Sintering timescales of crystal-bearing pyroclast mixtures under stress

The transformation of hot, pyroclastic deposits into dense, coherent magma is a primary way that permeability evolves in shallow conduits, governing sealing timescales, plug formation, and ultimately shifts in eruptive style. This densification occurs via sintering, modulated by competing processes that include diffusive outgassing and vesiculation. Vesiculation introduces hysteretic volume and rheological changes within particles that can ultimately inhibit sintering of hydrous particle packs. While the role of temperature, water content, and grain size distribution in governing viscous sintering kinetics are well constrained, natural deposits commonly contain rigid crystals and experience compressive stresses, whose effects on sintering timescales remain poorly quantified. Here, we experimentally investigate how sintering of coarse ash-to-lapilli (0.50-2.50 mm), hydrous, rhyolitic clasts is modulated with increasing crystal fraction and by the application of stress up to 1-3 MPa. Our results can be divided into two different suites. First, we show that in the absence of applied load increasing crystal content systematically reduces sintering efficiency, preserving permeable pathways for longer. But second, under an applied stress, we induce extremely rapid densification and suppress vesiculation. Textural analysis shows that under stress, grain boundaries are erased, and near-vesicle-free obsidian is formed, even if crystallinities are as high as 60%, which is in stark contrast to the relatively poor sintering of crystal-bearing samples achieved in the absence of applied stress. We thus demonstrate the effects of crystal cargo and shallow stresses on densification, suggesting they exert first-order controls on permeability evolution in the shallow conduit - such as in tuffisites - and providing a framework for interpreting the variable degrees of sintering seen in silicic volcanic environments.