QUANTITATIVE SOLUTE ANALYSIS OF SINGLE FLUID INCLUSIONS IN HALITE BY LA-ICP-MS AND CRYO-SEM-EDS - COMPLEMENTARY MICROBEAM TECHNIQUES

An interlaboratory study was carried out to assess the complementarine ss of laser ablation-inductively coupled plasma-mass spectrometry (LA- ICP-MS) and cryogenic SEM-energy dispersive analysis (cryo-SEM-EDS) fo r the quantitative solute analysis of single inclusions in halite. The halite samples were selected from diverse ancient and recent evaporit e environments to give a wide range of solute compositions, and were c haracterized by abundant, homogeneous populations of large (1-100 mu m diameter) primary brine inclusions. Homogeneity of the inclusions was important because both techniques are destructive and had to be carri ed out on different individual inclusions. Cryogenic SEM-EDS was chose n because it is one of the few, well established fluid inclusion techn iques 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, L i, B) into true concentration units. Normalization was achieved by ref erring to chlorine. The results confirm that LA-ICP-MS and cryo-SEM-ED S are perfectly complementary and yield values for Mg and K that agree within 1 sigma standard deviation. Calcium concentrations were genera lly too low to be measured by cryo-SEM-EDS, but values determined by L A-ICP-MS are consistent with the hypothesis that the brines are satura ted 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 an d halite saturation indices. Though the study utilized halite-saturate d inclusions, cryo-SEM-EDS and LA-ICP-MS can be applied equally to low er salinity fluids and, with care, to inclusions that contain daughter minerals.