Description: The opening of the Southern Ocean gateways allowed the emergence of the Antarctic Circumpolar Current (ACC), crucial for the onset of global Cenozoic cooling (e.g., Sijp et al., 2014, Glob. Planet. Change 119; Voigt et al., 2013, EPSL 369/370). South of Africa, the opening was associated with the formation of several large igneous provinces (LIPs) including the Mozambique Ridge, Agulhas Plateau and the smaller Northeast Georgia Rise and Maud Rise. Plate tectonic reconstructions imply that the latter two were once part of the much greater Agulhas Plateau and were separated by subsequent rifting (Parsaglia et al., 2008, Geophys. J. Int. 174). It is debated whether and to what extent the emplacement of these large volcanic features obstructed the exchange of water masses between the Atlantic and the Indian Ocean thereby delaying the onset of the ACC. The Agulhas Plateau was drilled during recent IODP Expedition 392 (Uenzelmann-Neben, Bohaty, Childress, et al., IODP Exp. 392 Preliminary Report, 2022). Igneous rocks were recovered at two sites on the southern part of the plateau (Sites U1579, U1580) and at one site near its northern edge (U1582). Preliminary data indicate that all sites returned tholeiitic or transitional basalts, formed by low pressure (shallow magma chamber) fractionation of mainly olivine and plagioclase (as typical for mid-ocean ridge basalts and many LIP lavas). The volcanic glass composition from extrusive (pillow) lavas cored at Site U1582 possesses a clearly tholeiitic composition with narrow compositional range ((La/Sm)n and (Sm/Yb)n ratios spanning from ~0.69-0.77 and 0.73-0.81, respectively (primitive-mantle normalized)). Their H2O/Ce ratios range from 422 to 629, which is obviously higher than found in normal mid-ocean ridge basalts or ocean island basalts (usually 〈250). Their δ11B composition indicate that these high H2O/Ce ratios are not caused by seawater assimilation. Whole rock samples from Sites U1579 and U1580 have a transitional composition. Accordingly, they show a more widespread geochemical composition with (La/Sm)n and (Sm/Yb)n ratios ranging from 0.69 to 0.75 and 0.81 to 1.19 (U1579) and 0.89 to 1.63 and 1.38 to 1.97 (U1580). Upcoming geochemical investigations, including Sr, Nd, Hf, Pb isotope analyses, will further reveal the nature and source of the magmatism.

Description: The opening of the Southern Ocean gateways has been debated as a key element in the emergence of the Antarctic Circumpolar Current (ACC), crucial for the onset of global Cenozoic cooling (e.g., Sijp et al., 2014, Glob. Planet. Change 119; Voigt et al., 2013, EPSL 369/370). South of Africa, the opening was associated with the formation of several large igneous provinces (LIPs) including the Mozambique Ridge, Agulhas Plateau and the smaller Northeast Georgia Rise and Maud Rise. Plate tectonic reconstructions imply that the latter two were once part of the much greater Agulhas Plateau and were separated by subsequent rifting (Parsaglia et al., 2008, Geophys. J. Int. 174). It is debated whether and to what extent the emplacement of these large volcanic features obstructed the exchange of water masses between the Atlantic and the Indian Ocean thereby delaying the onset of the ACC. The Agulhas Plateau was drilled during recent IODP Expedition 392 (Uenzelmann-Neben, Bohaty, Childress, et al., IODP Exp. 392 Preliminary Report, 2022, in press). Igneous rocks were recovered at two sites on the southern part of the plateau (Sites U1579, U1580) and at one site near its northern edge (U1582). Preliminary (shipboard) data indicate that all sites returned tholeiitic basalts, formed by low pressure (shallow magma chamber) fractionation of mainly olivine and plagioclase (as typical for mid-ocean ridge basalts and many LIP lavas). 40Ar/39Ar dating will help to answer fundamental questions regarding the emplacement age of the plateau and its temporal-spatial evolution. Geochemical investigations of the recovered rocks will reveal the nature and source of the magmatism. The proposed project will focus on the determination of radiogenic (Sr, Nd, Hf, Pb) isotope compositions to further constrain the magma source composition for comparison with other regional and global LIP magmatism. In particular, we want to address the open question whether the Agulhas Plateau, Northeast Georgia Rise and Maud Rise magmatism can all be attributed to the same magma source and setting (“Greater Agulhas”) to test the previous plate tectonic reconstruction models. Additional questions are whether a (deep-sourced?) mantle plume initiated the magmatism and its causal relationship to the regional continental breakup and opening of ocean basins. The isotope data (in combination with the results from age dating and major/trace element geochemistry) will help to constrain the origin and impact of this regional LIP magmatism on tectonic configuration, ocean circulation, and global climate in the mid to late Cretaceous.

Description: The Sorachi-Yezo belt, central Hokkaido, Japan, is composed of voluminous tholeiitic basaltic volcanics, and has been thought to be accreted fragments of an oceanic plateau formed in the Late Jurassic Pacific Ocean. Picrites have been reported as pillow lava and hyaloclastite from the Sorachi-Yezo belt. These picrites are characterized by very magnesian olivine phenocrysts (up to Fo [=100* Mg/(Mg+Fe)] = 94.1), which indicate that the primary magma was unusually Mg rich. The estimated MgO of the primary magma and the mantle potential temperature are as high as 29 wt% and 1700 °C, respectively, comparable to those of the Neoarchean komatiites, and higher than those of the Gorgona komatiites and picrites. The rare earth element patterns of the Sorachi-Yezo picrites are divided into two groups that are chemically akin to the Neoarchean komatiites and Gorgona komatiites and picrites, indicating different melting regimes in an extremely hot mantle plume. The Sorachi-Yezo picrites provide evidence for extremely high temperature magmatism, like that of Archean komatiite, caused by melting of the hottest mantle plume among the Phanerozoic oceanic large igneous provinces.