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Project Progress

The importance of micro analytical techniques is increasingly being recognized because it allows one to evaluate spatial heterogeneities and provides chemical information on a phase that can be integrated with the petrography of a sample. However, with ever increasing improvements in analytical performance of micro techniques, it is important to address if inferred chemical compositional heterogeneities are real or if they are produced by analytical artifacts. For example, in SIMS analysis workers have found that the accuracy of O, Fe, or S isotope analysis can be compromised in high symmetry minerals such as magnetite or sphalerite as a function of the angle of the primary ion beam with respect to the mineral lattice (Huberty et al., 2010; Kita et al., 2011; Kozdon et al., 2010; Lyon et al., 1998). In the case for Fe isotope analysis of magnetite these effects can produce at least a 0.7 ‰ in ⁵⁶Fe/⁵⁴Fe accuracy problem (Kita et al., 2011). Because of these observations we have begun to investigate methods for accurate Fe isotope analysis in a variety of Fe bearing phases including magnetite, hematite, and pyrite. To avoid the spatial effects associated with SIMS analysis in high symmetry minerals we have investigated laser ablation methods using a laser with a ~100 fs pulse width. The fs-LA process is fundamentally different as compared to the more commonly used nano second pulse width laser and we have chosen to use fs-LA because it should minimize possible isotope fractionation caused by heating of the sample substrate. Nanosecond pulses induce a progressive melting of the target and remove matter mainly via a mechanical process of hydrodynamic sputtering (Hergenroder, 2006b; Stuart et al., 1996). In contrast, the fs-LA process is non-thermal because the pulse energy is delivered in a time shorter than the thermal relaxation of the solid target which is > ~1ps (von der Linde et al., 1997).

Simple validation studies comparing the Fe isotope composition of magnetite determined by fs-LA and standard solution nebulization have been made on different layers of Isua BIFs and shows that there is no bias between analysis of the same layer by fs-LA and conventional methods (Fig. 1). The fs-LA analyses were made as rasters on areas of approximately 60 × 40 μm with an average depth of 6 μm as determined by white light interferometry (Fig. 2). We highlight that in this study we found no resolvable variability in Fe isotope compositions (0.15 ‰ in ⁵⁶Fe/⁵⁴Fe) within an individual layer and that inter-layer variability was relatively minor (Fig. 1). This observation stands in marked contrast to recent SIMS studies of magnetite from Isua BIFS that found significant ( >1 ‰) variability in ⁵⁶Fe/⁵⁴Fe ratios in magnetite grains that are separated by 10’s of μm. Although our study and that of Whitehouse and Fedo (2007) were not conducted on the same samples, our past work has shown that Fe isotope analysis of magnetite by SIMS methods can be prone to significant accuracy issues associated with orientation effects (Kita et al., 2011) and these differences highlight how important it is to evaluate accuracy and evaluate true isotope heterogeneity versus analytical artifacts.

Past work has identified three major factors that affect the accuracy and precision of chemical and isotopic analysis by laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) that include:

1) Is the aerosol produced by ablation a stoichiometric representation of the substrate?
2) Is the aerosol produced by the ablation process chemically homogenous with a particle size distribution suitable for transport to and ionization by the plasma?
3) How much does the ionization process contribute to inaccuracies in analysis by plasma loading, incomplete ionization, or other non-spectral effects?
We have begun evaluating the above processes by collecting aerosols generated by fs-LA based on their aerodynamic size using a MOUDI cascade impactor. The particle size distribution of aerosols generated by fs-LA of magnetite, shows that >75% by mass of Fe of particles produced have an diameter of < 100 nm (Figure 3). The size sorted Fe isotope composition of aerosols generated from magnetite have been analyzed using conventional solution nebulization and the mass balance of all the particles indicate that the aerosols sample is identical in its Fe isotope composition to that of the substrate. However, there is a correlation between particle size and Fe isotope composition. The smallest aerodynamic sized particles have lower δ⁵⁶Fe values as compared to larger particles and the total range is approximately 0.8 ‰ (Figure 3). These isotope fractionations are consistent with rapid condensation of Fe from the plasma generated by fs-LA where the first condensed particles have light Fe isotope compositions (e.g., Richter, 2004; Wiesli et al., 2007). Regardless of the origin of this isotope fractionation this variability in Fe isotope composition as a function of size highlights the importance of quantitative transport of the aerosol to the plasma. For example, the smallest sized aerosol particles have δ⁵⁶Fe values of ~ 0.1 ‰ and make up approximately 35% of the total mass of Fe, and the remaining 65% of Fe has an overall δ56Fe value of ~0.5 ‰. Two-component mixing between these particles would indicate that if the smallest aerosol particles are lost the measured Fe isotope composition would be too large as shown in Figure 4. We note that loss of such material should be apparent in decreasing iron ion signal (Fig. 4).

In addition to these studies that characterize the aerosols produced during fs-LA we have also started investigating possible non spectral effects that could lead to inaccurate Fe isotope measurements. For example, in Band Iron Formation (BIF), magnetite tends to be chemically pure (>98% Fe₃O₄) but it is inter grown with significant amounts of Si, occurring as small quartz blebs between magnetite grains that are ~10 μm in diameter and are likely to be ablated during a magnetite analysis (e.g., Fig. 2). Therefore, it is critical to establish if small amounts of Si will create non spectral interference that result in inaccurate Fe isotope analyses. Past studies using standard solution nebulization and MC-ICP-MS show matrix effects can impart significant isotope ratio accuracy issues (e.g., > 0.2 ‰ in ⁵⁶Fe/⁵⁴Fe) if samples and standards have different analyte concentrations, are in different acid concentrations, or have different levels of impurities in them (e.g., Albarede and Beard, 2004) and similar effects have been demonstrated with ns-LA ((Aeschliman et al., 2003; Guillong and Gunther, 2002; Guillong et al., 2003; Jackson and Gunther, 2003; Janney et al., 2011; Kroslakova and Gunther, 2007; Norman et al., 2006; Rodushkin et al., 2002) and in some fs-LA applications (Arrowsmith, 1987; Ikehata et al., 2008, 2011) In contrast, it has been claimed that there are not matrix effects when doing Fe isotope analyses by fs-LA (Horn et al., 2006; Steinhoefel et al., 2009). A preliminary study of possible non spectral interferences during fs-LA of magnetite has been made using a sample standard bracketing technique in which during the sample analysis different concentrations of solutions were aspirated into the aerosol stream using an Aridus II desolvating nebulizer, as compared to the bracketing standard which had blank acid aspirated during their analysis by LA. Our studies found that low atomic number elements such as Mg, Na, and Si had no effect on generating spurious isotope results and no significant effect on ion intensity. In contrast, it was found that heavy elements such as U could affect Fe isotope accuracy where a 1 ppm U solution resulted in a 4 ‰ shift in 56Fe/54Fe ratios and decreased the ion intensity of nearly 20%. Because this isotope effect appeared only for heavy elements we believe that this matrix effect is associated with space charge effects after the plasma has been sampled through the skimmer cone because if it was associated with plasma loading one would expect that similar quantities of any element would affect isotope ratios. A unique aspect of this data set, as compared to our past solution nebulization tests, is that we did not observe any effects of mass bias for fs-LA if low atomic mass elements were introduced which we have previously observed to affect solution nebulization (e.g., Albarede and Beard, 2004). This observation suggests that fs-LA is less prone to matrix effects as compared to solution nebulization and provides great promise for fs-LA being an accurate and precise method for isotope analysis.

Cited References

Aeschliman, D.B., Bajic, S.J., Baldwin, D.P., Houk, R.S., 2003. High-speed digital photographic study of an inductively coupled plasma during laser ablation: comparison of dried solution aerosols from a microconcentric nebulizer and solid particles from laser ablation. Journal of Analytical Atomic Spectrometry 18, 1008-1014.
Albarede, F., Beard, B., 2004. Analytical methods for non-traditional isotopes, in: Johnson, C.M., Beard, B.L., Albarede, F. (Eds.), Geochemistry of Non-Traditional Stable Isotopes, pp. 113-152.
Arrowsmith, P., 1987. Laser ablation of solids for elemental analysis by Inductively Coupled Plasma Mass-Spectrometry Analytical Chemistry 59, 1437-1444.
Guillong, M., Gunther, D., 2002. Effect of particle size distribution on ICP-induced elemental fractionation in laser ablation-inductively coupled plasma-mass spectrometry. Journal of Analytical Atomic Spectrometry 17, 831-837.
Guillong, M., Kuhn, H.R., Gunther, D., 2003. Application of a particle separation device to reduce inductively coupled plasma-enhanced elemental fractionation in laser ablation-inductively coupled plasma-mass spectrometry. Spectrochimica Acta Part B-Atomic Spectroscopy 58, 211-220.
Hergenroder, R., 2006b. Hydrodynamic sputtering as a possible source for fractionation in LA-ICP-MS. Journal of Analytical Atomic Spectrometry 21, 517-524.
Horn, I., von Blanckenburg, F., Schoenberg, R., Steinhoefel, G., Markl, G., 2006. In situ iron isotope ratio determination using UV-femtosecond laser ablation with application to hydrothermal ore formation processes. Geochimica Et Cosmochimica Acta 70, 3677-3688.
Huberty, J.M., Kita, N.T., Kozdon, R., Heck, P.R., Fournelle, J.H., Spicuzza, M.J., Xu, H., Valley, J.W., 2010. Crystal orientation effects in delta O-18 for magnetite and hematite by SIMS. Chemical Geology 276, 269-283.
Ikehata, K., Notsu, K., Hirata, T., 2008. In situ determination of Cu isotope ratios in copper-rich materials by NIR femtosecond LA-MC-ICP-MS. Journal of Analytical Atomic Spectrometry 23, 1003-1008.
Ikehata, K., Notsu, K., Hirata, T., 2011. Copper isotope characteristics of copper-rich minerals from Besshi-type volganogenic massive sulfide deposits, Japan, determined using a femtosecond LA-MC-ICP-MS. Economic Geology 106, 307-316.
Jackson, S.E., Gunther, D., 2003. The nature and sources of laser induced isotopic fractionation in laser ablation-multicollector-inductively coupled plasma-mass spectrometry. Journal of Analytical Atomic Spectrometry 18, 205-212.
Janney, P.E., Richter, F.M., Mendybaev, R.A., Wadhwa, M., Georg, R.B., Watson, E.B., Hines, R.R., 2011. Matrix effects in the analysis of Mg and Si isotope ratios in natural and synthetic glasses by laser ablation-multicollector ICPMS: A comparison of single- and double-focusing mass spectrometers. Chemical Geology 281, 26-40.
Kita, N.T., Huberty, J.M., Kozdon, R., Beard, B.L., Valley, J.W., 2011. High-precision SIMS oxygen, sulfur and iron stable isotope analyses of geological materials: accuracy, surface topography and crystal orientation. Surface and Interface Analysis 43, 427-431.
Kozdon, R., Kita, N.T., Huberty, J.M., Fournelle, J.H., Johnson, C.A., Valley, J.W., 2010. In situ sulfur isotope analysis of sulfide minerals by SIMS: Precision and accuracy, with application to thermometry of similar to 3.5 Ga Pilbara cherts. Chemical Geology 275, 243-253.
Kroslakova, I., Gunther, D., 2007. Elemental fractionation in laser ablation-inductively coupled plasma-mass spectrometry: evidence for mass load induced matrix effects in the ICP during ablation of a silicate glass. Journal of Analytical Atomic Spectrometry 22, 51-62.
Lyon, I.C., Saxton, J.M., Cornah, S.J., 1998. Isotopic fractionation during secondary ionisation mass spectrometry: crystallographic orientation effects in magnetite. International Journal of Mass Spectrometry 172, 115-122.
Norman, M.D., McCulloch, M.T., O’Neill, H.S., Yaxley, G.M., 2006. Magnesium isotopic analysis of olivine by laser-ablation multi-collector ICP-MS: composition dependent matrix effects and a comparison of the Earth and Moon. Journal of Analytical Atomic Spectrometry 21, 50-54.
Rodushkin, I., Axelsson, M.D., Malinovsky, D., Baxter, D.C., 2002. Analyte- and matrix-dependent elemental response variations in laser ablation inductively coupled plasma mass spectrometry – Part 2. Implications for multi-element analyses. Journal of Analytical Atomic Spectrometry 17, 1231-1239.
Steinhoefel, G., Horn, I., von Blanckenburg, F., 2009. Matrix-independent Fe isotope ratio determination in silicates using UV femtosecond laser ablation. Chemical Geology 268, 67-73.
Stuart, B.C., Feit, M.D., Herman, S., Rubenchik, A.M., Shore, B.W., Perry, M.D., 1996. Nanosecond-to-femtosecond laser-induced breakdown in dielectrics. Physical Review B 53, 1749-1761.
von der Linde, D., Sokolowski-Tinten, K., Bialkowski, J., 1997. Laser-solid interaction in the femtosecond time regime. Applied Surface Science 109-110, 1-10.
Whitehouse, M.J., Fedo, C.M., 2007. Microscale heterogeneity of Fe isotopes in > 3.71 Ga banded iron formation from the Isua Greenstone Belt, Southwest Greenland. Geology 35, 719-722.

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