Halogens have been a widely underutilized geochemical tool. Only now are we seeing the potential that halogens have to answer deep unresolved questions like How did they Earth form from meteorites? And how does the Earths volatiles get transported and stored.
The halogens are made up of Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At); they reside within group 17 of the periodic table. The halogens are what we call highly electronegative; this means they have an electron missing from their outer shell. This makes them highly reactive and unstable. The Halogens can therefore be thought of as the complete opposites of the noble gases. The noble gases react with other elements only under the most extreme of conditions while halogens are so reactive they are rarely found in nature without being bound to another element. So if they are always shacked up with other elements what use are they to us as geochemists.
Well interestingly the halogens behave similar to the noble gases when within geological reservoirs. They are highly incompatible due to their large ionic radii. This means they don’t like to be contained with a crystal structure due to their large size. That means at the first opportunity they will leave the crystal and join the melt phase within a geological system. They are also fractionated by biological processes which enable us to track water interaction within rocks. This has led to the halogens being spread throughout the Earth with different amounts in different reservoirs. As such the halogens just like the noble gases are powerful tracers within geological systems.
So what have we been using the halogens for and what is left for them to tell us? Halogens have been used for years ever since they were measured within volcanic eruption plumes. As such much of the early research focussed on how the halogens interacted with the atmosphere and what they could tell us about the inside of the volcano. More recently, work has focused on how they are able to trace volatiles which are subducted back into the mantle. Noble gases have been used to suggest that water and other volatiles contained within oceanic crust is then subducted and added back into the mantle. There is a slight problem with using the noble gases though; firstly they are quite rare in seawater and sediment so it would be hard to see if they are added back in and secondly it’s difficult to measure recycled noble gases as they are often contaminated by the atmosphere. By using the halogens we have been able to tell that seawater contained within the oceanic crust is infact taken back down into the earth through subduction. So what lies ahead for the Halogens? Well a group of us here at Manchester are working hard trying to discover more about how the earth and other solar system bodies function. Question being asked include; How far down do the volatiles from subduction travel into the mantle?, how do halogens get taken into rocks and minerals? and how did the Earth gain its halogen signature? was it from meteorites or something we haven’t discovered yet.
Finally I will just quickly mention how we are studying the halogens here in Manchester as it can be quite tricky. As with all good tracers the abundance of halogens within our samples is very low so we need a way of measuring then accurately. Well this is where I will get a bit technical. Basically to measure the halogens we turn them into noble gases. This might sound like alchemy but all we do is put our samples in a nuclear reactor (don’t worry it’s not located in a secret location within Manchester) for a period of time. Within the reactor the samples are bombarded by neutrons which are incorporated in to the nucleus of the atom. The atom becomes unstable and has to eject some other particle and there for changes its elemental structure. For instance 37Cl takes in a neutron and emits a beta particle (along with a gamma ray) to become 38Ar. Once the halogens are all converted into noble gases we can finally measure then through traditional noble gas mass spectrometry (discussed in countless of our other posts).