Origins of chemical diversity of back-arc basin basalts: a segment-scale study of the Eastern Lau Spreading Center

We report major, trace, and volatile element data on basaltic glasses from the northernmost segment of the Eastern Lau Spreading Center (ELSC1) in the Lau back-arc basin to further test and constrain models of back-arc volcanism. The zero-age samples come from 47 precisely collected stations from an 85 km length spreading center. The chemical data covary similarly to other back-arc systems but with tighter correlations and well-developed spatial systematics. We confirm a correlation between volatile content and apparent extent of melting of the mantle source but also show that the data cannot be reproduced by the model of isobaric addition of water that has been broadly applied to back-arc basins. The new data also confirm that there is no relationship between mantle temperature and the wet melting productivity. Two distinct magmatic provinces can be identified along the ELSC1 axis, a southern province influenced by a “wet component” with strong affinities to arc volcanism and a northern province influenced by a “damp component” intermediate between enriched mid-ocean ridge basalts (E-MORB) and arc basalts. High–field strength elements and rare earth elements are all mobilized to some extent by the wet component, and the detailed composition of this component is determined. It differs in significant ways from the Mariana component reported by E. Stolper and S. Newman (1994), particularly by having lower abundances of most elements relative to H_(2)O. The differences can be explained if the slab temperature is higher for the Mariana and the source from which the fluid is derived is more enriched. The ELSC1 damp component is best explained by mixing between the wet component and an E-MORB-like component. We propose that mixing between water-rich fluids and low-degree silicate melts occurs at depth in the subduction zone to generate the chemical diversity of the ELSC1 subduction components. These modified sources then rise independently to the surface and melt, and these melts mix with melts of the background mantle from the ridge melting regime to generate the linear data arrays characteristic of back-arc basalts. The major and trace element framework for ELSC1, combined with different slab temperatures and compositions for difference convergent margins, may be able to be applied to other back-arc basins around the globe

Origins of Chemical Diversity of Back-Arc Basin Basalts: A Segment-Scale Study of the Eastern Lau Spreading Center

We report major, trace, and volatile element data on basaltic glasses from the northernmost segment of the Eastern Lau Spreading Center (ELSC1) in the Lau back-arc basin to further test and constrain models of back-arc volcanism. The zero-age samples come from 47 precisely collected stations from an 85 km length spreading center. The chemical data covary similarly to other back-arc systems but with tighter correlations and well-developed spatial systematics. We confirm a correlation between volatile content and apparent extent of melting of the mantle source but also show that the data cannot be reproduced by the model of isobaric addition of water that has been broadly applied to back-arc basins. The new data also confirm that there is no relationship between mantle temperature and the wet melting productivity. Two distinct magmatic provinces can be identified along the ELSC1 axis, a southern province influenced by a wet component with strong affinities to arc volcanism and a northern province influenced by a damp component intermediate between enriched mid-ocean ridge basalts (E-MORB) and arc basalts. High field strength elements and rare earth elements are all mobilized to some extent by the wet component, and the detailed composition of this component is determined. It differs in significant ways from the Mariana component reported by E. Stolper and S. Newman (1994), particularly by having lower abundances of most elements relative to $H_2O$. The differences can be explained if the slab temperature is higher for the Mariana and the source from which the fluid is derived is more enriched. The ELSC1 damp component is best explained by mixing between the wet component and an E-MORB-like component. We propose that mixing between water-rich fluids and low-degree silicate melts occurs at depth in the subduction zone to generate the chemical diversity of the ELSC1 subduction components. These modified sources then rise independently to the surface and melt, and these melts mix with melts of the background mantle from the ridge melting regime to generate the linear data arrays characteristic of back-arc basalts. The major and trace element framework for ELSC1, combined with different slab temperatures and compositions for difference convergent margins, may be able to be applied to other back-arc basins around the globe.Earth and Planetary Science

Sites d’acquisition, de transformation et d’utilisation de la dolérite et des grès éocènes

L’année 2011 a été l’occasion de débuter à l’université de Nantes les analyses géochimiques de dolérite (AB, CL et GK) provenant du site de Beulin et d’échantillons provenant d’autres affleurements de dolérite limitrophes. L’objectif de ces analyses est d’établir leur éventuelle signature géochimique afin de faciliter l’identification des productions de l’atelier de Beulin. Par ailleurs, la publication de la carte géologique de Mayenne par le BRGM nous a permis de corriger la nature de la roc..

Les dorsales ultra-lentes, une réponseau jeu de la tectonique des plaques et de laconvection mantellique

International audienceUltra-slow spreading ridges such as the South West Indian ridge orthe Arctic ridge system are oddities amongst oceanic ridges. Indeed,conversely to faster oceanic ridges, petrographic and seafloor studieshave shown that they characterized by low melt supply and present lowcrustal thicknesses and heat flow; these features are interpreted as anevidence for a cooler sublithospheric mantle. In cartoonish sketches ofplate tectonics, ridges open above upwellings, subduction zones occurover downwellings, and plates are riding over the mantle convectioncells. In this study, we designed a simple yet dynamically consistentthermal convection model to test the impact of far-field forces on spreadingridges and show that this pattern is disrupted by plate tectonics. Inparticular, continental collisions modulate the spreading rates becauseresisting forces build up at plate boundaries. As a consequence, thismodifies the surface boundary conditions and therefore the underlyingmantle flow. We show that the ideal convection cell pattern quicklybreaks down when plate motion is impeded by continental collisions inthe far field. Not only the decreasing spreading rates are diagnostic, butin the same time, (i) the heat flow is decreasing at the ridge, (ii) the thermalstructure of the cooling lithosphere no longer matches the coolinghalf-space model, and (iii) the mantle temperature beneath the ridgedrops by more than 100 degrees. We compare our model predictions toavailable observables and show that this simple mechanism explains theatypical thermo-mechanical evolution of the South West Indian ridgeand Arctic ridge system. Last, the recent S wave seismic tomographymodel of Debayle and Ricard (2012) reveals that only away from thosetwo ridges does lithospheric thickening departs from the half-space coolingmodel, in accord with our model predictions

Ultraslow, slow, or fast spreading ridges: an interplay between plate tectonics and mantle convection

International audienceOceanic spreading rates are highly variable. These variations are known to correlate to a variety of surface observables,like magmatic production, heat flow or bathymetry, which lead to classify ridges into fast and slow spreadingridges, but also as the more peculiar ultraslow spreading regime. Here we explore the dynamic relationships betweenspreading ridges, plate tectonics and mantle flow. For this, we first focus on the thermal signature at deeperlevels that we infer from the global S-wave seismic tomography model of Debayle and Ricard (2012). We showthat the thermal structure of ridges gradually departs from the half-space cooling model for slow, and above allultraslow spreading ridges. We also infer that the sub- lithospheric mantle temperature decreases by more than180K from fast spreading to ultraslow spreading regimes. Both observations indicate that the mantle convectionpattern is increasingly altered underneath slow and ultraslow spreading ridges. We suggest that this is due to farfieldtectonics on the other ends of lithospheric plates. Not only it modulates the spreading rates but it also altersthe convection regime: collisions at active plate boundaries obstruct plate motion and decrease their velocities. Wethen test this hypothesis using a thermo-mechanical model that represents a convection cell carrying a positivelybuoyant continental lithosphere on top. The continent gradually drifts away from the spreading ridge, from whichthe oceanic lithosphere grows and cools while the continent eventually collides at the opposite side. In turn, thisevent drastically modifies the upper kinematic condition for the convecting mantle that evolves from a mobile lidregime to an almost stagnant lid regime. Implications on spreading ridges are prominent: heat advection is slowerthan thermal diffusion, which causes the oceanic lithosphere to thicken faster; the oceanic plates get compressedand destabilized by a growing number of small scale transient plumes, which disrupts the structure of the oceaniclithospheres, lowers the heat flow and may even starve ultraslow ridges from partial melting

Helium and trace element geochemical signals in the southweast Indian Ocean

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Dynamic topography, endoreism, and the 87Sr/86Sr curve

International audienceThe trend of the 87Sr/86Sr ratio in oceanic waters displays two maxima during the Cambrian and present-day, and a minimum during the Jurassic, overprinted by shorter wavelengths oscillations ( 40 Myrs). Those variations are interpreted as a direct proxy for the flux of continental sediments into the oceans associated to orogenic events. We interpret the frequent mismatch between theory and observations by the sequestration of sediments into continental basins, which consequently starves the ocean from 87Sr rich sediments. Not only the short-term, but also the long-term (Phanerozoic) evolution of the 87Sr/86Sr curve could evidence endoreic/exoreic cycles controlled by the dynamic deflection of the topography of continents above subduction zones. During endoreic phases, the erosional product is only partially, if any, redistributed in the global oceans. It is instead sequestrated in the form of widespread continental deposits. Conversely, during exoreic phases, the sedimentary flux into the oceans cumulates the contribution of the erosional product of the relief (which is directly exported into the ocean) and the release of the sediment load that was sequestrated during endoreic phases. Such cycles modulate the isotopic composition of the ocean waters accordingly. The dynamic deflection of the topography that is associated with orogenic cycles provides a good explanation for periods of short-term (20-40 Myrs) endoreism: widespread hinterland basins due to the dynamic topography above the subducting slabs that are associated to mountain building. Such basins are capable of storing tremendous amounts of sediments, a process that is further reinforced by the sequestration of sediments in intra-mountainous basins. This cycle is well illustrated by the foreland stratigraphy during the Variscan orogeny. The long wavelength of the 87Sr/86Sr trend is that of the Wilson cycle. During supercontinental aggregation, centripetal subductions zones promote the widespread development of dynamic basins above the slabs, in the center of Pangea. These basins have no outlet to the ocean, and thus starve the ocean from 87Sr rich sediments. Conversely, exoreism is expected to increase during continental breakup, as corroborated by the isotopic record. The 87Sr/86Sr ratio is thus a proxy for endoreism at various time scales, and as such shall not be regarded as an indicator of continental erosion

Abundance and distribution of platinum-group elements in orogenic lherzolithes ; a case study in a Fontete Rouge lherzolithe (French Pyrénées)

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