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Project Progress

We are evaluating the capabilities of the ion microprobe and femtosecond laser ablation (fsLA) for Fe isotope analysis of different Fe-bearing minerals. This work has focused on using the WiscSIMS facility and the newly installed fsLA system in the UW Radiogenic Isotope laboratory to determine the best instrument operating conditions. In parallel with this work, different Fe-bearing minerals are being evaluated for suitability of a Fe isotope standard. To date we have developed a series of Fe isotope mineral standards including magnetite, hematite, pyrite, and Fe carbonate (Table 1). This work has entailed back-scattered electron imaging and electron microprobe wavelength-dispersive analysis to determine the extent of chemical zonation, and major element composition.

We have established the best running conditions for magnetite analysis by ion microprobe. With a 25 micron sized O- primary beam, the ⁵⁶Fe/⁵⁴Fe ratio can be measured to an internal 2-standard error precision of 0.15 ‰ based on 4 minutes of sputtering time. Multiple analyses of a single mineral grain indicate that overall external reproducibility (2-standard deviations) is the same. However, multiple analyses of magnetite grains that are homogenous to +0.050 ‰ define a 0.6 ‰ of δ⁵⁶Fe values which are positively correlated with the ion yield (⁵⁶Fe ion current normalized to primary ion beam current) (Figure 1). This variability in δ⁵⁶Fe values in a chemically and isotopically homogenous material is probably the result of crystal orientation effects associated with ion channeling and implanting of O₂ into the crystal structure resulting in relative changes in ion yield (Kita et al., 2010). We are currently investigating methods to minimize fractionation by using different energy offsets and primary acceleration potentials as well as developing methods to correct for crystal orientation mass bias effects based the crystallographic orientation of the mineral. We are also investigating related effects for oxygen and sulfur isotopes in magnetite and sphalerite (see project report: New frontiers in micro-analysis of isotopic compositions of natural materials: Development of O, S, Si, and Li isotopes). We highlight that this crystal orientation effect is not expected for phases such as pyrite or Fe carbonate and thus these phases may be more amenable to analysis by ion probe.

We are in the initial stages of developing methods for Fe isotope analysis using fsLA coupled to multi-collection inductively coupled plasma mass spectrometry (MC-ICP-MS). So far, we have focused on magnetite because this is the most important oxide mineral in banded iron formations, as well as early Archean black shales. Preliminary data indicates that the fsLA is free of matrix effects associated with sample heating as well as chemical differences in the phase (Figure 2), confirming the promise that ultra-fast laser pulses of ~100 fs (10^-13 s) have clear advantages as compared to traditional nanosecond (10^-9 s) lasers. For example, magnetite analyses are accurate over a wide range of magnetite compositions (Fig. 2A) that range from nearly endmember magnetite to samples with high concentrations of Ti, Mg, Mn and Zn (Fig. 2B). The internal (2-SE) and external precision (2-SD) of each analysis is ~0.15 ‰ in ⁵⁶Fe/⁵⁴Fe, which, remarkably, is only slightly poorer than that obtained using solutions. Laser spot sizes ranged from 15 and 20 microns and the sample was scanned back and forth (~5 passes) over a 75 micron linear segment. An analysis lasts 400 seconds and is a preceded by a 60 second gas blank analysis which is subtracted from the on-peak analyte measurement. These results demonstrate that fsLA, coupled to MC-ICP-MS promises to revolutionize in situ isotopic analysis of geologic materials for elements amenable to MC-ICP-MS analysis. In contrast, elements not readily analyzed using MC-ICP-MS, such as O and C, will likely remain in the realm of ion probe analysis.