Chromatographic Separation and Multicollection-ICPMS Analysis of Iron. Investigating Mass-Dependent and -Independent Isotope Effects

Abstract

A procedure was developed that allows precise determination of Fe isotopic composition. Purification of Fe was achieved by ion chromatography on AG1-X8 strongly basic anion-exchange resin. No isotopic fractionation is associated with column chemistry within 0.02 per thousand /amu at 2sigma. The isotopic composition was measured with a Micromass IsoProbe multicollection inductively coupled plasma hexapole mass spectrometer. The Fe isotopic composition of the Orgueil CI1 carbonaceous chondrite, which best approximates the solar composition, is indistinguishable from that of IRMM-014 (-0.005 +/- 0.017 per thousand /amu). The IRMM-014 reference material is therefore used for normalization of the isotopic ratios. The protocol for analyzing mass-dependent variations is validated by measuring geostandards (IF-G, DTS-2, BCR-2, AGV-2) and heavily fractionated Fe left after vacuum evaporation of molten wüstite (FeO) and solar (MgO-Al(2)O(3)-SiO(2)-CaO-FeO in chondritic proportions) compositions. It is shown that the isotopic composition of Fe during evaporation of FeO follows a Rayleigh distillation with a fractionation factor alpha equal to (m(1)/m(2)()1/2), where m(1) and m(2) are the masses of the considered isotopes. This agrees with earlier measurements and theoretical expectations. The isotopic composition of Fe left after vacuum evaporation of solar composition also follows a Rayleigh distillation but with a fractionation factor (1.013 22 +/- 0.000 67 for the (56)Fe/(54)Fe ratio) that is lower than the square root of the masses (1.018 35). The protocol for analyzing mass-independent variations is validated by measuring terrestrial rocks that are not expected to show departure from mass-dependent fractionation. After internal normalization of the (57)Fe/(54)Fe ratio, the isotopic composition of Fe can be measured accurately with precisions of 0.2epsilon and 0.5epsilon at 2sigma for (56)Fe/(54)Fe and (58)Fe/(54)Fe ratios, respectively (epsilon refers to relative variations in parts per 10 000). For (58)Fe, this precision is an order of magnitude better than what had been achieved before. The method is applied to rocks that could potentially exhibit mass-independent effects, meteorites and Archaean terrestrial samples. The isotopic composition of a 3.8-Ga-old banded iron formation from Isua (IF-G, Greenland), and quartz-pyroxene rocks from Akilia and Innersuartuut (GR91-26 and SM/GR/171770, Greenland) are normal within uncertainties. Similarly, the Orgueil (CI1), Allende (CV3.2), Eagle Station (ESPAL), Brenham (MGPAL), and Old Woman (IIAB) meteorites do not show any mass-independent effect.

Iron isotope fractionation in planetary crusts

“…Between 5 and 10 mg of crushed material was fully digested by sequential mixtures of HF-HNO 3 and HNO 3 -HCl. Iron in the dissolved samples was then purified using anionexchange chromatography (see Table 2 for the detailed procedure; Strelow, 1980;Dauphas et al, 2004Dauphas et al, , 2009b. The purification was repeated twice, thus ensuring the removal of all the matrix elements.…”

Iron isotope analysis of red and black pigments on pottery in Nasca, Peru

“…Fe separations were done by anion-exchange chromatography using Biorad AG® 1-X8. Elements such as Cr and Ni wash out, while Fe(III) adheres to the anion exchange resin in strong normality HCl (e.g., Dauphas et al 2004). We use 8 N HCl to remove "matrix" followed by elution of Fe with 0.5 N HCl, H 2 O and 8 N HNO 3 to ensure full Fe recovery (Lesher et al 2009).…”

A procedural development for the analysis of 56/54Fe and 57/54Fe isotope ratios with new generation IsoProbe MC-ICP-MS

“…Recent studies have now indicated that Fe isotope fractionation can also occur abiotically in low temperature aqueous systems [12][13][14] and at high temperature. [16][17][18] The principal difficulty in measuring precisely and accurately Fe isotope ratios is the [27][28][29][30] and iv) high resolution multi-collection. [31][32][33] In this contribution, we detail a procedure developed for the analysis of Fe isotope ratios on a new generation GV Instruments (formerly Micromass) IsoProbe-P MC-ICP-MS at the Scottish Universities Environmental Research Centre (East Kilbride, UK).…”

“…The soft extraction mode has been available only on new generations of the IsoProbe, and has seen increasing interest since it considerably reduces molecular interferences and memory effects. 29 We investigated the stability of the measurement during 100 integrations of the analysis in both hard and soft extraction (Fig. 4).…”

“…In the literature, reported rf amplitudes vary from 40 % to 60 %. 29 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 10 329 330 331 332 333 334 335 336 337 338 …”