Crystal reaming during the assembly maturation a

Authors: Barry A Walker Erik W Klemetti Anita L Grunder John H Dilles Frank J Tepley Denise Giles

Publish Date: 2012/11/29

Volume: 165, Issue: 4, Pages: 663-682

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Abstract

Phenocryst assemblages of lavas from the longlived Aucanquilcha Volcanic Cluster AVC have been probed to assess pressure and temperature conditions of preeruptive arc magmas Andesite to dacite lavas of the AVC erupted throughout an 11millionyear arc magmatic cycle in the central Andes in northern Chile Phases targeted for thermobarometry include amphibole plagioclase pyroxenes and Fe–Ti oxides Overall crystallization is documented over 1–75 kbar ~25 km of pressure and ~680–1110 °C of temperature Pressure estimates range from ~1 to 5 kbar for amphiboles and from ~3 to 75 kbar for pyroxenes Pyroxene temperatures are tightly clustered from ~1000–1100 °C Fe–Ti oxide temperatures range from ~750–1000 °C and amphibole temperatures range from ~780–1050 °C Although slightly higher these temperatures correspond well with previously published zircon temperatures ranging from ~670–900 °C Two different Fe–Ti oxide thermometers Andersen and Lindsley 1985 Ghiorso and Evans 2008 are compared and agree well We also compare amphibole and amphibole–plagioclase thermobarometers Ridolfi et al 2010 Holland and Blundy 1994 Anderson and Smith 1995 the solutions from which do not agree well In samples where we employ multiple thermometers pyroxene temperature estimates are always highest zircon temperature estimates are lowest and Fe–Ti oxide and amphibole temperature estimates fall in between Maximum Fe–Ti oxide and zircon temperatures are observed during the middle stage of AVC activity ~5–3 Ma a time associated with increased eruption rates Amphibole temperatures during this time are relatively restricted ~850–1000 °C The crystal record presented here offers a timetransgressive view of an evolving multitiered subvolcanic reservoir Some crystals in AVC lavas are likely to be true phenocrysts but the diversity of crystallization temperatures and pressures recorded by phases in individual AVC lavas suggests erupting magma extensively reams and accumulates crystals from disparate levels of the middle to upper crustFunding for this project was provided by the National Science Foundation grants EAR9814941 to A Grunder EAR0610114 to A Grunder and F Tepley Alissa Morson collected about half of the Fe–Ti oxide data and was funded by an REU grant through the NSF OCE1004947 Thanks to “Big” Dale Burns for assistance in the electron probe laboratory at OSU We thank Keith Putirka Adam Kent George Bergantz and the VIPER group Volcanology Igneous Petrology and Economic Research at OSU for helpful discussions This paper benefited immensely from comments by Mike Krawczynski and an anonymous reviewer

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