The image on the left is a computer simulation of the earth’s magnetic field during ‘normal’ pole arrangement.
What is earth’s magnetic field and why do we care about it? In short it is the magnetic field that extends out from the earth into space and interacts and blocks out the solar wind and its charged particles, which would otherwise remove our protective ozone layer. Ozone intern protects us from most of the ultraviolet radiation of space, so we don’t all get skin cancer. Woo!
In order to understand how our planet and the universe works, it helps uncovering what was going on before our time. The earth’s magnetic field was not always orientated the same way. Magnetic North and South pole switch places, sometimes after millions of years, sometimes even just after a few thousand. There is a lot of ways of obtaining data on how the earth’s magnetic field was orientated, as long as there is sufficient material from that time to analyze. The most abundant and most representative materials that give us insight, contain magnetic minerals such as magnetite which store the orientation of the surrounding field when crystallizing, often found in igneous rocks around the world (please refer to my other blog which goes into a little more detail on this topic).
So far so good right? Well how far back in time can we get rocks with magnetic signatures? We know from various peer reviewed studies that there was a magnetic field at the end of the Archean, but what about before?
Rocks that are beyond 4 billion years of age tend to be very different now, then they were when they first formed. This often means that there is a lot of contamination within the minerals recording the past magnetic field, it becomes tricky to say what magnetic field they represent. This is where minerals like Zircon come in, which are a lot more resilient to processes that alter minerals like magnetite.
Zircons are most commonly used to get ages of rocks, especially old ones. They are tinys minerals that occur in most rocks throughout earths history, are very stable, which means they tend to stay unchanged throughout most processes and thus act like little time capsules.
Photograph of zircon crystals they are the size of a sand grain.
During their formation in igneous intrusions small amounts of ferrous minerals can be trapped as inclusions in these small ‘time capsules’ and record the field at time of formation.
But what are the oldest rocks….? If most rocks contain zircons what is the oldest zircon on earth?
The oldest zircons that we’ve found are detrital grains from the Jack Hills in Australia (picture on the right). The original host rock has been broken down and it’s broken down constituents (the detritus) i.e. the zircons have formed part of the new sedimentary rock.
A group of scientists from around the globe have been studying these specific zircons in order to answer the question in the title of this blog. Dr. Rich Taylor a scientist working on this project gave a talk last week about their progress.
The main aim of their study was to identify primary ferrous inclusions inside the zircons (these inclusions are coeval with the formation of the zircons as part of igneous processes, remember?). These inclusions are minuscule and as established before zircons are tiny little things themselves, which means that the signal would be incredibly weak. Using the most sensitive magnetometer to find a signal that might not even be there, it is therefore a pretty complicated and time consuming endeavour. “The hardest paleomagnetic project ever devised” as quoted from Rich Taylor.
Minerals like Zircons, Quartz and Feldspar act as armour for the magnetic particles because they themselves are non magnetic so produce the clearest signals. During their growth as crystals they will incorporate magnetic minerals some will be during the first growth stages in a magma chamber but some will be due to fluids later on, as mentioned earlier. Distinguishing between primary and secondary inclusions is key in this project.
A slightly more in depth explanation about primary and secondary overprinting for you guys wanting to go above and beyond with the science. There are two main ways in which grains can become magnetized one is through thermal remnant magnetization (TRM) which records the magnetic field as the grain passes through the curie point during cooling (refer back to my previous blog), which is what the scientists were looking for. The second one is chemical remnant magnetism (CRM) which normally manifests itself in magnetic overprinting from fluids containing dissolved ferrous material passing through. The extent and exact fingerprint for CRM is not fully understood yet so it is still pretty tricky to distinguish a true TRM signal from a later overprinted CRM. Makes sense right?
Taylor and his team devised various processes like cleaning the zircons with hot pressurized hydroflouric acid (really nasty stuff, especially when it’s heated) which removed any unwanted ferrous contaminants in the zircon that arrived as fluid overprints via cracks in the crystal, like hematite.
Using a Quantum Diamond Microscope, a Scanning Electron Microscope and some other fancy sci-fi-sounding tools the team was able to devise criteria to separate out garbage zircons from ones containing primary ferrous inclusions, so they thought. After going through 4,000 individual zircons picked for this study, two ended up passing all stages of assessment with flying colours (yes just 2). Now it’s time to see what secrets these two zircons can show us about our planets geomagnetic history… Unfortunately the ‘primary’ inclusions turned out to not coincide with the formation of the zircons meaning that they were also useless. From 4,000 to 0. Science is not always glamorous…
Now what? What does this mean in terms of the bigger picture?
Well, Taylor and his team discovered (the hard way) that the it is very difficult to get paleomagnetic data from before the Archean. They found that the Jack Hills Zircons don’t provide accurate insights on the Hadean magnetic field. Even if the two zircons turned out to have good data the sample size is far too small for it to actually mean anything. They helped ‘debunk’ controversial papers claiming to have already established a record for the Hadean magnetic field also using Jack Hills zircons, pointing out that these studies lumped overprinted and primary signals together. A non discovery (compared to your original motivation and hopes) is also a discovery!
Leon's Geology 