Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

M. Sankar Das ¹

Affiliations
1 Analytical Chemistry Division, Bhabha Atomic Research Centre, Bombay, IN

Abstract

Recent studies carried out in the Analytical Chemistry Division, BARC, in the field of elemental and isotopic analyses of samples of geological importance by the neutron activation methods are reviewed. The salient features of the analysis of a wide variety of samples for their uranium content by the delayed fission neutron counting technique is described. The technique is quite specific and sensitive down to 0.1 ppm of uranium, with the possibility of the extension of the limit down to 0.004 ppm with sample size of about 500 mg. Another technique of interest to geologists is the fission track counting method, which is quite simple and can easily be used by laboratories with limited facilities and situated away from the reactor centre. The lower limit of analysis by the solubilation and standard addition approach adopted in this work is governed by the purity, with reference to uranium, of the reagents used for sample decomposition.

In the field of geochronology, the uranium, and thorium contents of zircons have been determined by the neutron activation method and the lead content by anodic stripping voltametry. These measurements have permitted the determination of Th/U ratios and the chemical ages of zircons. The limitations of the technique are the same as applies to the Pb-α method, however, unlike this method, which normally assumes a Th/U ratio of 1, the present method uses the experimental Th/U ratios, which seems to vary with the genesis of the zircons. Using the neutron activation method, the isotopic analysis of Pb-204 and Pb-208 has been carried out and the data has permitted the dating of thorium minerals. Model ages of galenas have been determined using the data obtained. The limitation of techniques and the interpretation of the ages have been critically commented upon.

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R. Parthasarathy ¹, M. Sankar Das ¹

Affiliations
1 Bhabha Atomic Research Centre, Bombay, IN

Abstract

A number of zircon separates from-various localities of India were analysed for their uranium, thorium and lead contents. Uranium and thorium were analysed by neutron activation, and lead by anodic stripping methods respectively. Thorium, uranium and lead were found to vary between 60-250, 60-900, 8-90 ppm, respectively. The Th/U ratios are found to be in conformity with the accepted values for granitic or pegmatiticzircon. The elemental abundances have been used to calculate their chemical ages and the values compared with those reported in literature. The ages obtained in this study range from 400-800 m.y. These ages are comparable with the previous reported values.

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A. V. Murali ¹, P. B. Pawaskar ¹, G. R. Reddy ¹, K. V. Subbarao ², V. N. Vasudev ³, M. Sankar Das ¹

Affiliations
1 Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay-400085, IN
2 Geology Department, Indian lnstitute of Technology, Powai, Bombay-400 076, IN
3 Chitradurga Copper Company, 16, Ali Asker Road, Bangalore-560052, IN

Abstract

No Abstract.

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M. Sankar Das

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No Abstract.

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P. B. Pawaskar ¹, B. S. Manerkar ¹, M. Sankar Das ¹

Affiliations
1 Analytical Chemistry Division, BARC, Bombay 400085, IN

Abstract

No Abstract.

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R. Parthasarathy ¹, A. V. Murali ¹, M. Sankar Das ¹, A. M. Nair ², Narayanaswamy ²

Affiliations
1 Analytical Chemistry Division, BARC, Bombay 400085, IN
2 Geosciences Division, Centre for Earth Science Studies, Trivandrum, Kerala 695010, IN

Abstract

Zircon separates from a few whole rocks and sediments along with the parent sources have been analysed for Zr, Hf, Se, REE, Th and U concentrations by the neutron activation analysis technique. The major, minor and trace analyses of rock samples and their modal analyses are also presented. The chondritic normalised REE patterns of rock samples show positive Eu anomalies, while the zircon samples show negative Eu anomalies. The data have been used to explain the genetical relationship among the rocks under study. This study supports the hypothesis proposed from this laboratory that the detrital zircons of Kerala beach are time integrated cumulates from the whole-rocks of this region. The scandium data indicate the paragenetic sequence of crystallization of zircons in relation to cpx and magnetite.

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R. Parthasarathy ¹, M. M. Palrecha ¹, M. Sankar Das ¹

Affiliations
1 Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, IN

Abstract

Seven zircon separates from different geological environments have been analysed for U, Th and Pb contents. U and Th are determined by Neutron Activation Analysis (NAA) and Pb by Differential Pulse Anodic Stripping Analysis (DPASV). The.elemental abundances have been utilised to calculate the chemical ages of zircons,

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J. Arunachalam ¹, A. V. Murali ¹, M. Sankar Das ¹

Affiliations
1 Analytical Chemistry Division, Bhabha Atomic Research Centre Trombay, Bombay 400085, IN

Abstract

A pattern recognition (PR) approach based on the principal component analysis (PCA) is adopted for the petrogenesis of basalts using the relative behaviour among the REE, on the data-sets of tholeiites (MOR, ocean island and continental) and alkalic basalts (melilites, nephelinites and alkali olivine basalts). It is observed that the overall variability in the REE data requires two or three principal components; with the first component accounting for 80% of the variability. The coefficient patterns of these components reveal distinct features. The dominant (first) component of tholeiites, exhibits a poor covariance of La and Ce with MREE. Coefficient patterns of the dominant component of tholeiites show breaks at Sm and at Dy/Tb similar to those observed in high pressure partition coefficients for cpx. The coefficient patterns in this major component appear to describe the REE behaviour during partial melting and high pressure fractionation of opx or cpx and eclogite.

In contrast with the tholeiites, the dominant component in alkalic basalts, shows excellent covariance among the LREE and MREE. Among the alkalic basalts, the alkali olivine basalts, show a different HREE behaviour. Comparison of the coefficient patterns of the dominant component of tholeiites and alkalic basalts suggests distinct differences in their source/mechanism of formation.

Theoretical computations involving partial melting and fractional crystallisation of only the major mineral (ol, opx, cpx, garnet/spinel) prove to be inadequate to simulate the observed coefficient patterns. The inclusion of ⁸⁷Sr/⁸⁶Sr isotopic ratios along with REE in the PCA, generates 'features' suggestive of additional sources (interstitial liquid, accessory minerals in the mantle and/or granitic crustal contamination) as contributing factors.

The PR approach described in this work allows one to extract more information from the REE data-set than what is possible by simple chondritic normalisation.

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