HR: 0800h
AN: V31C-0630    [Abstracts]
TI: Correlated Oxygen Fugacity and Slab-Derived Fluid Flux Linked to Oceanic Fracture Zone Subduction at Nevado de Longavi Volcano, Southern Andes.
AU: * Selles, D
EM: Daniel.Selles@terre.unige.ch
AF: Mineralogy Department, University of Geneva, 13 Rue des Maraichers, Geneva, 1205 Switzerland
AU: Rodriguez, C
EM: Carolina.Rodriguez@terre.unige.ch
AF: Mineralogy Department, University of Geneva, 13 Rue des Maraichers, Geneva, 1205 Switzerland
AU: Dungan, M A
EM: Michael.Dungan@terre.unige.ch
AF: Mineralogy Department, University of Geneva, 13 Rue des Maraichers, Geneva, 1205 Switzerland
AU: Leeman, W
EM:
AF: Dept. of Earth Science, Rice University, 6100 Main St., Houston, TX 77005 United States
AB: Nevado de Longaví volcano (NLV, 36.2°S, Andean Southern Volcanic Zone) is located above the projection of the oceanic Mocha fracture zone (MFZ), which migrates southward at a rate of ~30 km/My. Over 1.5 My NLV magmas became progressively wetter and more oxidized. Oxygen fugacity increases from relatively reducing conditions (NNO-1) in the earliest lavas to NNO+2 in the Holocene. Whole-rock fluid tracers, mineralogy, and mineral chemistry record increases in slab-derived fluids correlate with this +3 log-unit increase in oxygen fugacity. We infer that these changes are due to the arrival of the MFZ, which supplied the mantle wedge with an increasing flux of oxidizing fluids from dehydration of transform-hosted serpentinite. Holocene dacitic magmas from this volcano are distinct from other comparably evolved Quaternary magmas of the region (33-41°S) by virtue of lower enrichments in incompatible elements because they have evolved mainly through crystal fractionation dominated by amphibole and calcic plagioclase at high water pressures. High proportions of slab-derived fluids in these magmas are inferred on the basis of high B concentrations and high B/Th and Ba/Zr. The earliest eruptive products contain anhydrous mineral assemblages and have lower B and B/Th. Melting degrees in the mantle source have also increased following enhanced fluid-flux, as inferred from fluid-immobile incompatible element abundances and ratios in mafic magmas. Silicate mineral chemistry also records these changes. Plagioclase phenocrysts from wet and oxidized magmas have low Fe and Mg contents relative to those from dry and reduced units, consistent with the late appearance of this phase in water-rich systems. Fe/Mg ratios in plagioclase correlate with Δ-NNO resulting from increased replacement of Fe³⁺ for Al. Calculated Fe³⁺ in amphibole increases with decreasing age at the expense of Ti⁴⁺ and A-site alkalis, leading to increasing Mg#'s in amphiboles crystallizing from residual rhyolitic liquids in dacites.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3651 Thermobarometry
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005