Articles

Séminaire de Gurumurthy, Birbal Sahni Inst. of Paleosciences, India

Silicate weathering in humid tropics and coastal biogeochemical processes affecting the elemental budgets to Oceans

The Western Ghats form a major mountain belt, next to the Himalayas, in controlling the flux of chemical elements and sediments to the northern Indian Ocean. Numerous small mountain streams flow towards the Arabian Sea draining the western slopes of Western Ghats because of the prevailing orographicity. The weathering process and the associated atmospheric CO2 drawdown due to silicate weathering have been deduced using the dissolved and particulate chemical composition in river basins having similar geomorpho-climatic settings but differing lithology (granite-gneisses, charnockites and basalts). The speaker discusses the silicates weathering process in the small mountain streams of Western Ghats, the rate controlling parameters and their role in geological carbon sequestration.

Further, the geochemical redistribution of redox sensitive elements during the weathering and fluvial transport process with special reference to molybdenum geochemistry in the riverine and estuarine environments will be discussed. The role of oxidative scavenging of dissolved molybdenum on to particulate phase in river, and its subsequent release/scavenging of metal elements in the coastal region will be discussed in detail to demonstrate the role of coastal/interface biogeochemical processes in the source-sink pathways of chemical elements, and the extent these biogeochemical processes affect the elemental budgets.

Email: gurumurthygp@bsip.res.in

27 novembre 2018

Séminaire de Kenni PEDERSEN (Univ. Aarhus, Danemark)

Models of plate tectonic processes and their interaction with the convecting mantle: magmatism, vertical movements and thermo-chemical structure

During this talk, I will present different results from 2D high-resolution numerical modeling of lithosphere/mantle-scale processes involving plate tectonics and convection in the Earth’s mantle at different scale. The talk will be subdivided into three themes that all employ this modeling technique.

1. Thermal subsidence of extensional basins in the presence of sublithospheric convection. Here, I present models of lithosphere extension above an asthenosphere mantle that is convecting with sufficient vigor to explain e.g. surface heat flow of old (‘steady state’) oceanic lithosphere. The models indicate that thermal subsidence in the presence of such convection can differ substantially from the well-known McKenzie model that only implicitly accounts for convective heat flow. For example, thermal subsidence can become strongly protracted, because the thermal boundary layer may become advected upwards during thinning and therefore transport more heat through the cooling lithosphere than in the case, where the thermal boundary layer is assumed at a constant depth (e.g. the McKenzie model). Furthermore, the presence of sublithospheric convection beneath a sedimentary basin also results in dynamic vertical movements in the order of 10-100 m at a time scale of 2-20 Myr. Such movements are a possible cause of stratigraphic cycles that are otherwise commonly attributed to glacial eustasy.

2. LIP formation as a consequence of rift-induced mantle overturn. Here, I present modeling results that provide yet another mechanism for anomalous magmatism such as that associated with LIP formation in the embryonic North Atlantic during the Paleocene/Eocene as well the protracted high degree of activity that followed and continues in Iceland today. Using thermomechanical modeling we show that the following line of events are physically plausible: Rifting along an old Caledonian suture with a mafic lower crust partly in eclogite facies may lead to rapid delamination of the mantle lithosphere. This again leads to a rapid, but not particularly voluminous phase of magmatism. The detached mantle lithosphere rapidly sinks into the lower mantle and induces a return flow due off lower mantle material into the upper mantle. If the lower mantle has elevated potential temperature relative to the upper mantle, the return flow is amplified by thermal buoyancy, and a partial overturn of the mantle is initiated. Within 6 Myr from onset of lithosphere delamination, hot lower mantle material rises to the base of the rifts and starts melting. This melt phase leads to fast production of large volumes of melt consistent with North atlantic LIP formation at 55 Ma. After this, lower mantle upwelling and melt productivity decreases, but still continues in the following time until the present, consistent with the long-lived melt anomaly of the North Atlantic and present day Iceland.

3. Syn-convergent extension and UHP exhumation in the D’Entrecasteaux Islands of the Woodlark Basin. In this part of the talk, I will briefly present a thermomechanical model for the exhumation of Ultra-High-Pressure (UHP) rocks in the young core complexes of the D’Entrecasteaux Islands that are situated just west of the oceanic Woodlark Basin. Plate kinematic constraints of the latter indicate that UHP exhumation was associated with North/South extension of at least 100 km since 4 Ma. This had led some authors to propose a mechanism of reverse subduction (eduction), where the Northern margin of the Australian continental plate was first subducted and then exhumed by normal motion along the former subduction plane. In the other hand, structural fabrics of the exhumed UHP units imply diapiric exhumation which however is at odds with the amount of extension required from plate kinematics. The model I here present shows that exhumation by reverse subduction led to ductile extrusion and diapir-like fabric, even though exhumation was largely driven by >100 km extension. The model further shows that despite continued convergence between the Australian and the Pacific plates, coeval extension due to the opening of the Woodlark Basin could have been accommodated by subduction further North in the New Britain trench, as subduction of the Australian margin was reversed.

References

Petersen, K., Armitage, J., Nielsen, S. & Thybo, H. Mantle temperature as a control on the time scale of thermal evolution of extensional basins. Earth and Planetary Science Letters 409, 61-70, (2015).

Petersen, K. D., Nielsen, S. B., Clausen, O. R., Stephenson, R. & Gerya, T. Small-Scale Mantle Convection Produces Stratigraphic Sequences in Sedimentary Basins. Science 329, 827-830, (2010).

Petersen, K. D., Schiffer, C. & Nagel, T. LIP formation and protracted lower mantle upwelling induced by rifting and delamination. Scientific Reports 8, 16578, (2018).

Petersen, K. D. & Buck, W. R. Eduction, extension, and exhumation of ultrahigh‐pressure rocks in metamorphic core complexes due to subduction initiation. Geochemistry, Geophysics, Geosystems 16, 2564-2581, (2015).

23 novembre 2018

Séminaire de Julien Alleon, Dept. of Earth, Atmospheric, and Planetary Science, MIT

Préservation chimique des plus anciennes traces de vie sur Terre

 

22 novembre 2018

Séminaire de Stéphane Molliex, Laboratoire Géosciences Océans, CNRS, UBO, UBS

Les nucléides cosmogéniques: un outil pour quantifier l’érosion quaternaire dans l’approche «source to sink».

14h30, Salle de séminaire LGO

16 novembre 2018

Séminaire de Mathieu Rosbapé

Interactions entre magmatisme et hydrothermalisme de haute température sous les dorsales océaniques: l’exemple de l’ophiolite d’Oman

Une part importante des échanges chimiques et thermiques entre les enveloppes internes et externes de la Terre (manteau, croûte, hydrosphère) se déroule au niveau des dorsales océaniques, en particulier à la transition entre manteau et croûte océanique (MTZ pour Moho Transition Zone). La MTZ agit comme un filtre réactif : les réactions entre fluides et/ou magmas issus de la fusion partielle profonde du manteau et harzburgites résiduelles amènent à la resorbtion des orthopyroxènes restitiques et à la formation d’un résidu essentiellement formé d’olivine, les dunites. Cette réaction s’accompagne d’une augmentation de porosité qui favorise la migration et l’extraction des magmas vers la croûte.

Les travaux récents portant sur l’étude de l’ophiolite d’Oman ont montré que des processus hydrothermaux de haute température, transitoires entre magmatisme anhydre et altération tardive de basse température, ont affecté la MTZ et la partie supérieure de la section mantellique (Python et al., 2007, 2011; Akizawa et al., 2016; Rospabé et al., 2017). L’occurrence d’orthopyroxène et d’amphibole parmi les phases minérales d’imprégnation au sein de la MTZ (i.e. produits de cristallisation fractionnée à partir des magmas interstitiels) renseigne sur le caractère hydraté en enrichi en silice du ou des magma(s) parent(s), contrastant avec le caractère MORB exprimé dans les cumulats crustaux et les filons mantelliques. La distribution spatiale de ces phases, plus abondantes à l’approche de la base de la croûte, suggère davantage la cristallisation à partir d’un magma hybride entre MORB ascendant et front hydrothermal descendant plutôt que d’un magma issu de la fusion partielle d’une source profonde hydratée (Rospabé et al., 2017 ; Rospabé et al., 2018b). La corrélation entre variation spatiale des signatures géochimiques et la présence de failles ou fractures synmagmatiques, qui peuvent être reliées au processus d’accrétion à la dorsale, renseigne sur la mise en place précoce de ces accidents tectoniques, et sur leur influence importante sur la structuration pétrologique et géochimique de la MTZ (Rospabé et al., soumis). Le forage des carottes CM1A et CM2B (400 + 300 m) à travers la MTZ de l’ophiolite d’Oman (Oman Drilling Project) a pour objectif une résolution d’échantillonnage jamais atteinte à ce jour concernant cette interface, ainsi qu’une meilleure quantification des flux élémentaires, notamment du Cr, engagés lors de tels processus d’hybridation.