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Using luminescence signals from bedrock feldspars for low-temperature thermochronology

  • Author(s): Brown, Nathan David
  • Advisor(s): Harrison, T. Mark
  • et al.
Abstract

Over the past several years, optically-stimulated luminescence signals (OSL) from quartz and infrared-stimulated luminescence (IRSL) from feldspar in bedrock have been investigated for their use in thermochronology. In this study, I propose using thermally-stimulated signals (thermoluminescence, or TL) from feldspar instead. Because TL is measured by gradually heating a sample, the luminescence emissions correspond to electron traps of increasing thermal stability. The primary goal of this dissertation is to describe how this signal can be optimally measured and interpreted to understand the recent thermal history of bedrock samples.

I first modified the laboratory luminescence reader to allow us to irradiate feldspar samples at a range of dose-rates, from 8.7 x 10^-5 to 1.2 x 10^-1 Gy/s to estimate the influence of dose-rate on the subsequent TL signals. Although less stable sites were not preferentially populated at higher dose-rates (an unexpected result), I did observe an increase in brightness at the lowest dose-rate, a result which may suggest that the dose-rate influences recombination or trapping competition probabilities. In natural TL signals, I observed the expected trend of greater site occupancy at lower measurement temperatures with higher dose-rates.

Next, I perform several experiments to relate the TL signal that I monitor to OSL and IRSL from feldspar. I propose that the TL signal preserves a more detailed structure of trapping stability than the optically-stimulated signals, which derive from the full range of TL stabilities. Moreover, preheating and phototransfer effects may redistribute trapped charge, leading to potential inaccuracies in IRSL or post-IR IRSL thermochronology techniques.

The kinetic parameters involved in natural and laboratory conditions are of primary importance when using TL signals to quantify thermal history. I develop a novel method for determining the activation energy, effective frequency factor, and kinetic order values for natural and regenerative TL signals. This method, termed `post-isothermal TL' analysis, reveals that for the blue-green emission of the low-temperature TL peak, the apparent trap depth in measured bedrock K-feldspar samples increases to a depth of about >=1.9 eV as measurement temperature increases, at which point it reaches a plateau in some samples. If this plateau value is the true depth of the trap, the frequency factors are measured to decrease as measurement temperature increases, an observation consistent with the recent conception that feldspar luminescence (IRSL and TL) results from excited-state tunneling to randomly-distributed centers.

Three archived drill cores were sampled at depths corresponding to burial temperatures ranging from -4.1 to 60.2 °C. The extracted feldspars from these samples yield TL signals that clearly relate to temperature. With higher ambient temperatures, there is a linear increase in the T1/2 value (measurement temperature at half-maximum emission intensity) and a reduction in signal intensity. This behavior can be replicated by isothermal treatments in the laboratory. I interpret this behavior as reflecting the continuum of trap lifetimes present in feldspar TL, an observation that I substantiate with additive dose experiments and a numerical model.

How glaciers erode is an active research question that can be informed by feldspar TL thermochronology. I collected bedrock samples along vertical and longitudinal profiles within the Rock Creek glacial valley in the Beartooth Mountains of Montana to investigate their thermal history. Using the relationship observed with the drill core samples, I can predict the ambient temperature of these samples from their T1/2 values. The calculated values are indistinguishable from the historical record of mean annual air temperature from the local weather station. The shapes of the resulting TL signals imply that the samples are in disequilibrium, i.e., that the signals are still growing through time. By measuring the single-aliquot regenerative (SAR) dose-responses of the T1/2 values, I estimated the maximum time that each sample has been at its current surface temperature. These ages correlate with periods of local glacial activity and offer insight into the erosional mechanisms involved. In particular, I observe post-glacial high-elevation plateau erosion, which supports a key prediction of the glacial buzzsaw hypothesis.

Although a maximum age is useful, a more desirable solution is a continuous T - t history. The final chapter pursues this goal with samples taken from the Yucaipa Ridge tectonic block (YRB) of the San Bernardino Mountains in Southern California. I introduce a multiple-aliquot additive-dose (MAAD) measurement protocol that can be used to estimate the degree of dose saturation as a function of measurement temperature, n/N(T). This MAAD TL n/N(T) method capitalizes on the earlier observation of feldspar TL, that site stability increases with measurement temperature. Using the same kinetic model used to describe the drill core samples, I simulate two previously-proposed geologic cooling scenarios for the YRB (as constrained by apatite (U - Th)/He ages and catchment-averaged cosmogenic 10Be denudation rates) and the model is found to be sensitive enough to discriminate between them. I then measure MAAD TL signals for several YRB samples, convert these to n/N(T) functions, and use Monte Carlo simulations to invert for each sample's thermal history. The resulting T - t histories from these five samples tell an internally consistent story of samples nearest the fault-parallel valley cooling recently and those nearer the ridge having been at present temperature for a longer time. Despite the vertical relief being only about 0.4 km between the highest and lowest samples, the difference in trap saturation is significant, suggesting that this technique may be well suited to resolving Quaternary landscape evolution. I interpret the exhumation histories of these samples to reflect a combination of post-uplift relaxation of isotherms and a lagged erosional response in the form of fluvial downcutting.

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