This project uses isotope geochemistry to determine the age and characteristics of basement rocks of the Nuvvuagittuq belt in Northern Quebec. These rocks may be up to 4.28 billion years old, making them amongst the oldest on earth. They have the potential to offer insight into processes of differentiation that occurred early in the earth?s history, such as the start of plate tectonics and determining whether the earth had a magma ocean. The broader impacts include training of a postdoctoral fellow in isotope geochemistry, and outreach to the general public and an indigenous Inuit community living in the field area.
This project supported sample collection and chemical and chronological studies of rocks from the Nuvvuagittuq region of northern Quebec, on the eastern shore of Hudson’s Bay. General regional geologic mapping by the Ministere des Resources Naturelles du Quebec in the early 2000’s discovered a small (~10 km2) area of old volcanic rocks roughly 25 km south of the Inuit village of Inukjuak. Granitic rocks surrounding the volcanic rocks gave ages of 3.6 billion years and provided a lower limit to the age of the volcanic rocks. The volcanic rocks could not be dated directly because they do not contain the U-rich mineral zircon that is routinely used to determine the age of rocks. Our project applied a different radioactive clock based on the decay of the mass 146 isotope of the element samarium to the mass 142 isotope of neodymium. The advantage of this radioactive clock for this application is that the type of volcanic rock in Nuvvuagittuq contains plentiful samarium and neodymium, but more importantly that because of its rapid decay (68 million year half life), the 146-Samarium that was present at Earth formation completely decayed away by around 4 billion years ago. This system is used routinely to date meteorites that formed over 4.5 billion years ago, but ours was the first application to date Earth rocks, mostly because most Earth rocks are less than 3.8 billion years old and hence did not contain 146-Samarium when they formed. The first step supported by this grant allowed the two project participants to spend two weeks in the Nuvvuagittuq area in July of 2009 to collect samples for dating. During the field trip, we also discovered a structure, known as pillow basalts, that forms when lava flows into liquid water showing that these volcanic rocks flowed into either a large lake or ocean existing at the time of their eruption. Chemical analyses of the collected volcanic rocks show them to split into three different groups with compositional similarities to rocks erupted in modern time in areas where oceanic plates are sinking back into Earth’s interior (e.g. Cascades, Japan). Our age dating defines the sequence of formation as follows: All three compositional groups of volcanic rocks were erupted between 4.3 and 4.4 billion years ago, making these the oldest preserved rocks on Earth and similar in age to the oldest rocks found on the Moon. This group of volcanic rocks was then intruded by new magmas of roughly similar composition 4.1 billion years ago. The whole belt was then swamped by large volumes of silica-rich magmas that intruded the area in 3 events at 3.76, 3.66 and 3.51 billion years ago. The whole area was then buried to of order 20-30 km depth at 2.6 to 2.7 billion years ago when it was also highly deformed. The Nuvvuagittuq belt thus records almost 1.8 billion years of Earth history dating back to shortly after Earth formation. One implication of these results is that Earth’s surface only 170 million years after planet formation had reached conditions not unlike those present today where liquid water was present. The work was featured in the "Birth of the Earth" episode of the History Channel’s chronicle of "How the Earth was Made" that included film footage of postdoctoral fellow Jonathan O’Neil in the field in Nuvvuagittuq.
