Eric Lewin

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. A 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. Different 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. By 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.

Stochastic melting of the marble cake mantle: evidence from local study of the East Pacific Rise at 12°50′N

Abstract Isotopes (Nd, Sr and Pb) and trace elements (REE, Ba, Sr, Rb) have been measured on a set of basaltic glasses from a restricted area (40 × 10 km) at 12°50′N on the East Pacific Rise. The huge variation of incompatible element concentrations (factor 70 for Ba concentrations), and the variable degrees of correlation between element concentrations cannot be explained by usual models of melting and fractional crystallization. A rough correlation between the Ce/Yb ratio and the isotopic ratios favors a “source effect” for the genesis of the glasses. We have developed a model including both partial melting process acting on a heterogeneous mantle source with two components (peridotites and pyroxenites; “marble cake mantle” of Allegre and Turcotte) and fractional crystallization. The purpose of this model is not to obtain values of the four parameters involved (degree of melting in the peridotites, in the pyroxenites, proportion of pyroxenites involved in the melting, degree of fractional crystallization) for each analyzed glass, but to model the whole set of glasses by stochastic genesis and sampling of liquids. We have used the stochastic procedure for the four controlled parameters, currently generating 10, 000 “samples”. Our preferred model for this portion of the East Pacific Ridge is obtained with a degree of melting in the peridotites and in the pyroxenites varying uniformly from 6 to 20%, and from 6 to 50% respectively. The degree of mixing between liquids issued from the two sources varies from 0 to 100%, and the degree of fractional crystallization remains small, without noticeable effect on the concentrations, varying from 0 to 6%.

Thorium/uranium ratio of the Earth☆

Through the study of komatiites and ophiolites, remnants of the old mantle, we review the evolution of the lead system in the depleted mantle through time. Using the 230Th232Th vs. 208Pb★206Pb★ diagram, (208Pb★206Pb★ = “radiogenic” lead ratio), we constrain the Th/U ratio of the bulk silicate Earth to 4.2. This value is also in full agreement with ratios inferred from the study of the various types of meteorites. Using this value, in the [208Pb★206Pb★, 143Nd144Nd] diagram, bulk-Earth plots on the oceanic basalt correlation line, as previously guessed. A comparison with Sr and Nd data shows that the Th/U fractionation correlates with that of Rb/Sr and Nd/Sm whereas the U/Pb fractionation is independent of other ratios.

Boron isotope systematics in large rivers: implications for the marine boron budget and paleo-pH reconstruction over the Cenozoic

Abstract The chemical composition of the oceans and long-term climate changes are believed to be linked. Reconstruction of seawater pH evolution is desirable as pH may be related to atmospheric pCO2, and hence to climate evolution. Boron isotopes in oceanic carbonates have been suggested to be a proxy for oceanic paleo-pH reconstruction. Nevertheless, the calculation of paleo-pH values over geological periods requires a precise knowledge of the boron isotopic composition of the oceans when calcite precipitated. We present the systematics of boron isotopic composition of the worlds main rivers. We deduce a continental boron flux to the oceans of 38×1010 gB/year with a mean isotopic composition of +10‰. These results lead to a balanced boron budget in the oceans and allow the development of a model for the marine boron secular evolution over the past 100 Myr. It is shown that the oceanic boron cycle is mainly controlled by the boron continental discharge and the boron uptake from the oceans during low temperature alteration of oceanic crust. However, the recent important increase of the clastic sediment supply, linked to the Himalayan erosion, impacts the oceanic boron budget by enhancing significantly the boron uptake by adsorption on sediments. We predict a boron isotopic composition in the oceans lower during the Cenozoic and slightly higher during the Cretaceous than today. The modelled values for the marine boron isotopes follow the variations of boron isotopes in carbonates over the Cenozoic era provided by previous studies, suggesting that the variations of the seawater pH may not have been important on this time scale. If this is the case, it involves that buffering mechanisms occur in the oceans to maintain seawater pH at a roughly constant value against past atmospheric pCO2 variations.

Chemical structure and history of the Earth: evidence from global non-linear inversion of isotopic data in a three-☐ model

Recent contributions [1–4] have shown the important role of budget equations in improving the isotopic and chemical ratio determinations for the principal geochemical reservoirs on the Earth. We now combine information from both contributions (SrNd isotopic couples [1,2] and the three Pb isotopic couples [3,4]) and complete them by adding the Hf and Os isotopic couples. The inversion of the budget equations and the model-ages-crustal age histogram relationships permits us to begin a systematics of the behaviour of chemical elements through the master geodynamical phenomenon of crust formation, and to calculate a parameter accounting for the “large scale” behaviour of each chemical element, that we call the sialic affinity. The use of a C1 chondrite chemical concentration set permits us to bound the concentration factor of the bulk silicate Earth to the depletion factor of the mantle. Using the best present estimates of the bulk Earth uranium content, we compute the size of the depleted reservoir; it is found to be larger than the upper mantle, defined to be above the 650 km discontinuity (about 1.2–1.6 times the upper mantle mass). This result implies that, for a layered Earth model, and exchange has occurred between the upper and lower mantles.

A coherent crust-mantle model for the uranium-thorium-lead isotopic system

Abstract When the Th/U ratio of the bulk silicated Earth is constrained to be 4.2, it provides a reference for the UThPb isotope systematics which permits the following budget approach for the crust-mantle system: the model gives the present-day continental crust/residual mantle isotopic balance using general inversion technique. Due to the nonlinearity of U decays, the model ages are expressed as functions of the mean age, spread and skewness of the statistical age histogram of the present-day continents. The mean age of the continents is found to be 1.9 ± 0.4 Ga, with a corresponding spread of 1.2 ± 0.6 Ga and a negative skewness (for the Atlantic-Pacific model, and similar values for the Indian model). A set of coherent isotopic ratios and element concentrations is given for U, Th and Pb in the residual mantle, primitive mantle and continental crust.

Isotopic systems and stirring times of the earth's mantle

Abstract We have developed a simple model which describes the evolution of isotopic and chemical heterogeneities of radiogenic systems in a reservoir undergoing differentiation, input and convection. We demonstrate that for parent-daughter isochron diagrams, such as those for mantle materials, the slope of the correlation line gives the approximate stirring time, defined as the characteristic time of homogenisation due to convection. In the case of the lead-lead isotopic diagram ( 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb) the slope of the correlation corresponds to an age of the order of twice the stirring time. We then constrain the stirring time of the whole upper mantle at around 0.9 Ga, and that for the MORB source reservoir alone at about 220 Ma.

Differences between oceanic basalts by multitrace element ratio topology

Trace element multidimensional analysis of ocean island basalts defines a structure delimited by the same four end members (Hawaiian Islands, St. Helena-Tubuai-Mangaia Islands, Kerguelen-Gough-Tristan da Cunha and the Society Islands) as determined by isotope ratios. In contrast to the results obtained for the distribution of isotopic ratios, the dispersions of trace element concentrations in mid-ocean ridge basalts are greater than those for ocean island basalts. This can be accounted for by a two-component mantle source composed of pyroxenite layers embedded in a peridotitic matrix, which melts to varying degrees; ocean island basalts are produced by a relatively uniform low degree of melting of the pyroxenite and limited isotopic exchange with the surrounding matrix, whereas mid-ocean ridge basalts are melts of both components with higher and more variable extents of melting and complete isotopic exchange between the pyroxenite strips and the peridotitic matrix.

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.