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Techniques and Applications for Dating Young Zircon from Death Valley, California and Taupō Volcanic Zone, New Zealand
Chan, Christine Florence
Chan, Christine Florence
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Abstract
Volcanoes provide a snapshot into processes that occur in the crust and mantle, while plutons capture a longer history of evolving magmatic systems. Over the past few decades, there has been a paradigm shift in our understanding of magmatic systems. The conventional view used to be that magma chambers were large, homogeneous, melt-rich bodies. However, contemporary models depict magma chambers as transcrustal magmatic systems that are temporally and spatially complex. This shift underscores the realization that these systems are not confined to a single, well-defined chamber but are intricate networks of interconnected crystal mush with melt pockets.The timing and duration of magmatic events also hold valuable clues to the tectonic processes responsible for the formation of mountains, generation of new crust at subduction zones, and the development of continental rifts. In this dissertation, I explore the development and application of high-precision zircon geochronology to document the timing of magmatic processes in different parts of the transcrustal magmatic system and within contrasting tectonic settings. Zircon's exceptional resistance to alteration, ubiquity in the Earth's crust, and the ability to date a wide range of geological events with precision make it an indispensable tool for geologists seeking to unravel Earth's complex history.Chapter 1 provides a comprehensive analysis of the spatial and temporal relationship between Miocene extension and magmatism in the Death Valley extensional region, California. Through new CA-ID-TIMS U-Pb zircon dating and TIMS-TEA zircon trace element analyses, this chapter reveals a nearly continuous period of plutonism between 13.2 and 10 Ma, culminating in a significant magmatic event at ~10.5 Ma. This focused magmatism facilitated extensional reactivation of an older detachment fault. A later episode of plutonism at ~8.2 Ma documents the shift from extension to dextral transtension in the Death Valley extensional region.Chapter 2 introduces a novel dual isotope dilution-thermal ionization mass spectrometer-inductively coupled plasma mass spectrometer (ID-TIMS-ICPMS) dating technique, which yields higher-precisions ages for 100 to 350 ka zircon. This technique measures Pb isotopes by ID-TIMS and U-Th isotopes by ID-ICPMS. The chapter documents the development and testing of the dual ID-TIMS-ICPMS dating technique, which yields higher precision zircon ages for this age range relative to ion microprobe U-Th ages and ID-TIMS U-Pb. This advancement enables higher age resolution to detect previously obscured age dispersion and thus offers a more accurate understanding of magma storage timescales.Chapter 3 investigates the rare phenomenon of paired volcanic eruptions, focusing on the ca. 240 ka Mamaku and Ohakuri caldera-forming eruptions in the Taupō Volcanic Zone (TVZ), New Zealand. These eruptions originated from two calderas located ~ 30 km apart and marked the end of the 350 to 240 ka ignimbrite flare-up. Attempts to produce ID-TIMS-ICPMS 206Pb-230Th-238U zircon ages for these ignimbrites provide critical insights into the rapid generation and extraction of rhyolitic melts from the Kapenga mush zone during accelerated rates of rifting. While challenges were encountered in the dating technique, this research sets the stage for future refinement and offers valuable implications for zircon age trends.Collectively, this dissertation uses high-precision zircon geochronology to shed light on the complex interactions between magmatism, tectonics, and volcanism, with implications for current transcrustal magmatic models and tempos of volcanic eruption cycles.
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2023-01-01
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University of Kansas
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Keywords
Geology, Geochemistry, isotope geochemistry, magmatism, tectonics, U-Th-Pb, zircon geochronology