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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 0.60–0.65 Fe 0.35–0.40 )Fe 2 O 4 ). 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. Plain Language Summary 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 wit

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. Key Points - Valdivia Bank is characterized by quasi-linear magnetic anomalies that are parallel to the inferred paleo-Mid-Atlantic Ridge - Magnetic anomalies imply that the plateau becomes younger E-W consistent with formation via seafloor spreading during anomalies C34n-C33r - Rift valleys, division of C33r, and anomaly curvature imply complex ridge tectonics and a ridge jump

Asymmetrically zoned hotspot tracks in the Pacifi c Ocean are interpreted to have formed from zoned plumes originating from the large-scale, lower-mantle, low-seismic-velocity anomaly (superplume?) beneath the southern Pacifi c, providing direct information about lowermantle compositional heterogeneity. New trace-element and Sr-Nd-Hf-Pb isotope data from the classic Tristan-Gough hotspot track in the South Atlantic also display a bilateral, asymmetric zonation with two distinct mantle source components, making it the fi rst zoned plume to be recognized overlying the African superplume. The plume zonation can be traced for 70 m.y., four times longer than recognized for Pacifi c zoned hotspot tracks. These fi ndings confi rm that the proposed zonation of Pacifi c hotspots is not simply a geochemical oddity, but could be a major feature of plumes derived from lower-mantle superplumes. We propose that the enriched southern Gough subtrack source with elevated 207Pb/204Pb and 208Pb/204Pb at a given 206Pb/204Pb, but low 143Nd/144Nd and 176Hf/177Hf (DUPAL-like composition), may refl ect the African superplume composition, whereas the more depleted northern Tristan subtrack source could represent a mixture of the superplume with the surrounding depleted mantle. Our results strengthen arguments that the enriched signature (DUPAL anomaly) in the South Atlantic could be derived from the lower mantle.

Discovering if hotspots observed on the Earth’s surface are explained by underlying plumes rising from the deep mantle or by shallow plate-driven processes continues to be an essential goal in Earth Science. Key evidence underpinning the mantle plume concept is the existence of age-progressive volcanic trails recording past plate motion relative to surface hotspots and their causal plumes. Using the icebreaker RV Polarstern, we sampled scattered hotspot trails on the 2,000 km-wide southeast Atlantic hotspot swell, which projects down to one of the Earth’s two largest and deepest regions of slower-than-average seismic wave speed – the Africa Low Shear Wave Velocity Province – caused by a massive thermo-chemical ‘pile’ on the core-mantle boundary.We showed recently using Ar/Ar isotopic ages – and crustal structure and seafloor ages – that these hotspot trails are age progressive and formed synchronously across the swell, consistent with African plate motion over plumes rising from the stable edge of a Low Shear Wave Velocity Province (LLSVP) (O’Connor et al., 2012). We showed furthermore that hotspot trails formed initially only at spreading boundaries at the outer edges of the swell until roughly 44 million years ago, when they started forming across the swell, far from spreading boundaries in lithosphere that was sufficiently weak (young) for plume melts to reach the surface. We concluded that if plume melts formed synchronous age progressive hotspot trails whenever they could penetrate the lithosphere, then hotspot trails in the South Atlantic are controlled by the interplay between deep plumes and the shallow motion and structure of the African plate. If the distribution of hotspot trails reflects where plume melts could or could not penetrate the continental or oceanic lithosphere then plumes could have been active for significantly longer than indicated by their volcanic chains. This provides a mechanism for extended late stage interplay between deep mantle proc

The origin of hotspot trails ranges controversially1 from deep mantle plumes rising from the core-mantle boundary2 to shallow plate cracking. But these mechanisms cannot explain uniquely the scattered hotspot trails on the 2,000 km-wide southeast Atlantic hotspot swell3, which projects down to one of the Earth’s two largest and deepest regions of slower-than-average seismic wave speed – the Africa Low Shear Wave Velocity Province, which marks a massive thermo-chemical ‘pile’ at the core-mantle boundary4,5,6. Here we use 40Ar/39Ar isotopic ages – and crustal structure and seafloor ages – to show that age progressive hotspot trails formed synchronously across the swell, consistent with African plate motion over plumes rising from the stable edge of a Low Shear Wave Velocity Province. We show also that hotspot trails formed initially only at spreading boundaries at the outer edges of the swell until roughly 44 million years ago, when they started forming across the swell, far from spreading boundaries in lithosphere that was sufficiently weak (young) for plume melts to reach the surface. We conclude that if plume melts formed synchronous age progressive hotspot trails wherever and whenever they could penetrate the swell lithosphere then hotspot trails in the South Atlantic are controlled by an interplay between deep plumes and the motion and structure of the African plate.

To the extent that a lower mantle origin is accepted for individual mantle plumes, they are our only means of investigating the chemical variability of lower mantle regions in space and time. Ultimately such mapping of the lower mantle should provide important constraints on the geological processes that led to the formation of these plume source regions. It is generally accepted that mantle plume sources contain differentiated recycled material from the surface of the Earth, but uncertainties remain as to the nature, composition and age of these recycled components. In addition, in the southern hemisphere plumes preferentially rise from the edges of ‘large low shear velocity provinces’ (LLSVP) (Thorne et al., 2004). It remains to be shown whether LLSVPs contribute material to rising mantle plumes and what their geochemical composition might be. The South Atlantic with four closely spaced mantle plumes in the vicinity to the African LLSVP could provide insights into these questions. Criteria in support of a lower mantle origin of these plumes are (1) presence of a flood basalt province (Tristan-Gough), (2) longevity of age-progressive volcanism (Tristan-Gough 130Ma, Shona 80 Ma, Discovery 40 Ma), as well as (3) enrichment of primordial 3He relative to MORB mantle (Discovery, Shona, Bouvet). The South Atlantic plumes are aligned and produce volcanism synchronously, consistent with their origin at the western edge of the African LLSVP. Geochemically the South Atlantic plumes are heterogeneous, spanning compositions in isotope space from EMI to Stracke’s FOZO (Stracke et al., 2005) and the extreme DUPAL signature with high delta 74 and delta 84 as represented by Gough Island. The extreme DUPAL is found in 3 of the 4 plume systems, indicating a common mantle source. The extreme DUPAL contributed to the Tristan-Gough plume system since 70 Ma and represents the southern component of the laterally zoned plume conduit (Rhode, personal comm. 2012). The Discovery plume is lat

Discovering if hotspots observed on the Earth’s surface are explained by underlying plumes rising from the deep mantle or by a shallow plate-cracking mechanism continues to be an essential goal in Earth Science. Key evidence underpinning the mantle plume concept is the existence of narrow, age-progressive volcanic trails recording past plate motion relative to surface hotspots and their deep causal plumes. Using the icebreaker RV Polarstern we sampled scattered hotspot trails on the 2,000 km-wide southeast Atlantic hotspot swell, which projects down to one of the Earth’s two largest and deepest regions of slower-than-average seismic wave speed – the Africa Low Shear Wave Velocity Province – caused by a massive thermo-chemical ‘pile’ on the core-mantle boundary. We showed recently using 40Ar/39Ar isotopic ages – and crustal structure and seafloor ages – that these hotspot trails are age progressive and formed synchronously across the swell, consistent with African plate motion over plumes rising from the stable edge of a Low Shear Wave Velocity Province (LLSVP) (O’Connor et al., 2012). We showed furthermore that hotspot trails formed initially only at spreading boundaries at the outer edges of the swell until roughly 44 million years ago, when they started forming across the swell, far from spreading boundaries in lithosphere that was sufficiently weak (young) for plume melts to reach the surface. We concluded that if plume melts formed synchronous age progressive hotspot trails whenever they could penetrate the lithosphere, then hotspot trails in the South Atlantic are controlled by the interplay between deep plumes and the shallow motion and structure of the African plate. Our observations reveal a plate tectonic-controlled cycle from the creation of deep thermo-chemical piles (LLSVP) and initiation of deep mantle plumes at the CMB to the shallow formation of the resulting hotspot trails. Moreover, suppression of plume melts from venting to the plate surface for te

The DUPAL anomaly (Hart 1984) in the South Atlantic has been attributed variably to deep sources related to mantle upwellings or shallow sources from continental material either from recent Gondwana breakup or ongoing erosions of the cratonic keels. Spatial distribution of the DUPAL anomaly provides an important constraint to distinguish between these possibilities. However, to this point sampling of South Atlantic mantle sources has been limited to the mid-Atlantic Ridge, young ocean islands, the Walvis Ridge and a single sample from Discovery Seamount, leaving uncertainties in the extent of the DUPAL anomaly, particularly its southern limit is poorly defined. Dredge samples from the effectively un-sampled Shona Ridge Meteor Rise Agulhas Ridge Cape Rise and Discovery seamounts were collected by the ANT XXIII/5 cruise of the FS Polarstern. Isotopic compositions of the new Discovery seamount samples form endmembers to the so-called Discovery and LOMU geochemical anomalies on the southern Mid-Atlantic Ridge, suggesting that the latter are formed from the same plume source through plume-ridge interaction (Class et al. 2009). Sr-Nd-Hf-Pb isotope data as well as preliminary Ar-Ar ages (J. OConnor work in progress) on the Shona aseismic ridges indicate the longevity of the Shona plume forming a zig-zag plume track (Hartnady & le Roex 1985). The new data integrated with literature data demonstrate an isotopically strongly heterogeneous source region for South Atlantic intraplate volcanism, including DUPAL, extreme EMI and HIMU. To this point the Shona plume signature on the mid-Atlantic Ridge was taken to be outside of the DUPAL region as its geochemical signature has HIMU affinity. All the Shona aseismic ridges sample this HIMU-like signature, however, each ridge has samples with DUPAL signature as well, suggesting that all the Shona-related bathymetric anomalies tap the DUPAL source. All components contributing to the Shona ridges are found in the 3000 km long Tristan pl

It has been more than two decades since White (1985) and Zindler and Hart (1986) proposed that the observed range of Sr-Nd-Pb isotope ratios of oceanic basalts can be described as mixtures of depleted mantle (DMM) with a limited number of enriched global endmember components (HIMU, EMI, EMII). There is no doubt that the global endmembers in isotope space represent extremes of the timing and magnitude of chemical fractionation processes in the Earths mantle. However, it remains a matter of debate how the intermediate isotopic compositions often evident on the local scale of individual islands are formed: (1) Do they represent mixtures between the limited number of global endmembers, or (2) do they reflect processes intermediate to the ones forming the global endmembers, or (3) does each individual ocean island basalt suite provide us with information about the timing and kind of geochemical differentiation forming that single source? Each possibility has important geodynamic implications. The Earths mantle is continuously differentiating through partial melting and remixing through plate tectonic recycling and convection, suggesting variable timing and composition of mantle sources. However, mantle sources might be formed by mixed lithologies and thus melt compositions might reflect mixtures of sources of more extreme compositions, possibly representing the global endmembers. We further address these questions based on two examples. Grande Comore Island is located on 140 Ma Indian Ocean lithosphere and its lavas reflect plume-lithosphere interaction. The Grande Comore plume component has Sr-Nd-Pb isotopic compositions intermediate between HIMU and EMI. Its extreme Os isotope ratios are among the highest measured in shield building-stage lavas of oceanic islands, giving further support for generally radiogenic Os isotope ratios in the EMI and HIMU compositions. A lack of correlation between OIB with high Os isotope ratios with inferred lithospheric thickness implies t