OIB/seamount recycling as a possible process for E-MORB genesis

International audienceThis paper deals with the origin of enriched MORB independent from any hot spot activity. Indeed, MORB enrichment was readily attributed to a ridge/hot spot interaction and in absence of identified neighboring hot spot, to more questionable processes (e.g., incipient plume or plume activity residue). More recently, the existence of enriched MORB away from any identifiable hot spot was attributed to different origins (i.e., recycled oceanic crust and/or enriched mantle after subduction metasomatism). Within this frame, we present here a new set of geochemical analyses of major and trace elements and Sr, Nd and Pb isotopes on samples collected by submersible on both intersections of the 15°20′N fracture zone with the spreading axis of the Mid-Atlantic Ridge. This area is characterized by an increasing enrichment of the lava compositions from north to south through the fracture zone. Results show that the geochemical enrichment observed with a different intensity on both sides of the fracture zone is linked to the 14°N topographic and geochemical anomaly. Our modeling shows that both trace element and isotopic compositions are consistent with a binary mixing between the regional depleted MORB mantle source and a recycled OIB/seamount, as previously proposed to explain the observed enrichment at 14°N. This model can also account for other enriched MORB i.e., the 18°-20°S region of the Central Indian Ridge, illustrating that it does not represent an isolated and local process. On the basis of our results and on the DMM isotopic evolution, the age of the recycled OIB/seamount is estimated to be ∼250 Ma, suggesting a recycling within the upper mantle. Considering the huge number of ocean islands and seamounts upon the ocean floor, their recycling into the upper mantle is a plausible process to produce enriched MORB

Differentiating Lithogenic Supplies, Water Mass Transport, and Biological Processes On and Off the Kerguelen Plateau Using Rare Earth Element Concentrations and Neodymium Isotopic Compositions

Distributions of dissolved rare earth element (REE) concentrations and neodymium isotopic compositions (expressed as εNd) of seawater over and off the Kerguelen Plateau in the Southern Ocean are presented. The sampling took place during the austral spring bloom in October–November 2011 (KEOPS2 project, GEOTRACES process study) and aimed to further the investigations of the KEOPS1 austral summer study in terms of sources and transport of lithogenic material, and to investigate the impact of local biogeochemical cycles on the REE distributions. The REE signature of the coastal eastern Kerguelen Islands waters was characterized by negative europium anomalies (Eu/Eu*) and negative εNd in filtered samples. By contrast, the unfiltered sample showed a positive Eu/Eu* and more radiogenic εNd. These distinct signatures could reflect either differential dissolution of the local flood basalt minerals or differential leaching of local trachyte veins. The dissolved Kerguelen coastal REE patterns differ from those observed close to Heard Island, these latter featuring a positive Eu/Eu* and a less radiogenic εNd (Zhang et al., 2008). These differences enabled us to trace the transport of waters (tagged by the Kerguelen REE signature) 200 km downstream from the coastal area, north of the Polar Front. Northward transport of the central Plateau shallow waters, enriched by both local vertical supplies and lateral advection of inputs from Heard Island, was also evident. However, the transport of Kerguelen inputs southeastward across the Polar Front could not be discerned (possibly as a result of rapid dilution or scavenging of REE signatures), although evidence for such transport was found previously using Ra isotopes (Sanial et al., 2015). Comparison of the REE patterns at stations sampled prior, during and at the demise of the bloom revealed diverse fractionations, including production of significant lanthanum and europium anomalies, which are tentatively ascribed to chemical reactions with various inorganic and biogenic phases, including surface coatings, barite crystals, and biogenic silica

Etude volcano-tectonique de la zone de Divergence Nord Tanzanienne (Terminaison Sud du Rift Kenyan).<br />Caractérisation pétrologique et géochimique du volcanisme récent (8 Ma – Actuel) et du manteau source.<br />Contraintes de mise en place.

The East African Rift System (EARS) is the best example worldwide of an active magmatic rift. It extends over more than 3500 km from the Afar province to the Mozambican Gulf, cutting through the Ethiopian and East Africa elevated plateaus that are the topographic expression of one or several Cenozoic mantle plumes beneath this part of Africa. Extensional strain initited along the eastern magmatic branch of the EARS at ca. 30 My in the Afar, and then propagated southwards by linkage of discrete magmatic cells centered on the Kenyan dome, to reach South Kenya 8 My ago. There, the North Tanzanian Divergence (NTD) corresponds to a dramatic change in structural style, associated with the abrupt disappearance of magmatism southwards. The transition zone from the NS axial valley into three diverging rift arms (Eyasi, Natron-Manyara and Pangani), dominated by tilted basement fault blocks, is outlined by a 200 km-long transverse volcanic chain, including major Neogene volcanic edifices such as the Ngorongoro, Mount Meru and Kilimanjaro.Our work presents new structural and geochemical results on magmatic rocks in the NTD, and its main originality is to integrate complementary approaches involving field investigations, petrological-mineralogical analyses, geochronology, Sr-Nd isotopic and trace elements geochemistry. That allows us to also precise the build-up history and mechanisms of a number of major volcanoes of the NTD (Ngorongoro, Mount Meru, Kilimanjaro) during Plio-Quaternary times. Applying this approach to a wider scale, and compiling published radiometric data with our own K-Ar age determinations, allows us to establish a kinematic rift propagation model to the NTD for the last 8 My. It is proposed that both magmatism and strain shifted eastwards along the transverse volcanic chain, up to the Pangani arm, from 8 My to Present, with the onset of magmatic activity in the whole NTD at 2.5 My. But, the main contribution of our study is devoted to the Kilimanjaro long-lived edifice that forms the prominent structure in the EARS, as a whole. Seventeen new K-Ar ages provide constraints for précising time relationships between its three main magmatic centres, and emphasis has been put on the Kibo central vent that recorded the last building phases in the time range 492-165 ky. In addition, new petrological and geochemical results on Kilimanjaro mafic lavas bring some new insights on the nature of mantle sources that are thought to be heterogeneous lithospheric material with residual amphibole and garnet. Two different metasomatic events, Precambrian and Plio-Quaternary in age, are inferred to have governed the evolution of the mantle sources, with the infiltration of melt from the underlying Neogene plume. These conclusions, extended at the scale of the NTD for young (Ce travail de thèse présente les résultats de l'étude volcano-structurale menée sur cette zone de rift divergent. L'originalité de notre approche provient de la combinaison d'outils complémentaires (terrain, pétrologie, radiochronologie, géochimie isotopique Sr-Nd et des éléments en traces), appliquées à une zone clé du REA. Ainsi, à l'échelle des principaux volcans de la DNT, et particulièrement pour le Ngorongoro et le Mt Meru, nous avons pu reconstituer leur activité Plio-Quaternaire en précisant les mécanismes de mise en place de leurs principales formations volcaniques. Cette approche locale a été étendue à l'échelle régionale en synthétisant les données radiochronologiques existantes, complétées par six nouveaux âges K-Ar. Nous avons ainsi pu reconstituer l'histoire volcano-tectonique de la DNT et proposer un modèle d'évolution spatio-temporel du volcanisme depuis 8 Ma. Ce dernier met en évidence une migration de l'activité magmatique vers l'Est entre 8 Ma et l'actuel avec une activité généralisée dans l'ensemble de la DNT à 2,5 Ma, qui accompagne l'extension de la déformation vers la branche de Pangani. Le Kilimandjaro, édifice majeur à l'échelle du Rift Africain, constitue le cœur de ce travail. La détermination de dix sept nouveaux âges K-Ar a permis de contraindre dans le temps les processus volcaniques ayant affecté les trois centres le constituant, et plus particulièrement le centre principal de Kibo pour lequel nous parvenons à reconstituer les dernières phases d'édification entre 492 ka et 165 ka. Grâce à l'approche pétrologique et géochimique réalisée sur les laves du Kilimandjaro, nous proposons un modèle d'évolution des sources mantelliques de ces magmas, en montrant qu'ils sont issus de la fusion partielle d'une source lithosphérique hétérogène à amphibole et grenat résiduels, ayant acquis ses caractéristiques géochimiques au cours de deux épisodes métasomatiques distincts : un premier, probablement d'âge Précambrien, et le deuxième, Plio-Quaternaire, provoqué par la percolation des magmas formés au sein du panache asthénosphérique sous-jacent. La généralisation de cette approche à l'échelle de la DNT, pour les laves primitives d'âges < 1 Ma, met en évidence des différences dans les processus pétrogénétiques à l'origine du magmatisme de cette région. Si sa manifestation, à l'Est de la DNT, présente des compositions caractéristiques d'une fertilisation de la lithosphère par l'activité sub-actuelle d'un panache, les laves émises à l'Ouest comportent des signatures géochimiques héritées d'un manteau lithosphérique métasomatisé au Précambrien, en accord avec les résultats obtenus sur les enclaves mantelliques. Le type de métasomatisme, sa localisation et sa période d'activité sont certainement liés à l'héritage structural, et particulièrement au positionnement relatif des blocs cratoniques Archéen et des zones transverses affectant les ceintures orogéniques Protérozoïques

Genesis of andesitic–boninitic magmas at mid-ocean ridges by melting of hydrated peridotites: Geochemical evidence from DSDP Site 334 gabbronorites

The gabbronoritic cumulates drilled at DSDP Site 334 (Mid-Atlantic Ridge off the FAMOUS area) are neither crystallization products of the associated basalts, nor from any MORB composition documented along ocean ridges. Their parent melts are richer in SiO2 than MORB at a given MgO content, as attested by the crystallization sequence starting with an olivine + calcic and sub-calcic pyroxene assemblages. These melts are issued from a source highly depleted in incompatible elements, likely residual peridotite left after MORB extraction. To understand the role of water in the genesis of these lithologies whose occurrence in a mid-ocean ridge setting is rather puzzling, we performed a geochemical study on clinopyroxene separates following an analytical protocol able to remove the effects of water rock interactions post-dating their crystallization. Accordingly, the measured isotopic signatures can be used to trace magma sources. We find that Site 334 clinopyroxenes depart from the global mantle correlation: normal MORB values for the 143Nd / 144Nd ratio (0.51307–0.51315) are associated to highly radiogenic 87Sr / 86Sr (0.7034–0.7067) ratios. This indicates that the parent melts of Site 334 cumulates are issued from a MORB source but that seawater contamination occurred at some stage of their genesis. The extent of contamination, traced by the Sr isotopic signature, is variable within all cumulates but more developed for gabbronorites sensus stricto, suggesting that seawater introduction was a continuous process during all the magmatic evolution of the system, from partial melting to fractional crystallization. Simple masse balance calculations are consistent with a contaminating agent having the characters of a highly hydrated (possibly water saturated) silica-rich melt depleted in almost all incompatible major, minor and trace elements relative to MORB. Mixing in various proportions of contaminated melts similar to the parent melts of Site 334 cumulates with MORB can account for part of the variability in the Sr isotopic signature of oceanic basalts, among other to the short wavelength isotopic “noise” superimposed on regional trends. We conclude that seawater introduction into residual peridotite at shallow depth beneath mid-ocean ridges can lead mantle rocks and their melts to follow complex P–T–fH2O paths that mimic petrogenetic contexts classically attributed to subduction zone environments, like the production of boninitic–andesitic magmas

Petrological and geochemical characteristics of DSDP Hole 37-334 gabbroic cumulates (Table 1)

The gabbronoritic cumulates drilled at DSDP Site 334 (Mid-Atlantic Ridge off the FAMOUS area) are neither crystallization products of the associated basalts, nor from any MORB composition documented along ocean ridges. Their parent melts are richer in SiO2 than MORB at a given MgO content, as attested by the crystallization sequence starting with an olivine+calcic and sub-calcic pyroxene assemblages. These melts are issued from a source highly depleted in incompatible elements, likely residual peridotite left after MORB extraction. To understand the role of water in the genesis of these lithologies whose occurrence in a mid-ocean ridge setting is rather puzzling, we performed a geochemical study on clinopyroxene separates following an analytical protocol able to remove the effects of water rock interactions post-dating their crystallization. Accordingly, the measured isotopic signatures can be used to trace magma sources. We find that Site 334 clinopyroxenes depart from the global mantle correlation: normal MORB values for the 143Nd/ 144Nd ratio (0.51307-0.51315) are associated to highly radiogenic 87Sr / 86Sr (0.7034-0.7067) ratios. This indicates that the parent melts of Site 334 cumulates are issued from a MORB source but that seawater contamination occurred at some stage of their genesis. The extent of contamination, traced by the Sr isotopic signature, is variable within all cumulates but more developed for gabbronorites sensus stricto, suggesting that seawater introduction was a continuous process during all the magmatic evolution of the system, from partial melting to fractional crystallization. Simple masse balance calculations are consistent with a contaminating agent having the characters of a highly hydrated (possibly water saturated) silica-rich melt depleted in almost all incompatible major, minor and trace elements relative to MORB. Mixing in various proportions of contaminated melts similar to the parent melts of Site 334 cumulates with MORB can account for part of the variability in the Sr isotopic signature of oceanic basalts, among other to the short wavelength isotopic ,,noise" superimposed on regional trends. We conclude that seawater introduction into residual peridotite at shallow depth beneath mid-ocean ridges can lead mantle rocks and their melts to follow complex P-T-fH2O paths that mimic petrogenetic contexts classically attributed to subduction zone environments, like the production of boninitic-andesitic magmas

Rift propagation at craton margin. Distribution of faulting and volcanism in the North Tanzanian Divergence (East Africa) during Neogene times

International audienceA revised kinematic model is proposed for the Neogene tectono-magmatic development of the North Tanzanian Divergence where the axial valley in S Kenya splits southwards into a wide diverging pattern of block faulting in association with the disappearance of volcanism. Propagation of rifting along the S Kenya proto-rift during the last 8 Ma is first assumed to have operated by linkage of discrete magmatic cells as far S as the Ngorongoro–Kilimanjaro transverse volcanic belt that follows the margin of cratonic blocks in N Tanzania. Strain is believed to have nucleated throughout the thermally-weakened lithosphere in the transverse volcanic belt that might have later linked the S Kenya and N Tanzania rift segments with marked structural changes along-strike. The North Tanzanian Divergence is now regarded as a two-armed rift pattern involving: (1) a wide domain of tilted fault blocks to the W (Mbulu) that encompasses the Eyasi and Manyara fault systems, in direct continuation with the Natron northern trough. The reactivation of basement fabrics in the cold and intact Precambrian lithosphere in the Mbulu domain resulted in an oblique rift pattern that contrasts with the orthogonal extension that prevailed in the Magadi–Natron trough above a more attenuated lithosphere. (2) To the E, the Pangani horst-like range is thought to be a younger (< 1 Ma) structure that formed in response to the relocation of extension S of the Kilimanjaro magmatic center. A significant contrast in the mechanical behaviour of the stretched lithosphere in the North Tanzanian diverging rift is assumed to have occurred on both sides of the Masai cratonic block with a mid-crustal decoupling level to the W where asymmetrical fault-basin patterns are dominant (Magadi–Natron and Mbulu), whereas a component of dynamical uplift is suspected to have caused the topographic elevation of the Pangani range in relation with possible far-travelled mantle melts produced at depth further N

Pliocene intraplate-type volcanism in the Andean foreland at 26°10′S, 64°40′W (NW Argentina): Implications for magmatic and structural evolution of the Central Andes

International audienceThe Antilla magmatic complex (26°10′S, 64°40′W, NW Argentina) attests to magma eruption at ca. 4.7 Ma in the Central Andes backarc region, 300 km E of the active arc. The Antilla lavas have an alkaline, predominantly mafic composition and record the most primitive isotopic ratios (87Sr/86Sr = 0.704360 and 143Nd/144Nd = 0.512764) of Central Andes Neogene backarc volcanism between 24°S and 27°S. They show trace-element patterns recalling backarc Pliocene-Quaternary intraplate mafic rocks, but they show lower silica and higher alkali contents, and are interpreted to derive from the depleted subcontinental mantle. A revision of the structural and volcanological characteristics of the Central Andes between 24°S and 27°S shows that this region was, during the Miocene-Pliocene, the site of lithospheric processes that account for partial melting in the mantle wedge, in the subcontinental mantle, and in the continental crust. The existing geophysical and petrological data agree with a model in which magma production was related to a process of lithospheric delamination. The Antilla rocks are the easternmost volcanic products with intraplate characteristics, located beside a large zone of partial melting of the continental crust, at the intersection of the NW-trending Archibarca and NE-trending Tucumán transversal lineaments. Their age of 4.7 Ma corresponds to the acme of mafic monogenetic and silicic ignimbrite volcanism in the backarc at the same latitude. This provides new constraints for the spatial and temporal reconstruction of deformation events in the crust and of the lithospheric delamination process and its bearing on magmatic activity

The Amsterdam-St. Paul Plateau: A complex hot spot/DUPAL-flavored MORB interaction

International audienceThe Amsterdam-St Paul (ASP) oceanic plateau results from the interaction between the ASP hot spot and the Southeast Indian ridge. A volcanic chain, named the Chain of the Dead Poets (CDP), lies to its northward tip and is related to the hot spot intraplate activity. The ASP plateau and CDP study reveals that ASP plume composition is inherited from oceanic crust and pelagic sediments recycled in the mantle through a 1.5 Ga subduction process. The ASP plateau lavas have a composition (major and trace elements and Sr-Nd-Pb-Hf isotopes) reflecting the interaction between ASP plume and the Indian MORB mantle, including some clear DUPAL input. The Indian upper mantle below ASP plateau is heterogeneous and made of a depleted mantle with lower continental crust (LCC) fragments probably delaminated during the Gondwana break-up. The lower continental crust is one of the possible reservoirs for the DUPAL anomaly origin that our data support. The range of magnitude of each end-member required in ASP plateau samples is (1) 45% to 75% of ASP plume and (2) 25% to 55% of Indian DM within 0% to a maximum of 6% of LCC layers included within. The three end-members involved (plume, upper mantle and lower continental crust) and their mixing in different proportions enhances the geochemical variability in the plateau lavas. Consequently, the apparent composition homogeneity of Amsterdam Island, an aerial summit of the plateau, may result from the presence of intermediate magmatic chambers into the plateau structure