Ultra-precise stable Fe isotope measurements by high resolution multiple-collector inductively coupled plasma mass spectrometry with a 57Fe–58Fe double spike

We have developed a method for high precision measurement of stable Fe isotopes by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), utilizing a 57Fe–58Fe double spike to correct for instrumental mass bias. Polyatomic interferences of 40Ar14N+, 40Ar16O+ and 40Ar16O1H+ on Fe isotopes are minimized by the use of a desolvating nebulizer and pseudo-high resolution mass spectrometry with typical mass resolutions of 3300, 3100 and 2700 on 54Fe+, 56Fe+ and 57Fe+, respectively. Sample-standard bracketing with double spiked IRMM-014 Fe isotope standards accounts for residual tails from these interferences beneath the Fe ion peaks.A range of analytical tests demonstrates that it is possible to attain internal precisions (2 se) of ± 0.02 to 0.04‰ on a single 15 min analysis. Long-term reproducibility (2 sd) of ± 0.02‰ was obtained by replicate (n = 51) analyses of IRMM-14 over a 6-month period. Replicate digestions and analyses of the basalt standard BHVO-2 (δ56Fe = + 0.128 ± 0.019‰) demonstrate that this external reproducibility is also obtainable on natural samples.Utilizing this new technique, we present high precision stable Fe isotope data for a range of international reference materials (BHVO-2, BCR-2, JB2, BIR-1, AGV-2, JA1, GSP-1, JG3a, JR1, RGM-1 and JF2). Data for this range of igneous standards (basalt, andesite, granodiorite, rhyolite) are in good agreement with previously published values obtained by high precision sample-standard bracketing techniques, and highlight that BIR-1 is the best calibrated standard available with a reference δ56Fe value of + 0.052 ± 0.009‰ (95% c.i.). Other reference values for BHVO-2, BCR-2, JB2, BIR-1, AGV-2 are also presented. Stable Fe isotope data for the JF2 alkali feldspar from a pegmatite are markedly heavy (δ56Fe = + 0.816 ± 0.025‰), perhaps due to the incorporation of isotopically heavy magmatic Fe3+ into the feldspar mineral structure during crystallization.

► Precision of ± 0.02‰ (2 sd) is attainable for Fe stable isotopes.
► Ultra-precise Fe stable isotopes are provided for 12 international reference materials.
► JF2 K-feldspar standard displays markedly heavy Fe signature (δ56Fe = + 0.816 ± 0.025).