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 Fe-57-Fe-58 double spike to correct for instrumental mass bias. Polyatomic interferences of (ArN+)-Ar-40-N-14, (ArO+)-Ar-40-O-16 and (ArOH+)-Ar-40-O-16-H-1 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 Fe-54(+), Fe-56(+) and Fe-57(+), 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 parts per thousand on a single 15 min analysis. Long-term reproducibility (2 sd) of +/- 0.02 parts per thousand 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 (delta Fe-56 = + 0.128 +/- 0.019 parts per thousand) 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 delta Fe-56 value of + 0.052 +/- 0.009 parts per thousand (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 (delta Fe-56 = + 0.816 +/- 0.025 parts per thousand), perhaps due to the incorporation of isotopically heavy magmatic Fe3+ into the feldspar mineral structure during crystallization. (C) 2012 Elsevier B.V. All rights reserved.