Hemispheric Geochemical Dichotomy of the Mantle Is a Legacy of Austral Supercontinent Assembly and Onset of Deep Continental Crust Subduction

• Published in: AGU advances 2022-12, Vol.3 (6), p.n/a
• Main Authors: Jackson, M. G., Macdonald, F. A.
• Format: Article
• Language: English
• Subjects: Continental crust; Dupal; Geochemistry; Gondwana; hotspot; Isotopes; mantle plume; Metamorphic rocks; Pangea; Plate tectonics; Sampling
• ISSN: 2576-604X; 2576-604X
• DOI: 10.1029/2022AV000664

Oceanic hotspots with extreme enriched mantle radiogenic isotopic signatures—including low 143Nd/144Nd indicative of subducted continental crust—are linked to plume conduits sampling the southern hemispheric mantle. However, the mechanisms responsible for concentrating subducted continental crust in the austral mantle are unknown. We show that subduction of sediments and subduction eroded material, and lower continental crust delamination, cannot generate this spatially coherent austral geochemical domain. However, continental collisions—associated with the assembly of Gondwana‐Pangea—were positioned predominantly in the southern hemisphere during the late Neoproterozoic appearance of widespread continental ultra‐high‐pressure metamorphic terranes, which marked the onset of deep subduction of upper continental crust. We propose that deep subduction of upper continental crust at ancient rifted‐passive margins during ca. 650‐300 Ma austral supercontinent assembly resulted in enhanced upper continental crust delivery into the southern hemisphere mantle. Similarly enriched mantle domains are absent in the boreal mantle plume source, for two reasons. First, continental crust subducted after 300 Ma—when the continents drifted into the northern hemisphere—has had insufficient time to return to the surface in plumes sampling the northern hemisphere mantle. Second, before the first known appearance of continental ultra‐high‐pressure rocks at 650 Ma, deep subduction of upper continental crust was uncommon, limiting its subduction into the northern (and southern) hemisphere mantle earlier in Earth history. Our model implies a recent formation of the austral enriched mantle domain, explains the geochemical dichotomy between austral and boreal plume sources, and may explain why there are twice as many austral hotspots as boreal hotspots. Plain Language Summary Volcanic hotspots with plume sources in the southern hemisphere exhibit enriched geochemical signatures—including low 143Nd/144Nd—associated with subducted upper continental crust in the Earth's mantle, but northern hemisphere plumes sources do not. The reason for this hemispheric geochemical dichotomy is not known. We show that the late Neoproterozoic to Paleozoic continental collisions associated with the assembly of Gondwana and Pangea were positioned predominantly in the southern hemisphere. This coincided with the appearance of widespread continental ultra‐high‐pressure metamorphic terranes, with pressures >