Figure 2 : co-variation plots of Al 2 O 3 vs. FeO, CaO and Mg# in whole rocks of Vitim ( )and Tariat ( ) peridotite xe- noliths, also shown are the field of on-craton peridotite (dark grey field) and off-craton peridotite xenoliths. Data are compared to experimental melting residues [4] and primitive mantle estimate [5]. Zn isotope fractionation in the fertile mantle L.S. Doucet a , N. Mattielli a and D.A. Ionov b a Laboratoire G-Time, Université Libre de Bruxelles, DSTE, CP160/02, 1050 Bruxelles, Belgium b Géooscience Montpellier, Université de Montpellier 2 & UMR-CNRS 5243, 34095, Montpellier, France 2. Geological setting and samples 1. Objectives of this study 6. Conclusions and perspectives We present here new Zn isotope data (δ 66 Zn) on 12 un-serpentinized (LOI=0.66% to 0.36%) peridotite xenoliths from Vitim (Siberia) and Tariat (Mongolia)(Fig. 1) considered as some of the most pristine fertile mantle peridotites available [1-3] to explore the homogeneity of fertile mantle. The garnet peridotites (Vitim) and spinel peridotites (Tariat) in this study typically plot on the off-craton field and have modal, major and trace element compostions close to the primitive mantle (Fig. 2)[1-3]. 4. Zn isotope fractionation Figure 1 : Location map of Vitim and Tariat in Eastern-Asia [1-2]. 3. Analytical methods and measurements Figure 3 : calibration for Zn purification - 100 mg of bulk-rock were disolved and Zn was purified from the matrix using ion exchange resin AG1X8 100-200 mesh following procedure exposed in Fig. 3. - The δ 66 Zn data were obtained by high-precision measurements on a Nu Plasma MC-ICPMS in wet mode using the sample-standard bracketing (in-house standard) and Cu doping methods. - Repeated in-house standard analyses give δ 66 Zn = +0.00±0.02 ‰ (2SD; n= 7); JMC-Lyon-3-0749 give δ 66 Zn = +0.11±0.03 ‰ (2SD; n= 17); IRMM-3702 give δ 66 Zn = +0.39±0.05 ‰ (2SD; n= 26); and measurements 2 total duplicates of BCR-2 give δ 66 Zn = +0.45±0.01 ‰ and +0.45±0.02 ‰. - Total procedural blanck is 10 ng. Figure 4 : δ 66 Zn data in bulk-rock perido- tites from Vitim and Tariat. - Spinel peridotites (Tariat) : δ 66 Zn = +0.29±0.09‰ (2SD) to +0.58±0.09‰ (av. 0.38‰). - Garnet peridotites (Vitim) : δ 66 Zn = +0.37±0.03‰ to +0.71±0.01‰ (av. 0.50‰). The δ 66 Zn values show correlation with Mg# WR , modal orthopyroxene, modal olivine and with modal clinopy- roxene. δ 66 Zn data do not correlate with the Zn or other trace element concentrations. The absence of correlation with Temperature or Pressure indicates that geotherm does not affect Zn isotope fractionation. Figure 5 : δ 66 Zn data vs. Mg#WR (a); modal olivine(b) and modal opx(c) and modal clinopyroxene (d). (1) the fertile mantle has more heterogeneous Zn isotope compositions than presumed, (2) the inter-mineral fractionation appears to be a key factor controlling Zn isotopes in the mantle, and (3) the observed δ 66 Zn co-variation trends suggest a mantle process that significantly fractionates Zn isotopes (Δ 66 Zn~0.4‰), but is not detected by traditional geochemical proxies. This study shows that Zn isotopes may be used as tracers for mantle processes, but further work on residual and metasomatized peridotites as well as on mineral separates is needed. References: [1] Ionov et al (2005) CG 217; [2] Ionov (2007) CMP 154; [3] Ionov and Hofmann (2007) EPSL 261; [4] Herzberg (2004) JPet 45; [5] McDonough and Sun (1995) CG 120;[6] Ben Othman et al (2006) GCA, 70; [7] Herzog et al (2009) GCA 73; [8] Chen et al (2013) EPSL 73369; [9] Pons et al (2013) Geobio 11. Modal Cpx (%) 10 12 14 16 18 20 22 0.889 0.891 0.893 0.895 0.897 Mg# in WR 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 (a) R 2 =0.2 δ 66 Zn in bulk-rocks (‰) 5. Zn isotope fractionation vs. geochemical parameters (c) 10 20 30 40 Modal Opx (%) R 2 =0.5 (b) 40 45 50 55 60 65 70 Modal Olivine (%) R 2 =0.4 0 1 2 3 4 5 Al 2 O 3 in WR, wt. % 9 8 7 6 5 30 % 38 % Experimental melting residues: Polybaric fractional melting 5-1 GPa 7-2 GPa 2-0 GPa 3-0 GPa (a) Horoman 20 % (b) 0 1 2 3 4 5 Al 2 O 3 in WR, wt. % 5 4 3 2 1 0 (c) 38 % 0.89 0.90 0.91 0.92 0.93 0.94 Mg# in WR 5 4 3 2 1 0 30 % 5-1 GPa 7-2 GPa 3-0 GPa on-craton off-craton FeOt ,wt. % CaO ,wt. % Al 2 O 3 in WR, wt. % Vitim = garnet peridotites; Tariat = spinel peridotites PM PM PM Norilsk Irkutsk Yakutsk Enisei Lena Lake Baïkal Laptev sea 100° E 70° N 60° N 140° E Vitim Exposed AR-PR1 crustal rocks Craton boundary Vitim (54°20N, 114°13E) Tariat (48°05N, 99°31E) Tariat Russia Mongolia China Oulan-Bator Harbin 50° N Siberian craton 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 Range for terrestrial basalts [6-9] δ 66 Zn in bulk-rocks (‰) Vitim Tariat 20% cpx (d) R 2 =0.6 error bar : ±2SD Range for terrestrial basalts HCl 1M (20ml) HCl 6M (10ml) HCl 8M (50ml) HNO3 1M / HBr 0.1M (10ml) 100 80 60 40 20 0 100 80 60 40 20 0 Zn Zn Cu Cu Fe Fe Pb Pb Cumulative Yield Yield The apparent homogeneity of the Zn isotope data on MORB, OIB and serpentinized orogeneric peridotites advocate for an ho- mogeneous fertile mantle [6-9]; in constrast, our results show : The fertile mantle, whose origin and evolution are still in debate, shows elemental and isotopic fractionations of transition metals (e.g. Fe) during mantle melt extraction, which remain unclear. Zinc enters in mantle minerals as trace elements, especially in olivine, and its single valence state ( +2 ) makes it insensitive to fO 2 and therefore the perfect target to investigate the isotope fractionation of transition metal in the fertile mantle and during melt extraction. ldoucet@ulb.ac.be Abstract N°249292 GSA2014 The fertile mantle tends to be enriched in heavy Zn isotopes rela- tive to basalts; identically garnet peridotites are commonly heavier than spinel peridotites