Effect of current density on the electron microprobe analysis of alkali aluminosilicate glasses

In a previous work (Morgan and London 1996), we proposed an optimized procedure for electron microprobe analysis (EMPA) of rhyolitic glasses using a broad (20 μm diameter), low-current (2 nA) fixed beam. Some important applications for EMPA of glass, such as vitreous inclusions in minerals and experimental run products, require smaller beam diameters that produce greater areal current densities (expressed as nA/μm2). For these situations, we have assessed the effect of areal current density on the migration of Na and its concomitant effects on other elements and their ratios during EMPA of granitic glasses. Anhydrous and hydrous glasses of a haplogranite composition (Ab38.23Or29.31Qtz33.37C0.10) were analyzed at 20 kV accelerating potential, using 2.50 nA beam currents, fixed beam diameters of 2.20 μm, and counting times scaled to yield similar analytical uncertainty at each condition (~2.6% relative for Na2O). There is almost no loss of Na (≤1.7.2.7% relative) using a current density of 0.006 nA/μm2, minor (7.9%) Na loss for current densities up to 0.1 nA/μm2, and increasing Na loss with higher current densities that becomes severe at >0.5 nA/μm2 (e.g., 48.63% relative loss from hydrous glass at 50 nA and 2 μm during 3.6 s of irradiation). Sodium migration is more pronounced in hydrous glasses than in anhydrous ones, with significant loss from hydrous glass occurring during the first second of irradiation. The migration of Na results in increased concentrations of Al and Si, but little or no change in the concentration of K; if not fully corrected for, these effects produce systematic errors in important elemental ratios. With current densities <0.01 nA/μm2, anhydrous glasses or crystalline materials are suitable standards and data correction may not be needed. Significant Na loss using current densities up to ~0.1.0.2 nA/μm2, especially in hydrous glasses, requires data correction or primary standardization utilizing a glass having composition and water content similar to the unknown. Current densities ≥0.5.1.0 nA/μm2 are not suitable for EMPA of glass because of large and uncertain corrections (~25% to >100% of the Na2O value obtained).