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Doctorat
États-Unis
2017
Isotopic Characterization of Bedrock, Sediments, and Minerals as Provenance Tools : Examples from an Inselberg in the Mojave Desert of SE California
Titre : Isotopic Characterization of Bedrock, Sediments, and Minerals as Provenance Tools : Examples from an Inselberg in the Mojave Desert of SE California
Auteur : Bonich-Wissink, Mariana.
Etablissement de soutenance : Syracuse University
Grade : Doctor of Philosophy (PhD) in Earth Science 2017
Résumé partiel
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.
Page publiée le 7 septembre 2017, mise à jour le 19 mars 2018