  Zircon crystals offer a unique combination of suitability for high-precision
radiometric dating and high resistance to alteration. Paleomagnetic experiments
on ancient zircons may potentially constrain the earliest geodynamo, which
holds broad implications for the early Earth interior and atmosphere. However,
the ability of zircons to record accurately the geomagnetic field has not been
fully demonstrated. Here we conduct thermal and room temperature alternating
field (AF) paleointensity experiments on 767.1 thousand year old (ka) zircons
from the Bishop Tuff, California. The rapid emplacement of these zircons in a
well-characterized magnetic field provides a high-fidelity test of the zircons
intrinsic paleomagnetic recording accuracy. Successful dual heating experiments
on nine zircons measured using a superconducting quantum interference device
(SQUID) microscope yield a mean paleointensity of 46.2 +/- 18.8 microtesla
(1sigma), which agrees closely with high-precision results from Bishop Tuff
whole rock (43.0 +/- 3.2 microtesla). High-resolution quantum diamond magnetic
mapping, electron microscopy, and X-ray tomography indicate that the bulk of
the remanent magnetization in Bishop Tuff zircons is carried by Fe oxides
associated with apatite inclusions, which would be susceptible to destruction
via metamorphism and aqueous alteration in older zircons. As such, while
zircons can reliably record the geomagnetic field, robust zircon-derived
paleomagnetic results require careful characterization of the ferromagnetic
carrier and demonstration of their occurrence in primary inclusions. We further
conclude that a combination of quantum diamond magnetometry and high-resolution
imaging can provide detailed, direct characterization of the ferromagnetic
mineralogy of geological samples.
