Reaction rims surrounding amphiboles in hornblendites allow for the investigation of the timescales associated with magmatic ascent beneath volcanoes and new insights into the magma dynamics beneath active volcanic centers. The hornblendites of this study originated from andesitic-dacitic lavas that erupted from the Quillicas Volcanoes of the Central Bolivian Andes approximately 1.4 million years ago. The hornblendite samples have previously been characterized for their textures, mineralogy, and geochemistry in Velázquez Santana et al. (2020). They consist of 90-95% amphibole and 5-10% plagioclase feldspar, with reaction rims occurring ubiquitously around the amphiboles. These rims were further investigated using a Zeiss Supra 35 VP Scanning Electron Microscope (SEM) at the Center for Advanced Microscopy and Imaging (CAMI) at Miami University. Mineralogical and textural analysis revealed the rims are gabbroic in nature and composed of pyroxenes, plagioclase feldspar, and oxides. The formation of the rims is interpreted to be the result of re-equilibration during magmatic ascent, due to changes in temperature and pressure. Uniquely, the width of the rims can be used as a proxy for magma ascent timescales. Based on 80 rim width measurements, the average width is 27.25μm (±26.71μm) with ~⅔ of measured rims 15-35μm in size. With a 10μm bin size, 50% of the measured rims are 20-30μm in width, while using a 5μm bin size, 36% are 20-25μm in size. Using the average width of 27.25μm, and based on the experimental results of Rutherford and Hill (1993), ascent timescales of 5 to 15 days are inferred. In future work, the results of these rim width measurements will be compared to the rim widths of amphiboles from other magmatic systems in the Andes, and more globally. To date, this study has provided new insights into the timescales associated with magmatic ascent, beneath the Quillicas volcano.
Authors: Mitchell P. Duncan, Liannie Velazques Santana, Raghad Al Gbory
Faculty Advisor: Claire L. McLeod, Geology and Environmental Earth Science
Graduate Student Advisor: Liannie Velazques Santana, Geology and Environmental Earth Science
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