ALBERT

Description: The waning stage(s) of the Tethyan ocean(s) in the Balkans are not well understood. Controversy centres on the origin and life-span of the Cretaceous Sava Zone, which is allegedly a remnant of the last oceanic domain in the Balkan Peninsula, defining the youngest suture between Eurasia- and Adria-derived plates. In order to investigate to what extent late-Cretaceous volcanism within the Sava zone is consistent with this model, we present new age data together with trace-element and Sr–Nd–Pb isotope data for the Klepa basaltic lavas from the central Balkan Peninsula. Our new geochemical data show marked differences between the Cretaceous Klepa basalts (Sava Zone) and the rocks of other volcanic sequences from the Jurassic ophiolites of the Balkans. The Klepa basalts mostly have Sr–Nd–Pb isotopic and trace-element signatures that resemble enriched within-plate basalts, substantially different from Jurassic ophiolite basalts with MORB, BAB and IAV affinities. Trace-element modelling of the Klepa rocks indicates 2–20% polybaric melting of a relatively homogeneously metasomatised mantle source that ranges in composition from garnet lherzolite to ilmenite+apatite bearing spinel–amphibole lherzolite. Thus, the residual mineralogy is characteristic of a continental rather than oceanic lithospheric mantle source, suggesting an intracontinental within-plate origin for the Klepa basalts. Two alternative geodynamic models are internally consistent with our new findings: i) if the Sava Zone represents remnants of the youngest Neotethyan Ocean, magmatism along this zone would be situated within the forearc region and triggered by ridge subduction; ii) if the Sava Zone delimits a diffuse tectonic boundary between Adria and Europe, which had already collided in the Late Jurassic, the Klepa basalts together with a number of other magmatic centres represent volcanism related to transtensional tectonics.

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: The Central American Volcanic Arc (CAVA) has been subject of intensive research over the past decades, leading to a large variety of different models for the origin of CAVA lavas with various source components. Based on a comprehensive new geochemical data set (i.e. major and trace elements and Sr-Nd-Pb-Hf-O isotope ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples from NW CAVA (Nicaragua to Guatemala), we present a new model for the NW Central American Volcanic Arc volcanism. Additional potential source component sample data from subducting Cocos Plate sediments, igneous oceanic crust and Guatemalan granitic and metamorphic continental basement further contributes to our new model. We find systematically increasing Pb isotope ratios and decreasing Nd and Hf isotope ratios along the arc from NW Nicaragua to Guatemala. BVF lavas generally have more radiogenic Pb and less radiogenic Nd and Hf isotopic compositions than related VF lavas, similar to what is observed for trace element ratios going northwards along the VF. Combined isotope and trace element data indicate the presence of three endmembers for the volcanism in NW Central America: (1) NW Nicaraguan VF samples with very high Ba/(La, Th) and U/Th, low La/Yb, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb, (2) NW Guatemalan VF and Guatemalan and Honduran BVF samples with low Ba/(La, Th) and U/Th, high La/Yb, radiogenic Sr and Pb but unradiogenic Nd and Hf, and elevated d18O, and (3) Honduran and Nicaraguan BVF samples with low Ba/(La, Th) and U/Th, high La/Yb, unradiogenic Sr but radiogenic Nd, Hf and Pb. We interpret the NW Nicaragua VF endmember to be dominated by a largely serpentinite-derived fluid flux from the subducting slab, possibly with small amounts (〈1 wt. %) of sediment melts, to a depleted N-MORB type of mantle wedge, resulting in large degrees of melting of primarily peridotitic material. Based on combined Hf and Nd and Hf and Pb isotope systematics, the isotopically enriched Guatemala VF and BVF endmember cannot be explained by the addition of subducted pelagic sediments to the source. Instead this endmember could be derived from pyroxenitic cumulates in the lithospheric mantle (and possibly lower crust) that were derived from parental magmas for plutonic rocks in NW Central America, which were melted during the Quaternary subduction-related volcanism. The isotopically depleted Honduras and Caribbean BA endmember could be derived from melting of young, recycled, oceanic crust in the asthenosphere upwelling in the back-arc, based on the OIB-like major and trace element but relatively depleted isotopic compositions of these samples. Mixing between these three endmember types of magmas can explain the observed systematic geochemical variations along and across the NW Central American Arc.

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: 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 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.

Description: The Canary Island Seamount Province forms a scattered hotspot track on the Atlantic ocean floor ,1300 km long and,350 km wide, perpendicular to lithospheric fractures, and parallel to theNWAfrican continental margin. New 40Ar/39Ar datings show that seamount ages vary from 133 Ma to 0.2 Ma in the central archipelago, and from 142 Ma to 91 Ma in the southwest. Combining 40Ar/39Ar ages with plate tectonic reconstructions, I find that the temporal and spatial distribution of seamounts is irreconcilable with a deep fixed mantle plume origin, or derivation from passive mantle upwelling beneath a mid-ocean ridge. I conclude that shallow mantle upwelling beneath the Atlantic Ocean basin off the NW African continental lithosphere flanks produced recurrent melting anomalies and seamounts from the Late Jurassic to Recent, nominating the Canary Island Seamount Province as oldest hotspot track in the Atlantic Ocean, and most long-lived preserved on earth.