Sutures in mafic pyroclasts

Micrometer-scale Fe (iron)-rich filaments are common in mafic pyroclasts, yet their origin remains debated. These filaments have been variably interpreted as sutured fractures in magma, the product of magma mingling, compositional boundary layers detached from plagioclase crystals, or oxide shells stripped from bubbles. Here, we compare new observations from several case studies with previous results, concluding that shared features point to a common origin of filaments in all eruptions. Key shared features of filaments are as follows: (1) Fe, Mg, and Ca enrichment; (2) variable degree of contortion, sharpness, and Fe oxide microlite content; (3) inclusion of small vesicles and microlite fragments; (4) transition into cracks and broken crystals; (5) connection to sharp concavities of vesicles and pyroclast edges; and (6) separation of regions within pyroclasts with distinct textures. Focused Ion Beam, micron-sized trenches cut perpendicular to the polished section, and X-ray computed microtomography images reveal that the two-dimensional filaments are complex three-dimensional surfaces resulting from magma suturing. These sutures form by fusing of molten, Fe-enriched surfaces, whether exposed crack surfaces within pyroclasts, the surfaces of colliding pyroclasts, or the surfaces of different portions of the same pyroclast folded over itself. During fusing, sutures entrap voids and crystal fragments present on the surfaces. After fusing, sutures evolve by diffusion, crystallization, and viscous deformation. Sutures witness the complex thermal and deformation history of pyroclasts during and after magma fragmentation and act as unique telltales of otherwise invisible processes that affect pyroclast final size, chemical composition, and vesicle and crystal size distributions.