Abstract:
The isotope composition of lead (Pb) the Earth's upper mantle (sampled by oceanic basalts) is far too radiogenic for evolution from chondritic (primitive solar system) material over 4.57 billion years, the so called "Pb paradox" [1]. Loss of Pb to the core [2] or arrival in a late veneer [3], have both been proposed as mechanisms to account for this imbalance. Alternatively, recent Pb isotope data for orogenic peridotites suggests that such rocks could serve as a complementary reservoir of unradiogenic Pb [4]. However, orogenic peridotites may not be representative of the asthenopshere underlying present-day mid-ocean ridges, furthermore, it is unclear why such material is not sampled by oceanic basalts. Here we show that sulphides trapped as inclusions in silicate minerals in abyssal peridotites from the North Atlantic ocean (ODP Leg 209; Site 1274A) preserve extremely unradiogenic Pb isotope compositions, some corresponding to an age of ca 1.8 billion years. These ages are indistinguishable from those preserved by Os isotopes in sulphides from the same abyssal peridotites [5], and demonstrate that both Pb and Os isotopes preserve an unequivocal record of ancient melt depletion in the sub-oceanic mantle. Therefore, at least, some of the Pb in the Earth's mantle is unradiogenic and complements the composition of oceanic basalts. That these sulphides contribute little of their Pb to the isotope composition of oceanic basalts may be due either to their entrapment in host silicate phases or else that they are generally present in refractory domains in the mantle that are little sampled by later melting events. [1] Allegre, C.J. Earth Planet. Sci. lett. 5, 261-269 (1969). [2] Vollmer, R. Nature 270, 144-147 (1977). [3] Albarede, F. Nature 461, 1227-1233 (2009). [4] Malaviarachchi, S. et al., Nature Geosc. 1, 859-863 (2008). [5] Harvey, J. et al. Earth Planet. Sci. lett. 244, 606-621 (2006).