57-7 Zircon Crystallization Patterns from U-Th Dates Inform U-Pb Estimates of Volcanic Eruption Ages

Session: Advances and Applications in Geochronology for Interpreting Stratigraphic and Basin Records, Part II

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Abstract:

Precise U-Pb dates of the mineral zircon are often used to estimate the depositional age of volcanic ashes and tuffs preserved in the stratigraphic record. Zircon U-Pb dates can be measured to high (better than 0.1%) precision and usefully span ages from < 1 Ma to > 4 Ga. However, a zircon U-Pb date reflects the time of zircon crystallization, which happens before the zircon crystal is erupted and deposited. Complicating matters, the history of zircon crystallization can be protracted, reflecting magmatic processes that may span hundreds of thousands of years or more. Strategies to relate zircon crystallization dates to depositional ages range from assuming zircon crystallizes close in time to the eruption, so that the U-Pb date can be used directly as an eruption/deposition age, to subtracting a sizeable correction factor from the U-Pb date (e.g., 90 ± 77 ka; Renne et al., 2010).

Whereas ID-TIMS zircon U-Pb dates published in decades past often relied on weighted means of many analyses that agreed within larger uncertainties, modern datasets frequently resolve extended zircon crystallization timescales. Zircon U-Th disequilibrium measurements by SIMS date < ~350 ka zircon crystallization at high spatial resolution. The U-Th date uncertainties are often just a few kyr, orders of magnitude better than U-Pb date uncertainties, offering the most highly resolved zircon crystallization records.

We have compiled a database of 4664 U-Th zircon crystallization dates from 133 samples with independently determined eruption ages from other chronometers (e.g., radiocarbon). For each sample, we quantify the shape of the zircon distribution using a maximum likelihood statistical approach. This approach peels back the expected scatter from measurement uncertainties, simultaneously estimating the cessation of zircon crystallization (CZC) and its uncertainty. We then compare the CZC with the independent eruption age to quantify the “eruption gap,” if it exists, between the two.

The resulting distribution of our 133 eruption gaps approximates an exponential distribution with a mean ≪ 1 kyr. The required correction for U-Pb crystallization dates is minimal if an estimate of the CZC is used instead of a high-MSWD weighted mean. The shape of the zircon U-Th age distributions systematically depends on the tectonic setting. We note that our conclusions may be subject to sampling bias and solicit suggestions for additional U-Th datasets with independent eruption ages.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9857

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