Intellectual Merit. The proposed research will use geochronology and geochemistry of cogenetic volcanic and plutonic rocks to understand critical aspects of caldera magmatism. During caldera magmatism, large volumes of compositionally diverse magmas are emplaced into the upper crust. Some fraction of this magma erupts and the remainder crystallizes to form a pluton. Much of our current understanding of caldera magmatic processes is from detailed studies of the vocanic rocks. Cogenetic plutonic rocks also record useful information about sub-caldera magmatic processes such as rejuvenation and modification of existing magma chambers, dike and sill emplacement, upwelling magma related to resurgence, and the waning stages of magmatism. None of these processes are directly recorded in the volcanic record. By establishing the temporal, chemical, and spatial relationship of both caldera-related volcanic and plutonic rocks, a more comprehensive model of caldera magmatism can be developed. Objectives of this proposal include investigating the depth of magma differentiation, exploring rates of magma emplacement into the upper crust, establishing the role of nonerupted magma in postcaldera magmatism, estimating the fraction of a caldera magma chambers that erupt, determining differences between zoned and nonzoned igneous suites, and understanding the relationship between large-volume silicic volcanism and plutonism. Three mid-Cenozoic caldera systems - Questa and Organ in New Mexico, and Mt. Aetna in Colorado, have been selected for investigating caldera related magmatism. Rio Grande-rift-faulting at these locations has exposed both intracaldera volcanic rocks and subcaldera intrusions. A challenge in developing caldera magmatism models is the likelihood that processes vary between different caldera systems. Because the field locations differ with respect to style of caldera magmatism and volume and depth of exposed volcanic and plutonic rocks, these caldera systems provide the necessary diversity to understand the associated magmatic processes. Fieldwork during the past several years has already yielded a comprehensive sample suite for each location. The Ar-Ar method will be used to establish the timing of volcanism and pluton emplacement and cooling histories. Laser ablation ICP-MS U-Pb dating will be used as a reconnaissance tool to identify samples for which high-precision ages will subsequently be determined using the CA-ID-TIMS method. Post-eruptive zircon modification in the plutonic rocks will be monitored using ion-probe O-isotope analysis. Geochemistry of coeval plutonic and volcanic rocks will be used to determine the roles of fractionation, partial melting, and assimilation in generating large volumes of silicic melt.
Broader Impacts. Understanding the magmatism associated with calderas is essential to assessing volcanic hazards. Comprehensive caldera magmatic models may also be used to explore for volcanorelated geothermal activity and economic ore deposits. Because this study will produce high-precision UPb and Ar-Ar ages for rapidly cooled rocks (e.g. ignimbrites, ring dikes), the proposed research will test the accuracy of recent efforts to intercalibrate the two radiometric systems. The project will provide the PIs, two doctoral students, two masters students, and at least three undergraduate students a collaborative research experience between New Mexico Tech, the University of North Carolina, the USGS and UCLA. Research will provide graduate students with useful laboratory training to develop skills necessary for professional careers. The PIs, along with help from the graduate students, will continue interacting with national and state parks to provide an education experience for the community. Data will be submitted to the NAVDAT database, which was, and will continue to be, used extensively in developing the ideas central to this proposal.
Intellectual Merit: This project was designed to seek new insights into the connections between magmas that erupt and form volcanic rocks and magmas that are trapped beneath the surface and form plutonic rocks. Conventional wisdom holds that when magmas erupt, they leave a mass 3-10X larger behind that crystallizes as a pluton. For the world’s largest eruptions (so called "super eruptions") that result in erupted volumes of 500-5000 km3 this model predicts accumulation of incredible volumes of magma in the shallow subsurface. We hypothesized that instead, when such huge volumes of magma did reach the shallow crust, that any magma chamber would be effectively emptied, leaving little or nothing behind in the plutonic rock record. We tested the hypothesis in the Southern Rocky Mountain volcanic field where the products of numerous super eruptions that occurred ~40-20 million years ago are preserved. In addition to these erupted rock (ignimbrites) recent faulting has exposed the shallow plutonic rocks of the province as well. We predicted that there would be an offset in the ages of the ignimbrites and the plutonic rocks, with the accumulation of most of the plutonic rocks in periods between ignimbrite eruption. Our work focused on the Mount Princeton area near Buena Vista, and the Questa area in northern New Mexico. There we completed comprehensive geochronologic analysis of both volcanic and plutonic rocks. The results of the work were exactly as we predicted. In both areas, the only plutonic rocks that are the same age as the erupted ignimbrites are very small dikes – that we interpret to be feeder dikes for the eruptions. The bulk of the volume of the plutonic rocks was typically younger than spatially associated ignimbrites. Remarkably, the largest pluton in the Buena Vista area (the Mount Princeton batholith) was assembled in essentially the only interval in the history of the field that is not characterized by ignimbrite eruption. This work challenges the dogma regarding the origin of super eruptions and should force rethinking regarding their cause and our ability to predict their occurrence. Broader Impacts: This research resulted in the training of a Ph.D. student (now working for NASA), two M.S. students (now at NASA and Chevron Mining), and two Honors B.S. theses (both students in Graduate School). In addition to academics, the PI is very active in community engagement in geology. He works closely with local scout troops in completing requirements for badges, and teaching the impact of geology on the Scouts’ lives. He collaborated with his daughter’s middle school science teacher to develop geology podcasts for middle school students nationwide (www.thewalkingclassroom.org/) and organizes geology activities for elementary school STEM nights in the area. Through this research he also met and initiated a collaboration in support of geology research at the Cherokee Ranch Geology Institute.
