ALBERT

Description: (GPF 20-3_080) 08.11.2020 –08.12.2020, Emden – Emden, King's Trough

Description: (GPF 20-3_080) 08.11.2020 –08.12.2020, Emden – Emden, King's Trough

Description: (GPF 20-3_080) 08.11.2020 – 08.12.2020, Emden – Emden, King's Trough

Description: Highlights • At least parts of the Beata Ridge formed during the main CLIP stage at 95–83 Ma. • Sampling of numerous volcanic rocks indicates a broad extrusive magmatic event. • Depleted and enriched geochemical signatures point to a heterogeneous mantle source. • Geochemical heterogeneities can occur on a small scale of tens of kilometers. Abstract The Caribbean Large Igneous Province (CLIP), a Cretaceous oceanic flood basalt province, presumably formed at the initiation of the Galápagos hotspot. During the M81 cruise of the German R/V METEOR, we sampled the Beata Ridge, a prominent submarine structure in the Caribbean Sea belonging to the CLIP. The ridge offers the opportunity to directly sample basement sequences of the central, submarine part of the CLIP, complementing numerous studies of accreted CLIP sequences exposed on land around the margins of this LIP. The majority of the recovered Beata Ridge samples are volcanic, implying that at least parts of the Beata Ridge were formed during a large extrusive event in contrast to previous assumptions that the structure is primarily composed of intrusive rocks. Several stratigraphically controlled profiles were sampled along the western slope of the Beata Ridge using the remotely operated vehicle (ROV) Kiel 6000 and revealed variously alternating sequences of magmatic rocks (lavas, pillow breccias, tuffs and gabbros) and sediment plains. We report new 40Ar/39Ar age and geochemical (major and trace element, Sr-Nd-Hf-Pb isotope) data for the recovered magmatic samples. Although the 40Ar/39Ar analyses display disturbed age spectra, they suggest an age range of 92.4–76.9 Ma. Thus our age data show for the first time that the Beata Ridge also formed during the main magmatic stage of the CLIP (~95–83 Ma). Previous studies suggested that the Beata Ridge was formed during a second, lower-volume magmatic phase of the CLIP (~81–71 Ma), possibly related to decompression melting during an extensional phase in the Caribbean. Most samples display relatively flat chondrite-normalized rare earth element (REE) patterns commonly observed throughout the CLIP, but light REE enriched and depleted compositions are also present. The occurrence of enriched and depleted incompatible element and radiogenic isotope signatures implies a heterogeneous mantle source region, as is observed for other LIPs worldwide. Since a high degree of geochemical variability is observed over short stratigraphic intervals within the ROV profiles, melt homogenization did not operate as effectively as commonly assumed for LIPs. Instead the plume head probably preserved some domains of enriched and depleted components, whereas most of the melts during the main stage have intermediate compositions (with flat REE patterns), representing mixtures of the enriched and depleted components.

Description: A long-standing question has been whether the Lower Nicaraguan Rise in the western Caribbean Sea represents thinned continental crust or is part of the Caribbean Large Igneous Province (CLIP). During the R/V Meteor cruise M81/2, the northeastern part of the Lower Nicaraguan Rise including the Hess Escarpment was extensively sampled. Only volcanic and carbonate rocks were recovered and no metamorphic rocks or rocks with continental crustal affinities, indicating that at least the uppermost basement is volcanic and belongs to a volcanic province, probably the CLIP. We provide a comprehensive geochemical dataset, including major and trace elements and Sr-Nd-Hf-Pb double spike isotope ratios, for the volcanic rocks. The basalts are geochemically subdivided into two groups: 1) depleted and 2) enriched. The majority belong to the depleted group with 6–9 wt% MgO and highly depleted incompatible element (e.g. (La/Yb)N = 0.15–0.76, (La/Sm)N = 0.21–0.65) compositions and depleted radiogenic isotope (e.g. εNdi = 8.9–11.0, εHfi = 13.6–16.5, 206Pb/204Pbi = 18.28–18.98) ratios. The subordinate enriched group basalts have 5–8 wt% MgO and were solely recovered from the lower part of the central Hess Escarpment. These rocks possess enriched incompatible element ((La/Sm)N = 1.15–1.60, (La/Yb)N = 1.40–2.57) and isotopic (εNdi = 7.52–8.34, εHfi = 12.6–12.9, 206Pb/204Pbi = 18.87–19.07) ratios resembling E-MORB. The depleted group is distinct from Pacific and Atlantic MORB based on combined 206Pb/204Pb, 143Nd/144Nd and 176Hf/177Hf data. It is similar, but not identical to the highly depleted Gorgona komatiites, thought to be generated during the CLIP event, and depleted Galápagos hotspot track lavas accreted to the Pacific margin of Central America. Ages of ~81 Ma from rocks drilled on the Hess Escarpment postdate the main CLIP phase (95-83 Ma), and indicate that at least the northeastern part of the Nicaraguan Rise formed during a later volcanic event. Lithospheric thinning after the main CLIP phase is suggested to have triggered upwelling of still anomalously hot, partially-melted or unmelted CLIP mantle material to form the younger depleted group of lavas. Our investigations imply that second-stage melting to generate depleted compositions during formation of oceanic large igneous provinces, as has been recently proposed for the Manihiki Plateau, is a more common process than originally thought.

Description: According to the classic mantle plume model, the evolution of a mantle plume comprises an initial plume head stage followed by a later plume tail stage. My thesis aims at deciphering the origin and evolution of submarine structures resulting from intraplate magmatism in two different areas: 1) the Caribbean Large Igneous Province (CLIP) formed during the plume head stage and 2) the Discovery Rise and adjacent structures in the South Atlantic representing the plume tail stage. The CLIP is assumed to have formed during the plume head stage of the Galápagos plume. Whereas this initial stage typically leads to the generation of huge continental flood basalt provinces and oceanic plateaus, the CLIP does not form such a coherent plateau. Instead, it consists of numerous oceanic flood basalt fragments accreted by tectonic processes on land around the margins of the LIP. In addition, large parts of the Caribbean seafloor are likely to be covered by CLIP sequences. In contrast to the better accessible subaerial fragments, the submarine sequences are poorly explored. Thus, my thesis addresses the detailed geochemical and geochronological investigation of two large submarine structures in the Caribbean Sea, the Beata Ridge and the Lower Nicaraguan Rise (LNR), to get new insights into the origin and evolution of the CLIP. My studies show for the first time that parts of the Beata Ridge formed during the main magmatic CLIP stage at ~89 Ma by widespread extrusive activity, contrasting previous assumptions that the ridge dominantly represents an intrusive complex formed during a later magmatic phase at ~76 Ma. My 40 Ar/ 39 Ar age dating yielded ages of ~92-77 Ma and thus cover both phases confirming long-term volcanism. Stratigraphically controlled sampling shows a high geochemical variability on a small scale (〈100 km) with trace element and radiogenic isotope compositions ranging from depleted to enriched. These observations point to the preservation of enriched and depleted mantle source components and small-scale heterogeneities within the plume head. A rough trend to more depleted compositions from older to younger samples indicates that the depleted component may have become more pronounced with time. All analyzed samples from the LNR are volcanic contradicting the suggestion of some authors that it belongs to the continental Chortís block in Central America. Instead, it is likely to be part of the CLIP. The majority of samples show strongly depleted IV incompatible element and radiogenic isotope compositions, and only a few samples have more enriched compositions similar to those found throughout the CLIP region. Combined with previously published seismic data and ages of ~81 Ma for similar depleted rocks drilled on the Hess Escarpment bordering the LNR, these results suggest that the enriched rocks represent the main CLIP event, whereas the depleted rocks were generated by second-stage melting ~10 Ma after the main stage. Lithospheric thinning probably led to upwelling of still hot, residual plume head material. The plume tail stage explored in my second study area in the South Atlantic typically leads to the formation of linear hotspot tracks. In contrast to the Tristan-Gough hotspot track, the other structures in this area do not form such linear seamount chains but are more irregular. As they are poorly investigated so far, I produced comprehensive geochemical and geochronological datasets for volcanic rocks from the Discovery Rise and from the Richardson Seamount, Agulhas Ridge and Meteor Rise in order to test if the structures are related to the Discovery and Shona hotspots, respectively. Thus far, these two plumes have largely been identified based on geochemical anomalies along the southern Mid-Atlantic Ridge (SMAR) indicating plume-ridge interaction. My new 40 Ar/ 39 Ar age data for the Discovery Rise show an age progression in the direction of plate motion from 23-40 Ma supporting a mantle plume origin. The rise consists of two parallel seamount chains. These display differences in incompatible element and Sr-Nd-Hf-Pb radiogenic isotope data indicating a spatial geochemical zonation of the Discovery plume. The northern chain has compositions similar to those of the nearby Gough subtrack and characteristic for the enriched DUPAL anomaly, a large geochemical domain in the Southern Hemisphere. In contrast, the southern chain is compositionally more enriched representing an extreme DUPAL-like component. These differences in composition are reflected in lavas from the close-by SMAR pointing to a geochemical zonation of the plume for ~40 Ma. The Richardson Seamount, Agulhas Ridge and Meteor Rise show geochemical characteristics ranging from Gough-like compositions to compositions similar to those for the Shona geochemical anomaly along the SMAR. My new 40 Ar/ 39 Ar age data range from 83-72 Ma, and combined with published age data from the Meteor Rise and Shona Ridge, they indicate that the structures roughly become younger toward the SMAR. These results suggest that the structures represent the hotspot track of the Shona plume. V My new geochemical data additionally provide new insights into the origin of the DUPAL anomaly. They indicate that detached or delaminated subcontinental lithospheric mantle (SCLM) and/or lower continental crust recycled through the lower mantle could represent the source of the anomaly. Furthermore, the South Atlantic plumes are located above the margins of the African Large Low Shear Velocity Province (LLSVP), and the geochemical zonation of the Tristan-Gough plume has been attributed to sampling of material from the LLSVP (DUPAL-like Gough component) and the ambient lower mantle (Tristan component). Similar DUPAL-like compositions for the other South Atlantic plumes suggest that these plumes also sample material from the LLSVP. The occurrence of an additional extreme DUPAL-like component in the southern Discovery Rise samples, however, points to a third lower mantle reservoir besides the Gough-type reservoir and the ambient lower mantle. Thus, the distinct geochemical domains may alternatively be independent of the LLSVP.