Crystallization Kinetics: Relationship between Crystal Morphology and the Cooling Rate—Applications for Different Geological Materials (original) (raw)
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Editorial: Research Topic Crystal Nucleation and Growth in Magmatic Suspensions
Frontiers in Earth Science, 2020
Editorial on the Research Topic Research Topic Crystal Nucleation and Growth in Magmatic Suspensions The differentiation, rheology and transport of magmas are strongly influenced by crystallization (nucleation and growth). Nucleation and crystal growth in a batch of magma evolve through time in response to changing environmental conditions, i.e., T, P, and fO 2. Investigations on naturally formed igneous rocks, combined with experiments where these environmental conditions can be controlled, are necessary to decipher, reconstruct and model the solidification of magmas. New advances continue to be made in both analytical techniques and experimental methods, with crystallization under conditions of strong disequilibrium being a particularly active area of experimental research. A host of difficulties confronts experimental studies of crystallization (nucleation, growth, and textural maturation), due to the high melting points, high degrees of chemical reactivity, and high redox sensitivities of natural (Fe-bearing) silicate liquids. These difficulties are compounded when the systems are investigated at elevated pressure or contain a high concentration of dissolved H 2 O. Frequently, solidification experiments are evaluated only post-quench; hence, the effect of a thermodynamic driving force (sensible cooling or decompression, if H 2 O-saturated) and kinetic limitations on the evolution of nucleation, crystal growth, and textures is observed mainly ex situ. The in situ observation requires relaxation of one or more of these constraints, but unlocks the opportunity to study environmental controls on individual phase appearance (or suppression), maturation of crystal populations, and overall transformation kinetics. This thematic issue contains five contributions to the study of magma crystallization. They share a reliance upon experimental data but differ in the application of in situ and ex situ methods, theoretical formulations, and the analysis of well-constrained natural volcanic samples. With Crystallization Kinetics of Alkali Feldspar in Peralkaline Rhyolitic Melts: Implications for Pantelleria Volcano, Arzilli et al. perform 'classic' ex situ isobaric cooling experiments at controlled fO 2 and various H 2 O concentrations. In this manner, they evaluate the kinetic crystallization of alkali feldspars in peralkaline rhyolitic magma over wide ranges in pressure and undercooling. This datarich contribution complements analogous studies on granite and granodiorite liquids by Fenn (1977) and Swanson (1977), quantifying the significant lag time between the imposition of a driving force for crystallization and the onset of crystallization. In The Onset and Solidification Path of a Basaltic Melt by in situ Differential Scanning Calorimetry (DSC) and ex situ Investigations, Giuliani et al. combine DSC to pinpoint the onset of crystallization in natural basalt (at ambient conditions) with electron microscopy-based textural analysis of run products. A strength of the DSC technique is the opportunity to precisely resolve transformation kinetics. They report that an increase in cooling rate by three orders of magnitude suppresses the timing of the onset of crystallization by two orders of magnitude. They further estimate the effects of cooling rate on the nucleation of spinel, plagioclase, melilite, and
Journal of Volcanology and Geothermal Research, 2006
Controlled cooling experiments at atmospheric pressure and fO2 of the NNO buffer were performed on three Stromboli lavas of calcalkaline (CA), high-K calcalkaline (HKCA) and shoshonitic (SHO) affinities. The experiments were conducted at cooling rates of 900 °C/h (fast cooling rate—FCR) and 1 °C/h (slow cooling rate—SCR), respectively, to investigate the kinetic factors governing textural and compositional features such as phase growth morphology, phase relations and phase compositions. The experiments produced different types of textures as both the cooling rate and the quenching temperatures were varied. In general, FCR experiments show a decrease of crystal size as well as a change in crystal shape from euhedral, to granular, to skeletal up to dendritic morphology with decreasing temperatures. SCR experiments produce less variable crystal size and crystal morphology. Measurements of plagioclases indicate that the crystal size and calculated crystal growth are functions of cooling rate; crystal growth rates are about two orders of magnitude higher in FCR experiments than SCR ones. Such textural responses are in agreement with the empirical calibration based on the plagioclase size dependence on cooling rate developed by Cashman (1993).Experimental phase relations display three main features of the crystallization process: (1) the delay in the crystallization of the liquidus phase in the FCR experiments with respect to the SCR ones; (2) crystallization, controlled by the multiphase cotectic relations (plagioclase + pyroxenes ± olivine), is more enhanced for FCR experiments compared to that of SCR ones at the lowest experimental temperature (1100 °C); (3) crystallization of augite and pigeonite is sensitive to cooling rate and system compositions; pigeonite, not found in the natural rocks, crystallizes in the CA sample across SCR and FCR runs, and in the HKCA sample just in the SCR experiments.FCR and SCR experiments produce similar liquid lines of descent for both CA and HKCA starting materials suggesting that the processes of nucleation and growth play reciprocal roles through mass balance as cooling rate varies. This is not true for the SHO composition where the trends of melt composition appear to be especially influenced by the different amounts of olivine crystallization at the different cooling rates. Data on mineral compositions show that plagioclase and clinopyroxene are the phases more strongly affected by the cooling gradients; crystals from SCR experiments and, to a lesser extent, from FCR experiments, show zoning patterns mainly at the lowest experimental temperature.
Plagioclase crystal size distribution (CSD) has been investigated in a quartz-diorite body, in the leucosome of migmatites and in the melanosome of un-melted contact metamorphic rocks from Gennargentu Complex (Sardinia, Italy). During the crystallization of the dioritic magma, a variety of competing kinetic processes determine the evolution of the igneous microstructure, but the relative contribution of each process remains elusive. Our approach was aimed to study the plagioclase crystallization from a liquid (quartz-diorites and migmatite leucosomes), comparing it to a crystallization at subsolidus conditions. CSD indicates that plagioclase in the quartz-diorite nucleated and grew in a cooling system at a constant cooling rate, producing straight-line CSD in a diagram of ln of population density vs. size range. The plagioclase crystallization continued until the latent heat was available and the temperature was high enough to allow the plagioclase growing. This can occur only when a crystal is held at temperature close to its liquidus for a long period of time. Under these conditions, the plagioclase nucleation rate is zero, but growth rate is high for crystal larger than the critical size. This does not necessarily mean that the temperature was held constant, just that the undercooling remained small (Ostwald ripening process). The aggregated small crystals, due to their high surface energy per unit volume, to minimise energy in the system dissolved and Periodico di Mineralogia (2014), 83, 3, 401-418 "fed" the growth of larger crystals. This process occurs because small grains have a higher surface energy per unit volume than do larger grains. The crystallization temperature (~900°C , 100 MPa) allows the formation of plagioclase as liquidus phase. From CSD measurements we calculated the different cooling ages for the different sample types.
Dynamic crystallization in magmas
Mineral reaction kinetics: Microstructures, textures, chemical and isotopic signatures, 2017
Undercooling and crystallization kinetics are recognized increasingly as important processes controlling the final textures and compositions of minerals as well as the physicochemical state of magmas during ascent and emplacement. Within a single volcanic unit, phenocrysts, microphenocrysts and microlites can span a wide range of compositions, develop complex zoning patterns, and show intricate textures testifying to crystallization far from equilibrium. These petrographic complexities are not associated necessarily with magma chamber processes such as mixing or mingling of distinctly different bulk compositions but, rather, may be caused by variable degrees of initial magma-undercooling and the evolution of undercooling through time. Heat-dissipation and decompression are the most effective driving forces of cooling and volatile loss that, in turn, exert a primary control on the solidification path of magma. Understanding these kinetic aspects over the temporal and spatial scales at which volcanic processes occur is therefore essential to interpret correctly the time-varying environmental conditions recorded in igneous minerals. This contribution aims to summarize and integrate experimental studies pertaining to the crystallization of magmas along kinetic or time-dependent pathways, where solidification is driven by changes in temperature, pressure and volatile concentration. Fundamental concepts examined in the last decades include the effect of undercooling on crystal nucleation and growth as well as on the transition between interface-and diffusion-controlled crystal growth and mass transfer occurring after crystals stop growing. We summarize recent static and dynamic decompression and cooling experiments that explore the role of undercooling in syn-eruptive crystallization occurring as magmas ascend in volcanic conduits and are emplaced at the surface. The ultimate aim of such studies is to decode the textural and compositional information within crystalline phases to place quantitative constraints on the crustal transport, ascent and emplacement histories of erupted and intrusive magmas. Magma crystallization under dynamic conditions will be assessed also through a comparative description of the disequilibrium features in minerals found in experimental and natural materials. A variety of departures from polyhedral growth, including morphologies indicating crystal surface instability, dendritic structures, sector zoning and growth twins are linked to the rate at which crystals grow. These have implications for the entrapment of melt inclusions and plausibility for interpreting the growth chronology of individual crystals. A simple ''tree-ring'' model, in which the oldest part of the crystal lies at the centre and the youngest at the rim, is not an appropriate description when growth is non-concentric. Further, deviation from chemical
Nucleation, crystal growth and the thermal regime of cooling magmas
Journal of Geophysical Research, 1984
Crystallization at the margin of a tures must depend in one way or another on local quiet cooling magma has been studied numerically, crystallization conditions. taking into account the kinetics of crystalli-The first attempts to study dynamic crystalzation.
We have investigated the effect of undercooling and deformation on the evolution of the texture and the crystallization kinetics of remelted basaltic material from Stromboli (pumice from the March 15, 2007 paroxysmal eruption) and Etna (1992 lava flow). Isothermal crystallization experiments were conducted at different degrees of undercooling and different applied strain rate (T = 1,157-1,187°C and _ c i = 4.26 s -1 for Stromboli; T = 1,131-1,182°C and _ c i = 0.53 s -1 for Etna). Melt viscosity increased due to the decrease in temperature and the increase in crystal content. The mineralogical assemblage comprises Sp ? Plg (dominant) ± Cpx with an overall crystal fraction (/) between 0.06 and 0.27, increasing with undercooling and flow conditions. Both degree of undercooling and deformation rate deeply affect the kinetics of the crystallization process. Plagioclase nucleation incubation time strongly decreases with increasing DT and flow, while slow diffusionlimited growth characterizes low DT-low deformation rate experiments. Both Stromboli (high strain rate) and Etna (low strain rate) plagioclase growth rates (G) display relative small variations with Stromboli showing higher values (4.8 ± 1.9 9 10 -9 m s -1 ) compared to Etna (2.1 ± 1.6 9 10 -9 m s -1 ). Plagioclase average nucleation rates J continuously increase with undercooling from 1.4 9 10 6 to 6.7 9 10 6 m -3 s -1 for Stromboli and from 3.6 9 10 4 to 4.0 9 10 6 m -3 s -1 for Etna. The extremely low value of 3.6 9 10 4 m -3 s -1 recorded at the lowest undercooling experiment for Etna (DT = 20°C) indicates that the crystallization process is growth-dominated and that possible effects of textural coarsening occur. G values obtained in this paper are generally one or two orders of magnitude higher compared to those obtained in the literature for equivalent undercooling conditions. Stirring of the melt, simulating magma flow or convective conditions, facilitates nucleation and growth of crystals via mechanical transportation of matter, resulting in the higher J and G observed. Any modeling pertaining to magma dynamics in the conduit (e.g., ascent rate) and lava flow emplacement (e.g., flow rate, pāhoehoe-'a'ā transition) should therefore take the effects of dynamic crystallization into account.
The kinetics of nucleation and crystal growth and scaling laws for magmatic crystallization
Contributions to Mineralogy and Petrology, 1987
Magmatic crystallization depends on the kinetics of nucleation and crystal growth. It occurs over a region of finite thickness called the crystallization interval, which moves into uncrystallized magma, We present a dimensional analysis which allows a simple understanding of the crystallization characteristics. We use scales for the rates of nucleation and crystal growth, denoted by I,, and Ym respectively. The crystallization time-scale % and length-scale dc are given by (y3.I~)-1/4 and 0c.%) 1/2 respectively, where ~c is thermal diffusivity. The thickness of the crystallization interval is proportional to this length-scale. The scale for crystal sizes is given by (Y~/Im) TM. We use numerical calcula-
Tracking crystallinity in siliceous hot-spring deposits
American Journal of Science, 2007
Siliceous hot spring deposits (sinters) entrap paleoenvironmentally significant components and are used as extreme-environment analogs in the search for early Earth and extraterrestrial life. However, sinters undergo a series of textural and mineralogical changes during diagenesis that can modify and overprint original environmental signals. For ancient hydrothermal settings including those close to the dawn of life, these transformations have long since occurred, so that study of diagenetic processes and effects is best undertaken in much younger deposits still undergoing change. Three young sinters preserve the entire diagenetic sequence of silica phases, from opal-A to quartz. The 6000 to ϳ 11,500 years BP ؎ 70 years sinter at Steamboat Springs, Nevada, the ϳ 1600-1900 ؎ 160 years BP Opal Mound sinter at Roosevelt Hot Springs, Utah, and the ϳ 456 ؎ 35 years BP deposit at Sinter Island, Taupo Volcanic Zone, New Zealand, provide an opportunity to track crystallographic, mineralogic and morphologic transitions of sinter diagenesis using standard and new analytical approaches. Worldwide, sinter forms from cooling, alkali chloride waters as noncrystalline opal-A, transforming first into noncrystalline opal-A/CT, then paracrystalline opal-CT ؎ moganite, paracrystalline opal-C, and eventually to microcrystalline quartz. In this study, these changes were identified by the novel and combined application of electron backscatter diffraction, X-ray powder diffraction, and scanning electron and optical microscopy techniques. We show that mineralogical changes precede morphological and accompanied crystallographic transformations. During this modification, silica particles grow and shrink several times from the micron-to nano-meter scales via dissolution, reprecipitation and recrystallization, and diagenesis follows the Ostwald Step rule. All deposits followed nearly identical diagenetic pathways, with time as the only variable in the march toward physicochemically stable quartz crystals. Diagenesis alters original environmental signatures trapped within sinters. After five silica phase changes, filamentous microfossils are modified but still remain recognizable within sinter from the Opal Mound and Steamboat Springs deposits, and during the opal-A to opal-CT silica phase transformations at Sinter Island. Therefore, delineating diagenetic components and how they affect sinters is necessary to accurately identify biosignals from ancient hot-spring deposits. introduction Siliceous sinter forms where hot alkali chloride waters discharge and cool at the Earth's surface. Silica deposits upon all exposed surfaces, including microbes, insects, pollen, plant debris, mineral detritus, as well as older sinters. Sinters are thus archives of past hydrologic, biologic and environmental conditions in terrestrial hydrothermal settings (
Effects of magma storage and ascent on the kinetics of crystal growth
Contributions to Mineralogy and Petrology, 1994
The size distributions of crystals of olivine, plagioclase and oxides of the 1991/93 eruption at Mt. Etna (Italy) are analyzed. The simultaneous collection of this information for different minerals gives precious insight into the cooling history of lavas. Three distinct episodes are detectable: a storage of the magma in a deep reservoir, characterized by nearly constant and low nucleation and growth rates (near to equilibrium); an ascent phase, with an ever increasing nucleation rate related to volatile exsolution; and finally a quenching phase. In addition to geochemical and geophysical evidence, the similarity of the crystal size distributions of the present eruption with those of previous ones of this century makes it possible to exclude that crystal size distributions of Etnean lavas are due to mixing of different populations. This strongly suggests that the main features of the volcano feeding system have not changed despite observed variations in the magma output rates.