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In situ Rb-Sr Age Mapping of Micas Captures Examples for Petro- and Thermochronology
Zack T (2020) Goldschmidt Abstracts, 2020 3065
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04h: Room 1, View in program
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Thomas, 2 questions. Is it possible to make Rb-Sr age rasters or maps using this approach? And do you get compromised trace element analyses because of the oxygen reaction gas? David Chew
Hi Dave! I would call the results for Högsbo and the Reynolds Range as Rb-Sr age maps, not sure if "only" 245 spots qualify as "maps" for you? We are limited to a minimum period of 5 seconds per age spot, to achieve a stable enough signal to get any age meaning. Of course, if the question is important enough, >1000 age spots could be generated for a higher resolution map, perhaps going down to 20 micron spot size in certain samples. The reaction gas N2O is pretty ideal for almost all trace elements. From the elements mentioned in the talk, it is only Li where a significant loss in sensitivity has to be accepted, as low mass ions are kicked out of the beam path by N2O (as Be and B). However, a good knowledge on the reaction products from reactions between ions and N2O is required- hope I can publish those findings soon...
Strangely enough authors can ask questions to their own presentations. Well, so I turn this into a message board: if you want to discuss Thermochronology, petrochronology, Rb/Sr or whatever, please find me on Discord, with some likelihood in the Rubidium room (this allows video chats). I am trying to be there as much as I would be at coffee breaks during Goldschmidt- so, a lot...
Hi Thomas, how do the Sr concentrations vary in the maps? Do you see increasing or decreasing Sr concentrations towards the rims? Is there a correlation with Pb concentrations?
The simple answer to the last question is "yes", there is a rough positive correlation between Sr (important note: here I mean 86Sr, the common Sr) and Pb. In detail, Pb seems to show a bit more scatter, but this could be counting errors (Pb has much lower dwell times than 86Sr). Common Sr and Pb do show an increase towards the rim, yes as well. You can see it in the slides pretty strongly for Högsbo, but much more subtlety for the Reynolds Range.
Is there much deposition of ablated material around your closely spaced pits for the grid-like mapping? Any concern that such build-ups may affect or overlap with the nearby analyses?
Just to repeat what I said in the Q&A session: the way the mapping is performed (5 seconds ablation per spot), it does not only drill through any possible ablation blanket, but it also allows to evaluate this in the time-resolved spectra. I do show a small example of those time-resolved spectra. Sometimes one can observe a spike at the first second- this should be the surface contamination.
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