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The processes leading to the building of the continental crust through magmatic underplating are fundamentally unknown, mainly because of the rare accessibility to deep level sections of the continental crust. The Italian Alps expose the Permian Mafic Complex, an 8-km-thick gabbronorite–diorite batholith that intruded the lower continental crust during the post-Variscan transtensional tectonics. We present here a petrological and geochemical study of a concentric dunite–pyroxenite–gabbronorite association, called Monte Mazzucco sequence, enclosed at deep levels of the Mafic Complex, thereby allowing us to provide new insights into the magmatic processes driven by emplacement of mantle melts in deep crustal continental areas. The studied sequence includes a ~ 60-m-thick dunite lens, in which olivine (82 mol % forsterite) is associated with accessory Cr-spinel including blebs and lamellae made up of magnetite. The dunite lens is permeated by mm- to cm-scale thick magmatic veins, which range in composition from hornblende lherzolite to olivine hornblendite and hornblende websterite. The lens is mantled by a m-scale ring consisting of amphibole-bearing (≤1 vol %) websterite, and the websterite ring is in turn enclosed by amphibole-free gabbronorites. Both magmatic veins within the dunites and mantling websterites typically include an oxide association of Al-spinel and magnetite. Remarkably, the hornblende websterite veins and the mantling websterites are typically plagioclase-free and include clinopyroxene and amphibole with chondrite-normalized rare earth element patterns characterized by negative Eu anomaly. The mantling websterites display a subtle, gradual outward decrease of Mg# for orthopyroxene, clinopyroxene and accessory olivine, coupled with an increase of the negative Eu anomaly in clinopyroxene and amphibole. The enclosing gabbronorites are amphibole-free and have a chemically evolved signature. We propose a petrogenetic scenario including two major events of melt–dunite interaction. The first resulted from focused reactive melt infiltration and formed the magmatic veins within dunites. The hornblende lherzolite and the olivine hornblendite veins were produced by focused reactive melt migration through dunite grain boundaries, involving dissolution of olivine and recrystallization of Cr-spinel into Al-spinel and magnetite, whereas the hornblende websterite veins crystallized from melts penetrating through narrow fractures and recording earlier plagioclase fractionation. Most likely, the infiltrating melts were overall derived from an evolving H2O-rich magma emplaced below the dunite body. The second event of melt–dunite reactive interaction developed the websterite ring around dunites. We envision that the outermost domain of the dunite body was replaced by websterites in response to reaction with an invading H2O-poor melt, which had previously undergone plagioclase fractionation. The dunite replacement occurred under dynamic conditions, which promoted the reaction progress, thereby leading to total or almost total dissolution of precursor olivine, and started to form the lens shape of the Monte Mazzucco ultramafic association. The gabbronorites closely adjacent to the websterite ring represent the crystallization products of the invading melt. [ABSTRACT FROM AUTHOR]

The widespread High Arctic Large Igneous Province (HALIP) exhibits prolonged melting over more than 50 Myr, an observation that is difficult to reconcile with the classic view that large igneous provinces (LIPs) originate from melting in plume heads. Hence, the suggested plume‐related origin and classification of HALIP as a LIP have been questioned. Here, we use numerical models that include melting and melt migration to investigate a rising plume interacting with lithosphere of variable thickness, that is, a basin‐to‐craton setting applicable to the Arctic. Models reveal that melt migration introduces significant spatial and temporal variations in melt volumes and pulses of melt production, including protracted melting for at least about 30–40 Myr, because of the dynamic feedback between migrating melt and local lithosphere thinning. For HALIP, plume material deflected from underneath the Greenland craton can re‐activate melting zones below the previously plume‐influenced Sverdrup Basin after a melt‐free period of about 10–15 Myr, even though the plume is already ∼500 km away. Hence, actively melting zones do not necessarily represent the location of the deeper plume stem at a given time, especially for secondary pulses. Additional processes such as (minor) plume flux variations or local lithospheric extension may alter the timing and volume of HALIP pulses, but are to first order not required to reproduce the long‐lived and multi‐pulse magmatism of HALIP. Since melting zones are always plume‐fed, we would expect HALIP magmatism to exhibit plume‐related trace element signatures throughout time, potentially shifting from mostly tholeiitic toward more alkalic compositions. Plain Language Summary: Typically, the arrival of a large mantle upwelling ("mantle plume") is expected to cause catastrophic large‐scale volcanism that lasts a few million years. However, a massive past volcanic event now distributed onshore and offshore across the Arctic (the High Arctic Large Igneous Province—HALIP) defies this definition. This wide‐spread magmatism exhibits dates spanning more than 50 Myr, with several pulses of activity. Based on this prolonged magmatism, it has been questioned whether all of it can be attributed to a mantle plume, despite the geochemistry of basalts indicating a plume source. Here, we show that a plume can cause prolonged and multi‐pulse magmatism if it interacts with an increase in lithosphere thickness. Once the plume moves below the thicker lithosphere, hot plume material is channeled along the base of the lithosphere toward the adjacent thinner part, where it can reactivate previous melting regions. At this time, the active plume can be about 500 km away from the melting region, hence plume‐related melt cannot be used as a proxy for the plume position at the given time. Based on the models, we suggest that the prolonged HALIP magmatism was caused by a plume interacting with the edge of a craton. Key Points: Mantle plumes interacting with changes in lithosphere thickness at craton edges can cause prolonged melting with pulses in the same regionRejuvenated melting happens underneath previously melt‐affected thinned lithosphere several hundred km downstream of the plume stemThe timing and duration of rejuvenated melting in models correspond to and therefore may explain observations of magmatic pulses from High Arctic Large Igneous Province [ABSTRACT FROM AUTHOR]

Stress‐driven melt segregation may have important geochemical and geophysical effects but remains a poorly understood process. Few constraints exist on the permeability and distribution of melt in deformed partially molten rocks. Here, we characterize the 3D melt network and resulting permeability of an experimentally deformed partially molten rock containing several melt‐rich bands based on an X‐ray microtomography data set. Melt fractions range from 0.08 to 0.28 in the ∼20‐μm‐thick melt‐rich bands, and from 0.02 to 0.07 in the intervening ∼30‐μm‐thick regions. We simulated melt flow through subvolumes extracted from the reconstructed rock at five length scales ranging from the grain scale (3 μm) to the minimum length required to fully encompass two melt‐rich bands (64 μm). At grain scale, few subvolumes contain interconnected melt, and permeability is isotropic. As the length scale increases, more subvolumes contain melt that is interconnected parallel to the melt bands, but connectivity diminishes in the direction perpendicular to them. Even if melt is connected in all directions, permeability is lower perpendicular to the bands, in agreement with the elongation of melt pockets. Permeability parallel to the bands is proportional to melt fraction to the power of an exponent that increases from ∼2 to 5 with increasing length scale. The permeability in directions parallel to the bands is comparable to that for an isotropic partially molten rock. However, no flow is possible perpendicular to the bands over distances similar to the band spacing. Melt connectivity limits sample scale melt flow to the plane of the melt‐rich bands. Plain Language Summary: When a mixture of solid olivine grains and basaltic melt is subjected to differential stress, the melt can collect into sheet‐like bands. Melt transport properties, including melt trajectories, chemical diffusion, and ductile creep, may all become anisotropic in the presence of these melt‐rich bands. We present here the first quantification of the melt network properties in three dimensions based on synchrotron X‐ray microtomography of experimentally deformed olivine/basalt aggregates. We show that the melt network presents no connection between melt‐rich bands, restricting flow to the directions contained in the planes of these bands. Even at grain scale, melt pockets are preferentially oriented along the plane of the bands. The relationship between melt content and permeability derived from undeformed rocks is not valid at sample scale in sheared aggregates. Key Points: 3D microtomography images of sheared partially molten rocks reveal scale‐dependent melt network propertiesThe permeability parallel to melt‐rich bands is similar to that of rocks with textural equilibrium melt geometriesMelt cannot flow over long distances perpendicular to melt‐rich band due to lack of melt connectivity in that direction and scale [ABSTRACT FROM AUTHOR]

Abstract The widespread High Arctic Large Igneous Province (HALIP) exhibits prolonged melting over more than 50 Myr, an observation that is difficult to reconcile with the classic view that large igneous provinces (LIPs) originate from melting in plume heads. Hence, the suggested plume‐related origin and classification of HALIP as a LIP have been questioned. Here, we use numerical models that include melting and melt migration to investigate a rising plume interacting with lithosphere of variable thickness, that is, a basin‐to‐craton setting applicable to the Arctic. Models reveal that melt migration introduces significant spatial and temporal variations in melt volumes and pulses of melt production, including protracted melting for at least about 30–40 Myr, because of the dynamic feedback between migrating melt and local lithosphere thinning. For HALIP, plume material deflected from underneath the Greenland craton can re‐activate melting zones below the previously plume‐influenced Sverdrup Basin after a melt‐free period of about 10–15 Myr, even though the plume is already ∼500 km away. Hence, actively melting zones do not necessarily represent the location of the deeper plume stem at a given time, especially for secondary pulses. Additional processes such as (minor) plume flux variations or local lithospheric extension may alter the timing and volume of HALIP pulses, but are to first order not required to reproduce the long‐lived and multi‐pulse magmatism of HALIP. Since melting zones are always plume‐fed, we would expect HALIP magmatism to exhibit plume‐related trace element signatures throughout time, potentially shifting from mostly tholeiitic toward more alkalic compositions.

Granitic magma bodies form in the ephemeral part of magma mush systems and are emplaced by a variety of mechanisms in different tectonic settings. This study investigates how granitic magma emplacement processes and tectonomagmatic interactions assert control over the architecture of mush state pluton‐scale magma transport pathways. The 1.45 Ga shallow‐crustal Götemar pluton is a 4.5 km diameter circular pluton that consists of three granite units: a coarse‐grained red granite, a medium‐grained pale to red granite, and fine‐grained pale microgranite sheets. We employed geological mapping supported by Anisotropy of Magnetic Susceptibility (AMS) to examine the magmatic and regional tectonic controls on late‐stage magma transport in the Götemar granitic magma mush system. Multiple parallel arcuate subhorizontal microgranite and medium‐grained granite sheets (from 0.1 to 10s of meters thick) were mapped within the pluton. The arcuate sheets pinch out from the northern part of the pluton toward the SE inferring magma propagation direction. A dominant set of vertical granitic sheets within the granite body strikes NW‐SE. The AMS fabrics are contact‐parallel in the main medium‐grained granite body and indicate inflation. Within the microgranite sheets, the AMS fabrics are parallel to the sheet strike and support a sheet propagation direction to the SE. The Götemar pluton displays a clear link between arcuate (concentric) magma‐transporting sheets and concentric strain‐partitioning related to the intrusion of medium‐grained granite magma. The vertical magma sheet orientations are consistent with an NE‐SW extensional stress field that is associated with the extensional back‐arc stress regime of the contemporary Hallandian Orogen. Plain Language Summary: The eruptive products of volcanoes are thought to be stored in pockets of melt in crystal‐dominated magmatic systems called crystal mushes prior to volcanic eruptions. An understanding of where magma is stored and how it is transported in mush systems is important in order to predict the eruptive behavior of the volcanic system. This contribution investigates the magma transport pathways in the Götemar granite in Sweden and its relationship to local magmatic deformation and regional deformation related to the Hallandian mountain building event. We show that magma is transported in vertical sheets parallel to the front of the Hallandian Orogen and laterally in sub‐horizontal arcuate sheets that reflect the circular shape of the granite pluton. Our study highlights the importance of understanding the shape and the formation of the magmatic granite body for deciphering the melt transport in the magma mush system under volcanoes. Key Points: Subhorizontal magma transport in a granitic magma mush controlled by magma emplacement structuresVertical magma sheets in granitic magma mush controlled by the regional stress fieldMagma transported in concentric magma fingers in the circular Götemar granite pluton [ABSTRACT FROM AUTHOR]

Prior to intrusion, magma migrates through the crustal plumbing system that likely contains layers or columns of crystal mush. To better understand the behavior of the crustal magmatic system during magmatic unrest, it is important to examine the process of melt migration within the crystal mush and the associated evolution in pressure and temperature. In this study I use an analytical model to explore the characteristics of transport of melt, pressure, and heat through an idealized crystal mush layer/column under uniaxial strain condition. The model invokes a thermo-poro-viscoelastic rheology and uses a frequency-domain method to explore two scenarios of magmatic unrest: harmonic perturbation of fluid pressure, and step-rise in fluid pressure at a source location. Several factors influence the transport of melt, pressure and heat, including the thermal-mechanical coupling arising from the mush rheology, the advection of heat by melt flows, the competition between thermal diffusivity and poroelastic diffusivity, and the viscoelastic relaxation of the crystalline framework. One key finding is the development of transport asymmetry: when a background temperature gradient exists, the transport properties become different for propagation along the background thermal gradient and propagation against the background thermal gradient. Analysis on an endmember case shows that the transport asymmetry is associated to the competition between the diffusion and advection of pore pressure, which determines a Peclet number that depends on the temperature difference across the mush and the thermal expansion coefficients. Because the temperature in magma mushes in the crust likely increase with depth, the observed propagation asymmetry suggests some intrinsic difference between a bottom-up vs. a top-down triggering mechanism for magmatic unrest. The results from this study highlight the importance for further exploration for a more complete description of the transport properties in the crystal mush.

Impacts into icy bodies often generate near‐surface melt chambers and thermal perturbations that soften the ice. We explore the post‐impact evolution of non‐penetrating impacts into Europa's ice shell. Simulations of viscous ice deformation show that dense impact melts founder before refreezing. If the transient cavity depth exceeds half the ice shell thickness, over 40% of the impact melt drains into the underlying ocean. Drainage of impact melts from the near‐surface to the ocean occurs on timescales of 103–104 years. The drainage of melts to the ocean occurs for all plausible ice shell thicknesses and ice viscosities, suggesting that melt foundering is a natural consequence of impacts on icy worlds. Post‐impact viscous deformation is an important process on icy worlds that affects cryovolcanism, likely modifies crater morphology, creates porous columns through the ice for surface‐to‐ocean exchange, and may supply the oxidants required for habitability to subsurface oceans. Plain Language Summary: Europa is an ocean world with an outer ice shell in our Solar System. Outer ice shells on ocean worlds serve as both a barrier and facilitator for internal ocean habitability and the detectability of its potential inhabitants. A prominent feature we observe on the surface of ice shells is highly variable craters from impacts that indent but do not penetrate the ice shell. Here, we find that many of the impacts into Europa's ice shell would have generated large near‐surface melt chambers, which rapidly drain into the underlying ocean and form a continuous surface‐to‐ocean melt column. This melt drainage occurs with any entrained surface materials and provides a consistent means of exchange between the surface and ocean, which likely increases the habitability of internal oceans. Furthermore, this drainage displaces large volumes of water from below impact craters, on the order of 10's of cubic kilometers. This large scale drainage may help to explain the wide range of collapsed, enigmatic crater morphologies observed on Europa, and other ocean worlds. Key Points: Impacts into ice shells generate melt chambers that viciously deform and sink through the ice shell, likely modifying crater morphologiesIf transient cavity depth exceeds half the ice thickness, impact melts drain to the ocean and may deliver oxidants needed for habitabilityDraining impact melts form a stable porous channel that connects the ocean to near‐surface and may allow for the escape of ocean fluids [ABSTRACT FROM AUTHOR]

Near-axis seamounts provide a unique setting to investigate three-dimensional mantle processes associated with the formation of new oceanic crust and lithosphere. Here, we investigate the characteristics and evolution of the 8˚20’N Seamount Chain, a lineament of seamounts that extends ~ 175 km west of the East Pacific Rise (EPR) axis, just north of the fracture zone of the Siqueiros Transform Fault. Shipboard gravity, magnetic, and bathymetric data acquired in 2016 are utilized to constrain models of seamount emplacement and evolution. Geophysical observations indicate that these seamounts formed during four distinct episodes of volcanism coinciding with changes in regional plate motion that are also reflected in the development of intra-transform spreading centers (ITSCs) along the Siqueiros transform fault (Fornari et al. 1989; Pockalny et al. 1997). Although volcanism is divided into distinct segments, the magnetic data indicate continuous volcanic construction over long portions of the chain. Crustal thickness variations along the chain up to 0.75 km increase eastward, inferred from gravity measurements, suggest that plate reorganization has considerably impacted melt distribution in the area surrounding the Siqueiros-EPR ridge transform intersection. This appears to have resulted in increased volcanism and the formation of the 8˚20’N Seamounts. These findings indicate that melting processes in the mantle and subsequently the formation of new oceanic crust and lithosphere are highly sensitive to tectonic stress changes in the vicinity of fast-spreading transform fault offsets. [ABSTRACT FROM AUTHOR]

The melt migration and solidification behavior on the surface of metal structure and in the debris voids is a key phenomenon of debris bed melting, which has an important impact on the assessment of reactor pressure vessel damage and melt leakage. In this study, the moving particle semi-implicit (MPS) method with high-order discretization scheme was coupled with a surface tension model that was established based on interface reconstruction. The migration and solidification behavior of UO 2 –ZrO 2 and Fe–Zr melts on the metal structure surface and in the debris voids were investigated with the improved MPS method. The effects of temperature, velocity, contact angle and droplet diameter on the melt migration and solidification were analyzed. The results showed that the molten UO 2 –ZrO 2 droplet on the metal structure surface presented three stages with solidification rate decreasing, while the solidification rate of molten Fe–Zr droplet had little change. The maximum spread factor and solidification rate increased with the increase of droplet falling velocity, but the spread factor was independent of the melt type at the same velocity. The high-temperature molten droplet penetrated the voids formed by the debris with diameter of 5 mm more easily compared to the voids formed by the debris with diameter of 3 mm. The influence of contact angle on the migration of molten droplet with initial velocity was small, and the maximum difference in droplet mass fraction was about 6%. Three groups of molten droplets with different diameters penetrated the voids, and the average increments of penetration mass fraction were 2.3%, 2.7% and 5.8%, respectively, with the increase of velocity. Near the bottom of the debris bed, the molten droplet without initial velocity solidified and blocked the flow channel, but the molten droplet with initial velocity of 0.5 m/s penetrated through the debris voids. The droplet with initial velocity of 0.5 m/s had a faster solidification rate in the voids compared to the droplet without initial velocity, and the droplet solidification mass fraction was 96.27% when the initial melt temperature was 1410 K. [ABSTRACT FROM AUTHOR]

• The behavior of cladding melting and melt migration in lead-cooled coolant was studied. • The viscosity function was used to evaluate the cladding viscosity at different enthalpies. • Surface tension promoted the molten cladding to adhere on fuel rod surface. • The "center-peaked distribution" case was easy to cause re-solidified metal in the upper part of the fuel rod. • Flowing coolant accelerated the cooling of melt and softened cladding. The behavior of cladding melting and melt migration is an important phenomenon during the early stage of core disruptive accidents (CDA) in lead-cooled fast reactors (LFRs), which can affect the accident evolution trend and severity. In this paper, the Moving Particle Semi-implicit (MPS) method with the advantage of phase-interface capture was employed to further reveal this key phenomenon. In the simulation, the cladding was regarded as a high-viscosity fluid, and the viscosity function related to solid fraction was used. Meanwhile, the modified algorithm with the viscosity term being placed after the pressure term was adopted to ensure the cladding was stationary before the temperature reached the melting point. The computational model was verified by the experiments of tin cladding tube melting and melt migration in seven-pin bundle. After that, the effects of surface tension coefficient, fuel initial temperature and coolant flow speed on cladding melting and melt migration in lead-bismuth coolant were investigated with the improved MPS method. The results indicated that surface tension promoted the melt to adhere on fuel rod surface and the melt-coolant contact area decreased when the surface tension was introduced. The fuel initial temperature with "center-peaked distribution" caused a temperature gradient in the flow channel and led to re-solidified metal at the upper part of the fuel rod. Compared with the "uniform distribution" case, the size of molten droplet in "center-peaked distribution" case was smaller and more fragmented droplets could be observed. When the lead-bismuth coolant was flowing, the enhanced heat transfer accelerated the cooling of melt as well as softened cladding, and the solidified melt blocked the flow channel at the midstream and downstream regions. [ABSTRACT FROM AUTHOR]

Jupiter's moon Europa is thought to have an ocean beneath its ice shell and the habitability of the internal ocean depends on the availability of redox gradients. Downward transport of radiolytic materials produced at the surface through the ice shell sets the flux of oxidants into the ocean. Here, we propose that oxidants are transported through the ice shell by the drainage of near‐surface brines formed concurrently with chaotic terrains. We estimate that Europa's porous regolith contains 3.7 × 1014 to 5.6 × 1018 mol (1.2 × 1013 − 1.8 × 1017 kg) of trapped O2. Simulations of coupled melt‐migration and eutectic phase behavior show that brines drain before they refreeze, delivering ∼85% of the surface oxidants to the ocean on timescales of 2 × 104 years. From the distribution of chaotic terrains and from Europa's surface age we estimate that brine drainage could deliver O2 to the ocean at rates of 2.0 × 106 to 1.3 × 1010 mol/yr (0.002−13.2 kg/s). Plain Language Summary: Some icy moons in the outer solar system likely contain an ocean beneath their ice shells. Jupiter's moon Europa may be especially suitable for life if radiolytic oxidants generated at its surface travel efficiently through the ice. We propose that oxidants can be transported through the ice by drainage of brines generated during the formation of chaotic terrains. These enigmatic surface features are thought to require the formation of large volumes of near‐surface brine. We show that the brines drain through the underlying ice before re‐freezing and transport oxidants in pulses of melt called "porosity waves." These pulses may propagate through the ice on timescales of 20,000 years. The rate of oxidant delivery is thus linked to chaotic terrain formation and is correspondingly spatially localized and episodic. Key Points: We synthesize available observations and show that Europa's near‐surface O2 abundance may be two times higher than previous estimatesNear‐surface brines generated during the formation of chaotic terrains drain downward through the ice shell in porosity wavesMore than 80% of surface oxidants entrained into the brines may be delivered to the internal ocean [ABSTRACT FROM AUTHOR]

Melt transport across the ductile mantle is essential for oceanic crust formation or intraplate volcanism. However, mechanisms of melt migration and associated chemical interaction between melt and solid mantle remain unclear. Here, we present a thermo‐hydro‐mechanical‐chemical (THMC) model for melt migration coupled to chemical differentiation. We consider melt migration by porosity waves and a chemical system of forsterite‐fayalite‐silica. We solve the one‐dimensional (1D) THMC model numerically using the finite difference method. Variables, such as solid and melt densities or MgO and SiO2 mass concentrations, are functions of pressure (P), temperature (T), and total silica mass fraction (CTSiO2 ${C}_{\mathrm{T}}^{{\text{SiO}}_{2}}$). These variables are pre‐computed with Gibbs energy minimization and their variations with evolving P, T, and CTSiO2 ${C}_{\mathrm{T}}^{{\text{SiO}}_{2}}$ are implemented in the THMC model. We consider P and T conditions relevant around the lithosphere‐asthenosphere boundary. Systematic 1D simulations quantify the impact of initial distributions of porosity and CTSiO2 ${C}_{\mathrm{T}}^{{\text{SiO}}_{2}}$ on the melt velocity. Larger perturbations of CTSiO2 ${C}_{\mathrm{T}}^{{\text{SiO}}_{2}}$ cause larger melt velocities. An adiabatic or conductive geotherm cause fundamentally different vertical variations of densities and concentrations, and an adiabatic geotherm generates higher melt velocities. We quantify differences between melt transport (considering incompatible tracers), major element transport and porosity evolution. Melt transport is significant in the models. We also quantify the relative importance of four porosity variation mechanisms: (a) mechanical compaction and decompaction, (b) density variation, (c) compositional variation, and (d) solid‐melt mass exchange. In the models, (de)compaction dominates the porosity variation. We further discuss preliminary results of 2D THMC simulations showing blob‐like and channel‐like porosity waves. Plain Language Summary: Melt transport across the ductile mantle is essential for oceanic crust formation or intraplate volcanism. However, melt transport processes are still incompletely understood and poorly quantified. Xenoliths (inclusions in igneous rock entrained during magma ascent) sampled by kimberlites (an igneous rock that erupted from the mantle) or intraplate basalts provide evidence that there is a reaction (metasomatism) between the rising melt and the solid mantle. However, the impact of chemical processes on melt migration remains unclear. Here, we present a new mathematical two‐phase (fluid‐solid) model, based on fundamental laws of physics and thermodynamics, which couples melt migration with chemical processes. We study melt migration around the lithosphere‐asthenosphere boundary and consider the solid (not molten) rocks as highly viscous fluids due to the high temperatures in these regions (at 80–100 km depth). We present one‐dimensional results of computer simulations and show that the variation of chemical components, such as silicon dioxide, changes the densities of the solid and melt, and can, hence, have a considerable impact on melt migration. We also present two‐dimensional simulations, which show the channelization of the rising melt. Key Points: We present a thermo‐hydro‐mechanical‐chemical model for melt migration by porosity waves coupled to thermodynamic P–T–X phase calculationsAdiabatic or conductive geotherms cause different vertical variations of densities and concentrations as well as different melt velocitiesVariations in Si concentration change solid and melt densities, trigger porosity waves and impact melt velocity, porosity and permeability [ABSTRACT FROM AUTHOR]

Abstract Melt transport across the ductile mantle is essential for oceanic crust formation or intraplate volcanism. However, mechanisms of melt migration and associated chemical interaction between melt and solid mantle remain unclear. Here, we present a thermo‐hydro‐mechanical‐chemical (THMC) model for melt migration coupled to chemical differentiation. We consider melt migration by porosity waves and a chemical system of forsterite‐fayalite‐silica. We solve the one‐dimensional (1D) THMC model numerically using the finite difference method. Variables, such as solid and melt densities or MgO and SiO2 mass concentrations, are functions of pressure (P), temperature (T), and total silica mass fraction (CTSiO2). These variables are pre‐computed with Gibbs energy minimization and their variations with evolving P, T, and CTSiO2 are implemented in the THMC model. We consider P and T conditions relevant around the lithosphere‐asthenosphere boundary. Systematic 1D simulations quantify the impact of initial distributions of porosity and CTSiO2 on the melt velocity. Larger perturbations of CTSiO2 cause larger melt velocities. An adiabatic or conductive geotherm cause fundamentally different vertical variations of densities and concentrations, and an adiabatic geotherm generates higher melt velocities. We quantify differences between melt transport (considering incompatible tracers), major element transport and porosity evolution. Melt transport is significant in the models. We also quantify the relative importance of four porosity variation mechanisms: (a) mechanical compaction and decompaction, (b) density variation, (c) compositional variation, and (d) solid‐melt mass exchange. In the models, (de)compaction dominates the porosity variation. We further discuss preliminary results of 2D THMC simulations showing blob‐like and channel‐like porosity waves.

X‐ray computed tomography (XCT) is a well known method for three‐dimensional characterization of materials that is established as a powerful tool in high‐pressure/high‐temperature research. The optimization of synchrotron beamlines and the development of fast high‐efficiency detectors now allow the addition of a temporal dimension to tomography studies under extreme conditions. Presented here is the experimental setup developed on the PSICHE beamline at SOLEIL to perform high‐speed XCT in the Ultra‐fast Tomography Paris–Edinburgh cell (UToPEc). The UToPEc is a compact panoramic (165° angular aperture) press optimized for fast tomography that can access 10 GPa and 1700°C. It is installed on a high‐speed rotation stage (up to 360° s−1) and allows the acquisition of a full computed tomography (CT) image with micrometre spatial resolution within a second. This marks a major technical breakthrough for time‐lapse XCT and the real‐time visualization of evolving dynamic systems. In this paper, a practical step‐by‐step guide to the use of the technique is provided, from the collection of CT images and their reconstruction to performing quantitative analysis, while accounting for the constraints imposed by high‐pressure and high‐temperature experimentation. The tomographic series allows the tracking of key topological parameters such as phase fractions from 3D volumetric data, and also the evolution of morphological properties (e.g. volume, flatness, dip) of each selected entity. The potential of this 4D tomography is illustrated by percolation experiments of carbonate melts within solid silicates, relevant for magma transfers in the Earth's mantle. [ABSTRACT FROM AUTHOR]

For rocky exoplanets, knowledge of their geologic characteristics such as composition and mineralogy, surface recycling mechanisms, and volcanic behavior are key to determining their suitability to host life. Thus, determining exoplanet habitability requires an understanding of surface chemistry, and understanding the composition of exoplanet surfaces necessitates applying methods from the field of igneous petrology. Piston‐cylinder partial melting experiments were conducted on two hypothetical rocky exoplanet bulk silicate compositions. HEX1, a composition with molar Mg/Si = 1.42 (higher than bulk silicate Earth's Mg/Si = 1.23) yields a solidus similar to that of Earth's undepleted mantle. However, HEX2, a composition with molar Ca/Al = 1.07 (higher than Earth Ca/Al = 0.72) has a solidus with a slope of ∼10°C/kbar (vs. ∼15°C/kbar for Earth) and as result, has much lower melting temperatures than Earth. The majority of predicted adiabats point toward the likely formation of a silicate magma ocean for exoplanets with a mantle composition similar to HEX2. For adiabats that do intersect HEX2's solidus, decompression melting initiates at pressures more than 4x greater than in the modern Earth's undepleted mantle. The experimental partial melt compositions for these exoplanet mantle analogs are broadly similar to primitive terrestrial magmas but with higher CaO, and for the HEX2 composition, higher SiO2 for a given degree of melting. This first of its kind exoplanetary experimental data can be used to calibrate future exoplanet petrologic models and predict volatile solubilities, volcanic degassing, and crust compositions for exoplanets with bulk compositions and ƒO2 similar to those explored herein. Plain Language Summary: The composition of rocky exoplanets can be approximated, to first order, from the composition of the stars they orbit. In this work, we conducted experiments to determine the types of minerals found in the interior of two hypothetical exoplanets that orbit stars with compositions different from our star, and the magma compositions produced from melting them. Our results suggest that in many ways exoplanets with these potential compositions have melting behavior similar to that of the Earth's interior. A key difference is that one composition produced melt at much lower temperatures than Earth, such that the exoplanet may form a magma ocean early in its history or begin melting at depths greater than are common in Earth today. These results provide constraints for modeling geologic processes on exoplanets and their ability to support life; this study underlines the need for further experimentation of this kind. The geochemical models that can be derived from this and similar studies will allow us to better interpret the data returned by satellites that will observe exoplanets in the near future. Key Points: First of their kind experiments examine phase equilibria and melt compositions for exoplanet silicate mantles at terrestrially relevant ƒO2The mineralogy and melts are similar to those of Earth's mantle. One composition melts at conditions more likely to produce magma oceansResults can be used to calibrate future exoplanet models for volatile solubility, volcanic atmosphere contributions, and crust composition [ABSTRACT FROM AUTHOR]

Abstract To investigate channelization during migration of a reactive melt, we performed a series of Darcy‐type experiments in which an alkali basalt infiltrated partially molten harzburgites and lherzolites at a confining pressure of 300 MPa, temperatures of 1200°C and 1250°C, and pore pressure gradients of ~2 to 60 MPa/mm. We compare our results to those from previously published experiments performed on wehrlites. In all experiments, irrespective of the exact mineralogy, a planar reaction layer composed of olivine + melt developed in which all of the pyroxene was consumed. Under specific conditions controlled primarily by the melt flow velocity, finger‐like channels composed of olivine + melt also developed. In wehrlites, these reaction infiltration instabilities formed at 1200°C and 1250°C at pressure gradients >25 and >5 MPa/mm, respectively. In harzburgites, channelization occurred only at 1250°C at a pressure gradient of 35 MPa/mm. In lherzolites, a planar melt‐filled vein developed at 1250°C; no finger‐like channels formed under a pressure gradient of ~25 MPa/mm. Both the finger‐like channels and the planar vein led to very efficient extraction of melt from the reservoir. Channelization established large compositional variations over short distances in the crystalized phases as well as in the local melt and greatly enhanced the abundance of the reaction product, olivine, similar to dunite channels in the Earth. The range of chemical‐mechanical responses displayed by this array of compositions provides a set of targets for reactive transport and mechanical modeling studies.

To investigate channelization during migration of a reactive melt, we performed a series of Darcy‐type experiments in which an alkali basalt infiltrated partially molten harzburgites and lherzolites at a confining pressure of 300 MPa, temperatures of 1200°C and 1250°C, and pore pressure gradients of ~2 to 60 MPa/mm. We compare our results to those from previously published experiments performed on wehrlites. In all experiments, irrespective of the exact mineralogy, a planar reaction layer composed of olivine + melt developed in which all of the pyroxene was consumed. Under specific conditions controlled primarily by the melt flow velocity, finger‐like channels composed of olivine + melt also developed. In wehrlites, these reaction infiltration instabilities formed at 1200°C and 1250°C at pressure gradients >25 and >5 MPa/mm, respectively. In harzburgites, channelization occurred only at 1250°C at a pressure gradient of 35 MPa/mm. In lherzolites, a planar melt‐filled vein developed at 1250°C; no finger‐like channels formed under a pressure gradient of ~25 MPa/mm. Both the finger‐like channels and the planar vein led to very efficient extraction of melt from the reservoir. Channelization established large compositional variations over short distances in the crystalized phases as well as in the local melt and greatly enhanced the abundance of the reaction product, olivine, similar to dunite channels in the Earth. The range of chemical‐mechanical responses displayed by this array of compositions provides a set of targets for reactive transport and mechanical modeling studies. Key Points: Reaction infiltration instability develops in experiments on wehrlites and harzburgitesChannelization establishes large compositional variations over short distances in the crystalized phases as well as in local meltChannelization greatly increases the modal abundance of olivine in the partially molten rock [ABSTRACT FROM AUTHOR]

We present the results of melting experiments on a suite of carbonaceous and ordinary chondritic compositions (CV, CM, CI, H and LL) performed at low pressure (0.1–13.1 MPa) and over a range of oxygen fugacity (log f O 2 − (log f O 2) IW = −2.5 to −1 and +0.8, IW being the iron-wustite buffer). These experiments constrain the composition of partial melts (F = 5–25 wt.%) of chondritic planetesimals. Most experiments (IW −2.5 to −1) were conducted in Molybdenum-Hafnium Carbide pressure vessels, which prevented the loss of alkali elements from the melt. The results show that all planetesimals not significantly depleted in moderately volatile elements relative to the sun's photosphere (e.g. CI, H and LL compositions) produced low-degree melts (<15 wt.%) rich in SiO 2 , Al 2 O 3 and alkali elements, regardless of the f O 2. Despite their high apparent viscosities (104-5 Pa.s), such low-density partial melts (2400–2500 kg/m3) were mobilized and, upon crystallizing, formed rocks containing up to 80 vol.% of plagioclase An 10-30 (i.e. oligoclase) such as the trachyandesite achondrites Graves Nunataks 06128/9, Northwest Africa 6698 and 11575, the Almahata Sitta clast ALM-A, as well as smaller "albitic clasts" in polymict ureilites and "alkali-silica-rich" inclusions in non-magmatic iron meteorites. We suggest that silica- and alkali- rich melts were widespread in small bodies of the early solar system but that much evidence was erased by subsequent stages of melting and planetary accretion and differentiation. [ABSTRACT FROM AUTHOR]

The size and distribution of chemical heterogeneities in the mantle source affect compositions of basalts erupted on the surface. To understand the effect of size and distribution of heterogeneous mantle source on the variations of isotope ratios and incompatible trace element abundances in mid-ocean ridge basalts (MORB), we develop a time-dependent two-porosity ridge model that features disequilibrium melting and channelized melt migration in a truncated triangular melting region. We run numerical simulations for a two-component mantle that consists of the depleted mantle and enriched blobs spanning a range of enriched compositions and sizes. The volume fraction of enriched mantle varies under 10% of the mantle source and the size of the melt production area that contributes to the pooled melt is also treated as a variable. Results show that higher abundance of incompatible trace element in the enriched mantle, larger size of enriched blobs, and narrower melt pooling area produce larger variations of isotope ratios in the pooled melt. We develop an empirical scaling equation that quantifies the fluctuation of isotope ratios as a function of elemental abundance, size and average spacing of enriched heterogeneities in the source, and the width of melt pooling area. The two-standard deviation of 143Nd/144Nd, 176Hf/177Hf, 87Sr/86Sr, and 206Pb/204Pb in global MORB not affected by hotspots can be produced by melting a two-component mantle with 5.2–8.2% enriched heterogeneities of 5.1–6.7 km mean size and 7.0(±1.6), 5.3(±1.2), 2.0(±0.5), and 9.8(±2.2) times higher abundances of Nd, Hf, Sr, and Pb than the depleted mantle, respectively. Incomplete mixing of melts at the base of the lithosphere is likely important for producing the variation of incompatible trace elements in MORB. The melt erupted at the ridge axis may represent an ensemble of the melt pooled from the center of the melt pooling area and melts pooled from local areas away from the ridge axis. The present study underscores the importance of the size and distribution of mantle heterogeneities to the interpretation of isotope ratios and trace element abundances in basalts. [ABSTRACT FROM AUTHOR]

The compositional variability of the lithospheric mantle at extensional settings is largely caused by the reactive percolation of uprising melts in the thermal boundary layer and in lithospheric environments. The Alpine-Apennine (A-A) ophiolites are predominantly constituted by mantle peridotites and are widely thought to represent analogs of the oceanic lithosphere formed at ocean/continent transition and slow- to ultraslow-spreading settings. Structural and geochemical studies on the A-A mantle peridotites have revealed that they preserve significant compositional and isotopic heterogeneity at variable scale, reflecting a long-lived multi-stage melt migration, intrusion and melt-rock interaction history, occurred at different lithospheric depths during progressive uplift. The A-A mantle peridotites thus constitute a unique window on mantle dynamics and lithosphere-asthenosphere interactions in very slow spreading environments. In this work, we review field, microstructural and chemical-isotopic evidence on the major stages of melt percolation and melt-rock interaction recorded by the A-A peridotites and discuss their consequences in creating chemical-isotopic heterogeneities at variable scales and enhancing weakening and deformation of the extending mantle. Focus will be on three most important stages: (i) old (pre-Jurassic) pyroxenite emplacement, and the significant isotopic modification induced in the host mantle by pyroxenite-derived melts, (ii) melt-peridotite interactions during Jurassic mantle exhumation, i.e. the open-system reactive porous flow at spinel facies depths causing bulk depletion (origin of reactive harzburgites and dunites), and the shallower melt impregnation which originated plagioclase-rich peridotites and an overall mantle refertilization. We infer that migrating melts largely originated as shallow, variably depleted, melt fractions, and acquired Si-rich composition by reactive dissolution of mantle pyroxenes during upward migration. Such melt-rock reaction processes share significant similarities with those documented in modern oceanic peridotites from slow- to ultraslow-spreading environments and track the progressive exhumation of large mantle sectors at shallow depths in oceanic settings where a thicker thermal boundary layer exists, as a consequence of slow-spreading rate. Image 1 • Alpine-Apennine peridotites are windows on mantle dynamics in slow spreading settings. • Melt-rock reactions cause multi-scale chemical-isotopic mantle heterogeneities. • Melt impregnation facilitates weakening and deformation of the extending mantle. [ABSTRACT FROM AUTHOR]

Abstract Rocky planetesimals in the early solar system melted internally and evolved chemically due to radiogenic heating from 26Al. Here we quantify the parametric controls on magma genesis and transport using a coupled petrological and fluid mechanical model of reactive two-phase flow. We find the mean grain size of silicate minerals to be a key control on magma ascent. For grain sizes ≳1 mm, melt segregation produces distinct radial structure and chemical stratification. This stratification is most pronounced for bodies formed at around 1 Myr after formation of Ca, Al-rich inclusions. These findings suggest a link between the time and orbital location of planetesimal formation and their subsequent structural and chemical evolution. According to our models, the evolution of partially molten planetesimal interiors falls into two categories. In the magma ocean scenario, the whole interior of a planetesimal experiences nearly complete melting, which would result in turbulent convection and core–mantle differentiation by the rainfall mechanism. In the magma sill scenario, segregating melts gradually deplete the deep interior of the radiogenic heat source. In this case, magma may form melt-rich layers beneath a cool and stable lid, while core formation would proceed by percolation. Our findings suggest that grain sizes prevalent during the internal heating stage governed magma ascent in planetesimals. Regardless of whether evolution progresses toward a magma ocean or magma sill structure, our models predict that temperature inversions due to rapid 26Al redistribution are limited to bodies formed earlier than ≈1 Myr after CAIs. We find that if grain size was ≲1 mm during peak internal melting, only elevated solid–melt density contrasts (such as found for the reducing conditions in enstatite chondrite compositions) would allow substantial melt segregation to occur. Highlights • Planetesimal melt transport regimes from coupled models of reactive two-phase flow. • Sub-lid magma sills develop on bodies formed at around 1 Myr after CAIs. • Earlier-formed planetesimals undergo body-wide magma ocean stages. • Mineral grain size key parameter controlling melt segregation rate (a crit ≈ 1 mm). • Substantial thermal inversions limited to bodies formed at t form ≤ 1 Myr after CAIs. [ABSTRACT FROM AUTHOR]

Core formation has left a lasting geochemical signature on the Earth. In order to constrain the composition of the Earth we must fully understand the processes by which newly formed Earth, and the bodies which accreted to it, differentiated. Percolation of iron-rich melt through solid silicate has been invoked as a mechanism for differentiation and core formation in terrestrial bodies in the early solar system. However, to date the contribution of percolation to core formation cannot be assessed due to the absence of data on Fe-rich melt migration velocities. Here we use a novel experimental design to investigate textural changes in an analog system, Au melt in polycrystalline h-BN, at 3 GPa, relevant to core formation in the early solar system. Using a combination of high resolution, in-situ X-ray tomography and fast 2-D radiographic imaging, we obtain the first direct data on melt migration velocities at high PT. Melt migration is highly variable and episodic, driven by variations in differential pressure during melt migration and matrix compaction. Smaller scale melt processes, representing migration of melt along pre-existing melt networks, give comparatively fast velocities of 0.6–60 μms−1. Ex-situ experiments are used to compare melt networks in analog systems to Fe-rich melt in silicates. Two competing processes for melt migration are percolation of melt along grain boundaries, and hydraulic fracturing induced by melt injection. Typically, both processes are noted in experimental and natural systems, although the relative importance of each mechanism is variable. Using a simple model for melt flow through a porous media, migration velocities determined here account for full differentiation of Earth-sized bodies within 101–103 Myr, for submicron diameter melt bands, or within a few Myr or micron-sized melt bands. This is consistent with rapid timescales inferred from geochemistry for core formation in planetesimals, implying that percolation may have had an important contribution to core differentiation in the Earth.

Abstract: High‐strain zones are potential pathways of melt migration through the crust. However, the identification of melt‐present high‐strain deformation is commonly limited to cases where the interpreted volume of melt “frozen” within the high‐strain zone is high (>10%). In this contribution, we examine high‐strain zones in the Pembroke Granulite, an otherwise low‐strain outcrop of volcanic arc lower crust exposed in Fiordland, New Zealand. These high‐strain zones display compositional layering, flaser‐shaped mineral grains, and closely spaced foliation planes indicative of high‐strain deformation. Asymmetric leucosome surrounding peritectic garnet grains suggest deformation was synchronous with minor amounts of in situ partial melting. High‐strain zones lack typical mylonite microstructures and instead display typical equilibrium microstructures, such as straight grain boundaries, 120° triple junctions, and subhedral grain shapes. We identify five key microstructures indicative of the former presence of melt within the high‐strain zones: (a) small dihedral angles of interstitial phases; (b) elongate interstitial grains; (c) small aggregates of quartz grains with xenomorphic plagioclase grains connected in three dimensions; (d) fine‐grained, K‐feldspar bearing, multiphase aggregates with or without augite rims; and (e) mm‐ to cm‐scale felsic dykelets. Preservation of key microstructures indicates that deformation ceased as conditions crossed the solidus, breaking the positive feedback loop between deformation and the presence of melt. We propose that microstructures indicative of the former presence of melt, such as the five identified above, may be used as a tool for recognising rocks formed during melt‐present high‐strain deformation where low (<5%) volumes of leucosome are “frozen” within the high‐strain zone. [ABSTRACT FROM AUTHOR]

Porosity waves arise naturally from the equations describing fluid migration in ductile rocks. Here, we show that higher-dimensional porosity waves can transport mass and therefore preserve geochemical signatures, at least partially. Fluid focusing into these high porosity waves leads to recirculation in their center. This recirculating fluid is separated from the background flow field by a circular dividing streamline and transported with the phase velocity of the porosity wave. Unlike models for one-dimensional chromatography in geological porous media, tracer transport in higher-dimensional porosity waves does not produce chromatographic separations between relatively incompatible elements due to the circular flow pattern. This may allow melt that originated from the partial melting of fertile heterogeneities or fluid produced during metamorphism to retain distinct geochemical signatures as they rise buoyantly towards the surface. [ABSTRACT FROM AUTHOR]

The Lower Zone of the Bushveld Complex comprises an up to 2-km-thick package of different ultramafic rock types with an approx. 90-cm-thick, sulphide-bearing noritic interval that occurs in the western and eastern limbs. The distribution and geometry of the zone are highly variable across the Complex, showing pronounced, yet laterally discontinuous layering on different scales. Together with the ubiquitous lack of large-scale fractionation in the Mg# of orthopyroxene, variable Sr isotope compositions and erratic Pt/Pd ratios, these observations strongly suggest an emplacement of the Lower Zone as a sill complex, as these contrasting geochemical characteristics are difficult to account for in a large Bushveld magma chamber, as previously suggested. It is more likely that these sills were episodically fed from a sub-Bushveld staging chamber, and variably contaminated, while passing through the crust before their final emplacement in the Lower Zone. Detailed mineralogical and microtextural work based on high-resolution elemental mapping of a set of samples, covering the entire Lower Zone stratigraphy of the western Bushveld shows that the variations in the late crystallising interstitial mineral mode are different from what would be expect, if all phases crystallised from a fixed initial mass of interstitial liquid. The interstitial mineral mode, represented by plagioclase, clinopyroxene and other late stage phases, shows variable ratios of these minerals ranging from ca. 21:15:64 to 75:17:8. In comparison to modelled expected ratios, most of the analysed rocks have higher amounts of early crystallising interstitial phases (e.g. plagioclase, clinopyroxene), relative to late crystallising phases (e.g. quartz, alkali feldspar). Therefore, interstitial melt must have migrated at different stages of fractionation during cumulate solidification, as a consequence of either compaction or displacement by convecting interstitial liquids. Two samples, however, show the opposite: late phases are relatively more abundant than early ones, which is consistent with a convection-driven replacement of primitive interstitial liquid by more evolved liquid. These results have important implications for the interpretation of the Lower Zone and, by extension, for layered intrusions in general: (1) interstitial sulphide mineralisation may be introduced into a cumulate through infiltrating melts, i.e. the liquid components of a sulphur-saturated crystal mush are not withheld from further migration, upon interaction with a cumulate pile; (2) most importantly, late stage minerals, such as zircon, rarely crystallise from trapped liquid that was initially in equilibrium with the cumulate. Therefore, dating of interstitial zircon from cumulates is unlikely to record the actual timing of emplacement, but merely the crystallisation of a later episode of residual melt that migrated through the cumulate. [ABSTRACT FROM AUTHOR]

Melt extraction from partially molten regions of the mantle occurs along high-permeability pathways. Melt-rock reactions can lead to the formation of high-permeability channels due to a positive feedback between melt flow and reaction. To study this process, we performed a series of Darcytype experiments in which a cylinder of partially molten rock sandwiched between a melt source and a porous sink was annealed at high pressures (P=300 MPa) and high temperatures (T=1200° or 1250°C) under a controlled pressure gradient (oP/ox=0-100MPa mm-1) for up to 5 h. The partially molten rock was formed from 50:50 mixtures of olivine (Ol) and clinopyroxene (Cpx) plus 4, 10 or 20 vol. % of alkali basalt. The melt source was a disk of alkali basalt undersaturated in silica with respect to the partially molten rock and the sink was a disk of porous alumina. During an experiment, melt from the source dissolved Cpx in the partially molten rock and precipitated Ol, thereby forming a Cpx-free reaction layer at the interface between the melt source and the partially molten rock. The melt fraction as well as the grain size in the reaction layer increased significantly compared with that present in the starting material, confirming that the reaction increased the local permeability of the partially molten rock, one of the prerequisites for the reaction infiltration instability process to operate. In experiments carried out under a small pressure gradient (and hence slow melt flow velocity), the reaction layer remained roughly planar and no channels developed. However, if the melt flow velocity by porous flow exceeded ∼0μ1 mms-1, the reaction layer locally protruded into the partially molten rock forming finger-like, melt-rich channels. The morphology and spacing of the channels depended on the initial melt fraction. In a partially molten rock with 20 vol. % melt, multiple, voluminous channels with an elliptical core of pure melt developed. At lower melt contents, fewer and thinner channels formed. Our experiments demonstrate that melt-rock reactions can lead to melt channelization in mantle lithologies, consistent with general predictions of the reaction infiltration instability theory. [ABSTRACT FROM AUTHOR]

Core complexes are domal structures that exhume crust and mantle during extension. They occur in tectonic settings such as mid-ocean-ridges and continental rifted margins and provide important windows into deep Earth processes. The Entia Dome, central Australia, is an enigmatic core complex since it formed during the contractional Alice Springs Orogeny. Here, we provide field, petrographic, geochemical, and experimental evidence that demonstrates melt-present deformation in the lower crust formed weak high strain gneisses at the Entia Dome. Syn-tectonic melt migration within the high strain zones induced melt-mediated reaction softening, grain size reduction, phase mixing and thermal softening, enhancing the dominant rheological weakening caused by the physical presence of melt. We combine our recognition of melt weakened lower crust with the structural architecture of the dome to suggest that NE- and SW-verging, upper crustal thrusting during the Alice Springs Orogeny was kinematically decoupled from the SE-directed extrusion of rheologically weak lower crust. For the first time, we place the evolution of the Entia Dome into the context of a tectonic extrusion hypothesis proposed for the Alice Springs Orogeny, where lower crustal extrusion provided the extensional setting necessary for core complex formation during a contractional orogeny. • High strain gneisses at the Entia Dome formed during melt-present deformation. • Melt-induced rheological weakening facilitated core complex formation. • Kinematic decoupling of extruding lower crust from converging upper crust. • Decoupling resolves core complex formation in a contractional orogen. [ABSTRACT FROM AUTHOR]

Geophysically detectible mid-lithospheric discontinuities (MLD) and lithosphere-asthenosphere boundaries (LAB) beneath cratons have received much attention over recent years, but a consensus on their origin has not yet emerged. Cratonic lithosphere composition and origin is peculiar due to its ultra-depletion during plume or accretionary tectonics, cool present-day geothermal gradients, compositional and rheological stratification and multiple metasomatic overprints. Bearing this in mind, we integrate current knowledge on the physical properties, chemical composition, mineralogy and fabric of cratonic mantle with experimental and thermodynamic constraints on the formation and migration of melts, both below and within cratonic lithosphere, in order to find petrologically viable explanations for cratonic mantle discontinuities. LABs characterised by strong seismic velocity gradients and increased conductivity require the presence of melts, which can form beneath intact cratonic roots reaching to ~ 200–250 km depth only in exceptionally warm and/or volatile-rich mantle, thus explaining the paucity of seismical LAB observations beneath cratons. When present, pervasive interaction of these - typically carbonated - melts with the deep lithosphere leads to densification and thermochemical erosion, which generates topography at the LAB and results in intermittent seismic LAB signals or conflicting seismic, petrologic and thermal LAB depths. In rare cases (e.g. Tanzanian craton), the tops of live melt percolation fronts may appear as MLDs and, after complete lithosphere rejuvenation, may be sites of future, shallower LABs (e.g. North China craton). Since intact cratons are presently tectonomagmatically quiescent, and since MLDs produce both positive and negative velocity gradients, in some cases with anisotropy, most MLDs may be best explained by accumulations (metasomes) of seismically slow minerals (pyroxenes, phlogopite, amphibole, carbonates) deposited during past magmatic-metasomatic activity, or fabric inherited from cratonisation. They may accumulate as layers at, or as subvertical veins above, the depth at which melt flow transitions from pervasive to focussed flow at the mechanical boundary layer, causing azimuthal and radial anisotropy. Thermodynamic calculations investigating the depth range in which small-volume melts can be produced relative to the field of phlogopite stability and the presence of MLDs show that phlogopite precipitates at various pressures as a function of age-dependent thermal state of the cratonic mantle, thus explaining variable MLD depths. Even if not directly observed, such metasomes have been shown to be important ingredients in small-volume volatile-rich melts typically penetrating cratonic lithospheres. The apparent sparseness of evidence for phlogopite-rich assemblages in the mantle xenolith record at geophysically imaged MLD depths, if not due to preferential disaggregation in the kimberlite or alteration, may relate to vagaries of both kimberlite and human sampling. [ABSTRACT FROM AUTHOR]

Differentiation of the continental crust begins with its partial melting. The products of crustal melting are silicic, hydrous, H 2 O-undersaturated, granitic liquids that are generated within matrices of residual crystals. Crustal differentiation requires that felsic magmas form and escape from these solid residua. An important question is whether granitic magmas collect into large batches, within or near their sources, which then give rise to ascent or, alternatively, bleed out of the sources in smaller streams or pulses. We demonstrate that the physical reality is closer to the second alternative, and thus question the validity of the concepts of magma segregation and source fertility, as they are sometimes visualised. Granitic plutons contain chemically distinct sub-populations formed by source-level entrainment of the peritectic assemblages into the melts. Rapid evacuation at source levels and high ascent rates protect the magmas from wall-rock interactions during their tenure at source depths and during their ascent through cool upper crust. The existence of different types of granites, and hence of clearly defined chemically different magmas within plutons, dictates that felsic magmas must separate efficiently from their anatectic sources and must ascend to the sites of their emplacement with minimal chemical interaction with crust through which the magma must move. Exposed deep-crustal sections are typically lithologically diverse, with the more fertile rocks commonly forming discrete layers surrounded by rocks with contrasting compositions. If the melts were required to segregate and accumulate into large volumes within their hot sources, especially by slow, gravity-driven melt percolation, it is very likely that they would be substantially modified by reaction with diverse source rocks. Thus, the processes mooted to occur in melting, assimilation, storage and homogenisation (MASH) or deep crustal hot (DCH) zones would act to erase original compositional heterogeneities and produce larger batches of more homogeneous magma, perhaps carrying evidence of extensive magma mixing. From various lines of evidence, we conclude that MASH and DCH zones may not exist and, even if they do, they cannot form the sources of most granitic magmas. The intrinsic heterogeneity in crustal source rocks, the likely occurrence of peritectic assemblage entrainment and the inefficiency of magma mixing decree that granitic rocks will retain source-inherited chemical and isotopic heterogeneities. Rapid, semi-continuous and disequilibrium withdrawal of small magma batches from source rocks means that elements of whole-rock, trace-element chemistry and accessory mineral chemistry will be decoupled from major-element variations. This also means that a large source with low nominal fertility (due to low hydrous mineral content, for example) could still produce a substantial granitic pluton, by additions of individually small increments of magma. [ABSTRACT FROM AUTHOR]

Dunite bands and dikes in ophiolitic mantle peridotites are interpreted as fossil melt channels within the suboceanic mantle. Concordant dunite bands (i.e. fossil melt channels transposed by outward transportation from the ridge axis via horizontal mantle flow) are particularly important as they possibly represent the melt channels through which the parental melts of mid-ocean ridge basalt (MORB) were transported to shallower depths beneath the paleo-ridge axis. We conducted field observations and sampling of concordant dunite bands (CDB) and their host harzburgite at selected outcrops covering a wide depth range in the mantle section along an inferred paleo-ridge segment in the northern to central part of the Oman ophiolite. The CDB increase in thickness and decrease in frequency upward. They are thicker and more frequent in the centre of the segment than near the segment ends when compared at the same stratigraphic level. The CDB consist mostly of olivine with minor spinel and very rare amounts of pyroxene. Clinopyroxene has a small grain size and an interstitial position relative to olivine. The constituent minerals in the CDB and their host harzburgite were analyzed by electron microprobe for major elements and by laser ablation inductively coupled plasma mass spectrometry for trace elements. Most of the CDB have refractory major element mineral compositions, such as high Fo [100 x Mg/(Mg / Fe)] in olivine (>90.5), high Cr# [Cr/(Cr / Al)] in chromian spinel (>0.50), and low Al2O3 (<3.5 wt %) in clinopyroxene. Chondrite-normalized trace element patterns of clinopyroxene in the host harzburgites consistently show a gentle decrease from heavy REE (HREE) to middle REE (MREE) and a sharp decrease from MREE to light REE (LREE) (= highly depleted), but those in the CDB show weaker LREE depletion, which is more variable depending on the stratigraphic level and position along the paleo-ridge segment. In contrast, the HREE concentrations in clinopyroxene in the CDB are higher than or similar to those of the host harzburgites. Trace element compositions of clinopyroxene in the CDB and their host harzburgites are evaluated with a one-dimensional, steady-state, opensystem decompressional melting-reaction model. The modeling results suggest that an LREE-enriched melt generated at high pressure was transported upwards through melt channels to the shallow mantle (up to the Moho transition zone), where it mingled with highly depleted melts accumulated from fractionally melted peridotites to generate normal (N)-MORB-like melts. The mantle started upwelling (= melting) in the garnet stability field in the segment centre, but either in the garnet or in the spinel stability field near the segment ends. This suggests a variation of geothermal gradient along the paleo-ridge segment: higher in the segment centre and lower near the segment ends. This inference is supported by the presence of thicker (up to 250 cm) CDB as well as more frequent occurrence of CDB in the segment centre than near the segment end and by the geochemical evidence for chromatographic N-MORB-like melt percolation into the host peridotite only in the uppermost horizons near the segment ends. [ABSTRACT FROM AUTHOR]

The petrologic and geochemical diversity of meteorites is a function of the bulk composition of their parent bodies, but also the result of how and when internal differentiation took place. Here we focus on this second aspect considering the two principal parameters involved: size and accretion time of the body. We discuss the interplay of the various time scales related to heating, cooling and drainage of silicate liquids. Based on two phase flow modeling in 1-D spherical geometry, we show that drainage time is proportional to two independent parameters: μ m / R 2 , the ratio of the matrix viscosity to the square of the body radius and μ f / a 2 , the ratio of the liquid viscosity to the square of the matrix grain size. We review the dependence of these properties on temperature, thermal history and degree of melting, demonstrating that they vary by several orders of magnitude during thermal evolution. These variations call into question the results of two phase flow modeling of small body differentiation that assume constant properties. For example, the idea that liquid migration was efficient enough to remove 26Al heat sources from the interior of bodies and dampen their melting (e.g. Moskovitz and Gaidos, 2011; Neumann et al., 2012) relies on percolation rates of silicate liquids overestimated by six to eight orders of magnitude. In bodies accreted during the first few million years of solar-system history, we conclude that drainage cannot prevent the occurrence of a global magma ocean. These conditions seem ideal to explain the generation of the parent-bodies of iron meteorites. A map of the different evolutionary scenarios of small bodies as a function of size and accretion time is proposed. • Differentiation of asteroids relies on the interplay of various time scales. • The main heat source, 26Al, is carried by the first silicate liquids. • Drainage time of silicate liquids controls the asteroid differenciation. • Drainage of silicate liquids is not as efficient as assumed in previous studies. • The bodies accreted during the first million years experienced a global magma ocean. [ABSTRACT FROM AUTHOR]

The Dinaride and Vardar zone ophiolite belts extend from the south-eastern margins of the Alps to the Albanian and Greek ophiolites. Detailed sampling of the Krivaja-Konjuh massif, one of the largest massifs in the Dinaride belt, reveals fertile compositions and an extensive record of deformation at spinel peridotite facies conditions. High Na2O clinopyroxene and spinel-orthopyroxene symplectites after garnet indicate a relatively high pressure, subcontinental origin of the southern and western part of Krivaja, similar to orogenic massifs such as Lherz, Ronda and the Eastern Central Alpine peridotites. Clinopyroxene and spinel compositions from Konjuh show similarities with fertile abyssal peridotite. In the central parts of the massif the spinel lherzolites contain locally abundant patches of plagioclase, indicating impregnation by melt. The migrating melt was orthopyroxene undersaturated, locally converting the peridotites to massive olivine-rich troctolites. Massive gabbros and more evolved gabbro veins cross-cutting peridotites indicate continued melt production at depth. Overall we infer that the massif represents the onset of rifting and early stages of formation of a new ocean basin. In the south of Krivaja very localized chromitite occurrences indicate that much more depleted melts with supra-subduction affinity traversed the massif that have no genetic relationship with the peridotites. This indicates that volcanics with supra-subduction affinity at the margins of the Krivaja-Konjuh massif record separate processes during closure of the ocean basin. Comparison with published compositional data from other Balkan massifs shows that the range of compositions within the Krivaja-Konjuh massif is similar to the compositional range of the western massifs of the Dinarides. The compositions of the Balkan massifs show a west to east gradient, ranging from subcontinental on the western side of the Dinarides to depleted mid-ocean ridge/arc compositions in the Vardar zone in the east. This is consistent with the hypothesis that both ophiolite belts originate in a single ocean, rather than from two separate basins. A distinct decrease in fertility occurs in the south of the Dinarides towards the Albanian ophiolites with supra-subduction affinity. [ABSTRACT FROM AUTHOR]