Occums razor might suggest that a very simple reason for the Na depletion matrix - has someone put down a matrix of analyses before you mapped it?
1. Reprep and map again.
Hi Les,
I assume you are talking about the Na map of the melt inclusion at the top of the topic? If so, please use the "quote" button for clarity next time...
Your speculation is not unreasonable (that it was previously mapped), but no, it is a virgin material and the artifact is due to some sort of sub surface charging dynamics during the stage scan acquisition. It is also present (though less apparent), in the K image (and Si and Al). The material is essentially homogeneous. See Julie Chouinard's poster from this summer (attached below).
2. If it is present, is it dose dependent? Kv or current?
We have not tried various conditions. If it is an ion migration artifact, and I'm pretty sure it is as these glasses show extreme time dependent intensity (TDI) effects, then beam current, size and even keV should affect the artifact "wavelength".
I have an idea for a TDI map acquisition, but so little time...