HR: 0830h
AN: V31C-0941    [PDF]
TI: Magmatic processes at Popocatepetl volcano, Mexico: petrology, geochemistry and Sr-Nd-Pb isotopes
AU: * Schaaf, P
EM: pschaaf@geofisica.unam.mx
AF: Instituto de Geof¡sica, UNAM, Ciudad Universitaria, Mexico-City, DF 04510 Mexico
AU: Stimac, J
EM: jstimac@unocal.com
AF: Philippine Geothermal Inc., 8741 Paseo de Roxas, Citibank Tower, Makati-City, xx Philippines
AU: Siebe, C
EM: csiebe@geofisica.unam.mx
AF: Instituto de Geof¡sica, UNAM, Ciudad Universitaria, Mexico-City, DF 04510 Mexico
AU: Mac¡as, J
EM: macias@geofisica.unam.mx
AF: Instituto de Geof¡sica, UNAM, Ciudad Universitaria, Mexico-City, DF 04510 Mexico
AB: Popocatepetl volcano is one of the most famous and most active stratovolcanoes of the Trans-Mexican Volcanic Belt (TMVB). It is located 60 km south-east of Mexico-City and 40 km west of the city of Puebla, both cities have more than 30 million inhabitants. In this contribution we present a study of Late Pleistocene to Recent products of Popocat‚petl (Popo) volcano and surrounding scoria cones to better establish their genetic relationship and magmatic history. Popo and flanking vents are located within the central portion of the Trans Mexican Volcanic Belt, which is related to oblique subduction of young oceanic lithosphere. Current activity of Popo can be understood in the context of its past eruptions and those from surrounding scoria cones. The latest cycle of eruption began Dec. 21, 1994 with continuous to pulsating emission of phreatic ash. The last important event happened on July 19, 2003, covering Mexico-City with a thin ash-layer. Both Popo and surrounding scoria cones produced moderate-K, calc-alkaline rocks, with the two groups differing mainly in degree of differentiation, water content, and oxidation state. Some vent samples on the immediate flanks of Popo and have phenocryst assemblages and compositions transitional between typical flanking vent and stratovolcano samples. Monogenetic vents produced mainly basaltic andesites to andesites, primarily by crystal fractionation of Ol (Fo80-90)+chromite, 2PyxñOl, and 2PyxñPlagñHb assemblages, with minor assimilation of crustal debris. The andesitic to dacitic rocks of Popo are dominated by Plag-2Pyx-2OxideñHbl assemblages, with variable amounts of Ol (Fo70-90)+chromite xenocrysts. A few Popo samples contain locally abundant xenolithic debris of cognate-granitoid intrusions and their metasedimentary wallrocks. The two suites share parental Mg-rich basaltic andesite magmas, with the Popo magmas reflecting longer residence in the crust, and enhanced hydration and oxidation due to the resulting processes of crystallization, recycling, assimilation, and degassing in relatively evolved magma chambers. The 1996 and 1997 dome eruptions confirm that dacitic magma currently resides beneath Popo and is episodically recharged by more mafic magma, fostering eruption and excess degassing. Two-oxide thermometry and the presence of FeCu sulfide globules confirm that these magmas erupted at T = 930§C and fO2 = -10.2 log, below anhydrite stability. 87Sr/86Sr, e-Nd and 206Pb/204Pb ratios are between 0.70365 and 0.70463, +6.4 and +3.0, and 18.618 and 18.781, respectively. Andesitic to dacitic rocks of Popo formed by mafic recharge, fractionation, and mixing of dacitic to basaltic magmas in mature crustal chambers. Plagioclase accumulation and recycling related to protracted fractionation and assimilation of earlier emplaced magmas and their wallrocks was also important.
DE: 1020 Composition of the crust
DE: 1040 Isotopic composition/chemistry
DE: 3640 Igneous petrology
DE: 8105 Continental margins and sedimentary basins
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting