Abstract Details
(2020) Applications and Limitations of U-Pb Thermochronology
Smye A
https://doi.org/10.46427/gold2020.2417
The author has not provided any additional details.
04h: Room 1, Friday 26th June 22:00 - 22:03
Andrew Smye
View all 2 abstracts at Goldschmidt2020
Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.
Submitted by Thomas Zack on Tuesday 16th June 22:27
Great Andy, splendid talk! First set of questions: how can we proceed with your conclusions? Look for the exception (error function diffusion profiles) and continue as usual? Try to quantify the amount of fast-diffusion pathways (I am think here of micas, of course; what a nightmare...)? Or something else? Cheers, Thomas
Thanks, Thomas! Excellent session. A defensible strategy would be to collect a significant number of U-Pb depth profiles and identify those that conform to an error function; if the thermal history is the dominant driver behind a profile shape, it could also be tractable to find multiple profiles with the same diffusive length scale. Regarding the effects of exsolution on diffusion systematics, I think we need to test the hypothesis by a microbeam analysis of Pb concentration proximal to individual lamellae....
Great Andy, splendid talk! First set of questions: how can we proceed with your conclusions? Look for the exception (error function diffusion profiles) and continue as usual? Try to quantify the amount of fast-diffusion pathways (I am think here of micas, of course; what a nightmare...)? Or something else? Cheers, Thomas
Thanks, Thomas! Excellent session. A defensible strategy would be to collect a significant number of U-Pb depth profiles and identify those that conform to an error function; if the thermal history is the dominant driver behind a profile shape, it could also be tractable to find multiple profiles with the same diffusive length scale. Regarding the effects of exsolution on diffusion systematics, I think we need to test the hypothesis by a microbeam analysis of Pb concentration proximal to individual lamellae....
Submitted by Matthias Konrad-Schmolke on Wednesday 24th June 14:48
Hi Andy, I liked your talk and findings. Are there any signs that indicate fast diffusion pathways visible in high contrast BSE images? Is there any chance of detecting them in rutile without TEM?
Hi Matthias, yes, exsolution lamellae (ilmenite and zircon) are common to rutile and can be present at microscopic-scale as well as sub-microscopic scale; these also tend to be highly visible under SEM.
Hi Andy, I liked your talk and findings. Are there any signs that indicate fast diffusion pathways visible in high contrast BSE images? Is there any chance of detecting them in rutile without TEM?
Hi Matthias, yes, exsolution lamellae (ilmenite and zircon) are common to rutile and can be present at microscopic-scale as well as sub-microscopic scale; these also tend to be highly visible under SEM.
Submitted by Freya George on Thursday 25th June 20:20
Thanks for an awesome presentation Andy! Obviously the focus here is rutile, but you mention the application of apatite and titanite thermochronology also. Have you found any signs that there are similarly heterogeneous controls on Pb diffuision in these phases?
Hi Freya! Yes for titanite; sub-grains and recrystallization are important here (see Rob Holder's work on this). I have less experience with apatite, but am led to believe that the same story applies (although common Pb contamination is an issue; see Dave Chew's work).
Thanks for an awesome presentation Andy! Obviously the focus here is rutile, but you mention the application of apatite and titanite thermochronology also. Have you found any signs that there are similarly heterogeneous controls on Pb diffuision in these phases?
Hi Freya! Yes for titanite; sub-grains and recrystallization are important here (see Rob Holder's work on this). I have less experience with apatite, but am led to believe that the same story applies (although common Pb contamination is an issue; see Dave Chew's work).
Submitted by Chloe Bonamici on Friday 26th June 15:12
Thanks for these nice examples in rutile. It seems like is some a nuance in your conclusion about Pb diffusion: Pb volume diffusion IS occurring, but that the diffusion domain size is often less than the size of the whole grain because of the propensity of rutile to exsolve ilmenite; and that preservation rutile grains with diffusion domain size equivalent to grain size is uncommon, though not absent. Is it fair to say your results suggest that it's not that the process of diffusion isn't occurring, only that it tends to be poorly preserved?
Hi Chloe, yes, absolutely. The combined effects of variable domain size (driven by exsolution) and flux-limited boundary conditions can result in a range of profile shapes.
Thanks for these nice examples in rutile. It seems like is some a nuance in your conclusion about Pb diffusion: Pb volume diffusion IS occurring, but that the diffusion domain size is often less than the size of the whole grain because of the propensity of rutile to exsolve ilmenite; and that preservation rutile grains with diffusion domain size equivalent to grain size is uncommon, though not absent. Is it fair to say your results suggest that it's not that the process of diffusion isn't occurring, only that it tends to be poorly preserved?
Hi Chloe, yes, absolutely. The combined effects of variable domain size (driven by exsolution) and flux-limited boundary conditions can result in a range of profile shapes.
Submitted by Jeremy Powell on Friday 26th June 16:21
Hi Andy, great presentation! I have one question about your rutile depth profile inversion procedure: is that code published or available online?
Hi Jeremy, yes, it's published (Smye, A. J., Marsh, J. H., Vermeesch, P., Garber, J. M., & Stockli, D. F. (2018). Applications and limitations of U-Pb thermochronology to middle and lower crustal thermal histories. Chemical Geology, 494, 1-18.) and also available on Pieter Vermeesch's website.
Hi Andy, great presentation! I have one question about your rutile depth profile inversion procedure: is that code published or available online?
Hi Jeremy, yes, it's published (Smye, A. J., Marsh, J. H., Vermeesch, P., Garber, J. M., & Stockli, D. F. (2018). Applications and limitations of U-Pb thermochronology to middle and lower crustal thermal histories. Chemical Geology, 494, 1-18.) and also available on Pieter Vermeesch's website.
Submitted by Bianca Németh on Friday 26th June 16:29
Hi Andy, great presentation, congratulations! I saw that most of your BDE images show reaction textures. Do you have equilibrium domains in the studied samples as well? And if yes, did you use them for geothermo-barometric calculations just to control these data? I mean I saw the diagrams also showing temperatures, but did you do other methods as well, and all of them show the same temperature range, or are there any differences between the used thermometers? (I just asked, because on the Bakony-Balaton Highland Volcanic Field, we study lower crustal granulite xenoliths, and the metapelites show somewhat lower temperature and pressure range compared to metabasics, but the metabasics seem to be UHT granulites with Teq=750-1100 °C , peq=1.0-1.6 GPa, and represent a continuous crustal section)
Hi Bianca, we have not undertaken conventional PT analyses on the Slave xenoliths, though I'm pretty sure others have done so (check out Bill Davis' work). Good suggestion re: linking PT with thermo results...
Hi Andy, great presentation, congratulations! I saw that most of your BDE images show reaction textures. Do you have equilibrium domains in the studied samples as well? And if yes, did you use them for geothermo-barometric calculations just to control these data? I mean I saw the diagrams also showing temperatures, but did you do other methods as well, and all of them show the same temperature range, or are there any differences between the used thermometers? (I just asked, because on the Bakony-Balaton Highland Volcanic Field, we study lower crustal granulite xenoliths, and the metapelites show somewhat lower temperature and pressure range compared to metabasics, but the metabasics seem to be UHT granulites with Teq=750-1100 °C , peq=1.0-1.6 GPa, and represent a continuous crustal section)
Hi Bianca, we have not undertaken conventional PT analyses on the Slave xenoliths, though I'm pretty sure others have done so (check out Bill Davis' work). Good suggestion re: linking PT with thermo results...
Submitted by Félix Gervais on Friday 26th June 21:28
Hi Andy, very interesting. What do you think controlled the variation in U-Pb ages between grains showing flat profiles in the Grenvillian sample? What happens to Pb once it moves out of a grain boundary? It it transported in fluids at grain boundaries or does it diffuse back in other phases, which have different Pb diffusivities?
Hi Andy, very interesting. What do you think controlled the variation in U-Pb ages between grains showing flat profiles in the Grenvillian sample? What happens to Pb once it moves out of a grain boundary? It it transported in fluids at grain boundaries or does it diffuse back in other phases, which have different Pb diffusivities?
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