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Cryptic-Magma-Recharge-Associated UNIVERSIDAD DE CHILE FACULTAD DE CIENCIAS FÍSICAS Y MATEMÁTICAS DEPARTAMENTO DE GEOLOGÍA CRYPTIC MAGMA RECHARGE ASSOCIATED WITH 20TH CENTURY ERUPTIONS AT VILLARRICA VOLCANO TESIS PARA OPTAR AL GRADO DE MAGÍSTER EN CIENCIAS, MENCIÓN GEOLOGÍA MEMORIA PARA OPTAR AL TÍTULO DE GEÓLOGO CHRISTIAN JAVIER PIZARRO ARAYA PROFESOR GUÍA: MIGUEL ÁNGEL PARADA REYES MIEMBROS DE LA COMISIÓN: ÁNGELO CASTRUCCIO ÁLVAREZ CLAUDIA CANNATELLI SANTIAGO DE CHILE 2018 RESUMEN DE LA MEMORIA PARA OPTAR AL TÍTULO DE: Geólogo y grado De Magíster en Ciencias, mención Geología POR: Christian Javier Pizarro Araya FECHA: 28/09/2018 PROFESOR GUÍA: Miguel Ángel Parada CRYPTIC MAGMA RECHARGE ASSOCIATED WITH 20TH CENTURY ERUPTIONS AT VILLARRICA VOLCANO El Volcán Villarrica es uno de los más activos de los Andes (ciclos de reposo de ~5.3 años), eruptando productos de composiciones basálticas a andesíticas basálticas, con estilos desde hawaiiano a estromboliano vigoroso. Estas características lo hacen un lugar destacado para determinar los parámetros pre-eruptivos y los procesos involucrados en sus erupciones. En esta tesis se compara la geoquímica de roca total y las abundancias, texturas y composiciones minerales de las lavas más relevantes del siglo XX, que fueron eruptadas en 1921, 1948 y 1971. Las lavas analizadas muestran un rango restringido en las composiciones de roca total (51.7-52.4 wt. % SiO2), sin embargo, presentan la siguiente tendencia creciente del contenido de MgO: ~5.3 wt % MgO (1921), ~6.0 wt % MgO (1971) and ~7.0 wt. % MgO (1948). Dos grupos de composiciones de plagioclasas se observan en todas las lavas: pobres en anortita (~An60) y ricas en anortita (~An80). Las composiciones pobres en anortita son las más abundantes en todas las lavas y fueron comúnmente formadas en las etapas tempranas (núcleos) y en las últimas etapas (bordes externos y fenocristales pequeños y casi no zonados) de cristalización de plagioclasa, mientras que las plagioclasas ricas en anortita se formaron alrededor de núcleos pobres en anortita o como cristales aislados durante etapas intermedias. Las temperaturas de equilibrio de plagioclasa-olivino-clinopiroxeno son de aproximadamente 1090 °C y fueron obtenidas en clots de cristales de las tres lavas. Los cálculos termométricos obtenidos con simulaciones de MELTS de formación de plagioclasa indican un calentamiento (thermal mixing) del reservorio a ~0.5 kbar de unos 100 °C con respecto a las temperaturas de los clots cristalinos. La incorporación de pequeñas cantidades de magma máfico más rico en volátiles y más caliente, de composición similar (cryptic mixing), en el reservorio del Villarrica, podría también calentarlo, probablemente, de una manera más eficiente que con la sola conducción de calor. Una interacción más larga del magma máfico caliente con el reservorio podría provocar cristalización de plagioclasa rica en anortita, disolución de plagioclasa pobre en anortita (es decir, un aumento de la razón modal de rica en An/pobre en An) y una modificación composicional hacia composiciones más ricas en MgO y volátiles, que últimamente resultarían en una mayor intensidad de la erupción. I Si me preguntas por qué, te diría que es por la belleza. Pero entiéndeme bien, es la belleza en sus distintos niveles. Uno es evidente. La reacción fisiológica provocada por el objeto de estudio es inevitable. Y yo soy adicto al asombro. Otro no lo es tanto. Yace en la estética del ejercicio argumentativo. En un plano similar al del montaje cinematográfico. También hay un tercer nivel, más críptico y profundo. Más permanente y enriquecedor. Ese que, como una maldición, se impregna en la médula del sujeto y lo transforma. II AGRADECIMIENTOS Al proyecto CONICYT-FONDAP 15090013, Centro de Excelencia en Geotermia de los Andes (CEGA) y al Departamento de Geología de la Universidad de Chile por financiar mis estudios de postgrado. Al Departamento de Postgrado y Postítulo de la Vicerrectoría de Asuntos Académicos por su ayuda para una estadía de investigación en School of Earth Sciences de University of Bristol, en el Reino Unido. Al profesor Miguel Ángel Parada por su inspiración, guía y apoyo. A los profesores Ángelo Castruccio y Claudia Cannatelli por su buena disposición y entusiasmo. A las profesoras Katharine Cashman y Alison Rust por recibirme en School of Earth Sciences de University of Bristol durante una estadía, por sus consejos e interés. A Stuart Kearns y Benjamin Buse por su ayuda con el uso de la microsonda en University of Bristol. A Joe Pickles por su ayuda con la microsonda en University of Exeter. A Claudio Contreras y Eduardo Morgado, coautores del manuscrito producto de esta investigación, quienes me guiaron y con quienes siempre pude contar. A mis compañeros de postgrado que directa o indirectamente colaboraron en esta tesis. A mi amada familia, mis queridos amigos y amigas, y las valiosas personas que conocí tanto en pregrado como en postgrado. III TABLA DE CONTENIDO Capítulo 1 : Introducción ............................................................................................................ 1 1.1. Estructura de la tesis .................................................................................................. 1 1.2. Objeto de estudio y motivación ................................................................................. 1 1.3. Objetivos ......................................................................................................................... 8 1.3.1. Objetivo general ...................................................................................................... 8 1.3.2. Objetivos específicos ............................................................................................... 8 1.3.3. Hipótesis .................................................................................................................. 8 1.4. Metodología .................................................................................................................... 9 Capítulo 2 : Resultados ............................................................................................................. 13 2.1. Geoquímica de roca total .............................................................................................. 13 2.2. Petrografía .................................................................................................................... 19 2.2.1. Composiciones y texturas de plagioclasa .............................................................. 23 2.2.2. Composiciones y texturas de minerales máficos ................................................... 27 2.3. Termobarometría .......................................................................................................... 29 2.4. Simulación termodinámica ........................................................................................... 31 Capítulo 3 Cryptic magma recharge associated with 20th century eruptions at Villarrica Volcano .......................................................................................................................................... 35 Abstract ................................................................................................................................ 36 IV Keywords ............................................................................................................................. 37 3.1. Introduction .................................................................................................................. 37 3.2. The studied lavas .......................................................................................................... 39 3.3. Analytical techniques and methodology....................................................................... 42 3.4. Whole-rock compositions ............................................................................................. 43 3.5. Petrography and Mineral chemistry ............................................................................. 46 3.5.1. Plagioclase compositions and textures .................................................................. 49 3.5.2. Mafic mineral compositions and textures .............................................................. 54 3.6. Thermometry and barometry ........................................................................................ 54 3.7. Discussion ..................................................................................................................... 59 3.7.1. Interpretation of plagioclase compositions and textures ........................................... 59 3.7.2. Magma recharge of the Villarrica reservoir .............................................................. 63 3.7.3. Final comments ......................................................................................................... 64 3.8. Conclusions .................................................................................................................. 65 Acknowledgments ............................................................................................................... 65 References ........................................................................................................................... 66 Appendix 1. Supplementary data......................................................................................... 73 Capítulo 4 : Conclusiones ....................................................................................................... 115 Bibliografía ............................................................................................................................
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