DOES CHEMICAL ABRASION ALWAYS WORK? INSIGHTS FROM OLD ZIRCON, SPLIT GRAINS, AND LARGE DATASETS (Invited Presentation)

The advent of chemical abrasion (CA) to selectively remove zircon domains that lost Pb has significantly improved reproducibility in high-precision U-Pb dating, and accordingly, has increased our confidence in making robust age interpretations. It is important to know whether all domains were entirely removed, especially when determining maximum depositional ages in samples that have only one or two young detrital grains. The common occurrence of discordant dates from old zircon shows that CA does not fully mitigate Pb loss in all grains with high alpha dose (Da), while the agreement of dates from zircon standards shows it works in pristine zircon with low Da. How do these observations translate to the zircon we typically analyze with a range of Da? We address this question using two datasets from samples that were not metamorphosed. Dataset 1 consists of 71 Phanerozoic grains (mostly detrital zircon) with low Da (generally <0.5 x 10^18/g) that were split into 2 fragments and analyzed separately. Dates from 70 of the 71 split grains agree within uncertainty, which suggests CA worked in nearly every grain. Dataset 2 consists of 245 grains with a wide range in Da from thirty-three 1.7-2.7 Ga rocks. Nearly all grains with low to moderate Da (0-3 x 10^18/g) yield dates that are concordant and equivalent within samples, which shows that CA was effective. However, the percentage of concordant dates decreases significantly at higher Da (3-6 x 10^18/g) and concordance is rare at very high Da. Collation of these two datasets gives confidence that CA successfully mitigated Pb loss in almost all of the ~16,000 grains that we have dated because most have low to moderate Da (0-3 x 10^18/g). For the small subset of grains that yield dates that are clearly too young to be accurate, Da must have been high or Pb loss was not the problem; the most likely culprit is new zircon growth, which cannot be remedied by CA.

Dataset 2 is ideal for assessing the revised 235U decay constant values proposed by Schoene et al. (2006) and Mattinson (2010) because many of the dates used the 202Pb-205Pb EARTHTIME tracer solution. Our value is in perfect agreement (upon adjustment of 238U/235U) with the previous determinations with a notable increase in precision. Mattinson recommended using the revised value, which has now been replicated in three labs using four tracer solutions, so perhaps it is time to start?