Comments on Problems with EPMA of Glass and related issues of quality control of EPMA data used in tephrochronology
December 26, 2006
To get a handle on the "state of the art", I did an informal survey of over 50 published articles that discussed tephra+glass+chemistry using some electron beam method (EPMA, SEM: EDS or WDS) and found a wide range of operations going on.
I was aware of some of the literature on the problems with epma of glasses, particularly of alkali loss -- mainly the work published in the Journal of Applied Physics going back to the 1960s, and the American Mineralogist papers by Charlie Nielsen and Haruldur Sigurdsson, Devine et al, and 2 papers by George Morgan and Dave London, as well as the mid-90s paper in the Canadian Mineralogist by Spray and Rae. I also knew of Froggatt's 1992 paper.
Ken clued me in to a separate set of literature that I was not aware of, Hunt and Hill, which specifically discussed issues of quality control vis a vis comparing glass compositions of tephra in order to infer time horizons from volcanic eruptions. Digging into all this in the weeks before AGU was an eye opener: not only did I find some very interesting and useful studies, but I also found that some of these researchers, working on the same topic, were not aware of each other.
My conclusion was that while they all used the electron microprobe as a research tool, because they published in different journals and attended different society's meetings, they were unfortunately unaware of each others work. I think it would be very useful to foster communication between and amongst all of us working on this topic: how to best utiize the electron microprobe for analysis of glass.
The tephrochronological community seems to have made significant advances in coming up with suggestions for improving the state of the art of epma of volcanic glasses. I would like to second that, and make sure that others not directly involved with those discussions, are aware.
One important link in all of this are the "technologists", the people who run the SEM and EPMA labs, who are (or should be) the "institutional" or "professional" memory that is passed on to the waves of students and researchers. Professional organizations of technologists like the Microbeam Analysis Society are one way that this information can be more broadly spread.
Added comments January 23, 2007 following some discussion on the sx50-users listserver:
One valuable technique that I did not highlight enough was the use of a cryo-stage for cooling down the sample. This was mentioned by Spray and Rae (1995, Canadian Mineralogist), and more recently by Kearns, Steen and Erlund (2005 M&M abstract volume), and has potential particularly for small bodies of glass where defocusing the beam is impossible. Although, the literature suggests it is not just heating but also charge implantation that is involved in the transient behavior of certain elements under epma. If users were to inform the probe manufacturers that this was a desired option, they would probably develop it. (Ian Steele had developed such a stage to optimize CL imaging on in SX50 in Chicago.)