Ancient lead and osmium in the oceanic mantle
Abstract
The isotope composition of lead (Pb) in the silicate Earth appears to be 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], storage in the lower continental crust [3], or arrival in a late veneer [4], have all been proposed as mechanisms to account for this imbalance. Recently is has been suggested that orogenic peridotites, and by implication the upper mantle itself, could serve as a complementary reservoir of the unradiogenic Pb [5]. 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. This study presents high-precision double-spike Pb isotope data for sulphides trapped as inclusions in silicate minerals in abyssal peridotites from the North Atlantic ocean (ODP Leg 209; Site 1274A). These sulphides preserve extremely unradiogenic Pb isotope compositions, some corresponding to an age of 1.83±0.23 billion years. These ages are indistinguishable from those preserved by Os isotopes in sulphides from the same samples [6], and demonstrate that both Pb and Os isotopes preserve an unequivocal record of ancient melt depletion in the sub-oceanic mantle. from abyssal peridotites exposed on the sea-floor the North Atlantic ocean.That these sulphides contribute little of their Pb to the isotope composition of oceanic basalts may be, in part, due to their entrapment in host silicate phases but also because they are present in refractory domains in the mantle that are little sampled by later melting events. If MORB do not sample these refractory domains then neither do they constrain the overall chemistry of the asthensophere, and will tend to underestimate the actual extent of depletion of the upper mantle. [1] Allègre, C.J. Earth Planet. Sci. lett. 5, 261-269 (1969). [2] Vollmer, R. Nature 270, 144-147 (1977). [3] O'Nions, R.K. et al. J. Geophys. Res. 84, 6091-6101. [4] Albarède, F. Nature 461, 1227-1233 (2009). [5] Malaviarachchi, S. et al., Nature Geosc. 1, 859-863 (2008). [6] Harvey, J. et al. Earth Planet. Sci. lett. 244, 606-621 (2006).
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V41G..04B
- Keywords:
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- 1025 GEOCHEMISTRY / Composition of the mantle;
- 1038 GEOCHEMISTRY / Mantle processes;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 1115 GEOCHRONOLOGY / Radioisotope geochronology