Bernard Dupré

Erosion sources determined by inversion of major and trace element ratios and strontium isotopic ratios in river water: The Congo Basin case

Dissolved and suspended load river material represents the integrated products of the erosion of drainage basins. To enlarge the study of erosion processes we have determined87Sr/86Sr ratios and the Cl, Na, Mg, Ca and Sr contents for the main tributaries of the Congo River Basin, both for water and suspended sediment. We have also analyzed 30 streams draining monolithological terranes. n nA systematic study of precipitation has permitted the estimation of a good rain correction factor. Sr isotopic ratios have shown that the seawater input correction based on riverine Cl content is not valid in the Congo Basin because a large part of the Sr, Ca and Mg come from a terrestrial source. The conventional atmospheric input correction by reference to the marine ratios underestimates the real atmospheric input because of the crustal elements carried by rainwaters. n nDifferent erosion source parameters have been obtained for carbonates, evaporites and silicates. An inversion scheme has been developed to compute the multimixing equations and allows the quantification of the input of each main reservoir (atmosphere, carbonates, evaporites and silicates) for each tributary and each element. For Ca and Mg, rainfall and carbonate dissolution are the main inputs. For Sr, the input is mainly controlled by rains and silicate weathering. n nBy using Sr isotopic systematics we have calculated the Sr isotopic composition of the silicate weathered crust for each of the main tributaries of the Congo Basin. We obtain uniform values for the main tributaries ranging between87Sr/86Sr= 0.7195 ± 0.001 and 0.7251 ± 0.005. These results allow the calculation of strontium model agesTSr, which differ from neodymium model agesTNd. UsingTNd, we have calculated the87Rb/86Sr of the silicate weathered crust. We obtain homogeneous values close to 0.75, which is in agreement with estimates for the average silicate crust. The discrepancy betweenTSr andTNd may be linked to the vegetation impact which fractionates Rb and Sr.

SrNdPb isotope systematics in Amazon and Congo River systems: constraints about erosion processes

Abstract 87 Sr 86 Sr , 143 Nd 144 Nd , 206 Pb 204 Pb , 207 Pb 204 Pb and 208 Pb 204 Pb isotopic ratios and Rb, Sr, Sm, Nd, U, Pb and Th concentrations have been measured in the suspended loads of the Congo and Amazon rivers and their tributaries. In the dissolved load, 87 Sr 86 Sr , Rb, Sr, Nd, Sm, U, Pb and Th concentrations are also reported. These results show that Nd, Sm, Th and Pb are almost insoluble and that their mass balance is controlled by particulates whereas Rb, Sr and U are fractionated between soluble and particulate phases. The 87 Sr 86 Sr ratios can only be interpreted after computing the amount of carbonate recycling and the partitioning for silicates between soluble and insoluble. This paper presents a method based on the lead isotopic system that can be used to test the steady state of erosion which is tacitly assumed in many river and erosion studies. The results presented show that the steady state is validated in each river of the Congo Basin and in the lowland rivers of the Amazon Basin, but in not verified in the rivers from the Andes (Rio Solimoes and Rio Madeira). 87 Sr 86 Sr , 206 Pb 204 Pb and 208 Pb 204 Pb ratios are positively correlated and negatively correlated with 143 Nd 144 Nd ratios. A number of arguments indicate that these correlations are mixing lines, the end-members being orogenic and shield components, respectively. For the Amazon Basin, analysis of the river loads shows that the orogenic zone is favoured some 5 times more than the shield on an equal area basis. This leads to a reinterpretation of Smue5f8Nd data for shales and casts some doubts upon the proposition of secular variations in the Sm Nd ratio of the continental crust.

Helium and lead isotope geochemistry of the Azores Archipelago

New helium and lead isotopic data for basalts from the Azores archipelago (North Atlantic) show that the Azores have 4He/3He ratios both higher and lower than MORB values. Good covariations of helium and lead isotopes are observed at the scale of the archipelago, and suggest the coexistence of two mantle components in the Azores which are identified by data from Sao Miguel and Terceira. The eastern part of Sao Miguel island displays radiogenic helium (4He/3He > 140,000, R/Ra 100,000) were observed at latitude higher than 40°N and may reflect the influence of the Sao Miguel component at the ridge.

Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics

Abstract In order to explore mantle heterogeneity through geological time, Pb isotopic compositions have been determined for 8 differentiated-layered intrusions whose ages are between 2.7 and 0.05 b.y.. The Pb-Pb ages of these intrusions and the U-Pb characteristics of their parent sources are discussed. The Pb-Pb dating method is found to be applicable for this type of basic or ultrabasic rocks and agrees satisfactorily with other available methods. Significant differences are found between the calculated values for the parent bodies of these intrusions. This could reflect either mantle heterogeneity since Archean times, or contamination of some of the bodies by continental crust. Discrimination between these hypotheses can be proposed from the positions of the initial Sr ratios of these massives in respect to the supposed “terrestrial” evolution line. The two intrusions which plot on that closed system Sr evolution line (Muntsche Tundra, U.S.S.R., and Skaergaard, Greenland) belong also to a simple two-stage evolution model for Pb, with a low first-stage μ value of 7.8. If these two bodies are considered as pieces of a “primitive” closed-system mantle, a 4.55 ± 0.01 age of the earth can be calculated from their Pb initial ratios.

The lead isotope systematics of ophiolite complexes

Samples of eleven ophiolites from the Mediterranean and the Pacific belts have been studied through PbPb systematics. n nThe ophiolites studied show both isotopic variations within one sequence and differences between the various complexes. The intra-sequence isotopic variations result principally from the in situ decay of uranium and thorium since the formation of the ophiolites. μ ratios (238U/204Pb) higher than 50 in the effusive part of ophiolites are required to explain the206Pb/204Pb spread within the sequences, and such values are notably higher than those presently measured in the oceanic crust. This uranium enrichment with respect to lead is due to the combined effect of the magmatic fractionations and of chemical exchanges during marine alteration. Pb loss must be an important factor for theU/Pb increase. Since the207Pb/204Pb ratio is not affected by recent radioactive decay, it represents a very useful tracer for the discussion of the genetic environment of the ophiolites. n nThree main types of Pb isotopic characteristics may be distinguished among ophiolite complexes, on the basis of their207Pb/204Pb: n n-|The ophiolite complexes of In Zecca (Corsica), Semail (Oman) and Toba (Japan) show207Pb/204Pb similar to the least radiogenic present-day MORB. These complexes probably originated in regions geochemically comparable to the present-day depleted mantle. However, the Semail ophiolite has a higher208Pb/204Pb ratio than MORB, which may be compatible with an origin in an interarc basin or premature arc. n n n-|The circum-Pacific ophiolites of Papua New Guinea, New Caledonia and Zambales (Philippines) have207Pb/204Pb comparable to the present-day “transitional” portions of the ridges. n n n-|Finally the circum-Mediterranean ophiolites of Troodos, Vourinos, and Antalya, display207Pb/204Pb ratios clearly higher than those of the present accretion zones. Thus, their genesis must have involved a significant continental component, and an origin in an island arc is therefore favoured. Some cases of large isotopic heterogeneities are also observed within the Antalya and Troodos complexes.

Variability of trace elements in basaltic suites

Abstract This paper discusses the distribution of trace element concentrations in cogenetic suites of lavas, based on the study of trace element histograms. For each histogram, the variability is calculated ( V = s/m , where m is the mean concentration and s its standard deviation). The study of two basaltic series, each specific of a single igneous process (fractional crystallization and partial melting), shows that trace element variabilities are directly related to bulk partition coefficients. Considering different trace element behaviour models, quantitative relationships are established between the variability and the descriptive parameters for the processes (bulk partition coefficients, mean degree of fractionation and related standard deviation). An important feature is that the variability is independent of the initial concentrations in the source. This method is applied for comparison to several basalt suites on which case studies of magmatic processes have been conducted. It is also applied to study the behaviour of trace elements in different geodynamical contexts (hotspot, mid-ocean ridge and arc volcanism). Finally, a set of numerical values of bulk partition coefficients is proposed that accounts for N-MORB genesis.

Application of element concentration variability to the study of basalt alteration (Fangataufa atoll, French Polynesia)

Abstract Lavas sampled in a drill hole at Fangataufa atoll, French Polynesia, have been studied to investigate trace-element behaviour during alteration processes. While most of these elements are not affected by alteration, U, Cs, Rb and Ba show various degrees of mobility due to alteration phenomena. Mobility is different for the different lithological units: hyalotuffs, hyaloclastites, submarine lava flows, and basaltic dykes and sills. Gain or loss of trace elements are studied using the properties of both variability, V ( V = σ m , where m is the mean concentration of an element and σ its standard deviation) and correlation coefficient between elements. The results allow a distinction to be made between open-system alteration, with chemical gains or losses, and closed-system alteration, in which dissolution-precipitation mechanisms keep concentrations unchanged: Cs is removed from subaerial basaltic intrusions. In submarine lava flows, alteration leads to U and Ba losses. In hyaloclastites, we notice Cs gain, U and Ba losses, and Rb redistribution. In hyalotuffs, alteration leads to U and Cs gains, and Ba loss.