213 research outputs found
Chemical Heterogeneities along the South Atlantic Mid-Ocean-Ridge (5-11°S): Shallow or Deep Recycling of Ocean Crust?
Between 5° and 11°S, the Mid-Atlantic Ridge displays
anomalous crustal thickness and geochemical compositions,
thought to be related to either small scale upper mantle
heterogeneities or a weak, diffuse mantle plume. We report
new high precision trace element and Sr, Nd and Pb (DS)
isotope data for 72 ridge axis samples and 9 off-axis seamount
samples along with UâThâRa disequilibria data for off axis
seamounts at c. 9.7°S. At least four distinct components are
needed to explain the geochemical variations along the ridge:
1) a common depleted (D-MORB-like) component near and
north of 4.8â7.6°S, 2) an enriched component upwelling
beneath Ascension Island and the northern A1 ridge segment
(segment numbers ascend from north to south), 3) an enriched
component upwelling beneath the A2 ridge segment, and 4) an
enriched component upwelling beneath the line of seamounts
east of the A3 segment and the A3 and A4 segments. The A1
and the A3+A4 segment lavas form well-defined mixing
arrays from Ascension Island and the A3 seamounts
respectively to the depleted D-MORB component. We
propose that the enriched components represent different
packages of subducted ocean crust and/or ocean island basalt
(OIB) type volcanic islands and seamounts that have either
been recycled through 1) the shallow mantle, upwelling
passively beneath the ridge system or 2) the deep mantle via
an actively upwelling heterogeneous mantle plume that
interacts with the ridge system
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 longâterm 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 longâterm 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 longâterm regional state of a subduction zone in determining slab dip and is consistent with mechanical models of subduction
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 SrâNdâPb 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 âradiogenicâ group with high 187Os/188Os from 0.152 to 0.183; (2) an âunradiogenicâ group with low 187Os/188Os from 0.129 to 0.138; (3) an âintermediateâ 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 25â35% of relatively young (âŒ1.2 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
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 âŒ20âMyr of interaction between the GalĂĄpagos hotspot and the adjacent GalĂĄpagos 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 GalĂĄpagos archipelago today. The seamount lavas are effectively modeled as small degree melts of a GalĂĄpagos 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 GalĂĄpagos plume. The seamounts have plume-like signatures because they tap underlying mantle previously infused with GalĂĄpagos 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
Volcanic CO2 output at the Central American subduction zone inferred from melt inclusions in olivine crystals from mafic tephras
The volatile contents of olivineâhosted (Fo89â71) melt inclusion glasses in rapidly quenched mafic tephras
from volcanic front volcanoes of the Central American Volcanic Arc (CAVA) in Guatemala, Nicaragua, and
Costa Rica, were analyzed by secondary ion mass spectrometry (SIMS) in order to derive the minimum eruptive
output of CO2, along with H2O, Cl, and S. Details of the analytical method are provided that establish
melt inclusion CO2 analyses with the Cameca ims6f at the Helmholtz Centre Potsdam. The highest CO2 concentrations
(up to 1800 mg/g) are observed in Nicaraguan samples, while melt inclusions from Guatemala and
Costa Rica have CO2 contents between 50 and 500 mg/g. CO2 does not positively covary with sediment/slab
fluid tracers such as Ba/La, Ba/Th, or U/La. Instead, the highest CO2 concentrations occur in the inclusions
with the most depleted incompatible element compositions and low H2O, approaching the composition
of midâocean ridge basalts (MORBs), whereas the most H2Oârich inclusions are relatively CO2âpoor
(<800 mg/g). This suggests that CO2 degassing was more extensive in the melts with the highest slab contribution.
CO2/Nb ratios in the least degassed CAVA melt inclusions are similar to those of primitive MORBs.
These are interpreted here as recording a minimum CO2 output rate from the mantle wedge, which amounts to
2.8 Ă 104 g/s for the âŒ1100 km long CAVA. Previously published estimates from quiescent degassing and
numerical modeling, which also encompassed the slab contribution, are 3 times higher. This comparison
allows us to estimate the proportion of the total CO2 output derived from the mantle wedge
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
Geodynamic evolution of the GalĂĄpagos hot spot system (Central East Pacific) over the past 20 m.y.: Constraints from morphology, geochemistry, and magnetic anomalies
[1] We report results of magnetic data from the Nazca Plate and of geochemical (major element and Sr-Nd-Pb-isotope) analyses of rocks dredged from the GalĂĄpagos hot spot tracks (Cocos, Carnegie, Malpelo and Coiba Ridges and adjacent seamounts) in the Central East Pacific. Magnetic anomalies indicate that the Malpelo and Carnegie Ridges were once attached and that seafloor spreading separated the two ridges between 14.5 Ma and 9.5 Ma. The variations in Sr-Nd-Pb isotopic composition show that three of the mantle components currently observed at the GalĂĄpagos (Central, Southern, and Eastern) existed in the hot spot for at least 20 m.y., whereas the Northern GalĂĄpagos mantle component has been present for at least âŒ15 Ma. Our data are consistent with the existence of a compositionally zoned/striped GalĂĄpagos plume since âŒ20 Ma. Combined constraints from the morphology of the hot spot tracks, the magnetic record, and the isotope geochemistry of the rock samples provide new insights into the hot spot-ridge geometry and interaction of the GalĂĄpagos hot spot with the Cocos-Nazca spreading center (CNS) over the past 20 m.y. At 19.5 Ma a ridge jump moved the spreading axis to the northern edge of the hot spot. Between 19.5 and 14.5 Ma, the spreading axis was located above the center of the hot spot. At 14.5 Ma, a new ridge jump moved the spreading axis to the south, splitting the paleo-Carnegie Ridge into the present Carnegie and Malpelo Ridges. The repeated ridge jumps reflect capture of the northwardly drifting spreading center by the GalĂĄpagos hot spot. At 11â12 Ma an offset of the spreading axis lay above the plume center. Spreading between the Carnegie and Malpelo Ridges continued until 9.5 Ma
BERING - A new international marine research project to investigate the magmatic and tectonic evolution of the Bering Sea and its margins
The BERING project is a new large project lead by the GEOMAR institution in Kiel and focused on marine and on-land investigations in Kamchatka, the Kurile and Aleutian Arcs, the Bering Sea, and the NW-Pacific. BERING is funded by the German Ministry of Education and Research with contributions from Russian and U.S. institutions. The overarching goal of BERING is to elucidate the magmatic and tectonic evolution of the Bering Sea and its margins over the past â„50 m.y. In particular, BERING investigates the physical and chemical conditions that control the development of subduction zones, including subduction initiation, evolution of mature arc systems, and the impact of subduction volcanism on the environment. To achieve this goal BERING will address the following major scientific questions in four major directions of study.............
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