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U-Th-Pb isotopes in continental mantle roots: Tracing lithospheric metasomatism, melting and anthropogenic contamination

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Sub-continental lithospheric mantle (SCLM) constitutes only ~2% of Earth mantle and is usually considered depleted relative to primitive mantle. Nevertheless, continental mantle roots are frequently invoked to substantially contribute to terrestrial volcanism in order to explain Nd, and particularly Sr-Pb isotope idiosyncrasies (i.e. “Enriched Mantle”) in mantle derived volcanic rocks (e.g., 1, 2). Sr and Nd isotope systematics are routinely investigated in mantle clinopyroxenes; however, the acquisition of precise and accurate Pb isotope data remains significantly more challenging due to low Pb abundances in peridotites and difficulties associated with mass spectrometry (3, 4). We studied U-Th-Pb elemental and isotope systematics of mantle clinopyroxene crystals and their grain-boundaries (e.g. Massif Central, Middle Atlas) utilizing new Pb double-spike techniques (4, 5). Leachates of all clinopyroxenes, regardless of the crystal-lattice hosted Pb isotope composition or geographic origin, are severely contaminated by anthropogenic Pb that strongly resembles “Enriched Mantle II”. Surprisingly, volcanism associated with these mantle xenoliths contributes insignificantly to the grain-boundary contamination. After careful removal of the grain-boundary contamination, individual suites of mantle clinopyroxenes exhibit highly heterogeneous Pb isotopes encompassing the entire range of Pb isotopes known from the convecting mantle. Importantly, “Enriched Mantle”-like Pb isotopes appear scarce in peridotitic clinopyroxenes. However, mantle clinopyroxenes have parent-isotope ratios (238U/204Pb up to 110; 232Th/204Pb up to 400) that are substantially more extreme than the source regions of terrestrial volcanism (238U/204Pb 10-25; 232Th/204Pb 25-85)(6). These clinopyroxenes have Rb-Sr, Sm-Nd and, to some extend, Lu-Hf isotopes that remain robustly affiliated with the convecting mantle. Therefore, U-Th-Pb isotope systematics of SCLM clinopyroxenes may place important restrictions on the residence time of extreme U-Th-Pb isotopes in continental roots and the contribution of SCLM to terrestrial volcanism. Numerical modeling requires the metasomatism responsible for the high U-Th-Pb isotope signature to be very young ( SCLM metasomatised producing high U-Th-Pb isotopes (e.g., 500 and 2500 Ma) would rapidly result in exceptionally radiogenic Pb isotopes that are not observed to-date. Moreover, calculated small-degree melts from SCLM using presentday U-Th-Pb elemental and isotope systematics yield average 238U/204Pb and 232Th/204Pb of 200 and 660, respectively. This is ubiquitously more enriched than melts generated from the convecting mantle (8). In summary, SCLM is marked by highly heterogeneous and often extreme U-Th-Pb isotopes, which originate from young metasomatism related to, but isotopically distinct, from the associated volcanism. SCLM is not a suitable source of terrestrial volcanism, especially if the most extreme U-Th-Pb isotopes are considered.

References: 1. C. Chauvel, B. M. Jahn, Geochimica et Cosmochimica Acta 48, 93-110 (1984). 2. S. Y. O’Reilly, W. L. Griffin, M. Zhang, G. Begg, 9th International Kimberlite Conference Extended Abstract No. 9IKC-A-00085, (2008). 3. D. G. Pearson, D. Canil, S. B. Shirey, in Treatise on Geochemistry, H. D. Holland, K. K. Turekian, Eds. (Elsevier, Amsterdam, 2003), vol. 2; The Mantle and Core, pp. 171-275. 4. J. A. Baker, D. W. Peate, T. Waight, C. M. Meyzen, Chemical Geology 211, 275-303 (2004). 5. N. Wittig, J. A. Baker, H. Downes, Geochimica et Cosmochimica Acta 71, 1290-1311 (2007). 6. M. F. Thirlwall, Chemical Geology 139, 51-74 (1997). 7. N. Wittig, D. G. Pearson, J. A. Baker, H. Downes, accepted in Geochimica et Cosmochimica Acta, (2009). 8. N. Wittig, D. G. Pearson, S. Duggen, J. A. Baker, K. Hoernle, submitted to Geochimica et Cosmochimica Acta, (2008).

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