ALBERT

Description: The Rio Grande Rise (RGR) isone of the most prominent bathymetric features in the South Atlantic Ocean.It has long been assumed that this massive oceanic plateau is a late-Cretaceous Large Igenous Province related to the Tristan-Gough mantle plume. But recent studies suggest that it might be a fragment of detached continental marginthathas been preserved asa'microcontinent'embedded in plume-influenced oceanic crust[1, 2].Here we present the first geochemical results from a combined geophysical and petrological cruise(MSM-82)carried out in spring 2019. We measured major and traceelement concentrations of 47whole rock samplesdredged from the flanks of a long rift valleythat cuts throughthe eastern (E) and western (W) parts of the RGR. Our preliminary data showthat theERGR differs in compositionfrom the WRGR. The latter rangesfrom trachybasaltto trachyandesite, while the former mainly consists of alkalinebasalt.Geophysical results show that the WRGR is underlain by thicker crust so the more evolved compositionsmight be due to longer residence times for melts in the associated magma chambers. Light rare earth elementenrichment[(La/Sm)N: 1.5 to 6.5]in all samples indicatesan enrichedmagmasource, consistent with the involvment of a mantle plume during the formation of the RGR. However, variable ratios ofimmobileincompatible trace elements (e.g. Nb/Zr) imply aheterogenoussource region.All samples have high Nb/Th (6.4 –14.3) similar to oceanic basalts, indicatingthat they have not undergone significant contamination bya continental component with low Nb/Th.Thus, our initial findingsare consistent withaplume-related magmatic origin for the RGRratherthana microcontinent.

Description: The age-progressive Tristan-Gough and Shona volcanic tracks on the southern South Atlantic seafloorshare a common Enriched Mantle (EMI-type) geochemicalsignaturethroughout their long histories(c. 132 Ma and 180Ma,respectively). Our new geochronological and geochemical data, combinedwith literature data, show that both EMI-type hotspot tracks are overlain by age-progressive volcanictrackswith St. Helena-type HIMU(high time-integrated 238U/204Pb)compositions, whichextend into southwestAfrica. The systematic age differenceof 30-40 Ma between the EMI and HIMU volcanism at any given location corresponds to a distance of 900-1200 km between the EMI and HIMU hotspots. Since the Gough-type-EMI plumes are located above the margin of the African Large Low Shear Velocity Province (LLSVP), their source reservoir could be located either within the LLSVP or represent the ambient mantle flowing against its steep outer margin. The common St. Helena-typeHIMU reservoir, however, must be located within the interior or on top of the LLSVP. Both EMI type hotspot tracks began with Large Igneous Provinces (Parana-Etendeka and Karoo)that are older than the oldest HIMU-type volcanism. Seismic tomographic images show a step (bulge) at shallower depthslocated ~1000-1500 km northeastfrom the African LLSVPmargin.We propose that removal of material by starting plume headsfrom within or on the African LLSVPled to instabilities that triggered smaller secondary plumes fromwithin or aboveinternal portions of the LLSVP.

Description: The vast majority of active volcanism that is located at plate boundaries can be easily explained by plate tectonic processes. Intraplate volcanism, which incorporates some of the smallest and largest volcanic events on Earth, cannot be successfully explained by a single process or model. The most volumetrically significant intraplate volcanic events are associated with the arrival of the head of a thermo-chemical anomaly rising from the deep mantle and impacting the base of the lithosphere. This event generates massive and short-lived magmatic activity over a wide area (up to 2000 km across), forming a large igneous province. A long-lived hotspot track can form over the mantle plume tail and is best illustrated by the formation of age progressive volcanic chains, such as the famous Hawaiian-Emperor chain. Millions of smaller solitary volcanic edifices, non-age progressive volcanic chains and provinces, on the other hand, have other potential mechanisms of origin. Potential models comprise decompression melting due to lithospheric extension, destabilization of fusible lithologies in the lithospheric mantle, small scale sub-lithospheric convection, or lithospheric delamination.