Andrea R Voegelin
email: voegelin@geo.unibe.ch

Thomas F Nägler

Thomas Pettke

Nadja Neubert

Marc Steinmann

Olivier Pourret

Igor M Villa

The impact of igneous bedrock weathering on the Mo isotopic composition of stream waters : Natural samples and laboratory experiments

River waters have been shown to be systematically enriched in the heavy molybdenum (Mo) isotopes when compared to typical granites and basalts, which generally possess Mo isotopic compositions (δ 98/95 Mo) of around 0 . This in-

consistency has been used to argue against weathering of crustal rocks as the cause for heavy riverine δ 98/95 Mo signatures. Incongruent dissolution of primary bedrock, however, may be an important process by which the anomalous Mo signatures of the river dissolved load are produced. This study therefore investigates the effect of igneous crustal rock weathering on the aquatic δ 98/95 Mo signal by comparing stream water and bedrock Mo isotope data to results of bulk rock leach experiments. For this purpose, stream water and bedrock (orthogneiss, granite, basalt), as well as soil and vegetation samples were collected in a small catchment in the French Massif Central. In accordance with the results of earlier studies on riverine Mo, both streams are isotopically heavier (δ 98/95 Mo = 0.5 to 1.1 ) than the typical granites and basalts. The excellent agreement of these data with those of Mo released during experimental leaching

Introduction

River transport is the main process controlling fluxes of most elements from continents to oceans [START_REF] Garrels | Evolution of Sedimentary Rocks[END_REF][START_REF] Gaillardet | Trace elements in river waters In Treatise in Geochemistry[END_REF]. The marine isotope and element inventory is thus strongly dependent on continental weathering processes and subsequent river transport to the ocean basins. In the case of the highly redox sensitive molybdenum (Mo), the continental contribution accounts for the predominant part of the marine Mo budget [START_REF] Morford | The geochemistry of redox sensitive trace metals in sediments[END_REF][START_REF] Mcmanus | Oceanic molybdenum isotope fractionation: Diagenesis and hydrothermal ridge-flank alteration[END_REF][START_REF] Mcmanus | Molybdenum and uranium geochemistry in continental margin sediments: Paleoproxy potential[END_REF]. Once dissolved Mo has entered the oceans, redox-dependent isotope fractionation accompanies its incorporation into sediments, covering all environments from oxic to strongly euxinic.

As each of these environments shows characteristic isotope signatures [START_REF] Barling | Natural mass-dependant variations in the isotopic composition of molybdenum[END_REF]Barling & Anbar, 2004;[START_REF] Siebert | Molybdenum isotope record as a potenial new proxy for paleoceanography[END_REF][START_REF] Siebert | Molybdenum isotope signatures in continental margin marine sediments[END_REF][START_REF] Neubert | Sulfidity controls molybenum isotope fractionation into euxinic sediments: Evidence from the modern Black Sea[END_REF], Mo isotopes in marine sediments have been used to investigate the evolution of atmospheric O 2 and to quantify the extent of seafloor anoxia in the geological record (e.g., [START_REF] Arnold | Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans[END_REF][START_REF] Siebert | PGE, Re-Os, and Mo isotope systematics in Archean and early Proterozoic sedimentary systems as proxies for redox conditions of the early Earth[END_REF][START_REF] Wille | Evidence for a gradual rise of oxygen between 2.6 and 2.5 Ga from Mo isotopes and Re-PGE signatures in shales[END_REF][START_REF] Pearce | Molybdenum isotopie evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early Jurassic[END_REF][START_REF] Voegelin | Molybdenum isotopes in late Archean carbonate rocks: Implications for early Earth oxygenation[END_REF]. All of these models rely upon the assumption of a fairly uniform long-term riverine Mo isotope input signature of around 0 based on the available δ 98/95 Mo data of crustal igneous rocks (-0.1 to +0.3 ;[START_REF] Siebert | Molybdenum isotope record as a potenial new proxy for paleoceanography[END_REF]. [START_REF] Arnold | Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans[END_REF] additionally included continental molybdenites (average of -0.1 , [START_REF] Barling | Natural mass-dependant variations in the isotopic composition of molybdenum[END_REF] in their model. [START_REF] Archer | The isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans[END_REF], [START_REF] Pearce | Molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain[END_REF] and [START_REF] Neubert | The molybdenum isotopic composition in river water: Constraints from small catchments[END_REF], however, have revealed not only a preferential enrichment of river waters in the heavy isotopes but also a large variability of their δ 98/95 Mo signature (-0.13 to 2.3 ). The pronounced discrepancy between the assumed crustal background and the aquatic signature thus emphasizes the need for a more thorough investigation of isotope fractionation processes during chemical rock weathering in the terrestrial environment.

Recent publications by

The heavy Mo isotopic composition of sedimentary source rocks was found to be reflected in the associated river water δ 98/95 Mo, suggesting a predominant control of catchment outcrop weathering [START_REF] Neubert | The molybdenum isotopic composition in river water: Constraints from small catchments[END_REF]. Thereby, sulfate weathering and sulfide oxidation were proposed to play a crucial role in liberating Mo from the different source rock types. Leach experiments performed by [START_REF] Liermann | Extent and isotopic composition of Fe and Mo release from two Pennsylvanian shales in the presence of organic ligand and bacteria[END_REF] on black shales document an enrichment of the solution in heavy Mo isotopes. The offset between the starting material and the leach solutions was interpreted to be caused by adsorption of dissolved Mo to Feand Mn-(oxyhydr)oxides, as they preferentially adsorb light Mo [START_REF] Barling | Natural mass-dependant variations in the isotopic composition of molybdenum[END_REF][START_REF] Siebert | Molybdenum isotope record as a potenial new proxy for paleoceanography[END_REF][START_REF] Goldberg | Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides[END_REF]). An analogous process, i.e. adsorption of Mo onto the suspended load during river transport, was suggested as a potential removal process of light Mo in natural environments [START_REF] Archer | The isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans[END_REF][START_REF] Pearce | Molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain[END_REF]. Finally, the same authors proposed that soil retention of light Mo is an important process to control river water Mo.

This study investigates Mo isotope fractionation processes during weathering of crustal igenous rocks (basalt, granite, orthogneiss) in a small catchment basin located in the French Massif Central. Although weathering of magmatic rocks has in the past not been associated with significant Mo isotope fractionation due to their small δ 98/95 Mo variability, stream waters analyzed here are enriched in the heavy isotopes. In order to identify the role of various Mo sources and weathering processes, stream water and bedrock data were complemented by measurements of the suspended load, soil material and vegetation. The Mo data of natural samples were compared to results of successive bulk rock leaching experiments. These experiments were conducted on all three bedrock lithologies in order to simulate the weathering behavior of different crustal igneous rock types and their role in generating the observed heavy aquatic δ 98/95 Mo signals.

Special emphasis was thereby placed on the effect of mineral dissolution and adsorption effects. To identify the Mo hosting phases and to constrain mass balance, laser ablation ICP-MS was used to obtain element concentrations of single mineral grains and sulfidic melt inclusions.

Study site and sampling

Geological setting

The mixed basaltic-granitic catchment basin is located in the southern part of the French Massif Central (Fig. 1). It covers an area of around 68 km

Sampling

Stream water was collected during two field campaigns in June 2003 (see [START_REF] Steinmann | Controls on transport and fractionation of the rare earth elements in stream water of a mixed basaltic-granitic catchment basin (Massif Central, France)[END_REF] and June 2010. Sampling locations are shown in Fig. 1 and listed with respect to their upstream distances from the catchment outlet in Table 1. All waters were filtered on site using 0.45µm nylon filters and Nalgene™ filtering units. Subsequently, they were acidified with distilled nitric acid and stored in pre-cleaned LDPE bottles for isotope and trace element analysis. Anion determinations were done on filtered, non-acidified aliquots.

Filters were dried and weighed prior to and after water sampling in order to determine the suspended load for each filter. Basaltic bedrock samples were collected in the riverbed of the study area. Additional basalt material was collected in a quarry located north of the catchment outlet (Fig. 1) as these rocks were particularly fresh. Well preserved granite and orthogneiss samples were collected during earlier fieldwork. One soil sample was taken in a swampy area on the basalt plateau. A soil sampler was used to recover the topmost ca. 30 cm. In the lab, root material for Mo analyses was extracted from the soil.

Rock sample descriptions

Basalt sample PA-1 is a fine grained olivine basalt, containing medium sized phenocrysts of olivine and clinopyroxene and small phenocrysts of plagioclase, embedded in a dense microcrystalline matrix. The opaque phases are predominantly titanomagnetites. Opaque sulfidic melt inclusions occur as small ( <30µm) droplets as identified in silicate phenocrysts. Basalt M29-R is mineralogically identical with smaller olivine, pyroxene and plagioclase phenocrysts when compared to sample PA-1. The biotite rich orthogneiss (AR-7) has a medium to coarse grained texture. It is composed of quartz, potassium feldspar and plagioclase and contains only few oxides and sulfides. The fine grained granite (LC-1) is dominated by quartz, potassium feldspar, biotite, plagioclase and contains some muscovite. Apatite and zircon occur as accessory phases and oxides and sulfides are rare.

Analytical methods

Leach experiments

In order to investigate the behavior of Mo isotopes during progressive rock weathering, rock samples were exposed to acid leach experiments under oxidizing conditions using 0.3 and 2 mol L -1 HCl and HNO 3 . The experiments were performed at low pH ( ≤1) to preclude secondary mineral formation [START_REF] Pistiner | Lithium-isotope fractionation during continental weathering processes[END_REF] and adsorption related Mo isotope fractionation (see section 5.2.3). These conditions should insure that the impact of primary mineral dissolution on the leach solution δ 98/95 Mo is isolated.

Experiments were conducted on both basalt grains and powders, and orthogneiss and granite powders. Samples were prepared by cutting off sections affected by rock weathering. Subsequently, slabs were crushed in a hydraulic press. Grains of 0.1-0.5 mm in size were separated, sonicated and rinsed repeatedly with high purity H 2 O ( >18.2 M Ωcm -1 ). Powders were ground in an agate ball mill, and multiple aliquots were processed to determine representative bulk rock δ 98/95 Mo compositions and concentrations for all rocks. All sample splits were weighed into screw-top PTFE beakers prior to leaching. Up to eight grams of basalt grains were immersed in 50 ml 0.3 mol L -1 HNO 3 and 0.3 mol L -1 HCl at room temperature for a period between 10 minutes and 2 months. Powder samples were processed using 0.3 mol L -1 HNO 3 at room temperature and 2 mol L -1 HNO 3 at a constant temperature of 50 • C. Periods of between 2 minutes and 7 days were chosen for these experiments. This setup produced leach solutions with a considerable range of fractional Mo release. Due to the overall low Mo content, particularly of the orthogneiss and granite ( <0.4 µg g -1 ), each leach experiment was performed on an individual sample split. Separation of acid solutions from rock materials was done by centrifugation and subsequent filtering through 0.2 µm nylon syringe filters. Finally, the Mo isotopic compositions and concentrations of leach solutions as well as selected residues were analyzed.

Preparation of water, rock, soil and vegetation samples

Prior to any sample processing and chemical Mo purification procedures a double spike with masses 100 Mo + 97 Mo (see Siebert et al., 2001 for details) was added to all sample types in order to account for any potential Mo isotope fractionation during column chemistry [START_REF] Anbar | Precise determination of mass-dependent variations in the isotopic composition of molybdenum using MC-ICPMS[END_REF][START_REF] Siebert | Determination of molybdenum isotope fractionation by double-spike multicollector inductively coupled plasma mass spectrometry[END_REF] and to correct for instrumental mass bias during measurement. Double (HF) and triple (HCl and HNO 3 ) distilled acids and 30% suprapure hydrogen peroxide were used for all digestion and purification steps. Powdered bulk rock samples were treated with concentrated HCl + H 2 O 2 and HF + HNO 3 dissolution steps to ensure complete digestion of the silicate matrix [START_REF] Siebert | Determination of molybdenum isotope fractionation by double-spike multicollector inductively coupled plasma mass spectrometry[END_REF][START_REF] Wille | Evidence for a gradual rise of oxygen between 2.6 and 2.5 Ga from Mo isotopes and Re-PGE signatures in shales[END_REF]. Soil material was dried at 60 • C and subsequently sieved to separate it from rock pebbles and root material. Roots were cleaned in an ultrasonic bath and rinsed repeatedly with high purity H 2 O to remove any residual soil particles.

Soil and root samples were digested using multiple HNO 3 and HF steps closely following the procedure described in [START_REF] Cenki-Tok | The impact of waterrock interaction and vegetation on calcium isotope fractionation in soil-and stream waters of a small, forested catchment (the Strengbach case)[END_REF]. Between 0.5 and 2 liters of filtered stream water were evaporated prior to column chemistry.

Chemical purification and isotope analysis of Mo and Sr

Mo was purified from all samples using an anion exchange column (1 mL Dowex 1X8 resin, 200-400 mesh). A cation exchange column (2 mL Dowex 50WX8 resin, 200-400 mesh) was used additionally to remove any residual iron (procedure after [START_REF] Siebert | Determination of molybdenum isotope fractionation by double-spike multicollector inductively coupled plasma mass spectrometry[END_REF][START_REF] Wille | Evidence for a gradual rise of oxygen between 2.6 and 2.5 Ga from Mo isotopes and Re-PGE signatures in shales[END_REF]. For Sr purification, eluted matrix solutions, obtained from Mo anion column separation, were evaporated and loaded onto Sr Spec™ columns. For MC-ICP-MS measurements, the evaporated Mo and Sr fractions were re-dissolved in 0.5 mol L -1 HNO 3 . Mo isotopic compositions and concentrations as well as Sr isotopic compositions were measured at the University of Bern on a double-focusing ®Nu-Instruments MC-ICP-MS connected to an Apex™ desolvating nebulizer. The Mo isotopic composition is measured relative to a standard solution (Johnson Matthey, 1000µg mL -1 (±0.3%) ICP standard solution, lot 602332B). This standard is 2.3 below modern open ocean seawater [START_REF] Siebert | Molybdenum isotope record as a potenial new proxy for paleoceanography[END_REF]. Final Mo isotopic data are reported as δ 98/95 Mo=[( 98 Mo/ 95 Mo) Sample /( 98 Mo/ 95 Mo) Standard -1]× 10 3 . The data presented were acquired with a preferred quantity of >60 ng and a minimum of 30 ng of Mo in solution. Total chemistry Mo blank was <1 ng. The external standard reproducibility of the δ 98/95 Mo ratio is ±0.1% at the 2σ level [START_REF] Siebert | Molybdenum isotope record as a potenial new proxy for paleoceanography[END_REF]. For Sr isotope analyses the samples were diluted down to an optimum amount of 50 to 100 ng. The external reproducibil-ity of the NIST SRM 987 standard during the period of the present analyses was 0.710235±0.000029 (2σ). The external reproducibility of of the NIST SRM 987 standard measured by [START_REF] Steinmann | Controls on transport and fractionation of the rare earth elements in stream water of a mixed basaltic-granitic catchment basin (Massif Central, France)[END_REF] was at 0.710259±0.000016 (2σ).

Major anion and cation analyses of stream waters and leach solutions

Major anion and cation contents of all river water samples (except M31-Ft and M31-Fu) were determined on 0.22µm filtered sample fractions at the laboratory of Chrono-Environnement, CNRS / University of Franche-Comté, Besançon (France). Na + , K + , Ca 2+ and Mg + were analyzed with a Perkin Elmer A Analyst 100 atomic absorption spectrometer (AAS) on sample aliquots acidified with HNO 3 to pH 2 after filtering. F -, Cl -, NO - 3 and SO 2- 4 were measured with a Dionex DX100 high-pressure ion chromatograph on unacidified sample aliquots. HCO - 3 concentrations were determined in the field within a few hours after sampling (in order to limit exchange with atmospheric CO 2 ) on unfiltered and unacidified samples by standard titrimetric methods considering total and carbonate alkalinity as equivalent and equal to HCO - 3 . Typical uncertainties including all error sources for major cations, anions and alkalinity lie between ±4

% and ±6 %, depending on the concentration levels [START_REF] Steinmann | Controls on transport and fractionation of the rare earth elements in stream water of a mixed basaltic-granitic catchment basin (Massif Central, France)[END_REF][START_REF] Binet | Variability of the groundwater sulfate concentration in fractured rock slopes: a tool to identify active unstable areas[END_REF]. Major anion and cation concentrations of river water samples M31-Ft and M31-Fu as well as SO 2- 4 concentrations of the leach solutions were determined using a Metrohm®861 Advanced Compact Ion Chromatograph at the University of Bern.

Laser ablation ICP-MS analyses of bedrock minerals

LA-ICP-MS analyses were performed on silicate phenocrysts, oxides, sulfide melt inclusions (entirely enclosed in the host mineral) and the fine grained matrix of basalt sample PA-1. Measurements were performed on a GeoLas-Pro 193 nm ArF excimer laser system in combination with a Perkin Elmer Elan DRC-e quadrupole mass spectrometer at the University of Bern. Instrumental conditions were similar to those reported in [START_REF] Pettke | Analytical protocols for elment concentration and isotope ratio measurement in fluid inclusions by LA-(MC)-ICP-MS[END_REF]. Major and trace element concentrations were measured using spot sizes between 16 and 120 µm, with the 120 µm beam used for bulk matrix compositions. Bracketing standardization using SRM 610 from NIST was used for instrument sensitivity calibration and drift correction. The standard contains sixty-one trace elements doped in a Si-Na-Ca-Al matrix, most of which are homogeneously distributed [START_REF] Eggins | Compositional heterogeneity in NIST SRM 610-617 glasses[END_REF]. Data quantification used the SILLS software package [START_REF] Guillong | Sills: A matlab-based program for the reduction of laser ablation ICP-MS data of homogenous materials and inclusions[END_REF], employing the major element oxide total of 100 wt% for internal standardization. For sulfide melt inclusions, data were treated following [START_REF] Halter | Major to trace element analysis of melt inclusions by laser-ablation ICP-MS: methods of quantification[END_REF]. The mixed inclusion plus host mineral signal was deconvolved by assuming that no SiO 2 is present in the sulfide melt inclusion; hence, the Mo concentrations obtained are those present in the pure sulfide inclusion. than in 2010 (0.02-0.15 ng g -1 ). Field parameters and further chemical compositions of the stream waters are listed in Table 2.

Results

4

Analyses performed on multiple splits of all three igneous rock types (Table 3) reveal different signatures ranging from a minimum of -0.28 , detected in an orthogneiss split (median δ 98/95 Mo=-0. for the orthogneiss and the granite. The basalts show median Mo concentrations of 3.5 µg g -1 (PA-1) and 2.4 µg g -1 (M29-R). Two basalt sample splits reached Mo concentrations as high as 7.7 and 4.8 µg g -1 , indicating a nugget effect on bulk rock Mo concentrations. The concentrations measured in this study are more variable compared to the assumed upper crust value of 0.78-1.5 µg g -1 (Rudnick & Gao, 2004, and references therein).

Bedrock Sr isotope data are taken from [START_REF] Steinmann | Controls on transport and fractionation of the rare earth elements in stream water of a mixed basaltic-granitic catchment basin (Massif Central, France)[END_REF]. Table 3 lists values typical for the individual rock types. A pronounced difference exists between the high 87 Sr/ 86 Sr ratios of orthogneiss and granite (typically between 0.720-0.750) and the low ratios of the basalt (ca. 0.703). The stream waters show the 87 Sr/ 86 Sr ratio typical of basalt on the plateau and a smalll increase of the 87 Sr/ 86 Sr ratio downstream (Table 1), reflecting the contribution of gneiss and granite.

Single grain and matrix element concentrations determined by LA-ICP-MS

Element concentrations of single minerals and the basalt matrix (averages) are given in Table 4, data of sulfidic melt inclusions in Table 5. The silicate phenocrysts have Mo concentrations reaching 0.1 to 0.2 µg g -1 in olivine and pyroxene, and between 1 and 2 µg g -1 in plagioclase. The Fe-Ti oxides contain between 0.5 and 1.2 µg g -1 Mo. The fine grained basalt matrix has an average Mo concentration of 3.5 µg g -1 with minimum values at 2.7 µg g -1 and a maximum at 5.5 µg g -1 . The sulfide melt inclusions have strongly variable Mo contents ranging from values <0.05µg g -1 to as high as 250 µg g

Soil, vegetation and suspended load

The soil sample, as well as root material extracted from it, show moderately negative δ 98/95 Mo values of -0.33 and -0.14 (Table 6). A concentration of 0.67 µg g -1 was found in the roots, the soil is more enriched with a concentration of 2.4 µg g -1 . Data of the suspended load are given in Table 7. The amount of suspended load lies between 1. of sample M29-R, the isotope compositions are slightly higher at lower leach fractions ( ≤ 15%) and decrease at higher fractions. Leach residues of PA-1 experiments show a broad range of δ 98/95 Mo values from 0.12 to -0.29 (Fig. 4). The lowest values were observed at the longest leach times, corresponding to the highest proportion of extracted heavy Mo.

The different leach setups produced different Mo release patterns (Fig. 6A).

HCl and HNO 3 grain leach solutions show increasing concentrations reaching a maximum of 0.3 µg g -1 . After a rapid initial Mo release the leaching rate slowed. The largest Mo release occured during the 2 mol L -1 HNO 3 powder experiments where maximum Mo concentrations of 1.9 µg g -1 (PA-1) and 0.7 µg g -1 (M29-R) were extracted from the basalts. The interaction between basalt powders and 0.3 mol L -1 HNO 3 produced solutions with considerably lower Mo concentrations (0.2-0.4 µg g -1 ). Also, a quick initial Mo release is followed by a concentration decrease with time. Sulfate concentrations of leach solutions increased with time, correlating with increasing Mo concentrations (R 2 =0.9 linear correlation; Fig. 6B).

Orthogneiss and Granite leach solutions

Powder leach experiments performed on the orthogneiss (AR-7) extracted between 1 and ca. 100% of the bedrock Mo inventory (Table 8, Fig. 5C). The isotopic compositions of the leach solutions vary between 0.11 and -0.15 .

Leach rates for the granite (LC-1) are much lower since only approximately 1-8 % of the total Mo were released applying the same leach times. Showing values between 1.03 and 1.38 , the granite leach solutions are isotopically much heavier than those of the orthogneiss (Fig. 5A).

Discussion

Natural samples: bedrock and stream waters

The catchment morphology promotes the formation of well developed weathering profiles on the plateau and hampers rock-water interaction in the steep granitic and orthogneissic hillsides. As a consequence, the mafic rocks, highly susceptible to weathering processes [START_REF] Gislason | Meteoric water-basalt interactions: I. a laboratory study[END_REF], are favored as the primary lithological control on the stream water geochemistry. Their predominance is shown by the aquatic Nd and Sr isotope data of [START_REF] Steinmann | Controls on transport and fractionation of the rare earth elements in stream water of a mixed basaltic-granitic catchment basin (Massif Central, France)[END_REF] and Sr data of this study (Table 1), which mainly reflect the signature of the basalt; the contribution from granites and orthogneisses is subordinate.

Data collected on all three riverbed rock types indicate an analogous basalt dominance for the river dissolved Mo system as the median Mo concentration of the mafic rocks of 2.4 and 3.5 µg g -1 is about one order of magnitude higher than that of the felsic bedrock types ( <0.4 µg g -1 ; Fig. 3). However, unlike the Sr and Nd isotope signatures, the δ 98/95 Mo of both the Malaval and the Séjallières do not concur with the isotopic composition of the basalt but are enriched in the heavy isotopes. With a median value of 0.58 the granite is isotopically slightly heavier than the previously reported magmatic rocks by [START_REF] Siebert | Molybdenum isotope record as a potenial new proxy for paleoceanography[END_REF] and represents the only rock type with a δ 98/95 Mo signature approximating that of the stream waters. Yet the granite cannot account for the isotopically heavy waters of the Séjallières, as it is crosscut only by the Malaval. The orthogneiss potentially contributes Mo to both the Séjallières and the lower segment of the Malaval. Nevertheless, the extremely low concentrations (median=0.08 µg g -1 , n=7) exclude the orthogneiss from being an important contributor to the overall aquatic Mo budget. The moderate stream water δ 98/95 Mo variations do not coincide with the sampling distance from the catchment outlet (Fig. 2). The lack of any systematic δ 98/95 Mo downstream trend of the river dissolved load, despite changing environmental conditions (i.e. bedrock type, soil cover, topography), as well as the overall small variability of the aquatic Mo signature may further support the idea of a single dominant source rock. The waters of both streams sampled in 2003 show a tendency to slightly higher δ 98/95 Mo values and a greater variability (0.7-1.1 ) than in 2010 (0.6-0.9 ). This marginal isotopic shift coincides with an increase in the amount of suspended particles, which was generally higher in 2003 than in 2010 (Table 7). Mo isotope fractionation through adsorption of Mo to (oxyhydr-)oxide particles (Barling & Anbar, 2004;[START_REF] Goldberg | Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides[END_REF] might thus be considered a process contributing to the isotopic composition of the river dissolved load. This process was previously proposed to cause the suspended load to be isotopically lighter than the dissolved load [START_REF] Pearce | Molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain[END_REF] in M29-R and 30-55% in PA-1; Table 4).

The Mo distribution among the different rock components is fairly heterogeneous. Thereby, moderately to strongly enriched phases can be distinguished from depleted phases (Tables 4 and5). Also, while the Mo content of the sulfide melt inclusions is highly diverse, all other rock constituents show minor Mo variability. Mass balance calculations indicate that when combined, the low concentration oxides and silicate phenocrysts only account for a minor fraction of the total Mo rock inventory (3-4 %; Table 4) and, hence, their role in producing the leach patterns is irrelevant. The Mo content of basalts is intimately connected to how evolved the parent basalt magma is [START_REF] Arnórsson | Molybdenum and tungsten in volcanic rocks and in surface and <100 • C ground waters in Iceland[END_REF][START_REF] Audétat | Source and evolution of molybdenum in the porphyry Mo(-Nb) deposit at Cave Peak[END_REF]. Therefore, the offset between the generally depleted silicate phenocrysts and the moderately enriched silicate matrix is attributed to fractional crystallization, which results in a progressive Mo enrichment of the magma [START_REF] Audétat | Source and evolution of molybdenum in the porphyry Mo(-Nb) deposit at Cave Peak[END_REF]. Accordingly, the depleted phenocrysts are likely to record the composition of the early magma, while the matrix represents the evolved, residual melt, where Mo accumulated due to its incompatibility. The three types of sulfide melt inclusions (Table 5) reflect different stages of silicate melt evolution because of their magmatic equilibrium coexistence. They

show distinct Mo concentration patterns with each stage, finally resulting in the pronounced Mo variability.

Due to the low Mo concentrations of silicate phenocrysts and oxides, the sulfide phases and the matrix remain primary sources of Mo and, hence, dominate the Mo budget of the solution. It has been argued previously that sulfide dissolution is an important weathering process, very likely to generate distinguishable geochemical patterns of the riverine dissolved load [START_REF] Neubert | The molybdenum isotopic composition in river water: Constraints from small catchments[END_REF]. The elevated Mo contents found in some of the melt inclusions and the rapid progression of sulfide weathering in oxygenated environments [START_REF] Anbar | A whiff of oxygen before the great oxidation event?[END_REF] here leads to the hypothesis that the preferential dissolution of the magmatic sulfides causes the isotopically heavy Mo in the basalt leachates. Increasing SO 2- 4 concentrations, which go along with heavy δ 98/95 Mo and increasing Mo concentrations of the liquid phase (Fig. 6A), support this interpretation. If the sulfides are the primary and most readily dissolved Mo source, then the isotopic composition of the leachates would require them to host at least 25 to 30 % of the total bedrock Mo (Fig. 4) despite being rare ( <1 % modal abundance).

Mass balance calculations are therefore used to check whether the experimentally derived data can be explained by the sulfides alone. Within the limits set by the geochemical and optical data (Tables 4 and5) the system shows a very high sensitivity to changes of both sulfide concentration and abundance.

Changing these parameters for all other components has a much smaller effect.

The calculations presented in Table 9 are thus based on constant, representative values for silicate phenocrysts, oxides and the matrix. By changing the sulfide parameters, the entire range of experimentally released heavy dissolved Mo can be reproduced. These data demonstrate that even few magmatic sulfide grains can account for a large fraction of the total bedrock Mo inventory. As leaching progresses the contribution from other rock components becomes progressively more important. Supporting evidence is given by the significant δ 98/95 Mo drop observed in the residues at leach amounts >30 %, suggesting that the heavy sulfide reservoir is exhausted and that the influence of an isotopically lighter component increases. This component is most likely the matrix, since it accounts for the largest fraction of the bulk rock Mo inventory associated with preferentially weathered devitrified glass. Gradual admixture of isotopically lighter matrix-derived Mo also explains the lowered δ 98/95 Mo of the solution at a leach amount of 54 % (δ 98/95 Mo = 0.42 , Fig. 4).

On its own, the experimental data (Figure 4) could theoretically be reconciled with equilibrium fractionation. In the context of all available information (sulfate and LA-ICPMS data as well as mass balance calculations), however, the proposed model is the most likely. A simple two-component model (solidliquid) does not realistically describe the data, as we are dealing with a threecomponent system (solution-sulfides-silicate matrix). Finally, sulfide is identified as the source of heavy δ 98/95 Mo due to the correlation with sulfate (R 2 =0.9 linear corrleation; Fig. 6B).

These data strongly suggest that Mo isotopes are fractionated between coexisting silicate and sulfide melts in basaltic systems, with sulfide melt being isotopically heavier than silicate melt. Therefore, the results indicate that Mo isotope fractionation is likely to occur at hot magmatic conditions. Iron isotope fractionation of similar magnitude has previously been observed at magmatic temperatures between sulfide (phyrrotite) and silicate melt [START_REF] Schuessler | The experimental calibration of the iron isotope fractionation factore between pyrrhotite and peralkaline rhyolitic melt[END_REF] as well as between silicate (fayalite) and magnetite [START_REF] Shahar | Equilibrium high-temperature Fe isotope fractionation between fayalite and magnetite: An experimental calibration[END_REF].

It remains to be established whether the observed Mo isotope fractionation is primarily due to crystallographic control of phases involved in such processes or is rather a consequence of changing intensive properties such as density.

The role of Mo adsorption to Fe-Ti oxide surfaces during experimental dissolution

As mentioned earlier, the low pH of the leaching agents should prevent secondary mineral formation during the experiments [START_REF] Pistiner | Lithium-isotope fractionation during continental weathering processes[END_REF] and isolate Mo isotope fractionation related to primary mineral dissolution. In particular, Fe 3+ does not precipitate and form additional phases [START_REF] Johnson | Isotopic constraints on biogeochemical cycling of Fe[END_REF]. Mo adsorption to metal (oxyhydr)oxide surfaces, however, is increased under these conditions (e.g., [START_REF] Goldberg | Molybdenum adsorption on oxides, clay minerals, and soils[END_REF][START_REF] Xu | Adsorption of molybdate and tetrathiomolybdate onto pyrite and goethite: Effect of ph and competitive anions[END_REF][START_REF] Kim | Adsorption of molybdate onto hematite: Kinetics and equilibrium[END_REF] and thus Mo adsorption to the preexisting titanomagnetite surfaces may have caused Mo isotope fractionation unrelated to mineral dissolution. Since Mo adsorption to Fe-Mn (oxyhydr)oxide surfaces preferably removes the light isotopes from solution [START_REF] Siebert | Molybdenum isotope record as a potenial new proxy for paleoceanography[END_REF]Barling & Anbar, 2004;[START_REF] Goldberg | Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides[END_REF], the observed isotope fractionation (∆ 98 Mo) between leach solutions and basalt could potentially be attributed to adsorption rather than to the dissolution of specific bedrock minerals. Assuming that the experimentally determined fractionation factor between magnetite and the dissolved phase (∆ 98 Mo=0.83 ±0.6 ; [START_REF] Goldberg | Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides[END_REF] is applicable to Ti bearing magnetite as well, the predicted basalt-leachate isotope fractionation agrees with the isotope fractionation between solid and solution observed in this study (∆ 98 Mo=0.4-0.9).

However, fractionation factors of [START_REF] Goldberg | Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides[END_REF] were determined at high pH (6.9-8) with Mo present as the tetrahedrally coordinated MoO 2- 4 . At the low pH, at which the experiments of this study were conducted ( ≤1), Mo is exclusively present in the octahedral form [START_REF] Cruywagen | Spectrophotometric determination of the thermodynamic parameters for the first two protonation reactions of molybdate[END_REF][START_REF] Cruywagen | Molybdenum (VI) equilibria at high perchloric acid concentration[END_REF][START_REF] Ozeki | Estimation of the dissolved structures and condensation reactivities of mononuclear molybdenum (VI) species in solution using the UV-vis absorption spectra and molecular orbital calculation DV-Xα[END_REF]. Since fractionation of Mo isotopes during adsorption to (oxyhydr)oxide surfaces occurs dominantly as a result of the transition from the tetrahedrally to the octahedrally coordinated Mo species [START_REF] Tossel | Calculating the partitioning of the isotopes of Mo between oxidic and sulfidic species in aqueous solution[END_REF][START_REF] Goldberg | Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides[END_REF], the fractionation factor between solid and solution decreases with more acidic pH, i.e. high proportions of octahedral Mo in solution. Consequently, ∆ 98 Mo is expected to be strongly reduced in the present experiments and the observed large fractionation cannot be attributed to adsorption onto the Fe-Ti oxide surfaces. This prediction is supported by the Mo concentration as well as isotope data. First, although the decreasing Mo contents observed in the 0.3 mol L -1 powder experiments is compatible with adsorption, no concomitant change in δ 98/95 Mo occurs (Fig. 4). Secondly, the increasing Mo concentrations observed in all other experiments suggest that adsorption is not significant. Fi-nally, in a two phase closed system equilibrium process exhibiting a constant fractionation factor, as it is the case in the experiments here, any liquid will show increased δ 98/95 Mo values with increasing Mo adsorption. The δ 98/95 Mo of the leach solutions, however, show no such trend (Fig. 4). Hence, the observed large isotope fractionation cannot be attributed to adsorption onto the Fe-Ti oxide surfaces. The discrepancy between the concentration data of the 0.3 mol L -1 and the 2 mol L -1 leaches (decreasing vs. increasing Mo concentrations, Fig. 6B) may conceivably be a function of variable pH. Adsorption reaches a maximum between pH 2 and 4 [START_REF] Goldberg | Molybdenum adsorption on oxides, clay minerals, and soils[END_REF] and the subsequent implied decrease could gradually limit adsorption at pH ≤2, resulting in the differing concentration patterns of the leachates.

Implications for the natural environment

The interpretation of the leach patterns indicates that incongruent phase dissolution of bulk rocks in general, and fast magmatic sulfide dissolution in particular, greatly influences how Mo isotopes are disproportionated during crustal bedrock weathering. With respect to the natural environment, the close match between the δ 98/95 Mo signatures of stream waters and experimental leach solutions suggests that the dissolution of isotopically heavy magmatic sulfides during basalt outcrop weathering is likely to be the key factor in producing the aquatic signatures of the Malaval and the Séjallières. These results raise important questions pertaining to the relevance of soil formation and adsorption processes, which have been considered vital in producing heavy river water δ 98/95 Mo signatures [START_REF] Archer | The isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans[END_REF][START_REF] Pearce | Molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain[END_REF]. According to the combined results of leach solutions and stream waters, soil formation and adsorption appear to have a minor effect on the isotopic signature of the dissolved Mo in the catchment basin under investigation. However, the soil sampled on the plateau exhibits a negative δ 98/95 Mo, typically produced by Mo adsorption to Fe-Mn (oxyhydr)oxides. Even so, this is most probably not the primary process causing the two streams to be enriched in the heavy Mo isotopes, and based on the experimental data an alternative process is proposed here. Incongruent chemical basalt weathering preferentially releases isotopically heavy magmatic sulfide Mo that dominates the dissolved Mo inventory. In the case of the Malaval and the Séjallières, the extraction of sulfide-bound Mo within the regolith cover is reflected in the aquatic sulfate concentrations, which are elevated in the lower parts of the catchment where streams are affected by regolith-derived waters (Table 2). Similarly, other stream water parameters also show an enrichment in the lower regions. This effect may be additionally amplified by rain water admixture to the surface waters on the plateau, because precipitation is generally depleted in Mo (e.g., [START_REF] Kawakubo | Physicochemical speciation of molybdenum in rain water[END_REF].

The isotopically heavy Mo of river waters is most likely the result of a complex interplay between various processes involved, and the mass balance between the competing factors likely controls the aquatic δ 98/95 Mo. The local system is highly dependent on rock mineralogy, degree of rock weathering and soil development, secondary mineral precipitation and the availability of mineral surfaces for adsorption. In the case presented here, magmatic sulfide mineral dissolution within the basaltic riverbed and regolith appears to dominate all other processes.

Finally, future studies will not only have to focus on Mo isotope fractionation during mineral dissolution and soil formation, but also on the importance of vegetation in the terrestrial Mo cycle, because the role of Mo as an essential micronutrient required by plants is well known (e.g., [START_REF] Mendel | Cell biology of molybdenum[END_REF][START_REF] Fitzpatrick | Molybdate transport throught the plant sulfate stranporter SHST1[END_REF]. The root and soil Mo isotope and concentration data presented here indicate that Mo uptake into the biomass is considerable and, therefore, may be an important additional pathway for the removal isotopically light Mo in the weathering environment.

Consequences for the global Mo isotope budget

The observation that most Mo isotopic compositions of dissolved river loads are heavier than most of the silicate magmatic rocks analysed so far raises two related problems. Firstly, the fate of the light Mo isotopes missing in the balance needs to be evaluated; secondly, the effect of isotopically heavy river water on the constancy of the continental input needs to be quantified. Different explanations for the heavy Mo isotopic composition of river waters have been put forward. [START_REF] Archer | The isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans[END_REF] proposed retention of light Mo isotopes in soils, while [START_REF] Neubert | The molybdenum isotopic composition in river water: Constraints from small catchments[END_REF] proposed the Mo isotopic composition of catchment source rocks as the predominant control on the dissolved riverine δ 98/95 Mo, stressing that sedimentary rocks (especially marine carbonates and sulfates) have a different δ 98/95 Mo from the magmatic silicates. The hypothesis of incongruent rock weathering presented in this paper is not meant to be a substitute for the former explanation, but rather a complementary process. The relative effect of the processes that affect the Mo isotopic composition of rivers on the global Mo isotopic cycle depends on the time scale. On a global scale, the total volume of soil (or weathered rock volume) at any given time is limited. If light Mo isotopes are preferentially retained in soil and/or weathering residues, erosion will liberate them sooner or later. When the integrated volume of soil formation is equal to the integrated soil erosion, the net Mo isotope effect will be zero. This is certainly true for very long time-scales. Likewise, whenever the globally integrated incongruent weathering of (sulfide bearing) fresh rocks is balanced by erosion of already weathered rocks, the Mo isotope effect will also be zero. The fact that to date there are no reports of magmatic rocks with a particularly low δ 98/95 Mo (i.e. residues of intense sulfide weathering) may well be related to an intentional sampling bias: to constrain the magmatic Mo isotopic composition of such rocks, fresh samples were collected and measured, not highly weathered ones. The only way to bias the total continental runoff towards an isotopic composition heavier than the continental crust (magmatic plus sedimentary rocks) requires a continuous net increase of the volume of soils and weathered rocks on a global scale, which, however, is not a long-term steady state that can be maintained over geological times. On the other hand, the hypothesis that the δ 98/95 Mo of catchment rocks accounts for the heavy Mo isotopic composition of river water is independent of the need to achieve a steady state. It applies to the whole geological period, for which marine sediments exposed to subaerial erosion show a heavy δ 98/95 Mo signature; basically this includes post Archean times.

Even if soil processes and incongruent weathering are transient, there is ample possibility that they do influence the Mo isotopic composition of the continental runoff in selected periods of Earth's history, namely when the Earth system was exceptionally far from steady state. Amongst the conceivable examples are large subaerial basaltic eruptions (LIPs: e.g., Deccan, Siberian plateau, etc.).

Large volumes of magmatic rocks were first produced and then weathered. The short-lived ocean anoxic events at the Permo-Triassic or Cretaceous-Paleogene transitions are temporally related to LIP volcanism. Modelling of anoxic events could potentially be refined by testing for a short-period δ 98/95 Mo modification, which could be attributed to a weathering "peak". As another example, physical weathering during large glaciations could lead to non-steady-state intensive sulfide weathering on the newly exposed rock surfaces. This is in line with the observation of an increased Mo concentration in the Tista river [START_REF] Neubert | The molybdenum isotopic composition in river water: Constraints from small catchments[END_REF], which was interpreted by the authors as physical weathering by a glacier. In any case, it remains to be established if disseminated sulfide minerals generally have a heavier Mo isotopic composition than the coexisting silicates because of equilibrium fractionation, or if this effect is restricted to basalts. In this respect, the study presented here is rather an identification of a process than the quantification of a Mo source.

Conclusions

The for more details on data range. The missing Mo in the mass balance is attributed to the sulfide melt inclusions (Tables 5 and9). . . a 2SE of individual measurements, the external reproducibility of the analytical procedure is ±0.1 (2σ). The 2SE of the Sr-isotope ratios refer to the last two digits.

b87 Sr/ 86 Sr data from [START_REF] Steinmann | Controls on transport and fractionation of the rare earth elements in stream water of a mixed basaltic-granitic catchment basin (Massif Central, France)[END_REF].

Table 2:

Upstream Rock pH Eh EC T F - Cl - NO - 3 SO 2- 4 HCO - 3 Ca 2+ Na + K + Mg 2+ Charge distance type Balance [km] [mV] [µS cm -1 ] [ • C] [mg L -1 ] [mg L -1 ] [mg L -1 ] [mg L -1 ] [mg L -1 ] [mg L -1 ] [mg L -1 ] [mg L -1 ] [mg L -

2

 2 and includes two streams, the Séjallières and the Malaval. The eastern and most elevated part (1301 m a. s. l.) is formed by Quaternary basalts. Downstream and to the west the catchment basin drops to an altitude of 714 m a. s. l. and is characterized by deep and narrow valleys with Hercynian granitic and orthogneissic bedrock. Due to its low inclination, the basalt plateau is covered by well developed soils and swampy areas. The steep orthogneissic and granitic hillsides to the west are covered by forests and exhibit a poorly developed soil cover.

  2 , n=7), to a maximum of 0.70 (median δ 98/95 Mo=0.58 , n=5) in the granite. By contrast, the two basalts PA-1 and M29-R show median δ 98/95 Mo values of 0.14 (n=6) and 0.07 (n=5), respectively. Hence, the basalts are the only igneous bedrock to comply with the Mo isotope signature of the basalts and granites measured by Siebert et al. (2003) ( δ 98/95 Mo = -0.1 to 0.3 ). The three different bedrocks exhibit strongly variable Mo concentrations, with median values of 0.08 µg g -1 and 0.33 µg g -1

  -1 . According to their Fe, Cu and Ni concentrations they can be subdivided into three different inclusion types. Type 1 has high Fe/Cu and Ni/Cu ratios and Mo concentrations <2 µg g -1 , type 2 exhibits moderate Fe/Cu and Ni/Cu ratios and very variable Mo concentrations between 0.3 and 250 µg g -1 , type 3 shows very low Fe/Cu and Ni/Cu ratios and intermediate Mo concentrations between 16 and 80 µg g -1 .

  3 and 19.7 mg L -1 in the 2003 samples, while in 2010 the suspension weight was below detectable limits by weighing. Also, most of the suspended load samples taken in 2010 had Mo concentrations not exceeding Mo blank levels ( <1 ng). Due to the low amount of suspended load in 2010, no Mo isotope composition and concentrations could be determined. Samples taken in 2003 were not available for measurement. 4.4. Laboratory leach experiments 4.4.1. Basalt leach solutions and residues Data of all leach solutions and residues are listed in Table 8. Experiments performed on grains of sample PA-1 show identical leach solution δ 98/95 Mo values for both the HCl and the HNO 3 leach (Fig. 4), all of which are isotopically significantly heavier than the starting material. Mo isotopic compositions lie between 0.61 and 0.84 with up to 9 % of the total rock Mo inventory extracted. Powder leach solutions of the same basalt reveal a similarly narrow range of Mo isotopic compositions (0.42-0.88 ). In these experiments, up to 54 % of the total bedrock Mo were released (Fig. 4). Leaching of basalt M29-R liberated up to around 30 % of Mo and shows solution δ 98/95 Mo signals consistent with those of PA-1 (0.53-0.97 ; Fig. 5). The evolution of the Mo isotope ratio as a function of increased leach fractions is nearly identical for both basalt samples. Between the initial, extremely rapid Mo release and Mo extraction of 30 %, no significant trend appears in the δ 98/95 Mo signal of PA-1. Only at a Mo release of approximately 54 % Mo the δ 98/95 Mo decreases. In the leach series

(

  2003), the basalt and granite leach solutions produced in the present study are not identical to the δ 98/95 Mo compositions of the bulk rock but show a considerable enrichment in the heavy isotopes (Figs.4 and 5A & B). In contrast, Mo freed from the orthogneiss matrix is only very weakly fractionated when compared to the starting material (Fig.5C). These results indicate that the isotopic composition of Mo released during the partial dissolution of igneous rocks is dependent on the bedrock mineralogy. Accordingly, the weathering of igneous crustal rocks is incongruent and likely controls the river dissolved Mo signature. In fact, the striking consistency between the δ 98/95 Mo of the basalt leach solutions (0.5 to 1 ; Figs.4 and 5B) and that of the stream waters (0.6 to 1.1 ; Fig.2) strongly suggests that the δ 98/95 Mo in these streams is controlled by bedrock weathering. Furthermore, it corroborates the interpretation that basalt is the single most important source contributing to the stream water Mo geochemistry in the catchment investigated here. Consequently, Mo host phases and their respective importance for mass balance need to be constrained.5.2.2. Mo released from basalts: host phases and mass balanceThe interpretation of the basalt leach patterns (Figs.4 and 5B) requires knowing the Mo content of the individual rock components. This permits the performance of mass balance calculations to identify the relevant Mo host phases. The discussion focuses on basalt PA-1 (Fig.4), as the larger sized phenocrysts facilitate a precise determination of Mo concentrations of individual components by LA-ICP-MS. Note, that although the two basalts have identical mineralogical compositions and their leach solutions show the same δ 98/95 Mo variability, the modal abundance of the individual rock components may vary between samples M29-R and PA-1. This is expressed by the difference in the fraction of Mo released at identical leach times (2 mol L -1 HNO 3 leach: 20-30%

  Figure 1:

Figure 2 :

 2 Figure 2:

1 ]

 1

  Figure 1:

  Mild acid leach experiments performed by[START_REF] Siebert | Molybdenum isotope record as a potenial new proxy for paleoceanography[END_REF] on granite did not generate a heavy dissolved Mo pool, but released Mo with a δ 98/95 Mo composition indistinguishable from that of the crustal starting material. These data have consequently been used to assume that igneous crustal rock weathering has no importance in producing the heavy δ 98/95 Mo of river waters (e.g.,[START_REF] Nägler | Sedimentary Mo isotope record across the Holocene fresh-brackish water transition of the Black Sea[END_REF][START_REF] Pearce | Molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain[END_REF]. In contrast to the data ofSiebert et al.

	5.2. Experimental primary mineral dissolution
	5.2.1. Incongruent bedrock weathering

, in return increasing the stream water δ 98/95 Mo values. While this may explain the small difference in δ 98/95 Mo between 2003 and 2010, it cannot account for the 0.5 to 1 offset between the basalt bedrock and the stream waters. Even in 2010, when the amount of suspended load is negligible, a large δ 98/95 Mo offset exists between the bedrock and the river dissolved load.

Table 1 :

 1 Rock Upstream δ 98/95 Mo 2SE a Mo 87 Sr/ 86 Sr 2SE a

	. 40

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table 3 :

 3 Splitδ 98/95 Mo 2SE a Mo typical 87 Sr/ 86 Sr values taken from[START_REF] Steinmann | Controls on transport and fractionation of the rare earth elements in stream water of a mixed basaltic-granitic catchment basin (Massif Central, France)[END_REF]. 2SE of the Sr-isotope ratios refer to the last two digits.

	87 Sr/ 86 Sr

b

Table 7 :

 7

		Upstream Rock Suspension
		distance	type weight
		[km]		[mg L -1 ]
	Samples June 2003
	M10	4.9	GSC 1.3 a
	M11	4.8	GSC 5.5 a
	S10	3.8	OG	19.7 a
	S11	3.7	OG	5.6 a
	Samples June 2010
	M28	8.9	BA	b.d.l.
	M29	8.4	BA	b.d.l.
	M31-Ft 4.9	GSC b.d.l.
	M31-Fu 4.8	GSC b.d.l.
	M31-Fd 4.7	GSC b.d.l.
	M32	2.5	OG	b.d.l.
	M33	2.6	OG	b.d.l.
	M34	2.4	OG	b.d.l.

a 2003 data taken from

[START_REF] Steinmann | Controls on transport and fractionation of the rare earth elements in stream water of a mixed basaltic-granitic catchment basin (Massif Central, France)[END_REF]

Table 8 :

 8 Concentrations relative to grams of sample material leached. b difference between bulk rock δ 98/95 Mo composition and δ 98/95 Mo composition of calculated mass balance.

	leach time	leaching agent	Temp.	Mo leached	δ 98/95 Mo in solution	2SE	Mo in solution	δ 98/95 Mo in residue	2SE	SO 2-4 in solution	δ 98/95 Mo M B
				[%]	[ ]		[µg g -1 ] a	[ ]		[µg g -1 ] a	[ ] b
	Basalt: PA-1 grain leach
	10min	0.3 mol L -1 HNO	cold	0.4	0.65	0.04	0.01	0.10	0.04	b.d.l.	-0.04
	0.5h	0.3 mol L -1 HNO	cold	0.8	0.63	0.06	0.03	0.09	0.05	4.4	-0.05
	12h	0.3 mol L -1 HNO	cold	2.3	0.80	0.04	0.08	0.08	0.03	5.6	-0.05
	24h	0.3 mol L -1 HNO	cold	2.7	0.80	0.07	0.09	0.09	0.03	6.3	-0.04
	72h	0.3 mol L -1 HNO	cold	6.1	0.68	0.03	0.21	0.06	0.04	20.0	-0.05
	1w	0.3 mol L -1 HNO	cold	7.1	0.68	0.04	0.25	0.03	0.05	31.7	-0.07
	1m	0.3 mol L -1 HNO	cold	8.5	0.68	0.05	0.30	-0.10	0.05	55.0	-0.18
	2m	0.3 mol L -1 HNO	cold	8.7	0.74	0.05	0.30	0.00	0.05	61.7	-0.09
	10min	0.3 mol L -1 HCl	cold	0.3	0.24	0.05	0.01	-	-	-	-
	0.5h	0.3 mol L -1 HCl	cold	0.8	0.61	0.05	0.03	-	-	-	-
	12h	0.3 mol L -1 HCl	cold	1.9	0.84	0.06	0.07	-	-	-	-
	1w	0.3 mol L -1 HCl	cold	5.8	0.70	0.06	0.21	-	-	-	-
	1m	0.3 mol L -1 HCl	cold	7.0	0.72	0.06	0.23	-	-	-	-
	Basalt: PA-1 powder leach
	10min	0.3 mol L -1 HNO	cold	8.5	0.61	0.05	0.30	0.10	0.06	-	0.00
	100min	0.3 mol L -1 HNO	cold	10.5	0.88	0.07	0.37	0.12	0.02	-	0.05
	1000min	0.3 mol L -1 HNO	cold	7.3	0.88	0.07	0.26	0.09	0.05	-	0.00
	10'000min	0.3 mol L -1 HNO	cold	5.1	0.85	0.05	0.18	-0.05	0.04	-	-0.15
	10min 100min 1000min 10'000min	2 mol L -1 HNO 3 2 mol L -1 HNO 3 2 mol L -1 HNO 3 2 mol L -1 HNO 3	50 • C 50 • C 50 • C 50 • C	28.6 29.2 44.1 54.3	0.57 0.85 0.64 0.42	0.03 0.16 0.04 0.06	1.03 1.02 1.54 1.90	--0.04 -0.29 -0.26	-0.06 0.08 0.10	----	-0.03 -0.09 -0.09
	Basalt: M29-R powder leach
	2min	0.3 mol L -1 HNO	cold	14.2	0.79	0.06	0.34	-	-	-	-
	10min	0.3 mol L -1 HNO	cold	7.0	0.97	0.04	0.23	-	-	-	-
	100min	0.3 mol L -1 HNO	cold	10.5	0.82	0.05	0.25	-	-	-	-
	1000min	0.3 mol L -1 HNO	cold	8.8	0.87	0.06	0.21	-	-	-	-
	10min 100min 1000min 10'000min	2 mol L -1 HNO 3 2 mol L -1 HNO 3 2 mol L -1 HNO 3 2 mol L -1 HNO 3	50 • C 50 • C 50 • C 50 • C	25.4 20.8 29.4 30.0	0.57 0.60 0.56 0.53	0.07 0.06 0.08 0.04	0.62 0.60 0.72 0.72	----	----	----	----
	Orthogneiss: AR-7 powder leach
	10min	0.3 mol L -1 HNO	cold	7.9	0.08	0.07	0.01	-	-	-	-
	100min	0.3 mol L -1 HNO	cold	6.3	0.11	0.05	0.005	-	-	-	-
	10'000min	0.3 mol L -1 HNO	cold	1.4	-0.08	0.10	0.001	-	-	-	-
	100min 1000min 10'000min	2 mol L -1 HNO 3 2 mol L -1 HNO 3 2 mol L -1 HNO 3	50 • C 50 • C 50 • C	26.8 50.9 100	0.02 -0.04 -0.15	0.08 0.07 0.15	0.02 0.04 0.10	---	---	---	---
	Granite: LC-1 powder leach
	10min	0.3 mol L -1 HNO	cold	1.8	1.18	0.04	0.01	-	-	-	-
	100min	0.3 mol L -1 HNO	cold	1.3	1.31	0.05	0.004	-	-	-	-
	10min 100min 1000min 10'000min	2 mol L -1 HNO 3 2 mol L -1 HNO 3 2 mol L -1 HNO 3 2 mol L -1 HNO 3	50 • C 50 • C 50 • C 50 • C	3.7 3.8 5.3 7.6	1.38 1.03 1.06 1.07	0.06 0.08 0.06 0.05	0.01 0.01 0.02 0.03	----	----	----	----

a

Table 9 :

 9

Acknowledgements

This work was financed by funds from the Swiss National Science Foundation (Grant 200021-126759 and 200020-113658). Special thanks go to Ulrich Linden for his tremendous help during field work. Ludovic Chaux and Thomas Routier kindly provided us with rock samples. We thank Priska Bähler for anion and cation determinations, as well as Barbara Zihlmann for XRD analysis. Nicolas Greber and Pierre Dèzes are acknowledged for their constructive comments on language and outline. This manuscript benefited from the comments of two anonymous reviewers.