Role of cryptic amphibole crystallization in magma differentiation at Hudson volcano, Southern Volcanic Zone, Chile |
| |
Authors: | David J Kratzmann Steven Carey Roberto A Scasso Jose-Antonio Naranjo |
| |
Institution: | 1. Graduate School of Oceanography, URI, S. Ferry Rd., Narragansett, RI, 02882, USA 2. Dpto. de Cs. Geológicas, FCEN, Univ. de Buenos Aires Cuidad Univ., Pab 2, 1 Piso, 1428, Buenos Aires, Argentina 3. Serv. Nacional Geol. y Minería, Casilla, Santiago, 10465, Chile
|
| |
Abstract: | Hudson volcano (Chile) is the southern most stratovolcano of the Andean Southern Volcanic Zone and has produced some of the
largest Holocene eruptions in South America. There have been at least 12 recorded Holocene explosive events at Hudson, with
the 6700 years BP, 3600 years BP, and 1991 eruptions the largest of these. Hudson volcano has consistently discharged magmas
of similar trachyandesitic and trachydacitic composition, with comparable anhydrous phenocryst assemblages, and pre-eruptive
temperatures and oxygen fugacities. Pre-eruptive storage conditions for the three largest Holocene events have been estimated
using mineral geothermometry, melt inclusion volatile contents, and comparisons to analogous high pressure experiments. Throughout
the Holocene, storage of the trachyandesitic magmas occurred at depths between 0.2 and 2.7 km at approximately ~972°C (±25)
and log fO2 −10.33–10.24 (±0.2) (one log unit above the NNO buffer), with between 1 and 3 wt% H2O in the melt. Pre-eruptive storage of the trachydacitic magma occurred between 1.1 and 2.0 km, at ~942°C (±26) and log fO2 −10.68 (±0.2), with ~2.5 wt% H2O in the melt. The evolved trachyandesitic and trachydacitic magmas can be derived from a basaltic parent primarily via fractional
crystallization. Entrapment pressures estimated from plagioclase-hosted melt inclusions suggest relatively shallow levels
of crystallization. However, trace element data (e.g., Dy/Yb ratio trends) suggests amphibole played an important role in
the differentiation of the Hudson magmas, and this fractionation is likely to have occurred at depths >6 km. The absence of
a garnet signal in the Hudson trace element data, the potential staging point for differentiation of parental mafic magmas
i.e., ~20 km (e.g., Annen et al. in J Petrol 47(3):505–539, 2006)], and the inferred amphibolite facies ~24 km (e.g., Rudnick and Fountain in Rev Geophys 33:267–309, 1995)] combine to place some constraint on the lower limit of depth of differentiation (i.e., ~20–24 km). These constraints suggest
that differentiation of mantle-derived magmas occurred at upper-mid to lower crustal levels and involved a hydrous mineral
assemblage that included amphibole, and generated a basaltic to basaltic andesitic composition similar to the magma discharged
during the first phase of the 1991 eruption. Continued fractionation at this depth resulted in the formation of the trachyandesitic
and trachydacitic compositions. These more evolved magmas ascended and stalled in the shallow crust, as suggested by the pressures
of entrapment obtained from the melt inclusions. The decrease in pressure that accompanied ascent, combined with the potential
heating of the magma body through decompression-induced crystallization would cause the magma to cross out of the amphibole
stability field. Further shallow crystallization involved an anhydrous mineral assemblage and may explain the lack of phenocrystic
amphibole in the Hudson suite. |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|