Current estimates for the composition of the lower continental crust show significant variation for the concentrations of the highly incompatible elements, including large uncertainties for the heat-producing elements. This has consequences for models of the formation of lower crust. For example, is lower continental crust inherently poor in incompatible elements or has it become so after extraction of partial melts caused by thermal incubation? Answering these questions will require better agreement between estimates for the chemistry of the lower crust. One issue is that granulite samples may have been altered during ascent. Xenoliths often experience contamination from the entraining alkaline magma, potentially resulting in elevated concentrations of incompatible trace elements when analysed by conventional bulk rock techniques. To avoid this, we assessed an in situ approach for reconstructing whole rock compositions with granulites from the Kapuskasing Structural Zone, Superior Province, Canada. As terrain samples, they have not been affected by host magma contamination, and as subrecent glacial exposures, they show minimal modern weathering. We used scanning electron microscope electron dispersive spectroscopy (SEM-EDS) phase mapping to establish the modal mineralogy. Major and trace element concentrations of mineral phases were determined by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), respectively. These concentrations were combined with the modal mineralogies to obtain reconstructed whole rock compositions, which were compared to conventional bulk rock analyses. The reconstructed data show good reproducibility relative to the conventional analyses for samples with massive textures. However, the conventional bulk rock chemistry systematically yields higher K concentrations, which are hosted in altered feldspars. Thus, even in terrain samples, minor alteration can lead to elevated incompatible element estimates that may not represent genuine lower continental crust.

Much of the continental lithosphere developed during the Archean, which was an Eon of change in terms of global geodynamics and geochemical cycles. Uncovering the causal links between crust forming processes and prevailing geodynamic mechanisms is crucial for understanding the origins and composition of the present-day continental lithosphere. Pristine Archean crust is scarce yet can be found in cratons worldwide. Many of these occurrences comprise rocks of the tonalite-trondhjemite-granodiorite (TTG) suite, which represent a prevalent component of the Archean continental crust. TTGs are generally considered to have formed by partial melting of amphibolite or eclogite source rocks that had basaltic precursors originally extracted from a depleted mantle (e.g., [1]). The age of the source rocks (i.e., the time between the basalt extraction from the mantle and TTG formation) can be determined from the initial radiogenic isotope compositions of TTGs, provided that the P/D ratio of the source can be reliably estimated and is significantly different from that of the depleted mantle.Based on this principle, we estimated the age of basaltic sources of TTGs from cratons of different age and paleogeography from initial 87Sr/86Sr compositions determined by in-situ Sr isotope analysis of primary igneous apatite (LA-MC-ICPMS). The 87Sr/86Sr of these apatites show that prior to 3.4 Ga TTGs were derived from relatively old mafic sources and that the average time between formation of basaltic material from the mantle and subsequent remelting under amphibolite to eclogite facies conditions decreased drastically during the Paleoarchean. This secular change indicates a rapid global increase in the efficiency of TTG production or the emergence of a new TTG-forming process at c. 3.4 Ga [2].In this contribution we explore this hypothesis by comparing the 87Sr/86Sr signature of the TTGs with their trace-element compositions, as well as with 176Hf/177Hf zircon data for these rocks and contemporary TTGs from other studies. This combined geochronological, isotope and geochemical analyses will provide new constraints on the age of TTG sources during the Archean and will allow investigation into the nature and probable causes of the apparent rejuvenation at 3.4 Ga, as indicated by Sr isotopes.[1] Hoffmann, J.E. et al. (2011) Geochim. Cosmochim. Acta 75, 4157-4178.[2] Caton, S., et al., (in review) Chem. Geol.

This thesis investigated the poorly constrained composition and evolution of the lower continental crust in Queensland through geochemistry, geochronology, and petrological modelling. The analysis of deep crustal rocks brought to the surface by explosive volcanism shows that the lower crust in Queensland is nearly devoid of many geologically significant trace elements. The modelling demonstrates that this lower crust formed through solid-melt interactions at the base of the crust. The combined data show that this depleted, hybridised crust could be more common than previously recognised and that hybridisation is likely a major driver of continent formation.

Compared to the well-studied upper continental crust, the composition of the lower crust is much more poorly constrained. Geophysical constraints and geochemical data from granulite xenoliths indicate that the lower crust is, on average, mafic and depleted in most incompatible elements, including the heat-producing elements (HPE). However, the extent of this depletion is not well known. The large uncertainties associated with lower crustal estimates have important implications for the Earth’s evolution, as the lower crust is often proposed to be a “hidden reservoir” (e.g., for unradiogenic Pb) needed to close mass balance discrepancies for the Bulk Silicate Earth.In this study, we analysed granulite xenoliths from Queensland, eastern Australia, and the Kola Peninsula, northwest Russia, using a reconstitution approach that corrects for host magma contamination. This method also provides detailed insight into which minerals control elemental distribution and concentrations of the xenoliths. The major element compositions of both suites of granulite xenoliths highlight their mafic nature, with SiO2 contents similar to previously published estimates. However, the concentrations of the most incompatible elements, including the large ion lithophile elements (LILE) and HPE, are very low. Some elements are more depleted by an order of magnitude than the most popular composites used in the literature. Zircon and monazite are rare in these mafic granulites, while apatite and rutile have relatively low Th and U concentrations. The absence of hydrous silicates (e.g., mica and amphibole) and the relatively high anorthite contents of feldspar in the xenoliths is a controlling factor in the low LILE concentrations, particularly for Rb and Cs. If this composition is representative of typical lower continental crust, then such highly refractory compositions limit the ability of the lower crust to act as a significant contributor for planetary mass balance considerations because it does not contain enough Pb, Nb, Ta, Cs and Rb to balance other inventories of the differentiated bulk silicate Earth.