High precision determination of chromium isotope ratios in geological samples by MC-ICP-MS

The present work describes methodology for the precise and accurate determination of chromium isotope ratios (Cr-50/Cr-52, Cr-53/Cr-52, and Cr-54/Cr-52) by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Mass discrimination and instrument drift are corrected for using a "standard-sample-standard'' bracketing technique and normalization against NIST SRM 979 Cr(NO3)(3)center dot 9H(2)O. The precisions (2s) of ratio measurements for Cr-50/Cr-52, Cr-53/Cr-52, and Cr-54/Cr-52 are +/- 0.11 parts per thousand, +/- 0.06 parts per thousand, and +/- 0.10 parts per thousand (110 ppm, 60 ppm and 100 ppm), respectively. For geological samples of relatively high concentration (300-17 000 ppm), Cr was separated from the matrix using columns of anion and cation exchange resins, providing a recovery of >95%. Inexplicably, for some geological samples, the aforementioned methodologies were not effective in reducing the concentrations of Fe and Ti to below 2% relative to the Cr concentration in the eluate. As a consequence, only Cr-53/Cr-52 was determined in such samples. Significant differences in isotope ratios were found among standard (NIST SRM 979), chemically pure metal chips of Cr (Alfa Aesar), a Late Proterozoic gabbro, a Cretaceous oil shale and natural products of combustion metamorphism of that oil shale. The igneous rocks and the pure metal have slightly negative delta Cr-53/52 values, the oil shale is +1.01 parts per thousand, and the products of combustion metamorphism are all somewhat lighter, suggesting that the heavier isotopes of chromium were removed in the process of metamorphism.