ASSESSMENT OF THE UNCERTAINTIES AND LIMITATIONS OF QUANTITATIVE ELEMENTAL ANALYSIS OF INDIVIDUAL FLUID INCLUSIONS USING SYNCHROTRON X-RAY-FLUORESCENCE (SXRF)

Synchrotron X-ray Fluorescence (SXRF) analysis is a nondestructive ana lytical technique that provides compositional information from single fluid inclusions. A protocol for conducting quantitative analyses of m etal concentrations in individual fluid inclusions has been developed. This has led to an understanding of the accuracy, precision, and dete ction Limits of this technique, as well as the optimal shapes, sizes, and geometries required for reliable fluid inclusion analysis. Aqueous fluid inclusions containing known concentrations of SrCl2 were synthe sized for the development and the standardization of this technique. S trontium chloride was selected because it is highly soluble, its freez ing-point depression is well known (allowing us to confirm the inclusi on composition using microthermometric analyses), and the energetic Sr X-rays are only mildly attenuated by quartz. To confirm the compositi on of the synthetic standards, solutions were measured before and afte r each hydrothermal run using Atomic Absorption Spectroscopy (AAS), an d the freezing-point depression for each fluid inclusion was measured. SXRF analyses were performed on beam line X26A of the National Synchr otron Light Source (NSLS) at Brookhaven National Laboratory using an 8 x 12 mu m white X-ray beam. The analytical volume was calculated base d on known beam dimensions and fluid inclusion geometry determined usi ng a modified spindle stage. Elemental concentrations were determined by ratioing the Sr counts from an inclusion to the counts obtained fro m capillaries of known diameter containing similar solutions. Numerous inclusions from five samples, each with a different Sr concentration, were analyzed. Within a single population the mean is very close to t he known concentration, but the precision is poor, with standard devia tions (1 sigma) from 10-39% of the mean. Errors in determining the inc lusion geometry are the main contributor to the poor precision. The po or precision requires that numerous inclusions within one population b e analyzed and averaged to accurately estimate the metal concentration for that population. Selection of flat-lying, equant, regularly-shape d inclusions for analysis minimizes errors resulting from inclusion ge ometry if quantitative results are sought. The detection limit for Sr in synthetic fluid inclusions (typically 4-15 mu m thick, and 5-100 mu m below the upper polished surface) is approximately 2,000 ppm Sr.