High-Precision Fe Isotope Analysis On MC-ICPMS Using A 57Fe-58Fe Double Spike Technique

Herein we report procedures based on multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) for high-precision Fe isotopic analysis using a 57Fe-58Fe double spike technique. Iron purification was achieved using AG1-X8 in HCl media following previously or newly established procedures. In the new procedure, smaller columns with 4 mm diameter were used, containing 0.4 mL AG1-X8, thus greatly reducing the operation time and the amount of acid and resin consumed compared to the previously established method using 1 mL resin. Potential trace Ni interference on 58Fe was suppressed by increasing the total Fe ion intensity to ≥ 120 V. Measurements of GSB Fe solutions doped with mono-elements demonstrated that a mass bias correction by the 57Fe-58Fe double spike was robust if Ca/Fe ≤ 1.0, Al/Fe ≤ 1.0, Cu/Fe ≤ 1.0, Co/Fe ≤ 0.1, Ni/Fe ≤ 10-4, and Cr/Fe ≤ 10-4. Monitoring of pure Fe standard solutions, viz. IRMM-014 and NIST3126a, and geological reference materials, viz. JP-1, BHVO-2, W-2a, GSP-2, and COQ-1, over nine months yielded δ56Fe (relative to IRMM-014) values of 0.003 ± 0.013‰ (2 SD, N = 20), 0.368 ± 0.011‰ (2 SD, N = 30), 0.019 ± 0.018‰ (2 SD, N = 15), 0.109 ± 0.017‰ (2 SD, N = 30), 0.049 ± 0.018‰ (2 SD, N = 17), 0.155 ± 0.018‰ (2 SD, N = 14), and -0.066 ± 0.022‰ (2 SD, N = 20), respectively, consistent with the recommended values within quoted errors. Based on repeated analyses of the standards, the long-term precision of our double spike method is better than 0.02‰ for δ56Fe on average, proving its ability to distinguish small isotope fractionation among high-temperature samples.