Abyssal peridotite Hf isotopes identify extreme mantle depletion

• Published in: Earth and Planetary Science Letters
• Main Authors: Stracke, Andreas, Snow, J. E., Hellebrand, E., von Der Handt, Annette, Bourdon, Bernard, Birbaum, K., Günther, D.
• Other Authors: Institute of Geochemistry and Petrology ETH Zürich, Department of Earth Sciences Swiss Federal Institute of Technology - ETH Zürich (D-ERDW), Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology Zürich (ETH Zürich)-Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology Zürich (ETH Zürich), Institut für Mineralogie, Westfälische Wilhelms-Universität Münster = University of Münster (WWU), Department of Geosciences Houston, University of Houston, Department of Geology and Geophysics, University of Hawaii, Institute of Geoscience, Albert-Ludwigs-Universität Freiburg, Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE), École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Université Jean Monnet - Saint-Étienne (UJM)-Centre National de la Recherche Scientifique (CNRS), Laboratory of Inorganic Chemistry, Max-Planck Institute for Chemistry, Deutsche Forschungsgemeinschaft (DFG), National Science Foundation (NSF)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2011
• Subjects: mantle; Hf-Nd isotopes; abyssal peridotite; mid ocean ridge; Gakkel Ridge; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; [SDU.STU.GP]Sciences of the Universe [physics]/Earth Sciences/Geophysics [physics.geo-ph]; [PHYS.PHYS.PHYS-GEO-PH]Physics [physics]/Physics [physics]/Geophysics [physics.geo-ph]; [SDE.MCG]Environmental Sciences/Global Changes; Arctic Ocean; Ocean Island
• Online Access: https://insu.hal.science/insu-00674838; https://doi.org/10.1016/j.epsl.2011.06.012

International audience The isotopic composition of radiogenic systems is a powerful tool to investigate Earth's evolution through time. Most of our understanding of the processes that affect the isotopic composition of the oceanic mantle comes from the study of basalts. Far fewer isotope analyses of actual oceanic mantle rocks (i.e. abyssal peridotites) exist, owing to their scarcity and often altered state. Here we present new Hf and Nd isotope analyses of clinopyroxene (cpx) from abyssal peridotites from the Gakkel Ridge in the Arctic Ocean, the Southwest Indian ridge and the South Atlantic. The Hf and Nd isotope ratios in the Indian Ocean and the South Atlantic peridotite cpx mostly overlap those of MORB, whereas coupled depletions in Nd and Hf isotope ratios in the Gakkel Ridge samples (epsilon(Hf) and epsilon(Nd) values of 60.4 and 20.5, respectively) extend the MORB and ocean island basalt (OIB) Hf Nd isotope array to considerably more depleted values. Some samples from the Gakkel Ridge range to extreme Hf isotope values up to epsilon(Hf) of 104, but lack the corresponding depletion in Nd isotopes (epsilon(Nd) of about 8). The Hf, rather than the Nd isotope ratios of the Gakkel Ridge peridotites correlate with major and trace element indices of depletion (e.g. Al(2)O(3) and Yb content, spinel Cr#) and their previously determined Os isotope ratios. Hence the Hf and Os isotope compositions of these samples preserve a record of ancient mantle depletion, whereas their Nd isotope signatures often do not. The example of the Gakkel Ridge peridotites suggests that the Sm/Nd and Nd isotope ratios in abyssal peridotites in general are very susceptible to resetting by melt-rock interaction, and that Nd isotope ratios in abyssal peridotites rarely preserve ancient mantle depletion to a similar extent as the Os and Hf isotope ratios. The oceanic mantle could thus range to more depleted Hf isotope signatures, and is thus isotopically more variable than inferred from oceanic basalts alone. MORB may therefore ...