ALBERT

Beschreibung: A Bragg Crystal Spectrometer (BCS) using a gas flow proportional counter as its primary detector is among the instruments under development for AXAF. The BCS will employ windows of 1-micron-thick polyimide coated on both sides with 200 A of Al; this window composition, while X-ray transmitting, will leak gas at a lower rate than the polypropylene film-based windows formerly employed. Accounts are given of the results obtained with additional innovative X-ray window materials currently under development, including diamond and Si-enriched Si3N4.

Beschreibung: Resolving the time–space (and compositional) evolution of volcanism along long-lived South Atlantic hotspot trails is important to understanding the connection between hotspot volcanism and mantle plumes. 40Ar/39Ar ages are reported here for rocks dredged from a line of five individual seamounts along an ∼290 km northeast to southwest line extending from the vicinity of Saint Helena Island, and also for Circe Seamount. These seamounts were created in a midplate setting and could have formed rapidly (≤1 Myr). The St. Helena Seamount ages reveal a remarkably linear migration rate of volcanism of 20±1 mm/yr for at least the past 19 Myr, which is interpreted as the absolute motion of the African plate. Because this is much slower than estimated for earlier African plate migration it also represents the first evidence based on seamount ages for a significant deceleration (∼33%) of the African plate since at least 19 Ma. However, this change could have occurred as early as 30 Ma when the limited data for the Tristan/Gough hotspot chain are also considered. This deceleration supports a relationship between African plate speed and the upsurge of hotspot volcanism on the African continent at ∼25 Ma. We suggest that the increased number of oceanic African hotspots between ∼19 and 30 Ma points to a link also between major changes in plate motion and the onset and continuation of oceanic hotspot volcanism. Our study supports the assumption that chains of individual, rapidly (?) formed seamounts have considerably more potential of providing clear insights into how mantle plumes interact with overriding lithosphere than do those consisting of uninterrupted, more massive lines of hotspot volcanism.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Ocean islands, seamounts and volcanic ridges are thought to form above mantle plumes.Yet, this mechanism cannot explain many volcanic features on the Pacific Ocean floor1 and some might instead be caused by cracks in the oceanic crust linked to the reorganization of plate motions1–3. A distinctive bend in the Hawaiian–Emperor volcanic chain has been linked to changes in the direction of motion of the Pacific Plate4,5, movement of the Hawaiian plume6–8, or a combination of both9. However, these links are uncertain because there is no independent record that precisely dates tectonic events that a�ected the Pacific Plate. Here we analyse the geochemical characteristics of lava samples collected from the Musicians Ridges, lines of volcanic seamounts formed close to the Hawaiian–Emperor bend. We find that the geochemical signature of these lavas is unlike typical ocean island basalts and instead resembles mid-ocean ridge basalts. We infer that the seamounts are unrelated to mantle plume activity and instead formed in an extensional setting, due to deformation of the Pacific Plate. 40Ar/39Ar dating reveals that the Musicians Ridges formed during two time windows that bracket the time of formation of the Hawaiian–Emperor bend, 53–52 and 48–47 million years ago.We conclude that the Hawaiian–Emperor bendwas formed by plate–mantle reorganization, potentially triggered by a series of subduction events at the Pacific Plate margins.

Beschreibung: 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.