| Abstrakt: |
Abstract An inter laboratory study was carried out to assess the complementariness of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and cryogenic SEM-energy dispersive analysis (cryo- SEM-EDS) for the quantitative solute analysis of single inclusions in halite. The halite samples were selected from diverse ancient and recent evaporite environments to give a wide range of solute compositions, and were characterized by abundant, homogeneous populations of large (1-100 µm diameter) primary brine inclusions. Homogeneity of the inclusions was important because both techniques are destructive and had to be carried out on different individual inclusions. Cryogenic SEM-EDS was chosen because it is one of the few, well established fluid inclusion techniques to provide absolute concentrations for the major solutes (Na, K, Mg, Ca, Cl, SO4), and could be used to convert the relative element concentration ratios determined by LA-ICP-MS (K, Mg, Ca, Cl, Br, Sr, Li, B) into true concentration units. Normalization was achieved by referring to chlorine. The results confirm that LA-ICP-MS and cryo-SEM-EDS are perfectly complementary and yield values for Mg and K that agree within \a standard deviation. Calcium concentrations were generally too low to be measured by cryo-SEM-EDS, but values determined by LA-ICP-MS are consistent with the hypothesis that the brines are saturated with respect to anhydrite. Overall analytical precision (relative standard deviation) for the major solutes is 2-3 times better for cryo-SEM-EDS than for LA-ICP-MS (5-15% vs. 10-35% respectively). Internal consistency of results was checked by charge mass balance estimates and halite saturation indices. Though the study utilized halite-saturated inclusions, cryo-SEM-EDS and LA-ICP-MS can be applied equally to lower salinity fluids and, with care, to inclusions that contain daughter minerals |
