ALBERT

Description: The equilibrium model of prograde metamorphism, in which rocks are regarded as departing only negligibly from equilibrium states as they recrystallize, has generated a wealth of petrologic insights. But mounting evidence from diverse approaches and observations over a range of scales has revealed that kinetic impediments to reaction may prevent metamorphic rocks from attaining rock-wide chemical equilibrium along their prograde crystallization paths. To illustrate the resulting potential for inaccurate interpretation if kinetic factors are disregarded, we briefly review several case studies, including: out-of-sequence, metastable, and displaced isograds in contact aureoles; paragenetic sequences documenting overstepped, disequilibrium reaction paths; patterns of compositional zoning in garnet demonstrating partial chemical equilibrium; petrologic incongruities between observation and thermodynamic prediction; and inhibited reaction progress revealed by petrologically constrained numerical simulations of garnet crystallization. While the equilibrium model provides an indispensable framework for the study of metamorphic systems, these examples emphasize that all reactions require departures from rock-wide equilibrium, so all rocks must traverse kinetically sensitive reaction paths during recrystallization. Mindfulness of the potential significance of kinetic influences opens new avenues for petrologic investigation, thereby enhancing both analysis and interpretation.

Description: Garnet in a metapelite from the aureole of the Vedrette di Ries tonalite, Eastern Alps, is found in two distinctive microstructural settings relative to a large single crystal of biotite: it forms a narrow polycrystalline layer in a reaction zone between plagioclase and biotite, and nearby it occurs in clusters of crystals interspersed with the biotite. In the reaction zone, morphological and microchemical evidence document multiple nucleation events along the interface between biotite and plagioclase, with ilmenite inclusions marking the original plagioclase–biotite boundary. Electron backscatter diffraction analysis shows that for all garnet in both microstructural settings, (110) grt || (001) bt with [100] grt || [100] bt . This single, recurring orientation for all garnet crystals adjacent to the biotite grain implies that nucleation was dominantly epitaxial during crystallization of the garnet. The occurrence is remarkable for the clear genetic relationships revealed by the microstructures and for its preservation of the mica substrate on which the garnet nucleated, which allows unambiguous determination of not only the coincident lattice planes but also the coincident lattice directions involved in the epitaxy.

Description: The combination of high-resolution X-ray computed tomography (HRXCT) with electron backscattered diffraction (EBSD) analysis has the potential to reveal evidence for epitaxial nucleation of garnet porphyroblasts containing an included fabric. HRXCT measures the fabric orientation near the inferred nucleation site at the crystal centre, whereas EBSD establishes the crystallographic orientation of the porphyroblast relative to the attitude of the included fabric. Using the orientation of the central fabric as a proxy for the attitudes of the basal planes of micas on which the garnet may have nucleated, one can determine if low-index planes in garnet—(100), (110), or (111)—are parallel to (001) of mica, which would correspond to known epitaxial relationships between the two minerals. We test the feasibility of this approach with a set of exploratory measurements for six garnet porphyroblasts from Passo del Sole (Switzerland) and five from the Nevado Filabride Complex (Spain); we also report unsuccessful attempts at such measurements in rocks from Harpswell Neck (USA). The results do not deviate appreciably from those expected of a set of measurements drawn from a parent population with a random distribution of orientations. However, considering the size of the present measurement uncertainties in relation to the precision required to recognize an epitaxial relationship unambiguously, it is not possible in this data set to exclude epitaxy as a nucleation mechanism. The principal limitation to this approach is the need for high precision in determining the attitude of the included fabric at the site of nucleation, which depends largely upon the choice of suitable samples. Crystals ideal for such studies would contain a non-crenulated, planar (not helicitic) foliation, defined by numerous small, well-aligned inclusions. With careful sample selection, this approach could be a viable technique for evaluating the prevalence in nature of epitaxial nucleation of garnet porphyroblasts.

Description: Numerical simulation of the evolution of stranded diffusion profiles in partially resorbed garnet crystals from the aureole of the Makhavinekh Lake Pluton, Labrador, yields quantitative determinations of rates of diffusion of Y, rare earth elements (REEs), and Cr at ~700–900 °C, 0.53 GPa. Diffusion coefficients for these trivalent cations are 0.5–1.5 log 10 units smaller than those for major divalent cations measured in the same crystals, but diffusivities for trivalent cations are all equal to one another to within ±0.25 log 10 unit. Integration of these new data with previously published results resolves some prior inconsistencies, and defines the dependence of diffusivities for Y, the REEs, and Cr on temperature, pressure, and oxygen fugacity, while accounting for minor effects of ionic radius and host-crystal composition. Nd, Sm, and Eu—elements that are strongly depleted in rims of relict garnet crystals due to preferential partitioning out of garnet during resorption—evolve small but distinct maxima from initially nearly flat profiles; this "uphill diffusion" results from cross-coupling with Y and the other REEs, which are strongly concentrated in relict garnet rims by resorption. The weak dependence of diffusivity on ionic radius and host-crystal composition, the near-equivalence of diffusivities of Y+REEs with that of Cr, and the strong positive cross-coupling among Y+REEs are all explained by a diffusion mechanism that links the mobility of VIII Y+REEs with that of VI Al; this is likely a consequence of the dominance of the menzerite-(Y) component as a means of incorporating Y+REEs in the garnet structure. However, the variety of possible substitution mechanisms that may enable Y+REE incorporation into garnet, and the degree to which each may be favored in different regimes of temperature, pressure, and oxygen fugacity, imply a potential for great complexity, as the net diffusional flux may result from the superposed effect of multiple diffusion mechanisms, each with different kinetics.

Description: The ubiquity of quartz in continental crust, and the involvement of SiO 2 in multiple metamorphic processes such as reactions, fluid flux, and solution-transfer processes, makes quartz an obvious choice for reconstructing prograde metamorphic conditions in various rock types. Recent studies have shown the usefulness of analyzing Ti distribution in quartz to constrain pressure-temperature-(relative) time-deformation ( P-T-t-D ) in metamorphosed tectonites. New high-precision single-crystal X-ray diffraction volume constraints on Ti-doped and chemically pure quartz provide further evidence for substitution of Ti 4+ for Si 4+ in the tetrahedral site in quartz, with resultant lattice strain on the structure. Recent applications of the Ti-in-quartz thermobarometer to dynamically recrystallized quartz have identified recrystallized subgrains that contain lower Ti concentrations ([Ti]) than their host porphyroclasts. In addition, [Ti] are lower than expected for the temperatures of recrystallization. Atomistic simulations that estimate energetic perturbations resulting from Ti incorporation into the quartz lattice indicate that significant increases in strain energy occur only at very high [Ti]; the strain-energy increase is negligible for [Ti] typical of quartz grown under mid-crustal conditions. This suggests that lattice strain rarely provides an appreciable driving force for Ti loss from quartz; instead, it appears that subgrain boundaries and dislocation arrays migrating through recrystallizing quartz crystals can promote localized re-equilibration, thermodynamically regulated by the composition of the intergranular medium (typically undersaturated in Ti). It therefore appears that analyses from dynamically recrystallized quartz cannot be meaningfully interpreted until methods are developed that can account quantitatively for the reduction of Ti resulting from crystal plastic flow.

Description: Numerical simulation of stranded diffusion profiles in partially resorbed garnet crystals from the aureole of the Makhavinekh Lake Pluton (Labrador, Canada) yields quantitative rates of intracrystalline diffusion for Li in garnet. Diffusion coefficients for Li at 700–900 °C and 0.53 GPa are 0.5–1.5 log 10 units lower than those for divalent cations in the same samples, and match those for Y and Yb. This correspondence likely stems from coupled substitution of Li and Y (or REE) ions in the garnet structure: the requirement of local electroneutrality limits Li mobility to be no faster than that of Y+REEs. Because of this coupling, Li zoning in garnet is retained to comparatively high temperatures, making garnet a valuable monitor of the behavior of Li in deep crustal systems.

Description: Yttrium and the rare-earth elements (Y+REEs) are incorporated into aluminosilicate garnet as trivalent ions replacing divalent Mg, Fe, Mn, or Ca ("M 2+ ") in dodecahedral sites, which requires some form of coupled substitution to maintain electroneutrality. We compare the energetic costs of competing coupled-substitution schemes, using lattice dynamics calculations to assess defect energies and exchange energies for each scheme. Substitutions with relatively low energetic costs introduce menzerite-like components via the exchange vector [YM –1 ·(Mg,Fe)Al –1 ], or alkali components via the exchange vector [Y(Na,Li)M –2 ]. Substitutions with substantially higher energetic costs introduce a vacancy component via the exchange vector [Y 2 M –3 ], or the yttrogarnet (YAG) component via the exchange vector [YM –1 ·AlSi –1 ], or a component with octahedral Li via the exchange vector [Y 2 M –2 ·LiAl –1 ]. Energetic costs decrease significantly as the host-garnet unit-cell dimension expands, decrease very modestly as temperature rises or pressure falls, and decrease substantially with the contraction in ionic radius across the lanthanide series. These results, combined with critical re-examination of arguments cited in favor of each substitution scheme in natural occurrences, suggest that Y+REE incorporation in natural garnet is dominated by coupled substitutions that introduce menzerite and alkali components, that the YAG substitution plays only a subsidiary role, and that the other schemes are likely to be of very minor importance.

Description: Garnet bears witness to the importance of kinetics during metamorphism in its microstructural features, compositional zoning, and diffusional response to thermal events. Porphyroblastic textures carry quantitative signals of protracted nucleation and sluggish intergranular diffusion, key impediments to reaction progress that may result in crystallization under conditions well removed from equilibrium. Growth zoning in garnet reveals partial chemical equilibration with matrix minerals: intergranular transport keeps pace with garnet growth for some elements but not for others, leading to variable degrees and length scales of chemical equilibration. Partial relaxation of compositional zoning by intracrystalline diffusion is a sensitive and quantitative indicator of thermal history, constraining rates and timescales of peak metamorphic heating, processes of burial and exhumation, and retrogression on cooling.