Éditeur(s) : Elsevier
Résumé : The combined use of Lu-Hf and Sm-Nd isotope systems potentially offers a unique perspective for investigating continental erosion, but little is known about whether, and to what extent, the Hf-Nd isotope composition of sediments is related to silicate weathering intensity. In this study, Hf and Nd elemental and isotope data are reported for marine muds, leached Fe-oxide fractions and zircon-rich turbidite sands collected off the Congo River mouth, and from other parts of the SE Atlantic Ocean. All studied samples from the Congo fan (muds, Fe-hydroxides, sands) exhibit indistinguishable Nd isotopic composition (epsilon(Nd)similar to - 16), indicating that Fe-hydroxides leached from these sediments correspond to continental oxides precipitated within the Congo basin. In marked contrast, Hf isotope compositions for the same samples exhibit significant variations. Leached Fe-hydroxide fractions are characterized by epsilon(Hf) values (from -1.1 to +13) far more radiogenic than associated sediments (from -7.1 to -12.0) and turbidite sands (from -27.2 to -31.6). epsilon(Hf) values for Congo fan sediments correlate very well with Al/K (i.e. a well-known index for the intensity of chemical weathering in Central Africa). Taken together, these results indicate that (1) silicate weathering on continents leads to erosion products having very distinctive Hf isotope signatures, and (2) a direct relationship exists between epsilon(Hf) of secondary clay minerals and chemical weathering intensity. These results combined with data from the literature have global implications for understanding the Hf-Nd isotope variability in marine precipitates and sediments. Leached Fe-hydroxides from Congo fan sediments plot remarkably well on an extension of the 'seawater array' (i.e. the correlation defined by deep-sea Fe-Mn precipitates), providing additional support to the suggestion that the ocean Hf budget is dominated by continental inputs. Fine-grained sediments define a diffuse trend, between that for igneous rocks and the,seawater array', which we refer to as the 'zircon-free sediment array' (epsilon(Hf)=0-91 epsilon(Nd)+3.10). Finally, we show that the Hf-Nd arrays for seawater, unweathered igneous rocks, zircon-free and zircon-bearing sediments (epsilon(Hf)= 1.80 epsilon(Nd)+2.35) can all be reconciled, using Monte Carlo simulations, with a simple weathering model of the continental crust. (C) 2008 Elsevier B.V. All rights reserved.
Earth and Planetary Science Letters (0012-821X) (Elsevier), 2009 , Vol. 277 , N. 3-4 , P. 318-326

Droits : 2009 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-6451.pdf
DOI:10.1016/j.epsl.2008.10.028
http://archimer.ifremer.fr/doc/00000/6451/

Éditeur(s) : HAL CCSD Taylor & Francis
Résumé : Modern geochronology has moved beyond the acquisition of dates: the goal is to understand the significance of these numbers for the geodynamic evolution of Earth at all scales. The coupling of the laser-ablation microprobe (LAM) to inductively coupled plasma mass spectrometers (ICPMS, multicollector (MC)-ICPMS) has revolutionised geochronology and geochemistry over the last 10 years. These systems enable the rapid and precise in situ analysis of trace-element patterns and isotopic systems, while adding information related to microstructural context and major-element composition. The integration of these multiple sources of data is crucial in constraining the origin of the sample and the processes leading to its formation, so that we can understand the meaning of a date in terms of geological events. LAM-ICPMS measurement of U-Pb ages and trace-element patterns in zircon, coupled with LAM-MC-ICPMS analysis of Hf isotopes in the same grains, gives new insights into the processes of magma genesis. Applied to detrital zircons from modern drainages or sedimentary rocks (the TerraneChron approach), it becomes a powerful tool to investigate problems of crustal evolution on scales ranging from single terranes to continents. The in situ analysis (LAM-MC-ICPMS) of Re-Os systematics in single grains of sulfides in mantle-derived peridotites has demonstrated that most mantle rocks contain several generations of Os-bearing sulfides; whole-rock analyses are mixtures reflecting multiple melting and metasomatic events in the lithospheric mantle. These deep-seated events are commonly mirrored in the crust; Os model-age spectra from xenolith suites show age 'peaks' that correspond to the ages of thermal/tectonic events in the overlying crust, suggesting strong linkages between crust and mantle. Integrated studies of the timing and nature of crustal and mantle events, using these techniques, will be important for understanding the large-scale dynamics of the Earth.
ISSN: 0812-0099

hal-00411328
https://hal.archives-ouvertes.fr/hal-00411328
DOI : 10.1080/08120090802097450

Éditeur(s) : HAL CCSD
Résumé : International audience
In the context of climate change, this study presents the ability of major/trace elements together with strontiumisotopes to trace back water paths at small scale and to deconvolve the geochemical signal of a small watershedsubject to intense flash floods episodes (Peyne, Hérault, France). Two small sub-basins draining distinct lithologiesin their heads (Plio-Villafranchian conglomerate versus Triassic gypsum-rich marls and dolomites) and the sameMiocene lithology downstream are investigated.Major elements and Ca/Na vs. Mg/Na ratios classically applied at large scale to distinguish carbonate from silicateweathering, allow here discriminating the three main lithologies from the two sub-basins. Trace elements Rband Sr coupled to calcium, also allow this lithological discrimination but in addition the Ca/Rb vs. Sr/Rb tracersappear to be much more discriminant for the various hydrological conditions. Thus, in combination with detailedlithological descriptions, they allow identifying the different facies that imprint the water signature through water-rock interaction according to the hydrological conditions.Strontium isotopes and Rb/Sr ratio, discriminate more precisely the drained lithologies of the 2 sub-basins. Firstly,the 87Sr/86Sr ratios allow identifying the nature of the lithologies and their main component(s) contributing to theSr budget in water and thus imprint the isotopic signature. Secondly, Sr isotopes evidenced two distinct Miocenefacies: the detritric faction (sandy marls), and the marine carbonates.The geochemical signatures of the brook samples draining both compartments were compared to the signatureof the Peyne River outlet just before the confluence into the Hérault River. It appears that the signature of thePeyne River, integrating all the water draining the basin, is relatively stable whatever the hydrological conditionsand mainly marked by the Miocene formations. Sr isotopes further highlight that this signature seems to resultfrom the mixing of both Miocene facies present in the lower part of the Peyne watershed, i.e. the sandy marls andthe carbonates. The typical signatures of the Plio-Villafanchian conglomerates and Triassic gypsiferous marls anddolomites of the headwaters of each sub-basin are completely hidden by the Miocene signatures. This should berelated to the large drainage area of the Miocene compared to other lithologies, despite the high solubility of thegypsum formations releasing large quantities of dissolved elements in solution.Understanding water paths at small scale is even more efficient when the geochemical approach is coupled with adetailed geological description. Indeed, the geochemical tracers are extremely dependent on the facies sequences(morphology) and of the soil nature (mineralogical composition), this is particularly sensitive at small scale.In the context of climate change increasing the intensity of rain events in the Mediterranean region, and resultingin catastrophic floods events (flash floods), tracing the origin of water contributing to the runoff is of primaryimportance and this must be investigated at small scale. Geochemical and isotopic fingerprinting thus constituteexcellent tools which can help to define the area of interest to be monitored in the framework of flood forecast andwarning networks
EGU General Assembly 2015
Vienne, Austria

hal-01118622
https://hal-brgm.archives-ouvertes.fr/hal-01118622

Éditeur(s) : HAL CCSD
Résumé : When the upper crust below the mountain ranges shows crustal extension with a direction collinear to the convergence direction, it is generally accepted that it is the result of gravitational collapse. However, recent studies challenge this paradigm by showing that erosion induces uplift and extension in the central part of the low convergent mountain ranges. In this study, we investigate the impact of denudation on the seismotectonic regime of mountain ranges.In order to identify a relationship between seismotectonic regime and erosion, the first part of this work presents a compilation of data in the mountain ranges. Based on these observations, a simple kinematic model is proposed to predict the seismotectonic regime of the study ranges.For low convergence rate and moderate mean elevation ranges, this model predicts an extension regime when the denudation rate is 15% higher than the convergence rate.The second part is devoted to the development of thermomechanical 2D finite element models to study the impact of surface processes on the deformation of the Pyrenees. The results show that the isostatic response to erosion reactivates pre-existing structures. The kinematics of an inherited fault plane can be predicted due to the gradient of the horizontal surface velocity profile. Thus, a plane located in the eroded zone is reactivated in normal fault when near the border of the erosion area, this same plane is reactivated in reverse fault. These results suggest that the current deformation in the North Pyrenean Zone could be the result of surface processes.Given the small number of studies quantifying erosion rates in the Pyrenees, the models developed in the second part suffer from high uncertainty. To remedy this, we study the central Pyrenees through a study that combines two types of data : watershed denudation rates from cosmogenic isotopes concentration, and incision rates from sediments buried in the karst. These results are presented in chapter 3. Denudation profiles obtained are consistent with reactivation of a normal fault plane located in the North Zone of the Central Pyrenees.From our study in the Pyrenees, a good correlation appears between the value of the rate of erosion and the glacier occupation. Given that the glaciers where more developed in the Alps, it raises the question of the impact of the late-würmian deglaciation on the present deformation of this range. A numerical model detailing this relationship is presented in the fourth chapter. Theresults show that deglaciation of the western Alps is controlled by the rheological heterogeneity of the crust. Some of our models predict uplift rates consistent with those highlighted by geodesy.
Lorsque l’extension de la croûte sous les parties hautes des chaînes de montagnes est colinéaire à la direction de convergence, il est traditionnellement admis que le moteur est l’effondrement gravitaire. Pourtant, des études récentes remettent en cause ce paradigme en montrant que l’érosion induit un soulèvement et de l’extension dans la partie centrale des chaînes de montagne à faible taux de convergence. L’objectif de notre étude est d’étudier l’impact de la dénudation de la topographie sur le régime sismo-tectonique des chaînes de montagnes.La première partie de ce travail présente une compilation de données dans les chaînes de montagnes afin de dégager des relations entre régime sismo-tectonique et érosion. Sur la base de ces observations, un modèle cinématique simple permettant de prévoir le régime de la chaîne est proposé. Ainsi, pour les chaînes à faible taux de convergence et d’élévation moyenne, ce modèleprédit de l’extension lorsque le taux de dénudation est 15% plus élevé que le taux de convergence.La deuxième partie est consacrée au développement d’un modèle thermo-mécanique 2D en éléments finis pour étudier l’impact des processus de surface sur la déformation des Pyrénées.Les résultats montrent que la réponse isostatique à l’érosion permet de réactiver des structurespré-existantes. La cinématique d’un plan de faille hérité peut être prédite grâce au gradient du profil des vitesses de surface horizontales. Ainsi, un plan situé dans la zone d’érosion est réactivé en faille normale alors qu’en bordure de cette zone une faille est réactivée en régime inverse. Ces résultats suggèrent que la déformation actuelle des Pyrénées pourrait être la conséquence d’unprocessus d’érosion.Compte tenu du faible nombre d’études quantifiant les taux d’érosion dans les Pyrénées, les modèles développés dans la deuxième partie souffrent d’une forte incertitude. Pour y remédier, nous avons cherché à les quantifier dans les Pyrénées Centrales grâce à une étude qui combine deux types de données : taux de dénudation des bassins versants à partir des isotopes cosmogéniques, et vitesses d’incision à partir des sédiments piégés dans les karsts. Ces résultats sontprésentés dans la troisième partie. Les profils de dénudation obtenus sont compatibles avec unrejeu en faille normale d’un plan situé dans la Zone Nord Pyrénéenne des Pyrénées Centrales.Notre étude dans les Pyrénées, montre une bonne corrélation entre la valeur du taux d’érosion et la cartographie de l’occupation glaciaire. Les glaciers ayant été plus développés dans les Alpes, cela pose la question de l’impact de la déglaciation tardi-Wurmienne sur la déformation actuelle de cette chaîne. Un modèle numérique détaillant cette relation est présenté dans le quatrième chapitre. Les résultats montrent que la déglaciation des Alpes occidentales est contrôléepar l’hétérogénéité rhéologique de la croute. Certains de nos modèles prédisent des vitesses de surrection compatibles avec celles mises en évidence par la géodésie.
https://hal.archives-ouvertes.fr/tel-01262333

tel-01262333
https://hal.archives-ouvertes.fr/tel-01262333
https://hal.archives-ouvertes.fr/tel-01262333/document
https://hal.archives-ouvertes.fr/tel-01262333/file/these-Genti-2015-postjury.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The New Caledonia ophiolite (Peridotite Nappe) is dominated by mantle lithologies, composed of forearc-related refractory harzburgites and minor lherzolites in both the spinel and plagioclase facies.In this study, a comprehensive geochemical data set (major, trace elements and Sr–Nd isotopes) is used to constrain the mantle evolution of the lherzolites and their relationships with the basalts from the Poya Terrane, which tectonically underlies the mantle rocks. The majority of the lherzolites are low-strain porphyroclastic tectonites. They likely record an asthenospheric origin followed by re-equilibration at lithospheric conditions, as supported by geothermometric estimates (T = 1100–940 °C and 920–890 °C for porphyroclastic and neoblastic spinel-facies assemblages, respectively). Olivine composition (Fo = 88.5–90.0 mol%), spinel Cr# ([molar 100 • Cr/(Cr + Al)] = 13–17) and relatively high amounts (7–8 vol%) of Al2O3- and Na2O-rich clinopyroxene (up to 0.5 and 6.5 wt.%, respectively) indicate a moderately depleted geochemical signature for the spinel lherzolites. Bulk rock and clinopyroxene rare earth elements (REE) patterns display a typical abyssal-type signature, i.e. steeply plunging LREE accompanied by nearly flat HREE to MREE. Clinopyroxene REE compositions of the spinel lherzolites may be reproduced by small amounts of fractional melting of a garnet lherzolite precursor (~ 4%), followed by 4%–5% melting in the spinel peridotite field. The plagioclase lherzolites show melt impregnation microstructures, Cr- and Ti-rich spinels and incompatible trace element enrichments (REE, Ti, Y, and Zr) in bulk rocks and clinopyroxenes. Impregnation modelling for these elements suggests that the plagioclase lherzolites originated from residual spinel lherzolites by entrapment of highly depleted (non-aggregated) MORB melt fractions in the shallow oceanic lithosphere. Nd isotope compositions of the investigated peridotites are consistent with derivation from an asthenospheric mantle source that experienced a recent MORB-producing depletion event. This evolution was most likely accomplished in a spreading ridge. However, geochemical trace element modelling and Nd isotopes do not support a genetic mantle–crust link between the lherzolites and enriched-MOR-type basalts from the Poya Terrane.
ISSN: 0024-4937

hal-01412440
https://hal.archives-ouvertes.fr/hal-01412440
DOI : 10.1016/j.lithos.2016.04.030

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Cratons represent the oldest preserved lithospheric domains. Their lithosphere (lithospheric mantle welded to overlying Precambrian crystalline basement) is considered to be particularly robust and long-lived due to the protecting presence of buoyant and rigid “keels” made up of residual harzburgites. Although the cratons are mostly assumed to form in the Archaean, the timing of their formation remains poorly constrained. In particular, there are very few datasets describing concurrently the age of both the crustal and mantle portions of the lithosphere. In this study, we report new U–Pb ages and Hf isotope compositions for zircons in crustal xenoliths from the Udachnaya kimberlite in the central Siberian craton; this dataset includes samples from both the upper and lower portions of the crust. The zircon ages agree well with model melt-extraction Re–Os ages on refractory peridotite xenoliths from the same pipe; taken together they allow an integrated view of lithosphere formation. Our data reveal that the present day upper crust is Archaean, whereas both the lower crust and the lithospheric mantle yield Paleoproterozoic ages. We infer that the deep lithosphere beneath the Siberian craton was not formed in a single Archaean event, but grew in at least two distinct events, one in the late Archaean and the other in the Paleoproterozoic. Importantly, a complete or large-scale delamination and rejuvenation of the Archaean lower lithosphere (lower crust and lithospheric mantle) took place in the Paleoproterozoic. This further demonstrates that craton formation can be a protracted, multi-stage process, and that the present day crust and mantle may not represent complementary reservoirs formed through the same tectono-magmatic event. Further, deep cratonic lithosphere may be less robust and long living than often assumed, with rejuvenation and replacement events throughout its history.
ISSN: 0012-821X

hal-01467648
https://hal.archives-ouvertes.fr/hal-01467648
DOI : 10.1016/j.epsl.2016.09.046

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Ocean island basalts (OIB) from the Comores archipelago (Indian Ocean) display mineralogical and geochemical features different from the other Indian OIB. We present here new geochronological data (40Ar/39Ar), major and trace element compositions and isotopic (Sr, Nd, Pb) ratios of silica-undersaturated alkaline rocks from Mayotte, the oldest island of the Comores. Two trends are defined using major element composition: (1) a highly silica-undersaturated trend which includes basanites, (melilite-bearing) nephelinites, intermediate lavas and phonolites from the southern part of the island and (2) a moderately silica-undersaturated trend which is mainly represented by alkali basalts, basanites, intermediate lavas and phonolites from the northern part of the island. Both trends could be explained, to some extent, by variable degrees of partial melting. Normative larnite-bearing olivine melilitites and nephelinites exhibit, in addition to their high silica-undersaturation, elevated concentrations in CaO (>12wt.%) and P2O5 (up to 1.35wt.%). These exceptional rocks would result from low degree deep partial melting of a CO2-metasomatized source in the presence of carbonate (probably dolomite) and apatite. Igneous rocks from southern and northwestern shield volcanoes are characterized by a radiogenic Pb composition, revealing the existence of a HIMU (high μ=238U/204Pb) component in their source. Its influence decreases from the main building stage (>10.6-~3.0Ma) to the post-shield stage for the benefit of a depleted MORB-mantle (DMM) component, especially in the north central rocks. This feature would reflect increasing melting degrees of the depleted dominant source, bearing small-scale HIMU heterogeneities progressively consumed with time. The HIMU signature might have been introduced in the Comorian lithospheric mantle by thermal erosion or delamination of a continental lithosphere during the Gondwana break-up. The other islands of the Comores archipelago (Moheli, Anjouan and "La Grille" type lavas from Grande Comore) display also a DMM-HIMU mixing trend. Only a few lavas from Grande Comore ("Karthala" type) and one sample from Mayotte show the clear EM1 contribution (87Sr/86Sr>0.7035) of the Comorian plume.
ISSN: 0024-4937

insu-01089376
https://hal-insu.archives-ouvertes.fr/insu-01089376
https://hal-insu.archives-ouvertes.fr/insu-01089376/document
https://hal-insu.archives-ouvertes.fr/insu-01089376/file/Pelleter%20et%20al%201.pdf
DOI : 10.1016/j.lithos.2014.09.012

Éditeur(s) : Pergamon-elsevier Science Ltd
Résumé : Quantitative analyses on calcareous nannofossil assemblages on high temporal resolution (600-800 years) have been carried out from Core KC01B in the Ionian Sea (Eastern Mediterranean) throughout Marine Isotope Stage (MIS) 13-9, between 500 and 300 ka. This is an interval of considerable climate changes, known as the Mid-Brunhes event, which includes MIS 11, considered as a possible analogue for future interglacial conditions. In the Mediterranean core, the interval is characterized by the dominance of Gephyrocapsa spp., as also known from ocean records. Calcareous nannofossil abundance fluctuations have been interpreted in terms of modification of sea surface primary productivity/water stratification and temperature. Specifically, the increase in abundance of Gephyrocapsa caribbeanica and small Gephyrocapsa coupled with decreases of Florisphaera profunda, Syracosphaera spp. and Rhabdosphaera spp. suggests enhanced primary productivity/mixed surface waters during interglacial stages and at the Terminations, which can be interpreted in terms of interaction between climate changes and mesoscale oceanographic circulation. On the other hand, distributions of Calciosolenia spp., Oolithotus spp. and Umbilicosphaera sibogae, which are in phase with delta O-18 curve showing higher abundances at the lighter values of the marine isotope record, are considered as warm-water indicators. Biotic proxies point to peculiar climate dynamics through the MIS 12-11 transition (i.e. Termination V). High productive, cool, low salinity and turbid surface waters characterize the early MIS 11(421-408 ka) and likely reflect enhanced continental humidity/monsoon activity over North Africa and increased runoff into the basin. A late surface water warming with respect to Termination V is established at about 403 ka, during a period of low insolation forcing, almost coeval with the deposition of sapropel S11. A climate optimum, lasting about 15 ky (403-389 ka), is then recorded during a period of low insolation forcing centred at 398 ka. The later part of MIS 11 is characterized by climate deterioration at about 389 ka. Comparison with results from subtropical to polar Atlantic records suggests a remarkable relation between Mediterranean and Atlantic climate regimes.
Quaternary International (1040-6182) (Pergamon-elsevier Science Ltd), 2013-03 , Vol. 288 , P. 97-111

Droits : 2011 Elsevier Ltd and INQUA. All rights reserved.
http://archimer.ifremer.fr/doc/00181/29194/27593.pdf
DOI:10.1016/j.quaint.2011.12.007
http://archimer.ifremer.fr/doc/00181/29194/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The eastern part of the Cordillera Occidental of Ecuador comprises thick buoyant oceanic plateaus associated with island-arc tholeiites and subduction-related calc-alkaline series, accreted to the Ecuadorian Continental Margin from Late Cretaceous to Eocene times. One of these plateau sequences, the Guaranda Oceanic Plateau is considered as remnant of the Caribbean–Colombian Oceanic Province (CCOP) accreted to the Ecuadorian Margin in the Maastrichtien. Samples studied in this paper were taken from four cross-sections through two arc-sequences in the northern part of the Cordillera Occidental of Ecuador, dated as (Río Cala) or ascribed to (Macuchi) the Late Cretaceous and one arc-like sequence in the Chogòn-Colonche Cordillera (Las Orquídeas). These three island-arcs can clearly be identified and rest conformably on the CCOP. In all four localities, basalts with abundant large clinopyroxene phenocrysts can be found, mimicking a picritic or ankaramitic facies. This mineralogical particularity, although not uncommon in island arc lavas, hints at a contribution of the CCOP in the genesis of these island arc rocks. The complete petrological and geochemical study of these rocks reveals that some have a primitive island-arc nature (MgO values range from 6 to 11 wt.%). Studied samples display marked Nb, Ta and Ti negative anomalies relative to the adjacent elements in the spidergrams characteristic of subduction-related magmatism. These rocks are LREE-enriched and their clinopyroxenes show a tholeiitic affinity (FeOT–TiO2 enrichment and CaO depletion from core to rim within a single crystal). The four sampled cross-sections through the island-arc sequences display homogeneous initial Nd, and Pb isotope ratios that suggest a unique mantellic source for these rocks resulting from the mixing of three components: an East-Pacific MORB end-member, an enriched pelagic sediment component, and a HIMU component carried by the CCOP. Indeed, the ankaramite and Mg-basalt sequences that form part of the Caribbean-Colombian Oceanic Plateau are radiogenically enriched in 206Pb/204Pb and 207Pb/204Pb and contain a HIMU component similar to that observed in the Gorgona basalts and Galápagos lavas. The subduction zone that generated the Late Cretaceous arcs occurred far from the continental margin, in an oceanic environment. This implies that no terrigenous detrital sediments interacted with the source at this period. Thus, the enriched component can only result from the melting of subducted pelagic sediments. We have thus defined the East-Pacific MORB, enriched (cherts, pelagic sediments) and HIMU components in an attempt to constrain and model the genesis of the studied island-arc magmatism, using a compilation of carefully selected isotopic data from literature according to rock age and paleogeographic location at the time of arc edification. Tripolar mixing models reveal that proportions of 12–15 wt.% of the HIMU component, 7–15 wt.% of the pelagic sediment end-member and 70–75 wt.% of an East-pacific MORB end-member are needed to explain the measured isotope ratios. These surprisingly high proportions of the HIMU/CCOP component could be explained by the young age of the oceanic plateau (5–15 Ma) during the Late Cretaceous arc emplacement. The CCOP, basement of these arc sequences, was probably still hot and easily assimilated at the island-arc lava source.
ISSN: 0895-9811

insu-00353692
https://hal-insu.archives-ouvertes.fr/insu-00353692
DOI : 10.1016/j.jsames.2008.06.003

Éditeur(s) : HAL CCSD Annual Reviews
Résumé : International audience
The record of the continental lithosphere is patchy and incomplete; no known rock is older than 4.02 Ga, and less than 5% of the rocks preserved are older than 3 Ga. In addition, there is no recognizable mantle lithosphere from before 3 Ga. We infer that there was lithosphere before 3 Ga and that ∼3 Ga marks the stabilization of blocks of continental lithosphere that have since survived. This was linked to plate tectonics emerging as the dominant tectonic regime in response to thermal cooling, the development of a more rigid lithosphere, and the recycling of water, which may in turn have facilitated plate tectonics. A number of models, using different approaches, suggest that at 3 Ga the volume of continental crust was ∼70% of its present-day volume and that this may be a minimum value. The continental crust before 3 Ga was on average more mafic than that generated subsequently, and this pre-3 Ga mafic new crust had fractionated Lu/Hf and Sm/Nd ratios as inferred for the sources of tonalite-trondhjemite-granodiorite and later granites. The more intermediate composition of new crust generated since 3 Ga is indicated by its higher Rb/Sr ratios. This change in composition was associated with an increase in crustal thickness, which resulted in more emergent crust available for weathering and erosion. This in turn led to an increase in the Sr isotope ratios of seawater and in the drawdown of CO2. Since 3 Ga, the preserved record of the continental crust is marked by global cycles of peaks and troughs of U-Pb crystallization ages, with the peaks of ages appearing to match periods of supercontinent assembly. There is increasing evidence that the peaks of ages represent enhanced preservation of magmatic rocks in periods leading up to and including continental collision in the assembly of supercontinents. These are times of increased crustal growth because more of the crust that is generated is retained within the crust. The rates of generation of continental crust and mantle lithosphere may have remained relatively constant at least since 3 Ga, yet the rates of destruction of continental crust have changed with time. Only relatively small volumes of rock are preserved from before 3 Ga, and so it remains difficult to establish which of these are representative of global processes and the extent to which the rock record before 3 Ga is distorted by particular biases.
ISSN: 0084-6597

hal-01622692
https://hal.archives-ouvertes.fr/hal-01622692
DOI : 10.1146/annurev-earth-063016-020525

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Kimberlites of the Udachnaya-East pipe (Siberia) include a uniquely dry and serpentine-free rock type with anomalously high contents of chlorine (Cl ≤ 6.1 wt%), alkalies (Na2O + K2O ≤ 10 wt%) and sulfur (S ≤ 0.50 wt%), referred to as a “salty” kimberlite. The straightforward interpretation is that the Na-, K-, Cl- and S-rich components originate directly from a carbonate-chloride kimberlitic magma that is anhydrous and alkali-rich. However, because brines and evaporites are present on the Siberian craton, previous studies proposed that the kimberlitic magma was contaminated by the assimilation of salt-rich crustal rocks. To clarify the origin of high Cl, alkalies and S in this unusual kimberlite, here we determine its sulfur speciation and isotopic composition and compare it to that of non-salty kimberlites and kimberlitic breccia from the same pipe, as well as potential contamination sources (hydrothermal sulfides and sulfates, country-rock sediment and brine collected in the area). The average δ34S of sulfides is − 1.4 ± 2.2‰ in the salty kimberlite, 2.1 ± 2.7‰ in the non-salty kimberlites and 14.2 ± 5.8‰ in the breccia. The average δ34S of sulfates in the salty kimberlites is 11.1 ± 1.8‰ and 27.3 ± 1.6‰ in the breccia. In contrast, the δ34S of potential contaminants range from 20 to 42‰ for hydrothermal sulfides, from 16 to 34‰ for hydrothermal sulfates, 34‰ for a country-rock sediment (Chukuck suite) and the regional brine aquifer. Our isotope analyses show that (1) in the salty kimberlites, neither sulfates nor sulfides can be simply explained by brine infiltration, hydrothermal alteration or the assimilation of known salt-rich country rocks and instead, we propose that they are late magmatic phases; (2) in the non-salty kimberlite and breccia, brine infiltration lead to sulfate reduction and the formation of secondary sulfides – this explains the removal of salts, alkali-carbonates and sulfates, as well as the minor olivine serpentinization; (3) hydrothermal sulfur was added to the kimberlitic breccia, but not to the massive kimberlites. In situ measurements of sulfides confirm this scenario, clearly showing the addition of two sulfide populations in the breccia (pyrite-pyrrhotites with average δ34S of 7.9 ± 3.4‰ and chalcopyrites with average δ34S of 38.0 ± 0.4‰) whereas the salty and non-salty kimberlites preserve a unique population of djerfisherites (Cl- and K-rich sulfides) with δ34S values within the mantle range. This study provides the first direct evidence of alkaline igneous rocks in which magmatic sulfate is more abundant than sulfide. Although sulfates have been rarely reported in mantle materials, sulfate-rich melts may be more common in the mantle than previously thought and could balance the sulfur isotope budget of Earth's mantle.
ISSN: 0009-2541

hal-01571982
https://hal.archives-ouvertes.fr/hal-01571982
DOI : 10.1016/j.chemgeo.2016.10.037

Éditeur(s) : HAL CCSD Geological Society of America
Résumé : If the net flux to the island arc crust is primitive arc basalt, the evolved composition of most arc magmas entails the formation of complementary thick ultramafic keels at the root of the island arc crust. Dunite, wehrlite, and Cr-rich pyroxenite from the Jijal complex, constituting the Moho transition zone of the Kohistan paleo–island arc (northern Pakistan), are often mentioned as an example of high-pressure cumulates formed by intracrustal fractionation of mantle-derived melts, which were later extracted to form the overlying mafic crust. Here we show that calculated liquids for Jijal pyroxenites-wehrlites are strongly rare earth element (REE) depleted and display flat or convex-upward REE patterns. These patterns are typical of boninites and are therefore unlike those of the overlying mafic crust that have higher REE concentrations and are derived from light rare earth element (LREE)–enriched melts similar to island arc basalt. This observation, along with the lower 208Pb/204Pb and 206Pb/204Pb ratios of Jijal pyroxenites-wehrlites relative to gabbros, rejects the hypothesis that gabbros and ultramafic rocks derive from a common melt via crystal fractionation. In the 208Pb/204Pb versus 206Pb/204Pb diagram, ultramafic rocks and gabbros lie on the same positive correlation, suggesting that their sources share a common enriched mantle 2 (EM2) signature but with a major depleted component contribution for the ultramafic rocks. These data are consistent with a scenario whereby the Jijal ultramafic section represents a Moho transition zone formed via melt-rock reaction between subarc mantle and incoming melt isotopically akin to Jijal gabbroic rocks. The lack in the Kohistan arc of cogenetic ultramafic cumulates complementary to the evolved mafic plutonic rocks implies either (1) that a substantial volume of such ultramafic cumulates was delaminated or torn out by subcrustal mantle flow from the base of the arc crust in extraordinarily short time scales (0.10–0.35 cm/yr), or (2) that the net flux to the Kohistan arc crust was more evolved than primitive arc basalt.
ISSN: 0091-7613

hal-00406688
https://hal.archives-ouvertes.fr/hal-00406688
DOI : 10.1130/G23675A.1

Éditeur(s) : HAL CCSD National Academy of Sciences
Résumé : International audience
The Isua Supracrustal Belt, Greenland, of Early Archean age (3.81-3.70 Ga) represents the oldest crustal segment on Earth. Its complex lithology comprises an ophiolite-like unit and volcanic rocks reminiscent of boninites, which tie Isua supracrustals to an island arc environment. We here present zinc (Zn) isotope compositions measured on serpentinites and other rocks from the Isua supracrustal sequence and on serpentinites from modern ophiolites, midocean ridges, and the Mariana forearc. In stark contrast to modern midocean ridge and ophiolite serpentinites, Zn in Isua and Mariana serpentinites is markedly depleted in heavy isotopes with respect to the igneous average. Based on recent results of Zn isotope fractionation between coexisting species in solution, the Isua serpentinites were permeated by carbonate- rich, high- pH hydrothermal solutions at medium temperature (100-300 degrees C). Zinc isotopes therefore stand out as a pH meter for fossil hydrothermal solutions. The geochemical features of the Isua fluids resemble the interstitial fluids sampled in the mud volcano serpentinites of the Mariana forearc. The reduced character and the high pH inferred for these fluids make Archean serpentine mud volcanoes a particularly favorable setting for the early stabilization of amino acids.
ISSN: 0027-8424

insu-00674714
https://hal-insu.archives-ouvertes.fr/insu-00674714
DOI : 10.1073/pnas.1108061108

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Chromitite pods in the Mayarí-Cristal ophiolitic massif (eastern Cuba) were formed in the Late Cretaceous when island arc tholeiites and MORB-like back-arc basin basalts reacted with residual mantle peridotites and generated chromite-rich bodies enclosed in dunite envelopes. Platinum-group minerals (PGM) in the podiform chromitites exhibit important Os-isotope heterogeneities at the kilometric, hand sample and thin section scales. 187Os/188Os calculated at the time of chromitite crystallization (~90 Ma) ranges between 0.1185 and 0.1295 (γOs = −7.1 to +1.6, relative to enstatite chondrite), and all but one PGM have subchondritic 187Os/188Os. Grains in a single hand sample have initial 187Os/188Os that spans from 0.1185 to 0.1274, and in one thin section it varies between 0.1185 and 0.1232 in two PGM included in chromite which are only several millimeters apart. As the Os budget of a single micrometric grain derives from a mantle region that was at least several m3 in size, the variable Os isotopic composition of PGM in the Mayarí-Cristal chromitites probably reflects the heterogeneity of their mantle sources on the 10-100 m scale. Our results show that this heterogeneity was not erased by pooling and mingling of individual melt batches during chromitite crystallization but was transferred to the ore deposits on mineral scale. The distribution of the Os model ages calculated for PGM shows four main peaks, at ~100, 500, 750 and 1,000 Ma. These variable Os model ages reflect the presence of different depleted domains in the oceanic (Pacific-related) upper mantle of the Greater Antilles paleo-subduction zone. The concordance between the age of crystallization of the Mayarí-Cristal chromitites and the most recent peak of the Os model age distribution in PGM supports that Os in several grains was derived from fertile domains of the upper mantle, whose bulk Os isotopic composition is best approximated by that of enstatite chondrites; on the other hand, most PGM are crystallized by melts that tapped highly refractory mantle sources.
ISSN: 0010-7999

hal-00822742
https://hal.archives-ouvertes.fr/hal-00822742
DOI : 10.1007/s00410-010-0575-2

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Tso Morari serpentinites in the Ladakh area, northwest Himalaya, originated from the forearc mantle overlying the northward subducting Neo-Tethys lithosphere and the margin of the Indian continent. The serpentinites are characterized by high concentration of fluid-mobile elements (FME: As, Sb, B, Li, and U) compared to ophiolitic or abyssal serpentinites. The Pb isotopic compositions of serpentinites show influence of the subducted Indian continental lithosphere. Trace element concentrations of antigorite determined in situ with Laser Ablation High Resolution Inductively Coupled Mass Spectrometer (LA-HR-ICP-MS) show high contents of FME including Pb, in contrast to the spatially associated iron oxides. Rare earth elements (REE) and compatible elements, such as Sc and Co, remained immobile during the hydration, allowing the identification of the primary minerals (olivine or orthopyroxene) from which serpentine formed. Serpentinized olivine displays higher Sb and As concentrations (up to 1000 x PM) than serpentinized orthopyroxenes that are enriched in Pb, Cs and Li (2 to up to 10 x PM).;We propose that the observed FME distribution in two types of serpentine reflect the differential incorporation of FME during the downward movement of the serpentinite along the subduction plane. At temperature lower than 400 degrees C, at shallow depths, olivine is preferentially serpentinized and incorporates elements that are fluid soluble at low temperatures, such as Sb and As. Above 400 degrees C, orthopyroxene is hydrated and incorporates Pb, Cs, Li and possibly Ba. Boron and U are incorporated in both types of serpentine suggesting that they are released from slabs at temperatures around 300-400 degrees C. The serpentine acts as a sink for water, but also for FME and transports them to deeper and hotter levels in the mantle, down to the isotherm 600-650 degrees C where dehydration occurs.
ISSN: 0009-2541

hal-00475565
https://hal.archives-ouvertes.fr/hal-00475565
DOI : 10.1016/j.chemgeo.2009.10.002

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Roussillon sedimentary Basin (South France) is a complex multi-layered aquifer, close to the Mediterranean Sea facing seasonally increases of water abstraction and salinization issues. We report geochemical and isotopic vertical variability in this aquifer using groundwater sampled with a Westbay System® at two coastal monitoring sites: Barcarès and Canet. The Westbay sampling allows pointing out and explaining the variation of water quality along vertical profiles, both in productive layers and in the less permeable ones where most of the chemical processes are susceptible to take place.The aquifer layers are not equally impacted by salinization, with electrical conductivity ranging from 460 to 43,000 μS·cm− 1. The δ2H–δ18O signatures show mixing between seawater and freshwater components with long water residence time as evidenced by the lack of contribution from modern water using 3H, 14C and CFCs/SF6. S(SO4) isotopes also evidence seawater contribution but some signatures can be related to oxidation of pyrite and/or organically bounded S. In the upper layers 87Sr/86Sr ratios are close to that of seawater and then increase with depth, reflecting water–rock interaction with argillaceous formations while punctual low values reflect interaction with carbonate. Boron isotopes highlight secondary processes such as adsorption/desorption onto clays in addition to mixings. At the Barcarès site (120 m deep), the high salinity in some layers appear to be related neither to present day seawater intrusion, nor to Salses-Leucate lagoonwater intrusion. Groundwater chemical composition thus highlights binary mixing between fresh groundwater and inherited salty water together with cation exchange processes, water–rock interactions and, locally, sedimentary organic matter mineralisation probably enhanced by pyrite oxidation.Finally, combining the results of this study and those of Caballero and Ladouche (2015), we discuss the possible future evolution of this aquifer system under global change, as well as the potential management strategies needed to preserve quantitatively and qualitatively this water resource.
ISSN: 0048-9697

hal-01685219
https://hal-brgm.archives-ouvertes.fr/hal-01685219
DOI : 10.1016/j.scitotenv.2016.05.016

Éditeur(s) : HAL CCSD Elsevier
Résumé : The ultramafic–mafic complexes located at the base of obducted paleo-island arcs are commonly interpreted as evidence for intra-crustal fractionation of primitive (high-Mg#) mantle melts. The present study, realized on the Jijal mafic–ultramafic basal section of the Cretaceous Kohistan oceanic arc (N Pakistan), discards a crystal fractionation model from a single parental magma to take into account the geochemical and isotope variations observed between the plutonic crust and the underlying ultramafic section. The basal ultramafic rocks, i.e. dunites, wehrlites and clinopyroxenites, show high Mg#, extremely depleted REE patterns, no marked HFSE anomalies and high LILE enrichment. Cpx and leached whole rocks from this unit yield restricted εNd values (+ 5.5 to + 6.8), a large εSr range (− 5.9 to + 8.8) and mainly radiogenic 207Pb/204Pb ratios. The overlying mafic section, i.e. gabbroic rocks, displays low Mg# (46 < Mg# < 54), enriched REE patterns and marked HFSE anomalies. These plutonic rocks have also very homogeneous Sr and Pb isotopic compositions defining a restricted domain, which does not overlap that of the ultramafic samples. Moreover, Cpx from the basal ultramafics yield an Sm–Nd isochron at 117 ± 7 Ma interpreted as dating crystallization of the ultramafic section. Together, these results indicate that the mafic rocks on one hand and the ultramafic rocks on the other hand, originate from separate sources with specific emplacement underneath the subduction zone. A lherzolite lens, collected from the top of the mafic section, plots on the early Cretaceous Sm–Nd isochron and therefore yields the same 143Nd/144Nd initial ratio. This suggests that the ultramafic part of the lower Kohistan arc complex sampled the initiation stages of the subduction with production of magmas with boninite-like features. At this stage, the mantle wedge has been already modified by percolation of the first slab-derived fluids. On such a scenario the lherzolite lens would correspond to the lithospheric mantle underneath the arc before its initiation. A multi-stage history is proposed for the evolution for the Kohistan arc through not, vert, similar 30 Ma. Stage #1 reflects the spontaneous initiation of subduction associated with extensive boninitic affinity magmatism, stages #2 and #3 are related to arc building by tholeiitic melts and, a fourth step (stage #4) corresponds to the intra-crustal differentiation.
ISSN: 0012-821X

hal-00407206
https://hal.archives-ouvertes.fr/hal-00407206
DOI : 10.1016/j.epsl.2007.06.026

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In orogenic systems, thrust faults play a major role in stacking different tectonic units and may act as conduits for the expulsion of large amounts of fluid of different origins (metamorphic, diagenetic, meteoric). This study focuses on the Monte Perdido thrust unit emplaced in the Paleogene Jaca thrust-sheet-top basin, in the SW-central Pyrenees. We aim to decipher the mechanisms and P-T conditions of deformation in fault zones and characterize the related fluid involvement, through combined microstructural, geochemical and microthermometry analyses. Two thrust faults cutting platform limestones, marls and siliciclastic turbidites of the lower part of the basin-fill (Paleocene-lower Eocene) have been studied. The fault zones are characterized by metre-thick shear zones with highly deformed, foliated clay-rich sediments. Foliation is underlined by preferentially oriented phyllosilicates. Several generations of shear and extension calcite, quartz and chlorite-bearing veins attest to fluid-rock interactions during a multi-stage deformation. Microstructural observations and stable isotope analyses on calcite from veins and host sediments suggest that deformation was aseismic and dominated by diffusive mass transfer from pressure solution sites along cleavage and stylolites to the precipitation sites in veins, with mineralizing fluids in equilibrium with the host sediments. Our results suggest an essentially closed hydrologic system, and imply the absence of significant fluid flow along the studied fault zones. Microthermometric study on fluid inclusions present in calcite and quartz veins, and calcite-quartz oxygen isotopic fractionation determined for the first generation shear veins, allow a geothermal gradient of 34 degrees C/km to be estimated. Analytical results demonstrate an evolution of the fault zones in three stages. The first stage was related to the emplacement of the Monte Perdido thrust unit during the middle Eocene at a temperature of similar to 208 degrees C and a burial depth of similar to 5.7 km. The second stage corresponds to a fault reactivation at a temperature of similar to 240 degrees C and a burial depth of similar to 6.5 km. The latter deformation may have been related to folding of the Monte Perdido thrust unit during the emplacement of the underlying Gavarnie thrust unit during the late Eocene-early Oligocene, with deeper burial resulting from aggradation of the thrust-sheet-top basin-fill. The last event corresponds to the formation of a dilatant vein system likely related to the exhumation of the massif.
ISSN: 0191-8141

hal-00644828
https://hal.archives-ouvertes.fr/hal-00644828
DOI : 10.1016/j.jsg.2011.06.006

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The N-S Languedoc volcanic province between the French Massif Central and the Mediterranean Sea is characterized by sporadic scattered low volume (similar to 2 km(3)) and geochemically homogeneous alkali basaltic activity spanning from 161 to 05 Ma The existence of magmatic activity of such a long duration within such a small area (similar to 140 km long and similar to 60 km wide) in spite of the extensive shift to the East of the European plate (about 2500 km during the last 160 Ma) is problematic Trace-element abundances in lavas suggest low degrees of melting (1-5%) in the spinet-garnet transition zone of an enriched Iherzolitic source The lavas display rather large ranges in Sr isotopic ratios (0 70307-0 70436) The Nd-143/Nd-144 ratio variations are smaller (0 51268-0 51300) and these of Pb-206/Pb-204 Pb-208/Pb-204 and Pb-207/Pb-204 are 18 745-19 515 38 53239 228 and 15 567-15 680 respectively The Languedoc lithospheric mantle as sampled by xenoliths is globally similar to the Pyrenees lithosphere The xenoliths show also rather large Sr Nd and Pb isotopic variations (Sr-87/Sr-86 0 70287-0 70578 Nd-143/Nd-144 0 51256-0 51414 Pb-208/Pb-204 37772-39 041 Pb-206/Pb-204 17 901-19 353) except for Pb-207/Pb-204 (15 570-15 620) The Pb-206/Pb-204 and La/Sm ratios are positively correlated both in xenoliths and lavas The increase of the Pb-206/Pb-204 (which could be interpreted as participation of the European Asthenospheric Reservoir EAR) is probably related to volatile-rich (carbonated?) fluid percolation This is corroborated by LILE and HFSE patterns observed in several xenoliths Therefore our data on lavas and xenoliths suggest a lithospheric origin for this long lived magmatism We propose (1) that the role of the asthenosphere in the Languedoc volcanism was restricted to volatile-rich fluid supplying and (2) that the fluid injection within the lithosphere may be related to the arrival of the Central Atlantic Plume head beneath Western Europe about 70 Ma ago In this model the isotopic signature of the oldest lavas (>70 Ma) would be that of the mantle lithosphere inherited from Hercynian processes The signatures of the subsequent lavas would be driven by the metasomatic component stored within the lithosphere and preferentially mobilized during incipient melting This metasomatised lower lithosphere was close to its solidus and small changes in P (or T) triggered incipient melting leading to basaltic volcanism Successive local re adjustments of the lithospheric blocks which accompanied the Meso-Cenozoic evolution of the Thetys Ligurian margin towards the present Mediterranean margin are the probable cause of these changes and so the sporadic volcanic activity in Languedoc is unrelated to deep asthenospheric processes
ISSN: 0024-4937

hal-00558884
https://hal.archives-ouvertes.fr/hal-00558884
DOI : 10.1016/j.lithos.2010.04.009

Éditeur(s) : HAL CCSD Elsevier
Résumé : Kyanite and zoisite eclogites from the Limousin area in the Variscan French Massif Central (FMC) have been formed by deep subduction to 100 km depth at peak pressure-temperature conditions around 660 degrees C and 2.9 GPa. They belong to a thin tectonic unit comprising ophiolitic bodies devoid of HP-UHP markers and forming a transported suture zone now separating the two main lithotectonic units of the FMC with contrasted tectono-metamorphic features (namely the lower and upper allochthons). Composition of kyanite-eclogites is similar to supra-subduction-zone basalts found in back-arc basins or island arcs (LREE depleted, negative Nb-Ta anomalies, epsilon Nd: +6 to + 9). Zoisite eclogites have the major-element fingerprint of ultrabasic iron-rich plagioclase cumulates common in continental intrusions or forming Fe-Ti oceanic gabbros. The distribution of immobile trace-elements is however more comparable to alkaline Si-undersaturated rocks, but the Sr-Nd isotopic composition (Sr-87/Sr-86: 0.706; epsilon Nd: -3 to -5) suggests the intervention of an old radiogenic crustal component during their petrogenesis. The chemical composition of hydrous zoisite eclogites does not correspond closely to a common magmatic precursor. It is thus suggested that, by analogy with well exposed worldwide HP-UHP terranes, at least a part of the trace-element signature and the isotopic compositions are controlled by channelled fluid-rock interaction at UHP conditions, leading to localised segregation of zoisite that hosts nearly all Sr and LREE budgets of the bulk eclogite. Sr-Nd isotopic composition of these samples is thus controlled by zoisite and probably represents the isotopic composition of the fluid phase present during UHP recrystallisation. Zircons extracted from a zoisite eclogite were dated in-situ by LA-ICP-MS. Crystallisation of the magmatic precursor is bracketed between 489 and 475 Ma and the UHP event is dated at 412 +/- 10 Ma, in agreement with published ages for the Eo-Variscan HP stage in the FMC. A late resetting of the U-Pb isotopic system at 382 +/- 7 Ma is related to an anatectic high pressure-medium pressure event, well known in the Limousin area. The oceanic eclogites dated in this study are tentatively linked to the Galicia-South Brittany Lower Paleozoic Ocean; they show evidence of the existence of an intra-oceanic subduction zone at the Lower Ordovician and of partial closure of this oceanic domain by ocean-continent subduction during Early Devonian.
ISSN: 0024-4937

hal-00512835
https://hal.archives-ouvertes.fr/hal-00512835
DOI : 10.1016/j.lithos.2010.05.013