The overarching goal of this research is to investigate rear-arc petrogenetic processes in the Northern Volcanic Zone (NVZ) of the South American Andes. The NVZ is ideal for this study because it is similar in many ways to other well-studied arcs, yet it exhibits characteristics that are unique to the NVZ. This research is important because we have the opportunity to learn about the processes that mark the transition from dominantly subduction-influenced melting to the regime of MORB-type petrogenesis in South America. In addition to the primary objective, timescales will be evaluated for magma ascent and degassing in an active, calc-alkaline rear-arc volcano (El Reventador), which is important for constraining the timescales of magmagenesis in the rear-arc region. The new data set will be used to test existing models of rear-arc magmatism, while developing new models that can be applied to future arc research. The proposed research will comprise a regional, in-depth, geochemical study of Ecuadorian rear-arc volcanoes using a combination of field research, analytical work and geochemical modeling. Petrogenetic processes will be characterized using major and trace elements combined with radiogenic isotopes (Pb, Sr, Nd) and U-series on whole rocks and mineral separates. Special attention will be given to minerals in order to characterize melt inclusions and compositional variation that reveal fine-scale petrogenetic fingerprints. Timescales (i.e. crystallization and degassing rates, ages) will be determined by measuring 238U-, 230Th-, 226Ra, and 210Pb- and 210Po nuclides in mineral separates and whole rocks, using facilities at WHOI and the Universities of Wyoming and Iowa. Rigorous characterization of minerals and will be accomplished using electron microprobe and secondary ion mass spectrometry facilities at UCLA. These data will be used to evaluate contrasting models for melt formation.
Intellectual Merit. This research is critical for understanding the nature of rear-arc processes in the Andean continental arc and arcs in general. The uniqueness of the NVZ lies in the compositional diversity of magmas (calc-alkaline and alkaline), the location of the rear-arc at the boundary of the South American craton, and the (possible) influence of the subducting Carnegie Ridge. Similarities to other rear-arc volcanoes provide a strong foundation for comparing our data and testing models. The application of U-series nuclides to igneous geochemistry has proven useful for determining melting and differentiation timescales in arc volcanoes, and more recently for identifying melting and differentiation processes. These currently active volcanoes in Ecuador present an opportunity to use U-series nuclides to investigate melting processes and magmatic timescales in a rear-arc setting.
Broader Impacts. The Broader Impacts of this project include promoting geoscience to students, fostering international communication and emphasizing the importance of science to society. Incorporation of an interactive website as part of the field research, will bring the field experiences directly into classrooms in the U.S. and Ecuador, and provide students a sense of scientific discovery that could have a long lasting and positive impact on their lives. This project has the potential to strengthen the scientific foundation of K-12 students, with a large impact on minority students in a dominantly Hispanic community. Additionally, this funding will support a graduate student and provide funding for a junior faculty member at California State University, Los Angeles.
This award is co-funded by the Office of International Science and Engineering (OISE).
The overarching goal of this project was to investigate causes of along-arc geochemical variation in volcanoes from the rear-arc of Ecuador and to use isotopes to investigate timescales of melting processes and petrogenetic history at El Reventador, Sumaco and Pan de Azucar volcanoes. During our 3-year project, we funded a post-doctoral researcher and a Master's student from University of Wyoming, two minority undergraduate students from CSULA and an international student from Ecuador. We connected with K-12 students in Laramie and Los Angeles, and collaborated with scientists in Ecuador, which was valuable for fostering international relations. As a result of this funding, we accomplished our research goals as well as generate several new projects that are ideal for graduate or undergraduate research. Our project outcomes for this research are described in the context of the Merit Review criteria: Intellectual Merit: Our findings significantly increase the understanding of petrogenetic processes in Ecuador and other rear-arc settings. Our main findings include; 1) There is significant geochemical variation among the three rear-arc volcanoes in Ecuador; specifically the concentration of incompatible trace elements increases from north to south along the arc. The best explanation for this variation is an increase in slab dip and concomitant decrease in the amount of H2O and degree of mantle melting from north to south. High concentrations of Sr (> 4,000 ppm) can only be explained by the addition of another component to the mantle prior to melting, and the 176Hf/177Hf isotope data indicate that lavas from Sumaco contain a geochemical component similar to MORB. Seismic data (USGS) show steepening of the slab beneath Sumaco and possibly a rift or tear that could result in decompression melting of upwelling mantle and a small degree of eclogite melting in a possible slab window. 2) The calc-alkaline characteristics of the northernmost volcano (El Reventador) are typical of arc magmas and result from dehydration melting of peridotite mantle, whereas the phonolite-series alkaline lavas in the southern rear-arc (Sumaco Volcano) have mineralogy that is similar to that of volcanoes like Mount Erebus, Laacher See and ocean islands related to plume activity. In particular, lavas from Sumaco contain the sulfate mineral hauyne that is uncommon in arc lavas and indicates that future eruptions from Sumaco Volcano are likely to be high in sulfur. 3) U-series isotopes were used to address timescales of petrogenetic processes at El Reventador, and to constrain ages of rocks at Sumaco and Pan de Azucar volcanoes, for which there exist no eruption ages. On the basis of the 226Ra-230Th isotope data, Sumaco lava ages are constrained to less than 8,000 years old for many lavas, and between 8,000 and 250 ka for most lavas. The El Reventador data show that the most mafic samples have 230Th excesses that shift to 238U excesses in the most evolved samples, indicating that differentiation of accessory minerals (likely apatite) contribute to the 238U excesses at this volcano. The initial 230Th excesses can be generated by partial melting of garnet peridotite mantle. U-series data from this project has been presented at AGU and also at a U-series conference in Sydney Australia in Feb 2014. Broader Impacts. Garrison developed and maintains a project website (www.calstatela.edu/programs/ecuadorvolcanoes/our_project.html) to disseminate results and project updates. Our Broader Impact results are summarized below: 1) As a result of this research, two minority (Hispanic, one female) undergraduates from an Hispanic Serving Institution (HSI) participated in an expedition to Ecuador and both students have gone on to graduate school. An Ecuadorian student was given a stipend to join our field expedition, and this same student was invited to the University of Wyoming to assist with sample preparation in Sims’s clean lab during summer 2011 in Year 2. The undergraduate CSULA students presented research at the 2012 AGU meeting in San Francisco, and the Master’s student from the University of Wyoming completed his thesis and is employed in the oil industry. Garrison continues to visit an elementary school in Los Angeles to talk with students about volcanoes, and has used release time to work on two parallel submissions that are 80% and 30 % competed. A third paper is being prepared for publication by the Master's student. 2) We have a complete collection of rock samples from the rear-arc of Ecuador from hard-to-access sample locations and from the volcano summit that we reached by helicopter using NSF Supplemental funds in Year 1. We also have a complete set of data for these rocks. 3) Future research will be directed at combining geophysics with geochemistry to resolve the geometry of the subducting slab and exploring the role of decompression melting in rear-arc settings. Potential student projects include using δ34S isotopes to investigate the source of sulfur at Sumaco, and applying Lu-Hf isotopes to other Ecuador volcanoes to test the rear-arc decompression melting model.
