IRON OXIDATION STATE OF HYDROUS BASALTIC GLASS: TIME-DEPENDENT INTENSITY FLANK METHOD

摘要

The ferric-ferrous ratio of magmas controls phase relations, melt physical properties (e.g., viscosity and density) and volatile solubility and can be linked to the oxygen fugacity of the melt. Melt inclusions are small pockets of melt trapped inside crystals during the ascent of the magma that can be used as a window into the pre-eruptive conditions of the magma. Many of these melt inclusions are small (< 20 μm, Fig. 1) precluding measurement of the ferric-ferrous ratio by bulk techniques such as wet chemistry, calorimetry or Mossbauer spectroscopy. XANES is able to analyse such melt inclusions but this involves access to a synchrotron which is not widely available. A technique for measuring the iron oxidation state using the electron microprobe would be useful due to its availability and high spatial resolution. The position and height of the Fe La- and LP-peaks are related to the total Fe concentration and oxidation state of the glass [1]. EPMA techniques for measuring the oxidation state of Fe in minerals and glasses use either the Peak Position [2] or the Flank Method [2] by calibrating a set of independently constrained standards. Unfortunately, hydrous basaltic glass is unstable under the beam conditions required to measure the Fe L-peaks and the glass tends to change oxidation state during analysis [3]. Large glass samples can be analysed with either the Peak Position or Flank Method techniques by spreading the beam power across the sample (using either multiple points or stage movement) to reduce beam damage. This is not possible for melt inclusions due to their size. Instead, the Fe Lβ/α-ratio (Flank Method) can be measured over time using multiple spectrometers and interpolated to time = 0 for a single spot analysis (Fig. 2). A set of glasses with various bulk compositions, water contents and oxidation states were measured using EPMA. The iron oxidation state of these glasses had been independently constrained by XANES, wet chemistry or Mossbauer spectroscopy. Analytical conditions were an accelerating voltage of 15 kV, a beam current of 50 nA and a spot size of 10 μm. Two TAP crystals were used to measure the Fe La-peak, a single TAPH crystal for the Fe Lβ and a LIFL for the Fe Ka-peak. Analyses were repeated ten times per glass. Results of the EPMA derived Fe(II)/FeT agree favourably with the other techniques (Fig. 3).