Multimethod U–Pb baddeleyite dating: insights from the Spread Eagle Intrusive Complex and Cape St. Mary's sills, Newfoundland, Canada

Abstract

<jats:p>Abstract. Baddeleyite (ZrO2) is widely used in U–Pb geochronology but analysis and age interpretation are often difficult, especially for samples which have experienced post-intrusive alteration and/or metamorphism. Here, we combine high spatial resolution (secondary ionization mass spectrometry, SIMS) and high-precision (isotope dilution thermal ionization mass spectrometry, ID-TIMS) analyses of baddeleyite from the Spread Eagle Intrusive Complex (SEIC) and Cape St. Mary's sills (CSMS) from Newfoundland. Literature data and our own detailed microtextural analysis suggest that at least seven different types of baddeleyite–zircon intergrowths can be distinguished in nature. These include secondary baddeleyite inclusions in altered zircon, previously unreported from low-grade rocks, and likely the first discovery of xenocrystic zircon inclusions mantled by baddeleyite. 207Pb∕206Pb baddeleyite dates from SIMS and ID-TIMS mostly overlap within uncertainties. However, some SIMS sessions of grain mounts show reverse discordance, suggesting that bias in the U ∕ Pb relative sensitivity calibration affected 206Pb∕238U dates, possibly due to crystal orientation effects, and/or alteration of baddeleyite crystals, which is indicated by unusually high common-Pb contents. ID-TIMS data for SEIC and CSMS single baddeleyite crystals reveal normal discordance as linear arrays with decreasing 206Pb∕238U dates, indicating that their discordance is dominated by recent Pb loss due to fast pathway diffusion or volume diffusion. Hence, 207Pb∕206Pb dates are more reliable than 206Pb∕238U dates even for Phanerozoic baddeleyite. Negative lower intercepts of baddeleyite discordia trends for ID-TIMS dates for SEIC and CSMS and direct correlations between ID-TIMS 207Pb∕206Pb dates and the degree of discordance may indicate preferential 206Pb loss, possibly due to 222Rn mobilization. In such cases, the most reliable crystallization ages are concordia upper intercept dates or weighted means of the least discordant 207Pb∕206Pb dates. We regard the best estimates of the intrusion ages to be 498.7±4.5 Ma (2σ; ID-TIMS upper intercept date for one SEIC dike) and 439.4±0.8 Ma (ID-TIMS weighted mean 207Pb∕206Pb date for one sill of CSMS). This first radiometric age for the SEIC is consistent with stratigraphic constraints and indicates a magmatic episode prior to opening of the Rheic Ocean. Sample SL18 of the Freetown Layered Complex (FLC), Sierra Leone, was investigated as an additional reference. For SL18, we report a revised 201.07±0.64 Ma intrusion age, based on a weighted mean 207Pb∕206Pb date of previous and new baddeleyite ID-TIMS data, agreeing well with corresponding SIMS data. Increasing discordance with decreasing crystal size in SL18 indicates that Pb loss affected baddeleyite rims more strongly than cores. Our SL18 results validate that the SIMS in situ approach, previously used for Precambrian and Paleozoic samples, is also suitable for Mesozoic baddeleyite. </jats:p>

Metamorphic titanite–zircon pseudomorphs after igneous zirconolite

“…Commonly, remnants of baddeleyite crystals are surrounded by polycrystalline zircon coronas. Metamorphic zircon-forming reactions from other Zr-bearing accessory minerals, specifically zirconolite, are less commonly discussed but have been reported by Pan (1997) and Pohlner et al (2020). Under subgreenschist conditions, the breakdown of zirconolite may involve changes by dissolution, alteration, and replacement (Gieré et al, 1998).…”

“…While zircon growth after baddeleyite is a comparably well-known reaction (Davidson and van Breemen, 1988), there are to this date only a few reports (Pan, 1997;Pohlner et al, 2020) of metamorphic mineral growth after zirconolite, CaZrTi 2 O 7 . This may be due to zirconolite and pseudomorphs after zirconolite being either extremely rare or, possibly, overlooked.…”

Geochronology of mesozoic large Igeous Provinces in NE Brazil

“…However, baddeleyite is not as reliable as the zircon chronometer because of secondary Pb-loss problems (Rioux et al, 2010). This limitation hinders its use for highly accurate and precise (≤0.1% 2 error) geochronology (Davis and Davis, 2010;Li et al, 2010;Schaltegger and Davies, 2017;Pohlner et al, 2020). With these recognized limitations of U-Pb baddeleyite geochronology, high-precision/high-accuracy U-Pb dating thus often relies on finding zircon crystals from segregated pods or evolved melts of mafic rocks.…”

Petrogenesis of Mafic-Ultramafic Intrusions in the Southern Goiás Magmatic Arc, Central Brazil

“…However, these analyses and age interpretations are often difficult, particularly if the samples have experienced postmagmatic alteration and metamorphism as in the Mangabal Complex and other maficultramafic intrusions in the area. In addition, baddeleyite geochronology by in-situ SIMS can be challenging due to analytical limitations for in-situ methods such as beam overlap on adjacent phases and possible bias in the U ∕ Pb relative sensitivity calibration factor (RSF) (e.g., Pohlner et al, 2020;Schmitt et al, 2010).…”