Volcanological and geochemical evolution and hazard assessment of the Diamante Caldera-Maipo Volcano Complex (34°10'S, 69º50'W)

El Complejo Caldera Diamante-Volcán Maipo (34° 10´ S, 69º 50´ O) se halla situado en los Andes argentino-chilenos, en el extremo norte de la Zona Volcánica Sur, donde el espesor de la corteza es de ~ 55 km. Constituye un sistema magmático complejo cuya actividad eruptiva se remonta a 450/150 ka y cuyo registro histórico es incierto y controvertido. En el transcurso de su evolución se distinguen dos etapas: 1) Diamante, en la cual tiene lugar la formación de una caldera de colapso (20x16 km de diámetro) y el emplazamiento de ignimbritas de gran volumen y 2) Maipo, durante la cual ocurre la construcción de un estratovolcán, con domo anular y conos adventicios asociados y con variedad de productos lávicos y piroclásticos de escaso volumen. Esta etapa abarca los últimos 100 ka y comprende siete eventos eruptivos principales. Los primeros cuatro han sido datados por Ar/Ar sobre roca total y corresponden al registro de actividad preglacial (86±10 ka/88±7 ka, 75±16 ka, 45±14 ka y 28±17 ka), mientras que los últimos tres eventos han sido asignados a la etapa postglacial (<14 ka) en base a observaciones de campo. Actualmente, los dos cráteres cuspidales se hallan cubiertos de hielo y no hay evidencias de actividad fumarólica o hidrotermal. Los estudios realizados han permitido reconocer flujos de escoria,aglomerados localmente soldados y depósitos de caída de tefra (ceniza y lapilli) como registro de actividad explosiva discreta, ocurrida en tiempos recientes, posiblemente históricos. Tentativamente, la última erupción ocurrió en 1912. Las volcanitas del Complejo Caldera Diamante-Volcán Maipo definen una serie calco-alcalina de alto K, con un rango en el contenido de SiO2 de 54% a 74%. La serie Maipo es continua y está integrada por andesitas basálticas, andesitas y dacitas con plagioclasa, piroxenos, olivina, hornblenda, biotita, sanidina y minerales opacos. Como rasgo característico, los fenocristales presentan texturas de desequilibrio (cribada periférica y/o central y zonación oscilatoria en plagioclasa, redondeamiento en olivina, reabsorción y coronas de reacción en minerales máficos). Se verifican ajustadas correlaciones positivas de K2O, Ba, Rb, La, y negativas de FeO, CaO, MgO, TiO2,. Sr, con el contenido de SiO2. El comportamiento de los elementos compatibles indica un fuerte control por parte de la cristalización fraccionada de la paragénesis mineral reconocida como principal mecanismo de diferenciación de la serie. Además, las texturas de desequilibrio y las periódicas oscilaciones composicionales indican que la mezcla de magmas debe haber tenido una influencia significativa en la evolución magmática del sistema. Las altas relaciones Sr 87/Sr 86 sugieren que la asimilación cortical también tuvo un rol importante. Los cálculos de P y T° indican que los magmas andesíticos se han equilibrado en un rango de profundidad de ~12-22 km, mientras que las dacitas adquirieron esa condición a niveles más someros (~4-15 km). Se discuten dos escenarios posibles ante una eventual reactivación del Complejo Caldera Diamante-Volcán Maipo.The Caldera Diamante-Maipo volcanic complex (34°10' S, 69º50' W) is located at the northern end of the South Volcanic Zone. The eruptive activity started 450/150 ka ago and its historic record remains uncertain. At present, neither fumarolic activity nor hydrothermal manifestations are detected. Two main stages are distinguished in the evolution of the volcanic complex: 1) "Diamante stage" corresponds to the emplacement of large-volume ignimbrites associated to a 20 by 16 km in diameter collapse caldera and 2) the "Maipo stage" represents andesite-dacite stratocone-building lavas and pyroclastics, a ring-fault dome and parasitic cones emplaced during the last 100 ka of the complex lifetime (4 pre-glacial events: 86 ± 10 ka / 88 ± 7 ka, 75 ± 16 ka, 45 ± 14 ka, 28 ± 17 ka and 3 post-glacial events <14 ka). Scoria flows and fall deposits near the summit are assigned to the recent explosive record. The last eruption tentatively occurred in 1912. The volcanics define a high-K, calc-alkaline suite ranging in silica from 54% to 74%. The Maipo series encompasses two pyroxene with minor olivine andesites and two pyroxene and hornblende dacites. Magmatic differentiation is strongly controlled by fractional crystallization. However, periodic magma mixing and crustal assimilation should have been significant in producing cyclic chemical variations. P-T° calculations indicate that andesitic and dacitic magmas have equilibrated at a depth of ~12-22 km and ~4-15 km, respectively. On the case of an eventual reactivation of the volcanic complex, two possible scenarios are discussed.Fil: Sruoga, Patricia. Secretaría de Industria y Minería. Servicio Geologico Minero Argentino; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Etcheverría, Mariela P.. Secretaría de Industria y Minería. Servicio Geologico Minero Argentino; ArgentinaFil: Feineman, Maureen. State University Of Pennsylvania; Estados UnidosFil: Rosas, Mario. Secretaría de Industria y Minería. Servicio Geologico Minero Argentino; ArgentinaFil: Bukert, Cosima. Geomar-helmholtz Centre For Ocean Research Kiel; AlemaniaFil: Ibañes, Oscar Damián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Luis; Argentin

Insights into crustal assimilation by Icelandic basalts from boron isotopes in melt inclusions from the 1783–1784 Lakagígar eruption

Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 94 (2012): 164-180, doi:10.1016/j.gca.2012.07.002.The boron isotope system has great potential for tracing alteration and assimilation in basaltic systems due to the very low concentrations of B in mantle-derived melts and the strong isotopic contrast between the mantle and surface materials. However, variability in B concentrations and isotope ratios in basalts can also be interpreted to reflect inputs from enriched regions of the mantle, as the extent of mantle heterogeneity with respect to boron remains poorly delineated. We have determined boron concentrations and isotope ratios in fresh, glassy, plagioclase-hosted melt inclusions and unaltered scoriaceous matrix glasses from four localities associated with the 1783-1784 Lakagígar (Laki Fissure) eruption, Iceland. Boron concentrations range from 0.59-1.25 ppm in the melt inclusions, and from 1.25-1.65 ppm in the matrix glasses, while δ11BNBS-951 ranges from - 7.8‰ to -16.5‰ in the melt inclusions and -10.5‰ to -16.9‰ in the matrix glasses. In contrast to previous studies of boron in basaltic melt inclusions from other fissure swarms in Iceland (Gurenko and Chaussidon, 1997, Chem. Geol. 135, 21-34), the Lakagígar melt inclusions display a significant range of boron concentrations and isotope ratios at constant K2O wt.%, which is more consistent with B addition by assimilation of altered basalt than it is with mixing between depleted and enriched mantle sources. Assimilation of freshwater-altered crustal materials at depth may impart a light δ11B signature such as that observed in the Lakagígar melt inclusions and tephra host glasses. Considering boron concentrations and isotope ratios in the Lakagígar glasses and previously studied altered Icelandic basalts, together with volatile equilibration depths of the Lakagígar melt inclusions, we propose that a) mantle-derived magmas formed beneath Lakagígar assimilated ~5-20% altered crust at a depth of ~3-4 km or more, probably during magma accumulation in sills formed at the boundaries of low-density hyaloclastite layers; and b) the magma subsequently underwent extensive mixing and homogenization prior to eruption, quite possibly within the magma chamber beneath the Grímsvötn central volcano, assimilating an additional ~10% of altered crust at that time, for a total of up to 30% crustal assimilation. We hypothesize that volatiles including H2O, CO2, S, F, and Cl, which were responsible for the majority of the considerable casualties attributed to the Lakagígar eruption, were added together with isotopically light B by assimilation of hydrothermally altered crustal materials.This work was supported by a PSU-EMS Miller Award to MDF, NSF 761 EAR 09117456 to PCL, and awards to MNB by the GSA Northeast Division Grant for Undergraduate Research, the Dept. of Geosciences Undergraduate Senior Thesis Grant, and PSU Undergraduate Research Discovery Grant

ExTerra at AGU 2011: Understanding Convergent Margin Processes Through Studies of Exhumed Terranes

Workshop held on December 7, 2011 in San Francisco, C