Melt Escape Research Articles

Multiphase rheology as a cause for stick-slip like melt extraction

When partial melting is parametrized in numerical thermo-mechanical models in a straightforward manner, it directly relates to variations in temperature. However, melt extraction is, to our knowledge rarely taken into account in these models, even if melt extraction and escape have a key influence on the mechanical behaviour of the entire system. Here, we propose a first step towards a numerical simulation for melt extraction in partially molten portions of the continental crust. We use a Lagrangian description, considering a discrete mineral distribution at sample scale. A cellular automaton mimics the mobile melt phase moving from cell to cell. An infinite source and sink for the melt and lateral periodic boundaries avoid melt accumulation. Time is represented by a proxy through successive iterations. The major controlling factors are the initial bulk melt percentage and a threshold controlling melt escape. We test pure and simple shear conditions to simulate strain partitioning between the melt and its matrix. The parameters, provided they do not vary, determine a regular or steady state amount of melt extraction after a few iterations. In contrast, for varying parameters during a single run, the quantity of melt extracted at each step presents alternate variations in amplitude (σμ) around an average value. Variations are neither periodic in the sense of a sinusoidal wave, nor of equal amplitude. Melt extraction behaves similar to a stick-slip motion in dry friction. Since we cannot have access to the prediction of melt amount, w test the origin of the sporadic motion. We examine variations of the input parameters (initial melt ϕ, permeability ζ, applied stress σ or τ). The transient effects mark by spikes (Δμ) with two contrasted situations. Return to a steady state is exponential, similar to the discharge of a capacitor. Nevertheless, a first mode occurs when the changes are made on separate runs, reinitializing the parameters between them. These experiments show a linear dependence of the variation in amplitude of σμ with the parameters. Alternatively, when the changes are successive during the same run, then σμ decreases accordingly. The exponential variation is interpreted as a memory effect or hysteretic behaviour of melt extraction. Similarly, the amplitude of the stiction spikes (Δμ) does not show the same behaviour when permeability increases or decreases. By analogy with friction, the change from sticking to slipping is interpreted as a manifestation of pinning and depinning, leading to an avalanche-like phenomena. Mechanically they correspond to a change in friction velocity. Physically, it points to the change in the free energy of the system, with a first derivative akin to velocity and a second derivative akin to acceleration. During melting, the mechanical barriers correspond to the energy variations due to chemical reactions between melt and crystals. Depinning corresponds to enhanced melting whereas pinning links with back reactions leading to garnet growth. The sporadic melt extraction links dry friction to poroelastic flow, or more generally to a consequence of two-phases rheology. Hence the sporadic motion of the weak phase originates from interactions with its matrix. Hence the flow (velocity) suffers modulations due to interactions with chemical potential (back-reaction, melting), entropy (ordering through dislocation creep), and strain (aperture of porous flow). Such interactions should be considered when dealing with quantitative mush (melt+ matrix) modeling (rejuvenation, extrusion).

Evolution of fluids and melts in deeply subducted continental crust: Insights from an UHP eclogite–vein system in the Dabie terrane, China

Both slab-derived high-pressure (HP) or ultrahigh-pressure (UHP) aqueous fluids and hydrous melts are important agents for material cycling in subduction zones. Although numerous studies have investigated the genesis of both fluid phases, the temporal–spatial relationship and the chemical difference between aqueous fluids and hydrous melts during subduction-zone metamorphism remain mysterious. In this study, we present detailed petrological and geochemical studies on the UHP eclogites and enclosed epidote-quartz veins in the Dabie terrane (eastern China) to understand the compositions and behaviors of fluids/melts in subduction zones. The eclogites consist mainly of garnet, omphacite, and hydrous minerals (epidote, phengite, and paragonite). Two stages of epidote, including epidote inclusions in omphacite (Ep-in) and porphyroblastic epidote (Ep-p), are recognized in the eclogites. The obviously higher contents of strontium (Sr) and light rare earth elements (LREEs) in Ep-p than in Ep-in demonstrate the previous coexistence of lawsonite and Ep-in. The combination of petrographic analyses, epidote Sr isotopes, and phase equilibria modeling indicates lawsonite in the eclogite breakdown at ~2.2–2.7 GPa and 620–750 °C during the initial exhumation, accompanied by the significant release of H2O-rich fluids. This process led to the growth of Ep-p in eclogite and the precipitation of vein omphacite, quartz, and epidote (Ep-v1). On the other hand, many lines of evidence for partial melting in the eclogites are found, including (1) multiphase solid inclusions (MSIs) of Ky + Kfs ± other phases, which represent the in situ products of phengite partial melting; (2) MSIs of Kfs ± Pl ± Qz, which represent the crystallized products of hydrous melts; and (3) veinlets of Kfs ± Di that crosscut the omphacite. Importantly, the MSIs in texture (1) are exclusively hosted in the Ep-p, which formed by lawsonite breakdown. This distinctive phenomenon indicates that the partial melting of phengite was most likely triggered by the injection of H2O from the lawsonite breakdown (i.e., water-fluxed melting). Phase equilibria modeling suggests that the partial melting of phengite occurred at ~1.5–2.0 GPa and 700–750 °C during the further exhumation stage of the eclogites. The occurrence of Ep (Ep-v2) + Pl + Kfs in the veins indicates that melts derived from the eclogites infiltrated the vein system. The compositions of the fluids and melts are quantitatively constrained using the epidote compositions (Ep-v1 and Ep-v2, respectively) and epidote–fluid/melt partition coefficients. The results indicate that the fluids have trace element patterns similar to those of the melts with high contents of large ion lithophile elements (LILEs), Th, U, Pb, and LREEs. Thus, aqueous fluids have the ability to transfer significant amounts of mobile elements from the slab like the hydrous melts, if the appropriate P–T conditions and chemical characteristics of wallrocks can be satisified.Our study reveals multistage metamorphic-fluid/melt evolution in the UHP eclogites, involving metamorphic dehydration and subsequent partial melting during the exhumation of subducted continental crust. We provide petrological evidence linking fluid liberation to the occurrence of partial melting in eclogites. Moreover, this study indicates that the development of vein systems in HP–UHP rocks may provide high-permeability channels that allow the flow and escape of melts formed during later-stage process. We propose that the occurrence of early-stage dehydration events in deeply subducted slabs facilitates the generation of hydrous melts and their transport in subduction zones.

Approaches to simulate destruction of a fuel rod with different fuel types

Due to the revival of interest to the development of fast reactors cooled by liquid metals, the theoretical research for improving their safety is actual. A detailed calculation of all stages of an accident from the beginning to the end requires knowledge of the laws for modelling physical processes occurring in the reactor in case of emergency. The most serious are accidents with core destruction. Simulation of a severe accident in a nuclear reactor is the key element in safety analysis of nuclear power plants. Destruction of fuel rods is one of the most important processes during core degradation that should be calculated. For different types of fuels the mechanisms of the degradation are different too. For example, oxide and metallic fuels usually melt congruently at high temperature, but nitride fuel dissociates. The main objective of the proposed research is to develop models and numerical algorithms for calculation of the destruction of fuel rods with oxide, metallic and nitride fuels. The models of destruction and some calculation results are presented in the paper. The processes are investigated for the first phase of severe accidents, covering the period from the onset of fuel-rod melting to the melt escape from the core center.

Extraction of high-silica granites from an upper crustal magma reservoir: Insights from the Narusongduo magmatic system, Gangdese arc

AbstractThe genesis of high-silica igneous rocks is important for understanding the behavior of shallow magmatic systems. However, although many such studies have focused on the eruption of crystal-poor high-SiO2 rhyolites, the origin of high-silica granites (HSGs) has received comparatively little attention. Here, we present a detailed study of HSGs from the Narusongduo volcanic complex, Gangdese arc. Combining zircon U-Pb geochronology with stratigraphic investigations, we show that the Narusongduo magmatic system was constructed over a period of ≥3.7 Myr with or without lulls. On the basis of zircon textures and ages, diverse zircon populations, including antecrysts and autocrysts, are recognized within the HSGs and volcanic rocks. All of the igneous rocks within the Narusongduo volcanic complex have highly radiogenic Sr–Nd isotopic compositions. Our results indicate the presence of an andesitic magma reservoir in the upper crust at a paleodepth of ~8 km. Ubiquitous zircon antecrysts in the HSGs, combined with compositional similarities between the HSGs and evolved melts of the andesitic magma reservoir, indicate that the Narusongduo HSGs represent melts extracted from the shallow magma reservoir. In addition, our results suggest that magma recharge promoted the escape of high-silica melts to form the Narusongduo HSGs. This work presents an excellent case that kilometer-scale high-silica granites are the differentiated products from an upper crustal magma reservoir. It would make a contribution to contemporary debates concerning the efficiency of crystal–melt separation in upper crustal magmatic systems.

Quantifying frozen melt in crustal rocks: A new melt-o-meter based on zircon rim volumes

Quantifying the distribution of granitic melt at all scales in mid- to lower crustal migmatitic terranes is critical to understand crustal melting processes, chemical differentiation of the crust and its rheological behavior during deformation. We propose a new method to determine the fraction of frozen granitic melt on a hand specimen scale based on the relative volumes of newly precipitated to total zircon (FPZ = Fraction of newly Precipitated Zircon) as obtained by image analysis on dated zircon cores and rims. Using the calculated Zr-solubility [Zr]sat in the melt at the inferred melting temperature and the Zr concentration in the bulk sample [Zr]bulk, the fraction of melt Fmelt can be determined through Fmelt = FPZ × [Zr]bulk / [Zr]sat. The such obtained Fmelt corresponds to the melt fraction in the hand specimen at the time the system closed for melt mobility. Thermodynamic modelling further allows estimation of H2O-contents required to maintain the melt fraction obtained from the melt-o-meter in a molten stage. The applicability of this method has been tested on eight migmatitic samples with peak temperatures between 725 and 925 °C. Most of the lower temperature migmatites (<750 °C) retained Fmelt of 0.15–0.25 (±0.03–0.05), a melt fraction below or at the static melt escape threshold. In contrast, most of the higher-temperature migmatites (>800 °C) retained Fmelt of 0.35–0.50 (±0.07–0.10). At these melt fractions, melt extraction and melt migration from and within the source should be efficient. Consequently, these samples are likely open-system migmatites affected by melt accumulation or depletion processes. The melt-o-meter requires that the rock types under consideration produced a granitic melt that remained zircon-saturated and is therefore restricted to migmatitic meta-sediments and meta-granitoids. When applied carefully, this melt-o-meter offers a new and powerful tool to not only quantify melt distribution but also evaluate the extent of melt mobility in migmatites.

Numerical Investigation of the Molten Metallic and Oxide Fuel Relocation along the Surface of Fuel Pin

This paper presents the results from numerical investigation of the molten (metal or oxide) fuel relocation during a thermal failure of a fuel rod under conditions close to a severe accident with a power increase in a fast rector. Since experimental investigations needed to determine the regularities of an accident with core destruction cannot be carried out under actual reactor conditions due to safety reasons, getting information on the specifics of molten fuel’s movement using mathematical simulation methods is urgent. The methods and approaches used in the considered problem to simulate molten fuel flow and heat transfer with the fuel-rod surface are briefly described. Although the main fuel in reactor units is an oxide fuel at present, the calculations were also performed for metallic (uranium) fuel to evaluate its effect on the development of an accident. In the calculations, account was taken for the different regularities of heat transfer of metal and oxide fuels that resulted from a considerable difference in the Prandtl numbers for melts of these materials. The processes were investigated for the first phase of severe accidents covering the period from the onset of fuel-rod melting to the melt escape from the core center. Fuel-rod melting and fuel flow were simulated for a single fuel rod.

Modelling komatiitic melt accumulation and segregation in the transition zone

Komatiites are probably produced in very hot mantle upwellings or plumes. Under such conditions, melting will take place deep within the upper mantle or even within the mantle transition zone. Due to its compressibility at such pressures, melt might be denser than olivine, but would remain buoyant with respect to a peridotitic mantle both above and below the olivine–wadsleyite phase boundary because of the presence of its higher temperature and denser garnet. We studied the physics of melting and melt segregation within hot upwelling mantle passing through the transition zone, with particular emphasis on the effect of depth-dependent density contrasts between melt and ambient mantle. Assuming a 1D plume, we solved the two-phase flow equations of the melt-matrix system accounting for matrix compaction and porosity-dependent shear and bulk viscosity. We assumed a constant ascent velocity and melt generation rate. In a first model series, the level of neutral buoyancy zneutr is assumed to lie above the depth of onset of melting, i.e. there exists a region where dense melt may lag behind the solid phases within the rising plume. Depending on two non-dimensional numbers (accumulation number Ac, compaction resistance number Cr) we find four regimes: 1) time-dependent melt accumulation in standing porosity waves that scale with the compaction length. The lowermost of these waves broadens with time until a high melt accumulation zone is formed in steady state. During this transient solitary porosity waves may cross the depth of neutral density and escape. 2) steady-state weak melt accumulation near zneutr, 3) no melt accumulation due to small density contrast or, 4) high matrix viscosity. In regime 4 the high mantle viscosity prevents the opening of pore space necessary to accumulate melt. In a second series, the rising mantle crosses the olivine–wadsleyite phase boundary, which imposes a jump in density contrast between melt and ambient mantle. A sharp melt porosity contrast develops and a large melt porosity accumulates immediately above the phase boundary. Both model series show 1) that not only melt density, but also porosity-dependent matrix viscosity controls the melt ascent or accumulation, 2) that there are parameter ranges and physical conditions which may lead to the accumulation of very large melt porosities (> degree of melting), 3) that in spite of melt being denser than olivine at some depths, in general these melts escape these regions and continue to percolate upward faster than the rising mantle. Melting and melt transport under the conditions predicted by the numerical modelling is able to reproduce the compositions of the main types of komatiite. Thus, the accumulation of large melt fractions, and sequential escape of melt from porosity waves, explains several puzzling features of the geochemical compositions of komatiites.

Formation and preservation of biotite-rich microdomains in high-temperature rocks from the Antananarivo Block, Madagascar

Highly restitic rocks from the Antananarivo Block in northern Madagascar are investigated in this study in order to unravel processes of H2O-rich biotite formation in HT rocks. Polyphase metamorphism and melt migration occurred at 0.6 GPa and 850 °C. Biotite remains stable together with orthopyroxene and makes up to 45 vol% of the rock. In addition, three well-characterised and delimited microdomains having different textural, chemical and petrological characteristics are preserved. Thermodynamic models using the specific bulk compositions of the domains are in agreement with petrological observations. These rocks provide evidence that the lower crust may be strongly heterogeneous, locally associated to the formation of hydrous restites controlled by episodes of melt production and melt escape. This has significant consequences for understanding of the lower crust.

Anisotropic tomography of Hokkaido reveals delamination‐induced flow above a subducting slab

AbstractWe present a new 3‐D anisotropic seismic model for the crust and upper mantle beneath Hokkaido (Japan) based on tomographic inversion of P and S arrival times from a regional seismic database. The P model is parameterized with three parameters at each point that describe the azimuthal anisotropy; the S model is represented isotropically. The isotropic P and S velocity anomalies match nearly perfectly. In the crust, they show a prominent linear anomaly in central Hokkaido along the Kamuikotan and Hidaka Belts, which represents the area of eastward underthrusting of the Japan Block underneath the Kuril fore arc. We interpret the high‐velocity anomaly beneath the Hidaka zone as being delaminated mafic crust and entrained mantle lithosphere, which developed due to crustal shortening in the collision zone. One of our vertical sections shows a very unusual configuration for a subduction zone: a low‐velocity slab is overlain by a high‐velocity body in the mantle wedge. We propose that the high‐velocity delaminated material sinking along the slab surface prevents the escape of fluids and melts from the upper part of the slab, where they are generated due to phase transitions. As a result, a large portion of the fluids is entrained downward and lowers the seismic velocities in the slab. The azimuthal anisotropy in the crust clearly corresponds to the major tectonic units and delineates the major suture zones. In the mantle, the anisotropy has a fan‐shaped configuration and most likely represents the deviating of flows starting in southern Hokkaido and splitting into three directions. The western and eastern flows proceed toward the two volcanic groups on Hokkaido, and they may carry with them additional material to trigger the characteristic caldera‐forming eruptions in these groups.

Phase equilibria constraints on the melt fertility of crustal rocks: the effect of subsolidus water loss

AbstractDuring regional prograde metamorphism, H2O generated by ongoing dehydration reactions is likely to be continuously lost from a rock by compaction. Classical melting experiments cannot easily simulate this phenomenon, because ideally, all run products are conserved within the experimental charge, although significant equilibration and H2O generation may occur during heating. Phase equilibria modelling is used to consider the effect of subsolidus water loss (SWL) on subsequent melting relationships of felsic lithologies (including metapelite, metagreywacke and metatonalite) in the suprasolidus. SWL drives the bulk composition towards the minimum saturation point on the boundary of the wet‐melting field and results in significantly reduced subsequent melt generation when compared to melting experiments involving conservation of subsolidus H2O. This effect is most significant at P–T conditions just above the solidus. For initially hydrated rocks, the reduction in melt production causes rheologically critical thresholds (e.g. melt connectivity threshold, melt escape threshold and the solid‐to‐liquid transition) to be intersected at temperatures generally more than 100 °C, higher than predicted by idealized experimental melting curves.

Settling and compaction of chromite cumulates employing a centrifuging piston cylinder and application to layered mafic intrusions

The time scales and mechanics of gravitationally driven crystal settling and compaction is investigated through high temperature (1,280–1,500 °C) centrifuge-assisted experiments on a chromite-basalt melt system at 100–1,500g (0.5 GPa). Subsequently, the feasibility of this process for the formation of dense chromite cumulate layers in large layered mafic intrusions (LMIs) is assessed. Centrifugation leads to a single cumulate layer formed at the gravitational bottom of the capsule. The experimentally observed mechanical settling velocity of a suspension of ~24 vol% chromite is calculated to be about half (~0.53) of the Stokes settling velocity, with a sedimentation exponent n of 2.35 (3). Gravitational settling leads to an orthocumulate layer with a porosity of 0.52 (all porosities as fraction). Formation times for such a layer from a magma with initial chromite contents of 0.1–1 vol% are 140–3.5 days, equal to a growth rate of 0.007–0.3 m/day for grain sizes of 1–2 mm. More compacted chromite layers form with increasing centrifugation time and acceleration through chemical compaction: An increase of grain contact areas and grain sizes together with a decrease in porosity is best explained by pressure dissolution at grain contacts, reprecipitation and grain growth into the intergranular space and a concomitant expulsion of intergranular melt. The relation between the porosity in the cumulate pile and effective pressure integrated over time (Δρ · h · a · t) is best fit with a logarithmic function, in fact confirming that a (pressure) dissolution–reprecipitation process is the dominant mechanism of compaction. The experimentally derived equation allows calculating compaction times: 70–80 % chromite at the bottom of a 1-m-thick chromite layer are reached after 9–250 years, whereas equivalent compaction times are 0.2–0.9 years for olivine (both for 2 mm grain size). The experiments allow to determine the bulk viscosities of chromite and olivine cumulates to be of magnitude 109 Pa s, much lower than previously reported. As long as melt escape from the compacting cumulate remains homogeneous, fluidization does not play any role; however, channelized melt flow may lead to suspension and upward movement of cumulate crystals. In LMIs, chromitite layers are typically part of a sequence with layers of mafic minerals, compaction occurs under the additional weight of the overlying layers and can be achieved in a few years to decades.

The formation of pure anorthosite on the Moon

The giant impact hypothesis for the origin of the Moon points to the creation of a hot, young Moon, likely with a global magma ocean. Such a magma ocean would produce a flotation crust of plagioclase crystals, or an anorthositic crust. Early calculations of the expected anorthosite content of the lunar highlands crust did not match initial measurements of Apollo samples, and more recently have not matched Clementine measurements or SELENE measurements, because they assumed interstitial melt would freeze. We consider a physical model of a magma ocean with an accumulating flotation lid, and find that significant escape of melt can take place for reasonable physical parameters and timescales of melt migration, even allowing for the inhibiting effect of plausible compaction viscosities. This model permits more nearly pure lunar anorthosites, consistent with observations. The model encounters some difficulty in explaining the expulsion of melt for the near-surface crust (up to five-kilometers’ depth) that presumably dominates the Apollo samples, suggesting that impact mixing and tidal heating are needed to explain the discrepancy.

Structural characteristics and thermal stability of bimetallic Cu-Ru nanocluster

Classical molecular dynamics simulations were used to study the structural characteristics and thermodynamic stability of Cu-Ru nanoclusters of most common structural variants like decahedra, icosahedra and cuboctahedra. As a result, it is found that the three types of structures have consistent stability orders from the point of view of bimetallic elemental distributions. The Ru-core/Cu-shell structures and the eutectic-like structures are more stable than solid solution and Cu-core/Ru-shell. The formation energies of bimetallic clusters are consistently positive, which shows that they are not thermodynamically stable. All clusters including the unstable Cu-core/Ru-shell and solid solution clusters, and the relatively stable Ru-core/Cu-shell transform into eutectic-like cluster during heating process. Additionally, the large difference in melting point between pure Cu and pure Ru cause the Cu part in the formed eutectic-like clusters melt first and escape forming separate cluster in the further heating process.

Preferential magma extraction from K- and metal-enriched source regions in the crust

We compare melting of potassic alteration zones in metamorphosed gold deposits with that of unaltered rocks of the same protolith to examine their relative contributions to crust-derived magmas and to investigate the implications for ore genesis. Potassic hydrothermal alteration, at the crustal levels where orogenic gold deposits form, stabilizes a higher proportion of muscovite and biotite than is possible in unaltered rocks at high metamorphic grades. Because these micas contain water, they control the melt fraction generated through dehydration melting in that a greater proportion of micas permits more extensive melting. Orogenic gold deposits, in which mineralization is typically encapsulated by potassic alteration, form at deep-enough crustal levels to survive repeated tectonic activity, which can lead to their being metamorphosed. In the vicinity of this metamorphosed gold mineralization, the greatest proportion of felsic melt is generated in the more metal- and sulfur-rich rocks because of the associated potassic alteration. Ore minerals dissolve and are physically incorporated into the resulting felsic melt, which thereby becomes metal- and sulfur-enriched. Since melt fraction is the dominant control on strain partitioning and melt mobilization, increased melting in K-altered mineralized rocks implies that these sites will be the first to experience melt escape and will continue to be the focus of melt escape during ongoing metamorphism. This strain partitioning promotes shear zone development, and once shearing is localized to K-altered mineralized domains, it may attract external magma, allowing extension and linking with nearby active shear zones. In this way, mineralized zones may connect to a regional network of magma transfer, allowing metal enrichment of migrating magmas. Terrains that underwent widespread K alteration associated with mid-crustal gold enrichment are likely, when metamorphosed, to produce significant volumes of reduced, relatively metal- and sulfur-enriched felsic magma. The ages and relative tectonic preservation potential of different K alteration-associated ore types implies that Au, Ag, As, Sb, Bi, Te, and W may be recycled within the crust through this mechanism, whereas Cu and Mo are unlikely to be recycled and require mantle sourcing to form new intrusion-related ores. Silicate melt derived from preexisting zones of gold enrichment in the lower crust may contribute significantly to the metal budget of intrusion-related gold systems, and possibly some gold-rich porphyry deposits.

Dehydration and melting during continental collision: Constraints from element and isotope geochemistry of low- T/UHP granitic gneiss in the Dabie orogen

A combined study of petrography, whole-rock major and trace elements as well as Rb−Sr and Sm−Nd isotopes, and mineral oxygen isotopes was carried out for two groups of low- T/UHP granitic gneiss in the Dabie orogen. The results demonstrate that metamorphic dehydration and partial melting occurred during exhumation of deeply subducted continent. Zircon δ 18O values of − 2.8 to + 4.7‰ for the gneiss are all lower than normal mantle values of 5.3 ± 0.3‰, consistent with 18O depletion of protolith due to high- T meteoric-hydrothermal alteration at mid-Neoproterozoic. Most samples have extremely low 87Sr/ 86Sr ratios at t 1 = 780 Ma, but very high 87Sr/ 86Sr ratios at t 2 = 230 Ma. This suggests intensive fluid disturbance due to the hydrothermal alteration of protoliths during Neoproterozoic magma emplacement and the metamorphic dehydration during Triassic continental collision. Rb–Sr isotopes, Th/Ta vs. La/Ta and Th/Hf vs. La/Nb relationships suggest that Group I gneiss experienced lower degrees of hydrothermal alteration, but higher degrees of dehydration, than Group II gneiss. The two groups of gneiss have similar patterns of REE and trace element partition. Group I gneiss displays good correlations between Nb and LREEs but no correlations between Nb and LILEs (Rb, Ba, Pb, Th and U), indicating differential mobilities of LILEs during the dehydration. Thus the correlation between Nb and LREEs is inherited from protolith rather than caused by metamorphic modification. Relative to Group I gneiss, Group II gneiss has stronger negative Eu anomaly, lower contents of Sr and Ba but higher contents of Rb, Th and U. In particular, Nb correlates with LILEs (e.g., Rb, Sr, Ba, Th and U), but not with LREEs (La and Ce). This may indicate decoupling between the dehydration and LILEs transport during continental collision. Furthermore, dehydration melting may have occurred due to breakdown of muscovite during “hot” exhumation. Group II gneiss has extremely low contents of FeO + MgO + TiO 2 (1.04 to 2.08 wt.%), high SiO 2 contents of 75.33 to 78.23 wt%, and high total alkali (Na 2O + K 2O) contents (7.52 to 8.92 wt.%), comparable with compositions predicted from partial melting of felsic rocks by experimental studies. Almost no UHP metamorphic minerals survived; felsic veins of fine-grain minerals occurs locally between coarse-grain minerals, resulting in a kind of metatexite migmatites due to dehydration melting without considerable escape of felsic melts from the host gneiss. In contrast, Group I gneiss only shows metamorphic dehydration. Therefore, the two groups of gneiss show contrasting behaviors of fluid–rock interaction during the continental collision.

Magmatic structures in the Krkonoše–Jizera Plutonic Complex, Bohemian Massif: evidence for localized multiphase flow and small-scale thermal–mechanical instabilities in a granitic magma chamber

The present paper examines magmatic structures in the Jizera and Liberec granites of the Krkonoše–Jizera Plutonic Complex, Bohemian Massif. The magmatic structures are here interpreted to preserve direct field evidence for highly localized magma flow and other processes in crystal-rich mushes, and to capture the evolution of physical processes in an ancient granitic magma chamber. We propose that after chamber-wide mixing and hybridization, as suggested by recent petrological studies, laminar magma flow became highly localized to weaker channel-like domains within the higher-strength crystal framework. Mafic schlieren formed at flow rims, and their formation presumably involved gravitational settling and velocity gradient flow sorting coupled with interstitial melt escape. Local thermal or compositional convection may have resulted in the formation of vertical schlieren tubes and ladder dikes whereas subhorizontal tubes or channels formed during flow driven by lateral gradients in magma pressure. After the cessation or deceleration of channel flow, gravity-driven processes (settling of crystals and enclaves, gravitational differentiation, development of downward dripping instabilities), accompanied by compaction, filter pressing and melt segregation, dominated in the crystal mush within the flow channels. Subsequently, magmatic folds developed in schlieren layers and the magma chamber recorded complex, late magmatic strains at high magma crystallinities. Late-stage magma pulsing into localized submagmatic cracks represents the latest events of magmatic history of the chamber prior to its final crystallization. We emphasize that the most favorable environments for the formation and preservation of magmatic structures, such as those hosted in the Jizera and Liberec granites, are slowly cooling crystal-rich mushes. Therefore, where preserved in plutons, these structures may lend strong support for a “mush model” of magmatic systems.

Simulation of Crustal Melt Segregation Through Cellular Automata: Insight on Steady and Non-steady State Effects Under Deformation

We present a numerical model of melt segregation under variable deformation regimes (pure and/or simple shear) and strain rates. The model is adapted from a cellular automaton. The Lagrangian description requires some initial melt content and obeys two thresholds. One allows melt connection and subsequent melt motion. The other rules melt escape. The geometry of the flow is that of pure and/or simple shear. Deformation is a necessary melt-segregating factor, with contraction being more efficient than simple shear. The ratio of escaping melt to the initial melt content defines the coefficient of melt extraction. It varies linearly with initial melt content, thresholds and deformation, which are tested separately. During each run, transient states of the melt-extraction coefficient were characterized by exponential spikes whose amplitude depends on the initial melt content, thresholds and deformation. Departures in melt extraction from average volume are damped when initial conditions are continuously changed, reflecting a memory effect. We suggest that melt extraction relates to a pinning-depinning process. Melt extraction being discontinuous and irregular with time, we suggest that detailed chemistry does not properly document the melt evolution in source rocks (migmatites).

Processes and Conditions During Contact Anatexis, Melt Escape and Restite Formation: the Huntly Gabbro Complex, NE Scotland

The Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic, mid-crustal, layered, mafic intrusions, emplaced into Proterozoic metaclastic rocks of NE Scotland soon after the thermal peak of static, high-T, low-P regional metamorphism. This gabbro and its associated contact metamorphic rocks illustrate a variety of processes operating during contact anatexis and subsequent melt segregation and extraction. These processes may closely mirror those occurring at much larger scales in the deep crust during high-grade regional metamorphism and the generation of granitic magmas. The emplacement of the Huntly mafic magma resulted in high-grade contact metamorphism and, locally, anatexis of metapelites, leading to the formation of migmatites. The migmatites and country-rock schists were studied to establish the physical conditions of metamorphism and anatexis, the nature of the melting reactions, the compositions of the melts produced, and the extent to which melting was a closedor open-system process. The country-rock schists immediately to the south of the Huntly Complex contain mineral assemblages characteristic of the regional andalusite zone. Thermobarometry of an andalusite schist yields regional metamorphic conditions of 537 42 C and 0 27 0 12 GPa, consistent with previously published P±T estimates. The contact metamorphic rocks include sillimanite hornfelses, metatexites and diatexites. The metatexites consist of cordierite±K-feldspar hornfels melanosomes and K-feldspar-rich garnetiferous leucosomes. The diatexites consist of schollen of fine-grained granoblastic hornfels and metatexite suspended in igneous-textured matrix rocks composed of abundant sub/euhedral garnet, cordierite, plagioclase and, locally, orthopyroxene, with minor interstitial biotite, K-feldspar and quartz. The hornfels melanosomes and schollen retained their structural integrity during partial melting, but the matrix rocks did not. In the highestgrade diatexites, the assemblage Grt ‡ Opx ‡ Crd‡ Hc ‡ Pl characterizes both the hornfels schollen and the sub/euhedral minerals of the matrix rocks. Application of phase equilibria to Opx-bearing rocks yields estimated peak-metamorphic conditions of 900 50 C, 0 45 0 1 GPa and aH2O5 0 3. The pressure estimate implies an emplacement depth of 16 3 km. The prograde P±T path of contact-metamorphic rocks had a low, positive dP/dT slope, indicating that the gabbro intrusion increased the lithostatic load on the country rocks by overplating. Pseudomorph textures involving Al-silicates provide strong evidence that the diatexites evolved from andalusite schists via a sillimanite hornfels stage. Mineralogical changes reflect a sequence of dehydration reactions, followed by fluid-absent partial-melting reactions involving biotite breakdown. It was the fluid-absent reactions that generated the sub/euhedral minerals in the diatexites, as peritectic phases. In many of the highestgrade diatexites, quartz-free anhydrous solid assemblages were produced via reactions such as Bt ‡ Sil ‡ Grt ˆ Crd ‡ Hc ‡ Kfs ‡ melt and Bt‡ Grt ‡ Crd ˆ Opx ‡ Hc ‡ Kfs ‡ melt. Whole-rock major-element geochemical studies indicate that the Opx±Crd hornfelses and diatexite matrices are depleted in Si and K relative to their schist protoliths. Mass-balance calculations indicate that (1) the Opx±Crd hornfels xenoliths represent solid Ca-, Mg-, Fe-, Al-, Na-rich residues left after extraction of 60% melt; (2) the Opx-bearing diatexite matrix rocks are also residual, and represent restite-enriched crystal±liquid mushes left after extraction of 55% melt; (3) the Opx-free diatexite matrix rocks probably represent restite-enriched mushes that retained a higher proportion of residual melt; (4) the anatectic melts were of H2O-undersaturated, peraluminous, JOURNAL OF PETROLOGY VOLUME 44 NUMBER 6 PAGES 995±1029 2003

Contrasting magmas in metapelitic and metapsammitic migmatites in the Cooma Complex, Australia

Metapelite-derived migmatites (“bedded migmatites”) formed in the low-pressure/high-temperature (LPHT) Cooma Complex, southeastern Australia, contain magma (neosome and leucosome) confined to the metapelitic beds in which they were generated. The metapsammitic beds were more ductile than the metapelitic beds (and the metapelitic parts of graded beds), which underwent fracture and boudinage, thereby providing space for the magma, though some also occurs in axial surface folia. Transitions from bedded to stromatic migmatites can be seen, but the magma mainly remained in the metapelites, even in the most strongly deformed stromatic migmatites. This, together with boudinage and transposition of the leucosome, as well as microstructural evidence of quartz recrystallization, suggest that much or most of the stromatic layering was formed by solid-state deformation. In contrast, magmas (neosomes) formed by partial melting of feldspathic metapsammites at Cooma moved out of their parent rocks, and coalesced into veins and small intrusions of diatexite, because (1) the host rocks deformed more homogeneously, and no interboudin space was made for the melts, and (2) the melt escape threshold was exceeded, probably with the assistance of deformation. Metapsammite melting occurred after solidification of the metapelite-derived magma, and the mobile metapsammite-derived magma (diatexite) disrupted and incorporated fragments of the metapelitic migmatites. The metapsammite-derived magma, together with this solid metapelitic material, locally coalesced into bodies closely resembling the Cooma Granodiorite.

Boron in metapelites controlled by the breakdown of tourmaline and retrograde formation of borosilicates in the Yanai area, Ryoke metamorphic belt, SW Japan

Tourmaline-out isograd formed by the breakdown of tourmaline is defined in the upper amphibolite-facies metapelites in the Yanai area, Ryoke metamorphic belt, SW Japan. The rim composition of tourmaline progressively becomes aluminous with ascending metamorphic grade, and the chemical zoning of tourmaline is controlled by X□AlNa–1Mg–1 and MgTiYAl–2 vectors in low- to medium-grade zones where muscovite is stable, whereas it is controlled by Mg(OH)YAl–1O–1, CaMgOX□–1YAl–1(OH)–1 and MgTiYAl–2 vectors in further higher–grade, muscovite-unstable zones. The size of tourmaline increases drastically where breakdown of muscovite+quartz takes place, probably due to the growth of tourmaline during breakdown of muscovite. On the high-temperature side of the tourmaline-out isograd, depletion of whole-rock boron is observed. Escape of boron-bearing melt or the fluid evolved from the melt during its crystallization probably caused this depletion, although locally trapped, boron-bearing melt or fluid formed irregularly shaped tourmaline and dumortierite during retrograde metamorphism.