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Description
Radiometric dating estimates the age of rocks by comparing the concentration of a decaying radioactive isotope to the concentrations of the decay byproducts. Radiometric dating has been instrumental in the calculation of the Earth's age, the Moon's age, and the age of our solar system. Geochronologists in the School of Earth and Space Exploration at ASU use radiometric dating extensively in their research, and have very specific procedures, hardware, and software to perform the dating calculations. Researchers use lasers to drill small holes, or ablations, in rock faces, collect the masses of various isotopes using a mass spectrometer, and scan the pit with an interferometer, which records the z heights of the pit on an x-y grid. This scan is then processed by custom-made software to determine the volume of the pit, which then is used along with the isotope masses and known decay rates to determine the age of the rock. My research has been focused on improving this volume calculation through computational geometry methods of surface reconstruction. During the process, I created an web application that reads interferometer scans, reconstructs a surface from those scans with Poisson reconstruction, renders the surface in the browser, and calculates the volume of the pit based on parameters provided by the researcher. The scans are stored in a central cloud datastore for future analysis, allowing the researchers in the geochronology community to collaborate together on scans from various rocks in their individual labs. The result of the project has been a complete and functioning application that is accessible to any researcher and reproducible from any computer. The 3D representation of the scan data allows researchers to easily understand the topology of the pit ablation and determine early on whether the measurements of the interferometer are trustworthy for the particular ablation. The volume calculation by the new software also reduces the variability in the volume calculation, which hopefully indicates the process is removing noise from the scan data and performing volume calculations on a more realistic representation of the actual ablation. In the future, this research will be used as the groundwork for more robust testing and closer approximations through implementation of different reconstruction algorithms. As the project grows and becomes more usable, hopefully there will be adoption in the community and it will become a reproducible standard for geochronologists performing radiometric dating.
ContributorsPruitt, Jacob Richard (Author) / Hodges, Kip (Thesis director) / Mercer, Cameron (Committee member) / van Soest, Matthijs (Committee member) / Department of Economics (Contributor) / Computer Science and Engineering Program (Contributor) / Barrett, The Honors College (Contributor)
Created2016-05
Description
It has been hypothesized that the ~25 km Rochechouart-Chassenon impact structure (RCIS) in the NW Massif Central, France, was formed during a Late Triassic (ca. 214 Ma) terrestrial impact event that produced a catena of several large craters. Testing this hypothesis, and assessing its possible impacts on biological evolution, requires both accurate and precise dating of candidate impact structures. Like many of these structures, the age of the RCIS is controversial because geochronological datasets yield contradictory results, even when a single isotopic system is used; for example, the two most recent 40Ar/39Ar studies of RCIS yielded statistically inconsistent dates of 201 ± 2 Ma (2σ) and 214 ± 8 Ma (2σ). While the precision offered by various geochronometers used to date impact structures varies significantly, a fair way to assess the confidence scientists might have in the accuracy of an impact age is to establish whether or not multiple chronometers yield statistically indistinguishable ages when applied to that structure. With that in mind, I have applied the (U-Th)/He, U/Pb, and radiation damage chronometers to zircons separated from two different RCIS impactites. My best estimate of the zircon (U-Th)/He age of the impact event is 191.6 ± 9.1 Ma at the 95% confidence level. U/Pb zircon dates suggest that most zircons in the RCIS target rocks were not completely reset during impact, but a subset (n = 8) of zircons appear to have crystallized from the impact melt or to have been completely reset; these zircons indicate a U/Pb impact age of 202.6 ± 5.8 Ma (95% confidence level). Zircon radiation damage dates are highly variable, indicating that the RCIS event resulted only in partial annealing of pre-impact zircon in the country rock, but a small sub-population of zircons yielded a mean date of 211 ± 13 Ma (95% confidence level). These results – all statistically indistinguishable from the previously published 40Ar/39Ar date of 201 ± 2 Ma – collectively argue that the impact age was near the presently agreed upon Triassic-Jurassic boundary. This age raises the possibility that seismite and tsunamite deposits found in the present-day British Isles may be related to the RCIS.
ContributorsHorne, Audrey (Author) / Hodges, Kip V. (Thesis advisor) / van Soest, Matthijs (Committee member) / Wittmann, Axel (Committee member) / Arizona State University (Publisher)
Created2016
Description
Many radioactive decay schemes employed in geochronology prove imprecise when placing accurate age constraints on young basalt flows. The (U-Th)/He systematics of detrital zircon and apatite within baked zones is examined as an alternative. Parent-daughter radioisotope ratios within grains from baked zones can completely reset if subjected to temperatures high enough and long enough for bulk diffusive loss. Presented here is the reproducibility of initial attempts to date flows by examining the (U-Th)/He geochronology of grains within baked zones. We examine grains from two localities within the San Francisco Volcanic Field and the Mormon Volcanic Field in northern Arizona. Thirteen zircon and apatite grains yielded from locality 2 collected from the uppermost 10 cm beneath a 7m flow of a basalt yield an apparent age of 4.39 ± 0.28 Ma (2σ), which is within range of published Middle Pliocene ages. Twenty-nine grains from locality 1 collected from the uppermost 20 cm beneath a 2 to 5m flow yield dates ranging from 0.47 ± 0.02 Ma to 892.77 ± 27.02 Ma, indicating the grains were partially reset or not reset at all. The degree to which grains are reset depends on a variety of factors detailed in this study. With these factors accounted for however, our study confirms application of this indirect dating technique is a useful tool for dating basaltic flows.
ContributorsCronk, Stephanie Sarah (Author) / Hodges, Kip (Thesis director) / van Soest, Matthijs (Committee member) / Barrett, The Honors College (Contributor) / School of Earth and Space Exploration (Contributor)
Created2014-05
Description
The South Tibetan Detachment System (STDS) marks a major decoupling horizon in the Himalaya, separating the highly metamorphosed infrastructure in the footwall from the weakly to unmetamorphosed superstructure in the hanging wall. The STDS stretches the entire range and is likely one of the most significant deformational features of the orogen, but its spatial and temporal evolution remain relatively unconstrained. As its name suggests, the STDS is a system of faults which occur at slightly different structural levels and are often diachronous. Detailed studies on the different strands are needed to understand the slip history of the system as a whole, which in turn will improve understanding of Himalayan orogenesis, thus informing tectonic models for continental orogenesis in general. I focus on some of the best exposed strands of the STDS which are located in the Annapurna region of Nepal. Outcrops within the shear zones of basal structures in the Kali Gandaki and Marsyandi valleys – the Annapurna and Chame detachments – contain leucogranites that are variably deformed via ductile slip on the detachments. I used U/Pb zircon and Th/Pb monazite geochronology to constrain emplacement ages of these leucogranites, which suggest ductile slip ceased prior to 14.95 ± 0.78 Ma and 16.0 ± 1.1 Ma on the Annapurna and Chame detachments respectively. 40Ar/ 39Ar muscovite and biotite, (U-Th)/He zircon and apatite thermochronology data and resulting thermal-kinematic models for samples I collected in the shear zones and footwalls of these detachments suggest further slip was ongoing on both detachments until ca. 12 Ma, although the majority of slip on the Chame detachment likely ceased by ca. 15-14 Ma. I also collected samples in the footwall of a structurally higher detachment in the Marsyandi and the resulting cooling ages and thermal-kinematic models suggest slip was contemporaneous with that on the lower Chame detachment. The new constraints on N-S extension on the STDS in the Annapurna region presented in this dissertation call into question the popular idea of a geodynamic change from N-S to E-W extension in the central Himalaya during the early Miocene, and emphasize the importance of the STDS as a major decoupling horizon.
ContributorsPye, Alexandra Eleanor (Author) / Hodges, Kip (Thesis advisor) / Whipple, Kelin (Committee member) / Barboni, Melanie (Committee member) / van Soest, Matthijs (Committee member) / McDonald, Christopher (Committee member) / Arizona State University (Publisher)
Created2022
Description
A mineral’s helium content reflects a balance between two competing processes: accumulation by radioactive decay and temperature-dependent diffusive loss. (U-Th)/He dating of zircon and other uranium and thorium-bearing minerals provides insight into the temperature histories of rocks at or near Earth’s surface that informs geoscientists’ understanding of tectonic and climate-driven exhumation, magmatic activity, and other thermal events. The crystal structure and chemistry of minerals affect helium diffusion kinetics, recorded closure temperatures, and interpretations of (U-Th)/He datasets. I used empirical and experimental methods to investigate helium systematics in two minerals chronometers: zircon and xenotime.
The same radioactivity that makes zircon a valuable chronometer damages its crystal structure over time and changes zircon helium kinetics. I used a zircon, titanite, and apatite (U-Th)/He dataset combined with previously published data and a new thermal model to place empirical constraints on the closure temperature for helium in a suite of variably damaged zircon crystals from the McClure Mountain syenite of Colorado. Results of this study suggest that the widely-used zircon damage accumulation and annealing model (ZRDAAM) does not accurately predict helium closure temperatures for a majority of the dated zircons. Detailed Raman maps of Proterozoic zircon crystals from the Lyon Mountain Granite of New York document complex radiation damage zoning. Models based on these results suggest that most ancient zircons are likely to exhibit intracrystalline variations in helium diffusivity due to radiation damage zoning, which may, in part, explain discrepancies between my empirical findings and ZRDAAM.
Zircon crystallography suggests that helium diffusion should be fastest along the crystallographic c-axis. I used laser depth profiling to show that diffusion is more strongly anisotropic than previously recognized. These findings imply that crystal morphology affects the closure temperature for helium in crystalline zircon. Diffusivity and the magnitude of diffusive anisotropy decrease with low doses of radiation damage.
Xenotime would make a promising (U-Th)/He thermochronometer if its helium kinetics were better known. I performed classic step-wise degassing experiments to characterize helium diffusion in xenotime FPX-1. Results suggest that this xenotime sample is sensitive to exceptionally low temperatures (∼50 °C) and produces consistent (U-Th)/He dates.
The same radioactivity that makes zircon a valuable chronometer damages its crystal structure over time and changes zircon helium kinetics. I used a zircon, titanite, and apatite (U-Th)/He dataset combined with previously published data and a new thermal model to place empirical constraints on the closure temperature for helium in a suite of variably damaged zircon crystals from the McClure Mountain syenite of Colorado. Results of this study suggest that the widely-used zircon damage accumulation and annealing model (ZRDAAM) does not accurately predict helium closure temperatures for a majority of the dated zircons. Detailed Raman maps of Proterozoic zircon crystals from the Lyon Mountain Granite of New York document complex radiation damage zoning. Models based on these results suggest that most ancient zircons are likely to exhibit intracrystalline variations in helium diffusivity due to radiation damage zoning, which may, in part, explain discrepancies between my empirical findings and ZRDAAM.
Zircon crystallography suggests that helium diffusion should be fastest along the crystallographic c-axis. I used laser depth profiling to show that diffusion is more strongly anisotropic than previously recognized. These findings imply that crystal morphology affects the closure temperature for helium in crystalline zircon. Diffusivity and the magnitude of diffusive anisotropy decrease with low doses of radiation damage.
Xenotime would make a promising (U-Th)/He thermochronometer if its helium kinetics were better known. I performed classic step-wise degassing experiments to characterize helium diffusion in xenotime FPX-1. Results suggest that this xenotime sample is sensitive to exceptionally low temperatures (∼50 °C) and produces consistent (U-Th)/He dates.
ContributorsAnderson, Alyssa Jordan (Author) / Hodges, Kip (Thesis advisor) / van Soest, Matthijs (Committee member) / Till, Christy (Committee member) / Shim, Sang-Heon (Committee member) / Sharp, Tom (Committee member) / Arizona State University (Publisher)
Created2019