ALBERT

Description: 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 disequilibriumfeatures inminerals 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 equilibrium develops in response to kinetically controlled cation redistributions related to the partitioning ofmajor and trace elements between rapidly growing crystal and melt. The incorporation into the crystal lattice of chemical components in non-stoichiometric or non-equilibrium proportions has important implications for the successful interpretation of the conditions under whichmagmas crystallize and for the development of new equilibrium models based on mineral compositional changes. Finally, it is important to stress that the main purpose of this contribution is to ignite research exploring the causes and consequences of cooling and decompression-driven crystal growth kinetics in order to appreciate in full the evolutionary paths of volcanic rocks and interpret the textural and compositional characteristics of their mineral constituents.

Description: Published

Description: 373–418

Description: 3V. Proprietà dei magmi e dei prodotti vulcanici

Description: Crystal textures of volcanic rocks record the processes involved in magma storage and eruptive ascent. Syn-eruptive crystallization, in which groundmass crystals form and grow according to environmental factors such as thermodynamic undercooling, strongly influences the texture of erupted magma. This stage is difficult to isolate for study in natural rocks, but well-suited for laboratory experiments because the chemical compositions and crystallization timescales of eruptive processes can be emulated. This study examines the incipient stages of plagioclase crystallization in hydrous rhyodacite magma undergoing decompression-driven degassing. Experimental samples in which crystal growth at both near-equilibrium and far-from equilibrium conditions were examined using electron backscatter diffraction (EBSD) analysis to ascertain crystallographic lattice orientations of individual crystals. The crystal orientation investigation affirms a common assumption invoked in textural studies of crystal number density: contiguous crystals with parallel faces are crystallographically continuous, whereas contiguous crystals with non-parallel faces have unrelated crystal lattice orientations and as such, represent separate crystals. In the highly undercooled sample, twinning is identified in ~87% of the crystals examined; in the near-equilibrium sample, 38% of the crystals are twinned. We find that the observed twinning is unlikely to be the result of deformation, transformation, or synneusis, but rather a result of growth defects introduced during the incipient stages of crystallization. We suggest internal structural defects (twins) control macroscopic morphological defects (embayments, swallowtails, and melt inclusions) as a result of the high energy of the twin plane boundary. Formation of twins during the incipient stages of plagioclase crystallization is the single most important factor contributing to anhedral morphologies of feldspar microlites growing during magma decompression, and plays a role in the development of some plagioclase-hosted melt inclusions.

Description: Diffusion modeling in olivine is a useful tool to resolve the timescales of various magmatic processes. Practical olivine geospeedometry applications employ 1D chemical transects across sections that are randomly sampled from a given 3D crystal population, but the accuracy and precision with which timescales can be retrieved from this procedure are not well constrained. Here, we use numerical 3D diffusion models of Fe-Mg to evaluate and quantify the uncertainties associated with their 1D counterparts. The 3D diffusion models were built using both simple and realistic olivine morphologies, and incorporate diffusion anisotropy as well as different zoning styles. The 3D model crystals were sectioned along ideal or random planes, which were used to perform 1D models and timescale comparisons. Results show that the timescales retrieved from 1D profiles are highly inaccurate and can vary by factors of 0.1–25 if diffusion anisotropy is not taken into account. Even when anisotropy is corrected for, timescales can still vary between 0.2–10 times the true 3D diffusion time due to crystal shape and sectioning effects. Simple grain selection procedures are described to reduce the misfit between calculated and actual diffusion times, and achieve an accuracy and precision of ~5% and ~15–25% relative, respectively. Provided that the grains are carefully selected, about 20 concentration profiles and associated 1D models suffice to achieve this accuracy.

Description: A difficulty in performing high-temperature (〉900 °C) experiments on near-liquidus hydrous mafic melts in gas-medium cold-seal pressure vessels (CSPV) is the tendency for H 2 O in the fluid phase to dissociate and H 2 to diffuse through capsule material, leading to progressive oxidation of sample material. Negative consequences include premature stabilization of Fe-Ti oxide phases and commensurate deviation of the liquid line of descent toward silica enrichment. Moreover, time-variance of an intensive variable equal in importance to temperature or total pressure is an unwanted feature of any experimental study. Methodologies commonly employed to mitigate the oxidation problem, not without their own drawbacks, include incorporating CH 4 into the pressurizing gas, limiting run duration to 24 h, enclosing samples in Au-alloy capsules, and incorporating solid buffering assemblages to serve as indicators of f O 2 excursion. Using the Co-Pd-O system as a f O 2 sensor, we investigated progressive oxidation of basaltic andesite at 1010 °C and P H 2 O = 150 MPa. Our time-series of 12, 24, 36, 48, and 60 h run durations reveals that oxidation occurs at a very high rate (~3–4 log unit change in f O 2 in 48 h). Both the variability of f O 2 and magnitude of dehydration-oxidation are considered unacceptable for phase equilibria work. Incorporation of additional CH 4 serves only to offset the progressive oxidation trend toward a lower absolute range in f O 2 . Ultimately, rapid oxidation in CSPV hinders the chemical equilibration of experimental charges. To mitigate the issue, we propose the following solution: Incorporation of a substantial mass of Ni metal powder as an O 2 getter to the outer capsule successfully: (1) slows down oxidation; (2) stabilizes f O 2 at the nickel-nickel oxide (NNO) buffer after ~20 h; and (3) allows compositions to approach equilibrium. Runs much longer than 48 h may require one or more steps involving quenching and re-filling the pressure system with CH 4 .

Description: Author(s): P. Klos, S. König, H.-W. Hammer, J. E. Lynn, and A. Schwenk The authors explore the detectability (signatures) of true few-body resonances, involving the separation of all particles in the system, through avoided level crossings in energy spectra in a finite box. In so doing they adapt the numerical method of a discrete variable representation (DVR) for fermions and bosons in periodic boxes and describe thoroughly its implementation. [Phys. Rev. C 98, 034004] Published Mon Sep 24, 2018