416 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)
Initial results for the composition of the igneous basement of the Bowers and Shirshov Ridges (Bering Sea, NW Pacific)
The Bowers and Shirshov Ridges (hereafter BR and SR, respectively) are two prominent submarine structures of
unknown age and provenance in the Bering Sea. So far only a few geochemical data exist on the composition of
basement rocks from the SR (Silantyev et al., 1985) and none for the BR. Age and geochemical data are crucial
to evaluate if the ridges represent remnant island arcs (Cooper et al. 1981, Scholl 2007), former pieces of
Kamchatka rifted away through seafloor spreading (SR: Baranov et al. 1991) or parts of the Mesozoic Hawaiian
hot-spot (Steinberger & Gaina, 2007).
Here we report the first geochemical data on the composition of the basement rocks from the BR and SR,
recovered during KALMAR R/V SONNE cruise 201 (Legs 1b and 2) in 2009. Fresh to moderately altered
volcanic rocks were dredged from the northern slope of the BR, from seamounts on the western extension of the
BR and from the western slope of the central part of the SR. We studied the petrography of the samples and
carried out geochemical analyses of major and trace elements by XRF and ICPMS at ACME Lab (Vancouver,
Canada) and CAU (Kiel).
The rocks from the northwestern slope of the BR are clinopyroxene (cpx)-phyric basalts with minor amounts of
olivine (ol) and plagioclase (plag) microphenocrysts, as well as hbl-plag-cpx-bearing basaltic andesites and
trachyandesites. The rocks are strongly enriched in LREE (LaN/YbN = 3.2 – 8.5, N indicates normalization to
primitive mantle), fluid-mobile elements (Ba, U, K) relative to NMORB and exhibit clear negative anomalies of
HFSE (Nb, Ta and Ti) in primitive mantle-normalized incompatible element diagrams. The BR rocks also have a
moderate adakitic signature, as indicated by elevated SrN/YN ratios (6.9 – 12.9). Hbl-cpx-plag trachybasalts from
the SR have similar major and trace element compositions (LaN/YbN = 2.1 – 4.9) to the BR rocks. The other
magmatic series from the SR comprises massive trachyandesites, trachytes and dacites with rare phenocrysts of
plag and cpx. These rocks also have island-arc type incompatible element patterns and are distinct from other
rock types from the BR and SR with less LREE enriched patterns (LaN/YbN ~ 1.8) and a strong negative Eu
anomaly (Eu/Eu* = 0.74).
Rocks dredged from a seamount on the western extension of the BR have very distinctive petrographic and
geochemical characteristics. These are ol-phyric pillow basalts with minor (less than 5%) amounts of plag and
cpx. The freshest whole rocks and pillow-rim glasses have relatively smooth patterns of incompatible trace
elements, akin to intraplate oceanic basalts and in some characteristic incompatible element ratios (e.g. ThN/BaN
= 0.6, SrN/CeN = 1.2, LaN/YbN = 3.3) are similar to Hawaiian hotspot tholeiites.
In summary, petrography and preliminary geochemical results indicate an island-arc origin for major parts of the
BR and SR. The discovery of intraplate basalts suggests that fragments of the Emperor Seamount Chain could
also be preserved in the Bering Sea (Steinberger & Gaina 2007) as seamounts and in the BR and SR basement.
Our further studies will be focused on obtaining absolute age data for the studied rocks, which will allow
combining the petrologic data with tectonic and geodynamic models for the NW Pacific
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
Geochronology, geochemistry and Nd, Sr and Pb isotopes of syn-orogenic granodiorites and granites (Damara orogen, Namibia) – arc-related plutonism or melting of mafic crustal sources?
Highlights
• Geochemical data from high-T granodiorites and granites imply lower crustal amphibolite melting.
• New U-Pb zircon ages imply syn-orogenic intrusion
• New Sr-Nd-Pb isotope data imply ancient crustal sources and constrain AFC processes
Abstract:
The Gawib pluton (Damara Belt, Namibia) consists of two main intrusive rock types; magnesian, calc-alkaline, mostly metaluminous hornblende- and titanite-bearing granodiorites and magnesian to ferroan, metaluminous to slightly peraluminous calc-alkaline granites. Uranium-Pb zircon data obtained on the granodiorites gave concordant ages of 548.5 ± 5.6 Ma indicating that the pluton belongs to the early syn-orogenic magmatism in the Damara orogen. Major and trace element variations indicate that fractional crystallization was the major rock-forming mechanism for the granodiorites. In the absence of high-precision geochronological data, the granites may represent more advanced fractionation products of the granodiorites although distinct Ba-Sr-Rb relationships preclude a direct derivation of the granites from the exposed granodiorites. If the granites originated by extensive fractional crystallization from similar granodiorites, they can only be derived from high-Ba, high-Sr, low-Rb granodiorites. Crustal contamination was also important in the petrogenesis of both rock types (granodiorites: ε Nd(init.): -7 to -13; 87Sr/86Sr(init.): 0.708-0.713; granites: ε Nd(init): -14 to -18; 87Sr/86Sr(init.): 0.712-0.726). In contrast to the granodiorites, the granites show more radiogenic 87Sr/86Sr ratios and less radiogenic ε Nd values indicating different contaminants for both rock types. ε Nd vs. MgO relationships imply some genetic link to isotopically unevolved quartz diorites similar to those observed at the Palmental complex. This pluton, however, is located c. 80 km NE from the Gawib pluton and probably cannot be viewed as the direct source of the Gawib granodiorites. If such a relationship is allowed, the granodiorites must be viewed as hybrid rocks containing a juvenile component because they were derived from unevolved quartz diorites by fractional crystallization. In addition, AFC processe have also played a role implying that the granodiorites contain also a reprocessed crustal component. Alternatively, comparison with experimentally derived melts imply that the granodiorites are generated by dehydration melting of a mafic, amphibole-bearing lower crustal source. Chemical parameters of the granodiorites compared to experimental results indicate high temperatures of c. 1040 °C. Zirconium saturation temperatures obtained on the most primitive samples gave c. 830 °C whereas apatite saturation temperatures obtained on the same samples give temperatures of c. 960-980 °C; the latter seems to be a more reliable temperature estimate. Interpretation of geochemical and isotope data from the complex suggest that the early synorogenic Pan-African igneous activity in this part of the Damara Belt was a high-temperature intra-crustal event. In contrast to igneous processes along active continental margins that produce also intermediate plutons with calc-alkaline affinities, this igneous event was not a major crust-forming episode and the granodiorites represent mostly reprocessed crustal material
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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