High-precision 27Al/24Mg ratio determination using a modified isotope-dilution approach

The precision of the Al-26-Mg-26 system-one of the most widely used chronometers for constraining the relative timing of events in the early solar system-is presently limited by methods for the determination of Al-27/Mg-24 ratios, which have seen little improvement in the last decade. We present a novel method for the measurement of Al-27/Mg-24 ratios in unpurified sample solutions by multiple-collector inductively coupled plasma mass spectrometry. Because Al is monoisotopic we use a modified isotope dilution approach that employs a mixed spike containing isotopically enriched Mg-25 and natural Al-27 in an accurately known ratio. In order to determine the spike to sample ratio for Al, measurements of spiked aliquots are bracketed by unspiked aliquots, which negates the impact of elemental bias. Unlike conventional isotope dilution, samples do not require chromatographic separation prior to analysis, which both saves time and minimises the risk of contamination of other samples with spike (which is added immediately prior to analysis). Repeat measurements of the BHVO-2, BCR-2, and BIR-1 international rock standards, as well as a gravimetrically prepared Al-Mg reference solution, indicate that our method is both accurate and reproducible to 0.2%. This 4- to 10-fold improvement over previous methods translates directly to an equal gain in the resolution of the Al-26-Mg-26 chronometer. The approach presented here could, in principle, be applied to other monoisotopic elements such as the Mn-Cr system. Based on multiple measurements of a similar to 2.7 gram piece of the Ivuna CI chondrite, we present a new estimate for the Al-27/Mg-24 ratio of this meteorite of 0.09781 +/- 0.00029.