Preparación y calibración del trazador mixto 149 Sm/ 150 Nd, para relaciones isotópicas de rocas de las Sierras Pampeanas del Noroeste Argentino

Abstract

“ 149 Sm/ 150 Nd Combined Spike Preparation and Calibration, for Isotopic Relationships of Rocks from the Pampean Ranges of Northwestern Argentina”. Samarium (Sm) and neodymium (Nd) are rare earth elements with the same chemical and physical properties, and similar geochemical behaviour. However, during certain geochemical processes fractionation between them may occur. The most significant modification of the Sm/Nd relationship occurs during mantle- crust differentiation. This modification allows us to date (by Sm/Nd model ages) the time of upper mantle differentiation in the great majority of crustal rocks. In the present work a 149 Sm/ 150 Nd combined spike was prepared and calibrated. It was then used to obtain Sm/Nd model ages of rocks from the Pampean Ranges of NW Argentina. Isotopic Dilution with Thermal Ionisation Mass Spectrometry (ID-TIMS) was used for spike calibration. This is the most precise technique used for geochronometric dating. A relationship of 1:3 was obtained for the prepared spike, with a concentration of 0,1334 ?g/g for 149 Sm and 0,4719 ?g/g for 150 Nd. This spike was applied to rocks of diverse lithologies from the Velasco, Copacabana and Paimán ranges, obtaining two-stage model ages of around 1,59 Ga. This value agrees with the geological observations of the region.