HR: 0800h
AN: V51A-0328 INVITED    [Abstracts]
TI: Peak Shifts in Al, Mg, Si and Na Ka in Geologically Important Materials
AU: * Fournelle, J
EM: johnf@geology.wisc.edu
AF: UW-Madison Dept of Geology and Geophysics, 1215 W. Dayton St, Madison, WI 53706, United States
AB: In the course of troubleshooting slightly low analytical totals in common silicate minerals, problems with the ROM peaking routine were found, i.e. it did not always find the peak center, a central plateau, e.g. ~ 5 sin theta units wide for Al Ka. Ultimately chemical peak shifts in Al, Mg. Si and Na Ka of common minerals were found. There are published references to some Al Ka peak shifts between some minerals, but never before to shifts within the feldspar family. There is an Al Ka peak shift of 7 sin theta units (0.6 ev) between anorthite and Na/K feldspars, with most rock forming minerals falling between these end members. A shift of 7 sin theta units on a correctly centered peak could cause ~2 % error (doubled if O by stochiometry); on an incorrectly centered peak the error could be 2-3x. These experiments were done on an SX51 (#485) operating at 15 kV, on TAP crystals. Consistent results were found on two separate crystals, over a period of 2 years (4 experiments). Examination of Al Ka in garnets shows there is a 5 unit sin theta shift in the Al Ka peak of pyrope relative to almandine. Most silicate Si Ka peaks are close together, although there is a small distinct shift for K-feldspar relative to other silicates. For Mg Ka, there is a wider gap, spanning 10 sin theta units from chromite to pyrope (e.g., MgO is 7 units from pyrope). Initial work suggests a similar occurrence for Na Ka, with a 10 unit sin theta span between albite and jadeite. Here, however the peaks are much wider (18 units for albite), the count rates lower, and precision lower. The accuracy of this experimental technique was evaluated: the shift between SiO2 and Si, 8-9 sin theta units (0.5-0.6 ± 0.1 eV) is the same as has been found by other more sensitive techniques (0.62, 0.655 ev). One reason these issues may not have been recognized before is that minor errors in Z (optical) focus yield comparable errors to peak shifts.
DE: 1065 Major and trace element geochemistry
DE: 1094 Instruments and techniques
DE: 3630 Experimental mineralogy and petrology
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting