Hunting the Hadean magnetic field by Rich Taylor

26/09/2019

Jack Hills of Australia

Hunting the Hadean magnetic field by Rich Taylor

In short:

Rich Taylor and his team worked on measuring the paleaomagnetic field intensity in zircons of the Hadean after a controversial paper by Tarduno et al (2015). After selecting through 4000 detrital zircons from the Jack Hills in western Australia for very demanding criteria only 2 where initially thought to have been overprinted. Measuring their magnetic remanance lead to results similar to those of Tarduno et al. But after further analysis the magnetic remanace seemed to actually be from an later overprint, thus discarding those samples. This lead to the question as to whether Tarduno et al 's results were not overprints as well.So even though the presence of a Hadean magnetic field remains unproven, Rich's research improved the techniques in paleomagnetic measurements.

Main:

The Earth's magnetic field has been studied for many years ranging to uses in navigation, exploration, subsurface exploration (magnetic anomalies) and astronomy. It's remanance in the rock record is well known for the discovery of paleomagnetic inversions, used as dating method and further evidence of sea-floor spreading. It is then only natural that the question of when did the geomagnetic field start existing? It has been proven that the geodynamo was already functioning in the Archean, but when did it start? What are the timescales of it coming in action? Was it rapid? Was it's intensity stable or not? All of these question drove the research for the Hadean magnetic field. Rich Taylor and his team are trying to find evidence of it's existence and are trying to quantify it's intensity. It also is a follow up from the controversial paper from Tarduno et al in 2015 entitled "Paleomagnetism. A Hadean to Paleoarchean geodynamo recorded by single zircon crystals".



In order to find remanace of the Hadean magnetic field, Rich and his team first had to find samples dated of over 4 billion years old. Today, very few of this material is left and they used detrital zircons found in sedimentary rocks in the Jack Hills in western Australia. These zircons probably grew in magma chambers between 4.4 and 4 billion years ago and then got weathered out of their source rock and redeposited in the sedimentary rocks of the Jack Hills

Zircons are the best tools as they are easy to accurately date with uranium/lead dating and often have inclusions of ferromagnetic material, trapped during the growth of the crystal. These crystals are also very resistant to weathering allowing their study after such long time.

Resistant as they are, zircons still get damaged after such long timescales which lead to the first part of the work realized by a PHD student consisting of selecting which samples to use. This formed a huge part of the work as the criteria are very demanding. First of all, the zircons must not have too many cracks, which lead to the deposition of secondary minerals thus overprinting the actual data. Then the crystals need to be of the right age (U/Pb dating). They must not have a too strong iron zoning, too much damage from the radioactive decay, no thermal or hydrothermal overprint.. From 4000 initial zircons only 2 where deemed suitable and passed all the criteria. In the study from Tarduna et al not much was said about the zircon selection and they analyzed over 8 samples so this study was trying to figure out the accuracy of these results.

The remanant magnetisation of these 2 samples where then analyzed using a SQUID microscope and then with a Quantum Diamond microscope, allowing to "map" the charges in a section of the crystal quantifying it's remanant magnetic intensity. The results at this point seemed very similar to the ones from the paper by Tarduno et al in 2015, but the potential for magnetic overprint was still there.

Using an atom probe, Rich and his team mapped in 3D a small needle of a sample. In zircons the inclusions usually form in small packages in the crystal, concentrating impurities such as iron or uranium. This seemed to be useful as these groups could be dated separately with their U/Pb ration and remanant magnetization being together. These were found to be of younger age than the Hadean thus meaning it being a later overprint. The hypothesis is that these inclusions got connected through small cracks to the outside of the crystal caused by the radioactive decay of the uranium. So the 2 remaining zircons where finally discarded as having a later overprint and thus not reflecting the original magnetic field. As their results were similar to those of Tarduno et al and with Tarduno not having such an extensive study of the risk of overprint in the samples it questions if the magnetic field measured by him was not an overprint.

In conclusion the Rich and his team could not prove the presence of an Hadean magnetic field but lead to a better understanding of the overprinting criteria to consider when doing paleomagnetic research.