Newly Discovered mantle province beneath the Southern Ocean

Abstract

It is generally accepted that Earth’s upper mantle is characterized by Indian- and Pacific-type domains with distinctive isotope characteristics. The boundary between these two mantle regions has been hypothesized to be located at the Australian-Antarctic-Discordance (AAD), where regions west and east of the AAD are Indian- and Pacific-type, respectively. It was further posited that the Pacific mantle is flowing into the Indian mantle because isotopes show he boundary is moving westward with time. This story has important implications for the global mantle convection. However, our recent recovery of basalts from a 2,000-km sampling gap along the Australian-Antarctic Ridge (AAR), located east of the AAD on the Pacific side, challenges this story: Sr, Nd, Pb, and Hf isotopic compositions of AAR MORB are distinct from those of Pacific and Indian MORB. Rather, the AAR lavas show mixing relationships with Cenozoic volcanoes from the West Antarctic Rift System, the Balleny and Scott Islands, New Zealand, suggesting that mantle beneath this region in a state of dynamic mixing. A deep plume beneath the WARS may sustain the dynamic mixing. The dynamic mixing zone does not extend to adjacent Marie Byrd Land or east Australia because the Cenozoic volcanism in these regions shows slightly different isotopic trends than the AAR. In multi-dimensional isotopic space, however, these bordering regions also share isotopic space with the AAR that is distinct from both the Indian and Pacific mantles. These isotopic evidences suggest the presence of an isotopically distinct mantle province between the Indian and Pacific. The dynamic mixing zone further exhibits a distinct mixing relationship with the Hikurangi seamounts, which were erupted at ~90 Ma. According to tectonic reconstruction models, Hikurangi seamounts are related to super-plume activity that caused Gondwana to break up at ~ 90 Ma, suggesting that the newly discovered mantle province in question may be traced back to the Gondwana break-up. These results indicate that dynamics along the AAD should be reinterpreted in light of interaction with a super-plume.