187Re-187Os in Lesser Himalayan sediments: Measurement techniques and preliminary results

The applications of the¹⁸⁷Re-¹⁸⁷Os isotope pair as a petrogenetic and geologic tracer are increasing in recent years due to several advances in the chemical extraction and purification of Re and Os, occurring at ppb levels in environmental samples, and in the precise determination of the Os isotope composition. We have established in our laboratory; based on available methods, chemical procedures and Negative Thermal Ionisation Mass Spectrometric techniques for the measurement of Re-Os concentrations in environmental samples and the Os isotope composition in them. Using these techniques, we are able to determine¹⁸⁷Os/¹⁸⁶Os ratios with a precision of ∼ 1% (±2σ_μ; twice the standard error of the mean) in several tens of picogram of Os. Preliminary analysis of black shales from the Lower Tal section of the Maldeota phosphorite mine yields a mean¹⁸⁷Re-¹⁸⁷Os model age of 597 ± 30 Ma. The¹⁸⁷Os/¹⁸⁶Os and Os concentration in black shales of the Lesser Himalaya range from 8 to 96 and 0.02 to 13 ng g^-1 respectively. The mean¹⁸⁷Os/¹⁸⁶Os in these samples is ∼ 25, significantly higher than the crustal value of ∼ 10.5, suggesting that these black shales could be an important source of radiogenic Os to the rivers draining the Himalaya and to the steady increase in¹⁸⁷Os/¹⁸⁶Os of the oceans through the Cenozoic.