When I started getting into noble gas thermochronology (i.e. 40Ar/39Ar and (U-Th)/He) I realized that there were different types of geoscience literature involving noble gas research. First, there are the studies I was most interested in, involving either the behavior of radiogenic noble gases in common crustal minerals or their application to understanding tectonic problems. The other noble gas studies, which I tended to ignore, used them as geochemical tracers of a whole boatload of earth processes, including whole-earth degassing and formation of the atmosphere. In the past year through collaboration between my research group and some excellent geochemists, and in light of a recent and excellent Nature article announcing some really surprising findings, I have gained a new appreciation for the role of noble gases in geoscience.
The article, which came out in the September 20th issue of Nature, is called 40Ar retention in the terrestrial planets. It presents the results of a whole series of experiments examining the behavior of 40Ar in forsterite ( Mg2SiO4) and enstatite (MgSiO3), the two minerals that make up most of the mantle. The common perception of noble gases is that they are relatively incompatible in minerals. Not only do they diffuse out quickly, but during partial melting events, the noble gases are strongly partitioned into the melt. The melt then ascends (say at a mid-ocean ridge), the gas exsolves, and escapes into the atmosphere. For 40Ar, we should be able to calculate the percent of the planet that has degassed if we know the K content of the planet (40K being the radioactive parent of 40Ar, and therefore the source of most of the Ar) and the total amount of 40Ar in the atmosphere. Some studies have concluded that the earth may be only ~50% degassed. This is confusing if you accept that noble gas diffusion is relatively fast and noble gases are strongly partitioned into melt during partial melting events; why hasn’t the whole mantle degassed by now?
The data presented in this paper are discussed in terms both diffusivity (the speed at which 40Ar atoms move through the crystal lattice) and solubility (the total amount of 40Ar that could be stuffed into a crystal lattice given unlimited time, temperature, and 40Ar). The authors took highly polished slabs of minerals (both natural and synthetic crystals) and put them in 40Ar rich atmospheres at different pressures and temperatures. After set amounts of time, they removed the samples, and looked at concentration profiles of 40Ar in the crystal using Rutherford Backscattering Imaging. From this they are able to construct concentration versus depth plots for all of the various temperature and 40Ar pressure scenarios, which are then fit these profiles with equations relating to diffusive uptake, which then allow for the calculation of some of the fundamental parameters of diffusion (diffusivity and solubility). These measurements are different from the bulk loss profiles I am used to, where we infer the concentration profile based on step heating experiments. These instead are direct measurements of the distribution of 40Ar in the solid. The paper discusses many of the potential problems of the experiments and measurements, but I won’t go into that here. Their punchline is that 40Ar solubility is actually fairly high in both forsterite and enstatite, and that 40Ar diffusivity is actually fairly low. In fact, during partial melting events, 40Ar can almost be thought of as a compatible element, that is, it is not strongly partitioned into the melt at all; both forsterite and enstatite can hold onto significant amounts of their 40Ar during partial melting! As I mentioned earlier, this is in contrast to previous thoughts and experiments on the topic, but does at least fit with the suggestion that the earth is not fully degassed.
Like most Nature papers it is not terribly long (they have a maximum of 4 pages to work with), but well worth the read. The implications could be tremendous. Even from a thermochronology perspective, it makes me wonder about the validity of our diffusion experiments that try to infer the concentration profiles of gases in minerals indirectly. Hmmmm.
Because it just got printed I decided not to include any of the figures in this post, but anyone interested should check out the original paper (Nature has the advantage that even most public libraries carry it). Allegedly there is a much more detailed version in the works, I’ll keep you posted.