Abstract Details
(2020) Muscovite MDD: In Vacuo Argon Release via Volume Diffusion
Heizler M, Holland M & Long S
https://doi.org/10.46427/gold2020.1013
06n: Room 2, Wednesday 24th June 23:21 - 23:24
Matthew Heizler
View abstracts at 4 conferences in series
Mark Holland
Sean Long View abstracts at 3 conferences in series
Mark Holland
Sean Long View abstracts at 3 conferences in series
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 Yuri Amelin on Wednesday 24th June 11:30
How does the volume diffusion goes in muscovite: mostly along the sheets or across the sheets? How does your crushing of large pieces of mica and size separation of crushed material change the dimensions (area to thickness ratio) of the mica crystals? Do the crushed fragments have the same proportions as natural small muscovite grains? I expect that the diffusion in mica crystals is strongly heterogeneous, so these questions are about the relevance of these experiments to real life situations.
We suspect that diffusion is very anisotropic with the principle pathway parallel to the mica sheets. That is, grain thickness is not a controlling factor. The starting material was a large (many cm sized) book of muscovite and it was used to demonstrate physical grain size control on degassing behavior. Because natural samples, such as mylonites have a large grain size variation a bulk muscovite separation will also contain a range of grain sizes. Therefore, by demonstrating that the spatial distribution of argon can be extracted during bulk step-heating means our experiments are directly relevant to natural systems. In a natural, fine-grained rock, micas tend to be removed with only minor crystal breakage and thus there is an expectation that the particles analyzed in the lab will mimic the grain size that controlled the 40Ar distribution in nature. There's plenty of followup work to do to address yours and many other questions and this work is on-going. Thanks for your interest.
How does the volume diffusion goes in muscovite: mostly along the sheets or across the sheets? How does your crushing of large pieces of mica and size separation of crushed material change the dimensions (area to thickness ratio) of the mica crystals? Do the crushed fragments have the same proportions as natural small muscovite grains? I expect that the diffusion in mica crystals is strongly heterogeneous, so these questions are about the relevance of these experiments to real life situations.
We suspect that diffusion is very anisotropic with the principle pathway parallel to the mica sheets. That is, grain thickness is not a controlling factor. The starting material was a large (many cm sized) book of muscovite and it was used to demonstrate physical grain size control on degassing behavior. Because natural samples, such as mylonites have a large grain size variation a bulk muscovite separation will also contain a range of grain sizes. Therefore, by demonstrating that the spatial distribution of argon can be extracted during bulk step-heating means our experiments are directly relevant to natural systems. In a natural, fine-grained rock, micas tend to be removed with only minor crystal breakage and thus there is an expectation that the particles analyzed in the lab will mimic the grain size that controlled the 40Ar distribution in nature. There's plenty of followup work to do to address yours and many other questions and this work is on-going. Thanks for your interest.
Submitted by Thomas Zack on Wednesday 24th June 15:02
Convincing presentation, and my answer to your question is: yes, most likely volume diffusion. Still, I would feel much better if you could provide high-resolution BSE images of muscovites in its textural context. There simply are all kinds of secondary processes that may or may not be working on micas.
I agree that there are secondary processes at work as well. I fully recognize that ultra-high vacuum extraction of argon is a big leap to diffusion of argon from a mica in the middle crust under geological timescales. However, I remain struck by the fact that our simple kinetic model explains all aspects of the experiment as well as natural samples. I would argon that diffusion is the principle transport mechanism, but surely other factors must be at play and we hope to address them with additional work. My biggest concern in a natural sample would be mixing two or more generations of muscovite that are age discordant and different in grain size (i.e. neocrystalized crystals nucleating on larger crystals). In a situation like this, your point about high-resolution imaging in a textural context would be critical in understanding any derived thermal history. Thanks.
Convincing presentation, and my answer to your question is: yes, most likely volume diffusion. Still, I would feel much better if you could provide high-resolution BSE images of muscovites in its textural context. There simply are all kinds of secondary processes that may or may not be working on micas.
I agree that there are secondary processes at work as well. I fully recognize that ultra-high vacuum extraction of argon is a big leap to diffusion of argon from a mica in the middle crust under geological timescales. However, I remain struck by the fact that our simple kinetic model explains all aspects of the experiment as well as natural samples. I would argon that diffusion is the principle transport mechanism, but surely other factors must be at play and we hope to address them with additional work. My biggest concern in a natural sample would be mixing two or more generations of muscovite that are age discordant and different in grain size (i.e. neocrystalized crystals nucleating on larger crystals). In a situation like this, your point about high-resolution imaging in a textural context would be critical in understanding any derived thermal history. Thanks.
Submitted by Bryant Ware on Wednesday 24th June 15:53
Interesting approach! If volume diffusion is the case, is this MDD approach to a thermal history something that could then be applied to previously obtained data? What is the gain in the pre-degassing steps and what determines the length of time you are pre-degassing the sample?
Interesting approach! If volume diffusion is the case, is this MDD approach to a thermal history something that could then be applied to previously obtained data? What is the gain in the pre-degassing steps and what determines the length of time you are pre-degassing the sample?
Submitted by Matthew Heizler on Wednesday 24th June 17:44
If the original experiment was conducted with enough resolution and with "gentle" heating the data could be potentially inverted via MDD. Most recent muscovite age spectra are likely generated by step-heating with a laser and thus this data would not be amenable too MDD analysis. For furnace step-heating it has been shown that rapidly heating for short durations causes sample instability and thus loss of a diffusion mechanism. For our age spectra, each step is 20 minutes in duration and some steps are only incremented by 5°C especially in the critical temperature window (700-900°C) when the mica is undergoing dehydroxilation. Coarse grained mica delaminate and thus their physical instability makes them less suitable for MDD analysis. A typical sample is not degassed prior to analysis. In our experiment we wanted to show test if degassing of 40Ar in vacuo is potentially a diffusion mechanism. Therefore we induced argon loss with the pre-degassing and we choose temperatures and durations that would typically result in about 30-60% 40Ar loss. The main point is that we know the thermal history (i.e. 800°C, 1 hr, or 600°C, 10 days) of the degassing event. Following irradiation we measure the sample just like we would a "geological" sample. We use the loss of 39Ar to estimate the kinetic parameters and then impose the known thermal history onto these parameters in a forward model to see what the predicted age spectrum would be for the known thermal history. As you can see our models nearly perfectly match the measured spectra strongly suggesting a diffusion mechanism for argon transport.
If the original experiment was conducted with enough resolution and with "gentle" heating the data could be potentially inverted via MDD. Most recent muscovite age spectra are likely generated by step-heating with a laser and thus this data would not be amenable too MDD analysis. For furnace step-heating it has been shown that rapidly heating for short durations causes sample instability and thus loss of a diffusion mechanism. For our age spectra, each step is 20 minutes in duration and some steps are only incremented by 5°C especially in the critical temperature window (700-900°C) when the mica is undergoing dehydroxilation. Coarse grained mica delaminate and thus their physical instability makes them less suitable for MDD analysis. A typical sample is not degassed prior to analysis. In our experiment we wanted to show test if degassing of 40Ar in vacuo is potentially a diffusion mechanism. Therefore we induced argon loss with the pre-degassing and we choose temperatures and durations that would typically result in about 30-60% 40Ar loss. The main point is that we know the thermal history (i.e. 800°C, 1 hr, or 600°C, 10 days) of the degassing event. Following irradiation we measure the sample just like we would a "geological" sample. We use the loss of 39Ar to estimate the kinetic parameters and then impose the known thermal history onto these parameters in a forward model to see what the predicted age spectrum would be for the known thermal history. As you can see our models nearly perfectly match the measured spectra strongly suggesting a diffusion mechanism for argon transport.
If the original experiment was conducted with enough resolution and with "gentle" heating the data could be potentially inverted via MDD. Most recent muscovite age spectra are likely generated by step-heating with a laser and thus this data would not be amenable too MDD analysis. For furnace step-heating it has been shown that rapidly heating for short durations causes sample instability and thus loss of a diffusion mechanism. For our age spectra, each step is 20 minutes in duration and some steps are only incremented by 5°C especially in the critical temperature window (700-900°C) when the mica is undergoing dehydroxilation. Coarse grained mica delaminate and thus their physical instability makes them less suitable for MDD analysis. A typical sample is not degassed prior to analysis. In our experiment we wanted to show test if degassing of 40Ar in vacuo is potentially a diffusion mechanism. Therefore we induced argon loss with the pre-degassing and we choose temperatures and durations that would typically result in about 30-60% 40Ar loss. The main point is that we know the thermal history (i.e. 800°C, 1 hr, or 600°C, 10 days) of the degassing event. Following irradiation we measure the sample just like we would a "geological" sample. We use the loss of 39Ar to estimate the kinetic parameters and then impose the known thermal history onto these parameters in a forward model to see what the predicted age spectrum would be for the known thermal history. As you can see our models nearly perfectly match the measured spectra strongly suggesting a diffusion mechanism for argon transport.
If the original experiment was conducted with enough resolution and with "gentle" heating the data could be potentially inverted via MDD. Most recent muscovite age spectra are likely generated by step-heating with a laser and thus this data would not be amenable too MDD analysis. For furnace step-heating it has been shown that rapidly heating for short durations causes sample instability and thus loss of a diffusion mechanism. For our age spectra, each step is 20 minutes in duration and some steps are only incremented by 5°C especially in the critical temperature window (700-900°C) when the mica is undergoing dehydroxilation. Coarse grained mica delaminate and thus their physical instability makes them less suitable for MDD analysis. A typical sample is not degassed prior to analysis. In our experiment we wanted to show test if degassing of 40Ar in vacuo is potentially a diffusion mechanism. Therefore we induced argon loss with the pre-degassing and we choose temperatures and durations that would typically result in about 30-60% 40Ar loss. The main point is that we know the thermal history (i.e. 800°C, 1 hr, or 600°C, 10 days) of the degassing event. Following irradiation we measure the sample just like we would a "geological" sample. We use the loss of 39Ar to estimate the kinetic parameters and then impose the known thermal history onto these parameters in a forward model to see what the predicted age spectrum would be for the known thermal history. As you can see our models nearly perfectly match the measured spectra strongly suggesting a diffusion mechanism for argon transport.
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