Date of Graduation

Summer 2023


Master of Natural and Applied Science in Geography, Geology, and Planning


Geography, Geology, and Planning

Committee Chair

Gary Michelfelder


Petrographic textural analysis and geochemical microanalysis of plagioclase phenocrysts was conducted to model near-liquidus residence times and to gain further insight into processes that occurred within the magma chambers sourcing historical explosive eruptions at Láscar Volcano (NE Chile). Plagioclase crystals were analyzed from representative samples of Stages II (Soncor ignimbrite), III (Tumbres scoria deposit), and IV (Talabre lava, 1993 Plinian eruption). I used laser ablation-inductively coupled plasma mass spectrometry, which was combined with previously collected electron microprobe analyses and electron backscatter images to build a representative dataset of major- and trace element contents, as well as the zoning patterns and petrographic textures present. Analyzed phenocrysts can be grouped into five distinct populations, all of which record a major compositional change, evidenced by a section of reverse zoning and often resorbed zones, between the core and the mantle. A majority of the plagioclase also exhibit low-amplitude oscillatory zoning near the rim of the crystal. These textures have been interpreted as the result of a major magma mixing event that occurs once per stage, followed by smaller-scale fluctuations of conditions within the magma chamber. Major- and trace- element contents of plagioclase phenocrysts from Stages III and IV have also been modeled by diffusion geospeedometry using Python script published by Lubbers et al. (2022) to obtain paired Sr-Mg near-liquidus residence times. These models reveal that plagioclase resides in a warm and eruptible state, at the decadal-to-centennial scale, beneath Láscar prior to eruption. The average timescale has shortened through time, suggesting an increasing rate of small-volume injections of primitive magma, as proposed for Parinacota’s Young Cone; aligning with predictions of relatively short residence times via bulk rock geochemical analysis.


Láscar, plagioclase, diffusion, geospeedometry, magma mixing, residence times, Python coding language, petrographic textures

Subject Categories

Geology | Volcanology


© Bennett G. Van Horn

Available for download on Monday, July 15, 2024

Open Access