IntroductionKimberlite 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).

RB-SR AND SM-ND WHOLE-ROCK ISOTOPE COMPOSITION OF DEFORMED PERIDOTITE XENOLITHS FROM KIMBERLITE PIPE

UDACHNAYA

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Figure 2 The 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 

Agashev A M 1*, Surgutanova E A 1, Demonterova E.I 2, Golovin A V1, Pokhilenko N P1.1. Institute of Geology & Mineralogy SB RAS, Novosibirsk 630090, Russia (* correspondence: agashev@igm.nsc.ru)2. 2. Institute of the Earth Crust SB RAS, Irkutsk, Russia.

Fig 5. Primitive mantle normalized trace elements patterns of deformed peridotites in comparison with that of their host kimberlite of Udachnaya pipe.

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 (Cratonic Harzburgite) of McDonough and Rudnick (1998). Diamonds are indicates experimental melting residues (Herzberg, 2004) at 2 GPa (solid) and 7 GPa (open).

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.

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.

Olivine Mg# ranges from 86.4 to 91.3; it correlates positively with concentrations of NiO. Orthopyroxene have Mg# (88.2–92.9) that are slightly higher than in olivines and positively correlate with Mg# of olivines. Clinopyroxenes are low in CaO, with 35.9–43.3 mol %. of diopside component, Ca/(Ca + Mg). REE patterns of the cpx are enriched in LREE with a maxima at Ce-Nd, which is typical for cpx of deformed peridotites. The chemical composition of garnets shows great variability in their Cr2O3 contents (1.8-12.2 Wt%). Garnets can be divided in two groups based on the shape of their REE patterns. First group have a sinusoidal REE pattern which is a usual feature of harzburgitic (low CaO) garnets included in diamonds (Stachel 2008). The second group has a flat REE pattern from MREE to HREE, and a sharp decrease from Nd to La, which is common for garnet megacrysts and high-T lherzolites.

Minerals chemistry

Figure4. Composition of garnets from Udachnaya deformed peridotite xenoliths.

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.

CONCLUSIONS

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.

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 anchient carbonatite metasomatism 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.

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 6. Trace element variation diagrams for WR composition of deformed peridotites.

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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 Udachnaya kimberlites have similar to deformed peridotites isotope composition with slightly lower value of e Nd t.

Rb-Sr and Sm-Nd isotope systems

Fig. 8. Initial (367 Ма) Sr-Nd isotope composition of deformed peridotites

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