Experimental evidence supports mantle partial melting in the asthenosphere

The low-velocity zone (LVZ) is a persistent seismic feature in a broad range of geological contexts. It coincides in depth with the asthenosphere, a mantle region of lowered viscosity that may be essential to enabling plate motions. The LVZ has been proposed to originate from either partial melting or a change in the rheological properties of solid mantle minerals. The two scenarios imply drastically distinct physical and geochemical states, leading to fundamentally different conclusions on the dynamics of plate tectonics. We report in situ ultrasonic velocity measurements on a series of partially molten samples, composed of mixtures of olivine plus 0.1 to 4.0 volume % of basalt, under conditions relevant to the LVZ. Our measurements provide direct compressional (VP) and shear (VS) wave velocities and constrain attenuation as a function of melt fraction. Mantle partial melting appears to be a viable origin for the LVZ, for melt fractions as low as ∼0.2%. In contrast, the presence of volatile elements appears necessary to explaining the extremely high VP/VS values observed in some local areas. The presence of melt in LVZ could play a major role in the dynamics of plate tectonics, favoring the decoupling of the plate relative to the asthenosphere

Educational Fiscal Policy and Its Effects on How our Children Learn: Comparing Minnesota and Illinois

The study compares Illinois’ and Minnesota’s education fiscal policies. Illinois funds it’s education system mainly from the local level, whereas Minnesota funds it’s mainly from the state level. Thus, in Illinois, if there are discrepancies between household incomes in wealthier and poorer areas, the schools in wealthier areas would receive more money than those in poorer areas. Test scores are then compared. Illinois typically has lower scores than Minnesota. The conclusion is that Illinois’ policies are hindering their students’ learning, compared to Minnesota students, with some mixed results

Deformation Mechanisms, Microstructures, and Seismic Anisotropy of Wadsleyite in the Earth's Transition Zone

Wadsleyite is the dominant mineral of the upper portion of the Earth's mantle transition zone (MTZ). As such, understanding plastic deformation of wadsleyite is relevant for the interpretation of observations of seismic signals from this region in terms of mantle flow. Despite its relevance, however, the deformation mechanisms of wadsleyite and their effects on microstructures and anisotropy are still poorly understood. Here, we present the results of new deformation experiments on polycrystalline wadsleyite at temperatures of 1400–1770 K and pressures between 12.3 and 20.3 GPa in the laser-heated diamond anvil cell. We rely on multigrain X-ray crystallography to follow the evolution of individual grain orientations and extract lattice preferred orientations at the sample scale at different steps of the experiments. A comparison of experimental results of our work and the literature with polycrystal plasticity simulations, indicates that ⟨111⟩{101} is the most active slip system of dislocations in wadsleyite at all investigated conditions. Secondary slip systems such as [001](010), [100](001), and [100]{0kl}, however, play a critical role in the resulting microstructures and their activity depends on both temperature and water content, from which we extract an updated deformation map of wadsleyite at MTZ conditions. Lastly, we propose several seismic anisotropy models of the upper part of the MTZ, depending on temperature, geophysical context, and levels of hydration that will be useful for the interpretation of seismic signals from the MTZ in terms of mantle flow and water conten

Apatite solubility in carbonatitic liquids and trace element partitioning between apatite and carbonatite at high pressure

International audienc

Mesures simultanées de conductivité électrique et de vitesse d'ondes sismiques de matériaux géologiques partiellement fondus à haute pression et haute température : implication pour la fraction de liquide silicaté dans l'asthénosphère

International audienceUn débat important existe actuellement sur les capacités des systèmes partiellement fondus à augmenter la conductivité électrique et réduire la vitesse des ondes sismiques du matériel géologique, nécessaires pour expliquer la présence de zones d’anomalies géophysiques dans le manteau terrestre (zone de faible vitesse : LVZ de l’asthénosphère entre 70 et 200 Km [1],[2],[3]). De nombreuses hypothèses alternatives ont été proposées et sont basées sur des processus à l’état solide [4] telles la diffusion de l’hydrogène dans la structure cristalline. Ces théories suggèrent une asthénosphère sans fusion partielle. Alors que de fortes évidences physiques confirment la présence de fusion partielle du manteau asthénosphérique, comme la découverte de basaltes alcalins jeunes disposés sur la plaque océanique indienne beaucoup plus ancienne [5], les mesures expérimentales de conductivité électriques et de vitesse des ondes sismiques réalisées en laboratoire sur les systèmes partiellement fondus apportent des estimations antagonistes de la fraction de liquide silicaté impliqué dans l’asthénosphère. De plus, la source du désaccord entre les deux techniques géophysiques (conductivité électrique et vitesse sismique) demeure toujours mal contrainte.Utilisant les nouvelles techniques expérimentales développées sur la presse multi-enclumes au Laboratoire Magmas et Volcans de Clermont-Ferrand, des mesures couplées de conductivité électrique et de vitesse des ondes acoustiques ont été réalisées de façon in situ et simultanée sur le même échantillon. Nous avons alors étudié des systèmes partiellement fondus en utilisant des échantillons composés d’un mélange biphasé d’olivine de San Carlos (standard pétrologique) et d’un basalte de ride médio-océanique hydraté provenant de la ride Est-Pacifique (issu d’un forage) dans des conditions de haute pression : 2.5 GPa et de haute température: jusqu’à 1650 K

The Anomalous Seismic Behavior of Aqueous Fluids Released during Dehydration of Chlorite in Subduction Zones

International audienceDehydration and fluid circulation are integral parts of subduction tectonics that govern the dynamics of the wedge mantle. The knowledge of the elastic behavior of aqueous fluid is crucial to understand the fluid–rock interactions in the mantle through velocity profiles. In this study, we investigated the elastic wave velocities of chlorite at high pressure beyond its dehydrating temperature, simulating the progressive dehydration of hydrous minerals in subduction zones. The dehydration resulted in an 8% increase in compressional (Vp) and a 5% decrease in shear wave (Vs) velocities at 950 K. The increase in Vp can be attributed to the stiffening of the sample due to the formation of secondary mineral phases followed by the dehydration of chlorite. The fluid-bearing samples exhibited Vp/Vs of 2.45 at 950 K. These seismic parameters are notably different from the major mantle minerals or hydrous silicate melts and provide unique seismic criteria for detecting mantle fluids through seismic tomography

High-Pressure Sound Velocity Measurements of Liquids Using In Situ Ultrasonic Techniques in a Multianvil Apparatus

International audienceSound velocity and equation of state of liquids provide important constraints on the generation, presence, and transport of silicate and metallic melts in the Earth’s interior. Unlike their solid counterparts, these properties of liquids pose great technical challenges to high-pressure measurements and are poorly constrained. Here we present the technical developments that have been made at the GSECARS beamline 13-ID-D of the Advanced Photon Source for the past several years for determination of sound velocity of liquids using the ultrasonic techniques in a 1000-ton Kawai-type multianvil apparatus. Temperature of the sound velocity measurements has been extended to ~2400 K at 4 GPa and ~2000 K at 8 GPa to enable studies of liquids with very high melting temperatures, such as the silicate liquids

Hot mantle geotherms stabilize calcic carbonatite magmas up to the surface

International audienceThe eruption of calciocarbonatites at Earth’s surface is at odds with them being equilibrated with the mantle at depth because high-pressure experimental studies predict that significant magnesium contents should be expected. Here we report on new high-pressure experiments that demonstrate extreme calcium enrichment of carbonatites en route to the surface. We have monitored the decompression of partially molten carbonated peridotite using a multianvil apparatus coupled to synchrotron radiation. The experimental charge was molten at high pressure and high temperature, before being decompressed along a path that avoided the so-called “carbonate ledge” (a boundary that prevents carbonatitic melts from reaching the surface). Reaction with clinopyroxene yields calcium enrichment and magnesium depletion. The resulting Ca/(Ca + Mg) of the quenched melt reaches 0.95, which compares well with the composition of erupted calcic carbonatites [Ca/(Ca + Mg) ∼0.96–0.99] and of calcic melts trapped in mantle xenoliths from ocean islands [Ca/(Ca + Mg) ∼0.84–0.97]. Our results demonstrate that it is possible to bring carbonatites very close to the surface, without breakdown, and therefore without catastrophic CO2 release. Such occurrence appears to be favored by hot geotherms, meaning that higher temperatures tend to stabilize carbonatitic melts at shallow mantle pressure. Carbonatitic magmas are usually associated with low temperatures, because of the assumed low melting degree or low eruption temperature of the only active carbonatite volcano (i.e., Oldoinyo Lengai, Tanzania). Here we show that emplacement of carbonatites at or near the surface necessitates a hot environment

Effect of melt content and the melt texture on sound wave velocity and electrical conductivity

International audienceThe geophysical observations of elevated electrical conductivity (EC) and reduced seismic wave velocity (SWV) have long been discussed in conjunctions with partial melting in the Earth's asthenosphere. Alternative mechanisms based on solid state processes, such as anelastic relaxation and hydrogen diffusion in mantle minerals have also been proposed. However, the recent finding of young alkali basalt (< 10 Ma) on the 135 million-year-old Pacific-plate provides strong physical evidences for the partial melting at the top of the asthenosphere. Various experimental techniques have been used to constrain the melt fraction in the asthenosphere low velocity zone (LVZ) reported at depth of 70-220 km. Geochemical and petrological constrains suggest low degree melting (less than 1 %) in the asthenosphere, while laboratory based EC and SWV measurements suggest melt fractions ranging from 0.1 to 10 %. Moreover, the comparison between geophysical observations and laboratory measurements of EC and SWV also yields contradicting estimations of the melt fraction, highlighting potential disagreements between EC and SWV techniques. In this study we aimed at investigating the effect of melt fraction and the melt texture, key parameters governing the magnitude and the style of seismic velocity and electrical conductivity variations. We performed simultaneous sound wave velocity and electrical conductivity measurements on mixtures of San Carlos olivine and natural mid oceanic ridge basalt (MORB) at 2.5 GPa and up to 1650 K. This critical improvement enables us to understand how seismic and electrical measurments responses to melt volume fractions and the evolution of melt interconnectivity with time in a partially molten sample. With our results, we were able to determine the potential limitations associated with laboratory based electrical conductivity and seismic wave velocity measurements for estimation of the melt fraction. Overall, the seismic velocity measurements appear more appropriate method to determine the melt fraction in a partially molten system. Based on our results, the geophysical anomalies observed in the LVZ of the asthenosphere can be explained with 0.3 to 0.8 vol. % of MORB-like melt