High-precision analysis of chromium isotopes in terrestrial and meteorite samples by thermal ionization mass spectrometry

Abstract

We present a method for the chemical separation of Cr from meteorite and terrestrial samples for isotopic analysis by thermal ionization mass spectrometry (TIMS). After sample digestion, separation of Cr(III) is achieved by means of a two-column cation-exchange chromatography procedure using AG 50W-x8 resin. In a first column, Cr(III) is isolated from major elements and the majority of trace elements. In a second column, trace amounts of Fe, Al and Ti are further removed. Total procedural yields are > 80%. Cr isotopes are measured by TIMS in the static multicollection mode. Mn/Cr ratios are obtained by multi-collector inductively coupled plasma source mass spectrometry (MC-ICPMS). The accuracy of our protocol was tested by reference to terrestrial analogs and comparison of Cr isotopic data for samples that underwent Cr purification following the cation-exchange chromatography described here and an alternative separation method employing both a cationic and an anionic chromatography step. Using our technique, Mn/Cr ratios reproduce to <2% (2s) and 53 Cr/ 52 Cr and 54 Cr/ 52 Cr to 6 ppm and 12 ppm, respectively (2s). This highly precise procedure allows the variability of Cr isotopes in the inner solar system objects to be addressed. Our method enabled us to document an initial homogeneity for 50,52,53 Cr isotopes within 10 ppm, while 20-70 ppm deficits in 54 Cr abundances have been resolved for a number of meteorite samples.

The Chromium Isotopic Composition of the Ungrouped Carbonaceous Chondrite Tagish Lake

“…At the level of precision required for this study, the matrix effects resulting from the major elements of the various samples have negligible effects and the precision on the Mn/Cr ratio is better than 2%. Further details can be found in Trinquier et al (2008a).…”

“…The detailed description of the TIMS measurements, data acquisition, and data correction is given in Trinquier et al (2008a Notes. Cr isotopic compositions were measured by TIMS (TRITON, Finnigan, Université Paris VI), whereas Mn and Cr abundances were measured on MC-ICP-MS (Neptune, Finnigan, Université Paris VI) in high-resolution mode to remove isobaric interferences on Cr species as described in Trinquier et al (2008a). The 53 Cr and 54 Cr isotopic compositions are expressed in the ε-notation which is the relative deviation per 10,000 from the terrestrial standard.…”

A systematic look at chromium isotopes in modern shells – implications for paleo-environmental reconstructions

“…This solution was added to the extraction columns loaded with 2 mL of pre-cleaned Dowex AG50W-X8 cation resin (200-400 mesh). The extraction procedure principally adhered to that published by Bonnand et al (2011) and Trinquier et al (2008) with only small modifications. The final Cr-bearing liquid cut was dried down at 130 • C, ready to be loaded for Cr isotopic analysis on the thermal ionization mass spectrometer.…”

Chromium isotope fractionation during subduction-related metamorphism, black shale weathering, and hydrothermal alteration

“…Sample 8A-2 is rich in talc and chlorite and is 285 likely a physico-chemical admixture of ultramafic mélange matrix and metasomatised metamafic 286 mélange block material (e.g., Marschall and Schumacher, 2012 separated from Cr in 6 N HCl by passing it through a micro column filled with 0.3 mL AG1-X8 (100-306 200 mesh) anion exchange resin; sample Cr was collected immediately after loading onto the 307 column. Residual Ti and V were cleaned with a micro column filled with 0.3 mL AG 50W-X8 (200-308 14 400 mesh) cation resin following previous methods (Trinquier et al, 2008 samples dissolved in 0.7 N HNO3 with concentrations of ~250 µg/g were introduced to the plasma 322 with a PFA μFlow nebulizer (~50 μL/min) coupled with an Apex IR desolvating introduction 323 system (Elemental Scientific) without additional gas or membrane desolvation. With a standard 324 sample cone and X skimmer cone and under high-resolution mode, the obtained sensitivity was 325 ∼3×10 -10 A 52 Cr on 1 µg/g Cr solution.…”