Volcanic rocks provide an integrated, multi-scale record of the processes that govern how magmas form. Using both theoretical and analytical tools, this work will document the critical processes that led to the formation of magmas in the Aeolian Islands, Italy. The volcanoes of the Aeolians were chosen as the study sites because they are compositionally diverse and geologically young; they thus represent prime places to study to how and why young volcanoes erupt. In addition to augmenting our understanding of how these magmas form and evolve, this work will also help characterize features of this young magmatic system such as the depth at which the magma bodies reside under the surface of Earth and the volatile content of the magmas prior to eruption. Such data can provide information to volcanologists who explore when and how volcanic eruptions initiate, thereby potentially enhancing volcanic hazard assessment and mitigation.
This work utilizes two approaches. One is application of a phase equilibria model, the development of which was funded by the National Science Foundation, to document the thermodynamics of processes that led to formation of magmas in the Aeolian Islands. Results of computer modeling allow predictions to be made about particular characteristics of the volcanic rocks. Using state-of-the-art analytical tools, these predictions will be tested by collecting geochemical, textural and field data on a selected suite of rocks. Comparison of theoretical expectations and these new data will allow the strengths and weaknesses of the phase equilibria model to be assessed and will also lead to well documented hypotheses about how the Aeolian volcanoes formed. If this marriage of state-of-the-art theoretical and analytical tools is successful, it will impact how scientists approach studying complex magmatic systems by providing a methodology by which source to surface compositional diversity can be understood. Because improvements to the phase equilibria model are an on-going goal, this work will also provide input that will inform model improvements.
Volcanic rocks integrate the records of a variety of processes that govern the compositional diversity of magmas (molten rock) on Earth. In the layer called the crust, located in the upper ~30 kilometers, magmas change composition through magma mixing, crustal assimilation, and fractional crystallization. Magma mixing is the blending of two distinct magmas, whereas fractional crystallization is the process of removal of (solid) crystals from the magma chamber. Crustal assimilation occurs when heat from the magma melts some of the surrounding crustal wallrock; these melts are then mixed with magma in the chamber. Documenting the relative roles of each of these, as well as their chronology, is important because they lead to changes in magma chemistry and other characteristics, such as viscosity, which, in turn, are among the controls on eruption explosivity. The Aeolian Islands (Italy) is an island chain composed of volcanoes, three of which were studied to understand how their magmatic systems evolved: Salina, Alicudi and Filicudi. Field work, collection of chemical data on whole-rocks (fist-sized pieces of volcanic rocks) and very detailed chemical and textural work on individual crystals called plagioclase (a calcium-aluminum silicate mineral) revealed that each of these volcanoes has common magma chamber features, and each also has distinct characteristics. In all three magmatic systems, the processes of magma mixing, assimilation and fractional crystallization operated, and evidence from thermodynamic modeling suggests that each volcanic edifice had a magma chamber system within the upper few kilometers below the volcano's surface. The rocks studied from all three islands also provide evidence for a burst of assimilation prior to eruption of the volcanic products, but mostly after the plagioclase formed. Additional study of this late-stage event may yield information about the characteristics of the magmatic systems prior to eruption. At Salina and Filicudi, there is also evidence of a deeper stage of crystallization, likely at ~12 kilometers. Distinct trends in plagioclase from both of these volcanoes also suggest that the mineral crystallized only in the shallow part of the magmatic system. On Salina, in the sequence of rocks known as the Fossa delle Felci, there is also evidence to suggest that intrusion of a magnesium-rich magma called basalt may have triggered eruption of the series of lavas that compose this part of the volcanic island. On Alicudi, whole-rock and plagioclase data, combined with thermodynamic modeling suggest that earlier stages of magmatism involved a discrete set of smaller magma bodies. As the magma system evolved and matured, the magma bodies shoaled, thereby occupying shallower levels in the crust. Conditions also favored aggregation of the magma bodies, likely into larger, more connected masses of crystals and melt. A novel aspect of this study involved use of in-situ plagioclase data (fine spatial resolution data acquired from crystals using technology that involves lasers or electron beams), whole-rock compositional information and petrologic modeling tools. In situ data produce compositional profiles from the interior to the exterior of plagioclase crystals, thus allowing interpretations of which processes (for example, magma mixing or crustal assimilation) were important from early to late in the crystal's growth history. This chronology and whole-rock data inform the use of thermodynamic models that provide information about the location of the magma chambers and other conditions, such as the amount of H2O in the magma. In aggregate, better documentation of magma chamber location and size and the relative roles of magmatic processes improves understanding of the potential timing and magnitude of volcanic eruptions. Undergraduate and graduate geology students involved in this project received training in the scientific method, on state of the art analytical instruments and with computational modeling tools. They enhanced their scientific writing and speaking skills, augmented their problem solving skills and learned to work individually as well as in groups. Three MS theses and one undergraduate science honors project were aomng the outcomes of this work. This project also supported career development for students underrepresented in the geosciences. The principal investigator developed new international collaborations that have led to exciting new projects and will enahnce future opportunities for students.
