Fault zone heterogeneity takes several forms, such as surface roughness, lithological variations, and the intermixing of mineral phases with contrasting frictional properties within the fault core. Each is thought to affect earthquake nucleation and arrest, yet their effects on preparatory processes before failure and on earthquake recurrence remains insufficiently understood. Here, we conducted direct shear experiments on binary mixtures of soda-lime glass beads and quartz powder, systematically varying the gouge composition from 100 to 0 wt% glass beads at 313 ± 15 kPa normal stress. Frictionally contrasting phases are intermixed at the grain scale, making the mixtures spatially homogeneous but compositionally heterogeneous. Homogeneous glass beads, as one end member of the gouge mixtures, produce quasi-periodic stick-slip with pronounced pre-failure dilation, late-stage rapid slip acceleration, and time and slip predictable recurrence. In contrast, compositionally mixed gouges accelerate more gradually and exhibit shorter dilational phases with small amplitudes. Furthermore, preparatory slip starts earlier in the interseismic period and evolves towards an intermittent critical stress state prior to failure, which is associated with increasing deviation from time-predictable recurrence and a growing fraction of aseismically accommodated slip. The transition from stick-slip to stable sliding occurs within a narrow compositional range (80–85 wt% glass beads), with peak slip velocities differing by approximately 3.8 orders of magnitude between the two regimes. These results demonstrate that gouge composition strongly affects preparatory processes, and predictability of stick-slip recurrence in the laboratory and potentially in nature.
We investigate three unresolved questions in CM chondrite research: (1) what was the initial O-isotope composition of their accreted water? (2) How much water was accreted? And (3) Did aqueous alteration proceed under open or closed-system conditions? We approach this problem through numerical modelling of the aqueous alteration process using an O-isotope perspective. Our model simulates dissolution and mixing between dry rock and the fluid reservoir, secondary mineralisation (phyllosilicate and calcite) and, in some scenarios open-system evaporative loss. Previously published O-isotope measurements (CM chondrite bulk rock, and T1 and T2 calcite) and a suitable alteration extent (65–85 vol%) are used as constraints against which viable alteration scenarios must match. We find the most likely conditions for alteration were the accretion of water-ice with an approximately Earth-like composition (plotting on or close to the intersection between the TFL and the PCM) and with a δ18O value <30 ‰ and likely at ∼5 ‰, prograde conditions (increasing temperature with time), T1 formation before significant protolith was dissolved, an initial water-to-rock (mass) ratio of ∼0.6 and moderate open system evaporative water loss. The most surprising result of this study is the isotopically light (relatively 16O-rich) initial water composition. We show that this is consistent with fluid compositions independently determined from several other studies. This implies that outer Solar System water was not ubiquitously 16O-poor, as might have been expected based on the composition of isotopically anomalous cosmic symplectite found in Acfer 094.
Plinian eruptions involve magma fragmentation that breaks magma into gas and pyroclasts. These pyroclasts contain unique vesicle textures that preserve the only direct record of magma degassing during ascent. Yet the origin of these vesicle textures has remained unresolved. To address this, we develop a conduit flow model that incorporates disequilibrium degassing, including bubble nucleation and growth, without imposing constraints on conduit geometry. Focusing on crystal-free to crystal-poor silicic Plinian eruptions, we demonstrate that the vesicle textures preserved in their pyroclasts, including vesicle number densities and size distributions, are the natural outcome of repeated bubble nucleation during magma ascent that culminates in magma fragmentation. Bubble nucleation builds bubble number density up to a critical threshold ( ∼ 1014 bubbles m−3), at which rapid diffusion of H2O into bubbles sharply increases melt viscosity and pushes the melt across the glass transition. This critical BND threshold persists across the wide range of discharge rates examined for silicic Plinian eruptions.
