Depth dependent element ratios in fluid inclusion analysis by laser ablation ICP-MS

Ratios of element count rates used for quantification of LA-ICP-MS analysis of fluid inclusions inside minerals change between shallow inclusions, ablated at crater depth to diameter aspect ratios <3 and deep inclusions, with aspect ratios >4, by up to a factor of 2.5. The Cl/Na and the Pb/Na ratios were found to be affected the most. Ablating a scapolite sample with homogeneous Na, Pb and Cl concentrations showed a similar pattern but of smaller magnitude. Such strong fractionation behaviour of chlorine has not been reported to date for LA-ICP-MS analysis. This aspect ratio dependent elemental fractionation is likely due to condensation-deposition and remobilisation effects within the laser ablation crater and may account for some of the scatter obtained when analysing a homogeneous assemblage of fluid inclusions located at variable depths below the surface.