EPMA ANALYSIS OF FLUID INCLUSIONS IN VOLCANIC ROCKS

The investigation of unusual mineral assemblages found in volcanic rocks from Tibet or Kamchatka reveals that interactions between magma and methane-hydrogen fluids derived from the deep Earth may generate highly reducing conditions within some volcanic plumbing systems. More recently, further insights into these unrecognized fluid transfer processes via the super-reduced mineral system observed in Cretaceous volcanic rocks from Mount Carmel, Israel [1], reflect the local interaction of mantle-derived CH4−H2 fluids with basaltic magmas in the shallow lithosphere. These interactions produced desilication of the magma, supersaturation in Al2O3, leading to rapid growth of corundum. This particular sample has been analysed by EPMA (Electron Probe Micro Analysis).

The development of the Schottky emitter and its implementation as an electron source in the electron microprobe has significantly improved the characterisation of geological specimens. One of the advantages of the field emission (FE) source is to obtain a finely focussed electron beam at low beam voltage (≤10 keV) while maintaining a high and stable beam current. Under these experimental conditions, the penetration depth of the primary electrons, and thus the interaction volume in which electrons are scattered and generate X-rays, decreases to the sub-µm scale (compared to µm-scale of the traditional electron microprobe at 15 or 20 keV). The high beam current capability allows both major element and trace element analysis. Thanks to wavelength-dispersive X-ray spectrometry (WDS) with sub-10eV energy resolution, accurate qualitative and quantitative analysis can be achieved even on sub-µm phases at low beam energy and high lateral resolution using low energy X-ray lines. For the heavier elements this can be achieved by using L- or M-lines.

The analytical capabilities of the new SXFive FE TACTIS are illustrated by the analysis of the corundum (Al2O3) found in Mount Carmel volcanic ejecta.

[1] Griffin WL, Gain SEM, Adams DT, et al. Geology 2016;44:815–8. (https://doi.org/10.1130/G37910.1)

The authors acknowledge Professor William L. Griffin and his teams at the Macquarie University for the sample courtesy.