MEASURED VS CALCULATED WR GEOCHEMICAL COMPOSITION OF DEFORMED PERIDOTITE XENOLITHS: INFERENCES ABOUT METASOMATISM IN CRATONIC MANTLE. CONCLUSIONS. Agashev A M 1*, Ionov D A2, Pokhilenko N P1, Golovin A V1, Surgutanova E A 1, Sharygin I S.
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Agashev A M 1*, Ionov D A2, Pokhilenko N P1, Golovin A V1, Surgutanova E A 1, Sharygin I S.
1Inst. Geology & Mineralogy, Novosibirsk 630090, Russia (* correspondence: [email protected])
2Université J. Monnet, Saint-Etienne 42023, France
Deformed peridotites are located within a (≥170-220 km) depth range of CM. The degree of rock deformation is not correlated either with P or T and thus does not depend on the vertical position in the mantle. The difference in degrees of deformation could be a function of horizontal distance from the veins or diapirs of intruding asthenospheric melt.
In the complex evolution of deformed peridotites which were formed primary as a high degree melt extraction residues at least 4 different types of metasomatic enrichment are evident: hydrous or carbonatitic fluid, silicate, Fe-rich and K metasomatism.
All evidences indicate that the agent for silicate metasomatism should have an intermediate between kimberlites and HIMU OIB composition. Initially it could be OIB which later evolve though fractional crystallization to composition close to kimberlitic.
The traditional point of view that lower layer of CM composed by deformed peridotites could be an artifact arises from preferable sampling of metasomatically enriched veined part of CM base locally developed beneath kimberlite fields.
Deformations and silicate metasomatism seen in deformed peridotites are genetically linked with the crystallization of megacrysts association and kimberlite melting within a single event of thermal perturbation at the CM base.
Figure 3. 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.
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.
Olivine Mg# ranges from 86.4 to 91.3; it correlates positively with concentrations of NiO. Olivine Mg# define two distinct groups with Mg# 89.4-91.7 and 86.4-87.5. Orthopyroxene from deformed peridotites have Mg# (88.2–92.9) that are slightly higher than in olivines and positively correlate with Mg# of olivines. Clinopyroxenes from deformed peridotites 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 ( Burgess and Harte 2004).
Figure 9. chondrite- Normalized REE patterns of measured, calculated and calculated with 1% of kimberlite WR composition of deformed peridotites. REE patterns of four samples (Uv-3/01 example) could be reproduced by addition of 1% kimberlite to calculated WR composition, but the majority of samples (examples is Uv 33/04) could not be reproduced in this way.
Figure 10. 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.
Mantle metasomatism and nature of metasomatic agents
To evaluate extend of metasomatic enrichment we calculate reconstructed WR using modal mineralogy and composition of Gar and Cpx.Comparison displays large difference between calculated and measured WR composition. The most pronounced differences are observed for highly incompatible elements (Nb, Ta, Th, U, Rb and Ba), and their concentrations in the calculated composition never exceed 20% of the measured. Significant amounts of La and Ce are incorporated into Cpx, and calculated WR composition mostly contains 20-40% of these elements budget. The middle REE in calculated WR composition comprise from 60 to 100% of the rock budget. Most of HREE also incorporated into Cpx and Gar (50-90% of the rock budget).
Schmidberger and Francis (2001) explained discrepancies between measured and calculated WR compositions by infiltration of 0.4-2% of host kimberlite into the rock. Our modeling shows that the REE contents of only four samples could be closely reproduced by addition of kimberlite melt to the calculated WR (Fig. 9). Most of the samples require a metasomatic agent with a much lower La/Yb ratio and LREE abundances than kimberlites; The HIMU type basalts of St. Helena islands (Willbold and Stracke, 2006) are enriched in incompatible elements, and calculation shows that they could be an appropriate 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 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.
All samples are typical four phase peridotites, consisting of variable amounts of olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx) and garnet (Gar). The xenoliths have a porphyroclastic or mosaic-porphyroclastic textures (Harte 1977) composed by bigger grains of major phases that setting in matrix of fine-grained sugar-like olivine neoblasts. The degree of deformation show by samples is significantly variable as well as neoblasts to porphyroclasts ratios and their grain sizes. The rocks represent a sequence of transition from initial stages of deformation with relicts of granular textures (Uv-24/05) to strongly deformed fluidal textured specimens with minerals elongated and oriented into the chains and layers (Uv 3/01) (Figure 1). The grain size of porphyroclasts minerals are ranging from the 1-2 mm and up to 12mm in different samples.
This paper discuses the geochemical evolution and metasomatic processes in the lower layers of continental lithospheric mantle. The evidences provided here are based on detailed study of suite of extraordinary fresh (L.O.I ~0) and large deformed peridotite xenoliths from Udachnaya kimberlite pipe, Siberia.
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.
In the complex evolution of deformed peridotites which were formed primary as a high degree melt extraction residues four main stages of metasomatic modifications of their composition were deduced. 1. An old mostly cryptic metasomatose by melt/fluid of carbonatitic composition. At this stage the garnets with sinusoidal REE pattern (Smn/Ern >1) were formed. 2. Silicate metasomatism which led to significant changes in mineralogical and chemical composition of deformed peridotites. 3. Fe and Ti metasomatism just before entrainment of deformed rocks into kimberlite magma. 4. Enrichment of peridotites in LIL elements (K, Rb, Ba) and formation of kelyphitic rims around garnet.
Nature of metacomatic agent was evaluated from mass-balance of measured and calculated WR compositions, ratios between hardly incompatible elements (D< 0.1) and fractional crystallization modeling. All evidences indicate that the agent for silicate metasomatism should have an intermediate between kimberlites and HIMU OIB composition. Initially it could be OIB which later evolve though fractional crystallization to composition close to kimberlitic
The metasomatic process responsible for deformed peridotite formation and precipitation of megacryst suites should not be widespread at the CM base. Rather, they are local features only existing in the CM below kimberlite fields and localized along metasomatic vein systems.
Figure 5. Composition of garnets from Udachnaya deformed peridotite xenoliths.
WR trace elements chemistry
PM-normalized patterns are shown in Fig 7. 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. 7) 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.
Evidence from fractional crystallization modeling
To evaluate the parental melt composition we model fractional crystallization process. We used D’s datasets of (Le Roex et al, 2003) and (Keshav, 2005) to model crystallization of kimberlitic melts, and data of (Holliday et al 1995 and Hauri 1994) for modelling of basaltic melt crystallization. Our calculations show that the parental melt for Cpx must be of intermediate composition between kimberlite and HIMU picrobasalts, i.e highly incompatible element contents should be like in basalt, but middle and HREE contents should be closer to their abundances in the kimberlitic melt. In both cases, prior to Cpx’s crystallization, a fraction of crystals (at least 10% of the melt volume) with modal proportion of 90% Gar + 10% of Ilm have to be removed from the melt. In contrast to Cpx’s, the composition of Gar’s could not be reproduced by fractional crystallization of kimberlite melt, and only Garnets with Smn/Ern ≤1 could be modeled from HIMU picro-basalts melt composition (Fig.14). Attempts to reproduce the composition of garnet with Smn/Ern >1 failed. Those garnets could not be in equilibrium with any terrestrial silicate liquids including kimberlite.
Fig 7. Primitive mantle normalized trace elements patterns of deformed peridotites in comparison with that of their host kimberlite of Udachnaya pipe.
Figure 8. Trace element variation diagrams for WR composition of deformed peridotites.
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Figure. 14. Results of fractional crystallization modeling a. Cpx composition Modeled by crystallizing HIMU basalts. b. cpx composition Modeled by crystallizing kimberlite. c. Gar composition Modeled by crystallization of HIMU basalt melt composition. No preliminary removal of any crystal fraction was considered in calculations.
Figure. 2.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).
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