Early Planetary Crust Formation:
The generation of Earth's continental crust sets our planet apart from other bodies in the solar system. Additionally, the presence of continental crust plays a large role in the chemical evolution of other reservoirs, including the atmosphere and oceans. A large portion of my research is aimed at answering key questions about when and how the continental crust was generated, and the implications for silicate planet evolution.
My PhD work focused on mapping and geochemical signatures present in 4.02-2.95 billion-year-old rocks within the Acasta Gneiss Complex. I integrated basic mapping, geochronology, petrology and advanced isotopic analyses to evaluate how the very ancient rocks preserved in this region, some of the oldest rocks known to exist on Earth, formed. My results, combined with a wealth of the geochemical data collected from the Acasta Gneiss Complex over the past decades, suggest that the Acasta Gneiss Complex developed by thickening and internal differentiation of a Hadean basaltic plateau, eventually initiating an episode of voluminous "mobile-lid" magmatism that may have served to stabilize and preserve the proto-craton (Reimink et al., 2018, Earth and Planetary Science Letters; Reimink et al., 2019, Chapter 15: Earth's Oldest Rocks, Second Edition).
Our schematic model for the formation of >3.75 Ga tonalites from the Acasta Gneiss Complex. This model integrates several different datasets, including whole-rock petrology, zircon U-Pb, O, and Hf-isotope geochemistry, and whole rock extinct nuclide information (142Nd). From Reimink et al., 2014, Nature Geoscience and Reimink et al., 2019 Earth's Oldest Rocks, Second Edition
At 3.6 Ga, felsic crust formation in the Acasta Gneiss Complex began to occur in a fundamentally different way. Hydrated, juvenile, mafic rocks were melted at great depth beneath the prexisting Hadean crustal block. This requires some form of "mobile-lid" tectonics whereby surface material is recycled at depth, but not necessarily indicative of global plate tectonics. From Reimink et al,. 2019 Earth's Oldest Rocks Second Edition
The Acasta Gneiss Complex is well-known because it contains the world's oldest known zircon-bearing rocks. However, this means that our model is difficult to test by looking at other rocks. Luckily, zircons are often preserved in younger sediments and >4.0 Ga zircons are continually being found in various locations around the world. I am currently testing the global applicability of our Acasta model using detrital zircons found in Neoarchean sediments. Thus far I have used zircon trace element data (Reimink et al., 2020 EPSL) and Hf-isotope data (Bauer, Reimink et al., in press, GPL) to compare to other locations globally.
I will continue to use new and existing zircon geochemical data to determine how the earliest crust on Earth formed, and what tectonic regime drove such crust formation. This will include using developing new geochemical indicators, analyzing existing global datasets, and generating new data from areas which have not been yet explored. As more and more regions with >4.0 Ga detrital zircons are described, developing robust analytical techniques and interpretations that can be applied to single detrital zircons will become extremely important.
The images to the left show 4.02 Ga zircons extracted from rocks found in the Acasta Gneiss Complex. The important point here is that Acasta zircons are found directly in their parental igneous rock, so that the compositions of the zircons can be directly linked to the whole rock composition (e.g., using the classic I-type to S-type magma comparison). Detrital zircons cannot be linked as directly, so the parental magma must be inferred. By analyzing Acasta zircons, I provide a template for linking old zircons to old rock compositions, which can be used to compare with new zircons as they are identified.
Left: The top image shows a very well-preserved 4.02 Ga zircon grain from the Acasta region. This grain has the classic "tree-ring" growth zones known as oscillatory growth, which indicates that the zircon grew from a magma.
Left: The bottom image also has some oscillatory growth zones (on the top of the grain) but those growth zones are overprinted and washed out in the lower left region of the grain. This overprinting is associated with cracks running down the grain, cracks that can be very difficult to observe at times. The point is that getting good data from ancient grains can be difficult and requires careful documentation of all features of the grains.
Right: This figure shows the distribution of Ti-in-zircon temperature estimates from the Acasta zircon suites (my new data) compared to ancient detrital zircons from the Jack Hills region (data compiled from several sources) as well as those from modern Iceland (data from Carley et al., 2014). The Ti content of zircon is in part controlled by the temperature that the zircon crystallizes. Low Ti contents of zircons from the Jack Hills region have been used to argue that these very ancient zircons crystallized from minimum-melt granites, at the solidus. However, we show that zircons found in tonalites, trondhjemites, and granodiorites (three rock types common in the Archean) have very similar Ti contents and similar calculated temperatures of crystallization. In fact, the composition of Acasta zircons is very similar to zircon from the Jack Hills region across many geochemical parameters, both elemental and isotopic (Reimink et al., 2020, EPSL).