Application of the U-Pb, Lu-Hf, and Sm-Nd Isotope Systems to Understand Crust-Mantle Evolution on Early Earth in the Pilbara Craton
Creators: name: Salerno, Ross A
Publisher: name: Washington State University
Year: 2025
Subjects: Archean
Garnet
Geochronology
Hf isotopes
Nd isotopes
Pilbara

description: The origin of the early felsic crust has been a prominent issue in geology since the age of the Earth was first established. Radiogenic isotopes have been the primary tool for addressing this question, but there is still significant uncertainty in our interpretation of the early Archean geological record. The remaining ambiguity is often related to the pervasive thermal and tectonic overprinting, which has altered or erased the primary histories of most Archean rocks. However, the Pilbara Craton of Western Australia is a well-preserved Paleoarchean terrane, making it an ideal natural laboratory to study the processes related to the growth and stabilization of the early continental crust. This dissertation focuses on the processes surrounding the origin and preservation of Paleoarchean felsic continental crust in the Pilbara Craton. We examine the Hf isotope compositions of whole rocks and zircon, as well as Nd isotope compositions of whole rock, apatite, and titanite, within several well-preserved granitic rocks from the Mt. Edgar Granitic Complex. By integrating whole rock and in situ isotope data in single samples, we present an approach that demonstrates that these samples have remained closed systems and therefore reflect the isotope compositions of their parent melts, where all samples have εHf(i) ≈ εNd(i) ≈ 0. We also examine the deformation and metamorphic history of the supracrustal rocks in the Warrawoona Syncline, directly adjacent to the Mt. Edgar Complex, by integrating garnet Lu-Hf Sm-Nd geochronology with microstructural analyses. These data record the timings of several Paleoarchean crustal overturn events, closely related to the intrusion and stabilization of large volumes of sialic rock in the upper crust. Furthermore, we present a dataset integrating whole rock and in situ Hf-Nd isotope data, U-Pb geochronology, and bulk rock major and trace element chemistry in a suite of 3.46 to 3.25 Ga TTG-like rocks from the Mt. Edgar Granitic Complex. We show that the vast majority of these rocks have broadly chondritic initial Hf and Nd isotope compositions and a range of trace element compositions. These data reflect that the granitic magmas were extracted from a source with broadly chondritic Lu/Hf and Sm/Nd and that these melts were sourced over a range of depths in the middle crust. The implications are three-fold: in situ Sm-Nd isotope data provide essential context for interpreting closed whole rock systems and the Hf-Nd isotope record of early Earth; demonstrably undisturbed Paleoarchean rocks show no evidence for early (&gt;4.0 Ga) significant crust-mantle differentiation; and these granitic rocks are juvenile, indicating significant crustal growth in the Pilbara Craton during the Paleoarchean. Moreover, as these granitic rocks have juvenile Hf-Nd isotope compositions, this implicitly relates the timing of crustal overturn events in the Paleoarchean with episodes of crustal growth.