The Distribution and Behaviour of Pb-U-Th and Pb Isotopes in Mantle Minerals.

The chemistry of mantle derived basalts is largely controlled by elemental partitioning between melt and mantle minerals. For the Re-Os isotope system, parent and daughter elements may be strongly decoupled between oxide and sulphide, both in sub-continental and sub-oceanic mantle [1]. Both natural and experimental studies indicate that Th and U are mostly located in silicates. In contrast sulphide dominates the Pb budget in the same rocks [2]. This raises the possibility that Th, U and Pb may also be strongly decoupled in the mantle. 207Pb-204Pb double spike (DS) and TIMS methods have been used to measure the distribution and partitioning behaviour of Pb between silicate phases and sulphide in mantle xenoliths from Kilbourne Hole and San Carlos, USA and Lashaine, Tanzania. Calibration of the DS yields results for SRM 981 that are in good agreement with published values within analytical error [3, 4, 5]. Preliminary Pb results on a garnet lherzolite from Tanzania indicate that, similar to what is observed in the 187Re-187Os system, included sulphides preserve much older age information than the silicate phases yielding model ages close to 3 Ga and < 1 Ga, respectively. These results indicate that the Pb isotope signature generated by the melting of mantle rocks depends on fertility, degree of melting and the release or retention of less radiogenic Pb from included sulphide in the source. The differential behaviour of Th, U and Pb in mantle rocks may, at least in part, play a role in the decoupling of those elements in MORB. References: [1] Burton, K.W., Schiano, P., Birck, J.L. & Allegre, C.J. (1999). Earth. Plan. Sci. Let., 172, 311-322. [2] Meijer, A., Kwon, T.T., Tilton, G.R. (1990). J. Geoph. Res., 95, 433-448. [3] Galer, S.J.G. & Abouchami, W., (1998). Min. Mag. 62A, 491-492. [4] Thirwall, M.F. (2000). Chem. Geol., 163, 299-322. [5] Baker, J., Peate, D., Waight, T. & Meysen, C. (2004). Chem. Geol., 211, 275-303.