Dehydrogenation of kaersutitic amphibole under electron beam excitation recorded by changes in Fe³⁺/∑Fe: An EMP and SIMS study

We present in situ microanalyses of Fe³⁺/∑Fe in mantle-derived kaersutites as measured by electron probe microanalysis (EMP) based on the "self absorption induced FeLα peak shift" method. The EMP results are not in agreement with bulk (wet chemistry) data. The heterogeneities revealed for some kaersutite megacrysts, when comparing bulk and EMP Fe³⁺/∑Fe results, cannot explain the differences with the EMP measurements. It is thus proposed that any EMP overestimation of Fe^3-/∑Fe results from a beam-induced dehydrogenation and a subsequent oxidation of Fe²⁺ to Fe³⁺ according to the known relation: Fe²⁺ + OH^- = Fe³⁺ + O² + 1/2 H₂. To demonstrate this phenomenon, H losses were measured by secondary ion mass spectrometry (SIMS) after EMP irradiation at different beam currents on two amphiboles with 1.1 and 1.7 wt% H₂O, respectively. In both amphiboles, H losses were observed under high beam currents (240 and 100 nA). No dehydrogenation is observed under lower beam currents for the 1.1 wt% H₂O amphibole, but still occurs, down to at least 50 nA, for the amphibole with the greatest H₂O contents. Ampbiboles with low H₂O contents (below ∼0.5 H^_) are less affected by beam damage. For amphiboles with higher H₂O contents, the electron beam current density should be reduced with consideration given to the resulting high statistical errors.