339 research outputs found
The Agulhas Ridge: Connections between changing patterns of deep sea currents and volcanic-tectonic activity as well as origin of the "Dupal Anomaly" and intraplate volcanism - Cruise No. MSM19/3 - December 1st - December 23rd, 2011 - Cape Town (South Africa) - Cape Town (South Africa)
Northwestern Central American Volcanic Arc: Increased contribution of enriched lithosphere to lavas along the volcanic front from Nicaragua to Guatemala and behind the volcanic front
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
Crustal Evolution and Petrogenesis of Silicic Plutonic Rocks within the Oman Ophiolite – Petrological and Geochemical Investigations
Temporal variations in Galápagos plume-ridge interaction at the Cocos-Nazca spreading center
The major goals of cruise SO208 with the German research vessel Sonne were to investigate 1) plume-ridge interaction through time at the Cocos-Nazca spreading center (CNS) north of the Galápagos Islands by sampling across axis profiles of the seafloor and 2) off axis volcanism at the East Pacific Rise (EPR) versus far field effects of the Galápagos hotspot documented in seamounts off the coast of N Costa Rica and Nicaragua. Overall the nature of material transfer from the plume to the ridge and its large scale distribution throughout the Eastern Pacific is being investigated by means of major and trace element and Sr-Nd-Pb (double spike) isotope data.
The seamounts on the EPR generated part of the Cocos plate appear to originate on one hand from a depleted MORBlike source consistent with their formation near the EPR axis, while other seamounts formed through lower degrees melting of an enriched OIB source either more distant from the EPR or by intraplate volcanism. Geochemical profiles across the Western and Eastern CNS indicate the participation of two different Galápagos plume components with a change in the amount this material entering the CNS with time. While at the western profile element ratios of more to less incompatible elements show an overall decrease of a plume component, Wolf-Darwin or Northern domain [1], with increasing age, the opposite is observed at the eastern profile. The Central domain component [1] increases with increasing age of the crust in this area. These observations indicate variable flux of specific Galápagos plume components to the CNS over the past 800 000 years. Sr-Nd-Pb isotope data to verify these observations are currently being generated and will be presented at the conference.
[1] Hoernle et al. (2000) Geology 28, 435–43
Recycling of geochemically heterogenous oceanic crust: significance for the origin of ocean island basalts
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 U–Th–Ra 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 4.8–7.6°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
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