Challenges and insights from zircon and baddeleyite U-Pb ages and Hf isotope analyses of Karoo and Siberian LIP intrusive rocks

Abstract

The emplacement of Large Igneous Provinces (LIPs) has been interpreted as the cause of many environmental and biotic crises during Earth's history, largely due to their potential for introducing massive volumes of volcanogenic and thermogenic gases into the atmosphere. To assess this potential link, it is crucial to evaluate the potential coincidence of environmental perturbation, fluctuations in biodiversity, and extinction periods with LIP emplacement, requiring high-precision geochronology of LIP rocks. Volumetrically, most magmas and lavas associated LIPs are mafic, which presents as a significant challenge for high-precision geochronology, as mafic magmas rarely saturate U-bearing minerals, such as zircon or baddeleyite. In such a case, 40Ar/39Ar dating of plagioclase from these rocks is carried out, which has geological and analytical complications that may lead to inaccurate ages, and subsequently to flawed geologic models (Antoine et al., 2022). Recent work has shown that high-silica melt pockets in LIP intrusions, contaminated by wall rock at the emplacement level, can crystallize mineral phases useful for high-precision U-Pb dating. The process of sedimentary wall rock contamination directly affects the composition of these minerals, as anatexis of this material significantly shifts the Hf isotope composition of zircon (and baddeleyite) away from mantle composition. These interactions between LIP intrusions and their wall rock can cause significant complications within the age spectra of these mineral phases. Baddeleyite, the first U-rich phase to saturate during LIP magma crystallization, commonly yields anomalously young and scattering dates from partial loss of radiogenic Pb. Zircon is a rare mineral in these rocks. Its scatter in U-Pb dates is a combination of residual Pb loss after chemical abrasion and traces of xenocrystic components in the zircon grain, derived from inherited zircon during the contamination process. Zircon U-Pb ages are particularly significant, as zircon crystallization not only records igneous solidification, but also wall rock interaction and thermogenic gas generation, allowing for high-precision geochronology of potential climate altering events. We will present new data sets of high-precision zircon and baddeleyite U-Pb and Hf isotope data, from sills of the Karoo and the Siberian Trap LIPs and highlight the potential implications for thorough understanding of the intrusive emplacement mechanisms. The Hf isotope compositions from both LIPs indicate localized contamination of the magma, indicating entrainment, anatexis and assimilation of wall rock during emplacement and sill inflation. Interpreted U-Pb ages of samples throughout the intrusive structure of the Siberian Trap LIP and the Karoo LIP indicate that not only was magma emplacement protracted over a few 100 ky within their subvolcanic domains, but that there was a similar temporal-structural progression of downward migrating emplacement through intrusive assembly. Finally, intrusive emplacement and associated wall rock thermogenic reactions for both LIPs are coeval with well documented periods of global climate change and carbon cycle perturbation in the lower Toarcian and lower Triassic, respectively. Cited reference: Antoine et al. (2022) Chem. Geol. 610, 12108