A Homogeneous Primordial Mantle Revealed by Kimberlites?

Diagrammatic cross-section of the Earth’s mantle, showing its chemical zones (the core is not shown) and a plate-tectonic subduction zone and related volcano. Kimberlite lavas come from the lower mantle, which Woodhead, et al. show is ancient and chemically primitive. Credit image: University of Melbourne

The widely accepted model of the Earth’s evolution holds that its mantle has melted more or less continuously over time, and that its composition has changed over time. However, a new study by Jon Woodhead and others shows that a part of the Earth’s mantle has been mostly isolated from these processes for at least the last 2.5 billion years. The study analyzed the radioactive isotope characteristics of kimberlites, a rare kind of igneous rock that erupts from deep in the Earth’s mantle. Surprisingly, kimberlites of all ages from around the world originated in chemically similar and primitive mantle material, pointing to a common or uniform and chondritic-like volume of the mantle. The results suggest this primordial mantle reservoir has remained isolated from a continually evolving terrestrial mantle for the last 2.5 billion years. The study further suggests that the same isolated source may have been perturbed around 200 million years ago, possibly coinciding with subduction along the margins of the Pangaea supercontinent. If so, the conclusions provide a mechanism to explain why uncontaminated primordial mantle is not sampled during recent mantle-derived melts. READ MORE