Abstract Details
(2020) Peak Pressure Determined from Staurolite-Chloritoid Metapelites in the Monte Rosa Nappe: Thermodynamic and Geodynamic Implications
Vaughan Hammon J, Luisier C, Schmalholz S & Baumgartner L
https://doi.org/10.46427/gold2020.2670
The author has not provided any additional details.
04h: Room 5, Thursday 25th June 22:06 - 22:09
Joshua Vaughan Hammon
Cindy Luisier View abstracts at 2 conferences in series
Stefan Schmalholz View abstracts at 3 conferences in series
Lukas P. Baumgartner View all 4 abstracts at Goldschmidt2020 View abstracts at 4 conferences in series
Cindy Luisier View abstracts at 2 conferences in series
Stefan Schmalholz View abstracts at 3 conferences in series
Lukas P. Baumgartner View all 4 abstracts at Goldschmidt2020 View abstracts at 4 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 Sumit Chakraborty on Tuesday 23rd June 06:13
How were the pressures determined on the whiteschists, and what are the uncertainties of that? Also, what was the fO2 at which you did the calculations for staurolite and chloritoid?
Hello Sumit. Peak metamorphic conditions in the whiteschist bodies were determined also using the pseudosection approach based on the equilibrium assemblage ctd + phg + tlc + qz. The uncertainties associated with the peak pressure for this assemblage relate to the water activity assumed. For this C. Luisier assumed a aH2O of 1, which was supported by high and homogenous OH- measurements in phengite. I refer you to Luisier et al. (2019) https://www.nature.com/articles/s41467-019-12727-z. All calculations for staurolite and chloritoid bearing assemblages assumed water in excess. Thank you for your questions! Joshua
How were the pressures determined on the whiteschists, and what are the uncertainties of that? Also, what was the fO2 at which you did the calculations for staurolite and chloritoid?
Hello Sumit. Peak metamorphic conditions in the whiteschist bodies were determined also using the pseudosection approach based on the equilibrium assemblage ctd + phg + tlc + qz. The uncertainties associated with the peak pressure for this assemblage relate to the water activity assumed. For this C. Luisier assumed a aH2O of 1, which was supported by high and homogenous OH- measurements in phengite. I refer you to Luisier et al. (2019) https://www.nature.com/articles/s41467-019-12727-z. All calculations for staurolite and chloritoid bearing assemblages assumed water in excess. Thank you for your questions! Joshua
Submitted by Simon Cuthbert on Tuesday 23rd June 12:38
I understand that there are also metabasic eclogites in the Monte Rosa Nappe that recorded P-T conditions similar to the whiteschists. How do these fit into the mechanical pressure difference scenario that you propose?
Hello Simon. Yes, eclogite boudins reveal pressures of up to 2.7 GPa (e.g. Gasco et al. 2011). We ourselves would like to investigate some eclogite boudins from the study area, i.e. within close proximity and structurally continuous with whiteschist, metagranite and metapelite samples analysed so far. However, several field seasons have proved unfruitful in finding preserved eclogite assemblages within mafic boudins (most exhibiting amphibolite assemblages). Equally, one must take special care that meta-mafic boudins do not derive from the overlying Zermatt-Saas ophiolite unit above, especially close to the contact. Concerning our hypothesis of mechanical pressure variations, one important feature needed includes the existence of rheological variations, interestingly mafic boudins within a metapelitic matrix may provide such rheological contrasts. Thank you for your question! Joshua
I understand that there are also metabasic eclogites in the Monte Rosa Nappe that recorded P-T conditions similar to the whiteschists. How do these fit into the mechanical pressure difference scenario that you propose?
Hello Simon. Yes, eclogite boudins reveal pressures of up to 2.7 GPa (e.g. Gasco et al. 2011). We ourselves would like to investigate some eclogite boudins from the study area, i.e. within close proximity and structurally continuous with whiteschist, metagranite and metapelite samples analysed so far. However, several field seasons have proved unfruitful in finding preserved eclogite assemblages within mafic boudins (most exhibiting amphibolite assemblages). Equally, one must take special care that meta-mafic boudins do not derive from the overlying Zermatt-Saas ophiolite unit above, especially close to the contact. Concerning our hypothesis of mechanical pressure variations, one important feature needed includes the existence of rheological variations, interestingly mafic boudins within a metapelitic matrix may provide such rheological contrasts. Thank you for your question! Joshua
Submitted by Shah Wali Faryad on Tuesday 23rd June 15:01
Peak Pressure Determined from Staurolite-Chloritoid Metapelites in the Monte Rosa Nappe: Thermodynamic and Geodynamic Implications Vaughan Hammon J, Luisier C, Schmalholz S & Baumgartner L Not all minerals are mineral assemblages are suitable for PT estimate, mainly, if we are working with local equilibrium in a pseudomophs or mineral with specific composition like Zn staurolite, for which solution models are not clear. In your case, the field relations are clear and if the PT calculations are correct, they should be the same for both rocks. As granite is mostly dry, the reaction rate is slow and both new and relict phases can be present. Using of pseudosection for bulk rock composition is not appropriate.
Hello Shah. Yes, I agree with you that the presence of both reactants and products within the metagranite can create difficulties when analysing bulk compositions for a pseudosection approach. For the metagranite, Luisier et al. (2019) did not use the pseudosection approach, instead, two observations aided a pressure estimate of <1.6 GPa, these include: 1) the lack of jadeite and 2) Si in phengite barometry. I refer you to the paper of Luisier et al. https://www.nature.com/articles/s41467-019-12727-z. Thank you for your question! Joshua
Peak Pressure Determined from Staurolite-Chloritoid Metapelites in the Monte Rosa Nappe: Thermodynamic and Geodynamic Implications Vaughan Hammon J, Luisier C, Schmalholz S & Baumgartner L Not all minerals are mineral assemblages are suitable for PT estimate, mainly, if we are working with local equilibrium in a pseudomophs or mineral with specific composition like Zn staurolite, for which solution models are not clear. In your case, the field relations are clear and if the PT calculations are correct, they should be the same for both rocks. As granite is mostly dry, the reaction rate is slow and both new and relict phases can be present. Using of pseudosection for bulk rock composition is not appropriate.
Hello Shah. Yes, I agree with you that the presence of both reactants and products within the metagranite can create difficulties when analysing bulk compositions for a pseudosection approach. For the metagranite, Luisier et al. (2019) did not use the pseudosection approach, instead, two observations aided a pressure estimate of <1.6 GPa, these include: 1) the lack of jadeite and 2) Si in phengite barometry. I refer you to the paper of Luisier et al. https://www.nature.com/articles/s41467-019-12727-z. Thank you for your question! Joshua
Submitted by Joshua David Vaughan-Hammon on Wednesday 24th June 16:08
Hello Shah. Yes, I agree with you that the presence of both reactants and products within the metagranite can create difficulties when analysing bulk compositions for a pseudosection approach. For the metagranite, Luisier et al. (2019) did not use the pseudosection approach, instead, two observations aided a pressure estimate of <1.6 GPa, these include: 1) the lack of jadeite and 2) Si in phengite barometry. I refer you to the paper of Luisier et al. https://www.nature.com/articles/s41467-019-12727-z. Thank you for your question! Joshua
Hello Shah. Yes, I agree with you that the presence of both reactants and products within the metagranite can create difficulties when analysing bulk compositions for a pseudosection approach. For the metagranite, Luisier et al. (2019) did not use the pseudosection approach, instead, two observations aided a pressure estimate of <1.6 GPa, these include: 1) the lack of jadeite and 2) Si in phengite barometry. I refer you to the paper of Luisier et al. https://www.nature.com/articles/s41467-019-12727-z. Thank you for your question! Joshua
Submitted by Thomas Zack on Thursday 25th June 11:33
Hi Joshua, it remains unclear to me how overpressure can be sustained(!) in natural settings. I am not an experimental petrologist, but I do know that it is futile to have the slightest crack in your piston cylinder if you want to go to any elevated pressure. And if you even invoke free water, things get even more unreal to me. Well, this is for the time of an experiment (at most 30 days)- how about at geologic time scales? Perhaps this could be a discussion in the Discord channel- guess it is general enough for Theme discussion #4, but our #4h is appropriate as well.
Hello Thomas. The two end-member hypothesise we present require two mechanisms in order to sustain an over-pressurised state long enough for metamorphic reactions to take place. 1) stress induced mechanical overpressure will be sustained in a body, required that no dissipative processes occur i.e. viscous creep. So far, we have not observed any deformation within whiteschist bodies related to the high pressure phase that would relax these stresses. 2) reaction-induced mechanical overpressure will be sustained in a body, required that the surrounding medium (in our case lower pressure meta-granite) is impermeable, thus preventing the fluids produced by prograde dehydration from escaping. As stated within my presentation, it is most likely a mixture of the two mechanism. However, understanding the physics of these coupled processes (mechanics - chemistry - fluid flow) is still not fully understood. Thank you for your question! Joshua
Hi Joshua, it remains unclear to me how overpressure can be sustained(!) in natural settings. I am not an experimental petrologist, but I do know that it is futile to have the slightest crack in your piston cylinder if you want to go to any elevated pressure. And if you even invoke free water, things get even more unreal to me. Well, this is for the time of an experiment (at most 30 days)- how about at geologic time scales? Perhaps this could be a discussion in the Discord channel- guess it is general enough for Theme discussion #4, but our #4h is appropriate as well.
Hello Thomas. The two end-member hypothesise we present require two mechanisms in order to sustain an over-pressurised state long enough for metamorphic reactions to take place. 1) stress induced mechanical overpressure will be sustained in a body, required that no dissipative processes occur i.e. viscous creep. So far, we have not observed any deformation within whiteschist bodies related to the high pressure phase that would relax these stresses. 2) reaction-induced mechanical overpressure will be sustained in a body, required that the surrounding medium (in our case lower pressure meta-granite) is impermeable, thus preventing the fluids produced by prograde dehydration from escaping. As stated within my presentation, it is most likely a mixture of the two mechanism. However, understanding the physics of these coupled processes (mechanics - chemistry - fluid flow) is still not fully understood. Thank you for your question! Joshua
Submitted by Joshua David Vaughan-Hammon on Thursday 25th June 13:37
Hello Shah. Yes, I agree with you that the presence of both reactants and products within the metagranite can create difficulties when analysing bulk compositions for a pseudosection approach. For the metagranite, Luisier et al. (2019) did not use the pseudosection approach, instead, two observations aided a pressure estimate of <1.6 GPa, these include: 1) the lack of jadeite and 2) Si in phengite barometry. I refer you to the paper of Luisier et al. https://www.nature.com/articles/s41467-019-12727-z. Thank you for your question! Joshua
Hello Shah. Yes, I agree with you that the presence of both reactants and products within the metagranite can create difficulties when analysing bulk compositions for a pseudosection approach. For the metagranite, Luisier et al. (2019) did not use the pseudosection approach, instead, two observations aided a pressure estimate of <1.6 GPa, these include: 1) the lack of jadeite and 2) Si in phengite barometry. I refer you to the paper of Luisier et al. https://www.nature.com/articles/s41467-019-12727-z. Thank you for your question! Joshua
Submitted by Joshua David Vaughan-Hammon on Thursday 25th June 14:57
Hello Shah. Yes, I agree with you that the presence of both reactants and products within the metagranite can create difficulties when analysing bulk compositions for a pseudosection approach. For the metagranite, Luisier et al. (2019) did not use the pseudosection approach, instead, two observations aided a pressure estimate of <1.6 GPa, these include: 1) the lack of jadeite and 2) Si in phengite barometry. I refer you to the paper of Luisier et al. https://www.nature.com/articles/s41467-019-12727-z. Thank you for your question! Joshua
Hello Shah. Yes, I agree with you that the presence of both reactants and products within the metagranite can create difficulties when analysing bulk compositions for a pseudosection approach. For the metagranite, Luisier et al. (2019) did not use the pseudosection approach, instead, two observations aided a pressure estimate of <1.6 GPa, these include: 1) the lack of jadeite and 2) Si in phengite barometry. I refer you to the paper of Luisier et al. https://www.nature.com/articles/s41467-019-12727-z. Thank you for your question! Joshua
Submitted by Freya George on Thursday 25th June 15:20
Hi Joshua, great talk! This is along the lines of Thomas' comment, but do numerical and/or experimental studies tell us anything about how much overpressure can be sustained, and for how long? If the differences in the whiteschists are indeed related to mechanical effects, nearly 10 kbar of pressure seems quite remarkable, especially to not record any hints of it in the other lithologies! On a different note, I notice in your Assemblage Two (and as you've illustrated in your schematic) you have some pretty interesting and heterogenous garnet microstructures. Had any thoughts on those? Thanks!
Hello Freya. I am going to expand slightly more on the similar questions posed by yourself and Thomas during the webinar session. In the meantime I would encourage you to see my reply to Thomas. Concerning the garnet structure, they are very nice indeed but I have not investigated this any further. So far, no major element zoning patterns have been observed within these small 2nd generation garnets. Therefore, we think they may be some trapped inclusion of graphite, for example. Similar inclusion patterns can also be seen in the dissolution-precipitations rims of larger 1st generation garnets (seen in the same photomicrograph in the presentation). Thank you for your questions! Joshua
Hi Joshua, great talk! This is along the lines of Thomas' comment, but do numerical and/or experimental studies tell us anything about how much overpressure can be sustained, and for how long? If the differences in the whiteschists are indeed related to mechanical effects, nearly 10 kbar of pressure seems quite remarkable, especially to not record any hints of it in the other lithologies! On a different note, I notice in your Assemblage Two (and as you've illustrated in your schematic) you have some pretty interesting and heterogenous garnet microstructures. Had any thoughts on those? Thanks!
Hello Freya. I am going to expand slightly more on the similar questions posed by yourself and Thomas during the webinar session. In the meantime I would encourage you to see my reply to Thomas. Concerning the garnet structure, they are very nice indeed but I have not investigated this any further. So far, no major element zoning patterns have been observed within these small 2nd generation garnets. Therefore, we think they may be some trapped inclusion of graphite, for example. Similar inclusion patterns can also be seen in the dissolution-precipitations rims of larger 1st generation garnets (seen in the same photomicrograph in the presentation). Thank you for your questions! Joshua
Submitted by Joshua David Vaughan-Hammon on Thursday 25th June 15:24
Hello Shah. Yes, I agree with you that the presence of both reactants and products within the metagranite can create difficulties when analysing bulk compositions for a pseudosection approach. For the metagranite, Luisier et al. (2019) did not use the pseudosection approach, instead, two observations aided a pressure estimate of <1.6 GPa, these include: 1) the lack of jadeite and 2) Si in phengite barometry. I refer you to the paper of Luisier et al. https://www.nature.com/articles/s41467-019-12727-z. Thank you for your question! Joshua
Hello Shah. Yes, I agree with you that the presence of both reactants and products within the metagranite can create difficulties when analysing bulk compositions for a pseudosection approach. For the metagranite, Luisier et al. (2019) did not use the pseudosection approach, instead, two observations aided a pressure estimate of <1.6 GPa, these include: 1) the lack of jadeite and 2) Si in phengite barometry. I refer you to the paper of Luisier et al. https://www.nature.com/articles/s41467-019-12727-z. Thank you for your question! Joshua
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