Date of Award

June 2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Scott D. Samson

Keywords

Apatite, Detrital, Geochemistry, Isotope, Provenance, Zircon

Subject Categories

Physical Sciences and Mathematics

Abstract

Our understanding of the evolution of the Earth’s surface is driven by our knowledge and comprehension of the processes which shape the landscape. The source, formation, and transport pathways of sediment are critical components to understanding crustal processes and changes and are often grouped under the broad scope of provenance analysis. Much of what we know of how sediments evolve and shape our landscape stems from provenance analysis, which tries to trace the pathway of source to sink. However, complexities arise when the final composition or signature of the detrital material is altered, which results in provenance signature deviations from source rock to the sediment. Weathering, erosion, mixing, recycling, sorting, diagenesis, and/or lithification can all drive these changes in sediment signatures.

In the following chapters, I test both the strengths and limitations of some of the commonly used fingerprints in detrital analysis to a single case study of the Stepladder pluton. The Cretaceous Stepladder pluton (SE California) provides a unique environment to test some of the most basic assumptions of provenance analysis. The pluton should act as a point source for sediments collected downslope, which have experienced limited transport and have been deposited in in an arid (limited chemical alteration) environment.

In Chapter 1, I present zircon U-Pb age data of the Stepladder pluton and derived sediments. With limited transport and an essentially unimodal age source, sediments derived from the inselberg should theoretically have matching age distributions with the pluton itself. However, our results show an unexpected and dramatic difference in age distributions between bedrock and detrital samples. A clear secondary source, one unassociated with the ‘upstream’ bedrock, dominates the detrital distributions. The differences in age distributions between the source and sinks suggest that interpreting parent rock assemblages and paleogeographic reconstructions solely on zircon geochronology can be misleading or contain significantly more complexity than what researchers assume.

To better constrain the provenance of the Stepladder Pluton sediments, I present dual characterization of single-grain apatite in chapter 2 as a new potential fingerprint for detrital provenance. 87Sr/86Sr ratios and (U-Th)/He ages are two independent isotopic signatures. The first provides the diagnostic magmatic signature, while the second provides a low temperature thermochronometer, which describes the exhumation or cooling history of the apatite. The focus of this chapter is to present the methodological protocols used to obtain the 87Sr/86Sr ratios and (U-Th)/He ages, and to describe the various tests we used to determine the source of Sr excess developed during He degassing. Further work is needed to determine a Sr correction factor or search for an alternative He degassing method, as (U-Th)/He ages are not affected by Sr chromatography, but Sr isotopic ratios are clearly altered during the degassing process.

Research presented in chapter 3 focuses on using apatite and whole-rock geochemistry as fingerprints for provenance studies. Here, I collected whole-rock 87Sr/86Sr ratios, apatite 87Sr/86Sr ratios, and apatite (U-Th)/He ages from both the Stepladder pluton and its derived sediments. We find that minimal variations in whole-rock initial 87Sr/86Sr ratios of the bedrock are likely due to heterogeneities within the Stepladder pluton, while the much larger observed variations in detrital sample ratios are due to hydraulic sorting of altered biotite. Single grain and multi-grain apatite isotopic signatures are not affected by hydraulic sorting, and thus may provide a more accurate determination of source rocks. Strontium isotopic ratios of most detrital apatite grains are similar to that of the bedrock from which they are derived. Igneous and detrital apatite (U-Th)/He ages are also notably similar, confirming with two independent isotopic systems that apatite is a more robust fingerprint of source than zircon and whole-rock geochemistry.

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