Introduction

1. RB-SR AND SM-ND WHOLE-ROCK ISOTOPE COMPOSITION OF DEFORMED PERIDOTITE XENOLITHS FROM KIMBERLITE PIPE UDACHNAYA CONCLUSIONS Silicate metasomatism by proto-kimberlite melt is not fully erased isotope heterogenity of bottom layers of lithospheric mantle, especially it evident in initial Sr isotope composition of deformed peridotites. Radiogenic Sr isotope composition could be related to anchientcarbonatitemetasomatism and formation of garnets of sinusoidal REE pattern. The Sr-Nd isotope composition and incompatible elements ratios suggests that metasomatic agent for deformed peridotites could be low-degree melt of OIB-like source with composition intermediate between kimberlite and HIMU OIB. This astenospheric OIB like melt was interacted with lithospheric roots and prepared incompatible elements enriched source for kimberlite melt formation. Agashev A M 1*, Surgutanova E A 1, Demonterova E.I 2,Golovin A V1, Pokhilenko N P1. Institute of Geology & Mineralogy SB RAS, Novosibirsk 630090, Russia (* correspondence: agashev@igm.nsc.ru) 2. Institute of the Earth Crust SB RAS, Irkutsk, Russia. Minerals chemistry OlivineMg#rangesfrom 86.4 to 91.3; itcorrelatespositivelywithconcentrationsofNiO. OrthopyroxenehaveMg# (88.2–92.9) thatareslightlyhigherthaninolivinesandpositivelycorrelatewithMg#ofolivines. ClinopyroxenesarelowinCaO, with 35.9–43.3 mol %. ofdiopsidecomponent, Ca/(Ca + Mg). REE patternsofthecpxareenrichedin LREE withamaximaatCe-Nd, whichistypicalforcpxofdeformedperidotites. Thechemicalcompositionofgarnetsshowsgreatvariabilityintheir Cr2O3 contents (1.8-12.2 Wt%). Garnetscanbedividedintwogroupsbasedontheshapeoftheir REE patterns. Firstgrouphaveasinusoidal REE patternwhichisausualfeatureofharzburgitic (lowCaO) garnetsincludedindiamonds (Stachel 2008). Thesecondgrouphasaflat REE patternfrom MREE to HREE, andasharpdecreasefromNdtoLa, whichiscommonforgarnetmegacrystsandhigh-Tlherzolites. Rb-Sr and Sm-Nd isotope systems SAMPLES. Samples are garnet-bearing peridotite xenoliths with macroscopically recognizable deformed (sheared) textures. All xenoliths are fresh with no or very rare secondary alteration and serpentinisation. All of them are large enough (0.5 kg to several kg) and suitable for whole-rock (WR) analysis. Only central parts of xenoliths without margins at the host kimberlite were used for the study of WR chemical composition. Based on our T-P estimates, deformed peridotites are located within a broad depth range (≥170-220 km) near the base of the cratonic mantle.The degree of deformations is not correlated to the depths and temperatures of samples equilibrium conditions. Figure4. Composition of garnets from Udachnaya deformed peridotite xenoliths. Fig. 8. Initial (367Ма) Sr-Nd isotope composition of deformed peridotites WR trace elements chemistry PM-normalized patterns are shown in Fig 5. The deformed peridotites are enriched in the highly incompatible elements with bulk distribution coefficient (D) < 0,01. The degree of enrichment in particular elements ranges from 2-10 times PM abundances for K and Rb to 1-5 times PM for Ba, Th, U, Nb and La. The concentration of elements of middle incompatibility (MREE, Zr, and Hf) varies around PM model composition and even slightly depleted in most of the samples. Heavy REE and Y concentrations are lower than that of PM model. Normalized to PM trace elements patterns (Fig. 5) have maximums at Rb, K and Ti and minimum at Th. The shape of these patterns differs from that of host kimberlite. Concentrations of all highly incompatible elements, excepting LILE, well correlate between each other and with concentrations of P2O5 (Fig. 8). Concentrations of elements with middle incompatibility (Zr, Hf and MREE) well correlate with TiO2, CaO and Na2O . HREE abundances show good correlations with those of Al2O3 and CaO. Figure 1. Textural variations in deformed peridotite xenoliths from kimberlite pipe Udachnaya. a and b low deformation degrees. c) and d) medium-deformed samples e) High degree of deformation in sample Uv-1/04 containing very coarse garnet porphyroclasts. f) Sample Uv 3/01 displays the highest degree of deformation with broken garnets arranged as linear chains. Introduction Kimberlite pipe Udachnaya is a well known source of unique fresh mantle xenoliths. Deformed peridotites are compose lowermost layer of lithospheric mantle and experienced most significant metasomatism among the peridotite suite before taken by kimberlite melt. To clarify the nature of metasomatic agent we have studied a suite of deformed peridotite xenoliths for their whole-rock Rb-Sr and Sm-Nd isotope compositions. The chemical composition of those WR samples and their minerals has been studied earlier (Agashev et al. 2013). The present day Sr isotope ratios of deformed peridotites show radiogenic values (0.7075-0.711) that consistent with their Rb/Sr ratios and negative correlate with amount of CaO in the rock composition indicating Cpx control. 87Sr/86Sr isotope ratios calculated back to the time of kimberlite emplacement (367 Ma) are scatters from depleted to slightly enriched values (0.7032-0.7054) indicating heterogeneity of lithospheric mantle roots before kimberlite emplacement. Initial (367 Ma) Nd isotope composition is less variable being in range of 3.8-5.8 e Nd t units. On the initial (367 Ma) Rb-Sr and Sm-Nd isotope ratios diagram the composition of deformed peridotites scatters from the field of HIMU OIB composition toward radiogenic Sr isotope composition of EM2 (enriched mantle 2) source. Host Udachnayakimberlites have similar to deformed peridotites isotope composition with slightly lower value of e Nd t. Fig 5.Primitive mantle normalized trace elements patterns of deformed peridotites in comparison with that of their host kimberlite of Udachnaya pipe. Figure 6. Trace element variation diagrams for WR composition of deformed peridotites. Mantle metasomatism and nature of metasomatic agents Incompatible elements ratios The elements which do not enter into modal mineralogy and therefore do not fractionate against each other could provide most useful information about geochemical signatures of metasomatic agent. The ratios between Nb, Th, U, La and Rb can be used as an example. Most of the measured WR have Th/U ratios similar to HIMU basalts and little lower than in host kimberlite, but their Nb/La ratios are more similar to kimberlite although it intersects with HIMU OIB. Ratios between Nb and Th are similar in all discussed substances indicate that those elements do not fractionate against each other during metasomatic processes in the mantle. In contrast, Rb/Nb ratios in measured WR are much higher than that in calculated and in the kimberlites and HIMU basalts. This feature is explained by preferred incorporation of LILE into kelyphitic rims around garnet grains. Figure 2The P-T conditions of equilibrium for deformed peridotites were calculated from chemical composition of garnet and two pyroxenes using Brey and Kohler (1990) equations. The continental geotherms of 35 and 40 mW/m2 (Pollack and Chapman 1977), graphite/diamond transition line and solidus of carbonated peridotite are shown. Major elements Figure 7. Ratios between very incompatible elements in measured and calculated WR compositions of deformed peridotites compared to these ratios in HIMU basalts and Udachnaya kimberlite melts. Figure. 3.Major element variation diagrams for WR composition of deformed peridotite xenoliths form kimberlite pipe Udachnaya. Black circles indicate PM composition (McDonough and Sun 1995), open squares are CH (CratonicHarzburgite) of McDonough and Rudnick (1998). Diamonds are indicates experimental melting residues (Herzberg, 2004) at 2 GPa (solid) and 7 GPa (open).