Can Zircons be Suitable Paleomagnetic Recorders? - A Correlative Study of Bishop Tuff Zircon Grains Using High Resolution Lab X-ray Microscopes and a Quantum Diamond Microscope

Abstract

The silicate mineral zircon (ZrSiO 4) preserves a unique record of the Earth's ancient past owing to its resistance to metamorphism and weathering processes. Geochemical studies of zircons routinely provide important constraints on their crystallization environment and consistently provide highly accurate radiometric formation ages due to their high initial U to Pb ratio. Zircons from the Jack Hills of Western Australia, averaging about 200 µm across or less, represent the only known samples of the early Earth in the 4.37-4.0 Ga interval. Recently Tarduno et al. have claimed to measure paleointensities from Hadean zircons. However, a context study of the Jack Hills area has questioned the likelihood that primary paleomagnetic record may be retained in Jack Hills zircons. This has focused the need to understand the suitability of zircons as a host material for paleomagnetic recorders. In this study characterize the location and strength of magnetic signal carriers located in zircons from the Bishop Tuff formation that is analogous to the low temperature granitoid melts that likely formed the Jack Hills zircons. Importantly, since this emplacement is only 767.1 ka, we can characterize a much less deformed and aged set of zircons. These provide an important control for understanding how zircons evolve in time. As such, paleomagnetic records from the deposit are not subject to significant viscous overprinting, post-depositional metamorphism, or heterogeneities due to paleosecular variations. Here we present a correlative workflow going from two-dimensional chemical and magnetic mapping to locating buried magnetic signal carriers using high-resolution x-ray tomography. While EDS and the the quantum diamond magnetometer allow for surface information and the location of faint magnetic signals this 2D approach leaves many questions about the relationship between the magnetic signals (in particular buried ones like in Fig 1C) and host mineralogy. Using our correlative work flow we can begin to determine the relationship between magnetic signal carriers and their host zircon grain as well as other mineral inclusions. We establish that Fe-oxides (magnetic signal carriers) are hosted or form next to apatite inclusions in zircons. Importantly, this shows that zircons if not heavily deformed can be reliable paleomagnetic recorders [5].