Dr. Heather S Cunningham has been granted the NSF Earth Sciences Postdoctoral Fellowship to carry out a research and education plan at the University of Wisconsin - Madison. The investigation aims to detail the short- and long-term trends in caldera edifice development at Atitlán Caldera, Guatemala. Selected volcanic ash and lavas representing the large silicic eruptions, associated scoria layers and cone-growth phases will be collected during a field campaign for geochemical analysis. Major and trace element data will be integrated into petrologic models to determine how large silicic magma bodies develop and if they are related by shallow-level modification processes such as fractional crystallization, magma mixing or assimilation. Isotopic data (87Sr/86Sr and 143Nd/144Nd) will be measured to support petrologic models since these isotopic ratios are distinctive to source composition. To constrain eruptive age, 40Ar/39Ar isotopes will be analyzed on phenocrysts in distinctive eruptive units. 230Th/238U disequilibrium will be measured on volcanic ash to resolve whether large silicic bodies are rapidly differentiated from a homogeneous source, such as a deep crustal hot zone, formed by the coalescence of several magma bodies or the product of crustal assimilation.
By resolving the time scales and processes that lead to large-caldera eruptions and associated cone-building phases potential volcanic risk can be inferred. However, our understanding of the processes that lead to large caldera-forming eruptions is limited by the lack of recent, well-exposed deposits. Thus, the Atitlán volcanic complex is ideally suited to test conflicting models for the generation of large-volume silicic eruptions and more importantly time scales over which caldera-systems develop. In addition to the development of geochemical models, an educational exercise will be created. A Dynamic Digital Map of Atitlán Caldera will be made available to educators that will allow students hands-on access to real world data.
@font-face { font-family: "Cambria"; }p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0in 0in 10pt; font-size: 12pt; font-family: "Times New Roman"; }div.Section1 { page: Section1; } Constraining the time scales and processes of rhyolite and basalt development in a large caldera system, Atitlán, Guatemala Heather Cunningham Dept of Geology, University of Wisconsin at Madison, 1215 W. Dayton St. Madison, WI 53706 Project Summary The aim of this project was to use novel geochemical and isotopic systems to identify the sources and time scales of rhyolite and basalt development in a large caldera system. The most recent caldera-forming eruption, the Los Chocoyos, occurred 84 kyr (thousand years) and produced the largest eruption in Central America in the past 200 kyr. For the 40,000 people living in the region, identifying the frequency of eruptive activity for large violent caldera-forming eruptions and smaller basaltic block and ash flows is essential in land use and volcanic hazard planning. Although eruptive activity through stratocone Atitlán has been infrequent in the past 200 yrs, the eruption of 125 km3 of lava in the past 84 kyr suggests periods of frequent and profuse volcanic eruptions. Rock samples of basaltic lava flows and rhyolitic pumice blocks were collected during a 10 day field season from important localities around the caldera. Samples were then cleaned and processed for mineralogic, compositional, Sr and Pb isotopic data, 40Ar/39Ar age data and U-series time scale data. Teaching activities, submitted to the National Association of Geoscience Teachers, based on eruptive frequency, the relation between volcanic and plutonic rocks and the diversity of magmas in two adjacent volcanoes will be made available to educators. The results of this study differ from other well-studied caldera systems. In particular, while other caldera systems show evidence for the accumulation of basalt in the upper crust and differentiation into rhyolite, at Atitlán rhyolites form by melting in the lower crust. The two types of rhyolite (low K and high K) erupted during the Los Chocoyos caldera-forming eruption show distinct geochemical and isotopic differences that result from the processes that form them. The high K rhyolite displays trace element ratios and minerals suggestive of feldspar and zircon mineral growth. In contrast, the low K rhyolite has trace element ratios that formed through processes the lower crust. Not only are trace element ratios used to infer crustal melting such as Ce/Y and La/Yb elevated, but also the rhyolite contain mid crustal hornblende. This information can be used to improve monitoring techniques under the caldera. The basaltic stratocones also developed in the lower crust through the crystallization of minerals of orthopyroxene and clinopyroxene. Lavas erupted within the caldera margin show mineralogic and geochemical evidence for crustal contamination with the rhyolite. Age dating of the lava flows did not produce concise eruptive ages. The geochemical diversity of the Atitlán system suggests a very complex magmatic plumbing system aided by high flux rates and an old, thick crustal column. Further studies, including volatile contents of melt inclusions and trace element diffusion rates, will help to highlight the staging of magmas under a large caldera system.
