Abyssal peridotite Hf isotopes identify extreme mantle depletion

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 (-_Hf and -_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 -_Hf of 104, but lack the corresponding depletion in Nd isotopes (-_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. Al₂O₃ 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 underestimate the average Hf and Nd isotope composition of the depleted mantle (DM), which may present an, as of yet, unrecognized obstacle for global mass balance models of Earth's differentiation. If the average DM is more depleted than MORB suggest, these models generally overestimate its mass fraction and would permit prolonged timescales for early Earth differentiation (>30Ma).