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Bubble and crystal textures evolve during magma ascent, altering properties that control ascent such as permeability and viscosity. Eruption style results from feedbacks between ascent, bubble nucleation and growth, microlite crystallization, and gas loss, all processes recorded in pyroclasts. We show that pyroclasts of the mafic Curacautín ignimbrite of Llaima volcano, Chile, record a history of repeated autobrecciation, fusing, and crystallization. We identified pyroclasts with domains of heterogeneous vesicle textures in sharp contact with one another that are overprinted by extensive microlite crystallization. Broken crystals with long axes (l) >10 μm record fragmentation events during the eruption. A second population of unbroken microlites with l ≤10 μm overprint sutures between fused domains, suggesting the highly crystalline groundmass formed at shallow depths after autobrecciation and fusing. Nearly all pyroclasts contain plutonic and ancestral Llaima lithics as inclusions, implying that fusing occurs from a few kilometers depth to as shallow as the surface. We propose that Curacautín ignimbrite magma autobrecciated during ascent and proto-pyroclasts remained melt rich enough to fuse together. Lithics from the conduit margins were entrained into the proto-pyroclasts before fusing. Autobrecciation broke existing phenocrysts and microlites; rapid post-fusing crystallization then generated the highly crystalline groundmass. This proposed conduit process has implications for interpreting the products of mafic explosive eruptions.
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Flow‐ and Fracture‐Driven Bubble Throat Growth Rates and Dynamic Permeability in Crystallizing Magma
Abstract Pyroclasts typically exhibit coalesced vesicle textures, which are the evidence of bubble coalescence and the incomplete bubble wall retraction in magma during volcanic eruptions. The sizes of bubble throats or inter‐bubble apertures in permeable networks control the extent of magma outgassing, and therefore, quantifying the growth rates of the bubble throats is important but has remained poorly constrained. Using dynamically similar experiments with spontaneous bursting of a single bubble in rheologically well‐characterized particulate suspensions, we investigate the growth rate of bubble throats for a range of particle volume fractions. For suspensions with 0.50 particle volume fraction, a circular hole (bubble throat) forms following bubble bursting, which after an initial fast growth starts plateauing at a throat‐bubble size ratio of 0.20. The throat growth time scale overall increases with increasing particle volume fraction due to the increase in suspension viscosity. On the other hand, bubbles in suspensions with particle volume fraction near the maximum packing fraction (0.64) exhibit a fracture‐like opening. Thus, our experimental results suggest that the plateauing of the bubble throat growth in crystal‐poor to crystal‐rich magma likely contributes to the wide occurrence of the incompletely retracted vesicle walls in pyroclasts. The implications of the flow‐ to fracture‐like growth of bubble throats on the development of dynamic permeability in magma are discussed.
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- Award ID(s):
- 2202666
- PAR ID:
- 10569216
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 26
- Issue:
- 2
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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