Further Characterization of Four Natural Ilmenite Reference Materials for in situ Fe Isotopic Analysis

Iron (Fe) isotopic composition is a useful geochemical tracer. Because of the high spatial resolution and simple sample preparation, laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) is becoming increasingly popular for Fe isotopic analysis. However, the matrix effect is a challenge for the LA-MC-ICP-MS technique, and external standardization using a matrix-matched reference material is essential. Ilmenite is a common and significant Fe-rich mineral phase in terrestrial and lunar rock, which has variable Cr contents. Herein, four natural ilmenites (RUS10, CZE2, SIN1, and GER16) with Cr contents of 15-2420 mu g g(-1) were characterized for Fe isotopic compositions by both solution nebulization (SN) MC-ICP-MS and LA-MC-ICP-MS. The protocol of isobaric interference correction (Cr-54 on Fe-54) was optimized for in situ Fe isotopic analysis. The precision and accuracy of ns-LA-MCICP-MS measurements are comparable to those of fs-LA-MC-ICP-MS for Fe isotopic analysis. The individual spot internal precision (2SE) of delta Fe-56 was consistently better than 0.08% at Fe-56 signal intensity >= 10 V, and the analytical external precisions (2sd) of delta Fe-56 were in a range from +/- 0.10% to +/- 0.16% in multiple ilmenite fragments by both ns-LA-MC-ICP-MS and fsLA-MC-ICP-MS. The delta Fe-56(IRMM- 014) values of four ilmenites (RUS10, CZE2, SIN1, and GER16) were 0.22 +/- 0.14% (2sd, n=165), 0.14 +/- 0.14% (n=123), 0.00 +/- 0.14% (n=110) and 0.00 +/- 0.14% (n=164), respectively. The four ilmenites, found to be homogeneous in Fe isotopic compositions, are proposed as candidate reference materials to be shared with peers. These ilmenites with different Cr contents and positive Fe isotopic values would certainly be helpful as reference materials for the scientific community.