Helium ion microscope - secondary ion mass spectrometry for geological materials

Abstract

The helium ion microscope (HIM) is a focussed ion beam instrument with unprecedented spatial resolution for secondary electron imaging but has traditionally lacked microanalytical capabilities. With the addition of the secondary ion mass spectrometry (SIMS) attachment, the capabilities of the instrument have expanded to microanalysis of isotopes from Li up to hundreds of atomic mass units, effectively opening up the analysis of all natural and geological systems. However, the instrument has thus far been underutilised by the geosciences community, due in no small part to a lack of a thorough understanding of the quantitative capabilities of the instrument. Li represents an ideal element for an exploration of the instrument as a tool for geological samples, due to its importance for economic geology and a green economy, and the difficult nature of observing Li with traditional microanalytical techniques. Also Li represents a "best-case" scenario for isotopic measurements. Here we present details of sample preparation, instrument sensitivity, theoretical, and measured detection limits for both elemental and isotopic analysis as well as practicalities for geological sample analyses of Li alongside a discussion of potential geological use cases of the HIM-SIMS instrument.

Keywords: geoscience; helium ion microscopy (HIM); lithium; secondary ion mass spectrometry (SIMS).

Figure 1

Mass spectra of a natural zircon sample before rastering with the primary beam (red) and after rastering with the primary beam for 240 min (blue). Count rates increase dramatically after the sample coating has been removed.

Figure 2

a) Backscattered electron (BSE) map of the sample, SIMS location outlined in red; b) raw and processed ⁹⁰Zr and ⁵⁶Fe mass images after 20 min of rastered beam exposure on the sample; c) raw and processed ⁹⁰Zr and ⁵⁶Fe mass images after 240 min of rastered beam exposure on the sample. Initial maps were 512 × 512 pixels, covering a field of view of 70 µm. The scales show the absolute number of ions detected.

Figure 3

The calculated sputtering yield as a function of the atomic number for a 10 kV primary Ne beam impacting a silicate glass matrix for a low-density glass (2.2 g·cm⁻³, blue) and a high-density glass (3.3 g·cm⁻³, red), calculation after [17].

Figure 4

Reflected-light micrograph of the analysed Spodumene grain. The grain has relatively unaltered regions, separated by altered regions which intrude from the edge of the grain. Figure 5 was taken from within the region shown in red, across one such zone of alteration.

Figure 5

a) Raw (solid line), b) 32 × 32 regridded (dotted line) and c) 64 × 64 regridded (dashed line) maps of ⁶Li (red) and ⁷Li (green) from a Spodumene grain, see Figure 4.

Figure 6

The relative δ⁷Li ratios for regions within the same grain of Spodumene as in Figure 4 and Figure 5. Four connected regions with roughly similar counts have been identified, the purple region, an altered region, shows no Li, whilst the red region contains too few pixels to calculate a δ⁷Li value with a reasonable error. Values are not calibrated to an external standard, but instead to the average ratio across all measurements of the grain, which should lead to values of zero for isotopically homogenous material.

Figure 7

a) Backscatter electron image of the region of interest. The red square shows the region mapped using HIM–SIMS; b) Li EDS map of the region shown in a); c) Fe EDS map of the region shown in a); d) ⁶Li map of the region within the red square in a) using HIM–SIMS; e) ⁷Li map of the region within the red square in a) using HIM–SIMS.

Figure 8

Count rates relative to background count rates for ⁶Li (red, 3 μg/g) and ⁷Li (green, 37 μg/g) for the NIST 612 glass standard. Background counts were collected simultaneously on a detector positioned over a fractional mass/charge ratio with no real counts.

Figure 9

The relative δ⁷Li ratios for regions within the same grain of Spodumene. Values are not calibrated to an external standard, but instead to the average ratio across all measurements, which should lead to values of zero for isotopically homogeneous material. The internal statistical counting error is shown in red for each point with the external error across all measurements in grey.

Figure 10

a) Examples of deformed cleavage planes in a Li-rich biotite mica shown parallel to the c-axis and b) δ⁷Li values for vertical strips taken perpendicular to the scanning direction of the beam, along the green line in a). Each vertical grey line represents two microns of space left between each image, whilst red values are calculated using 1/8th width strips of the original maps and green values are calculated using 1/16th strips. Values are normalised to the average ratio across all measurements of the sample, rather than to an external standard.