Afina Lupulescu

Aqueous fluid connectivity and chemical transport in clinopyroxene-rich rocks

Abstract In order to assess the efficacy of fluid-assisted chemical transport in deep-seated, pyroxene-rich rocks, two types of experiment involving synthetic clinopyroxenites were performed at 900°–950°C and 1.5 GPa: (1) annealing of ferrosalite (CaMg 0.4 Fe 0.54 Mn 0.06 Si 2 O 6 ) or diopside (CaMg 0.94 Fe 0.06 Si 2 O 6 ) powder in the presence of aqueous fluids (water NaCl, CaCl 2 or CO 2 ) to produce near-equilibrium fluid/crystal dihedral angles (θs), and (2) diffusion-couple experiments in which an H 2 O-bearing cylinder of Fe-rich clinopyroxenite (ferrosalite) was placed against one of Mg-rich clinopyroxenite (diopside). ‘Control’ diffusion experiments on fluid-free clinopyroxenite and on single clinopyroxene crystals were also carried out. The dihedral angle experiments consistently yielded median apparent θ values (i.e., in 2D section) of 65–69°, well above the ‘pinch-off’ value of 60° and independent of additives NaCl, CaCl 2 and CO 2 . In all cases, however, the θ frequency distributions were broad enough to suggest a range of real θs on either side of the median. The textures were also complicated by frequent crystal faceting at pore margins. The diffusion-couple experiments, carried out with 0, 0.6, 1.3, 2.7, 4.0 and 8.0 vol% H 2 O (at run P and T ) showed no Fe = Mg interdiffusion for all but the most fluid-rich (8%) couple. This result is consistent with the determinations in showing that, for geologically realistic fluid abundances, aqueous fluids in clinopyroxenite form isolated (noninterconnected) pores. This conclusion implies that aqueous fluids play no role in larger-than-grain-scale chemical transport in pyroxene-rich rocks under conditions of mechanical equilibrium. The results also allow the possibility that water can be deeply subducted as a separate (but trapped) phase beyond the stability limit of amphibole.

CROSSOVER SCALING IN DENDRITIC EVOLUTION AT LOW UNDERCOOLING

We examine scaling in two-dimensional simulations of dendritic growth at low undercooling, as well as in three-dimensional pivalic acid dendrites grown on NASA’s USMP-4 isothermal dendritic growth experiment. We report new results on self-affine evolution in both the experiments and simulations. We find that the time-dependent scaling of our low undercooling simulations displays a crossover scaling from a regime different than that characterizing Laplacian growth to steady-state growth. [S0031-9007(99)09307-2]

Implications of the interface shape on steady-state dendritic crystal growth

Experimental results obtained as part of a series of experiments on dendritic growth in microgravity, the isothermal dendritic growth experiment (IDGE), indicate that Ivantsovs transport solution to the dendrite problem is valid to first order. The data reveal a Peclet (Pe) number vs. supercooling relationship that is both systematically lower than expected, and exhibits larger scatter than is accounted for by the uncertainties of the individual data points. A range of explanations have been offered, including residual convection, far-field chamber effects, and dendrite self-interaction. Although some of these explain certain features of the experimental data, quantitative evidence remains lacking concerning the cause of the two issues mentioned above. This investigation examines the dendritic growth data in the context of how the actual shape of the dendrite tip and its trailing side-branch structure may affect the transport process. The method of moving heat sources is used to describe the diffusive thermal transport processes around a crystalline body of revolution growing into its quiescent melt at a constant rate. Results indicate that when Ivantsovs assumed paraboloidal tip shape is modified to better reflect the actual observed tip shape, enhanced agreement can be obtained between the model and the experimental data. Additionally, these results support earlier work by Schaefer (J. Crystal Growth 43 (1978) 17) in predicting that under the conditions of the IDGE experiment, the side-branch region of a dendrite can contribute significantly to the thermal field at the tip. This suggests that the scatter in the IDGE data can be explained by stochastic variations in this influential side-branch structure. With these observations in hand, it is reasonable to claim that the basic transport solution describing dendritic growth is correct, provided adequate account has been taken of details such as container wall effects and dendrite self-interaction. Until now, these conclusions have not been completely supported by quantitative evidence.

Diffusion-limited crystal growth in silicate systems: similarity with high-pressure liquid-phase sintering

Abstract In natural partially molten silicate systems as well as in material systems, textural adjustments occur to reduce the total free energy of the system, including those contained in surfaces and interfaces. To assess the effect of water content on textural equilibration and crystal growth in solid–liquid silicate systems we conducted wetting-angle and diffusion experiments. These experiments provide information about the extent of the interfacial energy reduction associated with dissolution along grain edges. The wetting angle exhibits a constant median value of ca. 50°, reflecting overall reduction of interfacial energy of the two-phase silicate system during melt penetration. The nature and extent of the breakdown reaction, the proportion of the new phases, and the crystallographic development are related to the initial water content. The solid was initially impermeable, so the liquid–solid equilibration involves dissolution producing crystallite rearrangements on a local scale, with immediate and nearby reprecipitation. Because the process of dissolution/precipitation requires transport of the atomic components, liquid penetration, and the rate-limited growth of the new phases are found to be consistent with capillary-driven diffusion.

Capillary Mediated Melting of Ellipsoidal Needle Crystals

Measurements of video data on melting dendritic crystal fragments in reduced gravity show that a fragment’s ellipsoidal axial ratio, C/A, rises initially until it melts down to a pole-to-pole length of C ≈ 5 mm. At that point we observe a sudden fall in the C/A ratio with time, as the polar regions melt toward each other more rapidly than C/A times the melting speed, dA/dt, of the equatorial region. This accelerated melting allows the C/A ratio to fall from values around 10–20 (needle-like) towards values approaching unity (spheres) just before total extinction occurs. Analytical and numerical modeling will be presented that suggest that the cause of these sudden changes in kinetics and morphology during melting at small length scales is due to a crystallite’s extreme shape anisotropy. Shape anisotropy leads to steep gradients in the mean curvature of the solid-melt interface near the ellipsoid’s poles. These curvature gradients act through the Gibbs-Thomson effect to induce unusual thermo-capillary heat fluxes within the crystallite that account for the observed enhanced polar melting rates. Numerical evaluation of the thermo-capillary heat fluxes shows that they increase rapidly with the C/A ratio, and with decreasing length scale, as melting progresses toward total extinction.

Evidence for Eigenfrequencies in Dendritic Growth Dynamics

Microgravity dendritic growth experiments, conducted aboard the space shuttle Columbia, are described. In-situ video images reveal that pivalic acid dendrites growing in the diffusion-controlled environment of low-earth orbit exhibit a range of transient or non-steady-state behaviors. The observed transient features of the growth process are being studied with the objective of understanding the mechanisms responsible for these behaviors. Included in these observations is possible evidence for characteristic frequencies or limit cycles in the growth behavior near the tip of the dendrites. These data, and their interpretations, will be discussed.

Melting Kinetics of Prolate Spheroidal Crystals

The melting kinetics of a pivalic acid (PVA) dendritic mushy zone was observed for the first time under convection-free conditions. Video data show that PVA dendrites melt into fragments that shrink at accelerating rates to extinction. Individual fragments follow a characteristic time-dependence derived here for the diminishing length scales within a melting mushy zone. The melting kinetics against which the experimental observations are compared is based on the conduction-limited quasi-static process of melting under shape-preserving conditions. Agreement between analytic theory and experiment was found for the melting of a prolate spheroidal crystal fragment with an aspect ratio of C/A = 12.

Melting processes under microgravity conditions

The kinetics of melting pivalic acid (PVA) dendrites was observed under convection-free conditions on STS-87 as part of the United States Microgravity Payload Mission (USMP-4) flown on Columbia in 1997. Analysis of video data show that PVA dendrites melt without relative motion with respect to the quiescent melt phase. Dendritic fragments display shrinking to extinction, with fragmentation occurring at higher initial supercoblings. Individual fragments follow a characteristic time-dependence derived elsewhere. The microgravity melting kinetics against which the experimental observations are compared is based on conduction-limited quasi-static melting under shape-preserving conditions. Agreement between analytic theory and our experiments is found when the melting process occurs under shape-preserving conditions as measured using the CA ratio of individual needle-like crystal fragments.

Time-Dependent Behavior of Dendrites Under Diffusion-Controlled Conditions

Dendrites interact with hydrodynamic flows during solidification. In the presence of gravity, thermal and solutal buoyancy forces induce convective motion in the melt. The basic theories of dendritic growth, however, are best tested under diffusion-controlled conditions, where, ideally, gravitational acceleration and convection are absent, or at least drastically reduced. Microgravity experiments of the Isothermal Dendritic Growth Experiment (IDGE) were designed to measure convection-free dendritic growth. IDGE experiments to accomplish this were flown on United States Microgravity Payload Missions: USMP-2 (March 1994), USMP-3 (March 1996), and USMP-4 (December 1997).

Late-Stage Capillarity Effects During Crystallite Melting in Microgravity

The Isothermal Dendritic Growth Experiment (IDGE) flew in late 1997 as a primary experiment on the United States Microgravity Payload Mission (USMP-4). IDGE video data show that PVA dendrites melt into small fragments, or crystallites, without being subject to any detectable relative motion with respect to the surrounding melt phase. Although a mass density di! erence of about 4% exists between crystals and the melt, the lack of significant body forces on orbit precludes buoyancy e! ects, such as sedimentation and convection. Prior work reported shows that the observed dendritic melting and fragmentation occur by heat conduction through the melt. Specifically, in microgravity, melting occurs such that needle-like fragments shrink with an increasing C/A ratio, where C is the semi-major axis length, and A is the semi-minor axis length. We report new observations of late-stage melting kinetics a! ected by capillarity, where the previously rising C/A ratio of individual crystallites suddenly drops from ! 20 to ! 5 when the typical crystallite length, 2C, falls below about 5 mm. The sudden drop observed in the C/A ratios of ellipsoidal fragments undergoing late-stage melting is ascribed to capillary e! ects that accompany the melting of slender needle-like crystallites. Capillary-induced heat fluxes, received at the crystal-melt interface—both externally from the hotter melt, as well as internally—arise via the Gibbs-Thomson e! ect from the steep interfacial curvature gradients near the poles prior to total extinction. The nature of these unusual heat fluxes during late-stage melting was modeled using finite di! erence methods, the results of which will be discussed as elucidating an important phenomena during melting as observed in reduced gravity during space flight.