In situ Ti isotopic measurements by Laser Ablation MC-ICP-MS

Abstract

We have been developing Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) technique to measure titanium isotopic composition in situ. A principal aim of this work is to search for isotopic heterogeneities larger than the few epsilons (ε, in parts in 10 4) of the solar system. Our analytical precision of the ratios of46Ti,48Ti, and50Ti to49Ti after exponential-law mass discrimination correction normalizing47Ti/ 49Ti to 1.33375 were about 2.5 ε (2σ). Mixture solutions were prepared by adding the expected level of Ca, Cr, Mg, and Al to the Ti solutions to demonstrate that our interference correction is effective. We then applied our technique with 213 nm Nd-YAG laser ablation to five Ti-rich terrestrial solids, and all of them also showed titanium isotopic composition that was consistent with one another and agreed with that for the solution standard. It appears that the in situ laser technique did not significantly increase the long-term reproducibility beyond the 2.5 ε established using the solution method. This is an order of magnitude better than the typical precision of a few permil for secondary ion mass spectrometry (SIMS). The combination of the ability to perform in situ analysis on 30 μm spots with ε level precision is a niche for LA-MC-ICP-MS. We also ablated two lines on a fassaite grain from a large well studied CAI Egg-6 of the Allende meteorite. After the mass discrimination was corrected by normalizing47Ti/ 49Ti, the46Ti and48Ti are normal within about 2 ε while50Ti/49Ti shows a 9 ε excess. These data are in excellent agreement with thermal ionization mass spectrometry (TIMS) results. Comparing our ICP-MS results against the results from TIMS studies, we found that our normal titanium isotopic ratios were closest to the less precise data of Heydegger et al. (1979) who measured Ti+. We support the proposal to IUPAC to change the accepted Ti abundance to that measured by ICP-MS and TIMS without using Ti oxides.