Journal of Petrology, Volume 39, Issue 10: October 1 1998.

Enriched Subcontinental Upper Mantle beneath Southern India: Evidence from Pb, Nd, Sr, and C-O Isotopic Studies on Tamil Nadu Carbonatites

HELMUT SCHLEICHER¹, ULRICH KRAMM², ERNST PERNICKA³, MANFRED SCHIDLOWSKI⁴, FATEMEH SCHMIDT⁵,V. SUBRAMANIAN⁶, WOLFGANG TODT⁴ AND SHRIVINAS G. VILADKAR⁷

¹MINERALOGISCH-PETROGRAPHISCHES INSTITUT UNIVERSITÄT HAMBURG, GRINDELALLEE 48, D-20146 HAMBURG,GERMANY
²INSTITUT FÜR MINERALOGIE UND LAGERSTÄTTENLEHRE, RHEINISCH-WESTFÄLISCHE TECHNISCHE HOCHSCHULEAACHEN, AACHEN, GERMANY
³MAX-PLANCK-INSTITUT FÜR KERNPHYSIK, POSTFACH 10 39 80, D-69029 HEIDELBERG, GERMANY
⁴MAX PLANCK-INSTITUT FÜR CHEMIE, POSTFACH 3060, D-55020 MAINZ, GERMANY
⁵MINERALOGISCH-PETROGRAPHISCHES INSTITUT UNIVERSITÄT HEIDELBERG, IM NEUENHEIMER FELD 236,D-69120 HEIDELBERG, GERMANY
⁶STATE GEOLOGICAL SURVEY, MADRAS, TAMIL NADU, INDIA
⁷GEOLOGY DEPARTMENT, ST XAVIER'S COLLEGE, 400 001 BOMBAY, INDIA

The major, trace and rare earth element distributions as well as the Nd, Sr, Pb, C and O isotope geochemistry of four carbonatite occurrences (Sevattur, Jogipatti, Samalpatti, Pakkanadu) from Tamil Nadu, southern India, have been investigated. The majority of these carbonatites are highly enriched in light rare earth elements (LREE) and exhibit stable isotope patterns similar to those reported for primary mantle-derived carbonatitic melts. In Sevattur, a well-defined lead/lead isochron yields an age of 801 ± 11 Ma for ankeritic carbonatites. Most of the samples from the other Tamil Nadu carbonatites define an array in the lead system which is interpreted as a mixing line between two reservoirs, namely a depleted mantle and a U-enriched component (either crustal lead or an enriched mantle reservoir). The carbonatites are characterized by very low present-day [epsilon]_Nd values between -8·8 and -20·1 and high ⁸⁷Sr/⁸⁶Sr ratios (0·7045-0·7054). At 800 Ma, the isotopic signature was similar to EM I, but somewhat more enriched. In contrast to the lead data, the Nd-Sr isotope data show a clear signature of a mantle source excluding crustal contamination. The data are compatible with either an EM I-like mantle component 800 my ago or a mixing process between two mantle reservoirs.

Keywords: carbonatites;geochemistry;India; isotope geochemistry; REE

Pages 1765-1785