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Speakers: Cat Ross & Shuai Yan, Graduate Research Assistants, University of Texas Institute for Geophysics

Cat Ross: Zircon (U-Th)/He Impact Crater Thermochronometry and the Effects of Shock Microstructures on Helium Diffusion Kinetics

Abstract: Accurate and precise age determination of impact cratering events remains challenging and often contentious; less than half of all known craters are regarded as accurately and precisely dated. Zircon (U-Th)/He (ZHe) dating of impactites can be employed to date medium to large impact structures as ZHe ages can be fully reset in minutes at T >1000°C, a plausible scenario in the central melt pool. In contrast, complete resetting of ZHe at 200-300°C, encountered near the crater margins or due to post-impact hydrothermal overprinting, may take >10^3-6 years. To test the reliability of ZHe impact dating, we have quantified the effects of shock-induced microstructures on helium diffusion kinetics in well-characterized variably shocked zircon. We investigated samples from two impact structures, the Chicxulub multi-ring crater and Ries complex crater, to compare diffusion kinetics from structures with different size, age, and hydrothermal system longevity. Shock microstructures were characterized by backscattered-electron imaging prior to determining the He diffusion kinetics by prograde and retrograde fractional-release experiments via light-bulb furnace with incremental step-heating (10°C) from 300°C to 600°C. Next, we examine the internal interconnectivity and sizes of the diffusion domains by electron backscatter diffraction (EBSD). While we found that zircon with few shock microstructures exhibited no marked deviation from helium diffusion kinetics of undamaged zircon, zircon grains with planar microstructures and granular textures are characterized by a dramatic decrease in helium retentivity due to the reduction in the effective domain size and the introduction of interconnected fast diffusion pathways. A subset of grains were ZHe dated and showed that less deformed grains yielded a weighted mean age within error of the accepted impact ages, while the grains with planar microstructures or granular textures gave systematically younger ages. These new diffusion data and ZHe ages demonstrate that highly shocked grains are unsuitable for ZHe impact crater dating. Therefore, detailed characterization of impact-induced microstructures is critical for determining accurate ZHe impact ages and offers the possibility of investigating post-impact hydrothermal circulation.

Shuai Yan: A widespread subglacial hydrology system detected by airborne geophysics survey in Princess Elizabeth Land, East Antarctica

Abstract: Measurements of the subglacial topography, geology and hydrology of Princess Elizabeth Land (PEL), East Antarctica is critical for our understanding of the dynamics of the East Antarctic Ice Sheet (EAIS). However, PEL has been one of the least surveyed sectors of East Antarctica. Over the past years, several collaborative airborne geophysical surveys have been performed in PEL, aiming to fill in this gap. In this presentation, we propose to show the newly collected geophysical evidence suggesting the existence of a widespread subglacial hydrology system in PEL. This subglacial hydrology system is composed of a large subglacial lake (hereby referred as Lake Snow Eagle, LSE) that is over 40 kilometers long and 10 kilometers wide, and multiple smaller subglacial water bodies around LSE. The existence of these subglacial water bodies provides important constraints on the basal thermal condition in this region. We also propose to address the possibility that this subglacial hydrology system is connected to the coastal regions by an extensive subglacial channel network, which would make it the largest known-to-date interior Antarctic subglacial hydrology system that has a potential direct hydraulic pathway to the ocean.