HR: 0800h
AN: V31B-0492    [Abstracts]
TI: Long term storage of explosively erupted magma at Nevado de Toluca volcano, Mexico
AU: * Arce, J L
EM: jlarce@geologia.unam.mx
AF: Instituto de Geologia, UNAM, Cd. universitaria, Coyoacan, Mexico, DF 04510, Mexico
AU: Gardner, J
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences Jackson School of Geosciences The University of Texas, 1 University Station C1100, Austin, TX 78712-0254, United States
AU: Macias, J L
EM: macias@geofisica.unam.mx
AF: Instituto de Geofisica, UNAM, Cd. universitaria, Coyoacan, Mexico, DF 04510, Mexico
AB: Dacitic magmas production is common in subduction-related volcanoes, occurring in those with a long period of activity as a result of the magmatic evolution. However, in this evolution many factors (i.e. crystal fractionation, assimilation, magma mixing) can interact to produce dacites. Nevado de Toluca volcano (4,680 masl; 19°09'N; 99°45'W) Central Mexico has recorded a long period of time producing dacites explosively, at least during 42 ka of activity, involving several km³ of magma, with two important Plinian-type eruptions occurred at ~21.7 ka (Lower Toluca Pumice) and ~10.5 ka (Upper Toluca Pumice). Questions like, what was the mechanism responsible to produce voluminous dacitic magma and how the volatiles and pressure changed in the Nevado de Toluca system, remain without answers. Dacites from the Lower Toluca Pumice (LTP) contain plagioclase, amphibole, iron-titanium oxides, and minor resorbed biotite, set in a glassy-vesicular matrix and the Upper Toluca Pumice (UTP) dacites contain the same mineral phases plus orthopyroxene. Ilmenite- ulvospinel geothermometry yielded a temperature of ~860°C for the LTP dacite, a little hotter than the UTP (~ 840°C). Based on hydrothermal experiments data, amphibole is stable above 100 MPa under 900°C, while plagioclase crystallizes up to 250-100 MPa at temperatures of 850-900°C. Pyroxene occurs only at pressures of 200-100 MPa with its respective temperatures of 825-900°C. Water contents in the LTP magma (2-3.5 wt %) are similar to that calculated for the UTP magma (1.3-3.6 wt %). So, there are only small changes in temperature and pressure from ~21.7 ka to 10.5 ka. It is noteworthy that orthopyroxene is absent in the LTP, however reaction-rimmed biotite (probably xenocrystic) is commonly observed in all dacites. Hence, almost all dacitic magmas seem to be stored at relatively similar pressures, water contents, and temperatures. All of these data could suggest repetitive basic magma injections producing the melting of biotite- bearing silicic country rocks, through the 42 ka deposits of Nevado de Toluca volcano
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting