Fluid inclusion analysis by laser-ablation ICPMS and comparison with proton induced x-ray emission
A. Hack and J. Mavrogenes
Fluid inclusion analysis by excimer laser-ablation ICPMS (LA—ICPMS) has received much attention in the literature recently as it shows great potential for delivering detailed information on mass fluxes and processes in crust and mantle hydrothermal regimes. However, until now no data was available on which to judge the accuracy of the laser-ablation technique and thus its ability to provide true quantitative fluid compositions. This problem stems from the fact that concentrations can only be determined from ICPMS data with reference to an internal standard. Previously, internal standard procedures for the quantification of natural inclusions have involved estimating Na from freezing point depression measurements and an arbitrary calibration against an ideal NaCl-H2O system. Alternatively for synthetic fluid inclusions, certain trace elements have been added at known concentration to starting solutions and assumed to remain unmodified in the fluid throughout formation of inclusions. The validity of either procedure has not been rigorously demonstrated. To investigate the problem, synthetic fluid inclusions were prepared in a mineral-buffered system with a starting solution containing a range of trace elements (U, Th, La, Tm, Lu, Ba, Sr). The fluid inclusions were then analysed by both LA—ICPMS and proton induced x-ray emission (PIXE), an established non-destructive fluid inclusion analytical technique. LA—ICPMS derived fluid compositions, using the starting trace element composition as the internal standard, were consistently over-estimated by one or more orders of magnitude. This indicates that at the experimental conditions (700oC, 300MPa) the trace elements were not stable in solution and were lost. Several possible sinks have been identified such as, saturation, solid solutions with buffering phases and adsorption. Despite this, the data suggest that trace elements have attained equilibrium or near-equilibrium concentrations. Because trace element concentrations are modified by equilibration with the buffering assemblage, quantification is not possible using this technique. However, as PIXE provides an independent measurement of the fluid inclusion composition, the LA—ICPMS data can still be quantified.Comparison of the concentrations measured by PIXE and LA—ICPMS shows that the two techniques produce the same quantitative elemental abundances (with the exception of Fe) within their respective analytical uncertainties (Figure 1). The measurement of Fe by LA—ICPMS is unreliable due to large isobaric molecular interferences.
Figure 1: Comparison of LA—ICPMS and PIXE synthetic fluid inclusion data from three experiments at different run conditions for Cu, K, and Fe. LA—ICPMS data were quantified by using the K concentration determined by PIXE as an internal standard and NIST 612 silicate glass as the external analytical standard. Errors = 1s.The work confirms that LA—ICPMS provides a ready means of measuring fluid inclusion element ratios and its ability to quantify individual element concentrations provided that an internal standard is reliably known. Trace element doping of experimental starting fluids, as a means of internal standardisation is not as robust as previously believed. Further work will investigate the accuracy of the freezing point depression model as a technique for estimating internal standards for LA—ICPMS fluid inclusion quantification.