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SUMMARY:U-Th-Pb isotopes in continental mantle roots: Tracing lithospheric
  metasomatism\, melting and anthropogenic contamination - Nadine Wittig\, 
 Durham University
DTSTART:20081014T153000Z
DTEND:20081014T163000Z
UID:TALK14277@talks.cam.ac.uk
CONTACT:Francis Rowland
DESCRIPTION: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") i
 n mantle derived volcanic rocks (e.g.\, 1\, 2). Sr and Nd isotope systemat
 ics are routinely investigated in mantle clinopyroxenes\; however\, the ac
 quisition of precise and accurate Pb isotope data remains significantly mo
 re challenging due to low Pb abundances in peridotites and difficulties as
 sociated with mass spectrometry (3\, 4). We studied U-Th-Pb elemental and 
 isotope systematics of mantle clinopyroxene crystals and their grain-bound
 aries (e.g. Massif Central\, Middle Atlas) utilizing new Pb double-spike t
 echniques (4\, 5). Leachates of all clinopyroxenes\, regardless of the cry
 stal-lattice hosted Pb isotope composition or geographic origin\, are seve
 rely contaminated by anthropogenic Pb that strongly resembles "Enriched Ma
 ntle II". Surprisingly\, volcanism associated with these mantle xenoliths 
 contributes insignificantly to the grain-boundary contamination. After car
 eful removal of the grain-boundary contamination\, individual suites of ma
 ntle clinopyroxenes exhibit highly heterogeneous Pb isotopes encompassing 
 the entire range of Pb isotopes known from the convecting mantle. Importan
 tly\, "Enriched Mantle"-like Pb isotopes appear scarce in peridotitic clin
 opyroxenes. However\, mantle clinopyroxenes have\nparent-isotope ratios (2
 38U/204Pb up to 110\; 232Th/204Pb up to 400) that are substantially more e
 xtreme 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 conv
 ecting mantle. Therefore\, U-Th-Pb isotope systematics of SCLM clinopyroxe
 nes may place important restrictions on the residence time of extreme U-Th
 -Pb isotopes in continental roots and the contribution of SCLM to terrestr
 ial volcanism. Numerical modeling requires the metasomatism responsible fo
 r the high U-Th-Pb isotope signature to be very young (<20 Myr) and associ
 ated with the most recent volcanism in the respective regions (5\,7). Olde
 r 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 u
 sing presentday U-Th-Pb elemental and isotope systematics yield average 23
 8U/204Pb and 232Th/204Pb of\n200 and 660\, respectively. This is ubiquitou
 sly 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 isoto
 pically distinct\, from the associated volcanism. SCLM is not a suitable s
 ource of terrestrial volcanism\, especially if the most extreme U-Th-Pb is
 otopes are considered.\n\n*References*:\n1. C. Chauvel\, B. M. Jahn\, Geoc
 himica et Cosmochimica Acta 48\, 93-110 (1984).\n2. S. Y. O'Reilly\, W. L.
  Griffin\, M. Zhang\, G. Begg\, 9th International Kimberlite Conference Ex
 tended Abstract No. 9IKC-A-00085\, (2008).\n3. D. G. Pearson\, D. Canil\, 
 S. B. Shirey\, in Treatise on Geochemistry\, H. D. Holland\, K. K. Turekia
 n\, Eds. (Elsevier\, Amsterdam\, 2003)\, vol. 2\; The Mantle and Core\, pp
 . 171-275.\n4. J. A. Baker\, D. W. Peate\, T. Waight\, C. M. Meyzen\, Chem
 ical Geology 211\, 275-303 (2004).\n5. N. Wittig\, J. A. Baker\, H. Downes
 \, Geochimica et Cosmochimica Acta 71\, 1290-1311 (2007).\n6. M. F. Thirlw
 all\, Chemical Geology 139\, 51-74 (1997).\n7. N. Wittig\, D. G. Pearson\,
  J. A. Baker\, H. Downes\, accepted in Geochimica et Cosmochimica Acta\, (
 2009).\n8. N. Wittig\, D. G. Pearson\, S. Duggen\, J. A. Baker\, K. Hoernl
 e\, submitted to Geochimica et Cosmochimica Acta\, (2008). 
LOCATION:Tilley Lecture Theatre\, Department of Earth Sciences
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