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The Tristan-Gough plume system forms age-progressive volcanism on the African plate over ~130 Ma, extending to the active islands of Gough and Tristan-Inaccessible. Walvis Ridge forms massive ridges and plateaus that split into three narrower ridges of the Guyot Province. International Ocean Discovery Program (IODP) Expedition 391 Site U1577 sampled the extreme eastern flank of Valdivia Bank, an oceanic plateau within the Walvis Ridge. Here we report major and trace element data as well as Sr-Nd-Hf-Pb isotopic compositions of IODP 391 Site U1577. Three massive basalt flow subunits were drilled, separated only by thin chilled margins. The lack of any sediment or significant alteration at the contacts, and their consistent paleomagnetic inclination, all suggest that these flows were erupted in relatively quick succession. Accordingly, geochemical variations are minimal. Samples from Site U1577 form tight clusters that overlap in major and trace elements with previous dredge and Deep Sea Drilling Project (DSDP) drill site samples from the Walvis Ridge. All are less enriched in incompatible trace elements, i.e., Ti, K, P, Sr and Zr, relative to samples from Tristan and Gough islands and the Guyot province, consistent with Walvis Ridge samples formed by higher degrees of partial melting. In contrast to Walvis Ridge dredge samples, Site U1577 samples are shifted slightly towards higher 176Hf/177Hf and lower 208Pb/204Pb isotopic compositions, while overlapping in 207Pb/204Pb vs. 206Pb/204Pb as well as Sr-Nd isotopic compositions. Such elevated 176Hf/177Hf combined with lower 208Pb/204Pb isotopic compositions have otherwise only been reported from the Eastern Rio Grande Rise formed in near-/on-ridge position. Magnetic lineations imply formation of Valdivia Bank by seafloor spreading as well. Site U1577 samples provide geochemical support for this hypothesis whereas dredge samples lack signatures of plume-ridge interaction. Also, with Site U1577 on the eastern flank, it is farthest from the mid-Atlantic Ridge at the time of formation compared to the location of near-by dredge samples. With major and trace element data integrated on the same samples as isotopic compositions, we will address the contrasting possibilities of an integral depleted plume component versus evidence for plume-ridge interaction. 

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.