image: Zircons, a mineral nearly as old as Earth itself, is a time keeper, and also provides a chemical window into many geological phenomena, such as oxidation state. By determining the oxidation levels of the magmas that formed these detrital zircons, scientists are able to deduce the onset of crust to mantle recycling, weathering, and the supercontinent cycle.
Credit: ©Science China Press
This study is led by Dr. Rui Wang and his PhD student Shao-chen Wu (Institute of Earth Sciences, China University of Geosciences, Beijing), Dr. Roberto Weinberg and Dr. Peter Cawood (Monash University), and Dr. William Collins (Curtin University).
Zircons, a mineral nearly as old as Earth itself, crystalize when magmas (molten rocks) cool and can be found in trace quantities in magmatic rocks. The formation of magmas constitutes the mountains in the Earth. Through interactions with water and atmosphere, the mountains break down into sediments. Zircons are so durable and resistant to weathering and erosion that they rarely go away, and therefore this mineral in sediments (so call “detrital zircons”) holds the greatest insight into the history of the Earth. Zircon enriches with U (U-Pb dating) is a time keeper, and also provides a chemical window into many geological phenomena, such as oxidation state.
The team uses a new method of Loucks et al (2020) for determining the oxidation state of granitic magmas that uses ratios of Ce, U, and Ti in zircon to track oxidation state change of crustal magmas through Earth history. The calculation does not require ionic charge to be known, nor is determination of crystallization temperature, pressure, or parental melt composition required.
“Previous methods include Ce/Ce* and Eu/Eu* oxybarometers, but each has limitations related to temperature, pressure, host rock chemical compositional variations, or precision of REE elements needed to measure the Ce/Ce* and Eu/Eu* anomalies.” Bob Loucks from Western Australia says.
This improved oxybarometer allows a more confident evaluation of the variation in oxidation state, which can now be interpreted in terms of global tectonic changes with time. By determining the oxidation levels of the magmas that formed these detrital zircons, scientists are able to deduce the onset of crust to mantle recycling, weathering, and the supercontinent cycle.
The key point is that rocks that lay at the Earth's surface can be carried back down to deep in the Earth's mantle (hundreds to thousands of km below the surface. Our data shows that not only has this happening today but could have been going on for billions of years. Looking at zircons from the early Earth, 3-billion-year-old zircons, to those formed today we have found that the redox state of the magmas in which they formed. The oxidation state (expressed as ΔFMQ) of the detrital zircons rise at ~3.5 billion years followed by a consistent average ΔFMQ > 0 over the last 3 billion years, suggesting recycling of oceanic lithosphere back into the mantle in what eventually became established as subduction zones. It shows that the lower limit of redox state dropped dramatically at 2.6 billion years ago, marking the formation of well-defined continents and the burial of oceanic rocks back into the deep mantle of the Earth. Further to that we found a cyclicity of the redox patterns: every 600 million years or so, continents come together to form supercontinents, like Gondwana, Rodinia, Nura, and Superia.
Journal
Science Bulletin