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May 15, 2020 Mary Gates Hall boruco’s magmatic system before the event. Phase equilib- SESSION O-1I ria modelling suggests re-equilibration of orthopyroxenes ap- proximately in the upper 7 km during magma ascent. The LITHOSPHERE TO BIOSPHERE: compositional and textural variability indicate magma mix- ing, which is a common process leading to explosive activity VOLCANOES,GLACIERS,CLIMATE in other TMVB volcanoes such as Popocatepetl´ and Volcan´ CHANGE, AND INSECTS de Colima. Session Moderator: Kirsten Nicolaysen, Geology 11:00 AM to 12:30 PM * Note: Titles in order of presentation. Lavas, domes, and beetles: Probing the magmatic processes leading to the 1870-5 CE eruption of Ceboruco Volcano, Mexico. Gustavo Bejar Lopez, Senior, Geology, Whitman College Mentor: Kirsten Nicolaysen, Geology, Whitman College Mentor: Nick Varley, Universidad de Colima As one of the westernmost stratovolcanoes in the Trans- Mexican Volcanic Belt (TMVB), Ceboruco exhibits a wide range of compositions correlating to its variable eruption style. With around 55,000 people residing within 10 km to the vent, Ceboruco represents an important hazard. Magma- tism in the Ceboruco region is the consequence of both com- pressional and extensional tectonic regimes: the small Rivera plate subducts under North America, and regional crustal ex- tension promotes the formation of three intersecting grabens in western Mexico, one of which hosts Ceboruco. In this tectonic setting, compositions are variable, and types of eruption ﬂuctuate between moderately-effusive ones, like Ceboruco’s 1870-5 CE event, to large Plinian activity, like its caldera- forming eruption 1,060 ± 55 yr BP. Historically documented, the 1870-5 CE eruption led to the formation of a 7.5 km long lava ﬂow, a small plug dome, and a larger dome complex (El Escarabajo). All these lavas exhibit the same trachydacitic composition (˜68 wt.% SiO2, ˜9 wt.% Na2O+K2O), signal- ing that they were derived from the same magma batch. As the eruption sequence and the subsurface pre-eruptive condi- tions remain unexplored, I probe those processes using bulk and mineral compositional analyses, microtextural observa- tions, and phase equilibrium modelling. Higher crystallinity and higher Ba abundance in the dome complex suggest its derivation from the denser, deeper portions of the magma body hinting that El Escarabajo erupted late in the 5-year pe- riod. Reverse zoning in plagioclases (An67 − 72) and Ca- depleted rims in clinopyroxenes indicate perturbation in Ce- Undergraduate Research Program 1 www.uw.edu/undergradresearch .

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Mantle Peridotite Xenoliths
Earth and Planetary Science Letters 260 (2007) 37–55 www.elsevier.com/locate/epsl Mantle peridotite xenoliths in andesite lava at El Peñon, central Mexican Volcanic Belt: Isotopic and trace element evidence for melting and metasomatism in the mantle wedge beneath an active arc ⁎ Samuel B. Mukasa a, , Dawnika L. Blatter b, Alexandre V. Andronikov a a Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA b Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA Received 6 July 2006; received in revised form 3 May 2007; accepted 7 May 2007 Available online 13 May 2007 Editor: R.W. Carlson Abstract Peridotites in the mantle wedge and components added to them from the subducting slab are thought to be the source of most arc magmas. However, direct sampling of these materials, which provides a glimpse into the upper mantle beneath an active margin, is exceedingly rare. In the few arc localities where found, peridotite xenoliths are usually brought to the surface by basaltic magmas. Remarkably, the hornblende-bearing ultramafic xenoliths and clinopyroxene megaxenocrysts from El Peñon in the central Mexican Volcanic Belt were brought to the surface by a Quaternary high-Mg siliceous andesite, a rock type usually considered too evolved to be a direct product of mantle melting. The xenoliths and megaxenocrysts from El Peñon represent lithospheric mantle affected by significant subduction of oceanic lithosphere since as early as the Permian. Trace element and radiogenic isotope data we report here on these materials suggest a history of depletion by melt extraction, metasomatism involving a fluid phase, and finally, limited reaction between the ultramafic materials and the host andesite, probably during transport.
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