Small-volume baddeleyite (ZrO2) U–Pb geochronology and Lu–Hf isotope geochemistry by LA-ICP-MS. Techniques and applications

Abstract

U–Pb geochronology of baddeleyite (ZrO2) is an increasingly used tool in the Earth and planetary sciences for determining the crystallization and emplacement ages of mafic igneous rocks. Additionally, baddeleyite has a strong affinity for hafnium and preferentially excludes the REE's, making it an important repository of Hf isotopic compositions, which can be used to provide constraints on the origin of these rocks. In this contribution we introduce a technique for U–Pb dating and Lu–Hf isotopic analysis of baddeleyite by LA-MC-ICP-MS. A systematic study of crystals with known ages demonstrates that our methodology is capable of producing 207Pb/206Pb results that are precise and accurate to within 1% of their TIMS values at a 2-sigma confidence level, while Phanerozoic crystals can generally be dated to within 1.5 to 3.0% accuracy using their 206Pb/238U compositions. These results are routinely reproducible with a variety of laser-spot sizes ranging from 30 to 10μm in diameter and with crater depths as shallow as ~3μm in depth. This represents a significant improvement in the sampled volume generally used for LA-ICP-MS geochronology and is a critical step for dating small baddeleyite crystals either as mineral separates or identified in situ within thin sections. No orientation-dependent biases on the measured 206Pb/238U values were identified from our data, suggesting that LA-ICP-MS dating of Phanerozoic crystals can be routinely performed without the biases previously reported for SIMS instruments. Our data show that initial 176Hf/177Hf ratios can be determined with an accuracy as good as 0.5 εHf units by taking the mean of 10 to 30 individual spot analyses acquired with a 40μm beam-diameter. These U–Pb and Hf results are comparable to what can be routinely achieved for zircon by LA-MC-ICP-MS. In addition to the methodological approach, we report a high-precision U–Pb TIMS age and four solution-MC-ICP-MS Hf isotopic results for new and/or important baddeleyite localities. These new data are used here to assess the adequacy of our fractionation, interference and mass-bias corrections and can be used as a basis for addressing the accuracy of LA-ICP-MS U–Pb and Lu–Hf data for future inter-laboratory calibration efforts.