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Composition chimique des sédiments entrant dans la zone de subduction des Petites Antilles

Abstract : Lesser Antilles lavas display large chemical variability and their isotopic compositions suggest variable incorporation of old crustal material during their genesis. The Lesser Antilles arc is also characterized by a chemical zoning from north to south along the arc, and lavas from the southern islands generally display a more pronounced crustal fingerprint than northern lavas. The aim of the study is to determine whether a north-south chemical variation exists within the subducting sediment cover, and if potential change in the sediment compositions can explain the chemical variations observed in the lavas. We have undertaken a detailed geochemical study (major and trace elements, Sr-Nd-Hf-Pb isotopes) of sedimentary flux potentially entering the subduction zones at different latitudes. Sampling includes sediments drilled at sites 543 (northern forearc) and 144 (extreme southern forearc) during DSDP Legs 78A and 14 respectively, and sediments from Barbados island (southern forearc).
Samples display large lithological heterogeneity which can globally be explained by mixing in variable proportion of a detrital component and a biogenic one (carbonated or siliceous). Moreover, at site 144, some organic rich deposits (black shales) of upper Cenomanian to Santonian (~ 95 to 84 Ma) were drilled and correspond to sedimentary records of Cretaceous Oceanic Anoxic Events 2 and 3. We have shown that variable dilution of detrital component by biogenic one is the main factor controlling the observed variations of trace element concentrations. Moreover, we have revealed an extremely important enrichment of U in site 144 black shales. Nd-Hf-Pb isotopic compositions of studied sediments mainly represent those of the detrital component, whereas Sr isotope ratios, in the case of carbonate rich sediments, are dominated by the seawater signature. Sediments from the three sites exhibit highly radiogenic Pb isotopic compositions if compared with other sediments localized in front of the other major subduction zones, due in part to the important input of detrital material derived from the Guyana and Brazilian Shields. Moreover, Cretaceous black shales from site 144, U-enriched, are characterized by extremely high Pb isotopes ratios due to radioactive decay of U.
Mixing of depleted mantle with northern sediments (site 543) produce the isotopic compositions of lavas from the northern part of the arc. For the southern part of the arc, mixing of the depleted mantle and sediments from site 144 explain the compositions of lavas from the islands of Martinique to Grenada. An increase of black shales contribution from north to south is necessary and in agreement with the increasing age of subducting Atlantic crust from north to south along the Lesser Antilles trench. Finally, some sediments of Barbados island associated with sediments like those from site 543 may represent the appropriated sedimentary end member to explain isotopic composition of southern island lavas.
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https://tel.archives-ouvertes.fr/tel-00187335
Contributor : Marion Carpentier <>
Submitted on : Wednesday, November 14, 2007 - 2:24:47 PM
Last modification on : Thursday, November 19, 2020 - 3:54:26 PM
Long-term archiving on: : Monday, April 12, 2010 - 2:10:35 AM

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Marion Carpentier. Composition chimique des sédiments entrant dans la zone de subduction des Petites Antilles. Géochimie. Université Joseph-Fourier - Grenoble I; Université Libre de Bruxelles, 2007. Français. ⟨tel-00187335⟩

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