HR: 0800h
AN: V41D-0812    [Abstracts]
TI: Holocene processes and petrogenesis at a long-lived trachyandesitic magmatic system
AU: * Kratzmann, D
EM: davidk@gso.uri.edu
AF: Graduate School of Oceanography, Univ. of Rhode Island, South Ferry Road, Narragansett, RI 02882, United States
AU: Carey, S
EM: scarey@gso.uri.edu
AF: Graduate School of Oceanography, Univ. of Rhode Island, South Ferry Road, Narragansett, RI 02882, United States
AU: Scasso, R
EM: rscasso@gl.fcen.uba.ar
AF: Dpto. de Cs. Geologicas, FCEN, Univ. de Buenos Aires Cuidad Univ., Pab 2, 1 Piso, Buenos Aires, 1428, Argentina
AU: Naranjo, J
EM: jnaranjo@sernageomin.cl
AF: Serv. Nacional Geol. y Mineria, Casilla, Santiago, 10465, Chile
AB: Three major Holocene explosive eruptions of Hudson volcano in southern Chile (1991, 3.6ka BP, and 6.7ka BP) have produced a range of magma compositions from trachybasalt to trachydacite. The 1991 event generated both explosive and effusive eruptions of trachybasalt (phase 1, <1km³ DRE) and trachyandesite (phase 2, ~3km³ DRE). More evolved, compositionally homogeneous trachydacite was erupted during the 3.6ka BP event (~4km³). The 6.7ka BP eruption discharged trachyandesite, with some compositional diversity towards trachydacite, but generally less evolved than the 3.6ka BP magma. These Holocene events have consistently produced a mineral assemblage including plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides, and trace apatite ± olivine, with total crystallinities varying from ~10-40%. Geochemical modeling suggests that the dominant trachyandesite erupted in 1991 and 6.7ka BP can be produced by fractional crystallization of a parental basaltic magma similar in composition to that produced during 1991 phase 1 eruption. Similarly, the more evolved trachydacite can be derived by fractional crystallization of the trachyandesitic composition. However, variations in magma compositions between and within the major plinian eruptions require a combination of fractional crystallization and magma mixing that is likely to take place in a relatively shallow magma storage region. Evidence for magma mixing includes coexisting glasses in single samples, complex and oscillatory zoning in plagioclase phenocrysts, and more evolved melt inclusions relative to co- existing matrix glasses. Melt inclusion volatile contents from the three eruptions, as estimated by the difference method, range from <1 to 5 wt%, and when combined with comparisons of experimental phase petrology indicate pre-eruptive storage at 1200-1800 bars pressure and temperature ranges of ~940 - 972°C (log fO₂ -10.68 to -10.24). Geochemical and petrologic evidence suggests that a trachyandesite magma storage region is periodically recharged with more mafic magmas. This magma mixing is a likely cause for triggering of explosive eruptions at Hudson, as shown in particular by the 1991 eruption, and can lead to the simultaneous eruption of contrasting magmas at separate vents.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 8400 VOLCANOLOGY
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting