17 research outputs found
Experimental evidence for the shallow production of phonolitic magmas at Mayotte
Since May 2018 till the end of 2021, Mayotte island has been the locus of a major submarine volcanic eruption characterized by the offshore emission of more than 6.5 km of basanitic magma. The eruption occurred along a WNWâESE trending submarine ridge on the east flank of the island where, in addition, several seemingly recent phonolitic bodies were also identified close to the island. To define realistic scenarios of magma ascent and potentially predict the style of an upcoming event, it is crucial to have a precise understanding on the plumbing system operating below volcanoes. The putative relationships between basanites emitted by the new volcano and these recent phonolites have been experimentally explored by performing crystallization experiments on a representative basanite over a large range of pressures (up to 400 MPa). The results show that the crystallization of basanite at crustal levels (12â15 km) yields a phonolitic residual liquid containing up to 3â4 wt% after 65 wt% of an assemblage of olivineplagioclaseamphiboleclinopyroxenebiotitemagnetiteilmeniteapatite. The final iron content of the residual phonolitic liquids is strongly controlled by the depth/pressure of fractionation. Fe-rich phonolites from the submarine ridge are produced at 6â8 km depth, while a shallower differentiation (4â5 km) results in the production of liquids with trachyteâbenmoreite affinities. If the fractionation process occurs at depths higher than 8 km, the resulting phonolitic melts are progressively enriched in â but depleted in FeO*, ie unlike those erupted. We therefore conclude that phonolitic magma production and storage at Mayotte is a rather shallow process.Depuis mai 2018 et jusquâĂ la fin de lâannĂ©e 2021, lâĂźle de Mayotte a Ă©tĂ© le scenario dâune Ă©ruption volcanique sous-marine majeure caractĂ©risĂ©e par lâĂ©mission en mer de plus de 6,5 km de magma basanitique. LâĂ©ruption sâest produite le long dâune ride sous-marine orientĂ©e ONOâESE sur le flanc est de lâĂźle oĂč, entre autre, plusieurs corps phonolitiques dâ aspect rĂ©cent ont Ă©galement Ă©tĂ© identifiĂ©s Ă proximitĂ© de lâĂźle. Pour dĂ©finir des scĂ©narios rĂ©alistes dâascension du magma et potentiellement prĂ©voir le style dâun Ă©vĂ©nement Ă venir, il est crucial dâavoir une comprĂ©hension prĂ©cise du systĂšme de plomberie magmatique opĂ©rant sous les volcans. Les relations gĂ©nĂ©tiques potentielles entre les basanites Ă©mises par le nouveau volcan et ces phonolites rĂ©centes ont Ă©tĂ© explorĂ©es expĂ©rimentalement en effectuant des expĂ©riences de cristallisation sur une basanite reprĂ©sentative, et ce sur une large gamme de pressions (jusquâĂ 400 MPa). Les rĂ©sultats montrent que la cristallisation de la basanite Ă des profondeurs crustales (12â15 km) produit un liquide rĂ©siduel phonolitique contenant jusquâĂ 3â4 % en poids (pd.%) de HO, aprĂšs la prĂ©cipitation dâau moins 65 pd.% dâun assemblage dâolivineplagioclaseamphiboleclinopyroxĂšnebiotitemagnĂ©titeilmĂ©niteapatite. La teneur finale en fer des liquides phonolitiques rĂ©siduels est fortement contrĂŽlĂ©e par la profondeur/pression de cristallisation. Les phonolites riches en fer de la dorsale sous-marine sont produites Ă 6â8 km de profondeur, tandis quâune diffĂ©renciation moins profonde (4â5 km) entraĂźne la production de liquides Ă affinitĂ©s trachyteâbenmoreite. Si le processus de fractionnement se produit Ă des profondeurs supĂ©rieures Ă 8 km, les liquides phonolitiques rĂ©sultants sont progressivement enrichis en SiOâAlO mais appauvris en FeO*, câest-Ă -dire diffĂ©rents des phonolites naturelles. Nous concluons donc que la production et le stockage de magma phonolitique Ă Mayotte est un processus plutĂŽt superficiel
Strain localization in oceanic detachment faults : the extreme case of a magma-starved slow spreading ridge
La partie orientale de la dorsale sud-ouest indienne est particuliĂšrement pauvre en apports magmatiques et constitue de ce fait un laboratoire naturel pour eÌtudier l'accreÌtion oceÌanique dans une configuration extreÌme, pour laquelle la divergence des plaques est presque compleÌtement accommodeÌe par le jeu de failles normales aÌ grand rejet. Ces failles, eÌgalement appeleÌes failles de deÌtachement, exhument des peÌridotites serpentiniseÌes d'origine mantellaire sur le plancher oceÌanique. Les meÌcanismes de deÌformation des roches mantelliques au cours de leur exhumation, avant leur passage dans le domaine de circulation de fluides hydrothermaux, sont peu connus. Nous avons eÌtudieÌ les textures de deÌformation et les meÌcanismes de localisation, ainsi que les assemblages mineÌralogiques associeÌs, dans un corpus de presque 400 eÌchantillons de peÌridotites serpentiniseÌes dragueÌes pendant la campagne SMoothSeafloor (2010). Tous les eÌchantillons preÌsentent aÌ des degreÌs variables une deÌformation heÌteÌrogeÌne combinant des meÌcanismes de deÌformation cassants (fractures, kinks) et plastiques (extinction ondulante, sous-joints, recristallisation dynamique). Lâune des manifestations de cette deÌformation est la recristallisation syntectonique dâolivines et de pyroxeÌnes au sein de kinks et de microfractures. Ces textures reÌsultent de conditions de hautes contraintes et hautes tempeÌratures en base de lithospheÌre, dans la zone dâenracinement des deÌtachements. Ces observations microstructurales ont eÌteÌ inteÌgreÌes dans un modeÌle thermomeÌcanique 2D du domaine axial des dorsales lentes, qui explore deux meÌcanismes d'affaiblissement observeÌs dans les eÌchantillons: la serpentinisation (800°C). La combinaison de ces deux meÌcanismes permet la reproduction de failles de deÌtachement dans une lithospheÌre eÌpaisse (20-25 km), avec un relief et une freÌquence qui sont consistantes avec celles observeÌes sur notre zone d'eÌtude.The Southwest Indian Ridge in its eastern part has particularly low magmatic inputs and is therefore a natural laboratory to study oceanic accretion in an extreme configuration for which the divergence of the plates is almost completely accommodated by large offset normal faults, also called detachment faults. These faults exhume mantle-derived peridotites. Very little is known about the deformation mechanisms that operate at the lower levels of the lithosphere, prior to serpentinization.We studied the deformation textures, analyze strain localization mechanisms, as well as mineralogical assemblages associated with these mechanisms from a corpus of almost 400 samples of serpentinized peridotites dredged during the SMoothSeafloor cruise (2010). All samples display heterogeneous deformation combining brittle (fracturation, kinks) and plastic mechanisms (undulose extinction, subgrain boundaries, dynamic recrystallization). For example, microfracturation and kinks are locally accompanied by partial recrystallization of the primary minerals into fine-grained aggregates along orthopyroxene grains. These textures result from high stress and high temperature conditions at the base of the lithosphere, where the root of the active detachment.We use these microstructural observations to constrain a 2D thermomechanical model of lithospheric extension, in which we explore two weakening mechanisms seen in the samples: serpentinization (temperatures 800°C). Combining the two, we develop detachment faults in a thick lithospheric context (20-25 km), with fault topography and offsets that are consistent with geological observations in the eastern SWIR
Modes de localisation des dĂ©formations dans les Failles de dĂ©tachement ocĂ©aniques : le cas extrĂȘme d'une dorsale lente Ă trĂšs faible budget magmatique
The Southwest Indian Ridge in its eastern part has particularly low magmatic inputs and is therefore a natural laboratory to study oceanic accretion in an extreme configuration for which the divergence of the plates is almost completely accommodated by large offset normal faults, also called detachment faults. These faults exhume mantle-derived peridotites. Very little is known about the deformation mechanisms that operate at the lower levels of the lithosphere, prior to serpentinization.We studied the deformation textures, analyze strain localization mechanisms, as well as mineralogical assemblages associated with these mechanisms from a corpus of almost 400 samples of serpentinized peridotites dredged during the SMoothSeafloor cruise (2010). All samples display heterogeneous deformation combining brittle (fracturation, kinks) and plastic mechanisms (undulose extinction, subgrain boundaries, dynamic recrystallization). For example, microfracturation and kinks are locally accompanied by partial recrystallization of the primary minerals into fine-grained aggregates along orthopyroxene grains. These textures result from high stress and high temperature conditions at the base of the lithosphere, where the root of the active detachment.We use these microstructural observations to constrain a 2D thermomechanical model of lithospheric extension, in which we explore two weakening mechanisms seen in the samples: serpentinization (temperatures 800°C). Combining the two, we develop detachment faults in a thick lithospheric context (20-25 km), with fault topography and offsets that are consistent with geological observations in the eastern SWIR.La partie orientale de la dorsale sud-ouest indienne est particuliĂšrement pauvre en apports magmatiques et constitue de ce fait un laboratoire naturel pour eÌtudier l'accreÌtion oceÌanique dans une configuration extreÌme, pour laquelle la divergence des plaques est presque compleÌtement accommodeÌe par le jeu de failles normales aÌ grand rejet. Ces failles, eÌgalement appeleÌes failles de deÌtachement, exhument des peÌridotites serpentiniseÌes d'origine mantellaire sur le plancher oceÌanique. Les meÌcanismes de deÌformation des roches mantelliques au cours de leur exhumation, avant leur passage dans le domaine de circulation de fluides hydrothermaux, sont peu connus. Nous avons eÌtudieÌ les textures de deÌformation et les meÌcanismes de localisation, ainsi que les assemblages mineÌralogiques associeÌs, dans un corpus de presque 400 eÌchantillons de peÌridotites serpentiniseÌes dragueÌes pendant la campagne SMoothSeafloor (2010). Tous les eÌchantillons preÌsentent aÌ des degreÌs variables une deÌformation heÌteÌrogeÌne combinant des meÌcanismes de deÌformation cassants (fractures, kinks) et plastiques (extinction ondulante, sous-joints, recristallisation dynamique). Lâune des manifestations de cette deÌformation est la recristallisation syntectonique dâolivines et de pyroxeÌnes au sein de kinks et de microfractures. Ces textures reÌsultent de conditions de hautes contraintes et hautes tempeÌratures en base de lithospheÌre, dans la zone dâenracinement des deÌtachements. Ces observations microstructurales ont eÌteÌ inteÌgreÌes dans un modeÌle thermomeÌcanique 2D du domaine axial des dorsales lentes, qui explore deux meÌcanismes d'affaiblissement observeÌs dans les eÌchantillons: la serpentinisation (800°C). La combinaison de ces deux meÌcanismes permet la reproduction de failles de deÌtachement dans une lithospheÌre eÌpaisse (20-25 km), avec un relief et une freÌquence qui sont consistantes avec celles observeÌes sur notre zone d'eÌtude
Erratum to "How do detachment faults form at ultraslow mid-ocean ridges in a thick axial lithosphere?" [Earth Planet. Sci. Lett. 533 (2020) 116048]
The Publisher regrets that an error occurred in the representation of Fig. 8, this should appear as follows
How do detachment faults form at ultraslow mid-ocean ridges in a thick axial lithosphere?
International audienc
On spreading modes and magma supply at slow and ultraslow mid-ocean ridges
The ultraslow eastern Southwest Indian Ridge (SWIR) offers an opportunity to study the effect of magma supply on an ultraslow mid-ocean ridge starting from quasi-melt-free detachment-dominated spreading, and transitioning to volcanic spreading as one nears prominent axial volcanos. Detachments in the quasi melt-free mode extend along-axis 60 to 95 km and have a lifetime of 0.6 to 1.5 myrs. They cut into their predecessor's footwall with an opposite polarity, causing part of the footwall lithosphere to experience further deformation, hydrothermal alteration, sparse magmatism and possibly thermal rejuvenation, in a hanging wall position. The accretion of the oceanic lithosphere in this context therefore occurs in two distinct stages over the lifetime of two successive detachment faults. We examine the transition from this nearly amagmatic detachment-dominated mode to the more common volcanic mode of spreading, showing that it occurs along-axis over distances <= 30 km. It involves a significant thinning of the axial lithosphere and a gradual decrease of the amount of tectonic displacement on faults, as the magmatic contribution to the divergence of the two plates increases. We develop a conceptual model of this transition, in which magma plays a double role: it fills the space between the diverging plates, thus reducing the need for displacement along faults, and it modifies the thermal state and the rheology of the plate boundary, affecting its thickness and its tectonic response to plate divergence. Based on a comparison of the ultraslow eastern SWIR, with the faster spreading Mid-Atlantic Ridge, we show that the activation of the volcanic, or of the detachment-dominated modes of spreading is connected with the volume of magma supplied per increment of plate separation, over a range of axial lithosphere thickness, and therefore over a range of the M ratio defined by (Buck et al., 2005) as the relative contribution of magma and faults to plate divergence (M is smaller, for a given volume of melt per increment of plate separation, if the plate is thicker). We therefore propose that M does not fully explain the variability in faulting styles observed at slow and ultraslow ridges and propose that rheological changes induced by magma also play a key role (melt itself is weak, hydrothermally altered gabbro-peridotite mixtures are weak, and melt heat sustains more vigorous hydrothermal circulation), resulting in contrasted potentials for strain localization, footwall flexure on faults and the development of detachment faults. (C) 2019 Elsevier B.V. All rights reserved
Fluid-assisted grain size reduction leads to strain localization in oceanic transform faults
Oceanic Transform Faults are major plate boundaries representing the most seismogenic part of the mid ocean ridge system. Nonetheless, their structure and deformation mechanisms at depth are largely unknown due to rare exposures of deep sections. Here we study the mineral fabric of deformed mantle peridotites - ultramafic mylonites - collected from the transpressive AtobĂĄ ridge, along the northern fault of the St. Paul transform system in the Equatorial Atlantic Ocean. We show that, at pressure and temperature conditions of the lower oceanic lithosphere, the dominant deformation mechanism is fluid-assisted dissolution-precipitation creep. Grain size reduction during deformation is enhanced by dissolution of coarser pyroxene grains in presence of fluid and contextual precipitation of small interstitial ones, leading to strain localization at lower stresses than dislocation creep. This mechanism potentially represents the dominant weakening factor in the oceanic lithosphere and a main driver for the onset and maintenance of oceanic transform faults
Experimental evidence for the shallow production of phonolitic magmas at Mayotte
Since May 2018 till the end of 2021, Mayotte island has been the locus of a major submarine volcanic eruption characterized by the offshore emission of more than 6.5 km of basanitic magma. The eruption occurred along a WNWâESE trending submarine ridge on the east flank of the island where, in addition, several seemingly recent phonolitic bodies were also identified close to the island. To define realistic scenarios of magma ascent and potentially predict the style of an upcoming event, it is crucial to have a precise understanding on the plumbing system operating below volcanoes. The putative relationships between basanites emitted by the new volcano and these recent phonolites have been experimentally explored by performing crystallization experiments on a representative basanite over a large range of pressures (up to 400 MPa). The results show that the crystallization of basanite at crustal levels (12â15 km) yields a phonolitic residual liquid containing up to 3â4 wt% after 65 wt% of an assemblage of olivineplagioclaseamphiboleclinopyroxenebiotitemagnetiteilmeniteapatite. The final iron content of the residual phonolitic liquids is strongly controlled by the depth/pressure of fractionation. Fe-rich phonolites from the submarine ridge are produced at 6â8 km depth, while a shallower differentiation (4â5 km) results in the production of liquids with trachyteâbenmoreite affinities. If the fractionation process occurs at depths higher than 8 km, the resulting phonolitic melts are progressively enriched in â but depleted in FeO*, ie unlike those erupted. We therefore conclude that phonolitic magma production and storage at Mayotte is a rather shallow process.Depuis mai 2018 et jusquâĂ la fin de lâannĂ©e 2021, lâĂźle de Mayotte a Ă©tĂ© le scenario dâune Ă©ruption volcanique sous-marine majeure caractĂ©risĂ©e par lâĂ©mission en mer de plus de 6,5 km de magma basanitique. LâĂ©ruption sâest produite le long dâune ride sous-marine orientĂ©e ONOâESE sur le flanc est de lâĂźle oĂč, entre autre, plusieurs corps phonolitiques dâ aspect rĂ©cent ont Ă©galement Ă©tĂ© identifiĂ©s Ă proximitĂ© de lâĂźle. Pour dĂ©finir des scĂ©narios rĂ©alistes dâascension du magma et potentiellement prĂ©voir le style dâun Ă©vĂ©nement Ă venir, il est crucial dâavoir une comprĂ©hension prĂ©cise du systĂšme de plomberie magmatique opĂ©rant sous les volcans. Les relations gĂ©nĂ©tiques potentielles entre les basanites Ă©mises par le nouveau volcan et ces phonolites rĂ©centes ont Ă©tĂ© explorĂ©es expĂ©rimentalement en effectuant des expĂ©riences de cristallisation sur une basanite reprĂ©sentative, et ce sur une large gamme de pressions (jusquâĂ 400 MPa). Les rĂ©sultats montrent que la cristallisation de la basanite Ă des profondeurs crustales (12â15 km) produit un liquide rĂ©siduel phonolitique contenant jusquâĂ 3â4 % en poids (pd.%) de HO, aprĂšs la prĂ©cipitation dâau moins 65 pd.% dâun assemblage dâolivineplagioclaseamphiboleclinopyroxĂšnebiotitemagnĂ©titeilmĂ©niteapatite. La teneur finale en fer des liquides phonolitiques rĂ©siduels est fortement contrĂŽlĂ©e par la profondeur/pression de cristallisation. Les phonolites riches en fer de la dorsale sous-marine sont produites Ă 6â8 km de profondeur, tandis quâune diffĂ©renciation moins profonde (4â5 km) entraĂźne la production de liquides Ă affinitĂ©s trachyteâbenmoreite. Si le processus de fractionnement se produit Ă des profondeurs supĂ©rieures Ă 8 km, les liquides phonolitiques rĂ©sultants sont progressivement enrichis en SiOâAlO mais appauvris en FeO*, câest-Ă -dire diffĂ©rents des phonolites naturelles. Nous concluons donc que la production et le stockage de magma phonolitique Ă Mayotte est un processus plutĂŽt superficiel
Experimental evidence for the shallow production of phonolitic magmas at Mayotte
International audienceSince May 2018 till the end of 2021, Mayotte island has been the locus of a major submarine volcanic eruption characterized by the offshore emission of more than 6.5 km3 of basanitic magma. The eruption occurred along a WNWâESE trending submarine ridge on the east flank of the island where, in addition, several seemingly recent phonolitic bodies were also identified close to the island. To define realistic scenarios of magma ascent and potentially predict the style of an upcoming event, it is crucial to have a precise understanding on the plumbing system operating below volcanoes. The putative relationships between basanites emitted by the new volcano and these recent phonolites have been experimentally explored by performing crystallization experiments on a representative basanite over a large range of pressures (up to 400 MPa). The results show that the crystallization of basanite at crustal levels (â€12â15 km) yields a phonolitic residual liquid containing up to 3â4 wt% H2O after â„65 wt% of an assemblage of olivine+plagioclase+amphibole+clinopyroxene+biotite+magnetite+ilmenite+apatite. The final iron content of the residual phonolitic liquids is strongly controlled by the depth/pressure of fractionation. Fe-rich phonolites from the submarine ridge are produced at 6â8 km depth, while a shallower differentiation (â€4â5 km) results in the production of liquids with trachyteâbenmoreite affinities. If the fractionation process occurs at depths higher than 8 km, the resulting phonolitic melts are progressively enriched in SiO2âAl2O3 but depleted in FeO*, ie unlike those erupted. We therefore conclude that phonolitic magma production and storage at Mayotte is a rather shallow process.Depuis mai 2018 et jusquâĂ la fin de lâannĂ©e 2021, lâĂźle de Mayotte a Ă©tĂ© le scenario dâune Ă©ruption volcanique sous-marine majeure caractĂ©risĂ©e par lâĂ©mission en mer de plus de 6,5 km3 de magma basanitique. LâĂ©ruption sâest produite le long dâune ride sous-marine orientĂ©e ONOâESE sur le flanc est de lâĂźle oĂč, entre autre, plusieurs corps phonolitiques dâ aspect rĂ©cent ont Ă©galement Ă©tĂ© identifiĂ©s Ă proximitĂ© de lâĂźle. Pour dĂ©finir des scĂ©narios rĂ©alistes dâascension du magma et potentiellement prĂ©voir le style dâun Ă©vĂ©nement Ă venir, il est crucial dâavoir une comprĂ©hension prĂ©cise du systĂšme de plomberie magmatique opĂ©rant sous les volcans. Les relations gĂ©nĂ©tiques potentielles entre les basanites Ă©mises par le nouveau volcan et ces phonolites rĂ©centes ont Ă©tĂ© explorĂ©es expĂ©rimentalement en effectuant des expĂ©riences de cristallisation sur une basanite reprĂ©sentative, et ce sur une large gamme de pressions (jusquâĂ 400 MPa). Les rĂ©sultats montrent que la cristallisation de la basanite Ă des profondeurs crustales (â€12â15 km) produit un liquide rĂ©siduel phonolitique contenant jusquâĂ 3â4 % en poids (pd.%) de H2O, aprĂšs la prĂ©cipitation dâau moins 65 pd.% dâun assemblage dâolivine+plagioclase+amphibole+clinopyroxĂšne+biotite+magnĂ©tite+ilmĂ©nite+apatite. La teneur finale en fer des liquides phonolitiques rĂ©siduels est fortement contrĂŽlĂ©e par la profondeur/pression de cristallisation. Les phonolites riches en fer de la dorsale sous-marine sont produites Ă 6â8 km de profondeur, tandis quâune diffĂ©renciation moins profonde (â€4â5 km) entraĂźne la production de liquides Ă affinitĂ©s trachyteâbenmoreite. Si le processus de fractionnement se produit Ă des profondeurs supĂ©rieures Ă 8 km, les liquides phonolitiques rĂ©sultants sont progressivement enrichis en SiO2âAl2O3 mais appauvris en FeO*, câest-Ă -dire diffĂ©rents des phonolites naturelles. Nous concluons donc que la production et le stockage de magma phonolitique Ă Mayotte est un processus plutĂŽt superficiel