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The Walvis Ridge system consists of a series of seamounts, ridges, and plateaus formed during the opening of the southern Atlantic Ocean since ~135 Ma. International Ocean Discovery Program Expeditions (IODP) 391 and 397T drilled six sites along the length of the hotspot track to understand the magmatic processes associated with evolving plume-ridge systems. The oldest drilled segment of the ridge system – Frio Ridge – extends from the Etendeka flood basalts in Namibia westward into the Atlantic Ocean. Site U1575 is on the Frio Ridge and is the closest site to the African continent. The site drilled 118.9 m of igneous basement with 70.7 m (59.5%) of recovery. The recovered core consisted of alternating sequences of submarine pillow lavas and sheet flows, some of which were massive (up to 21 m thick). Preliminary major and trace element data demonstrate the basaltic lavas are fractionated (MgO = 4.8-6.4 wt. %) with modest TiO2 contents (1.5-2.7 wt. %). The upper 52 m of igneous section (214-267 mbsf) are geochemically consistent throughout the various eruptive styles. However, an abrupt compositional shift to lavas with lower incompatible element abundances (TiO2, Zr, Sr, Nb, La, etc.) from 274-311 mbsf demonstrates a clear shift in magmatic source contributions. Below this, the lavas return to compositions similar to the upper portion of the hole. Shipboard natural gamma radiation (NGR) and magnetic susceptibility (MS) measurements correlate with mineralogical and compositional changes. Specifically, decreases in NGR correlate well with decreases in K2O, Sr, Y, and Zr. MS is positively correlated with zones containing olivine. Trace element discrimination plots demonstrate a dual character: Ti-V relationships are strongly MORB-like while Th/Nb suggests the lavas have both MORB and plume characteristics, consistent with the formation of the Frio Ridge through plume-ridge interaction. Elevated Zr/Nb and Y/Nb values are also consistent with a hybrid source. The composition of this core contrasts sharply with cores recovered from the younger Guyot Province to the southwest. Sites U1578 and U1585 have episodes of higher TiO2 contents (>3.5 wt. %) with trace element signatures (e.g. low Zr/Nb & Y/Nb) indicative of a pronounced plume component, consistent with an intraplate setting for the formation of the Guyot Province. 

Walvis Ridge, a time-transgressive series of ridges, oceanic plateau, seamounts, guyots, and two active volcanic islands extending SW from the coast of Namibia, records the evolution of the Tristan-Gough-Walvis Ridge (TGW) hotspot and the opening of the South Atlantic since ~135 Ma. However, much of our current understanding of the interplay between geodynamic cycles, tectonism, and mantle plume generation along the TGW hotspot track is based upon a limited number of dredged rock samples. Here, we present preliminary whole rock major and trace element geochemistry and shipboard physical properties data from Site U1578, located on a Center track guyot in the Guyot Province. The 302 m of igneous section recovered from Site U1578 provides an extraordinary, > 1 Myr record of plume magmatism, submarine volcanism, and geochemical evolution. The chemical stratigraphy of core from Site U1578 provides important new perspectives on submarine volcanism, magma flux, and the transition between continental tholeiitic basalts of the Etendeka flood basalt province and alkaline lavas of the Guyot Province. Core from U1578 records the longest sequence of pillow, sheet, and massive lava flows in the TGW system. Eleven (of 12 total) lithologic flow units record shifts in major and trace element geochemistry and episodic cycles of recharge and fractional crystallization. Preliminary XRF and ICP-MS analyses indicate a dominantly pyroxenite source and document the shift between high TiO2 (>3.5 wt. %) to low TiO2 (<3.5 wt. %) alkaline basalts. Site U1578 core samples from the Guyot Province have lower Nb/Y and Zr/Nb compared to Walvis Ridge sites drilled closer to the African continent (Frio Ridge at Site U1575 and Valdivia Bank at Sites U1576 and U1577), coincident with a transition from plume-ridge interaction to intraplate magmatism with time. This shift resulted in a thicker lithospheric lid and thus deeper and lower degrees of melting, preferentially sampling the enriched plume component. Additionally, shipboard natural gamma radiation (NGR) and magnetic susceptibility (MS) measurements correlate well with observed lithologic characteristics and new ICP-MS and XRF analyses. A 100 m zone of high NGR values neatly overlaps high K2O, and olivine cumulate layers correlate to higher MS and higher concentrations of Cr and Ni. 

Abstract The past ∼200 million years of Earth's geomagnetic field behavior have been recorded within oceanic basalts, many of which are only accessible via scientific ocean drilling. Obtaining the best possible paleomagnetic measurements from such valuable samples requires an a priori understanding of their magnetic mineralogies when choosing the most appropriate protocol for stepwise demagnetization experiments (either alternating field or thermal). Here, we present a quick, and non‐destructive method that utilizes the amplitude‐dependence of magnetic susceptibility to screen submarine basalts prior to choosing a demagnetization protocol, whenever conducting a pilot study or other detailed rock‐magnetic characterization is not possible. We demonstrate this method using samples acquired during International Ocean Discovery Program Expedition 391. Our approach is rooted in the observation that amplitude‐dependent magnetic susceptibility is observed in basalt samples whose dominant magnetic carrier is multidomain titanomagnetite (∼TM_60–65, (Ti_0.60–0.65Fe_0.35–0.40)Fe₂O₄). Samples with low Ti contents within titanomagnetite or samples that have experienced a high degree of oxidative weathering do not display appreciable amplitude dependence. Due to their low Curie temperatures, basalts that possess amplitude‐dependence should ideally be demagnetized either using alternating fields or via finely‐spaced thermal demagnetization heating steps below 300°C. Our screening method can enhance the success rate of paleomagnetic studies of oceanic basalt samples. 

Abstract Valdivia Bank is an oceanic plateau in the South Atlantic formed by hot spot magmatism at the Mid‐Atlantic Ridge during the Late Cretaceous. It is part of the Walvis Ridge, an aseismic ridge and seamount chain widely considered to be formed by age‐progressive volcanism from the Tristan‐Gough plume. To better understand the formation and history of this edifice, we developed a bathymetric map of Valdivia Bank by merging available multibeam echosounder data sets with a bathymetry grid based mainly on satellite altimetry (SRTM15+). The bathymetric map reveals previously unresolved features including extensive rift grabens, volcanic mounds and knolls, and large‐scale sediment transport systems. After Valdivia Bank was emplaced and probably eroded at sea level, it underwent a period of rifting, followed by a secondary magmatic pulse that caused regional uplift to sea‐level, followed by subsidence to current depths. Shallow banks at depths of ∼1,000 m are the result of a thick sediment pile atop uplifted volcanic crust. Several shallower mounds (∼1,000–520 m) and a guyot (∼220 m) likely resulted from coral reef growth atop one or more volcanic pedestals formed during the younger Cenozoic magmatic event. As sediments accumulated on the shallow platforms, sediment transport systems developed as gullies, channels and mass transport deposits carved valleys and troughs, shedding sediment into abyssal fans at the plateau base. The new bathymetric map demonstrates that oceanic plateaus are geologically active long after initial emplacement.