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.
The focus of this NSF-funded research was to understand the origin of magmas that erupt to form calderas (supervolcanoes). Research was conducted at three caldera systems in New Mexico and Colorado, where large-scale faulting, uplift, and erosion has exposed the nonerupted components of the magmatic system. Volcanic and plutonic rocks were sampled and dated using the Ar/Ar and U/Pb dating techniques in order to understand their temporal relationships and assess models for caldera magmatism. Results indicate that caldera-forming magmas are generated at mid- to lower crustal levels as well as in the upper crust. At the Questa and Mt. Aetna caldera systems, located in northern NM and central CO, respectively, shallow plutonic rocks were previously interpreted to be the large volumes of nonerupted magma in caldera-forming magma chambers. Our results indicate that these shallow plutons were emplaced several hundred thousand to several millions of years before and/or after the caldera forming eruption. Ar/Ar and U/Pb ages suggest that these caldera-forming magmas were generated at deeper levels than current exposure depths, likely in the mid or lower crust. In contrast, large-volumes of shallow plutons rocks exposed at the Organ caldera, southern NM, are the same age as the caldera-forming volcanic rocks. The temporal relationships indicate that the Organ caldera magmas were generated in the upper crust. These results have significant implications for assessing caldera hazards. For example, one method to assess caldera hazards is to use geophysical techniques to image large (several 100 to several 1000 cubic kilometer) regions of magma in the upper crust. If caldera-forming magmas are generated at mid to lower crustal levels there may not be a protracted, distinct upper crustal seismic signature. Ages of volcanic and plutonic rocks indicate that postcaldera magmatism begins immediately (within analytical uncertainty) after the caldera eruption and continues for several hundred thousand to several millions of years after the caldera-forming events. Thus, calderas will remain potential hazards for a prolonged period. However, because these postcaldera magmas are the heat source for geothermal systems our research indicates that young calderas may be a long-lived viable source for alternative energy.
