Paper No. 171-6
Presentation Time: 9:00 AM-1:00 PM
MODELING CRYSTAL RESIDENCE TIMES RECORDED IN PLAGIOCLASE, LÁSCAR VOLCANO, CENTRAL ANDES VOLCANIC ZONE
Láscar Volcano (Volcán Láscar; 5592 m; 23°22’S, 67°44’W), located in northern Chile, is considered the most active volcano in the Central Andes Volcanic Zone (CVZ). Historic activity dates to 1848 and has involved mostly dome growth and vulcanian eruptions with one large plinian eruption in April 1993. In the pursuit of assessing explosive volcanic edifices such as Láscar, diffusion models are important tools in understanding the longevity of magmatic processes in the magma system. Here, we present preliminary diffusion chronometry modeling of plagioclase phenocrysts hosted in pumice from the April 1993 eruption and compare it to crystals from several historical eruptions: the Soncor ignimbrite (26.5 ka), the Tumbres eruption (9.3 ka), and the Tumbres-Talabre lava (7.1 ka). These models allow for the comparison of crystal residence times within Láscar’s historical eruptive history and provide insight into the timescales of magma influx prior to eruption. Plagioclase phenocrysts within polished thin sections of the chosen eruptions were analyzed via electron probe micro-analyzer (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP/MS) to obtain major and trace element contents. We use these data to model diffusion chronometry and thermodynamically constrained trace element content evolution from single plagioclase phenocrysts to determine the long-term evolution of Lascar and determine the timescales of magma storage in the crust. We present the initial diffusion model results using Sr, Ba, Mg and Fe compared to percent anorthite content to delineate residence times within the magma chambers underlying Láscar. Plagioclase trace element transects record a diverse suite of magmatic processes but are overall controlled by magma mixing. Preliminary chronometry results of Mg and Sr profiles suggest that plagioclase from the 1993 eruption only reside for short periods of time after magma replenishment. Diffusion chronometry of Sr, Ba, Mg, and Fe contents in plagioclase provide further insight and increased understanding of the activity of one of the most active volcanoes in South America. These results further constrain the processes and timescales controlling eruption at Lascar.