On- and off-axis chemical heterogeneities along the South Atlantic Mid-Ocean-Ridge (5-11 degrees S): Shallow or deep recycling of ocean crust and/or intraplate volcanism?.

Hoernle, Kaj , Hauff, Folkmar, Kokfelt, Thomas, Haase, Karsten M., Garbe-Schönberg, Dieter and Werner, Reinhard (2011) On- and off-axis chemical heterogeneities along the South Atlantic Mid-Ocean-Ridge (5-11 degrees S): Shallow or deep recycling of ocean crust and/or intraplate volcanism?. Earth and Planetary Science Letters, 306 . pp. 86-97. DOI 10.1016/j.epsl.2011.03.032.

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The Mid-Atlantic Ridge between the Ascension and Bode Verde Fracture Zones exhibits anomalous crustal
thickness and geochemical compositions, which could reflect the presence of either small, enriched
heterogeneities in the upper mantle or a weak, diffuse mantle plume. We report new trace element (106
samples) and Sr, Nd and Pb (double spike) isotope data from 72 ridge axis samples and 9 off-axis seamount
samples between 5 and 11°S, aswell asU–Th–Ra disequilibria data for the seamounts. The U-series data constrain
the age of one sample from Seamount D, furthest (120 km) east of the shallowest part of the ridge, to be
b10,000 yrs old and the samples from the other three seamounts closer to the ridge to be younger than
240,000 yrs. As can be most clearly discerned on a diagram of 208Pb/206Pb vs. 143Nd/144Nd, at least four distinct
components are required to explain the geochemical variations along the ridge: 1) a common depleted (D-MORBlike)
component in samples near and north of the Ascension Fracture Zone (4.8–7.6°S), representing the most
depleted compositions sampled thus far along the mid-Atlantic ridge, 2) an enriched component upwelling
beneath Ascension Island and the northern A1 ridge segment (segment numbers increase from A1 to A4 going
south from the Ascension Fracture Zone), 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, which extend from
Ascension Island and the A3 seamounts respectively to the depleted D-MORB component, interpreted to reflect
local ambient mantle. 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 shallowmantle, upwelling passively beneath the ridge systemor 2) the deep mantle via
an actively upwelling heterogeneous mantle plume that interacts with the ridge system.

Document Type: Article
Keywords: Volcanology; Southern Atlantic Mid-Ocean Ridge; Ascension hotspot; mantle plume-ridge interaction; trace element and Sr-Nd-Pb isotope geochemistry; U-series age dating
Research affiliation: Kiel University
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS Magmatic and Hydrothermal Systems
OceanRep > The Future Ocean - Cluster of Excellence
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/j.epsl.2011.03.032
ISSN: 0012-821X
Related URLs:
Projects: SPP 1144 "From Mantle to Ocean", Future Ocean
Date Deposited: 24 Nov 2010 13:52
Last Modified: 06 Mar 2017 09:26
URI: http://oceanrep.geomar.de/id/eprint/10015

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