Marion Garçon

Catherine Chauvel

Sarah Bureau

Beach placer, a proxy for the average Nd and Hf isotopic composition of a continental area

Keywords: Neodymium, hafnium, monazite, zircon, continental crust

Beach placer deposits concentrate detrital heavy minerals which are the erosion products of large areas of continental crust. Here, we report the first analyses of Nd-Hf isotopic ratios and trace element concentrations that we measured in a beach placer from Camargue, France and in its pure mineral separates. Both the bulk composition of the placer and those of its pure mineral separates were determined. We also report mineral proportions obtained using observations under a binocular microscope and micro X-ray fluorescence mapping.

Our results indicate that monazite totally controls the placer's Nd isotopic composition (eNd = -9.3) while zircon dominates its Hf isotopes (eHf = -13.0) even though both mineral phases represent only a small proportion of the heavy mineral assemblage (3.5 and 10% respectively).

We demonstrate that the Camargue placer provides a good estimate of the average Nd and Hf isotopic composition of the continental area drained by the Rhone River in western Europe (eNd ≈ -9 and eHf ≈ -13). Using these values, we calculate two stage model ages and show that almost all the placer minerals are derived from Proterozoic crustal protoliths. This provides valuable information on the history of the continental crust drained by the Rhone River. In particular, it suggests that little juvenile crust was created during the recent geological events that formed the Alps and the Massif Central, the two main massifs from which the placer minerals originate.

More generally, we propose that similar measurements made on other worldwide beach placer deposits could provide estimates of the present-day Nd and Hf isotopic composition of large continental areas, values that are difficult to obtain due to the well-known heterogeneity of continental material but are essential to model the growth of continental crust through Earth history or to model the impact of crustal material when recycled into the mantle.

Introduction

Modern beach placer deposits are surficial accumulations of clastic heavy minerals that mostly form in the foreshore of sandy beaches by gravity separation under the action of waves and currents. In some regions, beach placers contain valuable heavy minerals such as ilmenite, zircon or monazite and have been studied to identify economically viable deposits (e.g. [START_REF]Placers in the System of Sedimentogenesis[END_REF][START_REF] Levchenko | Specific Features of the Mineral Composition of Titanium-Zirconium Placers in Russia[END_REF][START_REF]Sedimentogenesis and Placer Formation[END_REF] or to better understand the mechanisms responsible for mineral sorting and depositional patterns (e.g. [START_REF] Komar | Processes of selective grain transport and the formation of placers on beaches[END_REF][START_REF] Frihy | Spatial variations in heavy minerals and patterns of sediment sorting along the Nile Delta, Egypt[END_REF][START_REF] Roy | Heavy Mineral Beach Placer in Southeastern Australia: Their Nature and Genesis[END_REF][START_REF] Hughes | Hydraulic sorting of heavy-mineral grains by swash on a medium-sand beach[END_REF][START_REF] Hou | Evolution of beach placer shorelines and heavy-mineral deposition in the eastern Eucla Basin, South Australia[END_REF][START_REF] Paine | Quantitative heavy-mineral analysis of a pliocene beach placer deposit in southeastern Australia using the autogeosem[END_REF]. Apart from [START_REF] Götze | Distribution of REE and trace elements in size and mineral fractions of high-purity quartz sands[END_REF] who studied a few trace elements in pure mineral fractions separated from mature quartz sands, no study has focused on the isotope and trace-element geochemistry of placers and their heavy minerals, even though this type of sediment obviously has the potential to provide valuable insights on broader issues such as fluvial transport processes, crust-mantle recycling or the composition of the upper continental crust.

Several previous studies (see for example, [START_REF] Goldstein | Nd and Sr isotopic systematics of river water suspended material: implications for crustal evolution[END_REF][START_REF] Gaillardet | A global geochemical mass budget applied to the Congo basin rivers: Erosion rates and continental crust composition[END_REF][START_REF] Allègre | Sr-Nd-Pb isotope systematics in Amazon and Congo River systems: constraints about erosion processes[END_REF][START_REF] Millot | Lead isotopic systematics of major river sediments: a new estimate of the Pb isotopic composition of the Upper Continental Crust[END_REF][START_REF] Kamber | A new estimate for the composition of weathered young upper continental crust from alluvial sediments, Queensland, Australia[END_REF] used river sediments as proxies to evaluate either the trace element content or the Nd and Pb isotopic compositions of the upper continental crust and its evolution through time. Other studies used Hf and Pb isotopes on individual zircon grains to constrain the timing of major crustal growth episodes as well as their mantle versus crustal sources [START_REF] Gehrels | Detrital zircon reference for Cambrian to Triassic miogeoclinal strata of western North America[END_REF][START_REF] Amelin | Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons[END_REF][START_REF] Bodet | Evolution of the SE-Asian continent from U-Pb and Hf isotopes in single grains of zircon and baddeleyite from large rivers[END_REF][START_REF] Belousova | Archaean and Proterozoic crustal evolution in the Eastern Succession of the Mt Isa district, Australia: U-Pb and Hf-isotope studies of detrital zircons[END_REF][START_REF] Hawkesworth | Using hafnium and oxygen isotopes in zircons to unravel the record of crustal evolution[END_REF][START_REF] Deer | From sediments to their source rocks: Hf and Nd isotopes in recent river sediments[END_REF]. However, no study ever considered taking advantage of the natural processes leading to the formation of zircon-rich placers to place constraints on both the composition and history of large-scale continental areas.

Here we present the first Nd and Hf isotopic analyses, as well as trace element concentrations, of several pure mineral separates that were isolated from a beach placer sampled near the Rhone delta in Camargue, France. The proportion of each heavy mineral species was estimated as precisely as possible using observations under a binocular microscope and micro X-ray fluorescence mapping. Using all these data, we (a) quantify the influence of the individual minerals on the trace element and isotopic composition of the entire placer, (b) we discuss and interpret the chemical characteristics of beach placers and (c) we demonstrate that placers are good proxies of the average Nd-Hf isotopic composition of large regions of the continental crust exposed to weathering and erosion.

Materials and Methods

Brief setting and placer sampling

The studied placer was sampled in 2003 next to the Rhone River delta on the Espiguette Beach in Camargue, France (see Figure 1) within the zone of wave swash where black-sand deposits are regularly observed after storms. Several kilos of mature sand, rich in black heavy minerals and mixed with very small proportions of light minerals, were collected using a spade on the beach. Previous studies (BRGM, 2000) showed that the Espiguette black-sands usually contain variable abundances of about twenty heavy minerals that are commonly found in beach placer deposits. These include ilmenite, magnetite, rutile, garnet, zircon, monazite and epidote (see Supplementary Table 1 for the complete list of minerals).

As shown in Figure 1, Espiguette beach is about 10 kilometres long and grows by accumulation of sediments transported from east to west by longshore currents [START_REF] Vella | Evolution of the Rhône delta plain in the Holocene[END_REF][START_REF] Sabatier | Sediment budget of the Rhône delta shoreface since the middle of the 19th century[END_REF]. The accumulated sediments essentially originate from the reworking of relict pro-deltaic lobes of the Rhone River and consist of materials directly eroded from the massifs drained by the Rhone River and its tributaries. Several studies (BRGM, 2000;[START_REF] Vassas | Mechanisms of enrichment of natural radioactivity along the beaches of the Camargue, France[END_REF] and more importantly the analyses of zircon typologies and apatite fission track densities performed by [START_REF] Thomassin | Evaluations dosimétriques de l'exposition potentielle liée à l'accumulation naturelle d'uranium et de thorium dans les sables de certaines plages du littoral de Camargue[END_REF] demonstrated that the Alps and the Massif Central are the two main sources for the heavy minerals that accumulate in the Camargue beach placers.

Separation of the mineral fractions

Before analysis and mineral separation, the placer sand was washed, placed in an ultrasonic bath and oven-dried to remove organic material and seawater. Using heavy liquids (bromoform and methylene iodide), the minerals were separated into three different fractions according to their density. In the heaviest fraction (d > 3.3 g.cm -3 ), the main minerals were isolated according to their magnetic susceptibility using a hand magnet and a Frantz Isodynamic Magnet Separator. Due to their similar magnetic properties, monazite, epidote and titanite were relatively difficult to separate from one another. However, the epidote and titanite grains were generally larger than the monazite grains and we separated them by collecting the fine and coarse fractions after simple 80µm-sieving. The isolated fractions were then purified by handpicking under a binocular microscope. We obtained six pure mineral separates consisting of garnet, ilmenite, rutile, zircon, epidote and monazite. However, due to the optical similarities of ilmenite and magnetite and because it is impossible to completely eliminate impurities with a hand magnet, we suspect that the pure ilmenite separate likely refers to a fraction containing a few magnetite grains.

Heavy mineral abundances

Mineral proportions were determined under a binocular microscope in a fraction containing all the heavy minerals isolated from the Camargue placer (d > 2.9 g.cm -3 ) using standard optical characteristics such as grain colour or grain shape. In this heavy mineral fraction, the mineral abundances were estimated as a percentage of the total counted grains.

As the identification of certain heavy minerals can be ambiguous in placers because of altered or rounded grains, we also developed a method that uses micro X-ray fluorescence mapping (µXRF EDAX Eagle III) to quantify the mineral abundances in the heavy fraction (d > 2.9 g.cm -3 ). Several hundred grains were analyzed by µXRF in order to identify their spectral signature and their mineralogical characteristics (see Supplementary File B). Then, using this information, we calculated the relative abundance of each mineral species as a percentage of the total grains analysed by µXRF.

Combining the observations under the binocular microscope with the results obtained by µ-XRF mapping, we estimated mineral abundances and converted them to mass percentage using specific mineral densities and average grain sizes.

Chemical analyses of the mineral fractions

Trace element concentrations and Nd-Hf isotopic compositions were measured in bulk fractions to evaluate the influence of each mineral on the geochemistry of the Camargue placer.

Measurements were performed on the six mineral separates (monazite, garnet, ilmenite, zircon, epidote and rutile), the total placer and on two fractions, one containing all the light minerals (d < 2.9 g.cm -3 ) and the other containing all the heavy minerals (d > 2.9 g.cm -3 ). To assess the efficiency of the purification by handpicking under a binocular microscope, the impure monazite, garnet, ilmenite and zircon fractions were also analysed.

Sample dissolution

The samples were first ground to fine powder by hand in an agate mortar to facilitate dissolution. They were then dissolved by acid attack in steel PARR bombs for at least 2 weeks at 135°C. Because of the well-known resistance of most placer minerals, we used samplespecific dissolution procedures in which the sample quantity, the acid mixture and the dissolution time in bombs were adapted to the mineral species (see Supplementary File C). In addition, in order to improve the efficiency of the dissolution, acid-mixtures were regularly removed from the bomb, stored in separated beakers and new acids were added every week.

After complete dissolution, two aliquots were taken from each dissolved sample, one to measure trace element concentrations and the other for Nd-Hf isotopic compositions.

Trace element concentrations

Trace element concentrations were measured in mineral fractions using an Agilent 7500ce ICP-MS in Grenoble, France following a procedure similar to that published by [START_REF] Chauvel | Comprehensive chemical and isotopic analyses of basalt and sediment reference materials[END_REF]. High precision measurements on ICP-MS were obtained by diluting samples in 2% HNO3 with traces of HF at a specific dilution factor that depends on the sample and the chemical element (see Supplementary File B). As described by [START_REF] Chauvel | Comprehensive chemical and isotopic analyses of basalt and sediment reference materials[END_REF], ICP-MS analytical drift was corrected by adding an internal standard (a multispike containing Be, Ge, In, Tm, Bi) to all samples. After correction for drift and oxide interferences, trace element concentrations were calculated using an external calibration based on the repeated measurements of the BR rock standard diluted 5000 times. To ensure the validity and the reproducibility of our measurements, rock standards AGV-1 and BR 24 were measured several times as unknown samples and several duplicates were performed (see Supplementary Table 2 and 3).

Nd and Hf isotopic compositions

Following the method described by [START_REF] Chauvel | Comprehensive chemical and isotopic analyses of basalt and sediment reference materials[END_REF], Hf and Nd were isolated from the same aliquot using ion exchange resins. For the pure mineral separates, the Hf isolation procedure was slightly modified since the standard method was not suitable for minerals that are much more concentrated in trace elements than usual rocks or sediments (see Supplementary File B). The total procedural blanks were always less than 20 pg for Nd and 120 pg for Hf and can thus be considered as negligible relative to the amount of Nd and Hf present in the samples. Nd and Hf isotopic ratios were measured on a Nu Plasma MC-ICP-MS at the ENS Lyon, France. The mass fractionation bias was corrected using the constant values of the ratios 146 Nd/ 144 Nd=0.7219 and 179 Hf/ 177 Hf=0.7325. Analytical drift was monitored by measuring the Ames-Rennes Nd and Ames-Grenoble Hf standards every two or three samples and was corrected using the recommended values published by [START_REF] Chauvel | A hafnium isotope and trace element perspective on melting of the depleted mantle[END_REF] and [START_REF] Chauvel | Comprehensive chemical and isotopic analyses of basalt and sediment reference materials[END_REF]. These two standards yielded a mean 143 Nd/ 144 Nd value of 0.511962 ± 13 (2σ, n = 18) and 176 Hf/ 177 Hf value of 0.282160 ± 10 (2σ, n = 11).

Results

Mineralogical composition of the heavy and light mineral fractions

Using the weight of each separated fraction, we estimated that the light minerals (d < 2.9 g.cm -3 ) represent only 5% wt of the total placer whereas the heavy minerals (d > 2.9 g.cm -3 ) constitute about 95% wt of the sample. Observation of the light mineral fraction under the binocular microscope shows that it mainly contains calcite, feldspar and quartz, but also a few residual heavy minerals (ilmenite, zircon, etc...) that were not properly separated from the light minerals by the heavy liquid separation procedure. Figure 2 shows the mineral abundances in the heavy mineral fraction determined under the binocular microscope and using the µXRF mapping. The mineral proportions estimated by combining the results from the two methods are given in the grey-coloured inset as a percentage of the total counted grains and as a mass percentage. The uncertainties on the mineral proportions and the details about mass percentage calculations are given in the Figure caption. As shown in Figure 2, the results obtained by µXRF mapping are remarkably consistent with the observations done under the binocular microscope.

The heavy mineral fraction mainly contains ilmenite-magnetite (44%) and garnet (28%) but also significant proportions of zircon (10%), rutile (7%), monazite (3.5%) and epidote (2%).

The µ-XRF determination provides precise abundances when the minerals are present in significant proportions (> 5%) and has the advantage to be quick and inexpensive. However, when the analysed grains have complex spectral signatures with too many peaks or variable proportions of chemical elements, as is the case for garnet and epidote, the method cannot accurately identify and quantify mineral abundances. In such a case, observation under the binocular microscope provides better results. Unfortunately, for ilmenite and magnetite, both methods remain inefficient because the two minerals are optically similar and their µ-XRF spectral signatures are highly variable depending on the degree of alteration [START_REF] Nair | Weathering of ilmenite from Chavara deposit and its comparison with Manavalakurichi placer ilmenite, southwestern India[END_REF][START_REF] Suresh-Babu | Alteration of ilmenite in the Manavalakurichi deposit, India[END_REF], hence indistinguishable with µ-XRF. The ilmenite percentage calculated in the heavy mineral fraction is probably overestimated because it includes some magnetite.

Trace-element concentrations

Trace-element concentrations measured in the placer, in the pure mineral separates and in the light and heavy mineral fractions are given in Table 1 and concentrations in the impure fractions of monazite, garnet, ilmenite and zircon are given in Supplementary Table 3. Except for garnet, the composition of the impure fractions is similar to that of the pure mineral separate but generally less concentrated in trace elements. To compare the composition of the entire placer with that of the pure mineral separates and the light and heavy mineral fractions, the concentrations of REE and other trace elements were normalized to the average chondrite composition of [START_REF] Evensen | Rare earth abundances in chondritic meteorites[END_REF] in Figure 3a and to the average upper continental crust values of [START_REF] Mclennan | Relationships between the trace element composition of sedimentary rocks and upper continental crust[END_REF] in Figure 3b. Compared to chondrites, the REE enrichment factors span several orders of magnitude from 10 to almost 400 000 (Figure 3a). All samples are characterised by a negative Eu anomaly. Compared to upper continental crust, enrichment factors range from about 0.01 to 4000 (Figure 3b) and almost all samples display significant depletions in Cs, Rb, Ba, Sr and Li elements that are all known to be highly mobile during alteration.

As can be seen in Figure 3b, trace-element concentrations measured in the total placer are about 100 times those of upper continental crust and are quasi-similar to those of the heavy mineral fraction. In contrast, the light mineral fraction has much lower trace element concentrations, quite similar to upper continental crust values. These three fractions display the same small negative anomalies in Nb, Ta and positive anomalies in Hf and Zr (Figure 3b).

Except for garnet, trace-element concentrations measured in the pure mineral separates strongly resemble those published in the literature when available and/or are consistent with partition coefficients determined in many studies.

• Monazite is extremely rich in Th-U and all REE but particularly the light REE (LREE) (e.g. [START_REF] Kamineni | The Geochemistry of Monazite Types from the Eastern Ghats Granulite Terrain, India[END_REF][START_REF] Ayres | REE fractionation and Nd-isotope disequilibrium during crustal anatexis: constraints from Himalayan leucogranites[END_REF][START_REF] Zhu | Monazite chemical composition: some implications for monazite geochronology[END_REF][START_REF] Nagy | Genesis and transformations of monazite, florencite and rhabdophane during medium grade metamorphism:examples from the Sopron Hills, Eastern Alps[END_REF][START_REF] Thöni | Sm-Nd isotope systematics of high-REE accessory minerals and major phases: ID-TIMS, LA-ICP-MS and EPMA data constrain multiple Permian-Triassic pegmatite emplacement in the Koralpe, Eastern Alps[END_REF][START_REF] Radulescu | The age of HP metamorphism in the Gran Paradiso Massif, Western Alps: A petrological and geochronological study of "silvery micaschists[END_REF].

• Ilmenite is characterized by positive anomalies in Nb-Ta that are consistent with the partition coefficients determined by [START_REF] Klemme | The partitioning of trace elements between ilmenite, ulvospinel,armalcolite and silicate melts with implications for the early differentiation of the moon[END_REF] and [START_REF] Melluso | Major and trace-element composition of olivine, perovskite, clinopyroxene, Cr-Fe-Ti oxides, phlogopite and host kamafugites and kimberlites, Alto Paranaiba, Brazil[END_REF] for these two elements.

• Rutile and epidote display quite similar trace-element concentrations except for their anomalies in Nb-Ta and in Zr-Hf which are positive for rutile and negative for epidote.

These features are in agreement with the partition coefficients determined by [START_REF] Foley | Rutile/melt partition coefficients for trace elements and an assessment of the influence of rutile on the trace element characteristics of subduction zone magmas[END_REF] and [START_REF] Klemme | Partitioning of trace elements between rutile and silicate melts: Implications for subduction zones[END_REF] for these four elements.

• Zircon differs from other minerals because of its strong depletion in LREE and its significant enrichment in Hf and Zr (Figure 3; see [START_REF] Hoskin | The Composition of Zircon and Igneous and Metamorphic Petrogenesis[END_REF] for a review).

• Garnet is more enriched in HREE than in LREE (Figure 3a), displays unusual positive Zr and Hf anomalies (Figure 3b) and its LREE concentrations are not as low as usually measured in typical metamorphic or magmatic garnets [START_REF] Jung | Trace element fractionation during high-grade metamorphism and crustal melting -constraints from ion microprobe data of metapelitic, migmatitic and igneous garnets and implications for Sm-Nd garnet chronology[END_REF]. In the pure garnet separate, the higher LREE concentrations associated with the Hf and Zr anomalies might be due to the presence of zircon inclusions and/or to an incomplete sorting of the fraction.

Nd and Hf isotopic compositions

Nd and Hf isotopic ratios measured in the placer, the pure mineral separates and the heavy and light mineral fractions are given in Table 2 and are plotted in a ƐHf vs. ƐNd diagram in Figure 4. The isotopic compositions of the impure fractions of monazite, garnet, ilmenite and zircon are given in Supplementary Table 3. Except for garnet, the compositions of the impure separates resemble those of their respective pure mineral separates. Nd and Hf isotopic compositions measured in the placer, the pure mineral separates and the heavy and light mineral fractions span a large range of values from 0.512150 (ƐNd = -9.4) to 0.512594 (ƐNd = -0.7) for Nd and from 0.282358 (ƐHf = -15.1) to 0.282942 (ƐHf = +5.6) for Hf. The pure zircon separate has the highest Nd isotopic ratio and one of the lowest Hf isotopic ratios whereas the pure monazite separate has the highest Hf isotopic ratio and one of the lowest Nd isotopic ratios. Considering the analytical uncertainties on the measurements, the total placer, the rutile and the light and heavy mineral fractions have similar Nd and Hf isotopic compositions which all lie on the mantle array in the ƐHf vs. ƐNd diagram (Figure 4). Ilmenite and garnet have more radiogenic Nd and Hf isotopic ratios that also fall on the mantle array in Figure 4. In contrast, epidote lies well above the mantle array due to the radiogenic decay associated to its high Lu/Hf ratio.

Discussion

Geochemical characteristics of the Camargue placer

As heavy minerals are important carriers of trace elements (e.g. [START_REF] Götze | Distribution of REE and trace elements in size and mineral fractions of high-purity quartz sands[END_REF][START_REF] Preston | The provenance of triassic continental sandstones frome the Beryl field, northern north sea: Mineralogical, geochemical and sedimentological constraints[END_REF][START_REF] Totten | Heavy Minerals in Shales[END_REF], we first decipher the contribution of each mineral species to the overall chemistry of the Camargue placer in order to further investigate the Sm-Nd and Lu-Hf systematics of beach placers.

The heavy minerals control the trace element budget of the placer

In Figure 5, we normalised the trace element concentrations of the heavy (high density, d > 2.9 g.cm -3 ) and light (low density, d < 2.9 g.cm -3 ) mineral fractions to the bulk composition of the Camargue placer to assess their respective contributions to the chemical budget of the placer. The enrichment factors calculated for the heavy mineral fraction are very close to 1 whereas they are quite low for the light mineral fraction -usually < 0.1. This clearly shows that heavy minerals almost totally control the bulk geochemistry of the placer by hosting most trace elements. Except for some mobile elements such as Cs, Rb, Ba, Sr and Li, the light minerals do not sequester large amounts of trace elements. The very low concentrations measured in this fraction indicate that it contributes to very little of the overall chemical budget of the placer but it still has the potential to significantly dilute overall concentrations [START_REF] Götze | Distribution of REE and trace elements in size and mineral fractions of high-purity quartz sands[END_REF]Vervoort et al., 1999). We also see that the spectrum for the light mineral fraction in Figure 3b is not really consistent with those usually found in the literature for quartz, feldspar or calcite, the main mineral species present in this fraction (see [START_REF] Götze | Distribution of REE and trace elements in size and mineral fractions of high-purity quartz sands[END_REF][START_REF] Govindaraju | Compilation of working values and sample description for 383 geostandards Geostandards[END_REF][START_REF] Korotev | A self-consistent composition of elemental concentration data for 93 geochemical reference samples[END_REF][START_REF] Monecke | Determination of Trace Elements in the Quartz Reference Material UNS-SpS and in Natural Quartz Samples by ICP-MS[END_REF][START_REF] Strnad | Determination of Trace Elements in Calcite Using Solution and Laser Ablasion ICP-MS: Calibration to NIST SRM Glass and USGS MACS Carbonate, and Application to Real Landfill Calcite[END_REF]. The light mineral fraction displays unusual HFSE contents that strongly resemble those found in the heavy mineral fraction (negative anomaly in Nb and Ta and positive anomaly in Zr and Hf, see Figure 3). This feature is best explained by the presence of few heavy minerals in the light mineral fraction, a feature not surprising since, unlike the pure mineral separates analysed in this study, the light mineral fraction was not carefully purified by hand-picking under a binocular microscope.

Trace elements hosted by pure mineral separates

In Figure 5, the placer-normalised patterns reflect the contribution of each heavy mineral to the bulk trace-element budget of the placer. With its very high concentrations, monazite obviously is the major carrier of Th, U and REE. Zircon mainly controls Zr, Hf and to a lesser extent U and HREE while rutile and ilmenite host most Nb and Ta in the placer. The other trace elements such as Cs, Rb, Ba, Sr, Pb or Li are carried in variable proportions by several mineral species and their concentrations in the total placer are not controlled by one or two specific mineral species (Figure 5).

To ensure that we isolated and analysed all the heavy mineral species that significantly contribute to the trace-element budget of the placer, we use the concentrations measured in the pure mineral separates and the mineral proportions estimated as mass percentages in Figure 2 to calculate a theoretical trace element pattern for the heavy mineral fraction. In Figure 6, we compare it to the measured values. We also take into account the uncertainties estimated on the mineral proportions (see footnote of Figure 6 for more detail) and we model a field of variability for the calculated heavy mineral fraction concentrations (blue field in Figure 6). With the exception of the HREE and some mobile elements, our results show that the measured traceelement pattern of the heavy mineral fraction is extremely well reproduced using the combined proportions of pure monazite, zircon, ilmenite, garnet, rutile and epidote. This confirms that, except for the HREE, the pure mineral separates account for the trace element budget of the heavy mineral fraction. For Nd and Hf, our calculation shows that their concentrations in the heavy mineral fraction are completely reproduced using only the mass proportions and contents measured in the pure monazite and zircon separates. Other minerals play close to no role in the overall abundance of Nd and Hf in the heavy mineral fraction. Monazite contains such high amounts of LREE and zircon such high amounts of Zr and Hf that even though they constitute only small percentages of the mineral assemblage, they are the controlling carriers of Nd and Hf in the beach placer. Finally, we interpret the misfit between calculated and measured concentrations for Cs, Rb, Ba and heavy REE (Figure 6) as being due to impurities in the garnet separate and/or to the presence of a mineral such as titanite, which we did not separate and analyse.

What controls the Nd and Hf isotopic systematic of beach placer?

Given that monazite and zircon are the carriers of Nd and Hf in the placer, they should control the isotopic compositions. To confirm this hypothesis, we calculated a mixing curve between these two minerals using the equation established by [START_REF] Faure | Principles of isotope Geology[END_REF]. This mixing curve and the mass proportion of monazite in the binary mixture are shown in Figure 4. Because monazite controls the Nd content and zircon the Hf content, the mixing hyperbola has such a strong curvature that it resembles two perpendicular straight lines in the ƐHf vs. ƐNd diagram. The first important observation is that, within uncertainties, the bulk composition of the placer, as well as those of the light and heavy mineral fractions plot at the point of maximum curvature on the mixing curve. A simple binary mixture of monazite and zircon without all the other mineral species thus properly reproduces their Nd and Hf isotopic compositions. This confirms that monazite totally controls the Nd isotopic composition in the placer as well as that of the heavy and the light mineral fractions while zircon totally dominates the Hf isotopic composition.

As a consequence of the concentration contrast between monazite and zircon for Nd and Hf, the proportion of monazite in the monazite-zircon mixture varies widely near the point of maximum curvature on the hyperbola (see the inset of Figure 4). Considering an uncertainty of 0.7 Ɛ units on the isotopic compositions, mixing calculations demonstrate that as long as there is 5 to 90% of monazite in the monazite-zircon mixture, the eNd of the mixture remains similar to that of the monazite and the eHf to that of the zircon. A very wide range of monazite-zircon ratio will thus produce Nd and Hf isotopic compositions that lie at the point of maximum curvature on the mixing hyperbola. This, together with the fact that heavy mineral proportions determined in the studied Camargue placer are comparable to those usually found in mature beach placer deposits, make us very confident in the reproducibility of our results. Hence, we are convinced that similar isotopic compositions would characterize placer samples taken from various locations on the same beach even if heavy mineral proportions vary locally. This latter observation potentially has major implications for beach-placer deposits in other parts of the world. Indeed, monazite and zircon are very common minerals in most coastal-marine placers [START_REF] Patyk-Kara | Inhomogeneity of the Mineral Field of Complex Polymineral Placers[END_REF][START_REF]Placers in the System of Sedimentogenesis[END_REF] where they represent 1-5 wt% and 5-20 wt% of the heavy mineral assemblage, respectively (e.g. [START_REF] Carpenter | Heavy Mineral Deposits in the Upper Coastal Plain of North Carolina and Virginia[END_REF][START_REF] Force | Geology of titanium-mineral deposits Boulder[END_REF][START_REF] Rao | Significance of heavy mineral ratios in some beach placers of Andhra Pradesh, East Coast of India[END_REF][START_REF] Roy | Heavy Mineral Beach Placer in Southeastern Australia: Their Nature and Genesis[END_REF][START_REF] Paine | Quantitative heavy-mineral analysis of a pliocene beach placer deposit in southeastern Australia using the autogeosem[END_REF][START_REF] Levchenko | Specific Features of the Mineral Composition of Titanium-Zirconium Placers in Russia[END_REF], proportions that are comparable to those estimated in the Camargue placer. Given that zircon is unequivocally the major carrier of Hf among common heavy minerals, we suggest that, as for the Camargue placer, the Hf elemental and isotopic budget of worldwide beach placers is likely entirely controlled by zircon. For Nd content and isotopic composition, the situation is more ambiguous because the percentage of monazite in the heavy mineral assemblages can sometimes be low and other minerals will influence the placer Nd isotopic composition if they are present in sufficiently large proportions. Apatite probably constitutes the most serious competitor to monazite in the REE budget of placers because it contains significant amounts of REE and is sometimes present at the percent level in some beach placers. However, the normal Nd content of monazite is at least 100 times that of typical apatite (see [START_REF] Ayres | REE fractionation and Nd-isotope disequilibrium during crustal anatexis: constraints from Himalayan leucogranites[END_REF][START_REF] Ayres | REE fractionation and Nd-isotope disequilibrium during crustal anatexis: constraints from Himalayan leucogranites[END_REF][START_REF] Belousova | Apatite as an indicator mineral for mineral exploration: trace-element compositions and their relationship to host rock type[END_REF] for Nd concentrations in apatites) and 1000 times that of other common placer minerals (cf Figure 3a; [START_REF] Götze | Distribution of REE and trace elements in size and mineral fractions of high-purity quartz sands[END_REF]. A simple calculation shows that even if a placer contains only 0.5% monazite, 5% apatite and 94.5% other minerals, 80% of the overall Nd budget is still control by monazite. We therefore suggest that the Nd and Hf isotopic compositions of worldwide beach placers should be controlled by monazite and zircon even though their abundances in the heavy mineral assemblage are often low.

Present-day Nd and Hf isotopic compositions of continental areas

The continental area drained by the Rhone River

Previous studies showed clearly that the present-day Hf-Nd isotopic compositions of crustal rocks plot on the extension of the mantle array in a Hf-Nd isotopic space and thus on the same trend as rocks coming from the mantle (Vervoort and Patchett, 1996;Vervoort et al., 1999;[START_REF] Vervoort | Hf-Nd isotopic evolution of the lower crust[END_REF] (Figure 4). In Figure 4, data for the Camargue placer lie on the mantle array and also fall in the field of crustal rocks, suggesting that its present-day ƐNd at -9 and its ƐHf at -13 are quite comparable to that of continental crust materials. Our ƐHf value is the first published estimate for the Hf isotopic composition of crustal rocks from Western Europe since no Hf data are available for the source rocks in the region drained by the Rhone River. This is not the case for ƐNd, however. Figure 7 shows that our ƐNd value is identical to the average value calculated from a compilation of ƐNd reported in the literature for continental crust rocksgranitoids and orthogneiss essentially -from the Massif Central and the Western and Central Alps. It also corresponds to the average Nd isotopic composition of metamorphic rocks, sediments and granitoids of the Hercynian fold belt of Central Europe [START_REF] Liew | Precambrian crustal components, plutonic associations, plate environment of the Hercynian Fold Belt of central Europe: Indications from a Nd and Sr isotopic study[END_REF]. We conclude that the placer's Nd and Hf isotopic compositions represent those of the average source region of the sediment, allowing us to provide the following estimate for the Western European crust: ƐHf ≈ -13 and ƐNd ≈ -9.

Can beach placers be used as proxies for other continental areas?

While we believe that the Camargue placer might be a good proxy for the average Nd and Hf isotopic composition of the continental area drained by the Rhone river, this might not be the case for other beach placers. For an estimate based on placer data to be robust requires a number of conditions that we will now examine. We demonstrated above that the Nd and Hf isotopic compositions of the Camargue placer are controlled by monazite and zircon, ubiquitous minerals that host most of the LREE and Hf in crustal rocks. However, it is also known that parent/daughter ratios such as Sm/Nd and Lu/Hf change when minerals crystallize from a magma [START_REF] Faure | Principles of isotope Geology[END_REF] and that sedimentary rocks do not necessarily have the same ratios as their sources [START_REF] Patchett | Hafnium/rare earth element fractionation in the sedimentary system and crustal recycling into the Earth's mantle[END_REF]. As time passes after formation of the minerals, the isotopic compositions of minerals evolve following their parent/daughter ratios, which might be significantly different from those of the whole rock. The impact of the difference in parent/daughter ratio needs therefore to be quantitatively evaluated before we can conclude that placers represent proxies for the average Nd and Hf isotopic composition of their crustal precursors.

In continental crust granitoids, Nd resides not only in monazite but also in other minerals such as apatite, allanite or feldspar [START_REF] Taylor | The Geochemical evolution of the continental crust[END_REF][START_REF] Bea | Residence of REE, Y, Th and U in Granites and Crustal Protoliths; Implications for the Chemistry of Crustal Melts[END_REF] that are not systematically concentrated in beach placers when continental crust is eroded. Nevertheless, as for most common granite-forming minerals (e.g. [START_REF] Taylor | The Geochemical evolution of the continental crust[END_REF][START_REF] Bayon | The control of weathering processes on riverine and seawater hafnium isotope ratios[END_REF], monazite has the same 147 Sm/ 144 Nd ratio as continental crust (see [START_REF] Mclennan | Relationships between the trace element composition of sedimentary rocks and upper continental crust[END_REF]Rudnick and[START_REF] Rudnick | Composition of the Continental Crust[END_REF] for continental crust ratios and [START_REF] Ayres | REE fractionation and Nd-isotope disequilibrium during crustal anatexis: constraints from Himalayan leucogranites[END_REF][START_REF] Thöni | Sm-Nd isotope systematics of high-REE accessory minerals and major phases: ID-TIMS, LA-ICP-MS and EPMA data constrain multiple Permian-Triassic pegmatite emplacement in the Koralpe, Eastern Alps[END_REF] and our own measurement for monazite ratios). Consequently, the monazite Nd isotopic composition never significantly differs from that of crustal rocks and can be considered as a good estimate of the average crustal value even if monazite does not dominate the Nd content of crustal rocks. In contrast, most of the Hf in continental crust granitoids is hosted in resistant zircons [START_REF] Patchett | Hafnium/rare earth element fractionation in the sedimentary system and crustal recycling into the Earth's mantle[END_REF][START_REF] Candela | Ore in the Earth's Crust[END_REF], which have low 176 Lu/ 177 Hf ratios compared to that of continental crust (0.002 versus 0.013) (see [START_REF] Mclennan | Relationships between the trace element composition of sedimentary rocks and upper continental crust[END_REF]Rudnick and[START_REF] Rudnick | Composition of the Continental Crust[END_REF] for continental crust ratios and [START_REF] Kinny | Lu-Hf and Sm-Nd isotope systems in zircon[END_REF] for zircon ratios). As a consequence, significant isotopic differences develop between mineral and crustal precursor if the zircon crystallization age is old. In practice, using a 176 Lu/ 177 Hf ratio of 0.002 for the zircon and 0.013 for the continental crust, our calculations show that if the zircon crystallization age is younger than 500 Ma, its isotopic composition differs from that of its crustal precursor by less than two ƐHf. This is simply because time is not sufficient for the difference in Lu/Hf to produce a Hf isotopic composition in the zircon very different from that of the crust. As a result, the Hf isotopic composition of a young zircon population, such as that in the Camargue placer, can be considered as a valuable proxy for the average crustal value of the drained area. In the case of an older zircon population, a correction could be applied provided that the average age of the population is known.

In summary, we believe that placers have the potential to provide naturally averaged information about the isotopic composition of large portions of the crust exposed to weathering and erosion, information that is not always available as revealed by the numerous papers that aim to estimate the composition of the continental crust (e.g. [START_REF] Taylor | The Geochemical evolution of the continental crust[END_REF][START_REF] Mclennan | Relationships between the trace element composition of sedimentary rocks and upper continental crust[END_REF][START_REF] Rudnick | Composition of the Continental Crust[END_REF]. Although we recognize that analytical procedure, particularly for sample dissolution, is sometimes difficult, we suggest that analyses of worldwide beach placer deposits may constitute a fairly simple way to constrain the Nd-Hf systematics of large continental areas. We recognize that this estimate does not take into account mafic and ultramafic materials that could exist in the drainage area because monazite and zircon, the two minerals controlling the Nd and Hf isotopic compositions of beach placers, are not ubiquitous in mafic and ultramafic rocks. Beach placer deposits approximate therefore the average isotopic composition of materials of continental affinity. Interesting sampling sites could be the deltas of large rivers where vast amounts of terrigenous material are deposited and where sea currents facilitate the formation of heavy mineral concentrates [START_REF] Komar | The Entrainment, Transport and Sorting of Heavy Minerals by Waves and Currents[END_REF].

Nd and Hf model ages of the drained continental area

Two stage model age calculations

Since minerals do not necessarily have Sm/Nd and Lu/Hf ratios similar to those of their whole rocks, Nd and Hf model ages of their crustal protolith must be calculated using a two stage-model, a first stage for the time spent in the crust and a second for the evolution of the mineral itself since crystallisation. For this calculation, we assume that the Sm-Nd and Lu-Hf isotopic systems evolved first in a continental crust-type reservoir extracted from the depleted mantle and then, after crystallisation, in the minerals themselves. An example of the calculation for the Lu-Hf isotopic system is shown in Figure 8 and the calculated Hf model ages are shown as a function of the Nd model ages in Figure 9. For the placer and for each mineral fraction, model ages were estimated using a Monte-Carlo procedure to propagate uncertainties related to (1) the isotopic composition of the depleted mantle (mean MORB-pole from the compilation made by [START_REF] Chauvel | Role of recycled oceanic basalt and sediment in generating the Hf-Nd mantle array[END_REF], (2) the 147 Sm/ 144 Nd and 176 Lu/ 177 Hf ratios of average continental crust and (3) the crystallisation ages of the placer minerals (see caption of Figure 9 for more details).

As a primary hypothesis, we considered that the crystallisation age of all minerals were, on average, Hercynian (300 ± 50 (2σ) Ma) given that (1) this period corresponds to the last major magmatic event forming widespread plutonic bodies and occurring both in the Eastern Massif Central and in the Western Alps [START_REF] Pin | Sources of Hercynian granitoids from the French Massif Central: Inferences from Nd Isotopes and consequences for crustal evolution[END_REF][START_REF] Debon | Evolution of the Mg/Fe Ratios in Late Variscan Plutonic Rocks from the External Crystalline Massifs of the Alps (France, Italy, Switzerland)[END_REF], the two main sources of the placer minerals and (2) numerous studies have reported crystallisation ages between 290 Ma and 340 Ma for monazite and zircon populations from the Massif Central and the Western-Central Alps (e.g. [START_REF] Bossart | Morphology versus U-Pb systematics in zircon: a high-resolution isotopic study of a zircon population from a Variscan dike in the Central Alps[END_REF][START_REF] Sergeev | Improving the resolution of single-grain U/Pb dating by use of zircon extracted from feldspar: Application to the Variscan magmatic cycle in the central Alps[END_REF][START_REF] Bruguier | Application of in situ zircon geochronology and accessory phase chemistry to constraining basin development during post-collisional extension: a case study from the French Massif Central[END_REF][START_REF] Mezeme | Electron microprobe monazite geochronology of magmatic events: Examples from Variscan migmatites and granitoids, Massif Central, France[END_REF][START_REF] Rossi | Variscan to eo-Alpine events recorded in European lower-crust zircons sampled from the French Massif Central and Corsica, France[END_REF][START_REF] Gébelin | Syntectonic crustal melting and high-grade metamorphism in a transpressional regime, Variscan Massif Central, France[END_REF]. Using this assumption, similar model ages are calculated for the two isotopic systems for the placer, the light and heavy mineral fractions and for each pure mineral separate, as shown by the distribution in Figure 9.

In contrast, if we assume an average Alpine crystallisation age of about 30-50 Ma or any age younger than 250 Ma or older than 350 Ma, large discrepancies occur between the Nd and Hf model ages. This would imply a systematic decoupling of the Sm-Nd and Lu-Hf isotopic systems in every mineral species, a situation that is obviously unrealistic. As an example for the pure zircon separate, taking a crystallisation age of 50 Ma leads to Nd model ages (0.7 to 1 Ga) much younger than the Hf model ages (1.3 to 1.7 Ga) while taking a crystallisation age of 500 Ma provides Nd model ages (1.7 to 2.0 Ga) that are far older than the Hf model ages (1.2 to 1.5 Ga). We are therefore confident that calculating model ages using a mean crystallisation age of 300 ± 50 (2σ) Ma for all the placer minerals provides robust information about the Nd and Hf isotopic systematics of the source region. Note that the bulk placer and some of the mineral fractions, such as the light and heavy mineral fractions, the pure ilmenite for Hf and the pure rutile for Nd, have parent/daughter ratios that resemble those of the continental crust, hence they have robust model ages that do not depend on the age of crystallisation we chose to calculate them.

Implications for the history of the drained continental area

Almost all mineral fractions, and the placer itself, display consistent Nd and Hf model ages (Figure 9) grouped around 1.4 -1.5 Ga, suggesting that they derive from similar crustal protoliths. The only exceptions are epidote and ilmenite both of which are characterised by slightly younger ages around 1.1 to 1.3 Ga. Assuming a younger crystallisation age for these two minerals does not significantly raise the Hf and Nd model ages. As a consequence, epidote and ilmenite may record more recent geological events or their parent/daughter ratios may have been disturbed by secondary processes.

Our calculated Nd and Hf model ages are fully consistent with the Proterozoic Nd model ages reported for granitoids from the Massif Central and the Alps (e.g. [START_REF] Bossart | Morphology versus U-Pb systematics in zircon: a high-resolution isotopic study of a zircon population from a Variscan dike in the Central Alps[END_REF][START_REF] Paquette | U-Pb zircon, Rb-Sr and Sm-Nd geochronology of high-to very-high-pressure meta-acidic rocks from the western Alps[END_REF][START_REF] Downes | Crustal evolution of the Hercynian belt of Western Europe: evidence for lowercrustal granulitic xenoliths (French Massif Central)[END_REF] and strongly resemble those of metamorphic rocks, sediments and granitoids from the European Hercynian fold belt whose model ages vary mainly between 1.4 and 1.7 Ga [START_REF] Liew | Precambrian crustal components, plutonic associations, plate environment of the Hercynian Fold Belt of central Europe: Indications from a Nd and Sr isotopic study[END_REF]. This suggests that the bulk composition of the Camargue placer provides a true estimate of both the Nd and Hf model ages of the large continental area drained by the Rhone River. More generally, Nd and Hf model ages obtained on placer sands from continents could provide similar information for the crustal evolution of large continental areas.

According to our measurements and calculations, the mean crustal residence age of the Nd and Hf now residing in the Camargue placer minerals is Proterozoic, hence far older than the age of the main orogenies that produced the Hercynian and Alpine terranes. As discussed by many authors studying the Hercynian magmatism in the Massif Central and the Alps (e.g. [START_REF] Bossart | Morphology versus U-Pb systematics in zircon: a high-resolution isotopic study of a zircon population from a Variscan dike in the Central Alps[END_REF][START_REF] Paquette | U-Pb zircon, Rb-Sr and Sm-Nd geochronology of high-to very-high-pressure meta-acidic rocks from the western Alps[END_REF][START_REF] Pin | Sources of Hercynian granitoids from the French Massif Central: Inferences from Nd Isotopes and consequences for crustal evolution[END_REF][START_REF] Sergeev | Improving the resolution of single-grain U/Pb dating by use of zircon extracted from feldspar: Application to the Variscan magmatic cycle in the central Alps[END_REF][START_REF] Downes | Sr, Nd and Pb isotopic evidence for the lower crustal origin of Hercynian granodiorites and monzogranites, Massif Central, France[END_REF], such model ages are best explained in terms of a two-component mixing between new crust and an older component at least Early Proterozoic in age. This suggests strongly that the main process occurring during the Hercynian and Alpine orogenies was reworking of old continental crust with little addition of new material. Considering that the mean age of the old crustal component is 1.8 -2.0 Ga [START_REF] Peucat | Continental crust formation seen through the Sr and Nd isotope systematics of S-type granites in the Hercynian belt of western France[END_REF] and that of the placer minerals is about 1.5 Ga, a simple calculation shows that a maximum of 20 -30% juvenile crust could have been added over the last 400 Ma i.e. during the recent Hercynian and Alpine events. Both Sm-Nd and Lu-Hf isotopic systems thus record clear evidence that the continental area drained by the Rhone River had a long history prior to the known Hercynian and Alpine events and that large amounts of old crustal material with a mean Proterozoic age were reworked during the genesis of rocks currently forming the Massif Central and the Alps.

Conclusions

This study provides the first analyses of Nd and Hf isotopes and associated trace element concentrations for a placer and the main mineral species it contains. Our results show that heavy minerals host most trace elements while light minerals only dilute concentrations. More specifically, we show that monazite and zircon control the Nd and Hf isotopic compositions, respectively, of the Camargue placer. A simple mixture of only these two minerals reproduces not only the isotopic compositions but also the Nd and Hf concentrations of the bulk placer. As the mass percentages of heavy minerals in the studied placer are typical of those commonly found in worldwide beach placer deposits, we suggest that monazite and zircon always dominate the Nd and Hf isotopic compositions of beach placer even though they are usually present in low proportions in the mineral assemblage.

Our data indicate that present-day Nd-Hf isotopic compositions of the Camargue placer are representative of the crustal rocks found in the Massif Central and the Alps and provide therefore a reliable estimate of the average isotopic composition of the continental area drained by the Rhone River (eNd ≈ -9, eHf ≈ -13). The calculated Nd and Hf model ages for all placer minerals are consistent and provide an average Proterozoic age for the history of this part of the continental crust. We finally suggest that other beach placers formed at the mouths of large rivers in other parts of the world could provide reliable estimates of the average Nd-Hf compositions of other large continental areas, values that are usually not easy to obtain. to measure and process µ-XRF data and Nicholas Arndt (LGCA, Grenoble) for constructive discussions that help to improve the content of this paper. Constructive comments provided by Jonathan Patchett, Laurie Reisberg and an anonymous reviewer greatly helped improving the overall quality of the manuscript. Financial support for this study was provided by the Observatoire des Sciences de l'Univers de Grenoble (OSUG) and an INSU-CNRS grant. Binocular microscope (about 300 grains)

µXRF mapping (about 700 grains)

Figure 2

Figure 2: Mineral abundances in the heavy fraction (d>2.9). A picture of the heavy fraction and the mineral proportions estimated as a percentage of the total grains counted under the binocular microscope are reported in the bottom left corner. The µ-XRF mapping showing the mineral species of the analysed grains on the adhesive surface and the mineral abundances calculated using this method as a percentage of the counted grains are presented in the top right corner. The heavy mineral abundances determined by combining the results from both methods are shown in the grey-coloured inset as a percentage of the total counted grains and as a mass percentage. For mass percentage calculations, we used the characteristic mineral densities given by [START_REF] Deer | From sediments to their source rocks: Hf and Nd isotopes in recent river sediments[END_REF] and we estimated, based on binocular microscope observations, that all the grains had roughly the same volume except for the epidote and titanite for which the radius were considered to be 1.5 times larger than that of the other grains. Taking into account all sources of errors, the uncertainties on the mass percentages are estimated at about 3%. [START_REF] Bennett | Nd isotopic evidence for transient, highly depleted mantle reservoirs in the early history of the Earth Earth and[END_REF], Vervoort et al. (1996), Vervoort and Patchett (1996), Vervoort and Blichert-Toft (1999) and [START_REF] Vervoort | Hf-Nd isotopic evolution of the lower crust[END_REF] and plot along an array of ƐHf = 1.50xƐNd -0.60.

Note that this Nd-Hf relation has the same slope as the mantle array of [START_REF] Chauvel | Role of recycled oceanic basalt and sediment in generating the Hf-Nd mantle array[END_REF] but passes below the BSE value defined by [START_REF] Bouvier | The Lu-Hf and Sm-Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets[END_REF]. Analytical errors are smaller than the symbol size. -12 -11.5 -11 -10.5 -10 -9.5 -9 -8.5 -8 -7.5 -7 -6.5 [START_REF] Downes | Sr and Nd isotope geochemistry of coexisting alkaline magma series, Cantal, Massif Central, France[END_REF]), Bossard et al. (1986), [START_REF] Downes | Isotopic and trace element arguments for the lower crustal origin of Hercynian granitoids and pre-Hercynian orthogneisses, Massif Central (France)[END_REF], [START_REF] Paquette | U-Pb zircon, Rb-Sr and Sm-Nd geochronology of high-to very-high-pressure meta-acidic rocks from the western Alps[END_REF], [START_REF] Pin | Sources of Hercynian granitoids from the French Massif Central: Inferences from Nd Isotopes and consequences for crustal evolution[END_REF]Duthou (1990), Tilton et al. (1991), [START_REF] Downes | Sr, Nd and Pb isotopic evidence for the lower crustal origin of Hercynian granodiorites and monzogranites, Massif Central, France[END_REF], [START_REF] Duchene | The Lu-Hf dating of garnets and the ages of the Alpine high-pressure metamorphism[END_REF], [START_REF] Henry | The erosion of the Alps: Nd isotopic and geochemical constraints on the sources of the peri-Alpine molasse sediments[END_REF] and [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]. ƐNd values are calculated using the CHUR composition of [START_REF] Bouvier | The Lu-Hf and Sm-Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets[END_REF]. 2003). λNd = 6.54*10 -12 yr -1 and λHf = 1.87*10 -11 yr -1 . For each mineral fraction and for the placer, colour dots show the location of model ages as calculated using the Monte Carlo simulation.

The distribution of Nd and Hf model ages for all fractions together, are shown as histograms along the x and y axes. 1 : 1 Tables Table 1: Trace-element concentrations measured in the various fractions (ppm).

Concentrations were calibrated to the BR standard composition using the reference values recommended by [START_REF] Eggins | A simple method for the precise determination of > 40 trace elements in geological samples by ICPMS using enriched isotope internal standardisation[END_REF]. Precision on the measured concentrations of most elements is usually better than 5% as described in details by [START_REF] Chauvel | Comprehensive chemical and isotopic analyses of basalt and sediment reference materials[END_REF] and values obtained for the rock standards analysed as unknown samples are similar to published values within 3% (see Supplementary Table 2). Standard deviations (2σ) were calculated between the n measurement compositions of the two rock standards analysed as unknown samples and calibrated to the BR standard using the reference values given in the first column of this Table . Differences between the published values and our measurement compositions (Diff) are also reported.

Diff (%) BR 24 n=4

AGV-1 n=4 [START_REF] Eggins | A simple method for the precise determination of > 40 trace elements in geological samples by ICPMS using enriched isotope internal standardisation[END_REF] LGCA values 2σ (%) 2σ (%) [START_REF] Eggins | A simple method for the precise determination of > 40 trace elements in geological samples by ICPMS using enriched isotope internal standardisation[END_REF] Diff (%)

FiguresFigure 1 :

 1 Figures

  Figure 3

Figure 3 :

 3 Figure 3: REE and trace elements in the mineral fractions isolated from the

Figure 4 :

 4 Figure 4

Figure 5 Figure 5 :

 55 Figure 5

Figure 6 :

 6 Figure 6: Modelling of the heavy-mineral trace-element pattern. The concentrations

Figure 7 :

 7 Figure 7

  Figure 8

Figure 8 :

 8 Figure 8: Sketch of Hf two stage model age calculations. CC: Continental crust. mx:

Figure 9 :

 9 Figure 9: Hf model ages versus Nd model ages obtained from 10 000 Monte Carlo

  Figure 9

Ilmenite Magnetite Apatite Monazite Zircon Rutile Garnet Epidote

			Ilmenite Magnetite Garnet Epidote	44% 30%
			Zircon	10%
				Rutile	7%
	2 mm		Monazite Apatite <1% 3.5%
			Unidentified	4.5%
			Grain proportion	Weight proportion
		Ilmenite Magnetite	44%	45% wt
	200 µm	Garnet Zircon Monazite Epidote Titanite Apatite Unidentified Rutile	28% 10% 7% 3.5% 2% 1% 3.5% <1%	24% wt 10% wt 7% wt 4% wt <1% wt 1% wt 5% wt 2% wt

Ilmenite Magnetite Rutile Garnet Monazite Zircon

	Ilmenite Magnetite Garnet	45-55% 20-30%
	Zircon	10-15%
	Rutile	3-8%
	Monazite	2-7%
	Epidote	1-3%
	Titanite	1-2%
	Apatite	1%
	Unidentified	1%

  -6

	Number of analyses	Alps Massif Central
	ƐNd

  The acronyms used in this equation are similar to those of Figure8. All the model ages were calculated using a Monte-Carlo procedure (n=10 000 for each mineral fraction and for the placer) to propagate the input parameter uncertainties. The distribution of crystallisation ages (tc) follows a Gaussian law with a mean value of 300 Ma and a standard deviation (2σ) of 50 Ma. After having randomly sampled a crystallisation age in the latter distribution,

					T DM =t c +	1 λ Hf	ln /	0 0 176 Lu 176 Hf 3 177 Hf 177 Hf 3	4 4 CC mx t c t c	-0 -0 176 Lu 176 Hf 3 177 Hf 177 Hf 3	t c DM DM 4 4 t c +1;
	0	143 >? 3 144 >?	4 @A B C , 0 147 DE 3 144 >?	4 @A B C , 0 176 FG 3 177 FG	4 @A B C	and 0 176 HI 3 177 FG	4	@A B C	were calculated using the
	following parameters: (a) we assume that the depleted mantle has linearly evolved from a bulk
	Earth value at Earth formation, (b) we use the CHUR values published by Bouvier et al. (2008)
	and (c) we sample randomly the 0 143 >? 3 144 >?	4 @A J	and 0 176 FG 3 177 FG	4 @A J	isotopic ratios in Gaussian
	distributions centered at the mean-MORB pole of Chauvel et al. (2008) with a standard
	deviation (2σ) of 2e units. 0 143 >? 3 144 >?	4 EK B C and 0 176 FG 3 177 FG	4	EK B C were calculated back in time
	using the Nd and Hf isotopic ratios and the Lu, Hf, Sm and Nd concentrations measured in this
	study. Finally, 0 147 DE 3 144 >?	4	LL B C and 0 176 HI 3 177 FG	4 LL B C were calculated using 0 147 DE 3 144 >?	4 LL J	and
	0	176 HI 3 177 FG	4 LL J	ratios randomly sampled in Gaussian distributions centered at 0.115 ± 0.005 (2σ)
	and 0.013 ± 0.004 (2σ), respectively (values from McLennan, 2001 and Rudnick and Gao,

Table 1 : Trace-element concentrations measured in the various fractions (ppm)

 1

	Sample	CAM Mnz	CAM Grt	CAM Il	CAM Zr	CAM Ru	CAM Ep	CAM HMC	CAM fd NT	Plac7
	Description	Monazite	Garnet	Ilménite	Zircon	Rutile	Epidote	Heavy minerals d > 2.9	Light minerals d < 2.9	Total placer
	Li	4.49	25.6	4.45	18.4	7.04	10.5	16.7	24.2	17.9
	Rb	3.48	2.09	4.22	2.53	8.23	5.95	6.53	19.9	9.09
	Sr	143	5.6	14.1	23	53.5	1525	108	509	170
	Y	17419	414	68.1	2137	285	313	1969	55.5	2363
	Zr	1509	2090	528	501294	23353	581	45508	1494	39432
	Nb	3.44	4.44	303	21.5	1366	42.7	295	2.29	360
	Cs	0.356	0.181	0.489	0.145	0.749	0.379	0.572	0.643	0.694
	Ba	10.3	4.32	22.5	6.7	67.2	19.7	26.7	85	50
	La	90255	7.28	56.7	17.7	433	350	2788	26.7	2345
	Ce	182339	15.4	113	60.6	866	699	5635	53	4833
	Pr	22283	1.87	12.8	7.19	104	84.6	652	6.69	569
	Nd	80352	7.42	46.2	35.2	374	312	2363	25.2	2039
	Sm	14649	2.85	11	18.9	68.7	62.6	432	5.27	360
	Eu	439	0.96	0.987	4.69	4.3	8.09	19.4	0.571	17
	Gd	9927	16	11.3	59.9	52.9	55	324	4.66	271
	Tb	1227	4.79	2.05	15.3	7.87	8.66	49.5	0.673	39.7
	Dy	4968	46.1	11.7	171	42.8	49.4	279	3.66	221
	Ho	656	12.7	2.4	60.1	8.9	9.6	58	0.725	45.5
	Er	1081	44.6	7.09	276	28.6	26.5	173	2.23	135
	Yb	344	50.6	7.05	495	39.1	23.6	168	2.94	134
	Lu	33	7.78	1.07	99.5	7.38	3.37	26.6	0.569	21.3
	Hf	50	47	14.6	11230	579	13.1	1012	42.2	889
	Ta	2.14	0.549	30.2	4.4	113	7.8	31	0.28	30.3
	Pb	1268	2.38	18.4	17.2	24.5	60.5	62.7	6.87	55.3
	Th	40663	4.58	20.1	284	306	142	1154	38.2	1006
	U	7233	5.33	5.54	720	146	42.4	265	3.96	217

Table 2 :

 2 Nd and Hf isotopic compositions of various fractions. Re-run stands for run duplicates. Uncertainties (± 2σ) reported in the Table correspond to in-run errors. Nd and Hf isotopes were calculated relative to the Ames-Rennes Nd and Ames-Grenoble Hf standard values given by[START_REF] Chauvel | A hafnium isotope and trace element perspective on melting of the depleted mantle[END_REF] andChauvel et al. (2011) ( 143 Nd/ 144 Nd = 0.511961 ± 13 (2σ) and 176 Hf/ 177 Hf = 0.282160 ± 12 (2σ)). The reproducibility calculated using re-run analyses and duplicates (see Supplementary Table3) is 0.000013 (2σ, n=7) for the176 Hf/ 177 Hf ratios and 0.000037 (2σ, n=7) for the 143 Nd/ 144 Nd ratios. ƐNd and ƐHf are calculated using the CHUR composition given by[START_REF] Bouvier | The Lu-Hf and Sm-Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets[END_REF]. Considering the reproducibility of the entire analytical procedure, uncertainties are estimated at ± 0.7 Ɛ units for ƐNd and ƐHf values.

Table 2 : Nd and Hf isotopic compositions of various fractions

 2

	Sample	Description	143 Nd/	144 Nd ± 2σ	ƐNd	176 Hf/	177 Hf ± 2σ	ƐHf
	CAM Mnz	Monazite	0.512158	± 9	-9.2	0.282942	±	+5.6
	CAM Mnz Re-run		0.512166	± 7	-9.0
	CAM Grt	Garnet	0.512363	± 9	-5.2	0.282559	±	-8.0
	CAM Il	Ilmenite	0.512359	± 8	-5.3	0.282593	±	-6.8
	CAM Il Re-run		0.512390	± 7	-4,7
	CAM Zr	Zircon	0.512594	± 5	-0.7	0.282417	±	-13.0
	CAM Zr Re-run						0.282409	±	-13.3
	CAM Ru	Rutile	0.512150	± 13	-9.4	0.282358	±	-15.1
	CAM Ep	Epidote	0.512279	± 5	-6.9	0.282743	± 19	-1.5
	CAM HMC	Heavy minerals d>2,9	0.512169	± 5	-9.0	0.282374	±	-14.5
	CAM HMC Re-run		0.512183	± 9	-8,7	0.282378	±	-14.4
	CAM fd NT	Light minerals d<2,9	0.512181	± 8	-8.8	0.282440	±	-12.2
	CAM fd NT Re-run						0.282441	±	-12.2
	Plac7	Total placer	0.512190	± 10	-8.6	0.282406	± 11	-13.4

Table B .1 : Sample-specific analytical procedures

 B

	Sample	Description BR ref	Dissolved Quantity	Acid mixtures Sample Dissolution AGV-1	Time in bombs	Trace element analyses Dilution factor	BR 24	Hf isolation Second Hf column
	Plac7	Total placer	≈ 50 mg		HF-HCl0 4 -HNO 3		2 months		10 000			x
	CAM HMC Li Rb	Heavy minerals 14.9 48.7	≈ 35 mg 11.0 69.9	0.8 0.4	HF-HCl0 4 -HCl-HNO 3 11.2 68.3	2.0 -2.4	2 months	500 000 for REE, U, Th, Y, Hf, Zr 7.17 1.6 5000 for the other elements 84.6 1.2	7.06 83.67	x	-1.6 -1.1
	CAM fd NT Sr	Light minerals 1364	≈ 35 mg 674	1.2	HF-HCl0 4 -HNO 3 655	-2.9	1 month	619	5000 1.8	618.71	x	-0.1
	Y	31.5			21.1		0.8	21	-0.5		30.9	0.6		30.96	0.2
	CAM Mnz Zr Nb	Monazite 283 119	≈ 35 mg 244 14.8	1.3 4.4	HF-HCl-HNO 3 240 14.9	-1.8 0.7	3 weeks	1 000 000 for REE, U, Th, Y 302 2.0 5000 for the other elements 40.2 3.4		303.5 39.6	0.6 -1.5
	CAM Zr Cs Ba La	0.823 1082 82.1	Zircon	≈ 10 mg 1.31 1220 38.4	2.2 3.4 0.8	HF-HCl0 4 -HCl-HNO 3 1.31 1223 38.5	-0.1 0.1 0.2	2 months	0.662 5 000 000 for Hf and Zr 3.4 394 2.0 5000 for the other elements 34.0 0.8	0.65 387.29 33.43	-1.9 -1.7 -1.6
	CAM Ru Ce Pr	152 17.4		Rutile	69.0 8.53	≈ 5 mg	0.2 0.4	68.5 HF-HCl0 4 -HNO 3 8.45	-0.7 -0.9	3 weeks	73.3 9.63	5000 1.6 1.4		72.66 9.59	-0.9 -0.5
	CAM Il Nd	Ilmenite 66.1	≈ 30 mg 31.9	0.9	HF-HCl0 4 -HCl-HNO 3 31.6	-1.1	3 weeks	39.8	5000 1.4		39.67	x	-0.3
	CAM Grt Sm Eu	12.1 3.58		Garnet	≈ 10 mg 5.86 1.59	0.9 3.0	HF-HCl0 4 -HCl 5.82 1.57	-0.6 -1.3	2 weeks	40 000 for REE 8.33 for the other elements 2.2 5000 8.33 2.5 1.2 2.48	x	0.0 -0.7
	Gd	9.57			4.62		1.8	4.73	2.3		7.41	1		7.32	-1.2
	CAM Ep Tb	1.29	Epidote	0.65	≈ 5 mg	2.3	HF-HCl0 4 -HCl 0.664	2.1	2 weeks	1.08	5000 2.8		1.06	x	-1.5
	Dy	6.3			3.53		2.9	3.54	0.2		5.68	1.8		5.67	-0.2
	Ho	1.09			0.678	1.7	0.69	1.7		1.05	2.2		1.06	0.6
	Er	2.59			1.84		3.1	1.86	0.8		2.72	1.2		2.71	-0.5
	Tm
	Yb	1.81			1.63		1.3	1.64	0.8		2.09	0.4		2.09	-0.2
	Lu	0.251		0.249	3.1	0.248	-0.3		0.303	3.4		0.3	-1.0
	Hf	5.62			5.00		1.3	5	0.1		6.62	1.2		6.69	1.0
	Ta	5.79			0.865	1.5	0.882	1.9		2.46	1		2.44	-0.9
	Pb	4.77			37.5		1.1	37.3	-0.7		3.75	0.8		3.69	-1.7
	Th	10.9			6.47		0.9	6.53	0.9		4.84	1.6		4.83	-0.1
	U	2.46			1.9		1.7	1.89	-0.8		1.22	1.6		1.23	1.0

Acknowledgements

We thank F. Senebier (LGCA, Grenoble) for the first separation and recognition of the mineral grains, P. Telouk (ENS, Lyon) for its assistance during Nd and Hf isotopic measurements in Lyon, M. Muñoz and M. Corrazi (LGCA, Grenoble) for their precious help

Supplementary File A

Determination of heavy mineral abundances by micro X-ray fluorescence

Several hundred grains were laid down on an adhesive surface and mapped by micro X-ray fluorescence (µXRF EDAX Eagle III) to identify spectral signatures that are characteristic for each mineralogical phase. The analyses were performed under primary vacuum with a 50µm X-ray spot size on a 512x400 pixel grid. Spectra were acquired on each pixel with a dwell time of 400 ms and a sampling step of 50 µm in x and y. On the adhesive surface, the grains were previously spaced at distances of at least 50µm from each other to insure better recognition of the spectral signature of each grain. X-ray tube operating conditions were maintained at 20 kV and 200 µA during all analyses. In the heavy mineral fraction (d > 2.9), the mineral species were discriminated using the X-ray fluorescence energies (Kα, Kβ, Lα or Lβ) of ten chemical elements (Si, Al, Ca, P, Zr, Ti, Fe, Mn, Nd and Ce) on a 0-20 KeV energy range. Among all spectra, we selected some representative reference spectra for the background noise and for each mineral of interest (ilmenite, magnetite, zircon, garnet, monazite and rutile). As used by [START_REF] Muñoz | Hyperspectral micro-XANES mapping in the diamond-anvil cell: Analytical procedure applied to the decomposition of (Mg,Fe)-ringwoodite at the upper/lower mantle boundary[END_REF] for micro-XANES mapping, our computer code then determines the best linear combination of reference spectra that reproduces the intensities measured by µXRF for the chosen chemical elements and for each pixel. If one of the linear coefficients is close to 1, the computer code associates the pixel with the corresponding mineral species (or the background); if not, the pixel is considered to be unidentified. Then, using this information and standard image processes, we created a map showing the nature of almost all the grains analysed by µXRF and we calculate the relative abundance of each referenced mineral species as a percentage of the total grains analysed on the adhesive surface.

Supplementary File B

Sample dissolution

Because placer minerals are quite difficult to dissolve at 150°C in bombs, only few mg of each mineral fraction were dissolved in acid mixtures combining pure concentrated HF, HCl, HClO4 and HNO3 (see Table B.1). For the six pure and homogeneous mineral fractions, we dissolved the quantity required to measure both trace element concentrations and Nd-Hf isotopic ratios. In contrast, for the total placer and the two fractions of light and heavy minerals, we dissolved about 30 to 50 mg of powders to minimize heterogeneities and obtain representative analyses.

Trace element concentrations

To measure trace element concentrations similar to those of the BR rock standard (external calibration), samples were diluted about 5000 times before being analysed on the ICP-MS. In contrast, when trace element concentrations were unusually high compared to the BR rock standard (Hf in zircons, REE in monazites, etc...), high dilution factors were preferentially used to calculate precise concentrations (see Table B.1). In that case, the dilution procedure was performed in several steps by carefully weighing the mass of each aliquot taken from the "mother" solution. Several procedural duplicates were then analysed to evaluate the reproducibility of the entire dilution procedure.

Chemical separation of Hf in the pure mineral fractions

Even if the dissolved quantity of sample powder was initially low (few mg), our calculations show that the resin used for the second Hf column would be saturated if we loaded some of the pure mineral separates on a column filled with 2 mL of anionic resin. To avoid resin-saturation problems that may cause loss of Hf, the standard Hf isolation procedure described by [START_REF] Blichert-Toft | Separation of Hf and Lu for high-precision isotope analysis of rock samples by magnetic sector-multiple collector ICP-MS[END_REF] and modified by [START_REF] Chauvel | Comprehensive chemical and isotopic analyses of basalt and sediment reference materials[END_REF] was adapted to each of the mineral fractions. For the rutile, monazite and zircon separates, the second Hf step that separates Hf from some major elements was removed because (1) these minerals do not contain high amount of these major elements and (2) there is a high risk to loose Hf during this step. For the epidote, garnet and ilmenite fractions, we kept the second Hf column because these minerals may contain significant amount of Fe, Ca or Al that need to be isolated from Hf to insure high precision on the Hf-isotope measurements (see Table B.1). However, to perform this step without saturating the resin, the ilmenite fraction was divided in two fractions loaded on two different columns that were subsequently merged to finish the chemical separation.

Supplementary Table 1: Heavy minerals identified in Espiguette black-sands (modified after BRGM, 2000)

Heavy minerals Abundance

Supplementary Table 3 Trace-element concentrations (ppm) and Hf-Nd isotopic compositions measured in monazite, garnet, ilmenite and zircon fractions before their purification by handpicking under a binocular microscope. Concentrations were obtained in the same way as for their respective pure mineral separates. The two dissolutions of the impure garnet fraction are not very reproducible most probably because of sample heterogeneity (unpurified fraction with a complex mineral assemblage