Causes of fragmented crystals in ignimbrites: a case study of the Cardones ignimbrite, Northern Chile

Abstract

Broken crystals have been documented in many large-volume caldera-forming ignimbrites and can help to understand the role of crystal fragmentation in both eruption and compaction processes, the latter generally overlooked in the literature. This study investigates the origin of fragmented crystals in the > 1260 km 3 , crystal-rich Cardones ignimbrites located in the Central Andes. Observations of fragmented crystals in non-welded pumice clasts indicate that primary fragmentation includes extensive crystal breakage and an associated ca. 5 vol% expansion of individual crystals while preserving their original shapes. These observations are consistent with the hypothesis that crystals fragment in a brittle response to rapid decompression associated with the eruption. Additionally, we observe that the extent of crystal fragmentation increases with increasing stratigraphic depth in the ignimbrite, recording secondary crystal fragmentation during welding and compaction. Secondary crystal fragmentation aids welding and compaction in two ways. First, enhanced crystal fragmentation at crystal-crystal contacts accommodates compaction along the principal axis of stress. Second, rotation and displacement of individual crystal fragments enhances lateral flow in the direction(s) of least principal stress. This process increases crystal aspect ratios and forms textures that resemble mantled porphyroclasts in shear zones, indicating lateral flow adds to processes of compaction and welding alongside bubble collapse. In the Cardones ignimbrite, secondary fragmentation commences at depths of 175-250 m (lithostatic pressures 4-6 MPa), and is modulated by both the overlying crystal load and the time spent above the glass transition temperature. Under these conditions, the existence of force-chains can produce stresses at crystal-crystal contacts of a few times the lithostatic pressure. We suggest that documenting crystal textures, in addition to conventional welding parameters, can provide useful information about welding processes in thick crystal-rich ignimbrites.

Fabrics, facies, and flow through a large-volume ignimbrite: Pampa De Oxaya, Chile

“…Primary welding is evidenced by the presence of devitrified remnant fluidal juvenile shard textures (Fig. 3c), the ubiquitous flattened juvenile fiamme and loading induced, in situ secondary crystal fragmentation (van Zalinge et al, 2018). The observed architecture is likely to arise from numerous factors relating to the generation, transport, emplacement and post-emplacement modification of the ignimbrite.…”

Volcanoes Erupt Stressed Quartz Crystals

“…Rarefaction waves produced during fragmentation would induce tensile stress in crystals (Best & Christiansen, 1997). Postfragmentation, high-velocity crystal collisions in the conduit or during deposition from pyroclastic density currents, may produce compressive deformation in the crystals (van Zalinge et al, 2018). Finally, magmas may effuse to form lava flows.…”

Crystal Size Distribution (CSD) Analysis of Volcanic Samples: Advances and Challenges

“…It is clear that the phenocrysts (sensu lato) often record more than one crystallization episode and may have time breaks, for example at resorption boundaries. Crystals in explosively erupted pyroclasts may also be extensively fragmented (e.g., Bindeman, 2005;Jerram et al, 2009;van Zalinge et al, 2018). Crystal cores may be drawn from different populations and therefore do not always represent a single crystallization event.…”