THE INSTRUMENTATION DILEMMA: A COMPARISON OF PAIRED LA-ICP-MS AND ID-TIMS U-PB DATES FROM ZIRCON

Zircon U-Pb geochronology is a commonly used dating technique with widespread application to igneous, metamorphic, and sedimentary systems. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and isotope dilution-thermal ionization mass spectrometry (ID-TIMS) are among the most used methods for acquiring U-Pb dates from zircon. Although ID-TIMS provides much higher analytical precision, dates acquired via LA-ICP-MS are much less expensive and may be acquired in sufficient quantity (i.e., >300 analyses per sample) for provenance characterization and identification of rare age modes (e.g., young zircon of near depositional age). However, recent case studies have demonstrated a tendency for the youngest LA-ICP-MS U-Pb dates from detrital zircons to skew younger than the corresponding ID-TIMS date, possibly a consequence of undetected Pb-loss that is mitigated in ID-TIMS via chemical abrasion.

We conducted a systematic characterization of the degree of agreement between dates acquired via LA-ICP-MS and ID-TIMS from the same zircon crystals. Our preliminary dataset consists of 787 date pairs analyzed from eight laboratories and 26 published studies. The dataset has a lithologic makeup of 53% igneous, 40% sedimentary, and 7% metamorphic rocks. Our compilation shows that most grains (~92%) yield a LA-ICP-MS date that is within 20% of its corresponding TIMS date. However, our preliminary results show LA-ICP-MS dates to be systematically shifted ~1.5% younger than their corresponding TIMS dates. When outliers are excluded, ~58% of LA-ICP-MS dates are younger than their corresponding TIMS date. These preliminary results highlight the potential effects of variable degrees of cryptic Pb-loss on LA-ICP-MS measurements, although inheritance may also play a role.