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The magmatic-hydrothermal system of Laguna del Maule

Hannah Ellis1, Alison Rust1

Abstract

The 17ka ignimbrite eruption of Laguna del Maule (LDM), Chile was the first and largest postglacial eruption of the LDM complex, resetting the magma system, erupting mafic juveniles, and bringing granitic lithics to the surface from depth. Previous geophysical studies into the LDM magmatic system reveal magnetotelluric anomalies of >1 S m−1 at depths of 3-5km, consistent with a saline fluid stored in porous rock. We propose that the erupted granites were brought to the surface rapidly from depths consistent with this MT anomaly, and have facilitated fluid storage and flow at depth. The granitic lithics vary in composition, texture and physical properties; from fine-grained granodiorites to coarse-grained monzogranites. Among these lithics are coarse-grained, hydrothermally altered, highly connected, porous monzogranites. Connected porosity estimated with helium pycnometry reveals pore volumes up to 6% in the monzogranites. 2 dimensional analysis with SEM reveals pore networks form a fine (5-25 micron) plexus of long grain boundaries dominantly associated with the quartz phase. Miarolitic cavities are also present, containing predominantly quartz crystals. We suggest the alpha-beta quartz transition as a mechanism for generating in-situ, highly connected, intergranular pore networks among the quartz phase of the monzogranites. Cl in quartz reveals complex zonation textures, and Ti in quartz reveals both magmatic and hydrothermal crystallization has taken place. Quartz thermometry offers crystallization temperatures of 400-800 (+/- 5). Amphibole geothermobarometry estimates crystallization depths of 3-5km at temperatures of 600-800(+/- 30) and pressures of 0.5-2.5 kbar (+/- 0.2 kbar), consistent with the depths of the mmagnetotelluric anomalies.