Melt Boundary Layer Research Articles

Variations in orientation relationships between rutile inclusions and garnet host relate to magmatic growth zoning

Rutile inclusions in almandine-spessartine garnet from a peraluminous pegmatoid from the Moldanubian zone (Bohemian Massif, AT) show distinct changes in aspect ratio, shape preferred orientations (SPO) and crystallographic orientation relationships (COR) along the transition between microstructurally different growth zones in the garnet core and rim. For identification of the COR characteristics we pool specific CORs based on their common axial relationship into three COR groups: Group 103R/111G, Group 001R/111G and Group 001R/100G. The rutile inclusions in the garnet core domains are elongated along the four Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document} directions and are dominated by COR Group 103R/111G. The garnet rim zone additionally contains rutile needles elongated along Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}100⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document}. Here, Group 001R/111G and 001R/100G are more abundant than in the garnet core. Needle-shaped rutile in the rim shows a systematic correlation between SPOs and CORs as needles elongated parallel to Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document} are dominated by Group 103R/111G and 001R/111G, whereas those needles elongated parallel to Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}100⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document} exclusively pertain to CORs of 001R/100G. Furthermore, the frequency of each particular SPO in the garnet rim clearly depends on the local growth direction of the particular Grt{112} sector. Facet-specific variations in rutile SPO frequencies in different sectors and growth zones of garnet were observed even between equivalent directions, indicating that the microstructures and textures of rutile inclusions reflect varying parameters of garnet growth. The characteristic differences in COR groups of different garnet growth zones are referred to compositional changes in the bulk melt or compositional boundary layer, associated with magmatic fractional crystallisation.

Purification of Si–Ti alloys via vacuum evaporation: A thermodynamic and kinetic study of Ca impurity removal

Vacuum refining is a green technology for separating Ca from Si–Ti alloys. This study investigated the kinetics of Ca evaporation from Si–Ti melts in vacuum refining, including the effect of holding time and melt temperature and composition. Although the Ca activity in Si–Ti melts is an essential thermodynamic property that affects the Ca evaporation kinetics, it has not been measured to date. Consequently, another objective of this study was to determine the Ca activity using a chemical equilibration technique and investigate its effect on Ca evaporation. The results show that the activity of Ca in the Si–Ti melt decreased gradually with increasing Ti content in the melt, which reduced the Ca removal ratio. The removal ratio of Ca increased with the temperature and holding time. The apparent evaporation rate of Ca was consistent with the first-order law, and the apparent evaporation rate constants of Ca in 15%Si–Ti melt at 1673 and 1773 K were 2.1 × 10−6 and 2.8 × 10−6 m/s, respectively. The apparent activation energy for Ca evaporation from Si–15 wt%Ti melt was determined to be 68.2 kJ/mol, and the rate-determining step to be Ca mass transfer at the melt boundary layer.

Fluorite Mineralization at the Dwyer Mine, Wilberforce Area, Ontario, Canada: Microtextural Indications of a Fluor-Calciocarbonatite

ABSTRACT The ore at the Dwyer fluorite mine, near Wilberforce, Ontario, consists of calcite–fluorite dikes that show clear signs of flowage. Those dikes and the large-scale development of fenites at the expense of a granite–monzonite pluton can only be explained by the existence of a subjacent body of carbonatite. The dikes consist of ribbons of calcite and fluorite and contain subhedral crystals of fluorapatite aligned with the ribbons. The dikes also carry crystals of aegirine-augite, titanite, and bastnäsite-(Ce). Both the fluorapatite and aegirine-augite contain micrometric globules of boundary-layer melt that crystallized in situ to calcite, fluorite, quartz, bastnäsite-(Ce), hematite, and titanite. Fragments of the REE-enriched fenite show signs of incipient rheomorphism at a temperature estimated to be at least 725 °C. The large-scale alkali metasomatism occurred toward the end of the Grenville orogenic cycle, at a time of crustal relaxation, roughly 200 million years after emplacement of a granite–monzonite pluton. By analogy with occurrences elsewhere, it is likely that the carbonatitic melt separated immiscibly from a nepheline syenitic parental melt. Fluor-calciocarbonatitic magmatism likely is genetically linked to the U and Th mineralization in the area and contributed to the unusual geological complexity of the Bancroft–Haliburton region.

Static layer melt crystallization: Effects of impurities on the growth behaviors of crystal layers

• Impurity concentration polarization occurred at solid–liquid interface. • Mass transfer coefficient of impurity in melt boundary layer was derived. • Growth rate model correlating growth rate with temperature gradient of crystal layer was proposed. • Latent heat of crystallization decreases during crystallization due to impurity concentration polarization. In this work, the effects of impurities on the growth behaviors of crystal layer during static layer melt crystallization (LMC) was studied. The crystal layer structure, growth rates, melt temperature curves and separation efficiency were experimentally investigated at different impurity concentrations and temperatures. With the experimental results, the mass transfer at the solid–liquid interface was discussed through: a) calculating the supercooling of the melt over the crystallization and b) deducing the mass transfer coefficient of impurity in the melt boundary layer. It was verified that the melt kept supercooled during the crystallization and concentration polarization occurred at the solid–liquid interface. A power law model was proposed to correlate the growth rate with the temperature gradient of crystal layer, which indicates that the concentration polarization results in the decrease of the latent heat of crystallization, and the mixture with lower impurity concentration and crystallization temperature would exhibit more serious concentration polarization.

Dephosphorization of Metallurgical-Grade Silicon by Electromagnetic Levitation

In this work, a dynamic model was established for the dephosphorization of metallurgical grade silicon by levitation refining. Particular emphasis was placed on the transport behavior of phosphorus in the liquid boundary layer, then at the melt surface and finally within the gas phase surrounding the levitated droplet. Within the melt boundary layer, and at the droplet surface, temperature was the controlling factor affecting phosphorus behavior. Within the gas phase, the controlling factors were temperature and gas velocity with temperature showing a more pronounced effect on the phosphorus mass transfer coefficient than gas velocity. The dynamic model shows that the dephosphorization of MG-Si was controlled by phosphorus diffusion within the gas phase. The values calculated from the model for the mass transfer coefficient were in good agreement with dephosphorization data obtained from levitation experiments conducted with metallurgical grade silicon. This work can provide a good foundation for application of electromagnetic levitation technology to the refining of metallurgical grade silicon.

Kinetic study of Mn vacuum evaporation from Mn steel melts

Considering the interference of Mn volatilization on the composition control during the vacuum treatment of medium/high Mn steel melt. The kinetic analysis of Mn volatilization from steel melt in various initial Mn contents, holding times, temperature and pressure regimes were investigated in lab scale experiments. The results show that Mn evaporation is accelerated significantly with the increase of temperature and initial Mn content in steel melt or with the decrease of chamber pressure. Furthermore, the logarithm of Mn content in steel melt has a linear function relationship with the holding time, indicating the first-order kinetic reaction of Mn evaporation. In addition, the overall mass transfer coefficient of Mn significantly decreases from 2.176E−05 to 5.932E−06 m s−1 with the increase of the initial Mn content from 2% to 30%. Moreover, high temperature and low partial pressure can obviously improve the overall mass transfer coefficient of Mn. Simultaneously, the apparent activation energy for Mn evaporation from steel melt is calculated to be 324.55 kJ mol−1. The theoretical kinetics model is established to fully analyze the evaporation mechanics of Mn, which is consistent with experimental results. In particular, with the increase of initial Mn content, the rate-limiting step transforms from mass transfer in the steel melt boundary layer to gas phase mass transfer, and mass transfer resistance in the gaseous phase is reached 91.22% with initial Mn content of 30%. Besides, the Mn evaporation process at high temperature and high partial pressure is determined by the mass transfer in the steel melt boundary layer and gas phase, respectively. This work systematically clarifies the factors influencing Mn loss in vacuum refining on high quality Mn steel manufacture.

A Silicocarbonatitic Melt and Spinel-Bearing Dunite of Crustal Origin at the Parker Phlogopite Mine, Notre-Dame-du-Laus, Quebec, Canada

The Parker phlogopite mine, located near Notre-Dame-du-Laus, Quebec, 74 km north of Ottawa, is well known among mineral collectors for its centimetric euhedral crystals of black spinel. Among the dozens of phlogopite mines active in the early 1900s in the Mont-Laurier–Bancroft corridor in the Central Metasedimentary Belt of the Grenville Province, the Parker mine is exceptional because of the association of forsterite + spinel with phlogopite. Euhedral crystals of these minerals are found “frozen” in a carbonate matrix. The carbonate dike and segregations are associated with spinel-rich dunite that contains accessory diopside, phlogopite, and pargasite, as well as ilmenite and apatite. The interstitial melt crystallized to calcite + dolomite. Hematite appeared as flakes in the melt owing to net loss of hydrogen, and the spinel underwent oxidation-induced exsolution. Our spinel crystal entrapped a domain of carbonate during growth. It also entrapped globules of boundary-layer melt that crystallized to a carbonate + sulfate + phosphate + silicate + oxide assemblage. Such globules, where present in the cumulate, are more pristine than in the coarse crystal of spinel, i.e., less affected by a hydrothermal overprint. We contend that the carbonate melt ultimately formed by the hydrous melting of marble, as supported by oxygen-isotope data on all major minerals. Melting occurred 1140 million years ago, at a time of tectonic relaxation following the Shawinigan compressive stresses.

Silicocarbonatitic melt inclusions in fluorapatite from the Yates prospect, Otter Lake, Québec: Evidence of marble anatexis in the central metasedimentary belt of the Grenville Province

Abstract We have investigated a locality very well known to mineral collectors, the Yates U-Th prospect near Otter Lake, Québec. There, dikes of orange to pink calcite enclose euhedral prisms of fluorapatite, locally aligned. Early investigators pointed out the importance of micro-inclusions in the prisms. We describe and image the micro-inclusions in two polished sections of fluorapatite prisms, one of them with a millimetric globule of orange calcite similar to that in the matrix. We interpret the globule to have been an inclusion of melt trapped during growth. Micro-globules disseminated in the fluorapatite are interpreted to have crystallized in situ from aliquots of the boundary-layer melt enriched in constituents rejected by the fluorapatite; the micro-globules contain a complex jigsawed assemblage of carbonate, silicate, and sulfate minerals. Early minerals to crystallize are commonly partly dissolved and partly replaced by lower-temperature phases. Such jigsawed assemblages seem to be absent in the carbonate matrix sampled away from the fluorapatite prisms. The pressure and temperature attained at the Rigolet stage of the Grenville collisional orogeny were conducive to the anatexis of marble in the presence of H2O. The carbonate melt is considered to have become silicocarbonatitic by assimilation of the enclosing gneisses, which were also close to their melting point. Degassing was important, and the melt froze quickly. The evidence points to a magmatic origin for the carbonate dikes and the associated clinopyroxenite, rather than a skarn-related association.

ANALYSIS OF FLOW AND HEAT TRANSFER IN THE BOUNDARY LAYER OF FLUID WITH ESSENTIAL DEPENDENCE OF VISCOSITY FROM TEMPERATURE

The introduction of new environmentally safe, durable and fire-resistant thermal insulation based on basalt fiber requires the improvement of technological processes for the production of basalt filaments in order to increase their energy efficiency. The quality of basalt fiber significantly depends on the properties of the basalt melt in the process. The process is considered to be more perfect than the more homogeneous and isotropic final properties of the melt. The conditions of flow and heat transfer in the bath and feeder of the melting furnace have a significant impact on the final properties of the melt.
 The paper presents the results of studies of heat transfer in the boundary layer on a flat plate with the flow of a liquid, the viscosity of which depends significantly on temperature. The system of differential equations, which describes the steady-state flow regime, is solved using symmetry analysis (analysis of Lie groups).
 On the basis of the developed mathematical model, new results were obtained which characterize the regularities of the flow and heat exchange of a highly viscous fluid in the range of temperature variations from 900 to 1450 ° C. The conditions for the formation of a low-mobility layer of liquid near a solid surface are determined. The influence of the value of a low-mobility fluid layer on the temperature distribution in the boundary layer of basalt melt is established.
 The results obtained allow us to adjust the temperature regimes in the melting furnace and improve the process of manufacturing ultrathin basalt threads.

Antarctic sub-shelf melt rates via PICO

Abstract. Ocean-induced melting below ice shelves is one of the dominant drivers for mass loss from the Antarctic Ice Sheet at present. An appropriate representation of sub-shelf melt rates is therefore essential for model simulations of marine-based ice sheet evolution. Continental-scale ice sheet models often rely on simple melt-parameterizations, in particular for long-term simulations, when fully coupled ice–ocean interaction becomes computationally too expensive. Such parameterizations can account for the influence of the local depth of the ice-shelf draft or its slope on melting. However, they do not capture the effect of ocean circulation underneath the ice shelf. Here we present the Potsdam Ice-shelf Cavity mOdel (PICO), which simulates the vertical overturning circulation in ice-shelf cavities and thus enables the computation of sub-shelf melt rates consistent with this circulation. PICO is based on an ocean box model that coarsely resolves ice shelf cavities and uses a boundary layer melt formulation. We implement it as a module of the Parallel Ice Sheet Model (PISM) and evaluate its performance under present-day conditions of the Southern Ocean. We identify a set of parameters that yield two-dimensional melt rate fields that qualitatively reproduce the typical pattern of comparably high melting near the grounding line and lower melting or refreezing towards the calving front. PICO captures the wide range of melt rates observed for Antarctic ice shelves, with an average of about 0.1 m a−1 for cold sub-shelf cavities, for example, underneath Ross or Ronne ice shelves, to 16 m a−1 for warm cavities such as in the Amundsen Sea region. This makes PICO a computationally feasible and more physical alternative to melt parameterizations purely based on ice draft geometry.

Trace element diffusion and kinetic fractionation in wet rhyolitic melt

Piston-cylinder experiments were run to determine the chemical diffusivities of 21 trace elements (Sc, V, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf, Th and U) in hydrous rhyolitic melts at 1 GPa pressure and temperatures from 850 to 1250 °C. Diffusion couple glasses were doped with trace elements in low concentrations to characterize the diffusivities of all cations in a single experiment. Laser ablation ICP-MS was used to evaluate the trace element concentration gradients that developed in the silicate glasses. All calculated diffusion coefficients correspond to the temperature dependence D = D0exp(-Ea/RT). Rhyolite liquids contained either ∼4.1 wt% or ∼6.2 wt% dissolved H2O; separate Arrhenius relationships are produced for each melt composition. Trace element diffusivities in the melt with 6.2 wt% H2O are roughly two times higher than those in the less hydrous melt.Calculated trace element diffusion coefficients cover nearly two orders of magnitude at a given temperature. The high field strength elements are the slowest diffusers, followed by the transition metals and heavy rare earth elements. The light rare earth elements have the fastest diffusion rates in hydrous rhyolitic melt. The measured diffusion coefficients range down to values sufficiently low to preclude diffusive homogenization over geochemically realistic time scales in some cases. The substantial differences in the diffusivities of individual cations may result in fractionated trace element signatures in rhyolite melt pockets. A simple model is used to explore the potential for kinetic fractionation of REE during growth of an apatite crystal in a diffusive boundary layer locally saturated in P2O5. The faster-diffusing light REE are more efficiently transported away from the crystal interface than the slower-moving heavy REE. Diffusion effects will enrich the melt boundary layer in slow-moving HREE relative to the faster LREE. The kinetic fractionation of REE in the melt growth medium will result in a precipitated apatite crystal with a disequilibrium trace element composition.

Diffusive fractionation of volatiles and their isotopes during bubble growth in magmas

Bubbles grow in decompressing magmas by simple expansion and by diffusive supply of volatiles to the bubble/melt interface. The latter phenomenon is of significant geochemical interest because diffusion can fractionate elements and isotopes (or isotopologues) of dissolved components. This raises the possibility that the character of volatile components in bubbles may not reflect that of volatiles dissolved in the host melt over the lifetime of a bubble—even in the absence of equilibrium vapor/melt isotopic fractionation. Recent experiments have confirmed the existence of an isotope mass effect on diffusion of the volatile element Cl in silicate melt [Fortin et al. (Isotopic fractionation of chlorine during chemical diffusion in a dacitic melt and its implications for isotope behavior during bubble growth (abstract), 2016 Fall AGU Meeting, 2016)], so there is a clear need to understand the efficacy of diffusive fractionation during bubble growth. In this study, numerical models of diffusion and mass redistribution during bubble growth were implemented for both “passive” volatiles—those whose concentrations are generally well below saturation levels—and “active” volatiles such as CO2 and H2O, whose elevated concentrations and limited solubilities are the cause of bubble nucleation and growth. Both diffusive and convective bubble-growth scenarios were explored. The magnitude of the isotope mass effect on passive volatiles partitioned into bubbles growing at a constant rate R in a static system depends upon R/DL, Kd and DH/DL (Kd = bubble/melt partition coefficient; DH/DL = diffusivity ratio of the heavy and light isotopes). During convective bubble growth, the presence of a discrete (physical) melt boundary layer against the growing bubble (of width xBL) simplifies outcomes because it leads to the quick onset of steady-state fractionation during growth, the magnitude of which depends mainly upon R∙xBL/DL and DH/DL (bubble/melt fractionation is maximized at R∙xBL/DL ≈0.1). Constant R is unrealistic for most real systems, so other scenarios were explored by including the solubility and EOS of an “active” volatile (e.g., CO2) in the numerical simulations. For plausible decompression paths, R increases exponentially with time—leading, potentially, to larger isotopic fractionation of species partitioned into the growing bubble. For volatile species whose isotope mass effects on diffusion have been measured (Cl, Li), predicted isotope fractionation in the exsolved vapor can be as large as −4‰ for Cl and −25‰ for Li.

High-Ti, bright-CL rims in volcanic quartz: a result of very rapid growth

Volcanic quartz commonly contains Ti-enriched zones and CO2-enriched melt inclusions, and crystallization temperatures and pressures derived from Ti-in-quartz geothermobarometry and H2O–CO2 glass geobarometry applied to these compositions are typically high. Consequently, these features are generally interpreted to represent high temperatures and/or pressures. Yet, growth rate estimates from some high-Ti/bright-CL quartz rims suggest they grew at rates orders of magnitude faster than did cores and interiors of the crystals. This observation is notable in light of studies that suggest that fast crystal growth rates can produce a boundary layer in the melt surrounding a growing crystal that is enriched in components that diffuse comparatively slowly in the melt. In these circumstances, the composition of zones or melt inclusions formed from such a boundary layer melt will not accurately represent that of the far-field melt, and temperatures and pressures estimated from these compositions will be anomalous. We use a numerical model based on the coupled growth–diffusion equation of Lasaga (1982) to assess the effect of growth rate on the production of high-Ti/bright-CL zones and high-CO2 melt inclusions in quartz in rhyolitic melts. Simulations span a wide range of growth rates (10−7 to 10−13 m/s) and timescales (1 minute–1 year), and results suggest that quartz growth at 10−10 m/s or faster can produce a boundary layer enriched in these components. This suggests that appropriate application of Ti-in-quartz and H2O–CO2 glass geothermobarometry is contingent upon the verification that the compositions used are not those of boundary layer melts. Applying our model to the Bishop Tuff, which contains quartz displaying high-Ti/bright-CL rims and high-CO2 rim-hosted melt inclusions, we find that growth rates of 10−7 to 10−9 m/s can produce the observed enrichments in these components over the timescales estimated for the growth of the rims (days–weeks); these growth rates are consistent with those estimated independently for these rims. Thus, it is not necessary to invoke changes in temperature and/or pressure to explain these features. More broadly, the results suggest that the high-Ti/bright-CL rims commonly observed in volcanic quartz may be a record of fast growth, potentially associated with the growth of groundmass crystal populations in response to eruptive decompression.

Discreditation of diomignite and its petrologic implications

Diomignite (Li 2 B 4 O 7 ) is discredited as a mineral species, and this discreditation has been approved by the International Mineralogical Association, Commission on New Minerals, Nomenclature and Classification. Diomignite was originally reported to occur in virtually every crystal-rich inclusion in spodumene from the Tanco pegmatite in southeastern Manitoba, Canada. However, detailed study of 30 randomly selected crystal-rich inclusions in the purported type material deposited at the U.S. National Museum of Natural History, 30 inclusions in the purported type material from the American Museum of Natural History, and several hundred inclusions in self-collected samples reveals that diomignite is absent in every inclusion examined. Because no holotype specimen exists and no neotype sample was provided by the surviving authors of the original description, the presence of diomignite could not be validated. The evidence provided in the original description to the IMA in 1984 is shown to be insufficient to support the existence of diomignite as a mineral species. The previously reported boron-rich (12 mass% B 2 O 3 ) composition of the melt represented as crystal-rich inclusions in spodumene and petalite from the Tanco pegmatite was predicated on the assumption that diomignite is a common daughter mineral that occurs in most inclusions and that the inclusions are primary melt inclusions. The nonexistence of diomignite, and the absence of other borate daughter minerals, in these crystal-rich inclusions indicates that the boron content was greatly overestimated and so comparisons to experimentally generated boron-rich (>10 mass% B 2 O 3 ) boundary-layer melts are unwarranted. Furthermore, the discreditation of diomignite negates the inferred role of a Li 2 B 4 O 7 -flux-rich melt in the generation of primary pegmatite textures and rare element oxide mineralization in the Tanco pegmatite.

On progress and rate of the peritectic reaction Fo + SiO2 → En in natural andesitic arc magmas

Using a high resolution ion microprobe with SCAPS imaging, the peritectic reaction of forsterite+silica to enstatite was studied down to submicron level in a natural andesitic tephra from the Central Plateau of North Island, New Zealand. The fayalitic component of natural olivines is stable in high-silica melts, and therefore the reaction is in fact a two-step progress: 1. Dissolution of Mg-rich olivine, rate-limited by Fe–Mg interdiffusion at the crystal rim, results in enrichment of Fe in the crystal rim and of Mg in the c. 1μm wide melt boundary layer around the crystal. 2. Magnesian pyroxenes preferentially but not exclusively nucleate in the melt boundary layer and grow; as soon as these microlites touch the rim of the dissolving olivine, they shield the crystals from the silica-rich melt, thereby preventing further olivine dissolution. At this point, Fe–Mg interdiffusion begins to destroy the Fe-enrichment of the olivine rim. The reaction is completed when the dissolving olivine crystal is completely mantled by magnesian pyroxene microlites. Thick pyroxene mantles are likely the result of pyroxene overgrowth rather than due to peritectic transformation. The morphology of the olivine rim preserves information about the reaction history of the grain. Modeling of Fe–Mg interdiffusion in the olivine rim following its shielding from the melt by pyroxene overgrowth may yield the rates of olivine dissolution and the rates of pyroxene growth if temperature is known. For the tephra we have studied, microlite thermometry yields a temperature of 1137 (±41)°C, indicating an olivine dissolution rate of 3–6×10−11ms−1 and an initially volumetric pyroxene growth rate of 2.2–5.6×10−21m3s−1. This points to timescales between olivine crystal uptake into the SiO2-rich melt and explosive eruption at the surface of a few hours to at most a day.

Dike propagation driven by melt accumulation at the lithosphere–asthenosphere boundary

This study examines the combined effects of melt migration by porous flow in the asthenosphere and dike propagation in the lithosphere. Melt collecting at the base of the lithosphere forms a decompacting boundary layer (DBL), in which the overpressure is sufficient to nucleate dikes that propagate buoyantly upward into the lithosphere. The asthenosphere melt flux determines the excess pressure and melt accumulation rate in the DBL, which together with the state of lithospheric stress, control dike growth rate, dike recurrence interval and the height to which dikes propagate. The vertical propagation and subsequent freezing of melt filled dikes heats and thins the lithosphere.Our model couples fundamental aspects of dike propagation and porous flow that are commonly treated separately. Our model allows us to estimate conditions under which vertically propagating dikes can thin the lithosphere, given a melt flux determined by the rate of melt production in the asthenosphere. The model also provides an estimate of the amount of melt present at the base of the lithosphere. We find that a steady state high-porosity boundary layer at the lithosphere–asthenosphere boundary, with a melt fraction about 2.5–4 times higher than the asthenosphere melt fraction. Diking occurs at melt fractions much less than the disaggregation limit, so dikes are only a few km tall and about 1 cm wide. Though dikes are small, their recurrence on the order of days can lead to lithosphere erosion rates on the order of a few km/Myr with a melt fraction of a few percent at the base of the lithosphere. The steady state boundary layer melt fraction is controlled by differences in lithospheric stress state and or asthenosphere melt flux, indicating that seismic discontinuities associated with melt accumulation at the lithosphere–asthenosphere boundary should vary systematically with variations in asthenosphere melt generation and tectonic setting.

Are silicate‐rich inclusions in spodumene crystallized aliquots of boundary layer melt?

AbstractThe constitutional zone refining model for the internal crystallization of granitic pegmatites stipulates that a highly fluxed and incompatible element‐enriched boundary layer melt evolves at the surface of crystal growth fronts in undercooled granitic melts. It has been argued that silicate‐rich inclusions in primary spodumene and petalite from the Tanco pegmatite are the crystallized products of an entrapped boundary layer melt. However, inclusions of identical character and composition to those in primary spodumene are also abundant in the spodumene component of SQI (spodumene + quartz intergrowths) that formed by the isochemical breakdown of petalite. Both textural types of spodumene at Tanco host fluid inclusion assemblages that display highly variable proportions of solid phases (dominantly cookeite, quartz, and zabuyelite) and a low salinity (approximately 6 wt% NaCl equivalent) aqueous fluid. The average major element content of three fluid inclusion assemblages is approximately Li2O (3 wt%), Al2O3 (11 wt%), SiO2 (30 wt%), CO2 (3 wt%) and H2O (53 wt%). The entrapment of these fluid inclusions is temporally and spatially unrelated to the flux‐rich melt that may have developed at the primary crystallization front of the pegmatite. Petrographic evidence and the mineralogical and chemical composition of spodumene‐hosted inclusions suggest that inclusions in spodumene are the products of hydrothermal dissolution of the host and the precipitation of quartz, cookeite, and zabuyelite prior to and during necking.

Vacuum Refining of Molten Silicon

AbstractMetallurgical fundamentals for vacuum refining of molten silicon and the behavior of different impurities in this process are studied. A novel mass transfer model for the removal of volatile impurities from silicon in vacuum induction refining is developed. The boundary conditions for vacuum refining system—the equilibrium partial pressures of the dissolved elements and their actual partial pressures under vacuum—are determined through thermodynamic and kinetic approaches. It is indicated that the vacuum removal kinetics of the impurities is different, and it is controlled by one, two, or all the three subsequent reaction mechanisms—mass transfer in a melt boundary layer, chemical evaporation on the melt surface, and mass transfer in the gas phase. Vacuum refining experimental results of this study and literature data are used to study the model validation. The model provides reliable results and shows correlation with the experimental data for many volatile elements. Kinetics of phosphorus removal, which is an important impurity in the production of solar grade silicon, is properly predicted by the model, and it is observed that phosphorus elimination from silicon is significantly increased with increasing process temperature.

Interaction of Kelvin force and transport across a melting substrate in a microgravity environment

Gradients in magnetic field applied to electrically insulating fluids result in an effective body force-the Kelvin force, which can potentially be used to control flow and thermal processes, particularly in microgravity environments. We study the influence of the Kelvin force on the mass, momentum, and energy transport of fluid arising from the melting of a semi-infinite solid substrate subjected to temperature and magnetic-field gradients. The governing equations of the magnetothermal free convection of the melt under the boundary layer approximation with a suitable transformation lead to a similarity solution. Closed-form analytical solutions for the limiting case of smaller Prandtl numbers are presented. The nonlinear similarity equations are solved numerically using an iterative boundary-value technique based on a finite-difference approach. Based on the numerical results, the effect of various characteristic nondimensional parameters on the structure of the melt boundary layer and transport rates of heat and mass across the substrate are elucidated and discussed.

Experiments and Models Bearing on the Role of Chromite as a Collector of Platinum Group Minerals by Local Reduction

Chromite is widely recognized to act as a collector for platinum group elements (PGE), which tend to be observed as discrete grains of platinum group minerals included within magmatic chromite grains. In the course of experiments involving the re-equilibration or growth of chromite and Cr-spinel in molten silicate, we observe that platinum group minerals (PGM; including metal alloys and laurite) form at the mineral–melt interface. The formation of PGM to the extent observed requires a mechanism involving sustained transport of PGE from a source within the experiment to the site of deposition. We propose that the driving force for this process is a redox gradient developed in response to mineral growth or re-equilibration with the surrounding melt. The mechanism is local reduction within the mineral–melt interfacial region as a consequence of the selective uptake of trivalent Cr and Fe from the melt by spinel relative to the divalent species. We have modeled the transient perturbation of fO2 in a compositional boundary layer melt around spinel for both crystal growth and diffusive re-equilibration of mineral and melt. We find that metal solubilities decrease by several per cent in the silicate melt at the melt–crystal interface during crystal growth, providing the driving force for PGM formation. In magmas that are saturated with PGM, as a result of falling temperature and oxygen fugacity during spinel crystallization, nucleation of PGM will be impeded by interfacial tension everywhere except in the reduced boundary layer around spinel crystals. The resulting concentration and trapping of alloy particles in the growing chromite crystals can produce apparent bulk chromite/melt partition coefficients exceeding 20 even if there is no solid solution of PGE in the chromite. The introduction of spinel grains, initially equilibrated with a mafic magma, into a more primitive magma, with higher Cr/Al, would lead to disequilibrium between chromite and melt. The perturbation of fO2 in the compositional boundary layer surrounding a chromite xenocryst would exceed 0·1 log unit, leading to local reduction of alloy solubility of the order of 13–18%. A small number of spinel xenocrysts could serve as collection sites for all of the excess PGE in the magma, leading to the eventual observation that a few chromite crystals contain many PGM inclusions, whereas the rest of the chromite population may be relatively free of PGM.