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Phase transformations are widely invoked as a source of rheological weakening during subduction, continental collision, mantle convection and various other geodynamic phenomena. However, despite more than half a century of research, the likelihood and magnitude of such weakening in nature remain poorly constrained. Here we use experiments performed on a synchrotron beamline to reveal transient weakening of up to three orders of magnitude during the polymorphic quartz to coesite (SiO2) and olivine to ringwoodite (Fe2SiO4) phase transitions. Weakening becomes increasingly prominent as the transformation outpaces deformation. We suggest that this behaviour is broadly applicable among silicate minerals undergoing first-order phase transitions and examine the likelihood of weakening due to the olivine-spinel, (Mg,Fe)2SiO4, transformation during subduction. Modelling suggests that cold, wet slabs are most susceptible to transformational weakening, consistent with geophysical observations of slab stagnation in the mantle transition zone beneath the western Pacific. Our study highlights the importance of incorporating transformational weakening into geodynamic simulations and provides a quantitative basis for doing so. 

We present the results of high-temperature (900°C), high-pressure (200 MPa) deformation experiments that identify the processes and deformation conditions leading to melt migration in crystal-rich mushes. This study is relevant to transport of magmas in transcrustal magma reservoirs. Experimental samples comprise juxtaposed pieces of soda-lime glass and densified mixtures of borosilicate glass and quartz sand, which, at elevated temperatures and pressures, have melt and shear viscosities similar to natural silicate melts and crystal-rich mushes. The synthetic mushes have crystal fractions of 0.60 to 0.83. Samples were deformed in torsion at shear strain rates of 10-5 to 10-4 s-1 to shear strains up to 2.7. Image analysis of experimental samples shows melt migrates into the mush during shear. In mushes with crystal fractions ≥ 0.75 shearing causes melt-filled mm-scale dikes to form and propagate into the mush. To our knowledge, these features are the first dikes formed in high-temperature, high-pressure deformation experiments. Dike formation results from shear-induced dilation, which causes the mush to become underpressurized relative to the melt, at an estimated pressure differential of 10 MPa. Experimental conditions indicate shear-induced dilation and diking occur while the mush is still viscous (i.e., Weissenberg number < 10-2). We apply our results to Soufrière Hills Volcano (Montserrat, West Indies) and use our analysis to predict the deformation conditions that would lead to diking and rapid, voluminous melt migration in that active volcanic system. 

Olivine lattice preferred orientation (LPO), or texture, forms in relation to deformation mechanisms such as dislocation creep and can be observed in the upper mantle as seismic anisotropy. Olivine is also mechanically anisotropic, meaning that it responds to stresses differently depending on the direction of the stress. Understanding the interplay between anisotropic viscosity (AV) and LPO, and their role in deformation, is necessary for relating seismic anisotropy to mantle flow patterns. In this study, we employ three methods to predict olivine texture (D-Rex, MDM, and MDM+AV) in a shear box model and a subduction model. D-Rex and MDM are two representative texture development methods that have been compared before, and our results are in line with previous studies showing that textures computed by D-Rex develop faster and are stronger and more point-like than textures calculated with MDM. MDM+AV uses the same isotropic mantle stresses and particle paths as D-Rex and MDM but includes the effect of AV for texture predictions. MDM+AV predicts a texture similar to MDM with a distinct girdle-like orientation for simple shear deformation or at low strain in the subduction model. At larger strains, MDM+AV’s textures are more point-like and stronger compared to the other two methods. The effective viscosity for MDM+AV drops by up to 60% in a shear box model and can be either strengthened or weakened relative to isotropic viscosity for different regions of the subduction model experiencing different patterns of deformation. Our results emphasize the significant role of AV in olivine texture development, which could substantially affect geodynamic processes in the upper mantle. 

Data accompanying manuscript of the same name. Here we present results of high-temperature, high-pressure experiments that test the conditions leading to melt migration in mushes. Our samples were made up of juxtaposed pierces of soda-lime glass and a densified mixture of borosilicate glass and quartz sand (X = 0.65 to 0.83). When these materials are subjected to high temperatures and confining pressures (900°C, 200 MPa) they are proxies natural silicate melts and mushes, respectively (Ryan et al., 2022). We deformed these samples in torsion and observed migration of melt into the mush as a result of shear. In samples with intermediate (X = 0.75) and high (X = 0.83) mush crystal fractions melt-filled dikes formed and propagated within the mush. To our knowledge these are the first instances of dike formation and propagation in high-temperature, high-pressure deformation experiments. The dikes formed as a result of shear-induced dilation, a process that was recognized in other granular media ∼150 years ago (Reynolds, 1885) but is rarely invoked as a potential deformation behavior for mushes (Petford et al., 2020). We use our experimental results to identify the conditions for shear-induced dilation and diking in mushes, apply this analysis to an active volcanic system (Soufrière Hills Volcano, Montserrat, W.I.) and, finally, consider the role of dike formation and propagation in mushes in the rapid generation and transport of crystal-poor magmas." Imaging: BSE mosaics of transverse sections of each experimental product were captured using a JEOL JXA-8530FPlus Electron Probe Microanalyzer (15 kV, 10 nA). Compositional differences between quartz, olivine, soda lime and borosilicate mean each phase is distinguishable based on its greyscale. Each sample was ground, polished and imaged four to ten times to produce serial sections. The area fraction of soda-lime glass that migrated into the mush (A) was quantified by thresholding and filtering BSE mosaics using ImageJ (Abramoff et al., 2004). Euclidean distance maps were thresholded to identify regions of soda-lime glass that have dimensions less than and greater than the estimated interparticle distance (40 μm; Supplement 2). Aintru is the area fraction of soda-lime glass in the mush with dimensions greater than the interparticle distance. The spatial distribution of soda-lime glass in the mush was quantified by overlaying rectangular grids on the BSE mosaics and measuring the area fractions greater and less than the interparticle distance (Supplement 2). 

The strength of lithospheric plates is a central component of plate tectonics, governed by brittle processes in the shallow portion of the plate and ductile behavior in the deeper portion. We review experimental constraints on ductile deformation of olivine, the main mineral in the upper mantle and thus the lithosphere. Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding (GBS), and diffusion-accommodated grain-boundary sliding (diffusion creep). Deformation in most of the lithosphere is dominated by GBS, except in shear zones—in which diffusion creep dominates—and in the brittle-ductile transition—in which low-temperature plasticity may dominate. We find that observations from naturally deformed rocks are consistent with extrapolation of the experimentally constrained olivine flow laws to geological conditions but that geophysical observations predict a weaker lithosphere. The causes of this discrepancy are unresolved but likely reside in the uncertainty surrounding processes in the brittle-ductile transition, at which the lithosphere is strongest. ▪ Ductile deformation of the lithospheric mantle is constrained by experimental data for olivine. ▪ Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding, and diffusion creep. ▪ Observations of naturally deformed rocks are consistent with extrapolation of olivine flow laws from experimental conditions. ▪ Experiments predict stronger lithosphere than geophysical observations, likely due to gaps in constraints on deformation in the brittle-ductile transition. 

Abstract Novel methods for sampling and characterizing biodiversity hold great promise for re-evaluating patterns of life across the planet. The sampling of airborne spores with a cyclone sampler, and the sequencing of their DNA, have been suggested as an efficient and well-calibrated tool for surveying fungal diversity across various environments. Here we present data originating from the Global Spore Sampling Project, comprising 2,768 samples collected during two years at 47 outdoor locations across the world. Each sample represents fungal DNA extracted from 24 m³of air. We applied a conservative bioinformatics pipeline that filtered out sequences that did not show strong evidence of representing a fungal species. The pipeline yielded 27,954 species-level operational taxonomic units (OTUs). Each OTU is accompanied by a probabilistic taxonomic classification, validated through comparison with expert evaluations. To examine the potential of the data for ecological analyses, we partitioned the variation in species distributions into spatial and seasonal components, showing a strong effect of the annual mean temperature on community composition. 

Abstract We conducted experiments to study melt migration in crystal‐rich mushes, with application to magma ascent within transcrustal magma reservoirs. Mushes with crystal volume fractions of 0.59–0.83 were prepared by hot‐pressing crushed borosilicate glass mixed with different proportions of quartz sand particles. Each experimental sample comprises stacked disks of mush and soda‐lime glass, a proxy for crystal‐free magma. Samples were subjected to confining pressures of 100–300 MPa and a temperature of 900°C (above the glass transition temperatures of the borosilicate and soda‐lime glasses) for up to 6 h. The bottom and circumference of the mush and soda lime disks experience the confining pressure, but the top of the mush disks is at room pressure, resulting in a pore‐pressure gradient across the mush layer. Following cooling and decompression, we determined the area fraction and morphology of soda‐lime melt that migrated into the mush layer during experiments. Melt fraction is more strongly correlated to crystal fraction than pore‐pressure gradient, increasing with crystal fraction before sharply decreasing as crystal fractions exceed 0.8. This change at 0.8 coincides with the transition from crystals in the mush moving during soda‐lime migration to crystals forming a continuous rigid network. In our experiments, melt migration occurred by viscous fingering, but near the mobile‐to‐rigid transition, melt migration is enhanced by additional capillary action. Our results indicate that magma migration may peak when rigid mushes “unlock” to become mobile. This transition may mark an increase in magma migration, a potential precursor to volcanic unrest and eruption.