Research

Zoned apatite from Vermeesch (2017)

Uranium Zoning Evaluation for Fission Track Dating

Uranium-238 is a radiogenic isotope that decays to lead via a series of alpha and beta decays. However, about 1 in 2 million uranium atoms undergoes spontaneous fission, where the nucleus of the uranium atom break apart and produces a ~15 micron long damage trail known as a fission track. I measure fission tracks in a calcium-phosphate mineral called apatite. In order to get an age for a given sample, I need to know both the track density (the number of tracks) and the concentration of uranium in the apatite grain we are analyzing. We measure uranium using laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS). The easiest and cheapest way to do this is to take a single spot analysis with the laser. However, a single spot analysis will not be representative if the grain is zoned, meaning the uranium concentration across the grain is not equal. In order to avoid mapping every grain for uranium (a timely and costly endeavor), we need to be able to say with statistical certainty is a grain is zoned or not.

To address this gap, I am currently developing a Python-based computational tool designed to assess zoning on the surface of a grain using the fossil fission track distribution as a proxy for uranium concentration. The program processes coordinate data representing fission tracks on the mineral internal surface and applies a statistical analysis using a correlation function to quantify spatial relationships between tracks and assess clustering and randomness. The program generates a series of random models with the same number of tracks and counted area and compares these distributions to the sample grain to calculate the probability that the tracks exhibit non-random clustering indicative of zoning. This allows the user to target grains for further investigation or alternative treatment, such as additional laser spot measurements. Most importantly, the zoning information revealed at the grain surface can be directly compared to the uranium concentration gradient obtained from LA-ICP-MS depth profiling. When both surface and vertical zoning are considered in tandem, fission-track thermochronologists can build a more comprehensive picture of the uranium zonation in all directions, paving the way for developing an improved basis for assigning uncertainties to single-grain ages.

Thermochronology of the Yukon-Tanana Terrane, Interior Alaska

The late-stage tectonic evolution of the Yukon-Tanana upland (YTU) in interior Alaska is complex and has received limited attention, due in large part to the inaccessibility of the area. The overlap of major tectonic events in the Eocene, including the development of regional strike-slip faults (Tintina and Denali) and oroclinal bending, complicates interpretations of geological datasets from this time. For my PhD project, I am investigating the upper crustal structure of the YTU from the Cretaceous to the Eocene using low-temperature thermochronology (apatite fission track and K-feldspar Ar/Ar). With these methods, I study the low-temperature cooling history of rocks (~300 - 20° C), which allows us to answer questions about tectonics and erosion. The ultimate goal of my project is to provide constraints on latest-stage tectonic configuration of the interior with the broader interest of how this all fits into the geologic history of the rest of Alaska.

Structural Evolution of the Hatcher Pass schist, Talkeetna Mountains, Alaska

The Hatcher Pass schist (HPs) is a regionally retrogressed chlorite-muscovite schist with plagioclase and garnet porphyroblasts located in the Willow Creek area in the southernmost Talkeetna Mountains of south-central Alaska. Most of south-central Alaska represents a subduction-accretion complex built upon the southern Wrangellia composite terrane. My MSc project was focused on constraining the origin and evolution of this schist in the context of regional southern Alaskan tectonics. We utilized geothermobarometry and forward modeling to provide constraints on the metamorphic evolution of the HPs, and conducted a detrital zircon analysis to better understand the nature of the protolith (Valdez group - turbidites).

WDS maps from the main foliation of the HPs generated from EPMA.

Structural observations in the field and petrologic data from the HPs suggest it was exhumed above a slab window in the Talkeetna Mountains. Spreading ridge subduction has been the presumed mechanism for the opening of a slab window under south-central Alaska. However, the historical "smoking gun" evidence for the opening of this slab window includes time-transgressive W-E plutonism across southern Alaska (the Sanak-Baranof plutonic belt). There is no evidence of these processes north of the large Border Ranges Fault where the HPs is located, so we suggest slab breakoff (first proposed by Terhune et al., 2019) as an alternative mechanism for the opening of a slab window in the southern Talkeetna Mountains. In our model, the lithosphere was weakened by high heat flow from the slab window, and as a result the HPs was exhumed along one of the many extensional faults concentrated in the hinge of the Alaska orocline.

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