instrumental techniques applied to mineralogy and geochemistry

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Analytical techniques applied to fluid inclusion studies: basics and applications 147 obtain the salinity of fluid inclusions. Thus, both the height and area ratios of hydrohalite 3423 cm –1 and 3405 cm –1 peaks against the ice 3098 cm –1 peak, and the total area ratios of hydrohalite peaks against ice 3098 cm 1 peak can be used to estimate the salinity of fluid inclusions. Analysis of individual fluid inclusions As has been shown above, fluid inclusions could contain fluids that have been trapped at different moments in the "life" of the host crystal. This gives rise to the existence of different populations of fluid inclusions. It is very common for a crystal not to contain a unique type of fluid. Instead it will reflect, to varying degrees, the different fluids characterizing the various processes that the fluid inclusion host crystal has undergone, from its formation up to the moment of observation. In addition, fluid inclusions may have suffered processes of re-equilibration during the evolution after their trapping. As a consequence, it becomes more and more frequent that, depending on the precision level decided according to the aims of the study proposed, characterization of the composition of fluid inclusions to bulk sample scale is not sufficient and the properties of individual fluid inclusions must be determined. For this proposal, a number of specific techniques, capable of analyzing individual fluid inclusions in a "point" form, can be found in specialized literature. Among these techniques, we can nowadays emphasize the laser ablation (LA) group (destructive techniques), and the microprobes group (non-destructive techniques). In relation to the first group, Fabre et al. (1999) explain that LA can be based on ICP- MS (inductively coupled plasma mass spectrometry) or on optical emission spectroscopy (OES). Both techniques are coupled with LA at microscopic scale to extract the liquid to be analyzed within the fluid inclusions. According to the same authors, LA-ICP-MS could be more adequate for determination of trace elements and LA-ICP-OES gives better detection limits results for major elements, especially Na and Ca. The limitations of LA-ICP-AES 5 arise from the lack of sensitivity of AES for some elements and the difficulty in determining absolute concentrations of components in the fluid inclusions 5 LA-ICP-AES and LA-ICP-OES (AES: atomic emission spectroscopy; OES: optical emission spectroscopy) could be considered the same technique.
148 Salvador Morales (Wilkinson et al. 1994). Also, detection limits (Günter et al., 1998) are higher for LA- ICP-AES (in the g g -1 to mg g -1 range) than for LA-ICP-MS (in the ng g -1 to g g -1 range). As a consequence, LA-ICP-MS is more frequently used today. Gagnon et al. (2003) indicate different advantages for LA-ICP-MS techniques: low capital cost, ease of use, ability to obtain data from depth fluid inclusions, etc. In addition, these authors point out the minimal sample preparation, the possibility of obtaining quantitative analysis of individual fluid inclusions, the high spatial resolution (down to 2m), the high sensitivity (less than g/g), moderate/high precision (5-30%), and the possibility of obtaining rapid multielement analysis over wide mass (6-240 amu) and dynamic (up to 11 order of magnitude) ranges, etc. In contrast, Gagnon et al (2003) indicate some weaknesses to overcome in the future: internal standardization, correction of host mineral contribution to the fluid inclusion signal, elimination of spectral interferences, etc. Günter et al. (1998) explained a stepwise opening procedure for a controlled ablation (a small hole is drilled for the partial release of liquid and vapour, followed by complete drilling using a pit covering the entire inclusion) in order to avoid the explosion and splashing of material, with the consequent loss of solid internal precipitates. Gagnon et al. (2003) propose an alternative strategy for ablation, consisting in a traversed opening. The method consists of traversing the sample surface until the laser beam is over the top of the fluid inclusion and then allowing the laser beam to drill into the inclusion. In practice, the most adequate procedure for ablation depends on each sample and some preliminary essays must be done prior to analyzing fluid inclusions. Günter et al. (1998) also explained the procedure for quantification of element concentrations. Element ratios calculated from integrated intensity are calculated from the elements ratios via an internal standard element, whose concentration was determined prior to ablation (in this case, concentration of Na determined using microthermometric measures). A complete description for quantification of element concentration can be found at Gagnon et al. (2003). These authors analyze several alternatives for calibration standard (including NIST and USGS synthetic glasses, synthetic fluid inclusions, desolvated solution standards, microcapillaries, microwells or fluid inclusion analogues) and for measurement of internal standards.
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