Synthetic fluid inclusions in natural quartz. IV. Chemical analyses of fluid inclusions by SEM/EDA: Evaluation of method

The compositions of individual synthetic fluid inclusions in the systems NaCl-KCl, NaCl-CaCl 2 and NaCl-KCl-CaCl 2 have been semi-quantitatively determined by energy dispersive analysis of precipitates produced during thermal decrepitation. Inclusions containing known mixtures of 20 wt.% total salinity were synthesized by healing fractures in natural quartz at 600–700°C and 5–7 kbars for 7–10 days. The two-phase, daughter-free inclusions homogenized at 170–250°C, began to decrepitate after about 100° of overheating and by 360–420°C a significant number of decrepitates had formed on the polished surface. Peak heights generated by EDA (raster mode) of these decrepitates were standardized using both single and mixed salt standards evaporated to dryness in a vacuum. Although the mixed salt standards better approximated the decrepitate compositions, difficulties were encountered in producing micronscale homogeneity and the single salts yielded more reliable results. Eight different solutions of 20–23 wt.% total salinity were run and in all the samples the average compositions of 10–20 discrete, single inclusion decrepitates fell with 6 wt.% (0.2 to 5.2) of the actual composition, suggesting that the decrepitates were chemically representative of their precursor inclusions. However, not all decrepitates analyzed provided similarly accurate results. Electron mapping revealed that fracture-aligned decrepitates were often chemically inhomogeneous and thus had to be avoided. A sample decrepitated at 500°C yielded spurious results suggesting that chloride volatility may become a significant problem when temperatures in excess of 450°C are required for decrepitation. Decrepitates with diameters between 10 and 30 μm yielded more consistent and accurate results than smaller or larger decrepitates on the same samples.