Adrian Finch, Nicola Horsburgh, Anouk Borst, and William Hutchinson from the University of St Andrews have worked together to complete a detailed study which investigated a sulphur fractionation in a Mesoproterozoic alkaline province in Greenland.
Published in Nature Communications last month, the study offers an alternative method to previous research in the quest to determine what happens to the crust when it enters the mantle.
In a statement made by the University of St Andrews, their research “reveals the fate of the Earth’s ancient crust” which “could help solve the mystery of how the Earth’s surface and mantle are connected.”
When a tectonic plate is subducted due to convergence, one plate is pushed down deep into the mantle. This research aims to discover the ultimate fate of this crust, as this is the key to understanding geochemical recycling as well as myriad effects on magma composition, plate tectonics, mantle dynamics and heat flow.
In their investigation, the team was able to identify crustal material in mantle-derived rocks, supporting the theory of crustal recycling.
Subducting portions of crust at sites of tectonic convergence carry volatile samples of water and other materials into the mantle. Therefore, as this crust has different isotopic signatures to the primitive mantle, it is possible to determine whether it resurfaces, i.e. whether it is “recycled”.
In the past, the most significant insights into the connection between the surface and mantle sulphur reservoirs have been gained through studying oceanic hotspots, particularly ocean island basalts (OIBs).
However, their restricted age rate(<200 Ma) means OIBs cannot reveal temporal variations in crustal recycling over the Earth’s history. Sulphur is associated with recycled sources and are found throughout the geological record in alkaline rocks.
This alternative method of study provides a more accurate way of identifying occurrences of crust recycling. The scientists from the University of St Andrews have provided work which demonstrates that alkaline rocks are a powerful data-set for understanding the relationship between the surface and mantle sulphur reservoirs.
Lead author Dr William Hutchinson, from the School of Earth and Environmental Sciences, said, “The beauty of our global dataset is that it extends back over two billion years and so these unique alkaline rocks represent an extremely powerful record of understanding crustal recycling over Earth history.”
This statement emphasises the advantages of studying alkaline rocks. The alkaline rock’s low viscosities, densities and temperatures promote their rapid rise to the surface, reducing any evidence of crustal contamination.
Their paper, titled “Sulphur isotopes of alkaline magmas unlock long-term records of crustal recycling on Earth”, provides a new perspective in understanding the fate of the Earth’s crust once it enters the mantle.
Their works brings Earth Sciences that much closer to answering the long-unanswered question: “How are the Earth’s surface and mantle connected?”