HR: 1330h
AN: V32C-1043    [PDF]
TI: Argon isotopes as recorders of magmatic processes
AU: * Layer, P W
EM: player@gi.alaska.edu
AF: Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775 United States
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712 United States
AU: Mora Chaparro, J C
EM: jcmora@geofisica.unam.mx
AF: Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Mexico City, DF 04510 Mexico
AU: Arce, J L
EM: arcejl@tonatiuh.igeofcu.unam.mx
AF: Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Mexico City, DF 04510 Mexico
AB: Argon isotopic ratios vary enough between different reservoirs (atmosphere, crust, mantle) and diffuse fast enough through most minerals at magmatic temperatures (700-1200 C) to make them ideal for looking at magma chamber dynamics. Indeed, diffusion is sufficiently fast to allow short time scales to be deciphered, setting argon apart from many other isotopic methods. A mineral's ability to retain "excess" argon (40Ar/36Ar ratios greater than the atmospheric value and apparent ages older than the known eruption age) during post-eruption cooling is key to Ar studies. Previous work shows that both phenocrysts (crystallizing in the magma chamber; e.g. Mt St. Helens; Layer and Gardner, 2001) and xenocrysts (introduced into the magma chamber; e.g Toba; Gardner et al., 2002) preserve excess argon, which enables magma chamber processes to be deciphered through the variable diffusion rates between crystal phases. Single crystal 40Ar/39Ar step-heating of biotite from the 10.5 ka eruption of Nevado de Toluca volcano, Mexico indicates that they are xenocrystic and resided for only a short ($<$ 1 year) time in the magma before it erupted. The biotite has reaction rims of hornblende, orthopyroxene and plagioclase, and failed to grow experimentally at pressure-temperature conditions of the magma, confirming the xenocrystic nature of this phase. Single-step fusion of plagioclase phenocrysts from eruptions of El Chichon volcano, Mexico, shows evidence of excess (mantle) argon, whereas hornblende from the same eruptions contains little or none. In this case, faster diffusion of Ar in plagioclase than in hornblende allow plagioclase to incorporate excess argon during magma recharge; hornblende does not. Combining such results with other isotopic systems may in fact better determine magma chamber processes. At El Chichon, Sr isotopes suggest magma recharges ocurred (Tepley et al., 2000), whereas the argon isotopes suggest such pulses occurred just before each eruption. The fast and variable diffusion of argon thus allows it to record events that other systems may miss or smear out with time. Gardner, J.E., P.W. Layer and M. Rutherford, 2002, Geology, 30, p. 347-350. Layer, P.W., and J.E. Gardner, 2001, Geophys. Res. Lett., 28, 4279-4282. Tepley, F. J., III, J.P. Davidson, R.I. Tilling, J.G. Arth, 2000, J. Petrol., 41, 1397-1411.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting