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 (e, in parts in 10^4) of the solar system. Our analytical precision of the ratios of 46Ti, 48Ti, and 50Ti to 49Ti after exponential-law mass discrimination correction normalizing 47Ti/49Ti to 1.33375 were about 2.5 e(2δ). 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 e 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 30μm spots with e 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 normalizing 47Ti/49Ti, the 46Ti and 48Ti are normal within about 2 e while 50Ti/49Ti shows a 9 e 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(superscript +). We support the proposal to IUPAC to change the accepted Ti abundance to that measured by ICP-MS and TIMS without using Ti oxides.
Highlights
During the past several decades, titanium isotopic compositions of meteorites have been studied to constrain their nucleosynthetic origins and mixing in the early solar system
Pioneering studies started from the measurements of isotopic composition of bulk chondrites, Ca-Al rich inclusions (CAIs), and chondrules by conventional thermal ionization mass spectrometry (TIMS) [Heydegger et al 1979; Niederer et al
We show that our current set up is capable of measuring in situ titanium isotopic composition to the precision of 2 - 3 e that secondary ionization mass spectrometric techniques (SIMS) is not currently able to achieve
Summary
During the past several decades, titanium isotopic compositions of meteorites have been studied to constrain their nucleosynthetic origins and mixing in the early solar system. The five titanium stable isotopes are thought to have come from several distinct stellar processes; and the refractory material makes Ti one of the earliest elements to condense in the solar system, or perhaps the last residue of pre-solar material to evaporate. Limited by analytical precision, they could only distinguish the larger variation in 50Ti mostly in what was called “Platy hibonites” phases (50Ti anomalies from -68 to 273 permil). Ti anomalies seem to be another vivid example of the cosmochemists’ rule of thumb: “The smaller the sample size, the larger the isotopic heterogeneity” (for an incompletely mixed solar system, Lee 1988)!
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