Compositional evolution of Earth's upper mantle shaped by plate tectonics

Compositional evolution of Earth's upper mantle shaped by plate tectonics
by Simon Mansfield
Sydney, Australia (SPX) Oct 17, 2024

The ongoing process of plate subduction on present-day Earth continuously alters the chemical makeup of the convecting mantle, with various mantle sources linked to these processes being extensively studied. Yet, questions remain about when the global chemical diversity of the mantle first emerged and how Earth's geodynamic evolution has shaped its composition over time.

A research team from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), along with collaborators from Australia, Switzerland, and the USA, has tackled these questions by compiling extensive geochemical and isotopic data from intracontinental basaltic rocks spanning the past billion years.

"This study sheds light on the long-term cycling of materials within Earth's interior over geologic time scales," stated Prof. LIU He of IOCAS, the corresponding author of the study. The research findings were published in *Science Advances* on Oct. 16.

The analysis revealed that intracontinental basalts exhibiting neodymium (Nd) isotope-enriched geochemical signatures (eNd < 0) began appearing around 300 million years ago. This period also coincides with the emergence of kimberlites containing signs of crustal material involvement, indicating a global compositional change in the mantle.

Observations showed that the locations of these enriched basalts and kimberlites were not influenced by their proximity to subduction zones. This led researchers to propose that the enriched geochemical signatures originated from older, more distant subduction events.

The modern style of plate tectonics, which includes continental crust subduction and deep slab break-off, is believed to have started in the late Neoproterozoic, approximately 700-600 million years ago. Under this tectonic regime, extensive cold subduction and crustal subduction, combined with increased subduction flux during supercontinent formation, introduced crustal and upper mantle materials into the lower mantle. After over 300 million years, these subducted materials could return to the upper mantle through mantle upwellings.

"This process may have fundamentally altered the composition of the convecting mantle and contributed to the formation of enriched magmas, eventually resulting in global-scale chemical heterogeneity of the mantle," explained Dr. CHEN Qian, the study's first author from IOCAS.

The emergence of enriched intraplate magmas approximately 300 million years after the onset of modern plate tectonics provides valuable insights into the processes driven by tectonic activity that contribute to the chemical diversity of the Earth's mantle. "The time frame aligns with models suggesting that it takes considerable time for subducted materials to influence the upper mantle composition through these dynamic processes," added Prof. LIU.

Research Report:Global mantle perturbations following the onset of modern plate tectonics