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The Comparison of Xenoliths in Eruptive Deposits at Dotsero and Cerro Overo Maars

Julia Bruno , Alison Graettinger

Abstract

Phreatomagmatic eruptions occur when magma explosively interacts with subsurface water, forming craters and displacing crustal rocks, some of which become xenoliths. This project investigates the relationship between eruptive styles and xenolith characteristics at two phreatomagmatic volcanoes: Dotsero Maar in Colorado, USA and Cerro Overo Maar in Chile. Dotsero Maar is characterized by three main pyroclastic deposits: welded spatter, red vesicular tuff breccia, and lithic-rich lapilli tuff. All deposits include quartz, sandstone, and siltstone xenoliths with evaporites in the later two deposit types. In contrast, Cerro Overo Maar features ignimbritic, dioritic, and granitic xenoliths, along with olivine crystals and fine scoria. To examine the thermal and eruptive history of Dotsero Maar, xenoliths ejected during different eruptive phases were analyzed, focusing on quartz- and evaporite-dominated xenoliths. Scanning Electron Microscopy was used to study the boundaries between sedimentary xenoliths and the igneous host, while X-Ray Diffraction was used to obtain mineralogical information about the quartz xenoliths and determine the quartz phase for comparison against the regional host rocks. Petrographic analysis was used to obtain textural evidence of magma-sediment mixing and signs of melting or chemical alteration of xenolithic material. The occurrence of evaporite xenoliths and quartz in Dotsero Maar's deposits provides insights into the thermal history of the eruption. These findings are compared to Cerro Overo Maar, where the composition of crustal rocks is less constrained, contributing to the understanding of phreatomagmatic eruptions and eruption dynamics.