ALBERT

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Highlights • High-Ti lavas have the same composition as Walvis Ridge and Gough Subtrack. • Low-Ti lavas are derived from a distinct source compare to the high-Ti lavas. • High-Ti and low-Ti basalts reflect the spatial zonation of the plume head. • Tristan-type composition has not been discovered in the plume head stage. • Sr-Nd-Pb-Hf isotopes from Etendeka flood basalts. Abstract The origin and distribution of geochemically distinct source components in continental flood volcanism (generally associated with the initial phase of a mantle plume head) are poorly understood. Here we present new geochemical (major and trace element and Sr-Nd-Pb-Hf isotope) data from the Etendeka flood basalts and associated dikes from northern and central Namibia that are believed to have been produced during the initial stage of the Tristan-Gough hotspot. Following earlier studies, the Etendeka lava flows and dikes are divided into high-Ti and low-Ti groups. The trace element and isotopic composition of the high-Ti tholeiitic basalts, exclusively outcropping in northern Etendeka (northwestern Namibia), are similar to the Gough-type enriched mantle I (EMI) composition found on the Walvis Ridge (the Atlantic type locality for the EMI end member). The low-Ti tholeiitic basalts, primarily outcropping in Southern Etendeka (central western Namibia), have higher 143Nd/144Nd and 207Pb/204Pb but lower 208Pb/204Pb ratios than the Gough composition. Combining our data with newly published 3He/4He data and estimates of the magma source’s potential temperature from 1520-1680◦C, we conclude that the source of the low-Ti basalts was also intrinsic to the Tristan-Gough plume, consistent with a spatially-zoned plume head. The low-Ti basalts were derived from a distinct EMI-type source component that has thus far only been detected in the initial Tristan-Gough plume head (∼132 Ma), but not the later submarine hotspot track.

Beschreibung: Age-progressive volcanism is generally accepted as the surface expression of deep-rooted mantle plumes, which are enigmatically linked with the African and Pacific large low-shear velocity provinces (LLSVPs). We present geochemical and geochronological data collected from the oldest portions of the age-progressive enriched mantle one (EMI)-type Tristan-Gough track. They are part of a 30- to 40-million year younger age-progressive hotspot track with St. Helena HIMU (high time-integrated U-238/Pb-204) composition, which is also observed at the EMI-type Shona hotspot track in the southernmost Atlantic. Whereas the primary EMI-type hotspots overlie the margin of the African LLSVP, the HIMU-type hotspots are located above a central portion of the African LLSVP, reflecting a large-scale geochemical zonation. We propose that extraction of large volumes of EMI-type mantle from the margin of the LLSVP by primary plume heads triggered upwelling of HIMU material from a more internal domain of the LLSVP, forming secondary plumes.

Beschreibung: The Rio Grande Rise in the western South Atlantic Ocean has been interpreted as either an oceanic plateau related to the Tristan-Gough mantle plume, or a fragment of detached continental crust. Here we present new major and trace element data for volcanic rocks from the western and eastern Rio Grande Rise and the adjacent Jean Charcot Seamount Chain. The eastern Rio Grande Rise and older parts of the western Rio Grande Rise are comprised of tholeiitic basalt with moderately enriched trace element compositions and likely formed above the Tristan-Gough mantle plume close to the southern Mid-Atlantic Ridge. Younger alkalic lavas from the western Rio Grande Rise and the Jean Charcot Seamount Chain were formed by lower degrees of melting beneath thicker lithosphere in an intraplate setting possibly during rifting of the plateau. There is no clear geochemical evidence that remnants of continental crust are present beneath the Rio Grande Rise.

Beschreibung: Highlights • A HIMU-like volcanism belt along the southwest Africa. • The HIMU-like volcanic complexes form age-progressive volcanic tracks. • EMI and HIMU mantle plumes are from different domains in the lower mantle. Abstract The origin of carbonatitic and highly silica-undersaturated volcanism, common along the SW coast of Africa extending from Angola through Namibia to the tip of South Africa, is still poorly understood. Here we present new geochemical data (major and trace element and Sr-Nd-Pb-Hf-O-C isotopes) from the Agate Mountain calcio- to magnesio‑carbonatites (∼83 Ma), Dicker Willem calcio‑carbonatites (49 Ma) and Swakopmund basanitic plugs (76–72 Ma) along the coast of Namibia that were emplaced after the EMI (enriched mantle one) type Etendeka flood basalts. The trace element and isotopic composition of Agate Mountain carbonatites and Swakopmund basanites indicate that they were derived from a HIMU-type (high time-integrated 238U/204Pb with radiogenic Pb isotope ratios) magma source, similar to the St. Helena global HIMU endmember in the South Atlantic. The Agate Mountain carbonatites form part of the late-stage Walvis Ridge HIMU hotspot track overlying the EM1-type Walvis Ridge basement forming part of the Tristan-Gough hotspot track. The Dicker Willem carbonatites, however, extend to higher 206Pb/204Pb than St. Helena, but have similar 206Pb/204Pb to Mangaia HIMU lavas in the Pacific. Compared to Mangaia HIMU, the Dicker Willem carbonatites with mantle-type O and C isotopes have higher 207Pb/204Pb and 87Sr/86Sr but lower 143Nd/144Nd, suggesting it may represent a new HIMU endmember flavor. The HIMU carbonatitic and silica-undersaturated rocks form a belt of age-progressive volcanic tracks, including: 1) from the Walvis Ridge, through NW Namibia to central Angola, 2) from the Vema Seamount via Dicker Willem carbonatite to Gibeon kimberlites and carbonatites, 3) from the Namaqualand to Bushmanland and to Warmbad volcanic centers in northwestern South Africa, and 4) along the older end of the Shona EMI-type volcanic track extending into South Africa. Geochemical and seismic tomographic data suggest that the EMI and HIMU mantle plumes are generated from different geochemical domains at the base of the lower mantle. The Tristan-Gough, Discovery and Shona EM1 volcanic tracks are derived from a common low-velocity anomaly (superplume-like structure with three branching arms) ascending from the outer margin, possibly lower primoridal layer, of the African large low-shear-velocity province (LLSVP). Seismic low-velocity anomalies can be traced from beneath the belt of HIMU volcanism to an internal and shallower part of the LLSVP, located ∼900–1200 km east of the outer LLSVP margin and suggest that HIMU-type (possibly subducted oceanic lithospheric) material overlies EMI-type (possibly primordial) material in the internal part of the LLSVP.