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Autobrecciation and fusing of mafic magma preceding explosive eruptions

Aaron A. Marshall, Michael Manga, Brittany D. Brand and Benjamin J. Andrews
Autobrecciation and fusing of mafic magma preceding explosive eruptions
Geology (Boulder) (July 2022) 50 (10): 1177-1181


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 Curacautin 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 mu m record fragmentation events during the eruption. A second population of unbroken microlites with l < or =10 mu m 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 Curacautin 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.

ISSN: 0091-7613
EISSN: 1943-2682
Serial Title: Geology (Boulder)
Serial Volume: 50
Serial Issue: 10
Title: Autobrecciation and fusing of mafic magma preceding explosive eruptions
Affiliation: Boise State University, Department of Geosciences, Boise, ID, United States
Pages: 1177-1181
Published: 20220727
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 33
Accession Number: 2022-050601
Categories: Igneous and metamorphic petrologyQuaternary geology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: NSF Grant EAR-1831143
Illustration Description: illus.
N38°42'00" - N38°42'00", W71°43'60" - W71°43'60"
Secondary Affiliation: University of California at Berkeley, USA, United StatesSmithsonian Institution National Museum of Natural History, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2022, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 202238
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