The significance of the Hawaiian-Emperor bend has been at the center of debates regarding mantle plume drift and plate motion direction and velocity changes since it was discovered. Yuryaku is the first seamount to occur after the Hawaiian-Emperor bend and is therefore a key volcano in defining Pacific plate motion and Hawaiian plume drift change at ~47 Ma. Yuryaku exhibits a puzzling bimodality in ages. Basalts with shield-stage geochemical characteristics erupted ~47–48 Ma, consistent with models of linear age progression of the Hawaiian mantle plume 1-3 . However, some samples from the same dredge hauls are alkalic in composition and have 40 Ar/ 39 Ar ages that are 16–17 million years younger than when the Hawaiian plume was located at Yuryaku 2-3 . This unusually large gap is too long to correspond with rejuvenated volcanism, which occurs within one million years of the postshield volcanic stage. This study presents the elemental and isotopic compositions of these bimodally aged rocks and interprets the data in the context of the regional tectonic framework and new high-resolution bathymetric maps. Some isotopic characteristics are consistent with a Hawaiian plume source with e Nd »7–8, epsilon e Hf »12, and 87 Sr/ 86 Sr » 0.70355–0.70358. In contrast, low chromium and nickel, and high aluminum and large ion lithophile element concentrations, as well as high 206 Pb/ 204 Pb (19.9–20.3

ABSTRACTUltraslow spreading ridges are poorly understood plate boundaries consisting of magmatic and amagmatic segments that expose mostly mantle peridotite and only traces of basalt and gabbro. The slowest part of the global spreading system is represented by the eastern Gakkel Ridge in the Central Arctic Ocean, where crustal accretion is characterized by extreme focusing of melt to discrete magmatic centers. Close to its eastern tip lies the unusual 5,310‐m‐deep Gakkel Ridge Deep (GRD) with limited sediment infill, which is in strong contrast to the broader sediment‐filled rift valleys to the east and west. Here, we report an 40Ar/39Ar age of 3.65 ± 0.01 Ma for a pillow basalt from a seamount located on the rim the GRD, confirming ultraslow spreading rates of ~7 mm/year close to the Laptev Sea as suggested from aeromagnetic data. Its geochemistry points to an alkaline lava, attributed to partial melting of a source that underwent prior geochemical enrichment. We note that the GRD extracts compositionally similar melts as the sparsely magmatic zone further west but at much slower spreading velocities of only ~6−7 mm/year, indicating the widespread occurrence of similarly fertile mantle in the High Arctic. This enriched source differs from subcontinental lithospheric mantle that influences magmatism along the Western Volcanic Zone (Goldstein et al., 2008, https://doi.org/10.1038/nature06919) and is similar to metasomatized mantle, shown to influence melt genesis along the Eastern Volcanic Zone.

We report on seismic and petrological data that provide new constraints on the geological evolution of the Amerasia Basin. A seismic reflection transect across the Makarov Basin, located between the Mendeleev and Lomonosov Ridges, shows a complete undisturbed sedimentary section of Mesozoic/Cenozoic age. In contrast to the Mendeleev Ridge, the margin of the Lomonosov Ridge is wide and shows horst and graben structures. We suggest that the Mendeleev Ridge is most likely volcanic in origin and support this finding with a 40 Ar/ 39 Ar isotopic age for a tholeiitic basalt sampled from the central Alpha/Mendeleev Ridge. Seismic reflection data for the Makarov Basin show no evidence of compressional features, consistent with the Lomonosov Ridge moving as a microplate in the Cenozoic. We propose that the Amerasia Basin moved as a single tectonic plate during the opening of the Eurasia Basin.

Asymmetrically zoned hotspot tracks in the Pacific Ocean are interpreted to have formed from zoned plumes originating from the large-scale, lower-mantle, low-seismic-velocity anomaly (superplume?) beneath the southern Pacific, providing direct information about lower-mantle 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 first 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 Pacific zoned hotspot tracks. These findings confirm that the proposed zonation of Pacific 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 Pb-207/Pb-204 and Pb-208/Pb-204 at a given Pb-206/Pb-204, but low Nd-143/Nd-144 and Hf-176/Hf-177 (DUPAL-like composition), may reflect 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 processes and the passive margin and adjacent continents that might explain extensive magmatism, lithospheric thinning and phases of post-rift uplift on continental margins and nearby continents.

Establishing if and when South Atlantic hotspots interacted with surface processes during rifting and continental breakup is important for understanding the mechanisms that control the evolution of passive margins and their adjacent continents. One approach is to reconstruct the volcanic history of hotspot trails located on the African Superswell in order to find the locations of hotspots during rifting and breakup to determine if, for example, they caused extreme fluxes of magma and post-rift uplift along the continental margin. However, because hotspot trails located south of the classical Tristan-Gough are virtually un-sampled we don’t know how many hotspots might have existed or for how long, and whether they originated from the core-mantle boundary or much shallower depths. In 2006 we dredge sampled hotspot trails located on the African Superswell using the RV Polarstern, an icebreaker capable of working in the poor weather conditions in the Southern Ocean. Combining new and existing Ar/Ar isotopic ages shows that volcanism migrated synchronously along co-parallel hotspot trails consistent with northeastern African plate motion relative to the leading edges of the African Superswell and an underlying stable Superplume (large low-shear-velocity province) extending from the core-mantle boundary. Between roughly 132 and 100 million years ago only the Tristan-Gough hotspot trail developed where rifting and breakup facilitated the rise of hotspot melts to the surface, while along rest of the leading edge hotspot volcanism was suppressed by the African continent. Such a notion implies that the African passive continental margin was migrating relative to the leading edge of the African Superplume for as long as 30 million years after continental rifting and breakup had facilitated the 132 Ma Parana-Etendeka continental flood basalts and initiation of the Tristan-Gough hotspot trail. This provides a mechanism for extended late stage interplay between deep mantle processes and the passive margin and adjacent continents that might explain extensive magmatism, lithospheric thinning and phases of postrift uplift.

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 laterally zoned since 40 Ma and the extreme DUPAL signature represents the northern component. In the Shona plume system, the extreme DUPAL component is found in the seamounts east of the Meteor Rise (31-43Ma) as well as the Agulhas Ridge (70-80 Ma). The EMI plume component is found on the Walvis Ridge (80Ma) and the Discovery seamounts (40Ma) and spreading ridge anomaly (0Ma) and shows a high variability in 207Pb/204Pb indicating an old age for this component. Plume dynamics suggest far-reaching sampling of lower mantle sources by plumes (Farnetani et al., 2012), however, the dynamics of the closely space South Atlantic plumes rising from a dense LLSVP remains to be investigated. The distribution of geochemical components along the South Atlantic plume tracks suggests that the DUPAL and EMI signatures are restricted to the African LLSVP and, moreover, a direct contribution of Pb from the LLSVP is indicated by Pb components formed during early Earth.

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 plume trail as well where the single location with HIMU affinity might simply reflect limited sampling. The extent of the DUPAL anomaly is discussed in the context of constraints on the mantle flow field as well as the composition of subcontinental lithospheric mantle and lower crust. Dynamic models require a shallow continental DUPAL anomaly to originate from the African continent, which is not supported by available data. Ultrapotassic rocks have been argued to represent the composition of the South American subcontinental lithospheric mantle showing the DUPAL signature. We argue this an insufficient argument for a shallow origin of the anomaly. Instead, the extent of the DUPAL anomaly along the mid Atlantic ridge as well as the bathymetric anomalies supports a deep origin of the DUPAL signature. Hart, S.R., 1984. Nature 309, 753-757.Class, C. and le Roex, A.P., 2009. Geochim Cosmochim Acta 73 (13) A229.Hartnady, C.J.H. and le Roex, A.P.,, 1985. Earth Planet. Sci. Lett. 75, 245-257.

The Shona Ridge system (Shona Ridge-Meteor Rise-Agulhas Ridge-Cape Rise seamounts) and the Discovery Rise to the north have been argued to represent the surface expressions of the Shona and Discovery mantle plumes, based on enriched samples along the southern Mid-Atlantic Ridge, a single sample from Discovery Rise and tectonic reconstructions. Leg PS69 ANT XXIII/5 of the R.V. Polarstern dredged the first comprehensive suite of samples from the Discovery Rise and Shona Ridge system, comprising alkali basalt through trachybasalt to trachyandesite and trachyte. Although many samples are moderately to heavily altered [loss on ignition (LOI) = 3-10%)], some incompatible elements (Nb, Ba, Zr, Y, Hf) appear to have remained sufficiently immobile that their ratios reflect the mantle source composition. Immobile element ratios for least altered samples (LOI < 2%) demonstrate the geochemically enriched nature (La/Sm-n > 2; La/Yb-n > 3; Zr/Nb < 10; Y/Nb < 2) of the underlying mantle. Samples from Discovery Rise include primitive basalt, trachyandesite and trachyte. Mafic lavas are relatively unaltered (LOI < 2%) and have high La/Sm-n (> 4 center dot 5), Ba/Nb (> 15) and Ba/Th (> 130) coupled with low Zr/Nb (< 6 center dot 5) and Ce/Nb-n (< 1). In accord with data from the single previously published Discovery Rise sample, these new samples confirm that the mantle source of Discovery Rise magmatism appears remarkably similar to that giving rise to Gough Island lavas some 400 km to the north. All lavas dredged from the aseismic Shona Ridge system are geochemically enriched relative to mid-ocean ridge basalt, with least altered samples having La/Sm-n > 2, Zr/Nb < 10 and Y/Nb < 2. They are heterogeneous in their immobile incompatible trace element ratios, and can be subdivided into four geochemical groups: Group A and B lavas (found on the Shona Ridge and Agulhas Ridge) have Ba/Nb < 10 coupled with Ce/Nb-n < 1; Group C lavas (restricted to the Meteor Rise) have Ba/Nb > 10 with Ce/Nb-n < 1; Group D lavas (Meteor Rise and Cape Rise) have Ba/Nb > 10 and Ce/Nb-n > 1. These differences are interpreted to reflect two physical components in the mantle source region: (1) a plume component (Shona plume; Group A and B lavas) comprising deep-seated upwelling mantle, itself heterogeneous and composed of typical, geochemically enriched, ocean island basalt-like mantle (La/Sm-n > 1; Ba/Nb < 10; Ce/Nb-n < 1), but including domains containing a recycled oceanic sediment component (Group C lavas); (2) a delaminated and recycled, metasomatized, subcontinental lithospheric mantle component, possibly rafted within the shallow convecting asthenospheric mantle, that has given rise to the Group D lavas. These new data are consistent with an earlier postulation that the Shona Ridge system represents the surface expression of an underlying mantle plume.

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 that they are not solely controlled by melt dynamics of a pyroxenite-peridotite source, but require variable proportions of pyroxenite in individual sources. New isotope data from the second example, the Discovery Seamounts in the South Atlantic, reveal a continuum in compositions between the extreme EMI composition of Walvis Ridge DSDP 525A and the LOMU extreme of the Discovery ridge anomaly (Douglass et al., 1999) and require a range of extreme composition outside the mixing tetrahedron of the global endmembers. In the global context, each individual island or volcano with enriched mantle affinity seems to form a trend towards its own unique enriched mantle endmember, inconsistent with mixing between narrowly defined global endmembers. The spectrum of enriched mantle endmembers is consistent with a dynamic Earth, continuously recycling varying proportions of oceanic crust, sediment and some continental lower crust or mantle. Douglass, J., Schilling, J.-G. and Fontignie, D., 1999. J. Geophys. Res., 104: 2941-2962.White, W.M., 1985. Geology, 13: 115-118.Zindler, A. and Hart, S., 1986. Ann. Rev. Earth Planet. Sci., 14: 493-571.