Scientists Track Earth's History Through Durable ZirconsNew Study Reveals Ancient Earth Processes Through Zircon Analysis. Credit: scitechdaily.com

Utilizing detrital zircons, a recent study reveals the ancient geological processes that have long shaped our Earth, from crust-to-mantle recycling to the formation of supercontinents. Leading the research was Dr. Rui Wang and his Ph.D. student Shao-chen Wu from the Institute of Earth Sciences at China University of Geosciences in Beijing, along with Dr. Roberto Weinberg and Dr. Peter Cawood from Monash University, and Dr. William Collins from Curtin University.

Zircons, a mineral that dates back to the dawn of Earth, form when molten rocks cool and are found in small amounts in magmatic rocks. These mountains are created by the formation of magma within the Earth and are eventually broken down into sediments through processes involving water and the atmosphere.

Due to their durability and resistance to weathering and erosion, zircons remain preserved and hold valuable information about the history of our planet. By analyzing the uranium content within zircons through U-Pb dating, researchers can gain insight into various geological phenomena, such as the oxidation state of the Earth.

The team developed a new method, based on the work of Loucks et al (2020), that uses the ratios of cerium, uranium, and titanium in zircons to track changes in the oxidation state of crustal magmas throughout Earth's history. Unlike previous methods, this calculation does not require knowledge of the ionic charge or specific details about the magma's composition.

According to Bob Loucks from Western Australia, previous methods like the Ce/Ce* and Eu/Eu* oxybarometers had limitations related to temperature, pressure, and chemical variations in the host rock, as well as the precision of rare earth elements needed for the measurements.

This improved oxybarometer allows for a more accurate assessment of changes in the oxidation state, which can then be interpreted in terms of global tectonic changes over time. By determining the oxidation levels of the magmas that formed the detrital zircons, scientists can track the onset of crust-to-mantle recycling, weathering, and the supercontinent cycle.

The research reveals that rocks at the Earth's surface can be carried deep into the mantle (hundreds to thousands of kilometers below the surface), indicating ongoing recycling processes that have been occurring for billions of years. By examining zircons ranging from 3 billion years old to those formed in present-day, the team discovered a consistent increase in the redox state (expressed as ΔFMQ) of the magmas in which they formed.

Around 3.5 billion years ago, the oxidation state started to rise, and for the past 3 billion years, it has remained at a consistent average of ΔFMQ > 0. This suggests that the oceanic lithosphere has been recycled back into the mantle through subduction zones. Interestingly, the lower limit of the redox state dropped significantly 2.6 billion years ago, coinciding with the formation of well-defined continents and the burial of oceanic rocks into the deep mantle.

Furthermore, the team observed a cyclical pattern in the redox state, with a supercontinent forming every 600 million years. This supports the idea of a supercontinent cycle, where continents come together and then break apart, forming new supercontinents over time, such as Gondwana, Rodinia, Nura, and Superia.

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