Analysis - Stable isotopes


Kinetic processes affect abundances of stable isotopes such that they may be different between two coexisting minerals, or between mineral and fluids, etc. These fractionations are also temperature dependent. Kinetic mass fractionation is mass dependent and so lighter isotope systems have greater potential because of the larger fractional difference in mass. Common systems for analysis are C, O, and S.

The stable isotopes generally emit strongly as negative ions requiring use of the Cs primary beam and charge neutralization in the case of the analysis of insulators. Precisions around 0.1 permil and accuracy of the order of 0.3 permil can be obtained on SHRIMP in O and S isotopic analysis.

Instrumental effects easily fractionate light isotope ratios measured on SHRIMP. One such is the steering required for maximum beam transport through the source slit, which is a function of the distance the sample sits away from the extraction plate. To be sure that each of the unattended analyses hits the correct location and is in focus a post analysis image or snap (shot) is taken. Below the out of focus sample is an obvious outlier and can be rejected on the basis of the post analysis snap. The average 2 standard deviation of the remaining bracketing Amelia albite standards is much better than 0.3‰ with an overall session statistics of δ18O/16O of 10.7‰ ± 0.34‰ (2 standard deviations).



Updated:  26 October 2011/ Responsible Officer:  SHRIMP Group leader/ Page Contact:  WebAdmin