Can you compare closely to other elements? If this is in fact diffusion, then you should expect a complimentary pattern. Specifically, Na diffuses much more easily than many other elements, and you may have another cation diffusing oppositely to the Na to balance charge. If so, then the patterns for all elements should not be the same, but they should be complimentary.
If they are all the same, then I'm going to put my money on an instrumental fluctuation: like (to make stuff up) the emission from your electron gun is varying, or your detector amp has some noise on it. You maybe can check this by looking at the amplitude of the signal. I notice the "Wt%" of the Na is varying by about 1/2 Wt%. So, do other nearby elements also vary by about that? Compute the variance of the concentration as a function of element Z. Maybe you get an overvoltage curve....
At the risk of being wrong I'd have to say it's not an instrumental effect as the Na signal does not vary at all in the host phase (olivine)- though it is essentially zero.
Also I recall Si and Al do not seem to diffuse as much as Na and K under normal TDI measurements. Typically one will see TDI variations of Na and K several times larger than Si and Al. See here for examples:
http://probesoftware.com/smf/index.php?topic=116.msg461#msg461On the other hand, maybe the images *are* consistent with that hypothesis- though wouldn't that mean that the ion migration is real, rather than an instrumental artifact? But then what causes the periodicity? Is it related to the size of the inclusion for some reason?
Of course, I've always assumed that it is the thermal heating of a glass which mobilizes the alkali ions (which is why we see Kearns' so called "incubation time" as seen here):
http://probesoftware.com/smf/index.php?topic=116.msg454#msg454And subsequently those alkali ions are then attracted to the sub-surface (dynamic) primary electron charge, hence the ions are drawn *deeper* into the sample, therefore exhibiting greater absorption losses, while the Si and Al atoms are less mobile and only show a minor increase in intensity due to the reduced degree of absorption by Na and K?
But, here's some of the other elements plotted so you can see them yourself: