Complexities of (U-Th)/he zircon thermochronology through the lens of zonation and deep time

Thurston, Olivia Grace

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Description

Title

Complexities of (U-Th)/he zircon thermochronology through the lens of zonation and deep time

Author(s)

Thurston, Olivia Grace

Issue Date

2021-04-23

Director of Research (if dissertation) or Advisor (if thesis)

Guenthner, William R

Doctoral Committee Chair(s)

Guenthner, William R

Committee Member(s)

• Garver, John I
• Anders, Alison M
• Lundstrom, Craig C
• Johnson, Thomas M

Department of Study

Geology

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Thermochronology
• zircon
• deep-time
• Grand Canyon
• annealing
• annealing kinetics
• zonation
• radiation damage

Abstract

The zircon (U-Th)/He (zircon-He) system is a dating tool used to date low-temperature geologic events that occur in the upper four kilometers of Earth’s crust. The zircon-He system is defined by the kinetic interactions of radiation damage accumulation and annealing, and He diffusion. The kinetics of the zircon-He system were developed using idealized samples and conditions, which do not always translate well to the complex samples and settings that researchers attempt to date when using the zircon-He system. I seek to better understand how the defining elements of the zircon-He system interact when applied to complex natural samples and deep-time thermal histories. I first explore the role of radiation damage zonation in zircon on annealing kinetics. Annealing is the thermally activated process by which chemical bonds reform after being broken by radiation damage. Radiation damage directly impacts the diffusion of He from the crystal lattice and is a key factor in defining the kinetics of the zircon (U-Th)/He system. Damage accumulates within a crystal as a function of time and U and Th concentration. The total level of radiation damage in a zircon crystal is governed by the thermally-activated, kinetic process of annealing, which in turn influences the interpretation of zircon (U-Th)/He dates for thermal histories. Several annealing models have been defined for the zircon system based on measurements in natural crystals; however, few studies have investigated how multiple levels of radiation damage due to zonation of actinides within a crystal may influence the annealing process. Here I use Raman spectroscopy to map the full-width half maximum (FWHM) of the 3(SiO4) band, a proxy for radiation damage, in zircon crystals from the Lucerne pluton (Maine, USA) with heterogeneous distributions of U and Th. I compare FWHM maps before and after annealing these crystals at laboratory times and temperatures. These maps show that each damage zone within a single zircon acts as an isolated domain that is dictated by an independent set of annealing kinetics. Thermally activated annealing decreases radiation damage in all radiation damage zones; however, the rate of annealing is not consistent across all zones. I identify specific FWHM damage levels present post-annealing regardless of laboratory time temperature conditions: FWHM modes at 2-5 cm-1, 10-15 cm-1, and 25-30 cm-1. I attribute these persistent damage modes to variable annealing kinetics that are partially dependent on the level of pre-annealing damage, combined with the inability of high-damage crystals, or zones within crystals, to fully recover their crystallinity. These findings therefore show that zircon crystals with non-uniform distributions of U and Th can anneal to create long-lived damage zones at specific damage levels, which has implications for treating the zircon (U-Th)/He chronometer as a multi-domain diffusion system. Next, I apply the zircon-He system to the geologically complex deep-time thermal history of the Eastern Grand Canyon. I demonstrate the power of zircon-He thermochronology to resolve cooling events of Precambrian basement exposures below the Great Unconformity surface in the Grand Canyon. I combine new zircon-He data with previous 40Ar/39Ar mica and K-spar results to model the < 250 °C, > 1 Ga, thermal history of these basement rocks. Forward and inverse models of zircon-He date-effective uranium (eU) concentration, a proxy for radiation damage, suggest that the main phase of Precambrian cooling to <200 °C was between 1350 and 1250 Ma, after the Yavapai orogeny. This result agrees with K-spar 40Ar/39Ar thermochronology showing rapid post-1400 Ma cooling and both are consistent with the 1255 Ma depositional age for the Unkar Group. The data and models are highly sensitive to late-stage reheating due to burial beneath ~ 3-4 km of Phanerozoic strata prior to the Laramide orogeny; models that best match observed date-eU correlations show maximum temperatures of 140-160 °C, in agreement with apatite (U-Th)/He and fission-track data. Forward and inverse models also test for the age of carving of Grand Canyon; they support a multi-stage cooling model involving 25-15 Ma cooling from 100 to 50 °C during partial carving of Eastern Grand Canyon, with post- 6 Ma rapid cooling indicated by 3 to 7 Ma zircon-He dates over a wide range of high eU. The zircon-He data capture basement exhumation below the Great Unconformity during the Mesoproterozoic (1300-1250 Ma), and “young” (20-0 Ma) carving of Grand Canyon. Finally, I assess the role of zonation in creating secondary dispersion in zircon-He data. High-levels of secondary dispersion can be seen in the zircon-He data collected from the Eastern Grand Canyon. Dispersion in zircon-He data can lead to greater levels of uncertainty in modeled thermal histories. Dispersive datapoints were determined as those datapoints that deviate from the average date-eU trendline of the binned data by more than 2 standard deviations. All dispersive datapoints were targeted as potentially having severe zonation, which can create unaccounted for complexities in the kinetics applied to the zircon-He system. CL images were collected for zircon separates from the Eastern Grand Canyon and showed a range of zonation patterns from no zonation and concentric zonation to chaotic, non-concentric zonation. I ran two inverse models to see if the addition of zonation information increases the probability of various thermal events in the Eastern Grand Canyon thermal history. One inverse model used data without any zonation information and the other used data with zonation information based on anonymous zonation styles identified in CL image. I also created forward models to see if the level of secondary dispersion could be modeled by artificially creating zircon-He data with zoned eU information data. I found that the addition of zonation information to my inverse models does not increase the probability of known thermal events but creates a less complex thermal history. The artificially zoned data does not fully explain the level of secondary dispersion seen in the measured zircon-He data. Of the seven dispersive datapoints identified in the zircon-He data, only three datapoints were captured by the artificially zoned data, leaving the two oldest and two youngest dispersive datapoints unexplained. I attribute the decrease in complexity of the zoned thermal history, and the inability of artificially zoned data to account for all secondary dispersion, to an overly simplified morphology of zonation in both my inverse and forward models. I suggest that He diffusion kinetics should include the option for the use of a multi-domain diffusion (MDD) model where domains are separated by fast-pathways of diffusion. The introduction of a non-nested MDD would allow for zonation information from chaotic zones to be better modeled and could ultimately reduce dispersion in zircon-He data. The collective results of this dissertation show both the utility of zircon-He thermochronology and areas where the method can be expanded and improved upon. Proper kinetic models for describing radiation damage annealing and He diffusion in zircon with heterogeneous uranium and thorium distribution would allow for zircon-He users to better understand the complexities and dispersion found in their zircon-He data. The potential for zoned zircon to produce additional dates and thermal history information using the zircon-He method, particularly in deep-time settings, makes improving the communal understanding and application of the underlying kinetics of the zircon-He methods all the more imperative.

Graduation Semester

2021-05

Type of Resource

Thesis

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Copyright 2021 Olivia Thurston

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