Giant magmatic water reservoirs at mid-crustal depth inferred from electrical conductivity and the growth of the continental crust

International audienceThe formation of the continental crust at subduction zones involves the differentiation of hydrous mantle-derived magmas through a combination of crystallization and crustal melting. However, understanding the mechanisms by which differentiation occurs at depth is hampered by the inaccessibility of the deep crust in active continental arcs. Here we report new high-pressure electrical conductivity and petrological experiments on hydrated andesitic melt from Uturuncu volcano on the Bolivian Altiplano. By applying our results to regional magnetotelluric data, we show that giant conductive anomalies at mid-crustal levels in several arcs are characterized by relatively low amounts of intergranular andesitic partial melts with unusually high dissolved water contents (≥8 wt.% H2O). Below Uturuncu, the Altiplano-Puna Magma Body (APMB) displays an electrical conductivity that requires high water content (up to 10 wt.%) dissolved in the melt based on crystal-liquid equilibria and melt H2O solubility experiments. Such a super-hydrous andesitic melt must constitute about 10% of the APMB, the remaining 90% being a combination of magmatic cumulates and older crustal rocks. The crustal ponding level of these andesites at around 6 kbar pressure implies that on ascent through the crust hydrous magmas reach their water saturation pressure in the mid-crust, resulting in decompression-induced crystallization that increases magma viscosity and in turn leads to preferential stalling and differentiation. Similar high conductivity features are observed beneath the Cascades volcanic arc and Taupo Volcanic Zone. This suggests that large amounts of water in super-hydrous andesitic magmas could be a common feature of active continental arcs and may illustrate a key step in the structure and growth of the continental crust.One Sentence Summary: Geophysical, laboratory conductivity and petrological experiments reveal that deep electrical conductivity anomalies beneath the Central Andes, Cascades and Taupo Volcanic Zone image the ponding of super-hydrous andesitic melts which contributes to the growth of continental crust

Late magmatic strain localization: An experimental approach

International audienceThe transfer and emplacement dynamics of highly crystallised (φ >∼0.6) magmas are mostly dependent on the interconnected crystal-framework controlling the tortuosity of the residual melt flow and capable of transmitting deviatoric high stresses. Crystal fraction, size distribution, and strain rate control the development of crystal fabrics and strain localisation. How localised structures potentially promote the transfer of residual gas and melts in hardly moveable late magmatic mushes is a critical question that remains largely undocumented. We present high temperature (650˚ to 750˚ C) and high pressure (P=300 MPa) deformation experiments on both natural and synthetic dioritic, hydrous (3 wt% H2O) suspensions with markedly euhedral anisometric crystals (0.43> φ >1) submitted to simple shear. Quantitative structural analysis, including grain shape fabric, clusters formation, and shear zones geometry, were performed by 2D SEM imagery and by 3D high resolution X-ray Computed Tomography. Crystal fraction φ ∼0.75, a modification of the mechanical behaviour is evidenced with a nearly solid like behaviour associated with a dramatic change of the microstructures. Strain localisation leads to the development of S/C' like structures progressively replaced by tensions gashes and Riedel cataclastic shears when approaching the full crystallization stage. Residual melt, and fluids when present, migrate from compressive regions to transtensive zones favouring potential outgassing and residual melt escape at near-solidus conditions. Our results stress the importance of strain localisation structures for residual fluids and melt transfers in magmatic suspensions submitted to high stresses

Late magmatic strain localization: An experimental approach

International audienceThe transfer and emplacement dynamics of highly crystallised (φ >∼0.6) magmas are mostly dependent on the interconnected crystal-framework controlling the tortuosity of the residual melt flow and capable of transmitting deviatoric high stresses. Crystal fraction, size distribution, and strain rate control the development of crystal fabrics and strain localisation. How localised structures potentially promote the transfer of residual gas and melts in hardly moveable late magmatic mushes is a critical question that remains largely undocumented. We present high temperature (650˚ to 750˚ C) and high pressure (P=300 MPa) deformation experiments on both natural and synthetic dioritic, hydrous (3 wt% H2O) suspensions with markedly euhedral anisometric crystals (0.43> φ >1) submitted to simple shear. Quantitative structural analysis, including grain shape fabric, clusters formation, and shear zones geometry, were performed by 2D SEM imagery and by 3D high resolution X-ray Computed Tomography. Crystal fraction φ ∼0.75, a modification of the mechanical behaviour is evidenced with a nearly solid like behaviour associated with a dramatic change of the microstructures. Strain localisation leads to the development of S/C' like structures progressively replaced by tensions gashes and Riedel cataclastic shears when approaching the full crystallization stage. Residual melt, and fluids when present, migrate from compressive regions to transtensive zones favouring potential outgassing and residual melt escape at near-solidus conditions. Our results stress the importance of strain localisation structures for residual fluids and melt transfers in magmatic suspensions submitted to high stresses

Late magmatic strain localization: An experimental approach

International audienceThe transfer and emplacement dynamics of highly crystallised (φ >∼0.6) magmas are mostly dependent on the interconnected crystal-framework controlling the tortuosity of the residual melt flow and capable of transmitting deviatoric high stresses. Crystal fraction, size distribution, and strain rate control the development of crystal fabrics and strain localisation. How localised structures potentially promote the transfer of residual gas and melts in hardly moveable late magmatic mushes is a critical question that remains largely undocumented. We present high temperature (650˚ to 750˚ C) and high pressure (P=300 MPa) deformation experiments on both natural and synthetic dioritic, hydrous (3 wt% H2O) suspensions with markedly euhedral anisometric crystals (0.43> φ >1) submitted to simple shear. Quantitative structural analysis, including grain shape fabric, clusters formation, and shear zones geometry, were performed by 2D SEM imagery and by 3D high resolution X-ray Computed Tomography. Crystal fraction φ ∼0.75, a modification of the mechanical behaviour is evidenced with a nearly solid like behaviour associated with a dramatic change of the microstructures. Strain localisation leads to the development of S/C' like structures progressively replaced by tensions gashes and Riedel cataclastic shears when approaching the full crystallization stage. Residual melt, and fluids when present, migrate from compressive regions to transtensive zones favouring potential outgassing and residual melt escape at near-solidus conditions. Our results stress the importance of strain localisation structures for residual fluids and melt transfers in magmatic suspensions submitted to high stresses

Late magmatic strain localization: An experimental approach

International audienceThe transfer and emplacement dynamics of highly crystallised (φ >∼0.6) magmas are mostly dependent on the interconnected crystal-framework controlling the tortuosity of the residual melt flow and capable of transmitting deviatoric high stresses. Crystal fraction, size distribution, and strain rate control the development of crystal fabrics and strain localisation. How localised structures potentially promote the transfer of residual gas and melts in hardly moveable late magmatic mushes is a critical question that remains largely undocumented. We present high temperature (650˚ to 750˚ C) and high pressure (P=300 MPa) deformation experiments on both natural and synthetic dioritic, hydrous (3 wt% H2O) suspensions with markedly euhedral anisometric crystals (0.43> φ >1) submitted to simple shear. Quantitative structural analysis, including grain shape fabric, clusters formation, and shear zones geometry, were performed by 2D SEM imagery and by 3D high resolution X-ray Computed Tomography. Crystal fraction φ ∼0.75, a modification of the mechanical behaviour is evidenced with a nearly solid like behaviour associated with a dramatic change of the microstructures. Strain localisation leads to the development of S/C' like structures progressively replaced by tensions gashes and Riedel cataclastic shears when approaching the full crystallization stage. Residual melt, and fluids when present, migrate from compressive regions to transtensive zones favouring potential outgassing and residual melt escape at near-solidus conditions. Our results stress the importance of strain localisation structures for residual fluids and melt transfers in magmatic suspensions submitted to high stresses

On the conditions of mafic-felsic magmas mixing and its bearing on andesite production in the crust

International audienceMixing between magmas is thought to affect a variety of processes, from the growth of continental crust to the triggering of volcanic eruptions, but its thermophysical viability remains unclear. Here, using high pressure mixing experiments, we show that mixing only occurs at low viscosity contrast, when the touching crystal network of the more viscous magma breaks down. Using thermal calculations, we show that hybridization requires injection of high proportions of the replenishing magma during short periods. The incremental growth of crustal reservoirs limits the production of hybrids to the waning stage of pluton assembly and to small portions of it. Large scale mixing appears to be more efficient at lower crustal conditions, but requires higher proportions of mafic melt, hence produces hybrids more mafic than in shallow reservoirs. Altogether, hybrid arc magmas correspond to periods of enhanced magma production at depth

Evidence for a persistent magma reservoir with large melt content beneath an apparently extinct volcano

ISSN:0012-821XISSN:1385-013