Measurements of Fe2⁺/∑Fe in natural and experimental silicate glasses by EPMA: further developments of the flank method

The redox state of mantle-derived primary magmas and their derivative liquids has been extensively studied over the past decades, yet remains a challenging topic. Insights into magma redox conditions are primarily derived from analyses of iron speciation in pillow glasses or mineral phases. Among the various techniques available to determine Fe³⁺/Fe²⁺ ratios, the EPMA-based flank method stands out for offering high spatial resolution and accuracy at relatively low cost. Nevertheless, its routine application is still limited by the requirement of well-calibrated standards and a precision that is generally lower than that of XANES or Mössbauer spectroscopy.

In this contribution, we present further improvements of the flank method approach using a JEOL JXA-iHP200F EPMA. To address this, we synthesized a set of new dry and H₂O-bearing basaltic to andesitic glass standards ranging in Fe²⁺/∑Fe from ~0 to 1. In addition, we used natural boninitic and fore-arc basaltic pillow glasses (IODP EXP 352) as well as Shatsky Rise pillow glasses to test the beam stability of different sample types and investigate their performance.

We demonstrate that, while dry basaltic glasses can endure high beam energies, hydrous glasses only endure about a tenth of beam energies before alteration. The precision in Fe²⁺/∑Fe ratio obtained for dry basaltic glasses (~ ±0.05) is significantly lower compared to hydrous glasses (~ ±0.12) due to their higher stability allowing longer measurement times and stronger beam currents. Our flank method results for dry basaltic glasses generally agree with previously obtained XANES data of the same samples.