ALBERT

Description: RECENT advances in 40Ar/39Ar dating1,2 have made it possible to date individual K-feldspar grains from Pleistocene tephra, a capability that greatly improves the reliability and temporal resolving power of the method. Here we apply these new techniques to the dating of a phonolite tephra from the East Eifel volcanic field in West Germany, which is sandwiched between loess and palaeosol (alfisol) deposits, and which was therefore erupted during the transition from a glacial to an interglacial period. Our age estimate for this transition is 215±4 kyr (1 σ), which has important implications for the marine δ18O timescale and for models of global climate change during the Pleistocene. The results show that single-grain dating can detect and compensate for the large quantities of xenocrystic contaminants which are found in many tephra deposits. This technique could be used to date the tephra layers found in marine sediment cores and the results could greatly enhance the reliability of the marine δ18O timescale for more rigorous Fourier analysis testing of the Milankovitch hypothesis.

Description: Thin oceanic crust is formed by decompression melting of the upper mantle at mid-ocean ridges, but the origin of the thick and buoyant continental crust is enigmatic. Juvenile continental crust may form from magmas erupted above intraoceanic subduction zones, where oceanic lithosphere subducts beneath other oceanic lithosphere. However, it is unclear why the subduction of dominantly basaltic oceanic crust would result in the formation of andesitic continental crust at the surface. Here we use geochemical and geophysical data to reconstruct the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70Myr. We find that the geochemical signature of erupted lavas evolved from basaltic to andesitic about 10Myr ago - coincident with the onset of subduction of more oceanic crust that originally formed above the Galápagos mantle plume. We also find that seismic P-waves travel through the crust at velocities intermediate between those typically observed for oceanic and continental crust. We develop a continentality index to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust globally. We conclude that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone - a process probably more common in the Archaean - can produce juvenile continental crust.

Description: Resolving flow geometry in the mantle wedge is central to understanding the thermal and chemical structure of subduction zones, subducting plate dehydration, and melting that leads to arc volcanism, which can threaten large populations and alter climate through gas and particle emission. Here we show that isotope geochemistry and seismic velocity anisotropy provide strong evidence for trench-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua. This finding contradicts classical models, which predict trench-normal flow owing to the overlying wedge mantle being dragged downwards by the subducting plate. The isotopic signature of central Costa Rican volcanic rocks is not consistent with its derivation from the mantle wedge1, 2, 3 or eroded fore-arc complexes4 but instead from seamounts of the Galapagos hotspot track on the subducting Cocos plate. This isotopic signature decreases continuously from central Costa Rica to northwestern Nicaragua. As the age of the isotopic signature beneath Costa Rica can be constrained and its transport distance is known, minimum northwestward flow rates can be estimated (63–190 mm yr-1) and are comparable to the magnitude of subducting Cocos plate motion (approx85 mm yr-1). Trench-parallel flow needs to be taken into account in models evaluating thermal and chemical structure and melt generation in subduction zones.

Description: [1] Although most Central American magmas have a typical arc geochemical signature, magmas in southern Central America (central Costa Rica and Panama) have isotopic and trace element compositions with an ocean island basalt (OIB) affinity, similar to the Galapagos-OIB lavas (e.g., Ba/La 〈 40, La/Yb 〉 10, 206Pb/204Pb 〉 18.8). Our new data for Costa Rica suggest that this signature, unusual for a convergent margin, has a relatively recent origin (Late Miocene ∼6 Ma). We also show that there was a transition from typical arc magmas (analogous to the modern Nicaraguan volcanic front) to OIB-like magmas similar to the Galapagos hot spot. The geographic distribution of the Galapagos signature in recent lavas from southern Central America is present landward from the subduction of the Galapagos hot spot tracks (the Seamount Province and the Cocos/Coiba Ridge) at the Middle American Trench. The higher Pb isotopic ratios, relatively lower Sr and Nd isotopic ratios, and enriched incompatible-element signature of central Costa Rican magmas can be explained by arc–hot spot interaction. The isotopic ratios of central Costa Rican lavas require the subducting Seamount Province (Northern Galapagos Domain) component, whereas the isotopic ratios of the adakites and alkaline basalts from southern Costa Rica and Panama are in the geochemical range of the subducting Cocos/Coiba Ridge (Central Galapagos Domain). Geological and geochemical evidence collectively indicate that the relatively recent Galapagos-OIB signature in southern Central America represents a geochemical signal from subducting Galapagos hot spot tracks, which started to collide with the margin ∼8 Ma ago. The Galapagos hot spot contribution decreases systematically along the volcanic front from central Costa Rica to NW Nicaragua.

Description: The 40Ar/39Ar ages for 35 volcanic rocks and 14C ages for two charcoal samples from the Madeira Archipelago and Ampère Seamount (eastern North Atlantic) are presented. The volcanic evolution of Madeira can be divided into a voluminous shield stage (〉4.6–0.7 Ma) and a subsequent low-volume posterosional stage (〈0.7–0 Ma). Volcanism during the shield stage originated from a two-armed rift system, composed of the E–W oriented Madeira rift arm and the N–S oriented Desertas rift arm. Average growth rates for the submarine (5500 km3/Ma) and subaerial (100–150 km3/Ma) shield stages on Madeira are among the lowest found for ocean island volcanoes. It is proposed that Madeira represents the present location of a 〉70 Myr old hotspot which formed Porto Santo Island (11.1–14.3 Ma), Seine, Ampère (31 Ma), Corral Patch and Ormond (65–67 Ma [Féraud et al., 1982, 1986]) Seamounts, and the Serra de Monchique (70–72 Ma [McIntyre and Berger, 1982]) complex in southern Portugal. Age and spatial relationships result in a calculated absolute African plate motion above the hotspot of 1.2 cm/yr around a rotation pole located at 43°36′N/ 24°33′W.

Description: Alternative reconstructions of the Jurassic northern extent of Greater India differ by up to several thousand kilometers. We present a new model that is constrained by revised seafloor spreading anomalies, fracture zones and crustal ages based on drillsites/dredges from all the abyssal plains along the West Australian margin and the Wharton Basin, where an unexpected sliver of Jurassic seafloor (153 Ma) has been found embedded in Cretaceous (95 My old) seafloor. Based on fracture zone trajectories, this NeoTethyan sliver must have originally formed along a western extension of the spreading center that formed the Argo Abyssal Plain, separating a western extension of West Argoland/West Burma from Greater India as a ribbon terrane. The NeoTethyan sliver, Zenith and Wallaby plateaus moved as part of Greater India until westward ridge jumps isolated them. Following another spreading reorganization, the Jurassic crust resumed migrating with Greater India until it was re-attached to the Australian plate ∼95 Ma. The new Wharton Basin data and kinematic model place strong constraints on the disputed northern Jurassic extent of Greater India. Late Jurassic seafloor spreading must have reached south to the Cuvier Abyssal Plain on the West Australian margin, connected to a spreading ridge wrapping around northern Greater India, but this Jurassic crust is no longer preserved there, having been entirely transferred to the conjugate plate by ridge propagations. This discovery constrains the major portion of Greater India to have been located south of the large-offset Wallaby-Zenith Fracture Zone, excluding much larger previously proposed shapes of Greater India.

Description: The Messinian salinity crisis—the desiccation of the Mediterranean Sea between 5.96 and 5.33 million years (Myr) ago1—was one of the most dramatic events on Earth during the Cenozoic era2. It resulted from the closure of marine gateways between the Atlantic Ocean and the Mediterranean Sea, the causes of which remain enigmatic. Here we use the age and composition of volcanic rocks to reconstruct the geodynamic evolution of the westernmost Mediterranean from the Middle Miocene epoch to the Pleistocene epoch (about 12.1–0.65 Myr ago). Our data show that a marked shift in the geochemistry of mantle-derived volcanic rocks, reflecting a change from subduction-related to intraplate-type volcanism, occurred between 6.3 and 4.8 Myr ago, largely synchronous with the Messinian salinity crisis. Using a thermomechanical model, we show that westward roll back of subducted Tethys oceanic lithosphere and associated asthenospheric upwelling provides a plausible mechanism for producing the shift in magma chemistry and the necessary uplift (approx1 km) along the African and Iberian continental margins to close the Miocene marine gateways, thereby causing the Messinian salinity crisis.

Description: The Madeira-Tore Rise, located ∼700 km off the NW African coast, forms a prominent ridge in the east Atlantic. The age and origin of the rise are controversial. This study presents major and trace element, Sr, Nd, Pb, Hf isotope and 40Ar/39Ar age determinations from volcanic rocks dredged from different sites along the rise. In addition, isotopic compositions of rock samples from Great Meteor Seamount in the central Atlantic are presented. The new radiometric and paleontologically constrained ages identify two major episodes of volcanism: The first is the base of the rise (circa 80 to 〉95 Ma) and the second is seamounts on the rise (0.5–16 Ma). It is proposed that interaction of the Canary hot spot with the Mid-Atlantic spreading center formed the deep basement of the Madeira-Tore Rise and the J-Anomaly Ridge west of the Atlantic spreading center in the Mid-Cretaceous. Age and geochemical data and plate tectonic reconstructions suggest, however, that the recovered Late Cretaceous volcanic rocks represent late stage volcanism from the time when the Madeira-Tore Rise was still close to the Canary hot spot. Long after moving away from the influence of the Canary hot spot, the Madeira-Tore Rise was overprinted by late Cenozoic volcanism. Miocene to Pleistocene volcanism at the northern end of the rise can be best explained by decompression mantle melting beneath extensional sectors of the Azores-Gibraltar Fracture Zone (African-Eurasian plate boundary). The geochemical compositions of these volcanic rocks suggest that the magmas were variably contaminated by enriched material within or derived by melting of enriched material underplated at the base of the lithosphere, possibly originating from the Cretaceous Canary plume. Alternatively, these late Cenozoic volcanic rocks may have derived from decompression melting of enriched pyroxenitic/eclogitic material in the upper mantle. Isotopically more depleted Pliocene to Pleistocene volcanism at the southern end of the Madeira-Tore Rise may be related to the nearby Madeira hot spot.

Description: New (40)Ar/(39)Ar ages combined with chemical and Sr, Nd, and Pb isotope data for volcanic rocks from Syria along with published data of Syrian and Arabian lavas constrain the spatiotemporal evolution of volcanism, melting regime, and magmatic sources contributing to the volcanic activity in northern Arabia. Several volcanic phases occurred in different parts of Syria in the last 20 Ma that partly correlate with different tectonic events like displacements along the Dead Sea Fault system or slab break-off beneath the Bitlis suture zone, although the large volume of magmas and their composition suggest that hot mantle material caused volcanism. Low Ce/Pb (〈20), Nb/Th (〈10), and Sr, Nd, and Pb isotope variations of Syrian lavas indicate the role of crustal contamination in magma genesis, and contamination of magmas with up to 30% of continental crustal material can explain their (87)Sr/(86)Sr. Fractionation-corrected major element compositions and REE ratios of uncontaminated lavas suggest a pressure-controlled melting regime in western Arabia that varies from shallow and high-degree melt formation in the south to increasingly deeper regions and lower extents of the beginning melting process northward. Temperature estimates of calculated primary, crustally uncontaminated Arabian lavas indicate their formation at elevated mantle temperatures (T(excess) similar to 100-200 degrees C) being characteristic for their generation in a plume mantle region. The Sr, Nd, and Pb isotope systematic of crustally uncontaminated Syrian lavas reveal a sublithospheric and a mantle plume source involvement in their formation, whereas a (hydrous) lithospheric origin of lavas can be excluded on the basis of negative correlations between Ba/La and K/La. The characteristically high (206)Pb/(204)Pb (similar to 19.5) of the mantle plume source can be explained by material entrainment associated with the Afar mantle plume. The Syrian volcanic rocks are generally younger than lavas from the southern Afro-Arabian region, indicating a northward progression of the commencing volcanism since the arrival of the Afar mantle plume beneath Ethiopia/Djibouti some 30 Ma ago. The distribution of crustally uncontaminated high (206)Pb/(204)Pb lavas in Arabia indicates a spatial influence of the Afar plume of similar to 2600 km in northward direction with an estimated flow velocity of plume material on the order of 22 cm/a.

Description: The origin of the Christmas Island Seamount Province in the northeast Indian Ocean is enigmatic. The seamounts do not form the narrow, linear and continuous trail of volcanoes that would be expected if they had formed above a mantle plume1, 2. Volcanism above a fracture in the lithosphere3 is also unlikely, because the fractures trend orthogonally with respect to the east–west trend of the Christmas Island chain. Here we combine 40Ar/39Ar age, Sr, Nd, Hf and high-precision Pb isotope analyses of volcanic rocks from the province with plate tectonic reconstructions. We find that the seamounts are 47–136 million years old, decrease in age from east to west and are consistently 0–25 million years younger than the underlying oceanic crust, consistent with formation near a mid-ocean ridge. The seamounts also exhibit an enriched geochemical signal, indicating that recycled continental lithosphere was present in their source. Plate tectonic reconstructions show that the seamount province formed at the position where West Burma began separating from Australia and India, forming a new mid-ocean ridge. We propose that the seamounts formed through shallow recycling of delaminated continental lithosphere entrained in mantle that was passively upwelling beneath the mid-ocean ridge. We conclude that shallow recycling of continental lithosphere at mid-ocean ridges could be an important mechanism for the formation of seamount provinces in young ocean basins.