François Chabaux

U-Th-Ra Fractionation During Weathering and River Transport

The potential of radioactive disequilibria as tracers and chronometers of weathering processes has been recognised since the 1960’s (e.g., Rosholt et al. 1966; Hansen and Stout 1968). This interest results from the dual property of the nuclides of the U and Th radioactive series (1) to be fractionated during water-rock interactions and (2) to have radioactive periods of the same order of magnitude as the time constants of many weathering processes and chemical transfers to ground and river waters. Therefore, the study of radioactive disequilibria in surface environments should help to bring information about the nature, the intensity but also the time-scale of the water-rock interactions produced during weathering and related chemical transfers. These different properties have justified many of the studies on U-Th series in weathering profiles and river waters. Rosholt (1982), Scott (1982) and Osmond and Ivanovich (1992) gave a synthesis of the studies of U-series in weathering and surface hydrology up to the 1980’s and the 1990’s respectively. Here, we present the main directions taken in these domains over the last decade. They were partly stimulated by the analytical developments made in the measurement of the medium half-life nuclides of 238U series (i.e., 234U-230Th-226Ra) in the mid-1980’s, namely, the use of thermal ionisation mass spectrometry (TIMS) (e.g., Chen et al. 1986; Edwards et al. 1987; Cohen et al. 1991; Chabaux 1993;Chabaux et al. 1994) and more recently the use of MC ICP-MS (Turner et al. 2001; Robinson et al. 2002) for U and Th isotope analysis. Details of these developments are given in this volume (Goldstein and Stirling 2003). Compared to the radioactive counting methods previously used, the new techniques permit (1) a reduction by one order of magnitude or more of the size of the sample required for …

(234U/238U) activity ratios in freshwaters as tracers of hydrological processes: the Strengbach watershed (Vosges, France)

Abstract (234U/238U) and 87Sr/86Sr isotopic ratios, as well as major and trace (U, Ba, Sr, Rb) element concentrations were analyzed in dissolved loads of the Strengbach stream (Vosges, France) in order to constrain the sources of U isotopes (234U–238U) to river waters. The variations of the (234U/238U) activity ratios along the stream indicate a clear dependence between the (234U/238U) activity ratios of the surface waters and the different types of rocks forming the watershed, with near equilibrium values for the waters draining granites and high (234U/238U) disequilibria (1.4) for those flowing over carbonates. The high (234U/238U) disequilibria are related to a supply of 234U-enriched groundwaters located within the carbonate rocks. The (234U/238U) activity ratios of the waters at the outlet of the catchment collecting the sources of the stream on the granitic lithology, decrease from 1.02 to 0.96 when the discharge of the stream increases. Such a tendency requires mixing between a water body enriched in 234U which weathered the granitic bed rock at secular equilibrium, and a water with a (234U/238U) activity ratio below unity representing a mobilization of U from material that has already been weathered. Comparison of the geochemical characteristics of waters collected during the year and those collected during a flood event, reveals the involvement of two different “weathered” end-members, depending on the hydrological conditions: during the year, the dissolved U transported by the river originates from bed-rock and deep horizons of the weathering profile, whereas a significant part of U, during the flood event, is supplied by superficial horizons of soils, probably complexed by organic colloids. These results outline the potential of (234U/238U) activity ratios to investigate hydrological processes and emphasize that their use as tracers of rock weathering should be made cautiously.

The calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance

Calcium isotope ratios relative to seawater were studied in natural waters in order to constrain the Ca isotopic flux to the oceans and to discuss the oceanic Ca budget. This study includes the analysis of (1) Ca isotopes in continental waters at the regional scale of the Upper Rhine valley and (2) the determination of the δ44Ca values of some major rivers and hydrothermal vents. The data indicate that the global δ44Ca variability of the studied waters, with a maximal range of 1‰, is limited and must be linked to mass fractionation processes. At the scale of a small watershed (Aubure, Vosges, France), the δ44Ca river flux changes due to the variation of the proportion of waters resulting from chemical alteration of rocks and biologically fractionated soil solutions. Ca isotopes could therefore be an important tool to quantify the impact of the plants and the biosphere on the river water chemistry. At the regional scale of the Rhine valley as well as at the global scale of world rivers, the δ44Ca variability of river samples ranges from 0.5 to 1‰ without any relationship with the lithology of the watershed or the climate. Similarly, the δ44Ca value of hydrothermal vents is also uniform and close to the mean value of river waters. Consequently, the δ44Ca flux to the ocean should remain rather constant through time with a mean of −1.1±0.2‰. This value confirms that the Ca budget of the present-day seawater is in steady-state. This was not necessarily the case in the past. Non-steady-state periods should have been due to intensity variations of incoming and outcoming Ca fluxes rather than to the variations in their isotopic signatures.

Chemical mobilizations in laterites: Evidence from trace elements and 238U-234U-230Th disequilibria

Geochemical and mineralogical investigations, including measurements of major and trace elements, Sr isotope ratios, and 238U-234U-230Th activity ratios, were made on an old African laterite to reconstruct its formation steps and assess recent chemical mobilization. The present data support a scenario of discontinuous formation for the laterite, with different bedrock weathering conditions during the formation of each unit, rather than a scenario of continuous formation. Absolute accumulation of Fe, U, and lanthanides in the uppermost ferruginous unit suggests an autochthonous origin of this iron cap by leaching of an older overlying profile. Present chemical distributions of lanthanides, as well as of Rb, K, Ba, and Sr, within the profile cannot be linked to the mineralogical distribution of both relictual primary and authigenic secondary phases. Complementary lanthanide patterns indicate that these elements were primarily accumulated in the uppermost ferruginous unit before further remobilization and accumulation in the underlying horizons. These redistribution processes may be related to the chemical instability of the ferruginous cap. The 238U-234U-230Th disequilibria indicate that recent U mobilization occurs in the whole profile and that, as for lanthanides, there is a vertical redistribution of U from the uppermost ferruginous unit to the underlying horizons. Moreover, these data show that both U losses and gains exist at each level of the profile. A simple modeling of this double U mobilization process is proposed to interpret the 238U-234U-230Th data. Differences in the mobilization and fractionation intensities of the U input and removal processes can account for the two evolution trends, which distinguish the ferruginous unit from the underlying ones. Furthermore, on the basis of this modeling, the profile appears to be in a transient state because of recent changes in the U mobilization conditions, which could correspond to major Pleistocene climatic variations.

238U230Th226Ra disequilibria in volcanics: A new insight into melting conditions

Using new mass spectrometry techniques developed for the analysis of Ra isotopes, we present 238U230Th226Ra disequilibria data from a variety of volcanic settings, and we compare them with previously published data. Two correlations are observed with alkali volcanic data, one between [230Th238U] and [230Th226Ra] and another between the intensity of the disequilibria and the buoyancy flux of the underlying plume. These two correlations prove that partial melting is the major cause of UThRa fractionations in this volcanic context. The 238U230Th226Ra disequilibria then place new constraints on some parameters of the classical melting models (batch melting and dynamic melting). We show, nevertheless, that these estimates remain very speculative. The comparison of 238U230Th226Ra disequilibria in alkali volcanics, carbonatites and subduction zones shows a clear parallel between the disequilibria value and the type of volcanic context. Such a parallel reflects the diversity of the conditions of magma generation, and shows that the 238U230Th226Ra disequilibria systematics are very dependent on the chemical composition of liquids produced during magmatic processes. A systematic difference is observed between disequilibria in MORB and in alkali volcanics, which could indicate that the melting processes in these two volcanic contexts are very different.

Isotopic tracing of the dissolved U fluxes of Himalayan rivers: implications for present and past U budgets of the Ganges-Brahmaputra system

U activity ratios have been measured in the dissolved loads of selected rivers from the Himalayan range, in Central Nepal, and from the Bangladesh, as well as in some rain waters. A few European and Asian rivers have also been analyzed for their U activity ratios. The data confirm the negligible effect of rainwater on the budget of dissolved U in river waters. The results also indicate that rivers on each Himalayan structural unit have homogeneous and specific U isotope compositions: i) (234U/238U) activity ratios slightly lower than unity in the dissolved load of the streams draining the Tethyan Sedimentary Series (TSS); ii) values slightly higher than unity for waters from the High Himalaya Crystalline (HHC) and the Lesser Himalaya (LH); iii) systematically higher (234U/238U) activity ratios for waters from the Siwaliks. Thus, U activity ratios, in association with Sr isotopic ratios, can be used to trace the sources of dissolved fluxes carried by these rivers. Coupling of U with Sr isotope data shows (1) that the U carried by the dissolved load of the Himalayan rivers mainly originates from U-rich lithologies of the TSS in the northern formations of the Tibetan plateau; and (2) that the elemental U and Sr fluxes carried by the Himalayan rivers at the outflow of the highlands are fairly homogeneous at the scale of the Himalayan chain. Rivers flowing on the Indian plain define a different trend from that of the Himalayan rivers in the U-Sr isotopic diagram, indicating the contribution of a specific floodplain component to the U and Sr budgets of the Ganges and the Brahmaputra. The influence of this component remains limited to 10 to 15 percent for the U flux, but can contribute 35 to 55% of the Sr flux. The variations of the Sr and U fluxes of the Ganges-Brahmaputra river system in response to climatic variations have been estimated by assuming a temporary cut off of the chemical fluxes from high-altitude terrains during glacial episodes. This scenario would significantly decrease the dissolved U flux of the Ganges-Brahmaputra river system and increase its U activity ratio. Such a climatic dependence of the Himalayan U flux could induce a periodic variation of the mean U activity ratio of the world rivers on glacial-interglacial timescales

Chemical weathering of basaltic lava flows undergoing extreme climatic conditions: the water geochemistry record

This study was dedicated to the early stage of the weathering of historic basaltic flows located in Mount Cameroon. The combination of high relief (i.e. 0 to 4071 m) and high rainfall range (i.e. 1.8 to 12 m/year) lead to strong climatic contrast. Spring and rivers were sampled all around the volcano. We report here the basic chemistry of the waters as well as strontium and uranium isotopic ratios. The combination of the molar proportions of solute obtained with the modal amounts of the minerals in the basalts gives a prediction of what should be the relative molar concentrations of major compounds in the weathering waters issuing from Mount Cameroon. The measured Alkalinity/Si and Mg/Si ratios are higher than the calculated ones while the measured Ca/Si ratio is equal to the calculated value. We suggest that the Si-poor waters of Mount Cameroon are due to biological pumping, trapping of Si in Fe-silicate minerals such as Si containing ferrihydrite and Si interaction with bacterial cell wall leading to the formation of allophane type minerals which were observed in Mount Cameroon soil profiles. Calcium uptake by plants explains the lower Ca/alkalinity ratios measured in the water samples. The water–rock ratio (R) calculated from the strontium isotopic compositions of the water samples, ranges from 29,452 to 367,450. The calculated weathering rates (WR) range from 1 to 20 mm/ky and from 1 to 103 mm/ky for high and low elevations, respectively, and agree with both the thickness and the age of paleosoils found in the same area and with previously published estimates from coupled reaction-transport models. This difference emphasizes the role of vegetation and rainfall at lower elevations as compared to what happens at high elevations.

Chapter 3 U-Series Geochemistry in Weathering Profiles, River Waters and Lakes

Publisher Summary This chapter summarizes the recent applications of U-series nuclides to study weathering and river transport. The chapter focuses on the determination of time-scales of continental weathering and erosion, based on U-series analysis in soils, rivers, as well as in lakes. Analysis of U-series nuclides in soils and alterites can provide information on the behavior of radionuclides during weathering in the natural environment and also on the nature and time-scale of weathering processes. The chapter also indicates that determination of the time-scales of weathering processes from U-series data in soils requires a full understanding of the processes that control U–Th–Ra fractionation in regoliths. In parallel with the development of U-series disequilibrium studies in soils and weathering profiles U-series disequilibria in rivers have been used to establish weathering mass balances and/or to determine weathering time-scales at the scale of a watershed. Studies of U- and Th-series nuclides in lake waters provide information on their geochemical behavior and mixing characteristics of lakes. Some of these studies complement the investigations in river waters. Lake sediments, on the other hand, hold records of recent environmental changes. Measurements of U–Th nuclides in these sediments and associated mineral precipitates can help chronologically decipher the records in them. It has been shown that U-series disequilibria in lake water and sediment could also provide a wealth of information about the physico-chemical processes operating in lakes, such as the source and fate of elements in sediment and water, the residence time of particles or radionuclides in lakes, mixing or turnover time-scales of lakes. The chapter briefly presents some of the results obtained in these areas of research. These findings are certainly one of the major advances in U- and Th-series nuclide applications in the earth sciences and have laid the foundation for further developments in this field.

Dating of weathering profiles by radioactive disequilibria : contribution of the study of authigenic mineral fractions

Abstract Soil isotopic dating is important for understanding mass transfers related to rock weathering. Here we evaluate a method including TIMS analyses of (234U/238U) activity ratios on

High performance automated ion chromatography separation for Ca isotope measurements in geological and biological samples

Natural mass-dependent fractionation of calcium isotopes is a promising tool for investigating Ca pathways and cycling in geological and biological materials. But since natural isotope fractionation of Ca appears to be extremely limited (∼1.25‰/amu), excellent external precision and sensitivity are needed to make full use of its potential. Here, we describe a new Ca purification procedure that consists of a high selectivity automated ionic chromatography separation protocol, which is suitable for Ca isotope measurements by mass spectrometry and applicable to multiple natural matrixes (waters, mineral and organic samples). The analytical progress in this automated technique are multiple: (1) saving time with a minimum of handling, (2) unique operating protocol whatever the nature of the sample, (3) complete separation of Ca from K, Mg and Sr, avoiding isobaric interferences which are critical during TIMS analysis, and (4) Ca separation by peak recognition optimising the full recovery of Ca even if its retention time is shifted from one sample to another. The two latter advantages ensure a Ca recovery yield close to 100%, leading to the absence of any fractionation of Ca isotopes during the chemical clean-up. Thus, this chemical separation will be of special interest for applications not compatible with the use of the double spike technique such as MC-ICP-MS and 40Ca excesses measurements. Additionally this procedure leads to a twofold improvement of the long-term repeatability of the Ca isotopes determination by TIMS (±0.11 δ44/40Ca, 2SD) as compared with a classical resin chemistry protocol and is similar to the best repeatability published so far (±0.10 δ44/40Ca, 2SD).