The Tristan-Gough-Walvis Ridge volcanic chain, located in the South Atlantic Ocean on the African plate, is one of Earth's longest linear seafloor features. However, the origin of its magmatism remains poorly understood. To better understand its formation, temporal evolution, and geochemical characteristics, six sites were drilled along the Tristan-Gough-Walvis Ridge chain by International Ocean Discovery Program (IODP) Expeditions 391 and 397T: three sites on Valdivia Bank (U1575, U1576, and U1577) and three sites in the Guyot Province (U1578, U1584, and U1585). Among these, Site U1578, located on the deep northwestern flank of a "Center track" seamount, represents the deepest basement penetration (>300 m) of both expeditions. According to preliminary geophysical and biostratigraphic constraints, the stratigraphic sections recovered from Site U1578 span >2.0 m.y. of the Paleocene (ca. 64.8-62.3 Ma). Here, we present an integrated study of major and trace element concentrations in minerals, combined with in situ Sr isotopes of plagioclase (Pl) from Site U1578 basalts. Based on the olivine-spinel (Ol-Sp) thermometry and pyMelt modeling, the calculated mantle potential temperature of the Center track is >= 1400 degrees C, which is similar to 50 degrees C higher than that of mid-ocean-ridge basalts (MORBs), most likely reflecting the influence of a mantle plume. Based on geochemical compositions and crystal textures, we identified three distinct clinopyroxene (Cpx) groups: Group 1 is characterized by high Mg# (>83.4) and elevated Ni (>200 ppm) and Cr (>5000 ppm) contents; Group 2 has intermediate Mg# (76.6-83.4), Ni (100-200 ppm), and Cr (1000-5000 ppm); and Group 3 exhibits low Mg# (<76.6) and depleted Ni (<70 ppm) and Cr (<30 ppm) contents. Multiple episodes of magma recharge and mixing in the plumbing system beneath the Center track produced a diversity of mineral textures, including normal, reverse, sector, and oscillatory zoning. Additionally, Pl from this site is isotopically heterogeneous (Sr-87/Sr-86 of 0.7038 +/- 0.0001 to 0.7046 +/- 0.0001) and broadly similar to basalts from the northern Tristan track in the Guyot Province. A shift to more radiogenic Sr isotopic signatures from the lower to the upper igneous units suggests the increasing incorporation of radiogenic materials into the mantle source with time, potentially derived from enriched mantle typically associated with basalts of the southern Gough track. These findings indicate that basaltic lavas from the Center track originate from an anomalously hot and geochemically enriched mantle source associated with a mantle plume or hotspot activity. Diverse mineral textures and chemistry record complex fractional crystallization and mixing processes during magma storage prior to eruption.

The Rio Grande Rise (RGR) oceanic plateau exhibits distinctly broad morphology compared to the linear, age-progressive Walvis Ridge (WR), despite both originating from the Tristan-Gough (T-G) plume. New 40Ar/39Ar ages (84-41 Ma) from RGR demonstrate prolonged coeval formation with WR at the Mid-Atlantic Ridge (MAR) during a ridge-centered plume configuration. These plume-ridge interactions coincided with a temporary microplate eventually incorporated into the South American Plate, explaining RGR's broader spatial distribution. Backtracking reveals that absolute plate motion and southward plume drift shut off excess magma supply when the T-G plume moved south of large lateral MAR offsets, ending volcanism on the South American plate around 52 Ma-later than previously estimated. Although RGR's erupted volume matches large igneous provinces, magmatic production rates were moderate, resembling modern-day Iceland rather than high-flux terrestrial large igneous provinces. This supports a ridge-centered plume origin over a high-flux magma pulse, demonstrating how plume-ridge configuration and microplate tectonics controlled the evolutionary divergence of these related South Atlantic volcanic features.

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 past similar to 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 (similar to TM60-65, (Ti0.60-0.65Fe0.35-0.40)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 degrees C. Our screening method can enhance the success rate of paleomagnetic studies of oceanic basalt samples. Oceanic basalts are ideal recorders of the Earth's magnetic field. To decipher magnetic histories recorded in rocks, paleomagnetists need to isolate the magnetization directions and intensities within rocks by one of two possible methods. One method typically involves progressively heating the samples to high temperatures. The other method involves exposing samples to alternating magnetic fields with increasing peak field intensities. Both of these methods are ultimately destructive to the original magnetization preserved within rocks. However, without knowledge of a given rock's magnetic mineralogy, randomly choosing thermal or alternating field demagnetization methods may result in high failure rates. We developed a pre-screening method to help decide which cleaning method will likely be more successful for a given sample based on low-field magnetic susceptibility measurements. These measurements do not affect the original magnetic information recorded in a rock, thereby permitting subsequent paleomagnetic studies on the same sample. Our technique can be performed as rapidly as 2 min per sample, is non-destructive, and does not require complicated sample preparation. Paleomagnetic studies utilize either alternating field or thermal demagnetization, but it is difficult to choose the best protocol a priori Amplitude-dependence of magnetic susceptibility measurements permits preliminary magnetic mineralogy characterization in submarine basalts Rapid amplitude-dependence measurements may aid in deciding upon the best demagnetization protocol for submarine basalt samples

Images of the outside of hard rock whole-round sections were acquired using a linescan imager (Section Half Imaging Logger [SHIL]) and a special holder that allows each 90 degree segment of the outer surface to be positioned properly. The images were taken at a resolution of 20 lines/mm (50 micropixels). JRSO staff take these quadrant images and compile them into a side-by-side rollout photograph of the section. Composite images are available as both JPG and TIF image formats. Individual quadrant images are available as JPG images only through this report; contact the IODP-JRSO Data Librarian if quadrant TIF files (~160 MB) are needed.

Elemental concentration in interstitial water samples was measured by inductively coupled plasma - atomic emission spectroscopy (ICP-AES). Data are presented by element-wavelength pair (e.g., more than one calcium line may be reported). Elemental lines for which data do not exist for a particular expedition will not appear.

X-ray diffraction (XRD) is used to identify minerals and their proportions in sediment or hard rock sample powders on a Bruker AXS D4 Endeavor X-ray diffractometer. Results are returned as diffractograms in a viewable format (either PDF or PNG).

Shear strength was measured on section halves using a GEISA automated vane shear (AVS) frame and device controller. The device is suitable for sediment not affected by cementation (i.e., saturated, clay-rich, soft sediment). Shear strength measurements should be considered only approximate, particularly because the influence of pore pressure changes during the undrained experiment cannot be estimated. Report includes vane shear strength at the sample's failure point, maximum torque angle, penetration direction, and rate of vane rotation.

Abstract Valdivia Bank (VB) is a Late Cretaceous oceanic plateau formed by volcanism from the Tristan‐Gough hotspot at the Mid‐Atlantic Ridge (MAR). To better understand its origin and evolution, magnetic data were used to generate a magnetic anomaly grid, which was inverted to determine crustal magnetization. The magnetization model reveals quasi‐linear polarity zones crossing the plateau and following expected MAR paleo‐locations, implying formation by seafloor spreading over ∼4 Myr during the formation of anomalies C34n‐C33r. Paleomagnetism and biostratigraphy data from International Ocean Discovery Program Expedition 391 confirm the magnetic interpretation. Anomaly C33r is split into two negative bands, likely by a westward ridge jump. One of these negative anomalies coincides with deep rift valleys, indicating their age and mechanism of formation. These findings imply that VB originated by seafloor spreading‐type volcanism during a plate reorganization, not from a vertical stack of lava flows as expected for a large volcano.

Digital section images were taken of the flat face of split cores on the Section Half Imaging Logger (SHIL) using a linescan camera at a resolution of 20 lines/mm (50 micron pixels). Cores were imaged as soon as possible after splitting to minimize color changes that occur through oxidation and drying. The SHIL produces TIF files as well as reduced-size JPG files. The TIF files are not kept online but users may request them from the IODP-JRSO Data Librarian.

Color reflectance data were measured on section halves using an integration sphere and a UV-VIS spectrophotometer mounted on the Section Half Multisensor Logger (SHMSL). Spectral counts are recorded in the range of 380 to 700 nm, covering the visible spectrum, and binned in ~2 nm bins. Spectral data are reduced from spectra and recorded in tristimulus XYZ values, CieLAB L*a*b* values, and other units.

A digital composite image (PNG) is made for each core comprising core sections scanned using a line-scan camera. The composite layout is equivalent to traditional core table photos. Top left is top of core; color and meter rule references are included.

Magnetic susceptibility was measured on whole-round sections (and rarely section halves) on the Whole-Round Multisensor Logger (WRMSL) and/or Special Task Multisensor Logger (STMSL) using a Bartington MS2 meter and a 90 mm or 80 mm MS2C loop. As volume of the sample is not controlled for this experiment, susceptibility units are recorded in instrument units and are not volume-corrected.