Numerical models of the magmatic processes induced by slab breakoff

After the onset of continental collision, magmatism often persists for tens of millions of years, albeit with a different composition, in reduced volumes, and with a more episodic nature and more widespread spatial distribution, compared to normal arc magmatism. Kinematic modelling studies have suggested that slab breakoff can account for this post-collisional magmatism through the formation of a slab window and subsequent heating of the overriding plate and decompression melting of upwelling asthenosphere, particularly if breakoff occurs at depths shallower than the overriding plate. To constrain the nature of any melting and the geodynamic conditions required, we numerically model the collision of two continental plates following a period of oceanic subduction. A thermodynamic database is used to determine the (de)hydration reactions and occurrence of melt throughout this process. We investigate melting conditions within a parameter space designed to generate a wide range of breakoff depths, timings and collisional styles. Under most circumstances, slab breakoff occurs deeper than the depth extent of the overriding plate; too deep to generate any decompressional melting of dry upwelling asthenosphere or thermal perturbation within the overriding plate. Even if slab breakoff is very shallow, the hot mantle inflow into the slab window is not sustained long enough to sufficiently heat the hydrated overriding plate to cause significant magmatism. Instead, for relatively fast, shallow breakoff we observe melting of asthenosphere above the detached slab through the release of water from the tip of the heating detached slab. Melting of the subducted continental crust during necking and breakoff is a more common feature and may be a more reliable indicator of the occurrence of breakoff. We suggest that magmatism from slab breakoff alone is unable to explain several of the characteristics of post-collisional magmatism, and that additional geodynamical processes need to be considered when interpreting magmatic observations

Zinc isotope evidence for sulfate-rich fluid transfer across subduction zones

Subduction zones modulate the chemical evolution of the Earth?s mantle. Water and volatile elements in the slab are released as fluids into the mantle wedge and this process is widely considered to result in the oxidation of the sub-arc mantle. However, the chemical composition and speciation of these fluids, which is critical for the mobility of economically important elements, remain poorly constrained. Sulfur has the potential to act both as oxidizing agent and transport medium. Here we use zinc stable isotopes ( \ensuremathδ 66 Zn) in subducted Alpine serpentinites to decipher the chemical properties of slab- derived fluids. We show that the progressive decrease in \ensuremathδ 66Zn with metamorphic grade is correlated with a decrease in sulfur content. As existing theoretical work predicts that Zn-SO42- complexes preferentially incorporate heavy \ensuremathδ 66Zn, our results provide strong evidence for the release of oxidized, sulfate-rich, slab serpentinite-derived fluids to the mantle wedge.This work was supported by an ERC Starting Grant (HabitablePlanet; 306655) and a NERC Deep Volatiles Consortium Grant (NE/M0003/1) awarded to H.W. H.W. and P.B. also acknowledge salary support from a NERC Advanced Fellowship (NE/F014295/2) and ERC Starting Grant (279828, MASE), respectivel

The behavior of iron and zinc stable isotopes accompanying the subduction of mafic oceanic crust: A case study from Western Alpine ophiolites

Arc lavas display elevated Fe3+/ΣFe ratios relative to MORB. One mechanism to explain this is the mobilization and transfer of oxidized or oxidizing components from the subducting slab to the mantle wedge. Here we use iron and zinc isotopes, which are fractionated upon complexation by sulfide, chloride, and carbonate ligands, to remark on the chemistry and oxidation state of fluids released during prograde metamorphism of subducted oceanic crust. We present data for metagabbros and metabasalts from the Chenaillet massif, Queyras complex, and the Zermatt-Saas ophiolite (Western European Alps), which have been metamorphosed at typical subduction zone P-T conditions and preserve their prograde metamorphic history. There is no systematic, detectable fractionation of either Fe or Zn isotopes across metamorphic facies, rather the isotope composition of the eclogites overlaps with published data for MORB. The lack of resolvable Fe isotope fractionation with increasing prograde metamorphism likely reflects the mass balance of the system, and in this scenario Fe mobility is not traceable with Fe isotopes. Given that Zn isotopes are fractionated by S-bearing and C-bearing fluids, this suggests that relatively small amounts of Zn are mobilized from the mafic lithologies in within these types of dehydration fluids. Conversely, metagabbros from the Queyras that are in proximity to metasediments display a significant Fe isotope fractionation. The covariation of δ56Fe of these samples with selected fluid mobile elements suggests the infiltration of sediment derived fluids with an isotopically light signature during subduction

Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zones

Subducted serpentinites have the potential to control the exchange of volatile and redox sensitive elements (e.g., Fe, S, C, N) between the slab, the mantle wedge and the deep mantle. Here we examine the mobility of iron and zinc in serpentinite-derived fluids by using their stable isotopes (δ56Fe and δ66Zn) in high-pressure subducted meta-serpentinites from the Cerro del Almirez massif (Spain). This massif preserves a metamorphic front between antigorite (Atg-serpentinite) and antigorite-olivine-orthopyroxene (transitional lithologies) -bearing serpentinites, and chlorite-bearing harzburgite (Chl-harzburgite), displaying granofels, spinifex and fine-grained recrystallized textures. Those rocks were formed at eclogite facies conditions (1.6�1.9 GPa and 680�710 °C). The mean δ56Fe of all the Cerro del Almirez meta- serpentinites (+0.05 ± 0.01 �) is identical within an error to that of primitive mantle (+0.03 ± 0.03 �). A positive correlation between δ56Fe and indices of peridotite protolith fertility (e.g., Al2O3/SiO2) suggests that the δ56Fe values of Cerro del Almirez samples predominantly reflect protolith compositional variations, likely produced by prior episodes of melt extraction. In contrast, the Zn concentrations (Zn = 34�67 ppm) and isotope signatures (δ66Zn = +0.18 � +0.55 �) of the Cerro del Almirez samples show a broad range of values, distinct to those of the primitive mantle (Zn = 54 ppm; δ66Zn = +0.16 ± 0.06 �). The Atg- serpentinites (Zn = 34�46 ppm; δ66Zn = +0.23 ± 0.06 �) display similar Zn and δ66Zn values to those of slab serpentinites from other high-pressure meta-ophiolites. Both Zn and δ66Zn increase in transitional lithologies (Zn = 45�67 ppm; δ66Zn = +0.30 ± 0.06 �) and Chl-harzburgites with granofels (Zn = 38� 59 ppm; δ66Zn = +0.33 ± 0.04 �) or spinifex (Zn = 48�66 ppm; δ66Zn = +0.43 ± 0.09 �) textures. Importantly, Cerro del Almirez transitional lithologies and Chl-harzburgites display abnormally high Zn relative to abyssal peridotites and serpentinites (29�45 ppm) and a positive correlation exists between Zn and δ66Zn. This correlation is interpreted to reflect the mobilization of Zn by subduction zone fluids at high pressures and temperatures coupled with significant Zn stable isotope fractionation. An increase in Zn and δ66Zn from Atg-serpentinite to Chl-harzburgite is associated with an increase in U/Yb, Sr/Y, Ba/Ce and Rb/Ce, suggesting that both Zn and δ66Zn record the interaction of the transitional lithologies and the Chl- harzburgites with fluids that had equilibrated with metasedimentary rocks. Quantitative models show that metasediment derived fluids can have isotopically heavy Zn as a consequence of sediment carbonate dissolution and subsequent Zn complexation with carbonate species in the released fluids (e.g., ZnHCO3(H2O)5+ or ZnCO3(H2O)3). Our models further demonstrate that Zn complexation with reduced carbon species cannot produce fluids with heavy δ66Zn signature and hence explain the δ66Zn variations observed in the Chl-harzburgites. The most straightforward explanation for the heavy δ66Zn of the Cerro del Almirez samples is thus serpentinite dehydration accompanied by the open system infiltration of the massif by oxidized, carbonate-rich sediment-derived fluids released during prograde subduction-related metamorphism

Cadomian S-type granites as basement rocks of the Variscan belt (Massif Central, France): Implications for the crustal evolution of the north Gondwana margin

International audienceFrom the Neoproterozoic to the early Paleozoic, the northern Gondwana margin was sequentially shaped by the Cadomian accretionary and the Variscan collisional orogens which offers the opportunity to investigate the relative extent of crust production/reworking in both geodynamic settings. In the eastern part of the Variscan French Massif Central (FMC), the Velay Orthogneiss Formation (VOF) represents a consistent lithological unit of the pre-Variscan basement and comprises augen gneisses and leucogneisses. Such rocks constitute a unique record of the pre-Variscan magmatic history and bear critical information on the crustal evolution of the northern Gondwana margin.Here, we present whole–rock major and trace element compositions indicating that: (i) the VOF shows a remarkable geochemical homogeneity; (ii) the protolith of the augen gneisses corresponds to strongly peraluminous, “S-type” porphyritic granites originating from partial melting of an Ediacaran sedimentary sequence; (iii) the leucogneisses are former leucogranites generated by fractionation of the magma at the origin of the porphyritic granites; and (iv) the whole suite emplaced at shallow crustal levels (< 7 km). U–Pb LA–(MC–)ICP–MS analyses on zircon yielded similar emplacement ages of c. 542 Ma and a narrow range of εHf(t) clustering around 0 for the protoliths of both augen and leucogneisses. This homogeneous Hf isotope signature, notably uncommon for S-type granites, would originate from a sequential process of: (i) inherited zircon dissolution during melting and ascent in the crust due to Zr-undersaturated conditions, (ii) isotopic homogenization of the melt by advection and elemental/isotopic diffusion, followed by (iii) early saturation upon emplacement owing to rapid cooling at shallow crustal levels.We propose that partial melting of Ediacaran sediments occurred during inversion of a Cadomian back-arc basin and was promoted by the high thermal gradient typical of thinned crust domains. Therefore, the VOF and other Cadomian S-type granitoids from the northern Gondwana margin are indicative of substantial crust reworking away from any proper continental collision zone

Mantle Flow and Deforming Continents: From India-Asia Convergence to Pacific Subduction

The formation of mountain belts or rift zones is commonly attributed to interactions between plates along their boundaries, but the widely distributed deformation of Asia from Himalaya to the Japan Sea and other back-arc basins is difficult to reconcile with this notion. Through comparison of the tectonic and kinematic records of the last 50 Ma with seismic tomography and anisotropy models, we show that the closure of the former Tethys Ocean and the extensional deformation of East Asia can be best explained if the asthenospheric mantle transporting India northward, forming the Himalaya and the Tibetan Plateau, reaches East Asia where it overrides the westward flowing Pacific mantle and contributes to subduction dynamics, distributing extensional deformation over a 3,000-km wide region. This deep asthenospheric flow partly controls the compressional stresses transmitted through the continent-continent collision, driving crustal thickening below the Himalayas and Tibet and the propagation of strike-slip faults across Asian lithosphere further north and east, as well as with the lithospheric and crustal flow powered by slab retreat east of the collision zone below East and SE Asia. The main shortening direction in the deforming continent between the collision zone and the Pacific subduction zones may in this case be a proxy for the direction of flow in the asthenosphere underneath, which may become a useful tool for studying mantle flow in the distant past. Our model of the India-Asia collision emphasizes the role of asthenospheric flow underneath continents and may offer alternative ways of understanding tectonic processes

Des architectes urbanistes au cœur d’une lutte entre des écologies concurrentes

À l’heure d’une intensification des controverses socio-environnementales autour de projets d’aménagement, une nouvelle forme de militantisme anticapitaliste émerge depuis des « zones à défendre » (ZAD). Ces mobilisations remettent en cause l’intérêt général promu en résistant contre de « grands projets inutiles et imposés ». À l’heure des dérèglements multiples, à la fois locaux et globaux, de plus en plus de conflits opposent des écologistes à d’autres écologistes. Cette enquête propose de cartographier l’histoire, les régimes narratifs et les effets d’une mobilisation socio-environnementale : celle du « quartier libre des Lentillères » en lutte contre la deuxième tranche du projet d’« écocité Jardin des maraîchers » à Dijon. Après avoir précisé les histoires à l’origine des représentations concurrentes de ce sol contesté, selon la perception qu’en ont les acteurs, l’article se concentre sur les régimes narratifs de la mobilisation ainsi que sur sa réception auprès des professionnels de l’urbain. Cette mobilisation peut être considérée comme une mise au travail vivante de la pensée écologique, un laboratoire de l’habiter qui trouble la pratique des architectes urbanistes, mais peut aussi représenter une opportunité originale pour réinterroger l’activité de projet à l’heure du nouveau régime climatique.At a time when socio-environmental controversies around development projects are intensifying, new forms of activism are emerging from militant anti-capitalist occupations (“zones à défendre” or “ZAD”) in resistance to “large, ineffectual and decreed projects.” These new forms of activism and civic engagement, both local and global, have become increasingly divisive, often pitting environmentalists against one another. This essay proposes to examine the history, the narrative regimes and the unfolding consequences of a specific case of socio-environmental mobilization, the “quartier libre des Lentillères” on the outskirts of the French city of Dijon. The “quartier libre” is currently being threatened by the proposed “écocité Jardin des maraîchers,” an urban development project supported by the mayor of the city. After describing the competing viewpoints on this contested piece of land, the article focuses on the perspective of those mobilizing to oppose the development project as well as its reception by scholars and professionals working on urban questions. The form of environmental activism taking place in Dijon can be seen as a living actualization of environmental and ecological theories. While scenarios like this seem unsettling to the disciplines of architecture and urban planning, they may also represent opportunities for rethinking the role of architectural and urban interventions faced with the new realities of a heating climate

Des architectes urbanistes au cœur d’une lutte entre des écologies concurrentes

At a time when socio-environmental controversies around development projects are intensifying, new forms of activism are emerging from militant anti-capitalist occupations (“zones à défendre” or “ZAD”) in resistance to “large, ineffectual and decreed projects.” These new forms of activism and civic engagement, both local and global, have become increasingly divisive, often pitting environmentalists against one another. This essay proposes to examine the history, the narrative regimes and the unfolding consequences of a specific case of socio-environmental mobilization, the “quartier libre des Lentillères” on the outskirts of the French city of Dijon. The “quartier libre” is currently being threatened by the proposed “écocité Jardin des maraîchers,” an urban development project supported by the mayor of the city. After describing the competing viewpoints on this contested piece of land, the article focuses on the perspective of those mobilizing to oppose the development project as well as its reception by scholars and professionals working on urban questions. The form of environmental activism taking place in Dijon can be seen as a living actualization of environmental and ecological theories. While scenarios like this seem unsettling to the disciplines of architecture and urban planning, they may also represent opportunities for rethinking the role of architectural and urban interventions faced with the new realities of a heating climate