Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo

Abstract

Zircon crystals from the Jack Hills, Western Australia, are one of the few surviving mineralogical records of Earth’s first 500 million years and have been proposed to contain a paleomagnetic record of the Hadean geodynamo. A prerequisite for the preservation of Hadean magnetization is the presence of primary magnetic inclusions within pristine igneous zircon. To date no images of the magnetic recorders within ancient zircon have been presented. Here we use high-resolution transmission electron microscopy to demonstrate that all observed inclusions are secondary features formed via two distinct mechanisms. Magnetite is produced via a pipe-diffusion mechanism whereby iron diffuses into radiation-damaged zircon along the cores of dislocations and is precipitated inside nanopores and also during low-temperature recrystallization of radiation-damaged zircon in the presence of an aqueous fluid. Although these magnetites can be recognized as secondary using transmission electron microscopy, they otherwise occur in regions that are indistinguishable from pristine igneous zircon and carry remanent magnetization that postdates the crystallization age by at least several hundred million years. Without microscopic evidence ruling out secondary magnetite, the paleomagnetic case for a Hadean–Eoarchean geodynamo cannot yet been made.

Principles and techniques of the quantum diamond microscope

“…Followup work with the QDMs have demonstrated their utility in imaging magnetization carriers at the grain scale. Recent example applications have included the imaging of large magnetite grains to visualize multi-domain structure [104] and of zircons [117][118][119] to understand and constrain the history of Earth's dynamo. The full potential of the QDM as a rock magnetic instrument are only beginning to be explored, with ongoing experiments on terrestrial and extraterrestrial rock types being pursued.…”

Detectability of Remanent Magnetism in the Crust of Venus

“…Grains longer than~80-1,000 nm are in the pseudo-single domain state (Muxworthy & Williams, 2009). Individual micromagnetic simulations are required to compute the stability of pseudo-single domain grains, although results often predict strong stability that coincidentally matches predictions for single domain grains from Néel theory that does not capture the energy barrier related to the traverse of the vortex core (e.g., magnetization vectors curling around a central axis) across the grain axes (Nagy et al, 2017;Tang et al, 2019). Very small grains have extremely short relaxation times (Muxworthy & Williams, 2009).…”

Venus: A Thick Basal Magma Ocean May Exist Today

“…Compositional stratification in the BMO could lead to noncontinuous dynamo operation with a brief burst of activity before an extended pause (Laneuville et al, 2017). For now, whether a dynamo existed in the Hadean and/or Eoarchean is unknown (e.g., Tang et al, 2019;Weiss et al, 2015).…”