Subduction Duration and Slab Dip

The dip angles of slabs are among the clearest characteristics of subduction zones, but the factors that control them remain obscure. Here, slab dip angles and subduction parameters, including subduction duration, the nature of the overriding plate, slab age, and convergence rate, are determined for 153 transects along subduction zones for the present day. We present a comprehensive tabulation of subduction duration based on isotopic ages of arc initiation and stratigraphic, structural, plate tectonic and seismic indicators of subduction initiation. We present two ages for subduction zones, a long‐term age and a reinitiation age. Using cross correlation and multivariate regression, we find that (1) subduction duration is the primary parameter controlling slab dips with slabs tending to have shallower dips at subduction zones that have been in existence longer; (2) the long‐term age of subduction duration better explains variation of shallow dip than reinitiation age; (3) overriding plate nature could influence shallow dip angle, where slabs below continents tend to have shallower dips; (4) slab age contributes to slab dip, with younger slabs having steeper shallow dips; and (5) the relations between slab dip and subduction parameters are depth dependent, where the ability of subduction duration and overriding plate nature to explain observed variation decreases with depth. The analysis emphasizes the importance of subduction history and the long‐term regional state of a subduction zone in determining slab dip and is consistent with mechanical models of subduction

The Cocos and Carnegie Aseismic Ridges: a Trace Element Record of Long-term Plume-Spreading Center Interaction

The aseismic Cocos and Carnegie Ridges, two prominent bathymetric features in the eastern Pacific, record ∼20 Myr of interaction between the Galápagos hotspot and the adjacent Galápagos Spreading Center. Trace element data determined by inductively coupled plasma-mass spectrometry in >90 dredged seamount lavas are used to estimate melt generation conditions and mantle source compositions along the ridges. Lavas from seamount provinces on the Cocos Ridge are alkalic and more enriched in incompatible trace elements than any in the Galápagos archipelago today. The seamount lavas are effectively modeled as small degree melts of a Galápagos plume source. Their eruption immediately follows the failure of a rift zone at each seamount province's location. Thus the anomalously young alkalic lavas of the Cocos Ridge, including Cocos Island, are probably caused by post-abandonment volcanism following either a ridge jump or rift failure, and not the direct activity of the Galápagos plume. The seamounts have plume-like signatures because they tap underlying mantle previously infused with Galápagos plume material. Whereas plume heterogeneities appear to be long-lived, tectonic rearrangements of the ridge plate boundary may be the dominant factor in controlling regional eruptive behavior and compositional variations

Os Isotope Systematics in the Canary Islands and Madeira: Lithospheric Contamination and Mantle Plume Signatures

Osmium concentrations and isotopic signatures were measured in 28 primarily Holocene basalts (22 of which have been analyzed for Sr–Nd–Pb isotope composition), two carbonatites and two mantle xenoliths from the Canary Islands, Selvagen Grande and Madeira in the eastern North Atlantic. 187Os/188Os ratios in the basalts range from 0.129 to 0.183. The Os isotope systematics indicate that the basalts fall into three petrogenetic groups: (1) a ‘radiogenic’ group with high 187Os/188Os from 0.152 to 0.183; (2) an ‘unradiogenic’ group with low 187Os/188Os from 0.129 to 0.138; (3) an ‘intermediate’ group with 187Os/188Os between 0.139 and 0.151. The Os isotope systematics of the radiogenic group samples are consistent with minor contamination of the basalts by marine sediment. All samples in the unradiogenic group contain mantle xenoliths, and the unradiogenic Os can be explained by bulk assimilation of ≤ 5% mantle peridotite in the form of disaggregated xenoliths. The radiogenic and unradiogenic groups are also characterized by higher 87Sr/86Sr and 208Pb/204Pb but lower 143Nd/144Nd than samples with similar 206Pb/204Pb from the intermediate group, which is interpreted to reflect interaction of plume magmas with the lithospheric mantle. The intermediate group samples are believed to represent the isotopic signature of the mantle plume. The Os isotopic composition of the Canary plume is among the most radiogenic found in ocean island basalts, comparable with the endmember HIMU islands Mangaia and Tubuaii, but at significantly lower 206Pb/204Pb. The radiogenic Os and moderate 206Pb/204Pb signature of the Canary plume is consistent with a plume which contains 25–35% of relatively young (∼1.2 Ga) recycled oceanic crust. Variable degree of mixing of the Canary Island plume source with shallow depleted asthenosphere containing a component of Paleozoic oceanic crust produces the limited range in Os isotopic signatures observed in the Madeira and Canary Island basalts despite a large range in 206Pb/204Pb isotopic composition

Seismic and geochemical evidence for large-scale mantle upwelling beneath the eastern Atlantic and western and central Europe

Seismic tomography and the isotope geochemistry of Cenozoic volcanic rocks suggest the existence of a large, sheet-like region of upwelling in the upper mantle which extends from the eastern Atlantic Ocean to central Europe and the western Mediterranean. A belt of extension and rifting in the latter two areas appears to lie above the intersection of the centre of the upwelling region with the base of the lithosphere. Lead, strontium and neodymium isotope data for all three regions converge on a restricted composition, inferred to be that of the upwelling mantle

How and when plume zonation appeared during the 132 Myr evolution of the Tristan Hotspot

Increasingly, spatial geochemical zonation, present as geographically distinct, subparallel trends, is observed along hotspot tracks, such as Hawaii and the Galapagos. The origin of this zonation is currently unclear. Recently zonation was found along the last B70 Myr of the Tristan-Gough hotspot track. Here we present new Sr–Nd–Pb–Hf isotope data from the older parts of this hotspot track (Walvis Ridge and Rio Grande Rise) and re-evaluate published data from the Etendeka and Parana flood basalts erupted at the initiation of the hotspot track. We show that only the enriched Gough, but not the less-enriched Tristan, component is present in the earlier (70–132 Ma) history of the hotspot. Here we present a model that can explain the temporal evolution and origin of plume zonation for both the Tristan-Gough and Hawaiian hotspots, two end member types of zoned plumes, through processes taking place in the plume sources at the base of the lower mantle

Petrogenesis of Eocene Tamazert continental carbonatites (Central High Atlas, Morocco): implications for a common source for the Tamazert and Canary and Cape Verde Island carbonatites

The Tamazert Eocene alkaline complex of the Central High Atlas Range of Morocco hosts the largest outcropping occurrences of carbonatites in northern Africa. The complex consists of carbonatites and undersaturated ultramafic to syenitic alkaline to peralkaline silicate rocks. Mineralogically and geochemically the Tamazert carbonatites are classified as calciocarbonatites, magnesiocarbonatites and silicocarbonatites.They are enriched in light rare earth elements and large ion lithophile elements (Cs, Rb, Ba, U,Th), but depleted in high field strength elements (particularly, Ti, Nb and Ta). Stable and radiogenic isotope ratios vary in the range of δ13CPDB=-5·8 to 1·8 0/00, δ18OSMOW=6·9-23·5 0/00, initial 87Sr/86Sr=0·7031-0·7076, 143Nd/144Nd=0·5125-0·5129 and 206Pb/204Pb=18·29-19·89. Calciocarbonatites intruding Jurassic limestones have the highest δ13C and δ18O values and the most radiogenic initial 87Sr/86Sr, but least radiogenic 143Nd/144Nd, 206Pb/204Pb and 208Pb/204Pb isotope ratios, and are interpreted to have interacted with the limestones (crustal components). The magnesio- and silicocarbonatites have Sr, Nd and Pb isotope ratios that are nearly identical to those of low-87Sr/86Sr calciocarbonatites. The isotope signature of the high-Sr, low-87Sr/86Sr calciocarbonatites with mantle-type O and C isotopic compositions indicates the presence of HIMU- and EMI-type components in the mantle source of the Tamazert carbonatites, similar to what has been proposed for the Cape Verde and Canary Islands.The close similarity in carbonatite composition between the Cape Verde and Canary Islands and Tamazert suggests a common sublithospheric source for these carbonatites. We therefore propose that theTamazert carbonatites originated through melting of Canary plume material that may have flowed through a sub-lithospheric corridor extending from the Atlantic near the Canary Islands to the Middle Atlas, formed by the delamination of the subcontinental lithosphere in response to Africa-Europe collision at c. 42Ma. Seismic tomography data suggest that the common source may be within the lower mantle at depths >1000 km

Seamounts off the West Antarctic margin: A case for non-hotspot driven intraplate volcanism

Highlights: • Marie Byrd Seamounts (MBS) formed off Antarctica at 65-56 Ma in an extensional regime • MBS originate from HIMU-type mantle attached at the base of the Antarctic lithosphere • Continental insulation flow transferred HIMU mantle into the oceanic mantle New radiometric age and geochemical data of volcanic rocks from the guyot-type Marie Byrd Seamounts (MBS) and the De Gerlache Seamounts and Peter I Island (Amundsen Sea) are presented. 40Ar/39Ar ages of the shield phase of three MBS are Early Cenozoic (65 to 56 Ma) and indicate formation well after creation of the Pacific-Antarctic Ridge. A Pliocene age (3.0 Ma) documents a younger phase of volcanism at one MBS and a Pleistocene age (1.8 Ma) for the submarine base of Peter I Island. Together with published data, the new age data imply that Cenozoic intraplate magmatism occurred at distinct time intervals in spatially confined areas of the Amundsen Sea, excluding an origin through a fixed mantle plume. Peter I Island appears strongly influenced by an EMII type mantle component that may reflect shallow mantle recycling of a continental raft during the final breakup of Gondwana. By contrast the Sr-Nd-Pb-Hf isotopic compositions of the MBS display a strong affinity to a HIMU type mantle source. On a regional scale the isotopic signatures overlap with those from volcanics related to the West Antarctic Rift System, and Cretaceous intraplate volcanics in and off New Zealand. We propose reactivation of the HIMU material, initially accreted to the base of continental lithosphere during the pre-rifting stage of Marie Byrd Land/Zealandia to explain intraplate volcanism in the Amundsen Sea in the absence of a long-lived hotspot. We propose continental insulation flow as the most plausible mechanism to transfer the sub-continental accreted plume material into the shallow oceanic mantle. Crustal extension at the southern boundary of the Bellingshausen Plate from about 74 to 62 Ma may have triggered adiabatic rise of the HIMU material from the base of Marie Byrd Land to form the MBS. The De Gerlache Seamounts are most likely related to a preserved zone of lithospheric weakness underneath the De Gerlache Gravity Anomaly

Oxygen isotope heterogeneity of the mantle beneath the Canary Islands : insights from olivine phenocrysts

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 162 (2011): 349-363, doi:10.1007/s00410-010-0600-5.A relatively narrow range of oxygen isotopic ratios (δ18O = 5.05.4‰) is preserved in olivine of mantle xenoliths, mid-ocean ridge (MORB) and most ocean island basalts (OIB). The values in excess of this range are generally attributed either to the presence of a recycled component in the Earth’s mantle or to shallow level contamination processes. A viable way forward to trace source heterogeneity is to find a link between chemical (elemental and isotopic) composition of the earlier crystallized mineral phases (olivine) and the composition of their parental magmas, then using them to reconstruct the composition of source region. The Canary hotspot is one of a few that contains ~1-2 Ga old recycled ocean crust that can be traced to the core-mantle boundary using seismic tomography and whose origin is attributed to the mixing of at least three main isotopically distinct mantle components i.e., HIMU, DMM and EM. This work reports ion microprobe and single crystal laser fluorination oxygen isotope data of 148 olivine grains also analyzed for major and minor elements in the same spot. The olivines are from 20 samples resembling the most primitive shield stage picrite through alkali basalt to basanite series erupted on Gran Canaria, Tenerife, La Gomera, La Palma and El Hierro, Canary Islands, for which shallow level contamination processes were not recognized. A broad range of δ18Oolivine values from 4.6 to 6.1‰ was obtained and explained by stable, long-term oxygen isotope heterogeneity of crystal cumulates present under different volcanoes. These cumulates are thought to have crystallized from mantle derived magmas uncontaminated at crustal depth, representing oxygen isotope heterogeneity of source region. A relationship between Ni×FeO/MgO and δ18Oolivine values found in one basanitic lava erupted on El Hierro, the westernmost island of the Canary Archipelago, was used to estimate oxygen isotope compositions of partial melts presumably originated from peridotite (HIMU-type component inherited its radiogenic isotope composition from ancient, ~12 Ga, recycled ocean crust) and pyroxenite (young, <1 Ga, recycled oceanic crust preserved as eclogite with depleted MORB-type isotopic signature) components of the Canary plume. The model calculations yield 5.2 and 5.9±0.3‰ for peridotite and pyroxenite derived melts, respectively, which appeared to correspond closely to the worldwide HIMU-type OIB and upper limit N-MORB δ18O values. This difference together with the broad range of δ18O variations found in the Canarian olivines cannot be explained by thermodynamic effects of oxygen isotopic fractionation and are believed to represent true variations in the mantle, due to oceanic crust and continental lithosphere recycling.This work was supported by the CNRS “poste rouge” grant to AG, the NSF EAR-CAREER-0844772 grant to IB and the CRPG-CNRS and at its initial stage by the DFG (grant SCHM 250/64) and the Alexander von Humboldt Foundation (Wolfgang Paul Award to A.V. Sobolev who provided access to the electron microprobe at the Max Planck Institute, Mainz, Germany)

Two-stage evolution of mantle peridotites from the Stalemate Fracture Zone, northwestern Pacific

This paper reports the results of a mineralogical study of 14 mantle peridotite samples dredged in 2009 from the eastern slope of the northwestern segment of the Stalemate Ridge in the northwestern Pacific during cruise SO201-KALMAR Leg 1b of the R/V Sonne. The sample collection included four serpentinized and silicified dunites and ten variably serpentinized lherzolites. The compositions of primary minerals (clinopyroxene, orthopyroxene, and spinel) change systematically from the lherzolites to dunites. Spinel from the lherzolites shows higher Mg# and lower Cr# values (0.65-0.68 and 0.26-0.33, respectively) compared with spinel from the dunites (Mg# = 0.56-0.64 and Cr# = 0.38-0.43). Clinopyroxene from the lherzolites is less magnesian (Mg# = 91.7-92.4) than clinopyroxene from dunite sample DR37-3 (Mg# = 93.7). Based on the obtained data, it was concluded that the lherzolites of the Stalemate Fracture Zone were derived by 10-12% near-fractional melting of a DMM-type depleted mantle reservoir beneath the Kula-Pacific spreading center. The dunites were produced by interaction of residual lherzolites with sodium- and titaniumrich melt and are probably fragments of a network of dunite channels in the shallow mantle. The moderately depleted composition of minerals clearly distinguishes the lherzolites from the strongly depleted peridotites of the East Pacific Rise and indicates the existence of slow-spreading mid-ocean ridges in the Pacific Ocean during the Cretaceous-Paleogene