Multimethod U–Pb baddeleyite dating: insights from the Spread Eagle Intrusive Complex and Cape St. Mary's sills, Newfoundland, Canada
Authors: Pohlner Johannes E. , Schmitt Axel K. , Chamberlain Kevin R. , Davies Joshua H. F. L. , Hildenbrand Anne , Austermann Gregor 
Published: July 2020

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.