemical weathering fluxes. We present a comparison of the chemical composition and Sr isotopic composition of a set of rivers sampled in the Slave Province (Northwest Territories, Canada) and in the Grenville Province (Québec, Canada). The surface waters of these high latitude catchments are very dilute, with the Slave rivers about three times more concentrated than the Grenville rivers. A detailed analysis of the Sr isotopic composition and chemical signature of these rivers shows that silicate weathering reactions are not the only mechanisms that control solute concentrations. An atmospheric component, constituted by the dissolution of evaporite and carbonate aerosols, is necessary to explain the dispersion of chemical ratios such as Ca/Na, Mg/Na, Sr/Na and Cl/Na. These aerosols probably have a local origin. Chemical denudation rates for the Slave Province are four times lower than those found in the Grenville Province (0.35 and 1.55 tons/km 2/yr respectively). Compared to a panel of surface waters from other shield areas of the world, the Slave Province appears to have the lowest chemical denudation rate in the world. In a chemical weathering rate vs. temperature plot, shield rivers define a triangular relationship, hot climate being able to produce the most variable denudation rates. But no simple relationship between chemical weathering rates and temperature or runoff is observed, in contrast to rivers draining basaltic areas. We show that a global power law (0.66 exponent) exists between chemical denudation rates and physical denudation rates, indicating that the shield areas with low mechanical denudation (such as the Slave Province or Cameroon) have also low chemical denudation rates. These results give importance to physical denudation in determining the chemical weathering rates of silicates. We think that any further modeling on Earth's long term climate will have to take into account this fundamental coupling between mechanical and chemical weathering fluxes

Primitive neon isotopes in Terceira Island (Azores archipelago) [rapid communication]

International audienceWe present the first neon data, as well as new helium data, on Terceira Island (Azores archipelago, Portugal). Clear 20Ne and 21Ne excesses compared to air are observed ( 20Ne/ 22Ne > 11.2) and moreover, the samples show a more primitive 21Ne/ 22Ne ratio than MORB, confirming that the Azores hotspot can be considered as sampling a "primitive", relatively undegassed, reservoir. Most 4He/ 3He isotopic ratios range between 80,000 and 63,500 (∼ 9 to 11.5 R/Ra), being similar to those previously reported by [1] [M. Moreira, R. Doucelance, B. Dupré, M. Kurz, C.J. Allègre, Helium and lead isotope geochemistry in the Azores archipelago, Earth Planet. Sci. Lett. 169 (1999) 189-205] . A more primitive 4He/ 3He ratio of ∼ 50,000 (R/Ra ∼ 15) was obtained in one sample, but we cannot completely exclude the possibility of a cosmogenic 3He contribution for this sample. Our study illustrates that the neon systematics can be more capable than helium to constrain the ultimate origin of hotspots in geodynamic settings dominated by plume-ridge interaction

Os isotope systematics in Fogo Island: Evidence for lower continental crust fragments under the Cape Verde Southern Islands

International audienc

Rhenium–osmium isotope systematics in MORB from the Southern Mid-Atlantic Ridge (40–50 S)

International audienceWe present 27 osmium isotopic analyses and Re–Os contents in MORB glasses from the South Atlantic ridge (40°–50° S), an area showing an important mantle source heterogeneity expressed by large variations in trace element and Sr, Nd and Pb isotope compositions. Os concentrations are very low and range from 1.37 to 12.47 ppt and Re concentrations range from to 0.44 to 2.13 ppb. The 187Os / 188Os ratios vary from 0.1314 to 0.2316 and define a global positive correlation with 1 / Os. A multidimensional analysis of our dataset reveals that 187Os / 188Os ratio is independent from the other isotopic systems or that it provides complementary information on the nature of the processes affecting the upper mantle. Using trace element and Sr isotopic ratios to distinguish the different processes affecting the South Atlantic MORB source, our results enable an assessment of their effects on the Os isotopic composition. The variable 187Os / 188Os ratios of N-MORB samples could be attributed to metasomatism of the asthenospheric mantle by melt percolation related to an ancient subducting zone. The 187Os / 188Os ratio appears also to be very sensitive to plume-ridge interaction and its latitudinal variation along the 47.35°–48.5° S segment confirms the migration of material from the Discovery plume southward the Agulhas FZ. Finally, associated to Pb-Sr-Nd isotopic ratios, the 187Os / 188Os ratios measured in E-MORB from the 48.5°–49.2° S segment confirm that delaminated lower continental crust is responsible for the DUPAL signature observed in the South Atlantic and Indian Oceans

Lead isotopic systematics of major river sediments: a new estimate of the Pb isotopic composition of the Upper Continental Crust

International audienceThe lead isotopic composition of river sediments is reported in the present work for the Earth's major river basins, from old cratonic to young orogenic areas and from subarctic to tropical climates. Sediment samples from these large river basins provide a useful tool to calculate the average upper crustal composition because they are large-scale integrated samples of the weathering products of the present-day Upper Continental Crust (UCC). Two different and complementary calculations were done to estimate the average lead isotopic composition of the UCC. The first, based on the flux weighted average of particulate lead delivered by the rivers, gave values of 19.07, 15.74 and 39.35 for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios, respectively. To avoid over-estimating the contribution of orogenic areas, which produces a bias (because the flux of particulate lead depends strongly on the physical erosion rate), a second calculation was done by averaging with drainage areas of each river basin. This gave values of 18.93, 15.71 and 39.03 for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios, respectively. These direct calculations of the lead isotopic composition of the UCC are similar and are in agreement with previous estimates made using an indirect approach

Automated separation of Sr from natural water samples or carbonate rocks by high performance ion chromatography

An automated separation of strontium prior to isotopic analysis from liquid samples (river water, seawater or solutions resulting from natural carbonate dissolution) was developed using High Performance Ion Chromatography (HPIC) technique. The experimental set up consists of an automatic sampler, an ion chromatograph and a fraction collector. To evaluate the performance of the set up, we compare the separation of strontium performed by HPIC with routine techniques which are manually operated using Sr-Spec resin ([Birck, J.-L.,1986. Precision K–Rb–Sr isotopic analysis; application to Rb–Sr chronology. Chemical Geology, 56, 73–83; Horwitch et al., 1990; Pin, C. and Bassin, C., 1992. Evaluation of a strontium-specific extraction chromatographic method for isotopic analysis in geological materials. Analytica Chemica Acta, 269, 249–255]) for 18 river-waters with very different Sr concentrations and Ca / Sr ratios, 5 seawater samples, and two solutions obtained by partial dissolution of Cenozoic high carbonate content marine sediments. The 87Sr / 86Sr isotopic ratios were measured by TIMS. Results agree within error at acceptable statistical scatter that can mostly be accounted for by the uncertainty of the mass spectrometry. Blank values average at 127 pg, but contamination is more than 90% introduced by the eluent and can be reduced if necessary. We show also that the conductivity cell detector after eluent suppression is sensitive enough to accurately measure these very low concentrations and thus that blank values can be evaluate directly by the HPIC set up. The major advantage of this technique is the saved handling time. Moreover, this technique benefits also from a number of on-line controls. During the procedure, Sr and Ca concentrations in the sample are determined. Within the same analytical procedure, others major elements such as Na, K, or Mg concentration can also be evaluated. In addition, the efficiency and the purity of the separation can easily be monitored by determination of Rb, Ca, and Sr content of the collected fraction. We show that the HPIC-separation technique has a great potential for dating marine carbonate sediments and for other high-resolutions studies. Extension to rare earth and lead separations is foreseen in the near future as well as an extension to Mg isotopic analysis. Silicate analysis is anticipated to be a more difficult step