HR: 11:50h
AN: V11F-07    [PDF]
TI: Feldspar Zonation in Andesites from Monogenetic Cones and Long-lived Stratovolcanos (Andagua and El Misti, S. Peru) : Constraints for Eruption Triggers
AU: * Ruprecht, P
EM: pruprec@gwdg.de
AF: GZG, Abt. Geochemie, Goldschmidtstr. 1, Goettingen, 37077 Germany
AU: Woerner, G
EM: gwoerner@gwdg.de
AF: GZG, Abt. Geochemie, Goldschmidtstr. 1, Goettingen, 37077 Germany
AU: Martin, A
EM: nyira-audrey.martin@caramail.com
AF: Laboratoire Volcan et Magmas, Universit‚ Blaise-Pascal,, 5 rue Kessler, Clermont-Fer, 63038 France
AU: Kronz, A
EM: akronz@gwdg.de
AF: GZG, Abt. Geochemie, Goldschmidtstr. 1, Goettingen, 37077 Germany
AB: Recharge, mixing, and degassing are generally thought to be the major triggers for volcanic eruptions. Mixing can involve (1) newly injected magma, (2) mixing of magmas coexisting in a (zoned) magma chamber) or (3) mixing of magmas of similar composition but different temperatures, e.g. from thermal plumes originating at the interface to mafic magma underplating. We use high-resolution imaging of plagioclase zonation and quantitative analysis by electron microprobe to investigate these scenarios for andesites from the Central Andes of S. Peru. El Misti stratovolcano and the monogenetic Andagua centers are well-suited because they are similar in terms of (1) lava major element compositions (SiO2 from 57 - 65 wt. per cent (El Misti) 56 - 67 wt. per cent (Andagua) and (2) size (100 to 500 microns). An vs. Fe-systematics on feldspar zonation have been applied on lavas from the El Misti stratovolcano and from the Andagua monogenetic field (S. Peru). They allow distinction between these triggering processes: Large and correlated An-Fe contrasts indicate chemical mixing related to recharge, mixing and complete overturn. By contrast, repeated and correlated An-Fe at low compositional contrast may indicate intra-chamber mixing. Thermal mixing leads to variable An content at constant Fe. Despite the similarities in lava compositions, zonation patterns in plagioclase differ strongly between El Misti and Andagua lavas: While Andagua lavas show largely homogeneous Ab-rich cores sharply overgrown only by a 10-30 microns thick An-rich rim, El Misti lavas comprise complexly zoned plagioclases with multiple events of mixing. Growth rate estimates for the largest crystals suggest less than 100 years of growth even for very slow rates. Clearly, many mixing events occurred without ensuing eruption, because the average time in between eruptions at El Misti is much longer than 100 yr. Andagua plagioclase zonations indicate that once the mafic second batch intruded the reservoir, an eruption was always and immediately triggered. This is in agreement with the relative volume of the magma systems. While Andagua eruptions are fed from small volume reservoirs that are filled and erupted only once. The Andagua reservoirs apparently cannot accommodate a second batch and the system reacts against the additional pressure with an eruption. By contrast, a large magma chamber in the upper crust feeds El Misti volcano. El Misti can easily accommodate a new batch, the large system is buffered. These relations are clearly imaged in the feldspar zonation patterns. Hence, recharge and mixing will trigger eruptions especially in small systems, large systems record many mixing events and may need an additional process as an eruption trigger.
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting