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  1. The strategy of Expedition 391 was to core at six distributed locations on the Tristan-Gough-Walvis (TGW) hotspot track, providing new insights into the temporal, volcanologic, petrologic, geochemical and paleomagnetic evolution of the hotspot track (see Scientific objectives in the Expedition 391 summary chapter [Sager et al., 2023c]). At the youngest and westernmost of these locations, three sites were proposed as a transect across the northern Guyot Province seamounts and ridges immediately southwest of the morphologic split that occurs at about 2°E (Figure F1). Because of severe cuts to operational time on that expedition caused by COVID-19 mitigation, two proposed sites (GT-4A and TT-4A) were omitted (see Introduction in the Expedition 391 summary chapter [Sager et al., 2023c]). Only the middle site (U1578) was cored (Figures F1, F2). This omission was a major deficiency for Expedition 391, because the three holes were positioned to sample the isotopic split that first occurs farther northeast, at the location of the DSDP Leg 74 transect (Hoernle et al., 2015). By omitting the ends of the transect, only dredge samples are available to characterize the isotopic end-members, which correspond to the Tristan track (northern seamounts that connect Walvis Ridge with the Tristan Island group) and the Gough track (southern seamounts that connect Walvis Ridge to the Gough Island group). 
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    Free, publicly-accessible full text available October 11, 2024
  2. International Ocean Discovery Program Expedition 397T sought to address the shortage of drilling time caused by COVID-19 mitigation during Expedition 391 (Walvis Ridge Hotspot) by drilling at two sites omitted from the earlier cruise. A week of coring time was added to a transit of JOIDES Resolution from Cape Town to Lisbon, which would cross Walvis Ridge on its way north. These two sites were located on two of the three seamount trails that emerge from the split in Walvis Ridge morphology into several seamount chains at 2°E. Site U1584 (proposed Site GT-6A) sampled the Gough track on the southeast side of the hotspot track, and Site U1585 (proposed Site TT-4A) sampled the Tristan track on the northwest side. Together with Site U1578, drilled on the Center track during Expedition 391, they form a transect across the northern Walvis Ridge Guyot Province. The goal was to core seamount basalts and associated volcanic material for geochemical and isotopic, geochronologic, paleomagnetic, and volcanological study. Scientifically, one emphasis was to better understand the split in isotopic signatures that occurs at the morphologic split. Geochronology would add to the established age progression but also give another dimension to understanding Walvis Ridge seamount formation by giving multiple ages at the same sites. The paleomagnetic study seeks to establish paleolatitudes for Walvis Ridge sites for comparison with those published from hotspot seamount chains in the Pacific, in particular to test whether a component of true polar wander affects hotspot paleolatitude. Hole U1584A cored a 66.4 m thick sedimentary and volcaniclastic section with two lithostratigraphic units. Unit I is a 23 m thick sequence of bioturbated clay and nannofossil chalk with increasing volcaniclastic content downhole. Unit II is a >43 m thick sequence of lapillistone with basalt fragments. Because the seismic section crossing the site shows no evidence as to the depth of the volcaniclastic cover, coring was terminated early. Because there were no other shallow sites nearby with different characteristics on existing seismic lines, the unused operations time from Site U1584 was shifted to the next site. The seismic reflector interpreted as the top of igneous rock at Site U1585 once again resulted from volcaniclastic deposits. Hole U1585A coring began at 144.1 mbsf and penetrated a 273.5 m thick sedimentary and volcaniclastic section atop a 81.2 m thick series of massive basalt flows. The hole was terminated at 498.8 mbsf because allotted operational time expired. The sedimentary section contains four main lithostratigraphic units. Unit I (144.1–157.02 mbsf) is a bioturbated nannofossil chalk with foraminifera, similar to the shallowest sediments recovered at Site U1584. Unit II (157.02–249.20 mbsf), which is divided into two subunits, is a 92.2 m thick succession of massive and bedded pumice and scoria lapillistone with increased reworking, clast alteration, and tuffaceous chalk intercalations downhole. Unit III (249.20–397.76 mbsf) is 148.6 m thick and consists of a complex succession of pink to greenish gray tuffaceous chalk containing multiple thin, graded ash turbidites and tuffaceous ash layers; intercalated tuffaceous chalk slumps; and several thick coarse lapilli and block-dominated volcaniclastic layers. Befitting its complexity, this unit is divided into eight subunits (IIIA–IIIH). Three of these subunits (IIIA, IIID, and IIIG) are mainly basalt breccias. Unit IV (397.76–417.60 mbsf) is a volcanic breccia, 19.8 m thick, containing mostly juvenile volcaniclasts. The igneous section, Unit V (417.60–498.80 mbsf) is composed of a small number of massive basaltic lava flows. It is divided into three igneous lithologic units, with Unit 2 represented by a single 3 cm piece of quenched basalt with olivine phenocrysts in a microcrystalline groundmass. This piece may represent a poorly recovered set of pillow lavas. Unit 1 is sparsely to highly olivine-clinopyroxene ± plagioclase phyric massive basalt and is divided into Subunits 1a and 1b based on textural and mineralogical differences, which suggests that they are two different flows. Unit 3 also consists of two massive lava flows with no clear boundary features. Subunit 3a is a 10.3 m thick highly clinopyroxene-plagioclase phyric massive basalt flow with a fine-grained groundmass. Subunit 3b is a featureless massive basalt flow that is moderately to highly clinopyroxene-olivine-plagioclase phyric and >43.7 m thick. Alteration of the lava flows is patchy and moderate to low in grade, with two stages, one at a higher temperature and one at a low temperature, both focused around fractures. The Site U1585 chronological succession from basalt flows to pelagic sediment indicates volcanic construction and subsidence. Lava eruptions were followed by inundation and shallow-water volcaniclastic sediment deposition, which deepened over time to deepwater conditions. Although the massive flows were probably erupted in a short time and have little variability, volcaniclasts in the sediments may provide geochemical and geochronologic data from a range of time and sources. Chemical analyses indicate that Site U1585 basalt samples are mostly alkalic basalt, with a few trachybasalt flow and clast samples and one basaltic trachyandesite clast. Ti/V values lie mostly within the oceanic island basalt (OIB) field but overlap the mid-ocean-ridge basalt (MORB) field. Only a handful of clasts from Site U1584 were analyzed, but geochemical data are similar. Paleomagnetic data from Site U1585 indicate that the sediments and basalt units are strongly magnetic and mostly give coherent inclination data, which indicates that the basaltic section and ~133 m of overlying volcaniclastic sediment is reversely polarized and that this reversal is preserved in a core. Above this, the rest of the sediment section records two normal and two reversed zones. Although there are not enough basalt flows to give a reliable paleolatitude, it may be possible to attain such a result from the sediments. 
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    Free, publicly-accessible full text available October 11, 2024
  3. The strategy of Expedition 391 was to core at six distributed locations on the Tristan-Gough-Walvis (TGW) hotspot track, providing new insights into its temporal, volcanological, petrologic, geochemical, and paleomagnetic evolution (Sager et al., 2023b). At the youngest and westernmost of these locations, three sites were proposed as a transect across the northern Guyot Province seamounts and ridges immediately southwest of the morphological split that occurs at about 2°E (Figure F1). Because of severe cuts to operational time during Expedition 391 caused by COVID-19 mitigation, two sites (proposed Sites GT-4A and TT-4A) were omitted (Sager et al., 2023b). Only the middle site (U1578) was cored (Figure F1). This omission was a major deficiency for Expedition 391 because the three sites were positioned to sample the isotopic split that first occurs farther northeast at the location of the Deep Sea Drilling Project (DSDP) Leg 74 transect (Hoernle et al., 2015). By omitting the ends of the transect, only dredge samples are available to characterize the isotopic end-members, which correspond to the Tristan track (northern seamounts) and the Gough track (southern seamounts). 
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    Free, publicly-accessible full text available October 11, 2024
  4. 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 (∼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°C. Our screening method can enhance the success rate of paleomagnetic studies of oceanic basalt samples.

     
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    Free, publicly-accessible full text available February 1, 2025
  5. 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.

     
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