Browsing by Subject "Magma mixing"

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Evolution of Plinian magmas from Popocatépetl Volcano, México
(2011-08) Sosa Ceballos, Giovanni 1975-; Gardner, James Edward, 1963-
Fractional crystallization, magma mixing, assimilation of continental crust, and how those processes modify volatile budgets, control the evolution of magma. As a consequence, the understanding of these processes, their magnitudes, and timescales is critical for interpreting ancient magma systems, their eruptions, and the potential future volcanic activity. In this dissertation I present the results of three projects. The first explores how magmatic processes affect magma reservoirs and eruption dynamics. The second explores where in the storage system and how often these processes occur. The third explores how volatile budgets are modified by processes such as crystallization, mixing, and assimilation. Volcán Popocatépetl (central México) erupted ~14100 14C yr BP producing the Tutti Frutti Plinian Eruption (TFPE). The eruption tapped two different silicic magmas that mixed just prior and during the eruption. The influx of mass and volatiles generated during the mixing of both magmas overpressured the reservoir, which was weakened at the top. The weakened reservoir relaxed while magma was tapped and collapsed to form a caldera at the surface. Although it is known that fractional crystallization, mixing, and assimilation can greatly modify magmas, the frequency and intensity of these events is not known. I investigated the magmatic processes responsible for the evolution of magmas erupted during five Plinian events of Popocatépetl volcano. Results show that during the last 23 ky magma was stored in two different zones, and was variably modified by replenishments of mafic magma. Interestingly, little evidence for large mafic inputs triggering explosive eruptions was found. Each of these processes alters the abundances of volatiles and introduces different types of volatiles to the system. Hence, the volatile budget of magma can have a rich and complex history. To investigate how volatile budgets evolve in active magma systems, I analyzed the abundances of volatiles (H2O, CO2, F, Cl, and S) in numerous glass inclusions trapped in phenocrysts. Results show that the magmas that produced the last five Plinian eruptions at Popocatépetl volcano evolved by crystallization and magma mixing, assimilation of the local carbonate basement is not chemically appreciable. Mixing with mafic magmas added substantial CO2 and S to the system, dewatered the magma, yet produced little change in the F contents of the magmas.
Petrography, zircon U-Pb geochronology and geochemistry of the Ertsberg Pluton, Ertsberg-Grasberg Mining District, Papua, Indonesia
(2020-06-19) Makis, Jacob Paul; Cloos, Mark
The Ertsberg Pluton is the largest (>15 km³) and youngest (3.1 - 2.8 Ma) igneous body within the renowned Ertsberg-Grasberg Mining District of Papua, Indonesia. It is associated with ore-grade Cu-Au mineralization in the form of four large skarns and a localized zone of classic porphyry style mineralization. Overall the intrusion has long been described as a diorite that is relatively homogeneous and resulted from the emplacement of a single batch of magma that underwent fractionation. In this study, two NE-SW trending, nearly 1200 m long cores were sampled for petrographic, geochemical, and geochronologic study. More than 90% Ertsberg pluton samples from these cores are best classified monzonite or monzodiorite (± quartz). Five types of magmatic rocks were identified in the two cores and included a medium-grained intermediate phase, a fine-grained intermediate phase, a dark mafic phase, a light phase, and an aplitic phase. Numerous internal contacts exist within the cores, most of which are gradational. Features such as plagioclase sieve cores and sieve rings, as well as albitic cores surrounded by anorthitic rims indicated an intermediate composition magma chamber was recharged and well mixed by hotter, mafic magmas. Major and trace element geochemistry was combined with LA-ICPMS zircon U-Pb crystallization ages to constrain the emplacement and evolution of the composite pluton. A suite of 63 samples were analyzed, 27 of which are from two cores sampled in this study. Out of the entire suite, 31 samples have zircon U-Pb crystallization ages and 31 samples have Nd and Sr isotopic measurements. When samples are plotted on Harker (SiO₂) and Fenner (MgO) diagrams, fractional crystallization trends are apparent. However, when the geochemistry is paired with zircon U-Pb crystallization ages, the oldest samples are the most felsic and the youngest samples are the most mafic. Trace element compositions mimic this relationship with the youngest samples having high concentrations of Sc, V, Ni and Cr. Furthermore, the Nd and Sr isotopic data define a mixing line. When the compositions are plotted against U-Pb ages, εNd increased and ⁸⁷Sr/⁸⁶Sr decreased through time. These trends are opposite to those expected for a single batch of magma that underwent fractional crystallization. The formation of the Ertsberg Pluton is explained by incremental emplacement of an original intermediate composition magma that was mixed with increasing proportions of recharging mafic magma. Space for the pluton was likely created by a combination of magma chamber floor subsidence (> 2 km) and by minor amounts of roof lifting (< 1 km) which were accommodated by movement along near-vertical faults near the margins. The bulk of the Ertsberg Pluton was constructed by the amalgamation of dikes/sills emplaced into a hot crystal-rich mush