Variation in bridgmanite grain size accounts for the mid-mantle viscosity jump

A viscosity jump of one to two orders of magnitude in the lower mantle of Earth at 800–1,200-km depth is inferred from geoid inversions and slab-subducting speeds. This jump is known as the mid-mantle viscosity jump1,2. The mid-mantle viscosity jump is a key component of lower-mantle dynamics and evolution because it decelerates slab subduction3, accelerates plume ascent4 and inhibits chemical mixing5. However, because phase transitions of the main lower-mantle minerals do not occur at this depth, the origin of the viscosity jump remains unknown. Here we show that bridgmanite-enriched rocks in the deep lower mantle have a grain size that is more than one order of magnitude larger and a viscosity that is at least one order of magnitude higher than those of the overlying pyrolitic rocks. This contrast is sufficient to explain the mid-mantle viscosity jump1,2. The rapid growth in bridgmanite-enriched rocks at the early stage of the history of Earth and the resulting high viscosity account for their preservation against mantle convection5–7. The high Mg:Si ratio of the upper mantle relative to chondrites8, the anomalous 142Nd:144Nd, 182W:184W and 3He:4He isotopic ratios in hot-spot magmas9,10, the plume deflection4 and slab stagnation in the mid-mantle3 as well as the sparse observations of seismic anisotropy11,12 can be explained by the long-term preservation of bridgmanite-enriched rocks in the deep lower mantle as promoted by their fast grain growth

The effect of dynamic recrystallisation on the rheology and microstructures of partially molten rocks

This work was founded by the joint project “Rheology of the continental crust in collision”, funded by the Procope scheme of PHC Egide in France and by the DAAD PPP scheme in Germany. M-GL acknowledges the support of the Juan de la Cierva programme of the Government of Spain’s Ministry for Science, Innovation and Universities. EGR acknowledges the support of the Beatriu de Pinós programme of the Government of Catalonia's Secretariat for Universities and Research of the Department of Economy and Knowledge (2016 BP 00208). This work benefited from discussions with Pi L. Jolivet and E. Burov within the ERC project RHEOLITH. We thank Elisabetta Mariani and Marcin Dabrowski for their helpful comments, together with the editorial guidance of Dave Healy and Bill Dunne.Peer reviewedPostprin

The grain-scale distribution and behaviour of melt and fluid in crystalline analogue systems

The production, segregation and migration of melt and aqueous fluids (henceforth called liquid) plays an important role for the transport of mass and energy within the mantle and the crust of the Earth. Many properties of large-scale liquid migration processes such as the permeability of a rock matrix or the initial segregation of newly formed liquid from the host-rock depends on the grain-scale distribution and behaviour of liquid. Although the general mechanisms of liquid distribution at the grain-scale are well understood, the influence of possibly important modifying processes such as static recrystallization, deformation, and chemical disequilibrium on the liquid distribution is not well constrained. For this thesis analogue experiments were used that allowed to investigate the interplay of these different mechanisms in-situ. In high-temperature environments where melts are produced, the grain-scale distribution in “equilibrium” is fully determined by the liquid fraction and the ratio between the solid-solid and the solid-liquid surface energy. The latter is commonly expressed as the dihedral or wetting angle between two grains and the liquid phase (Chapter 2). The interplay of this “equilibrium” liquid distribution with ongoing surface energy driven recrystallization is investigated in Chapter 4 and 5 with experiments using norcamphor plus ethanol liquid. Ethanol in contact with norcamphor forms a wetting angle of about 25°, which is similar to reported angles of rock-forming minerals in contact with silicate melt. The experiments in Chapter 4 show that previously reported disequilibrium features such as trapped liquid lenses, fully-wetted grain boundaries, and large liquid pockets can be explained by the interplay of the liquid with ongoing recrystallization. Closer inspection of dihedral angles in Chapter 5 reveals that the wetting angles are themselves modified by grain coarsening. Ongoing recrystallization constantly moves liquid-filled triple junctions, thereby altering the wetting angles dynamically as a function of the triple junction velocity. A polycrystalline aggregate will therefore always display a range of equilibrium and dynamic wetting angles at raised temperature, rather than a single wetting angle as previously thought. For the deformation experiments partially molten KNO3–LiNO3 experiments were used in addition to norcamphor–ethanol experiments (Chapter 6). Three deformation regimes were observed. At a high bulk liquid fraction >10 vol.% the aggregate deformed by compaction and granular flow. At a “moderate” liquid fraction, the aggregate deformed mainly by grain boundary sliding (GBS) that was localized into conjugate shear zones. At a low liquid fraction, the grains of the aggregate formed a supporting framework that deformed internally by crystal plastic deformation or diffusion creep. Liquid segregation was most efficient during framework deformation, while GBS lead to slow liquid segregation or even liquid dispersion in the deforming areas.Das Verhalten von Schmelzen und Fluiden im Kornmaßstab bestimmt wichtige Parameter in teilgeschmolzenen Systemen wie z.B. deren Porosität-Permeabilität und Rheologie. Somit übt die kleinräumige (mm bis mm) Schmelzverteilung einen direkten Einfluss auf großräumige geologische Prozesse wie z.B. die Schmelzsegregation und Migration an Mittelozeanischen Rücken oder Subduktionszonen aus. Obwohl viele der grundlegenden Mechanismen der Gleichgewichtsschmelzverteilung im Kornmaßstab gut bekannt sind, ist der mögliche Einfluss von parallel ablaufenden Prozessen wie z.B. Rekristallisation, Deformation oder chemischem Ungleichgewicht auf die Schmelzverteilung bisher noch unsicher. Für die vorliegende Arbeit wurden Analogexperimente mit Norcamphor–Ethanol und partiell geschmolzenem KNO3–LiNO3 durchgeführt, die es ermöglichen, die Wechselwirkungen zwischen der Schmelzverteilung im Gleichgewicht und den verschiedenen modifizierenden Faktoren in-situ zu beobachten. Das Norcamphor–Ethanol System hat einen Benetzungswinkel von ca. 25° und gleicht in der Hinsicht natürlichen geschmolzenen System wie z.B. Quarz oder Olivin plus Schmelze. Statische Experimente mit Norcamphor–Ethanol ergaben, dass Rekristallisation kontinuierlich zu einer lokalen Ungleichgewichtsverteilung von der Schmelze führt (4. Kapitel). Der normalerweise als statisch angenommenen charakteristischen Benetzungswinkel in dem Analogsystem wurde ebenfalls durch Kornwachstum verändert. Der dynamische Benetzungswinkel ist hierbei eine Funktion der Geschwindigkeit, mit der sich die fest-fest-flüssig Grenze bewegt (5. Kapitel). Bei Deformationsexperimenten (6. Kapitel) wurden abhängig von der Schmelzfraktion drei unterschiedliche Deformationsregimes beobachtet. Bei über 10 Vol.% Schmelzanteil deformierte das Aggregate durch granuläres Fließen und Kompaktion. Bei einer Flüssigkeitsfraktion < 8-10 Vol.% wurde das Korngrenzgleiten in Scherzonen lokalisiert, die wenig deformierende Kornaggregate voneinander abgrenzten. Unterhalb einer systemspezifischen Flüssigkeitsfraktion bildeten sich zusammenhängende Aggregate aus allen Kristallen, welches intern z.B. kristallplastisch deformiert wurde. Die Schmelzsegregation in den Experimenten hing von dem jeweiligen Deformationsmechanismus ab. Korngrenzgleiten oder granuläres Fließen hielt eine bestimmte geometrisch notwendige Flüssigkeitsfraktion in dem deformierenden Gebiet (z.B. in einer Scherzone), während die Flüssigkeit sehr effizient aus kristallplastisch deformierenden Bereichen gepresst wurde (6. Kapitel). Während der Deformation durch granuläres Fließen oder Korngrenzgleiten kam es außerdem zu einem „dynamische Benetzung“ genannten Prozess, bei dem normalerweise nichtbenetzende Flüssigkeiten (z.B. Gasblasen im Norcamphor) ein temporäres Flüssigkeitsnetzwerk bilden konnten (7. Kapitel). Diese Prozesse wurden mit der Bildung und den Konsequenzen von dynamischen Benetzungswinkeln erklärt

First in situ observation of crystallization processes in a basaltic-andesitic melt with the moissanite cell

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Melt migration and melt-rock reactions in the deforming Earth’s upper mantle: Experiments at high pressure and temperature

International audienceCrucial to our understanding of melt migration and segregation from source rocks beneath mid-ocean ridges and volcanic provinces is the intimate relationship with ductile mantle deformation. For this reason, numerous experiments and theoretical studies have been performed to simulate the process of melt extraction from deforming mantle rocks, with the emerging view that segregation occurs into melt-rich bands that are inclined relative to the direction of deformation. This is seemingly in contrast, however, to observations made on mantle rocks exposed at the Earth’s surface, which display evidence for melt migration in bands that were originally parallel to the direction of deformation. Here, we present experimental evidence that reconciles these contradictory observations. Olivine aggregates containing 10 wt% of a melt that reacts with olivine to precipitate orthopyroxene were deformed at 2 GPa and 1150 °C in simple shear experiments. In agreement with previous studies, we observe the development of a melt-preferred orientation that is inclined (∼30°) with respect to the main compression axis σ1. However, the alignment of newly crystallized orthopyroxene aggregates defines a fabric that develops perpendicular to σ1 and rotates toward the shear direction with increasing shear strain (γ of 0.3–2). This misalignment significantly changes the interpretation of evidence for melt channeling and transport in exposed upper mantle rocks: the fabric formed by the phases that have crystallized from a melt during deformation cannot be used directly as a marker for the melt migration direction