Université du Québec à Montréal (UQAM) / Geotop: U-Pb geochronologist
davies.joshua@uqam.ca
Accessory Minerals
Baddeleyite
Baddeleyite (ZrO2) is a trace phase, found most commonly in mafic rocks. It can contain high amounts of uranium with almost no initial Pb, making it a highly suitable mineral for U-Pb geochronology.
Baddeleyite is often used to date large igneous provinces (LIPs) which can be linked to major global climate change events, oceanic anoxic events and mass extinctions. However, baddeleyite is known to suffer from lead loss resulting in inaccurate ages. The mechanisms causing the lead loss are not known. One avenue of research in my group is trying to understand how and why baddeleyite loses lead.
Petrological observations combined with high precision U-Pb geochronology can be useful in understanding how the U-Pb system behaves in baddeleyite. The SEM image show a baddeleyite crystal with a zircon overgrowth indicating that the zircon grew after the baddeleyite. However, when these minerals are dated, the zircon gives an older age suggesting that the baddeleyite has lost lead.
Click on picture for paper
Baddeleyite oxygen isotope analysis is a new tool developed by my group which may be useful for understanding how lead is mobilised in baddeleyite.
We showed that baddeleyite can record highly variable oxygen isotopic compositions which are not in equilibrium with their magmatic values. We interpreted this as possibly relating to hydrothermal alteration which may be associated with Pb loss.
Click on picture for paper
Baddeleyite crystals of different size were dated from the Freetown layered igneous complex in Sierra Leone. This intrusion is part of the Central Atlantic large igneous province (CAMP, see LIP research section). The igneous complex contains baddeleyite crystals of various sizes.
High precision U-Pb geochronology of these grains indicates that they are all younger than the rest of the CAMP province, and also younger than Ar-Ar plagioclase ages and U-Pb apatite ages from the same samples. The smaller baddeleyite grains also give younger ages than the larger grains. this suggests that volume diffusion may be a mechanism for lead loss in baddeleyite.
