Quantitative solute analysis of single fluid inclusions in halite by LA-ICP-MS and cryo-SEM-EDS: complementary microbeam techniques

Abstract

An interlaboratory 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 mu m 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, SO 4 ), 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 1sigma 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.