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Massimo Coltorti, University of Ferrara (Italy)
Costanza Bonadiman, University of Ferrara (Italy)
Suzanne Y. O'Reilly, GEMOC-Macquarie University (Australia)
William L. Griffin, GEMOC-Macquarie University (Australia)
Norman Pearson, GEMOC-Macquarie University (Australia)
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Mantle xenoliths from two Late Tertiary necks of Sal island (Cape Verde Archipelago) show about 50% of spinel-bearing harzburgites and lherzolites. Cpx modal distribution in these rocks is bi-modal with two maxima between 8-17vol% and 1-3vol%. This fact, together with major and trace element whole rock and mineral data, indicates that the compositional variation from lherzolites to harzburgites cannot be explained by a common progressive depletion process.
Large protogranular cpx and opx in lherzolites have quite high Cr2O3 contents and trace element distributions compatible with original equilibration within the garnet facies. In situ Re/Os analyses of intergranular sulfides from lherzolites yield values of 187Os/188Os varying from 0.1013+0.0013 to 0.1268+0.0022. TRD model ages define four groups: 2.7-3.5Ga, 2.0-2.2Ga, 0.95-1.1Ga and 0.5-0.7Ga. These Re-depletion ages mirror the tectonic history of the western margin of the West African Craton and the corresponding continental margin of Brazil, strongly suggesting that at least part of the Cape Verde archipelago is underlain by fragments of subcontinetal lithospheric mantle (SCLM) and that the lithosphere-asthenosphere boundary (LAB) in oceanic regions may be deeper than previously considered.
On the other hand Cape Verde harzburgites are similar to ultradepleted mantle lithologies, commonly sampled by ocean island alkaline basalts. Sulfides are typically absent from these rocks, thus Re/Os ages cannot be determined. These samples are characterized by partial melting beyond the cpx-out reaction and are on average significantly more refractory than abyssal peridotites. Thermobarometric estimates exclude that these highly refractory bodies would be the result of plume interaction with an already depleted abyssal peridotites. They appear analogous to recent arc-related peridotites, allowing a recycling mechanism for their origin.
These data point toward a quite heterogeneous composition of the oceanic lithosphere which can result from the juxtaposition of i) subcontinental lithospheric mantle, ii) residua after MORB extraction and iii) lithospheric mantle material recycled into the upper mantle through subduction. These disparate processes of formation for oceanic lithosphere, including the presence of old continental lithospheric domains, imply variable LAB depths may be found in oceanic regions.
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