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Whole rock major elements on ICP-OES by flux fusion or on trace element dissolutions, and trace elements by solution ICP-MS. Basaltic glass major elements by electron microprobe and trace elements by laser ICP-MS. Sr-Nd-Hf-Pb isotopes on MC-ICP-MS. 

Massive submarine basalt flows were sampled at five sites on the Tristan‐Gough‐Walvis hotspot track in the South Atlantic by International Oceanic Discovery Program Expeditions 391/397T, where the plume was interacting with a mid‐ocean ridge, a setting similar to that the of modern Iceland. High resolution XRF core scans document significant internal chemical variations with depth in these flows. Some of this reflects basal olivine accumulation. However, some examples have “scallop‐shaped” patterns that are interpreted to represent influxes of new magma during flow lobe inflation with successive lava injections focused toward the base of the flow unit. Olivine concentration in the deeper parts of the flow is interpreted to reflect top‐down tapping of a vertically zoned magma chamber, with the upper part of the chamber erupting first, and successive eruptive pulses tapping progressively deeper levels of the stratified chamber. The occurrence of massive submarine lava flows requires high eruptive fluxes relative to pillow lava formation. Propagation of these massive flows is favored by (a) high sea water confining pressures, which inhibit vesiculation and keep effective viscosity low and dissolved volatile content high, and (b) chill zones and thick viscoelastic crusts of quenched lava on the flow tops, which effectively insulate the flow interior from ambient temperatures. The formation of a thin film of super‐heated steam on the upper flow surface may similarly enhance the insulation. Evidence suggests that similar massive flows on the seafloor may extend many kilometers from their vents. 

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. 

2021 Goldschmidt Conference abstract 

Abstract Chemical events involving deep carbon- and water-rich fluids impact the continental lithosphere over its history. Diamonds are a by-product of such episodic fluid infiltrations, and entrapment of these fluids as microinclusions in lithospheric diamonds provide unique opportunities to investigate their nature. However, until now, direct constraints on the timing of such events have not been available. Here we report three alteration events in the southwest Kaapvaal lithosphere using U-Th-He geochronology of fluid-bearing diamonds, and constrain the upper limit of He diffusivity (toD ≈ 1.8 × 10⁻¹⁹cm²s⁻¹), thus providing a means to directly place both upper and lower age limits on these alteration episodes. The youngest, during the Cretaceous, involved highly saline fluids, indicating a relationship with late-Mesozoic kimberlite eruptions. Remnants of two preceding events, by a Paleozoic silicic fluid and a Proterozoic carbonatitic fluid, are also encapsulated in Kaapvaal diamonds and are likely coeval with major surface tectonic events (e.g. the Damara and Namaqua–Natal orogenies). 

Abstract Rio Grande Rise (RGR) and Walvis Ridge (WR) are South Atlantic large igneous provinces (LIPs), formed on the South American and African plates, respectively, mainly by volcanism from a hot spot erupting at the Mid‐Atlantic Ridge (MAR) during the Late Cretaceous. Both display morphologic complexities that imply their tectonic evolution is incompletely understood. We studied bathymetry, gravity, and vertical gravity gradient maps derived from satellite altimetry to trace faults providing indications of seafloor spreading directions and changes. We also examined magnetic anomalies for time constraint and reflection seismic data for structural information. Abyssal hill fabric and magnetic anomaly data indicate that the area between RGR and WR was anomalous between anomalies C34 (83.6 Ma) and C30 (66.4 Ma) owing to reorganization of a right‐lateral transform on the MAR. This event began ∼92 Ma as the transform shifted south to form multiple, short‐offset right‐lateral transforms, with the reorganization extending through anomaly C34 and ending before anomaly C30. Anomalous spacing of magnetic anomalies and discordant fault fabric indicate that a microplate formed with a core of Cretaceous Quiet Zone seafloor. As the MAR jumped eastward, this microplate was captured by the South American plate and now resides mostly in a basin between the main RGR plateau and a related ridge to the east (East Rio Grande Rise). The microplate is ringed by igneous massifs, implying a link with volcanism. The results presented here indicate that these two LIPs had a complex Late Cretaceous history that belies simple hot spot models.