Éditeur(s) : HAL CCSD Springer Verlag
Résumé : The activity of the Damavand volcano (Central Alborz, northern Iran) began 1.8 Ma ago and continued up to 7 ka BP. Although the volcanic suite is clearly of shoshonitic affinity, only two petrographic types can be distinguished in the studied lavas: (1) weakly differentiated absarokites (49 < %SiO2 < 51), scattered around the volcano but with a regional extension, (2) highly differentiated banakites (59 < %SiO2 < 63), which form the bulk of the 4,000 m thick volcanic pile. All lavas are alkalic (3.7 < %K2O < 5), REE and LILE-rich (e.g., 85 < La < 148 ppm; 9 < Th < 32 ppm) and show highly fractionated REE patterns (69 < La/Yb < 115) and pronounced Nb-Ta negative anomalies. The absarokites are characterised by Sr (0.7045-0.7046) and Nd (0.51266-0.51269) isotope compositions close to the Bulk Earth values, and distinct from those of the banakites (0.7047 < Sr-87/Sr-86 < 0.7049, 0.51258 < Nd-143/Nd-144 < 0.51262). The Pb isotope ratios are also slightly lower in the absarokites than in the banakites (18.71 < Pb-206/Pb-204 < 18.77, 15.62 < Pb-207/Pb-204 < 15.63, 38.85 < Pb-208/Pb-204 < 38.91, and 18.77 < Pb-206/Pb-204 < 18.84, 15.62 < Pb-207/Pb-204 < 15.64, 38.94 < Pb-208/Pb-204 < 39.06, respectively). Overall, there is a clear tendency towards higher Sr, Pb and lower Nd isotope ratios with increasing degree of differentiation. This study suggests that the absarokites result from a low degree of partial melting (similar to 5%) of a highly metasomatized mantle source, which inherited its characteristics from an old subduction setting. The initiation of volcanic activity 1.8 Ma ago results from variations in the lithospheric thermal regime, probably related to lithospheric delamination as proposed for Anatolia (Pearce et al. 1990). The banakites are mainly generated by extensive fractional crystallisation (similar to 70%) of the absarokitic magma, with a limited amount (a few percents) of assimilation of an old crustal component, in the form of bulk assimilation or AFC processes, which both can explain the Sr, Nd and Pb isotope data.
ISSN: 1437-3254

hal-00411553
https://hal.archives-ouvertes.fr/hal-00411553
DOI : 10.1007/s00531-006-0159-6

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The In-Ezzane volcanic district (EZD), located at the triple junction of Algeria, Niger and Libya belongs to the Eastern Hoggar, covers 350 km2 and includes 9 volcanic edifices that are probably aligned along NW-SE faults, parallel to the Tafassasset valley. The low volume (0.7 and 1 km3) of emitted lavas, the similar morphology of the monogenic cones and the lack of differentiated rocks indicate that the volcanic activity of the EZD was restricted in time and volume. The new K-Ar age (i.e. 2.86 ± 0.07 Ma) indicates that the EZD is contemporaneous with the last alkali volcanism paroxysm in Hoggar and with the nearby Libyan volcanics. The EZD alkali basalts (mainly basanite) show a remarkable homogeneous compositions both in major elements (44.8 ⩽ SiO2 ⩽ 45.8 wt.%; 5.2 ⩽ (Na2O + K2O) ⩽ 6.2 wt.%), trace elements (4.3 ⩽ Th ⩽ 5.5 ppm; 34.7 ⩽ La ⩽ 44.7 ppm; 16.1 < La/Yb(N) < 21.6) and radiogenic isotopes (0.70285 < 87Sr/86Sr < 0.70303; 0.51298 < 143Nd/144Nd < 0.51301; (19.212 < 206Pb/204Pb < 19.340, 15.589 < 207Pb/204Pb < 15.602 and 38.834 < 208Pb/204Pb < 38.903). Relative to the Hoggar alkaly basalts the EZD basalts appear systematically impoverished in incompatible elements and show a depleted signature both in Sr and Nd isotopes (almost MORB-like). The Pb isotopes are relatively enriched and intermediate between tholeiites and alkali basalts of the Hoggar. This unusual geochemical signature, is uneasy to reconcile with the known characteristics of the Hoggar swell, and would rather fingerprint a circum cratonic mantle lying beneath the west border of the Murzuq craton (Libya).
ISSN: 0899-5362

hal-01054340
https://hal.archives-ouvertes.fr/hal-01054340
DOI : 10.1016/j.jafrearsci.2013.12.001

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Sedimentary rocks and detrital minerals sample large areas of the continental crust, and they are increasingly seen as a reliable archive for its global evolution. This study presents two approaches to model the growth of the continental crust through the sedimentary archive. The first builds on the variations in U-Pb, Hf and O isotopes in global databases of detrital zircons. We show that uncertainty in the Hf isotope composition of the mantle reservoir from which new crust separated, in the 176Lu/177Hf ratio of that new crust, and in the contribution in the databases of zircons that experienced ancient Pb loss(es), adds some uncertainty to the individual Hf model ages, but not to the overall shape of the calculated continental growth curves. The second approach is based on the variation of Nd isotopes in 645 worldwide fine-grained continental sedimentary rocks with different deposition ages, which requires a correction of the bias induced by preferential erosion of younger rocks through an erosion parameter referred to as K. This dimensionless parameter relates the proportions of younger to older source rocks in the sediment, to the proportions of younger to older source rocks present in the crust from which the sediment was derived. We suggest that a Hadean/Archaean value of K = 1 (i.e., no preferential erosion), and that post-Archaean values of K = 4–6, may be reasonable for the global Earth system. Models built on the detrital zircon and the fine-grained sediment records independently suggest that at least 65% of the present volume of continental crust was established by 3 Ga. The continental crust has been generated continuously, but with a marked decrease in the growth rate at ~ 3 Ga. The period from > 4 Ga to ~ 3 Ga is characterised by relatively high net rates of continental growth (2.9–3.4 km3 yr− 1 on average), which are similar to the rates at which new crust is generated (and destroyed) at the present time. Net growth rates are much lower since 3 Ga (0.6–0.9 km3 yr− 1 on average), which can be attributed to higher rates of destruction of continental crust. The change in slope in the continental growth curve at ~ 3 Ga is taken to indicate a global change in the way bulk crust was generated and preserved, and this change has been linked to the onset of subduction-driven plate tectonics. At least 100% of the present volume of the continental crust has been destroyed and recycled back into the mantle since ~ 3 Ga, and this time marks a transition in the average composition of new continental crust. Continental crust generated before 3 Ga was on average mafic, dense, relatively thin (< 20 km) and therefore different from the calc-alkaline andesitic crust that dominates the continental record today. Continental crust that formed after 3 Ga gradually became more intermediate in composition, buoyant and thicker. The increase in crustal thickness is accompanied by increasing rates of crustal reworking and increasing input of sediment to the ocean. These changes may have been accommodated by a change in lithospheric strength at around 3 Ga, as it became strong enough to support high-relief crust. This time period therefore indicates when significant volumes of continental crust started to become emergent and were available for erosion and weathering, thus impacting on the composition of the atmosphere and the oceans.
ISSN: 0037-0738

hal-01622696
https://hal.archives-ouvertes.fr/hal-01622696
DOI : 10.1016/j.sedgeo.2017.06.001

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
We show here that the Amalaoulaou complex, in the Pan-African belt of West Africa (Gourma, Mali), corresponds to the lower and middle sections of a Neoproterozoic intra-oceanic arc. This complex records a 90-130-Ma-long evolution of magmatic inputs and differentiation above a subducting oceanic slab. Early c. 793 Ma-old metagabbros crystallised at lower crustal or uppermost mantle depths (25-30 km) and have geochemical characteristic of high-alumina basalts extracted from a depleted mantle source slightly enriched by slab-derived sedimentary components ((La/Sm)(N) < 1; epsilon(Nd): +5.4-6.2; Sr-87/Sr-86: 0.7027-0.7029). In response to crustal thickening, these mafic rocks were recrystallised into garnet-granulites (850-1,000A degrees C; 10-12 kbar) and subject to local dehydration-melting reactions, forming trondhjemititic leucosomes with garnet-clinopyroxene-rutile residues. Slightly after the granulitic event, the arc root was subject to strong HT shearing during partial exhumation (detachment faults/rifting or thrusting), coeval with the emplacement of spinel- and garnet-pyroxenite dykes crystallised from a high-Mg andesitic parental magma. Quartz and hornblende-gabbros (700-660 Ma) with composition typical of hydrous volcanic rocks from mature arcs ((La/Sm)(N): 0.9-1.8; epsilon(Nd): +4.6 to +5.2; Sr-87/Sr-86: 0.7028-0.7031) were subsequently emplaced at mid-arc crust levels (similar to 15 km). Trace element and isotopic data indicate that magmas tapped a depleted mantle source significantly more enriched in oceanic sedimentary components (0.2%). Exhumation occurred either in two stages (700-660 and 623 Ma) or in one stage (623 Ma) with a final exhumation of the arc root along cold P-T path (550A degrees C, 6-9 kbar; epidote-amphibolite and greenschist facies conditions) during the main Pan-African collision event (620-580 Ma). The composition of magmas forming the Cryogenian Amalaoulaou arc and the processes leading to intra-arc differentiation are strikingly comparable to those observed in the deep section of exposed Mezosoic oceanic arcs, namely the Kohistan and Talkeetna complex. This evolution of the Amalaoulaou oceanic arc and its accretion towards the West African craton belong to the life and closure of the Pharusian Ocean that eventually led to the formation of the Greater Gondwana supercontinent, a similar story having occurred on the other side of the Sahara with the Mozambique Ocean.
ISSN: 0010-7999

hal-00639447
https://hal.archives-ouvertes.fr/hal-00639447
DOI : 10.1007/s00410-011-0624-5

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The identification of a large OIB-type volcanic sequence on top of an obducted nappe in the Lesser Caucaus of Armenia helps us explain the obduction processes in the Caucasus region that are related to dramatic change in the global tectonics of the Tethyan region in the late Lower Cretaceous. The ophiolitic nappe preserves three distinct magmatic series, obducted in a single tectonic slice over the South Armenian Block during the Coniacian-Santonian (88-83 Ma), the same time as the Oman ophiolite. Similar geological, petrological, geochemical and age features for various Armenian ophiolitic massifs (Sevan, Stepanavan, and Vedi) argue for the presence of a single large obducted ophiolite unit. The ophiolite, shows evidence for a slow-spreading oceanic environment in Lower to Middle Jurassic. Serpentinites, gabbros and plagiogranites were exhumed by normal faults, and covered by radiolarites. Few pillow-lava flows have infilled the rift grabens. The ophiolite lavas have hybrid geochemical composition intermediate between Arc and MORB signatures: (La/Yb)(N) = 0.6-0.9: (Nb/Th)(N) = 0.17-0.57; (Nd-143/Nd-144)(i) = 0.51273-0.51291; (Sr-87/Sr-86)(i) = 0.70370-0.70565; (Pb-207/Pb-204)(i) = 15.4587-15.5411; (Pb-208/Pb-204)(i) = 37.4053-38.2336; (Pb-206/Pb-204)(i) = 17.9195-18.4594. These compositions suggest they were probably formed in a back-arc basin by melting of a shallow asthenosphere source contaminated by a deeper mantle source modified by subducted slab-derived products. Sr-87/Sr-86 ratios and petrological evidence show that these lavas have been intensely altered by mid-oceanic hydrothermalism as well as by serpentinites, which are interpreted as exhumed mantle peridotites. The gabbros have almost the same geochemical composition as related pillow-lavas: (La/Yb)(N) = 0.2-2.3; (Nb/Th)(N) = 0.1-2.8; (Nd-143/Nd-144)(i) = 0.51264-0.51276: (Sr-87/Sr-86)(i) = 0.70386-0.70557; (Pb-207/Pb-204)(i) = 15.4888-15.5391; (Pb-208/Pb-204)(i) = 37.2729-37.8713; (Pb-206/Pb-204)(i) = 17.6296-17.9683. Plagiogranites show major and trace element features similar to other Neo-Tethyan plagiogranites (La/Yb)(N) = 1.10-7.92; (Nb/Th)(N) = 0.10-0.94; but display a less radiogenic Nd isotopic composition than basalts [(Nd-143/Nd-144)(i) = 0.51263] and more radiogenic (Sr-87/Sr-86)(i) ratios. This oceanic crust sequence is covered by variable thicknesses of unaltered pillowed OIB alkaline lavas emplaced in marine conditions. Ar-40/Ar-39 dating of a single-grain amphibole phenocryst provides a Lower Cretaceous age of 117.3 +/- 0.9 Ma, which confirms a distinct formation age of the OIB lavas. The geochemical composition of these alkaline lavas is similar to plateau-lavas [(La/Yb)(N) = 6-14; (Nb/Th)(N) = 0.23-0.76; (Nd-143/Nd-144)(i) = 0.51262-0.51271; (Sr-87/Sr-86)(i) = 0.70338-0.70551; (Pb-207/Pb-204)(i) = 15.5439-15.6158; (Pb-208/Pb-204)(i) = 38.3724-39.3623; (Pb-206/Pb-204)(i) = 18.4024-19.6744].;They have significantly more radiogenic lead isotopic compositions than phiolitic rocks, and fit the geochemical compositions of hot-spot derived lavas mixed with various proportions of oceanic mantle. In addition, this oceanic + plateau sequence is covered by Upper Cretaceous calc-alkaline lavas: (La/Yb)(N) = 2.07-2.31; (Nb/Th)(N) = 0.08-0.15; (Nd-144/Nd-143)(i) = 0.51271-0.51282; (Sr-117/Sr-116)(i) = 0.70452-0.70478), which were likely formed in a supra-subduction zone environment. During the late Lower to early Upper Cretaceous period, hot-spot related magmatism related to plateau events may have led to significant crustal thickening in various zones of the Middle-eastern Neotethys. These processes have likely hindered subduction of some of the hot and thickened oceanic crust segments, and allowed them to be obducted over small continental blocks such as the South Armenian Block.
ISSN: 0024-4937

hal-00443855
https://hal.archives-ouvertes.fr/hal-00443855
DOI : 10.1016/j.lithos.2009.02.006

Éditeur(s) : Elsevier Science Bv
Résumé : For many trace elements, continental margins are the location of intense exchange processes between sediment and seawater, which control their distribution in the water column, but have yet to be fully understood. In this study, we have investigated the impact of fluid seepage at cold seeps on the marine cycle of neodymium. We determined dissolved and total dissolvable (TD) concentrations for REE and well-established tracers of fluid seepage (CH4, TDFe, TDMn), and Nd isotopic compositions in seawater samples collected above cold seeps and a reference site (i.e. away from any fluid venting area) from the Niger Delta margin. We also analyzed cold seep authigenic phases and various core-top sediment fractions (pore water, detrital component, easily leachable phases, uncleaned foraminifera) recovered near the hydrocast stations. Methane, TDFe and TDMn concentrations clearly indicate active fluid venting at the studied seeps, with plumes rising up to about 100 m above the seafloor. Depth profiles show pronounced REE enrichments in the non-filtered samples (TD concentrations) within plumes, whereas filtered samples (dissolved concentrations) exhibit slight REE depletion in plumes relative to the overlying water column and display typical seawater REE patterns. These results suggest that the net flux of REE emitted into seawater at cold seeps is controlled by the presence of particulate phases, most probably Fe-Mn oxyhydroxides associated to resuspended sediments. At the reference site, however, our data reveal significant enrichment for dissolved REE in bottom waters, that clearly relates to diffusive benthic fluxes from surface sediments. Neodymium isotopic ratios measured in the water column range from epsilon(Nd) similar to-15.7 to -10.4. Evidence that the epsilon(Nd) values for Antarctic Intermediate waters (AAIW) differed from those reported for the same water mass at open ocean settings shows that sediment/water interactions take place in the Gulf of Guinea. At each site, however, the bottom water epsilon(Nd) signature generally differs from that for cold seep minerals, easily leachable sediment phases, and detrital fractions from local sediments, ruling out the possibility that seepage of methane-rich fluids and sediment dissolution act as a substantial source of dissolved Nd to seawater in the Gulf of Guinea. Taken together, our data hence suggest that co-precipitation of Fe-Mn oxyhydroxide phases in sub-surface sediments leads to quantitative scavenging of dissolved REE at cold seeps, preventing their emission into bottom waters. Most probably, it is likely that diffusion from suboxic surface sediments dominates the exchange processes affecting the marine Nd cycle at the Niger Delta margin. (C) 2011 Elsevier B.V. All rights reserved.
Earth And Planetary Science Letters (0012-821X) (Elsevier Science Bv), 2011-12 , Vol. 312 , N. 3-4 , P. 443-452

Droits : 2011 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/00066/17754/15571.pdf
DOI:10.1016/j.epsl.2011.10.008
http://archimer.ifremer.fr/doc/00066/17754/

Éditeur(s) : Copernicus Gesellschaft Mbh
Résumé : Boundary Exchange (BE - exchange of elements between continental margins and the open ocean) has been emphasized as a key process in the oceanic cycle of neodymium (Nd) (Lacan and Jeandel, 2005a). Here, we use a regional eddy-permitting resolution Ocean General Circulation Model (1/4A degrees) of the North Atlantic basin to simulate the distribution of the Nd isotopic composition, considering BE as the only source. Results show good agreement with the data, confirming previous results obtained using the same parameterization of the source in a coarse resolution global model (Arsouze et al., 2007), and therefore the major control played by the BE processes in the Nd cycle on the regional scale. We quantified the exchange rate of the BE, and found that the time needed for the continental margins to significantly imprint the chemical composition of the surrounding seawater (further referred as characteristic exchange time) is of the order of 0.2 years. However, the timescale of the BE may be subject to large variations as a very short exchange time (a few days) is needed to reproduce the highly negative values of surface waters in the Labrador Sea, whereas a longer one (up to 0.5 years) is required to simulate the radiogenic influence of basaltic margins and distinguish the negative isotopic signatures of North Atlantic Deep Water from the more radiogenic southern origin water masses. This likely represents geographical variations in erosion fluxes and the subsequent particle load onto the continental margins. Although the parameterization of the BE is the same in both configurations of the model, the characteristic exchange time in the eddy-permitting configuration is significantly lower than the previous evaluations using a low resolution configuration (6 months to 10 years), but however in agreement with the available seawater Nd isotope data. This results highlights the importance of the model dynamics in simulating the BE process.
Ocean Science (1812-0784) (Copernicus Gesellschaft Mbh), 2010 , Vol. 6 , N. 3 , P. 789-797

Droits : Author(s) 2010. This work is distributed, EGU
http://archimer.ifremer.fr/doc/00013/12412/9201.pdf
DOI:10.5194/os-6-789-2010
http://archimer.ifremer.fr/doc/00013/12412/

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : We present whole-rock major- and trace-element and Nd–Sr–Pb radiogenic isotope data for Cretaceous igneous suites from eastern Cuba. These rocks are related to the Greater Antilles paleo-island arc magmatism and have three different igneous styles. Group 1 consists of tholeiitic basalts and rare basaltic andesites that have normal mid-ocean ridge basalt (N-MORB)-like compositions similar to those found in back-arc basin basalts (TiO2 = 1·2–2·9 wt%; La/Yb(N) = 0·7–0·9, Th/Nb = 0·06–0·08, and initial 208Pb/ 204Pb = 37·65–37·74). Group 2 comprises basaltic and rare basaltic andesitic subvolcanic dykes with major- and trace-element and isotopic compositions similar to those of island arc tholeiites (TiO2 = 0·7–1·4 wt%; La/Yb(N) = 0·6–0·9, Th/Nb = 0·06–0·68, and initial 208Pb/ 204Pb = 37·74–38·25). Group 3 is composed of low-Ti (TiO2 = 0·3–0·9 wt%) calcalkaline igneous rocks that have an unambiguous subduction-related character (La/Yb(N) = 1·1–5·0, Th/Nb = 0·35–1·55, and initial 208Pb/ 204Pb = 37·94–38·39). The parental magmas of the three groups formed by variable melting degrees (< 5–25%) of spinel lherzolite, with more depleted mantle sources for Groups 2 and 3 than Group 1. The trace-element and radiogenic isotope compositions of primitive Group 3 samples are strongly bimodal. One subgroup of samples is characterized by low Ta/Yb (0·02–0·03) and Th/La (0·10–0·13), slightly subchondritic Nb/Ta (13·3–17·3), and relatively high initial 206Pb/204Pb (18·57–18·62) and {varepsilon}Nd (7·6–9·4). The remaining primitive Group 3 samples have higher Ta/Yb (0·06–0·11) and Th/La (0·24–0·32), and highly subchondritic Nb/Ta (7·6–9·1), coupled with lower initial 206Pb/204Pb (18·24–18·29) and {varepsilon}Nd (3·4–5·5). These signatures were induced by two distinct slab components that mainly reflect the contributions of Cretaceous Atlantic marine and North American continental sediments, respectively. Nb/Ta in the first subgroup was influenced by melting of rutile-bearing subducted crust, whereas in the second it was inherited from recycled continental sediments. The involvement of Atlantic and North American sediments in Cuban Cretaceous magmatism indicates that the Proto-Caribbean (North American-Proto Atlantic) lithosphere subducted beneath the Greater Antilles arc during the Late Cretaceous (pre-Campanian), consistent with geotectonic models involving onset of SW-dipping subduction beneath the Greater Antilles paleo-arc during the Aptian. The variable mantle source depletion and magnitude of the subduction component probably reflect different settings across the arc, from the arc front to a back-arc spreading ridge.
ISSN: 0022-3530

hal-00406373
https://hal.archives-ouvertes.fr/hal-00406373
DOI : 10.1093/petrology/egm040

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Most peridotite massifs and mantle xenoliths show a wide range of isotopic variations, often involving significant decoupling between Hf, Nd and Sr isotopes. These variations are generally ascribed either to mingling of individual components of contrasted isotopic compositions or to time integration of parent-element enrichment by percolating melts/fluids, superimposed onto previous depletion event(s). However, strong isotopic decoupling may also arise during porous flow as a result of daughter-elements fractionation during solid-liquid interaction. Although porous flow is recognized as an important process in mantle rocks, its effects on mantle isotopic variability have been barely investigated so far. The peridotites of the Lherz massif (French Pyrenees) display a frozen melt percolation front separating highly refractory harzburgites from refertilized lherzolites. Isotopic signatures observed at the melt percolation front show a strong decoupling of Hf from Nd and Sr isotopes that cannot be accounted for by simple mixing involving the harzburgite protolith and the percolating melt. Using one dimensional percolation-diffusion and percolation-reaction modeling, we show that these signatures represent transient isotopic compositions generated by porous flow. These signatures are governed by a few critical parameters such as daughter element concentrations in melt and peridotite, element diffusivity, and efficiency of isotopic homogenization rather than by the chromatographic effect of melt transport and the refertilization reaction. Subtle variations in these parameters may generate significant inter-isotopic decoupling and wide isotopic variations in mantle rocks. (C) 2009 Elsevier B.V. All rights reserved.
ISSN: 0012-821X

hal-00413135
https://hal.archives-ouvertes.fr/hal-00413135
DOI : 10.1016/j.epsl.2008.12.033

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : Geochemistry, Geophysics, Geosystems, v. 9, p. Q12001, 2008. http://dx.doi.org/10.1029/2008GC002098
International audience
Rb/Sr, 40Ar/39Ar, and Sm/Nd isotopic data are reported in Caledonian eclogites from the Lindås Nappe, Bergen Arcs, Norway, in order to investigate processes controlling isotopic equilibrium at mineral scale in polymetamorphic rocks. The Bergen Arcs exposes Sveconorwegian ∼950–930 Ma granulites, partially overprinted by Caledonian eclogite-facies metamorphism at ∼425 Ma and amphibolite-facies metamorphism at ∼410 Ma. Geochemical and Rb-Sr data from more than 10 phengite fractions separated from one sample reflect the composition of the microdomain (a few hundred microns in size) in which phengite crystallized. Phengite crystallized after garnet or plagioclase by dissolution-precipitation processes yield apparent age between 700 and 600 Ma. At the time of their crystallization, these phengites inherited the isotopic composition of their precursor minerals, at a microdomain scale. Phengite from quartz veins, which crystallized from elements mobilized by the circulating fluid, yield an age closer to the eclogite-facies metamorphic age. The closed system evolution of the eclogitizing fluid, the segregation of textural and reactional microdomains, the high Sr content of the studied phengite, and the short duration of the recrystallization processes (<1Ma) are interpreted as the main factors responsible for the lack of a Rb/Sr isotopic equilibrium, at the scale of hand samples. Such equilibrium is nevertheless reached in quartz veins where the crystallization of minerals implies that the fluid circulation acted as a factor of isotopic homogenization. The in situ single mineral Ar/Ar datings revealed that both eclogite- and amphibolite-facies minerals are characterized by an excess of radiogenic argon (ages between 425 and 520 Ma). The excess of argon has been inherited from the previous granulite and has been only partially evacuated from the system by the circulating fluid. The apparent Sm/Nd ages from garnets inherited from the granulite-facies metamorphism (c. 930 Ma) are in agreement with previous estimates. This result confirms that the diffusion of Nd in garnet does not occur at temperatures lower than 700°C. This study highlights the complexity of radiochronometer behavior during HP metamorphism and demonstrates that coupling different radiochronometers, such as Ar/Ar and Rb/Sr, does not always guarantee the validity of the geochronological results.
ISSN: 1525-2027

hal-00407885
https://hal.archives-ouvertes.fr/hal-00407885
DOI : 10.1029/2008GC002098

Éditeur(s) : Amer Geophysical Union
Résumé : In this study, we undertook a reconnaissance study of sediments provenance in the Gulf of Lions focusing over the last 16 ka. We used geochemical and isotopic tracers to determine the source of sediments and give insight into the weathering conditions prevailing. Sediments samples were selected both onshore and offshore from the western, eastern, and central part of the Gulf of Lions. We analyzed bulk sediments, coarse and fine silt, and clay fractions. Elemental and Nd isotope compositions appeared to differ from one grain size fraction to another one. These are interpreted in terms of zircon addition in the coarse silt fraction for the elemental concentrations and variable sources influences for the Nd isotope compositions. Our results indicate that sediments in the Gulf of Lions mainly originated from the Rhone River watershed although a contribution of Saharan dust is seen in one sample. Influence of Pyrenean small rivers is minor in these samples. Some Sr isotope compositions shifts are interpreted as reflecting variable amounts of chemical weathering that are consistent with published paleoclimatic reconstructions.
Geochemistry Geophysics Geosystems (1525-2027) (Amer Geophysical Union), 2011-08 , Vol. 12 , N. Q08006 , P. 20 p.

Droits : 2011 by the American Geophysical Union
http://archimer.ifremer.fr/doc/00043/15376/12734.pdf
DOI:10.1029/2011GC003523
http://archimer.ifremer.fr/doc/00043/15376/

Éditeur(s) : Elsevier Science Bv
Résumé : Here we present a new geochemical study of Lopevi volcano, one the most active volcanoes in the Vanuatu island arc. We focus on the temporally well-defined sequence of lava flows emitted since 1960, and for the first time, on pre-1960 volcanic products, including high-MgO basalts and felsic andesites, the most evolved lavas sampled so far on this island. This work reports the first Pb and Hf isotopic study of lavas from Lopevi island. These lavas display correlations between differentiation indexes such as SiO2 content and isotopic ratios. The felsic andesites extend the known correlations with both the least (Sr-Pb) and the most (Nd-Hf) radiogenic isotopic compositions on the island. Our results confirm that the rising magma interacted with the sub-arc crust. Assimilation-Fractional Crystallization (AFC) quantitative modeling of trace element ratios and isotopic compositions requires 1% and 10% of assimilated partial melts of a mafic oceanic crust to account for the pre- and post-1960 lavas, respectively. The post-1960 lavas differ from the former lavas emitted similar to 20 years earlier by enrichments in fluid mobile elements (K, Ba, Rb ... ), Th, and Light Rare Earth Elements (LREE). We ascribe these features to slight variations in the metasomatic agent added to the sub-arc mantle and ultimately derived from the subducted lithosphere. However, the contrasting time scales involved in subducted lithosphere dehydration and magma genesis, relative to the time elapsed between eruptions of the two lava series, suggest that two different portions of mantle which have undergone slightly different metasomatism, gave birth to the Lopevi lavas. These distinct magmas are still present beneath the volcano.
Journal Of Volcanology And Geothermal Research (0377-0273) (Elsevier Science Bv), 2013-08 , Vol. 264 , P. 72-84

Droits : 2013 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/00167/27801/26223.pdf
DOI:10.1016/j.jvolgeores.2013.07.005
http://archimer.ifremer.fr/doc/00167/27801/

Éditeur(s) : HAL CCSD Elsevier
Résumé : Pb, Hf, Nd and Sr isotopes of basaltic lavas from the two Reunion Island volcanoes are reported in order to examine the origin of the sources feeding these volcanoes and to detect possible changes through time. Samples, chosen to cover the whole lifetime of the two volcanoes (from 2 Ma to present), yield a chemically restricted (compared to 0113 lavas) but complex distribution. Reunion plume isotopic characteristics have been defined on the basis of the composition of uncontaminated shield-building lavas from the Piton de la Fournaise volcano. The average epsilon(Nd), epsilon(Hf), Sr-87/Sr-86 and Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 isotope ratios calculated for this component are +4.4, +9.1, 0.70411, 18.97, 15.59 and 39.03, respectively. In Pb-Pb isotope space, each volcano defines a distinct linear trend but slight variation are also detected within the various volcanic sequences. The Piton des Neiges volcano yields a distinct and significantly more scattered isotopic distribution than Piton de la Fournaise for both Pb, Hf and Nd isotope tracers. A principal component analysis of the Pb isotope data from Piton de la Fournaise reveals a major contribution of the C and EM-1 components (with a clear Dupal flavor) as main components for the modem Reunion plume. The same components have been identified for Piton des Neiges but with a stronger participation of a depleted mantle component and a weaker EM-1 contribution. The compositional change of the lavas erupted by the Piton des Neiges and Piton de la Fournaise volcanoes is attributed to the impingement of two small-scale blobs of plume material at the base of the Reunion lithosphere. Compared to other hot-spots worldwide, in particular Hawaii and Kerguelen, magmas beneath Reunion are generated from a considerably more homogeneous, compositionally more primitive plume higher in Pb-206. Although shallow-level contamination processes have been locally detected they did not alter significantly the composition of the plume magmas. This is tentatively attributed to mantle dynamics producing small, high-velocity blobs that ascend rapidly through the lithosphere, and to the lack of a well-developed magma chamber at depth in the lithosphere.
ISSN: 0012-821X

hal-00412196
https://hal.archives-ouvertes.fr/hal-00412196
DOI : 10.1016/j.epsl.2007.11.018

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
Major and trace element signatures and Sr-Nd-Pb isotope data for muscovite (Ms)-bearing amphibolite blocks and associated muscovite-bearing trondhjemite and quartz-muscovite rocks from the Sierra del Convento melange (eastern Cuba) indicate that Proto-Caribbean oceanic crust underwent wet partial melting processes during Mesozoic subduction and after accretion to the upper plate. Trace element normalized patterns of Ms-bearing amphibolites are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE) and evidence variable trace element transfer from the Proto-Caribbean subducting slab to the mantle wedge. Ms-bearing trondhjemites show LREE enrichment and HREE depletion and have geochemical features similar to adakites, including SiO2>56 wt.%, high Na2O contents (5.5-9.0 wt.%) and high Sr/Y (16-644). We consider that the trondhjemites represent primary and natural melts formed by deep partial melting of the subducting slab which did not significantly react with the mantle wedge before intrusion in the subduction channel. The Ms-bearing trondhjemites show different geochemical and petrological signatures compared with the Ms-free tonalites-trondhjemites from Sierra del Convent, that are interpreted as primary slab melts. These differences support the idea that partial melting processes in the Sierra del Convento subduction channel were triggered by the infiltration of fluids derived from three distinct subducted sources: sediments, altered mid-ocean ridge basalts (MORB), and serpentinites. In this scenario, the pegmatitic quartz-muscovite rocks, which are highly enriched in LILE, probably represent the crystallization products of fluids derived by differentiation of the trondhjemitic melts.
ISSN: 0024-4937

hal-00669881
https://hal.archives-ouvertes.fr/hal-00669881
DOI : 10.1016/j.lithos.2011.07.011

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Volumetrically minor microsyenites, alkali microgranite and related trachytic dykes intrude early Pliocene OIB-like alkali basaltic and basanitic flows of the Meseta del Lago Buenos Aires in Central Patagonia (47 degrees S-71 degrees 30'W), and occur together with scarce trachytic lava flows. Whole-rock K-Ar ages between 3.98 and 3.08 Ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post-plateau basaltic sequence that they intrude, during a bimodal mafic-felsic magmatic episode devoid of intermediate compositions. Chemically, these rocks have A(1)-type granitoid affinities and are characterized by high silica and alkali contents (60-68 wt.% SiO2; 8.7-10.8 wt.% Na2O+K2O), major and trace elements patterns evidencing evolution by low-pressure fractional crystallization, and Sr and Nd isotopic signatures similar to those of coeval basalts ((Sr-87/Sr-86)o=0.70488-0.70571; (Nd-143/Nd-144)(o)=0.512603-0.512645). Nevertheless, some of them have the most radiogenic Sr values ever reported for a magmatic rock in the Meseta and even in the whole Neogene Patagonian Plateau Lavas province ((Sr-17/Sr-86)(o)= 0.70556-0.70571; (Nd-143/Nd-144)(o)=0.512603-0.512608). In addition, very high contents of strongly incompatible elements in the most evolved rocks, together with Sr isotopic ratios higher than those of coeval basalts, suggest the occurrence of open-system magmatic processes. Continuous fractional crystallization from a primitive basaltic source, similar to post-plateau coeval basalts, towards alkali granites combined with small rates of assimilation of host Jurassic tuffs (AFC) in a shallow magmatic reservoir, best explains the geochemical and petrographic features of the felsic rocks. Therefore, A(1)-type magmatic rocks can be generated by open-system crystallization of deep asthenospheric melts in back-are tectonic settings. In Central Patagonia, these similar to 3-4 Ma old alkaline intrusions occur aligned along a similar to N160-170 trending lineament, the Zeballos Fault Zone, stacking the morphotectonic front of one segment of the Patagonian Cordillera. Intrusion along this fault zone
ISSN: 0024-4937

hal-00412030
https://hal.archives-ouvertes.fr/hal-00412030
DOI : 10.1016/j.lithos.2007.07.002

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 : We report rare earth element (REE) and neodymium (Nd) isotope data for the organic fraction of sediments collected from various depositional environments, i.e. rivers (n=25), estuaries (n=18), open-ocean settings (n=15), and cold seeps (n=12). Sedimentary Organic Matter (SOM) was extracted using a mixed hydrogen peroxide/nitric acid solution (20%-H2O2 – 0.02M-HNO3), after removal of carbonate and oxy-hydroxide phases with dilute hydrochloric acid (0.25M-HCl). A series of experimental tests indicate that extraction of sedimentary organic compounds using H202 may be complicated occasionally by partial dissolution of sulphide minerals and residual carbonates. However, this contamination is expected to be minor for REE because measured concentrations in H2O2 leachates are about two-orders of magnitude higher than in the above mentioned phases. The mean REE concentrations determined in the H2O2 leachates for samples from rivers, estuaries, coastal seas and open-ocean settings yield relatively similar levels, with ΣREE = 109 ± 86 ppm (mean ± s; n=58). The organic fractions leached from cold seep sediments display even higher concentration levels (285 ± 150 ppm; mean ± s; n=12). The H2O2 leachates for most sediments exhibit remarkably similar shale-normalized REE patterns, all characterized by a mid-REE enrichment compared to the other REE. This suggests that the distribution of REE in leached sedimentary organic phases is controlled primarily by biogeochemical processes, rather than by the composition of the source from which they derive (e.g. pore, river or sea- water). The Nd isotopic compositions for organic phases leached from river sediments are very similar to those for the corresponding detrital fractions. In contrast, the SOM extracted from marine sediments display εNd values that typically range between the εNd signatures for terrestrial organic matter (inferred from the analysis of the sedimentary detrital fractions) and marine organic matter (inferred from the analysis of local surface seawater). A notable exception is the case of organic matter (OM) fractions leached from cold seep sediment samples, which sometimes exhibit εNd values markedly different from both terrigenous and surface seawater signatures. This suggests that a significant fraction of organic compounds in these sediments may be derived from chemosynthetic processes, recycling pore water REE characterized by a distinct isotopic composition. Overall, our results confirm that organic matter probably plays an important role in the oceanic REE budget, through direct scavenging and remineralization within the water column. Both the high REE abundances and the shape of shale-normalized patterns for leached SOM also suggest that OM degradation in sub-surface marine sediments during early diagenesis could control, to a large extent, the distribution of REE in pore waters. Benthic fluxes of organic-bound REE could hence substantially contribute to the exchange processes between particulates and seawater that take place at ocean margins. Neodymium isotopes could provide useful information for tracing the origin (terrestrial versus marine) and geographical provenance of organic matter, with potential applications in paleoceanography. In particular, future studies should further investigate the potential of Nd isotopes in organic compounds preserved in sedimentary records for reconstructing past variations of surface ocean circulation.
Geochimica Et Cosmochimica Acta (0016-7037) (Pergamon-elsevier Science Ltd), 2014-09 , Vol. 140 , P. 177-198

Droits : 2014 Elsevier Ltd. All rights reserved.
http://archimer.ifremer.fr/doc/00191/30250/28682.pdf
DOI:10.1016/j.gca.2014.05.016
http://archimer.ifremer.fr/doc/00191/30250/

Éditeur(s) : HAL CCSD Cambridge University Press (CUP)
Résumé : International audience
Upper Triassic volcanic and sedimentary rocks of the Mamonia Complex in southwestern Cyprus are exposed in erosional windows through the post-Cretaceous cover, where the Mamonia Complex is tectonically imbricated with the Troodos and Akamas ophiolitic suites. Most of these Upper Triassic volcanic rocks have been considered to represent remnants of Triassic oceanic crust and its associated seamounts. New Nd and Pb isotopic data show that the whole Mamonia volcanic suite exhibits features of oceanic island basalts (OIB). Four rock types have been distinguished on the basis of the petrology and chemistry of the rocks. Volcanism began with the eruption of depleted olivine tholeiites (Type 1) and oceanic island tholeiites (Type 2) associated with deep basin siliceous and/or calcareous sediments. The tholeiites were followed by highly phyric alkali basalts (Type 3) interbedded with pelagic Halobia-bearing limestones or white reefal limestones. Strongly LREE-enriched trachytes (Type 4) were emplaced during the final stage of volcanic activity. Nd and Pb isotopic ratios suggest that tholeiites and mildly alkali basalts derived from partial melting of heterogeneous enriched mantle sources. Fractional crystallization alone cannot account for the derivation of trachytes from alkaline basalts. The trachytes could have been derived from the partial melting at depth of mafic material which shares with the alkali basalts similar trace element and isotopic compositions. This is corroborated by the rather similar isotopic compositions of the alkali basalts and trachytes. The correlations observed between incompatible elements (Nb, Th) and {varepsilon}Nd and Pb isotopic initial ratios suggest that the Mamonia suite was derived from the mixing of a depleted mantle (DMM) and an enriched component of High µ (µ = 238U/204Pb, HIMU) type. Models using both Nd and Pb isotopic initial ratios suggest that the depleted tholeiites (Type 1) derived from a DMM source contaminated by an Enriched Mantle Type 2 component (EM2), and that the oceanic tholeiites (Type 2), alkali basalts (Type 3) and trachytes (Type 4) were derived from the mixing of the enriched mantle source of the depleted tholeiites with a HIMU component. None of the Mamonia volcanic rocks show evidence of crustal contamination. The Upper Triassic within-plate volcanism likely erupted in a small southerly Neotethyan basin, located north of the Eratosthenes seamount and likely floored by oceanic crust.
ISSN: 0016-7568

insu-00353657
https://hal-insu.archives-ouvertes.fr/insu-00353657
DOI : 10.1017/S0016756806002937

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : National audience
The late Lower to Middle Permian Panjal Traps (NW Himalaya, India-Pakistan) represent the greatest magmatic province erupted on the northern Indian platform during the Neotethys opening. New geochemical and isotopic analyses were performed on basalts from the eastern borders of the traps (SE Zanskar-NW Spiti area) in order to characterize this volcanism, to discuss its compositional variations in comparison to Panjal counterparts and its relationships with the opening of Neotethys. Lavas show features of tholeiitic low-Ti (< 1.6%) continental flood basalts with LREE, Th enrichments and Nb-Ta negative anomalies. Trace element ratios combined with {varepsilon}Ndi values (–3.6 to +0.9) and high Pb isotopic ratios suggest that these tholeiitic basalts were derived from an OIB-like mantle contaminated at various degrees by a continental crust component. Previous geochemical features are broadly similar to those of the coeval Panjal volcanic sequences identified westwards (Ladakh, Kashmir and Pakistan). Present geochemical constraints obtained for the Panjal Traps basalts suggest they originated from rapid effusion of tholeiitic melts during opening of the Neotethys Ocean. Similar magmatism implying an OIB-type reservoir is contemporaneously recognized on and along the adjacent Arabian platform. Both Indian and Arabian Permian volcanics were emplaced during coeval syn-rift to post rift transition. These Lower to Middle Permian south Neotethyan continental flood magmatism are regarded as associated to a passive rifting. In this scheme, OIB-type isotopic signature would be related either to a melting episode of syn-rift up-welling mantle plumes or to a melting of a regional abnormally hot and enriched mantle.
EISSN: 1777-5817

insu-00338862
https://hal-insu.archives-ouvertes.fr/insu-00338862
DOI : 10.2113/gssgfbull.179.4.383