Bulk Melt Research Articles

Thermal Expansion of BaO · 2B2O3 in the Vitreous and Crystalline States: I. Thermal Expansion and Density of Barium Diborate Prepared from Monolithic Glass of the Same Composition

The thermal expansion and density of vitreous and polycrystalline barium diborates prepared by crystallization of monolithic glasses of stoichiometric composition are investigated in the temperature range 20–700°C. The glass crystallizes in the form of the α and β modifications depending on the heat treatment conditions. The α phase nucleates on the surface of the monolithic sample and grows inward the sample in the form of textured layers. The β phase nucleates in the melt bulk and grows in the form of spherulites. Textured polycrystalline barium diborate in the form of the α phase possesses a pronounced anisotropy of thermal expansion with respect to the direction of crystalline-layer growth. This is associated with the negative expansion along one of the crystallographic axes. The effective temperature coefficients of thermal expansion in the temperature range 20–600°C increase in the order: α phase-glass-β phase. Upon heating above 690°C, the β phase transforms partially into the α phase. This is accompanied by a considerable increase in the volume. The densities of the glass and polycrystalline samples in the temperature range 20–700°C increase in the order: glass-α phase–β phase.

Dissolution of Quartz, Albite, and Orthoclase in H2O-Saturated Haplogranitic Melt at 800°C and 200 MPa: Diffusive Transport Properties of Granitic Melts at Crustal Anatectic Conditions

We have conducted experiments on dissolution of quartz, albite, orthoclase, and corundum into H2O-saturated haplogranite melt at 800 � C and 200MPa over a duration of 120–1488h with the aim of ascertaining the diffusive transport properties of granitic melts at crustal anatectic temperatures. Cylinders of anhydrous starting glass and a single mineral phase (quartz or feldspar) were juxtaposed along flat and polished surfaces inside gold or platinum capsules with � 10 wt % added H2O. Concentration profiles in glass (quenched melt) perpendicular to the mineral–glass interfaces and comparison with relevant phase diagrams suggest that melts at the interface are saturated in the dissolving phases after 384h, and with longer durations the concentration profiles are controlled only by diffusion of components in the melt. The evolution of the concentration profiles with time indicates that uncoupled diffusion in the melt takes place along the following four linearly independent directions in oxide composition space: SiO2 ,N a2O, and K2O axes (Si-, Na-, and K-eigenvectors, respectively), and a direction between the Al2O3, Na2O, and K2O axes (Al-eigenvector), such that the Al/Na molar ratio is equal to that of the bulk melt and the Al/(Na þ K) molar ratio is equal to the equilibrium ASI (¼ mol. Al2O3/[Na2O þ K2O]) of the melt. Experiments in which a glass cylinder was sandwiched between two mineral cylinders—quartz and albite, quartz and K-feldspar, or albite and corundum—tested the validity of the inferred directions of uncoupled diffusion and explored longrange chemical communication in the melt via chemical potential gradients. The application of available solutions to the diffusion equations for the experimental quartz and feldspar dissolution data provides diffusivities along the directions of the Si-eigenvector and Al-eigenvector of � (2� 0–2� 8) · 10 � 15 m 2 /s and � (0� 6–2� 4) · 10 � 14 m 2 /s, respectively. Minimum diffusivities of alkalis [� (3–9) · 10 � 11 m 2 /s] are orders of magnitude greater than the tetrahedral components of the melt. The information provided here determines the rate at which crustal anatexis can occur when sufficient heat is supplied and diffusion is the only mass transport (mixing) process in the melt. The calculated diffusivities imply that a quartzo-feldspathic source rock with initial grain size of 2–3mm undergoing hydrostatic, H2O-saturated melting at 800 � C (infinite heat supply) could produce 20–30 vol. % of homogeneous melt in less than 1–10 years. Slower diffusion in H2O-undersaturated melts will increase this time frame.

Polystyrene nanocomposites based on carbazole-containing surfactants

New organically-modified clays containing a carbazole unit were prepared and the number of long alkyl chains on the surfactant was varied. The clay was used to prepare polystyrene nanocomposites by both bulk polymerization and melt blending. The dispersion of these clays in the polymer matrix was evaluated using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The thermal stability of the clays and the nanocomposites were analyzed by thermogravimetric analysis (TGA) while the fire properties were evaluated by cone calorimetry. If more than two alkyl chains were present, the gallery spacing is apparently overcrowded, leading to poor dispersion. Bulk polymerization gave nanocomposites with better dispersion and reduced flammability when compared to the melt blending process.

Dynamics of partially faceted melt–crystal interfaces III: Three-dimensional computational approach and calculations

The modeling of partially faceted melt–crystal interfaces in bulk melt growth systems has been addressed in a number of recent publications. In particular, in Weinstein and Brandon [J. Crystal Growth 268(1–2) (2004) 299], a method for self-consistent two-dimensional dynamic analysis of such systems while accounting for both macro- and nano-scale phenomena, which result from the coupling between competing kinetic mechanisms and associated thermal fields, was presented. In this manuscript, we report on an extension of this approach to three-dimensional systems. The method is first described in detail after which it is applied to model processes involving the vertical gradient freeze growth both of silicon and of yttrium aluminum garnet. In axisymmetric situations, results are shown to successfully reproduce calculations obtained using the previous two-dimensional modeling approach. Additional results demonstrate a number of important three-dimensional nano- and macro-scale features of the melt–crystal interface. These include observations of the dominant role of the coldest dislocation step source in the case where more than one such dislocation line intersects an advancing facet, a demonstration of the effect of growth rate on the morphology of a multi-faceted interface, and a simple explicit analysis of step flow on an evolving facet.

Styrenic nanocomposites prepared using a novel biphenyl-containing modified clay

Montmorillonite was organically modified using an ammonium salt containing 4-acetylbiphenyl. This clay (BPNC16 clay) was used to prepare polystyrene (PS), acrylonitrile butadiene styrene (ABS) and high impact polystyrene (HIPS) nanocomposites. Polystyrene nanocomposites were prepared both by in situ bulk polymerisation and melt blending processes, while the ABS and HIPS nanocomposites were prepared only by melt blending. X-ray diffraction and transmission electron microscopy were used to confirm nanocomposite formation. Thermogravimetric analysis was used to evaluate thermal stability and the flammability properties were evaluated using cone calorimetry. By thermogravimetry, BPNC16 clay was found to show high thermal stability, and by cone calorimetry, a decrease in both the peak heat release rate and the mass loss rate was observed for the nanocomposites.

Evolution of particle morphology in semisolid processing

Experimental results are summarized of the formation of spheroidal grain structures in an Al-4.5 wt pct Cu alloy by (1) isothermally holding very fine equiaxed dendrites in the liquid-solid region and (2) direct spheroidal solidification from the melt. The fine-grained dendritic structure was obtained by very rapid solidification in a thin-section (0.8 mm) permanent-mold plate casting under conditions such that solidification took place during the filling process and, hence, during rapid fluid flow. Grains of approximately 30 µm in diameter (representing a grain density of about 3×104 mm−3) were obtained; these ripened to a spheroidal morphology in the liquid-solid region in less than 5 seconds. A similarly high grain count was then obtained in the bulk melt by vigorous agitation and rapid cooling near the liquidus temperature; the melt was thereafter cooled more slowly. Under these conditions, a spherical morphology formed early in solidification and grew in that form. The evolution of particle size with time in the liquid-solid region is shown to be essentially identical in the initially dendritic and spherical-growth experiments.

Atomic mobility in liquid gallium under nanoconfinement

Results of NMR studies of nuclear spin-lattice relaxation in liquid metallic gallium confined within random pore networks of two different porous glasses with 16 and 2 nm pore sizes are presented. The measurements were run in the temperature range from 330 K to confined gallium freezing. Relaxation for both gallium isotopes $^{71}\mathrm{Ga}$ and $^{69}\mathrm{Ga}$ was found to accelerate remarkably compared to the bulk melt, the dominant mechanism of relaxation changing from magnetic to quadrupolar. The correlation time of electric field gradient fluctuations caused by atomic motion was estimated at various temperatures using data for quadrupolar relaxation contribution and was found to increase drastically compared to bulk, which corresponded to a pronounced slowdown of atomic mobility in confined liquid gallium. The influence of confinement was more effective for smaller pore sizes. The temperature dependence of the correlation time for confined gallium was found to be noticeably stronger than in bulk, an additional slowdown of atomic mobility being observed at low temperatures.

Melilitite at Fort Portal, Uganda: Another dimension to the carbonate volcanism

Because the calciocarbonatite lavas at Fort Portal were the first ever described they have received great attention, with the pyroclastic rocks being relatively neglected. Volumetrically the lavas are minute, and the major deposit is a 2 m thick blanket of “flaggy” tuffs, long regarded as carbonatite tuff with crustal debris. Fresh examination shows these tuffs to contain melilitite previously unreported from Fort Portal. The rock is a mix of crust and mantle debris with near-isotropic lapilli, set in a matrix composed predominantly of carbonate. The low birefringence parts of the lapilli are devitrified melilitite glass. Compound lapilli are abundant, containing aggregates of globules, together with xenolithic/crystic fragments. In some, there are concentric zones of more carbonate rich material alternating with melilitite: tangential phlogopite flakes mark the outer zones, in marked contrast to their planar distribution through the enclosing rock matrix. Euhedral titano-magnetite (10–15%) is the most obvious cognate mineral. Devitrified melilitite contains abundant small crystals and microlites of melilite, apatite, magnetite, and carbonates, mostly formed during disequilibrium quench crystallisation. Because of this, and widespread fine grained accidental debris, a precise bulk melt composition is hard to obtain, but the average is close to melilitite with high P 2O 5. Mantle debris is largely disaggregated magnetite–phlogopite clinopyroxenite, which could give a bulk composition close to the melt. Low Mg and high Mg calcite are present in the melilitite lapilli, and in the enclosing carbonate rich matrix. Previously, high Mg calcite was reported only as cement in lapilli tuffs, while the lavas contain only low Mg calcite in the assemblage calcite–periclase (consistent with low pressure carbonate melt crystallisation). Carbonatite–melilitite magma left the mantle carrying restite debris. Melt fragmentation took place in the deep crust, with rapidly quenched droplets enclosing crust debris. Chemical covariations within the flaggy tuffs are uniform and explicable as carbonatite–melilitite plus a thoroughly mixed combination of crust and mantle debris. New links are indicated with the alkaline ultramafic-carbonate volcanism to the south, in Uganda, and parallels with that in Italy.

Trapped Field in Individual and Stacked Rings of Bulk Melt Processed Y-Ba-Cu-O

Single grain rare earth barium cuprate [(RE)BCO] high temperature superconductors can trap large magnetic fields. In principle, samples can be stacked to form a quasipermanent magnet with a high length to width aspect ratio for engineering applications. The flux trapping properties of a stack of ring-shaped samples of Y-Ba-Cu-O (YBCO) were measured both on the surface and through the bore. Significant discrepancies between the experimental results and the predictions of a simple theory based on the Biot-Savart model were observed. These differences can be accounted for by the presence of a high density of small cracks parallel to the ab planes, which inhibit the circulation of current around the sample.

Synergy between nanocomposite formation and low levels of bromine on fire retardancy in polystyrenes

An organically-modified clay has been prepared using ammonium salts which contain an oligomeric material consisting of vinylbenzyl chloride, styrene and dibromostyrene. The presence of dibromostyrene enhances the flame retardancy of polystyrene nanocomposites compared to both the virgin polymer and polystyrene nanocomposites prepared from non-halogen-containing organically-modified clays. The nanocomposites were prepared both by bulk polymerization and melt blending and they were evaluated by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis and cone calorimetry measurements. Bulk polymerization produced nanocomposites with reduced peak heat release rate, reduced total heat release and improved thermal stability. It is noteworthy that all these improvements were obtained with clay loading as low as 3% and bromine content less than 4%.

Numerical Simulation of Grain Structure Evolution in Solidification of an Al-5.0wt%Cu Alloy under Electromagnetic Stirring and Its Experimental Verification

A two-dimensional computational model, coupling a hmacroscopic heat and fluid flow analysis with a cellular automaton model under electromagnetic stirring, was developed. The dynamic evolution of solidification grain structures and transport phenomena during solidification were presented successfully. The Gaussian distribution was chosen to deal with nucleation events both on the mold wall and in the bulk melt. The KGT (Kurz-Giovanola-Trivedi) model, based on the calculated temperature field, was adopted to evaluate the growth velocity of a dendrite tip. The material used was an Al-5.0wt%Cu alloy. Both the computational and experimental results showed that grain refinement was improved remarkably with electromagnetic stirring. The fully equiaxed grain structures were obtained with a proper choosing of the casting process variables, such as pouring temperature, cooling rate and stirring intensity.

Crystal nucleation in hydrous rhyolite: Experimental data applied to classical theory

Feldspar nucleation rate data obtained by laboratory decompression of hydrous silicate melt are interpreted in view of the classical theory of nucleation (CNT) and a non-classical variation, the diffuse-interface theory (DIT). The nucleation rate data can be modeled by the CNT formalism only if the interfacial free energy (σ) is allowed to vary as a function of composition. The values thus obtained vary by a factor of four (0.024–0.100 J/m2) and decrease systematically over a sixfold increase in dissolved H2O content (0.8–4.8 wt%). This result is qualitatively consistent with the effects of dissolved H2O on the liquid-vapor interfacial free energy in haplogranite magma (Mangan and Sisson 2000) and the liquid-crystal interfacial free energy in the one-component Li-disilicate system (Davis et al. 1997). The DIT states that the interfacial region between the bulk solid and bulk melt has thermodynamic properties intermediate between these phases, and that σ is defined as the difference between the interfacial enthalpy ( H int) and interfacial entropy ( TS int). If the DIT model is correct, the nucleation rate data for feldspar may indicate that: (1) dissolved H2O content controls the spatial distribution of enthalpy and configurational entropy around incipient crystals, and (2) the spatial gradients of these potentials diverge during devolatilization. This study suggests that crystal nucleation studies may yield insights into the structure and thermodynamics of hydrous melts; likewise, experimental studies are important for refining a physical understanding of nucleation phenomena. Our results can be applied to quantitative numerical models of ascent-driven magma crystallization.

Purification of polycrystalline [formula omitted]-caprolactam particles

The purification of granules, which are polycrystalline particles, suspended in a melt has been studied experimentally. A stirred vessel was used as purification device, since the operating conditions can be manipulated easily in this configuration. The particles consist of a skeleton of caprolactam crystals. The impurities are mainly located in the interstitial space or pores between the crystals. A model that describes the purification of porous and crystalline particles is presented. The driving force for purification is the difference between the pore-averaged impurity concentration and the impurity concentration in the bulk melt. The model accounts for pore diffusion. The mass transfer coefficient has been related to the purification rate, taking the size and the shape of the particles into account. On average 94% of the impurities could be removed from the particles. The remaining 6% is attributed to inclusions (2%) and to equilibrium between the impurity concentration in the pores and the melt. The purification rate constant was in the order of magnitude of 5 × 10 - 4 s - 1 . The purification rate constant is affected by the overall water content of the particle, the purification temperature and the particle morphology. The presented model can be used for the design of a purifier of polycrystalline particles which may take the appearance of a stirred vessel or a gravity wash column.

Hawaiian hot-spot swell structure from seafloor MT sounding

Seafloor magnetotelluric (MT) data were collected at seven sites across the Hawaiian hot spot swell, spread approximately evenly between 120 and 800 km southwest of the Hawaiian-Emperor island chain. All data are consistent with an electrical strike direction of 300°, aligned along the seamount chain, and are well fit using two-dimensional (2D) inversion. The major features of the 2D electrical model are a resistive lithosphere underlain by a conductive lower mantle, and a narrow, conductive, ‘plume’ connecting the surface of the islands to the lower mantle. This plume is required; without it the swell bathymetry produces a large divergence of the along-strike and across-strike components of the MT fields, which is not seen in the data. The plume radius appears to be less than 100 km, and its resistivity of around 10 Ωm, extending to a depth of 150 km, is consistent with a bulk melt fraction of 5–10%. A seismic low velocity region (LVR) observed by Laske et al. [Laske, G., Phipp Morgan, J., Orcutt, J.A., 1999. First results from the Hawaiian SWELL experiment, Geophys. Res. Lett. 26, 3397–3400] at depths centered around 60 km and extending 300 km from the islands is not reflected in our inverse model, which extends high lithospheric resistivities to the edge of the conductive plume. Forward modeling shows that resistivities in the seismic LVR can be lowered at most to 30 Ωm, suggesting a maximum of 1% connected melt and probably less. However, a model of hot subsolidus lithosphere of 10 2 Ωm (1450–1500 °C) within the seismic LVR increasing to an off-swell resistivity of >10 3 Ωm (<1300 °C) fits the MT data adequately and is also consistent with the 5% drop in seismic velocities within the LVR. This suggests a ‘hot, dry lithosphere’ model of thermal rejuvination, or possibly underplated lithosphere depleted in volatiles due to melt extraction, either of which is derived from a relatively narrow mantle plume source of about 100 km radius. A simple thermal buoyancy calculation shows that the temperature structure implied by the electrical and seismic measurements is in quantitative agreement with the swell bathymetry.

Evidence for ocean water invasion into the Chicxulub crater at the Cretaceous/Tertiary boundary

Abstract— The possibility of ocean water invasion into the Chicxulub crater following the impact at the Cretaceous/Tertiary boundary was investigated based on examination of an impactite between approximately 794.63 and 894.94 m in the Yaxcopoil‐1 (Yax‐1) core. The presence of cross lamination in the uppermost part of the impactite suggests the influence of an ocean current at least during the sedimentation of this interval. Abundant occurrence of nannofossils of late Campanian to early Maastrichtian age in the matrices of samples from the upper part of the impactite suggests that the carbonate sediments deposited on the inner rim margin and outside the crater were eroded and transported into the crater most likely by ocean water that invaded the crater after its formation. The maximum grain size of limestone lithics and vesicular melt fragments, and grain and bulk chemical compositions show a cyclic variation in the upper part of the impactite. The upward fining grain size and the absence of erosional contact at the base of each cycle suggest that the sediments were derived from resuspension of units elsewhere in the crater, most likely by high energy currents association with ocean water invasion.

Evidence for ocean water invasion into the Chicxulub crater at the Cretaceous/Tertiary boundary

Abstract The possibility of ocean water invasion into the Chicxulub crater following the impact at the Cretaceous/Tertiary boundary was investigated based on examination of an impactite between approximately 794.63 and 894.94 m in the Yaxcopoil‐1 (Yax‐1) core. The presence of cross lamination in the uppermost part of the impactite suggests the influence of an ocean current at least during the sedimentation of this interval. Abundant occurrence of nannofossils of late Campanian to early Maastrichtian age in the matrices of samples from the upper part of the impactite suggests that the carbonate sediments deposited on the inner rim margin and outside the crater were eroded and transported into the crater most likely by ocean water that invaded the crater after its formation. The maximum grain size of limestone lithics and vesicular melt fragments, and grain and bulk chemical compositions show a cyclic variation in the upper part of the impactite. The upward fining grain size and the absence of erosional contact at the base of each cycle suggest that the sediments were derived from resuspension of units elsewhere in the crater, most likely by high energy currents association with ocean water invasion.

Shear wave attenuation and dispersion in melt‐bearing olivine polycrystals: 2. Microstructural interpretation and seismological implications

The torsional forced oscillation tests of melt‐bearing olivine aggregates reported by Jackson et al. [2004] consistently show a peak in attenuation that is absent from melt‐free aggregates tested under similar conditions and grain sizes. Characterization by SEM shows that the melt resides in triple junction tubules and larger pockets as previously described. TEM imaging and EDS analysis reveals that olivine‐olivine grain boundaries are characterized by a region ≤1 nm wide which is structurally and chemically distinct from olivine grain interiors. From the possible mechanisms that can produce an anelastic attenuation peak, melt squirt can be eliminated for our samples and experimental conditions. We attribute the observed attenuation peak to elastically accommodated grain boundary sliding, requiring that the grain boundaries are weak relative to olivine grain interiors but have a significantly higher viscosity than bulk melt. While the nanometer scale grain boundary structure in the melt‐bearing aggregates is essentially the same as for melt‐free aggregates studied previously, elastically accommodated sliding in the latter is apparently inhibited by tight three‐grain edge intersections. The exponentially increasing high temperature background attenuation in both types of aggregate is attributed to diffusionally accommodated grain boundary sliding. Extrapolation to mantle grain sizes shows that the broad peak may be responsible for nearly frequency independent attenuation in partially molten regions of the upper mantle.

Interface between End-Functionalized PEO Oligomers and a Silica Nanoparticle Studied by Molecular Dynamics Simulations

Fully atomistic molecular dynamics (MD) simulations have been carried out on a series of bulk melt pure PEO oligomers and PEO oligomer−silica systems, which differed by their macromolecular end groups. A realistic hybrid model of a silica nanoparticle combining an ionic core as well as the fine-tuning of the surface thickness and number of OH groups per unit surface area was used. The PEO oligomers were decorrelated in all systems under study in order to prevent any artifacts related to the preparation procedure. Significant changes were found to occur in the immediate vicinity of the interface with flattened PEO backbones arranged in densily packed shells and stabilized by the added PEO−silica interactions. Their conformations were also more coiled in order to better adapt to the surface structure. While methyl end groups did not show special characteristics other than their steric effect, hydroxyl end groups had a much higher affinity for the surface and tended to position themselves perpendicular to the surface, thus forming dynamic hydrogen-bonding complexes between the hydroxyl oxygens and the silanol hydrogens. The range of influence of the nanoparticle was evident for structural properties only up to two or three molecular layers, 10−15 Å, but was approximately twice that for dynamical ones.

The behavior of Li and B during planetary basalt crystallization

Light-lithophile-element zoning patterns in pyroxene in the Shergotty and Zagami martian basaltic meteorites have previously been presented as evidence for pre-eruptive martian water. We investigate the partitioning of Li and B in martian igneous minerals experimentally, using a composition approximating that of the Queen Alexandria Range (QUE) 94201 martian basalt in one-atmosphere gas-mixing furnaces. Partition coefficients obtained for pyroxene (D Li = 0.2, D B = 0.023), plagioclase (D Li = 0.37, DB = 0.024), olivine (D Li = 0.27, DB = 0.007) and merrillite (D Li = 0.7, DB = 0.03), measured using Secondary Ion Mass Spectrometry of experimental charges, demonstrate that Li and B are incompatible in these minerals. As a test of our experimentally determined, anhydrous partition coefficients, we analyzed pyroxene in the Pasamonte eucrite, an asteroidal basalt with an anhydrous petrogenesis. Coupling of the partition coefficients with Pasamonte pyroxene analyses gives B parental melt concentrations in relative agreement with bulk rock analyses; however, Li parental melt concentrations and bulk analyses do not agree, suggestive of post-crystallization modification of Li zoning trends. Our results demonstrate that Li and B zoning patterns in pyroxene in martian basalts cannot be explained by the uptake of Li and B by co-crystallizing plagioclase, olivine, or merrillite, and suggest that the petrogeneses of Shergotty and Zagami involved something other than simple, one-atmosphere, anhydrous crystallization

Order or chaos? Origin and mode of emplacement of breccias in floors of large impact structures

Breccias in the crater floor of large impact structures are pseudotachylites (sensu largo), authigenic monomict and polymict clastic-matrix breccias, so-called footwall breccias, and impact melt breccias. Pseudotachylite bodies in the center of large impact structures (e.g., Vredefort Dome, South Africa) appear to have a random distribution and orientation, but most dip steeply or vertically. Large bodies of pseudotachylite in the more distal sectors of the >200-km-diameter Sudbury Structure have been interpreted as ring and terrace collapse features. In the Vredefort Dome, networks of randomly distributed pseudotachylite veins accompany large (“mother lode”) pseudotachylite dikes. In general, pseudotachylites in the floors of central parts of impact craters may form through explosive transfer of thermal shock energy, in a process that could be termed “flash replacement melting”, whereas pseudotachylites at large distances from the centers of large impact structure are believed to have formed through friction leading to partial or complete melting, similar to the formation of tectonic pseudotachylites. In smaller structures (e.g., Ries and Slate Islands), clastic-matrix breccias instead of pseudotachylites occur as the most common breccias in the crater floors. They have a chaotic distribution pattern. Their dips are commonly also steep to vertical. Melt breccia dikes in the target rocks of the crater floor are associated with melt sheets that fill the lower part of the excavation cavity. At Vredefort, erosion has removed the coherent melt sheet, but melt breccia dikes (Vredefort Granophyre) in the crater floor are preserved. They are characterized by a remarkably homogeneous chemical composition and are believed to represent the initial, undifferentiated impact melt. Near the Vredefort collar, the Granophyre forms more or less concentric dikes. In the more central parts of the Dome, their orientation is more random, but, in places, may be controlled by the Archean fabric of the crater basement. The “Offset” dikes of the Sudbury Structure are associated with the Sudbury Igneous Complex that represents, in total or in part, a differentiated impact melt sheet. The dikes, in many aspects, are similar to Vredefort Granophyres, but their interpretation as undifferentiated bulk melt is problematic on geochemical and structural grounds. Chemically, these dikes are not homogeneous and, in places, they contain massive sulfide deposits that are unlikely components of an undifferentiated impact melt. Differentiation and precipitation of massive sulfides are slow processes compared with the presumably high energy and fast emplacement of a supposedly undifferentiated melt. Pseudotachylites of the central impact crater apparently are mainly produced in the compression phase of the impact process, whereas clastic-matrix breccias form during the uplift and crater modification phases. Impact melt breccia dikes contain pseudotachylite-bearing inclusions and cut across pseudotachylite bodies. Such impact melt breccia dikes were probably emplaced during uplift and crater modification.