Variations in the
contents of rare earth elements, Sm/Nd and εNd(0) values in peridotite xenoliths
have been used to reveal the isotope-geochemical
heterogeneity in the vertical section of the mantle continental lithosphere.
The section covers all the changes in the mineral composition of peridotites and their clinopyroxenes
from the upper plagioclase facies (Pl) to lower high
temperature garnet peridotite (Gt2) facies in the range ofÁ
Fig.1. The changes in the Sm/Nd ratio and εNd(0) values in peridotites from various depths correlate well reflecting the successive transition from the Ìprimary oceanic¨ (OM) value - Sm/Nd > 0.3247 (average for the primitive mantle) and εNd(0) >> CHUR=0 to the values reflecting the effect of the mantle metasomatismÁ and other secondary factors which is registered by Sm/Nd << 0.3247 andÁ appearance of εNd(0) < CHUR negative values.The comparison of Sm/Nd and εNd(0) values of peridotites and alkaline magmas of various composition and age (Fig.2) showed a sheer coincidence of these parameters for Phanerozoic alkaline rocks and carbonatites: (1)- (0.1-130 Ma) andÁ (2-) (239-404 Ma) and a deviation of the εNd(0) values to distinctly negative values up to -30 and -45 for Precambrian alkaline series, which reflects a dependence on the age of formation of these alkaline rocks in Precambrian: (3)- alkaline intrusions (1160-1884Ma), (4)- carbonatites (1299-1872Ma), (5)- volcanites of greenstoun belts (4) (2575-2681Ma) and (6)-carbonatites (2661-2677 Ma). [7- ocean mantle; 8- primitive mantle; HARZ Û LHER ãharzburgites and lherzolites].
The maximum range of variations in the REE distribution in the lithosphere peridotites in relation to the influence intensity of the mantle metasomatism on them was revealed from variations of the light and heavy REE (withÁ La and Yb as an example ãFig. 3 and 4). The optimal La enrichment in harzburgites and lherzolites exceeds the average mantle level by a factor of 15-30.This is equal to La concentrations in pyroxenites ã early mantle melts of lithosphere. If we compare this with the oceanic mantle, the difference reaches two orders.. Thus, the role of the processes of the lightÁ lanthanoid secondary supply into continental peridotite can be considered to be a real factor of formation of the anriched mantle zones of the lithosphere that are sufficiently enriched in these elements for considering them to be potential sources for generation ofÁ alkaline magmas with sharply increased concentrations of mobile rare elements, that is incompatible with Yb and other heavy REE.
DifferentiationÁ of La (Fig. 3) and Yb (Fig. 4) in mantle harzburgites (1) , lherzolites (2), primitive (3) and oceanic (4) mantle and pyroxenite xenolites (5) of lithosphere.
References (1-83) may to see in the paper õDevelopment of a Heterogeneity in the Lithosphere: Geochemical Evidenceæ (Balashov Yu. A.Á Petrology, 2009, Vol. 17, No. 1, pp. 90ã100).
This study was financially supported by RFBR grants 07-05-00572-a, 07-05-00759-a.