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The formation of adcumulates necessitates the continued growth of primocrysts down to low porosities. Gravitationally driven viscous compaction at the base of a crystal mushy layer on the magma chamber floor, driven by the weight of the mushy layer itself, is commonly suggested as a significant process acting to drive out interstitial liquid and promote adcumulate formation. Compaction necessitates viscous deformation, by either dislocation creep or diffusion-controlled processes such as pressure-solution: many studies suggest that the foliations preserved in cumulates are a consequence of recrystallization during compaction, completely overprinting primary magmatic fabrics. We test the compaction hypothesis by looking for microstructural evidence of viscous deformation. A detailed examination of cumulates from the Skaergaard intrusion, East Greenland, demonstrates only limited crystal plastic deformation, with no correlation between the extent of dislocation creep and the calculated volume fraction of trapped liquid left in the cumulates. Although the evidence for diffusion-controlled deformation is often cryptic, there is an anti-correlation between apparent aspect ratio of plagioclase and the extent of adcumulate crystallization, contradicting previous hypotheses involving transposition of original magmatic fabrics by dissolution-reprecipitation. This is supported by the spatial distribution of compositional zoning in plagioclase, which demonstrates that pressure-solution or related diffusion-controlled processes were insufficient to obscure primary magmatic fabrics. The Skaergaard adcumulates did not form by viscous compaction. Instead we suggest that they formed by primary processes involving mass transport in a thin mushy layer. Compaction is most likely to occur in slowly cooled intrusions in which the bulk magma crystallizes abundant dense minerals. We present preliminary observations of microstructures in norites from the lower Main Zone of the Bushveld Intrusion, South Africa, and in plagioclase-rich cumulates from the Fe-Ti oxide-rich Baima Intrusion, SW China. The evidence for dislocation creep in both intrusions is unambiguous, although deformation was insufficient to obliterate all traces of the primary magmatic fabrics and unlikely to have been sufficient to significantly reduce the volume of interstitial liquid. [ABSTRACT FROM AUTHOR]

Post-caldera eruptions of Santorini, Greece, over the past 3000 years resulted in the formation of the Kameni Islands, which comprise a series of compositionally similar dacitic lava flows. Each lava flow has a distinct population of partially-crystalline mafic enclaves, which we propose were derived from the break-up of a layer of replenishing magma responsible for triggering the eruption. Five of the recent flows (erupted in 1570, 1939, 1940, 1941 and 1950) include enclaves of essentially identical andesitic bulk compositions, which formed by crystallization of originally aphyric melts prior to eruption. Detailed examination of angles subtended at the junctions between pairs of plagioclase grains demonstrates that enclaves from each flow have a characteristic textural signature, with distinct differences in the extent of quench-related modification of the original population of dihedral angles formed by impingement of growing grains. These variations suggest that the temperature difference between the host dacite and the replenishing andesite at the time of layer overturn and eruption differed between flows. The uniformity of major element compositions of both the replenishing magma and the host dacitic lava flows demonstrates that the critical parameter in determining the timing of layer-overturn is the pre-eruptive H2O content of both host dacite and replenishing magma. We suggest that the replenishing magma responsible for two of the three eruptions in the period 1939-1941 was significantly wetter than that responsible for the later 1950 eruption. The enclaves with the least amount of quench-related modification occur in the 1570 flow. We suggest that in this case the intruding magma was relatively dry. [ABSTRACT FROM AUTHOR]

The crystallographic preferred orientation (CPO) of polycrystalline olivine affects both the viscous and seismic anisotropy of Earth's upper mantle with wide geodynamical implications. In this methods paper, we present a continuous field formulation of the popular directors method for modeling the strain‐induced evolution of olivine CPOs, assuming the activation of a single preferred crystal slip system. The formulation reduces the problem of CPO evolution to a linear matrix problem that can easily be integrated alongside large‐scale geodynamical flow models, and conveniently minimizes the degrees of freedom necessary to represent CPO fields. We validate the CPO model against existing deformation experiments and naturally deformed samples, as well as the popular discrete grain model D‐Rex. A numerical model of viscoplastic thermal convection is built to illustrate how flow and CPO evolution may be two‐way coupled, suggesting that CPO‐induced viscous anisotropy does not necessarily strongly affect convection time scales, boundary (lid) stresses, and seismic anisotropy, compared to isotropic viscoplastic rheologies. As a consequence, geodynamical modeling that relies on an isotropic rheology (one‐way coupling) might suffice for predicting seismic anisotropy under some circumstances. Finally, we discuss limitations and shortcomings of our method, such as representing D‐ and E‐type fabrics or modeling flows with mixed fabric types, and potential improvements such as accounting for the effect of dynamic recrystallization. Plain Language Summary: The orientation of olivine crystals in Earth's upper mantle can affect how seismic waves and mantle viscosity depend on the direction of propagation and stress, respectively, with implications for interpreting seismic data, the interactions between plate motions and the sublithospheric mantle, and the circulation around subducting plates, among other things. In this methods paper, we re‐write the popular directors method as a continuous field problem, allowing for efficient and seamless prediction of how grain orientations co‐evolve with large‐scale mantle flow. We build an idealized model of thermal convection to illustrate the effect of letting flow and grain orientations co‐evolve compared to decoupled models where grain orientations do not affect flow. Our results suggest that, for certain cases, decoupled models might suffice for understanding for example, large‐scale seismic data. Although our methodology has several shortcomings, such as the neglect of recrystallization, it presents a new path forward for understanding how grain orientations can affect flow, and vice–versa, generally thought to be important for understanding seismic signatures and mantle–plate dynamics. Key Points: The directors method is posed as a continuous field problem for large‐scale modeling of crystallographic preferred orientation (CPO)‐induced viscous and seismic anisotropiesA model of thermal convection demonstrates that seismic anisotropy and boundary stresses are affected when flow and CPO are two‐way coupledChallenges are discussed for representing D‐ and E‐type fabrics, dynamic recrystallization, and non‐uniform fabric types [ABSTRACT FROM AUTHOR]

Information on several papers discussed at a colloquium held at GeoForschungsZentrum in Potsdom, Germany on July 2, 2010 related to metamorphism is presented. Topics include granulite metastability, fractures triggered by fluid-present reactions, and importance of free water (H2O) on diffusion kinetics and microstructures. Several presenters were also featured including Bruce Yardley, Bjorn Jamveit, and Georg Dresen.