Zircon – Tiny but Telling: A Petrochronological Study

This thesis concerns the field of petrochronology, where time is linked to the physical conditions (e.g., pressure P and temperature T) of rock-forming events, to provide better constraints on key geological processes such as mountain building, metamorphism, and magmatism. The goal of this work is to enhance applications using the chemical composition of the mineral zircon in petrochronological studies of high- and ultrahigh-pressure ((U)HP) metamorphic rocks. The primary use of zircon in petrochronology is to determine rock ages through U-Pb dating. Zircon dating is considered particularly reliable because zircon is stable over a wide PT range in the Earth's interior and elemental and isotopic composition undergo little or no change during a variety of geological processes. A major challenge of zircon petrochronological studies, however, is to determine PT conditions of zircon growth. The methodical focus of this work is to investigate the extent to which equilibrium rare earth element (REE) distribution between zircon and melt, which is used here as an analogy to the REE partitioning between zircon and co-existing metamorphic minerals, is sensitive to pressure and temperature. By expanding a previously developed latÝce strain model-based prediction function for zircon-melt REE partition coefÏcients (DREE) – a function enabling the prediction of the overall REE partitioning behavior from only a small number of DREE values – with an empirically determined constraint, the function is made applicable to existing zircon-melt/bulk rock DREE data. Evaluation of literature data with this expanded prediction function confirms the previously discovered temperature dependence of zircon-melt DREE and allows the calibration of a zircon-melt DREE based geothermometer. Zircon synthesized in high-pressure-temperature experiments in equilibrium with magma reveal that, in addition to temperature, pressure and the major element composition of the coexisting magma affect zircon-melt DREE. The finding that pressure, alongside temperature, affects the partitioning behavior of REE between zircon and melt establishes a foundation for developing a mineral-mineral geothermobarometer based on zircon-melt DREE for (U)HP metamorphic rocks, offering a potential new method to link zircon ages with the PT conditions of zircon formation. Quantifying trace element contents in tiny zircon crystals from high-pressure-temperature experiments is challenging. To improve trace element determination in zircon using high-resolution techniques like NanoSIMS, the synthesis of a high-quality zircon standard from a Li-Mo flux was attempted. Although the synthesis of a suitable standard for quantitative trace element determinations was unsuccessful, it provides insights into the challenges for future synthesis attempts. In the applied part of the thesis, a detailed petrochronological study is conducted, reevaluating the zircon ages of the Earth's youngest known UHP coesite eclogite from the Papua New Guinea UHP terrane. Zircon ages of the coesite eclogite obtained in previous studies using different analytical techniques to determine U and Pb contents are inconsistent. Based on zircon REE contents, determined at the same spots as the U-Pb isotopic data for zircon dating, along with zircon textures, two distinct zircon generations are now identified: an older and a younger generation with ages ranging from 7.0±0.2 to 7.9±0.3 Ma and from 4.4±0.3 to 5.5±0.4 Ma, respectively. A thorough petrological analysis of the coesite eclogite identifies a primary mineral assemblage of garnet, omphacite, amphibole, coesite, phengite, rutile, and zircon. The middle to heavy REE patterns of the older zircon indicate it was part of the primary mineral assemblage. PT pseudosection modeling suggests that this assemblage formed under UHP conditions of P=3.1±0.2 GPa and T=765±30°C. The findings of this study underscore the unique role of zircon in petrochronology and pave the way toward a comprehensive tool for determining the PT conditions of zircon formation, to integrate it with age information obtained on the same zircon grain.