Baddeleyite microtextures and U-Pb discordance: 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 different patterns of discordance often hamper accurate age interpretations. This is also the case for baddeleyite from the Spread Eagle Intrusive Complex (SEIC) and Cape St. Mary’s sills (CSMS) from Newfoundland, which we investigated combining high precision and high spatial resolution methods. Literature data and our own observations suggest that at least seven different types of baddeleyite–zircon intergrowths can be distinguished in nature, among which we describe xenocrystic zircon inclusions in baddeleyite for the first time. Baddeleyite 207Pb/206Pb dates from secondary ionization mass spectrometry (SIMS) and isotope dilution thermal ionization mass spectrometry (ID-TIMS) are in good agreement with each other and with stratigraphic data, but some SIMS sessions of grain mounts show reverse discordance. This suggests that matrix differences between references and unknowns biased the U-Pb relative sensitivity calibration, possibly due to crystal orientation effects, or due to alteration of the 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 or volume diffusion. Hence, 207Pb/206Pb dates are more reliable than 206Pb/238U dates even for Phanerozoic baddeleyite. Negative lower intercepts of baddeleyite discordias and direct correlations between ID-TIMS 207Pb/206Pb dates and degree of discordance 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). Sample SL18 of the Freetown Layered Complex, Sierra Leone (associated with the Central Atlantic Magmatic Province) 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. Employment of SIMS or mechanical abrasion prior to ID-TIMS analysis may therefore produce more concordant baddeleyite data. We emphasize that the combination of high precision and high spatial resolution dating, along with detailed microscale imaging of baddeleyite, is powerful for extracting reliable age information from baddeleyite from rocks with a complex post-magmatic evolution. </jats:p>

Discovery of the Naturally Occurring Pure CaTiO3 Cubic Perovskite from Tazheran Massif

“…In situ analyses need calibration materials (i.e., standards), which must mimic the behavior (chemical compositions and crystal structures) in the unknowns. Previous studies on zircons and baddeleyites indicate that the chemical composition and crystal structure-related matrix effect may lead to U-Pb age offset by more than 4% [31][32][33][34] . Hence, perovskite U-Pb data analyzed by those in situ methods may be only valid if the sample and the used standards have identical crystal structures, either orthorhombic or perovskite-C. To date, perovskite standards used in most of the laboratories are orthorhombic; for example, orthorhombic Tazh perovskite is an international perovskite U-Pb dating standard, which is widely used in SIMS and LA-ICPMS labs worldly.…”

Jizerka Gemstone Placer—Possible Links to the Timing of Cenozoic Alkali Basalt Volcanism in Jizera Mountains, Czech Republic

“…The age of the zircon was found to be 31.2 ± 0.4 Ma (Figure 9), while a highly imprecise age of 31 ± 16 Ma was estimated for the baddeleyite rim (Figure 9). The high uncertainty of the baddeleyite U-Pb age is attributed to the low content of radiogenic Pb, low volume of the material available for dating and an excess of common lead due to intergrowth with other phases in the symplectitic mixture [42].…”

“…Minerals 2023, 13, 771 11 of 20 content of radiogenic Pb, low volume of the material available for dating and an excess of common lead due to intergrowth with other phases in the symplectitic mixture [42]. content of radiogenic Pb, low volume of the material available for dating and an excess of common lead due to intergrowth with other phases in the symplectitic mixture [42].…”

“…According to many authors, silicate inclusions in igneous minerals may have compositions corresponding to the chemistry of their parent rocks [42]. The Jizerka sapphire and its niobian rutile inclusion also contain scarce primary melt inclusions in the form of small droplets of salic glass that could have been trapped in these minerals during their growth in magma (Figure 5).…”

Authors’ response to reviewer comments

“…It may be that 206Pb-biased loss in baddeleyite is rare, or that it is ubiquitous, but rarely evident outside of bulk (unbiased) Pb loss and analytical error. Recent high-precision U-Pb data for baddeleyite and co-existing zircon from the Duluth gabbro (Hoaglund, 2010; Ibañez-Mejia and Tissot, 2019) require either 206Pb-biased loss in baddeleyite, or excess 207Pb due to the incorporation and decay of 231Pa, and in several early studies 206Pb loss was proposed to explain differences between dates of baddeleyite and co-existing minerals (Davis and Sutcliffe, 1985;Heaman and LeCheminant, 2000). Excess 207Pb cannot explain our data from FP6D.…”