U-Pb and Lu-Hf isotopic systems in zircons from some kimberlites of the Siberian platform and from Ebeliakh alluvial deposit: age and geochemic peculiarities of the source rocks.

Lokhov K.I.^*, Lukyanova L.I.^**, Shokalsky S.P.^**, Kapitonov I.N.^**,

Lepekhina E.N.^**,  Antonov A.V.^**,  Sergeev S.A.^**

* St.-Petersburg State University, geological faculty, St.-Petersburg, Russia;

** VSEGEI, St.Perersburg, Russia.

Kirill_Lokhov@vsegei.ru

 

The important practical task is determination of possible magmatic sources of diamonds in alluvial deposits. In alluvial deposits together with diamonds and specific deep origin minerals also are concentrating zircons, which carry unique genetic and age information by U-Pb and Lu-Hf isotopic systems. In this work we carried attempt of determination of the magmatic base source of the Ebeliakh alluvial deposit of diamonds, basing on comparison of zircons from this deposit with zircons from some kimberlite pipes of the Siberian platform. Among studied objects of the Jakutian diamond bearing province were investigated zircons from diamond -rich, -poor and –free kimberlites from different fields and of different age. In different pipes there are various distribution patterns of rounded and table-cut diamonds.

Isotopic investigation was carried out in Center of Isotopic Research (CIR) VSEGEI by methods of local isotopic analysis: at first was studied U-Pb isotopic system by SIMS SHRIMP II with spatial resolution beam diameter d=15 mkm and depth h=2 mkm, and then in the same locations by laser ablation LA-MCICPMS Finnigan Neptune/DUV-193 with d=40 mkm and h=30-40 mkm, following (Lokhov, e.a., 2009a). For isotopic Hf-Nd systematization some data on Sm-Nd isotopic system in kimberlites were taken from published materials (Agshev, e.a., 2000, Bogatikov, e.a., 2004, Kostrovitsky, e.a., 2007), some part of data were obtained in CIR VSEGEI.

Isotopic Hf-Nd systematization has shown, that among studied kimberlites Paleozoic and Mezozoic ones are corresponding to known type 1 (fig.1). Mid Triassic kimberlites are forming specific anomalous type, for them are characteristic some excesses of radiogenic hafnium relative to neodymium. There can be different reasons of these excesses, and one is that is a result of higher degree of partial melting of garnet bearing mantle source under impact of thermal field of Mid- Triassic plume, with which is connected trap magmatism on the South-West of the Siberian platform.

 

Fig.1. isotopic Hf-Nd systematization of some kimberlites of the Siberian platform. Gray dotted lines are marking correlation for magmatic rocks “Hf-Nd terrestrial array”, ellipses – fields corresponding to known kimberlites (Nowell, e.a., 2004) 1- first type, 2- second type, 3- transitional type.

 

For zircons from Ebeliakh alluvial deposit was obtained U-Pb age 232-242 Ma (one grain 127 Ma) and εHf(T)=+6.6 - +10.5,  hence, the base magmatic source can be genetically connected with anomalous kimberlites of Mid- Triassic age.

As in data set from (Nowell, e.a., 2004), studied kimberlite zircons are characterizing by exceptionally low values of ¹⁷⁷Lu/¹⁷⁶Hf<0.0001 (fig.2). The interesting fact is that in younger zircons this ratio is slightly increasing.

 

 

 

 Fig.2. Values of Lu/Hf ratio in zirconsfrom kimberlites and from alluvial deposit Ebeliakh.

 

Low value of luthetium – hafnium ratio in kimberlite zircons (¹⁷⁷Lu/¹⁷⁶Hf<0.0001) is important additional geochemical parameter for magmatic kimberlite zircons identification.

 

References:

Agashev А.М., Orihashi Y, Vatanabe Т. e.a. Isotopic geochemical characteristics of kimberlites of the Siberian platform in connection with problem of their origin. Geologiya I Geofizika 2000, v.  41. № 1, p. 90-99 (in Russ).

Bogatikov О. А., Kononova V. А. e.a. Petrochemical and isotopic variations of kimberlites of Jakutia and their reasons.  Geokhimiya 2004,  № 9, p. 915-939 (in Russ).

Kostrovitsky S.I., Morikio Т. Isotopic – geochemical systematization of kimberlites of the Siberian platform. Geologiya i Geofizika 2007, v. 48, № 3, p. 350-371 (in Russ).

Lokhov K.I., Kapitonov I.N., Prasolov E.M., e.a. Extremely radiogenic hafnium in zircons from Precambrian calsiphyres. Doklady Earth Sci. 2009а, v. 425A, № 3, с. 463 – 466.

Lokhov K.I., Saltykova Т.Е., Kapitonov I.N., e.a. Correct interpretation of U-Pb age by zircons basing on isotopic geochemistry of hafnium and neodymium (on example of some magmatic complexes of basement of East-European platform). Regionalnaya geologiya I metallogeniya 38, 2009b, p.62-72 (in Russ).

Nowell D.M, Pearson D.G., Bell D.R., e.a. Hf isotope systematics of Kimberlites and their Megacrysts: New constrains on their Source regions.// Journal of Petrology, 2004, aug 1, p. 1-30.