Suchergebnisse

Plate subduction and delamination, two key processes driving plate tectonics, are thought to be controlled by the buoyancy of the lithospheric mantle relative to the underlying asthenosphere. Most mantle delamination models consider a lithospheric density higher than the asthenosphere to ensure negative buoyancy (slab pull). However, mineral physics show that the continental lithospheric mantle density is lighter than the asthenosphere, and that only its pressure-temperature-composition dependence makes it become denser and unstable when sinking adiabatically. Here, we explore the controls on buoyancy using a 2D thermal-diffusive model of plate convergence, considering five chemical compositions and tectonothermal ages, namely Archon (>2.5 Ga), Proton (2.5–1.0 Ga), Tecton (<1.0 Ga), and two oceanic lithospheric plates of 30 Ma and 120 Ma. While the advection of colder rock in oceanic-like plates always results in negative buoyancy, Protons and Tectons exhibit an ability to slowly flip from negative to positive buoyancy at low convergence rates: they first favour the sinking due to advection and then become more buoyant because they are thinner and heat up faster during subduction. In contrast, the lighter density of cratons overprints this effect and hinders delamination or subduction, regardless of the convergence rate. This may explain why Archons are more stable during the Wilson cycle. ; This work has been supported by EU Marie Curie Initial Training Network 'SUBITOP' (674899-SUBITOP-H2020-MSCA-ITN-2015) and MITE (Spanish Government national research program; CGL2014-59516). ; Peer reviewed

Mantle plumes are hot buoyant upwellings that rise from Earth's core-mantle-boundary to its surface where they can produce large igneous provinces (LIPS) and volcanic tracks, such as the Siberian Traps and the Hawaiian Emperor chain, respectively. We show that flattened mantle plume heads, which can have radii of >1200 km in the uppermost mantle, can heat the overlying lithospheric mantle to temperatures above the diamond stability field. As a consequence, they can destroy diamonds within the roots of Archean cratons, the principal source of diamonds in kimberlites. We quantitatively demonstrate that there is a 'sour spot' for this effect that occurs when lithospheric thicknesses are 165-185 km and the plume has a temperature of >150 degrees C above background mantle. Our model explains why the kimberlites associated with the 370 Ma Yakutsk-Vilyui plume in the Siberian craton are diamondiferous whilst those associated with the younger 250 Ma Siberian Traps plume are barren. We also show that the time required to restore the pre-plume thermal structure of the lithosphere is ca. 75-120 Myr, and that destroyed diamonds may regrow once the plume's thermal effect dissipates. The 1100 Ma Kyle Lake and adjacent 180-150 Ma Attawapiskat kimberlites in the southern Superior craton exemplify this, where the older kimberlites are associated with a narrower diamond window (<30 km) in comparison with the ca. 85 km diamond window of the younger Attawapiskat field. ; REE has been partially supported by Mega-Grant 14.Y26.31.0012 of the government of the Russian Federation. DRD was funded by an ARC Future Fellowship (FT140101262). DRD and IHC were funded by ARC Discovery Grant DP17010058.