Abstract Details
(2020) Wet or Oxidizing? Deciphering the Dominant Determinant of Calc-Alkaline Differentiation Using a Monte Carlo MELTS Methodology
Pitcher B, Gualda G & Ghiorso M
https://doi.org/10.46427/gold2020.2090
The author has not provided any additional details.
05d: Room 2, Friday 26th June 22:18 - 22:21
Bradley Pitcher
View abstracts at 2 conferences in series
Guilherme A. R. Gualda View all 5 abstracts at Goldschmidt2020 View abstracts at 4 conferences in series
Mark Ghiorso View all 3 abstracts at Goldschmidt2020 View abstracts at 3 conferences in series
Guilherme A. R. Gualda View all 5 abstracts at Goldschmidt2020 View abstracts at 4 conferences in series
Mark Ghiorso View all 3 abstracts at Goldschmidt2020 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 Emily Johnson on Friday 26th June 05:04
Interesting research and talk! I'm curious about the Tholeiitic parent that can yield a more-calc alkaline liquid line of descent (given the right fo2, H2O): what is the biggest difference you see in the crystallizing phases in the MELTS runs (compared to the low fo2, low H2O conditions) that explains this?
Great question, Emily. I have some interesting figures that would answer your question. I will show those during the Q&A. If you aren't able to make it for the Q&A, please shoot me an email at: bradwpitcher@gmail.com. I would love to discuss further! I think this work has interesting implications for the Cascades!!!
Interesting research and talk! I'm curious about the Tholeiitic parent that can yield a more-calc alkaline liquid line of descent (given the right fo2, H2O): what is the biggest difference you see in the crystallizing phases in the MELTS runs (compared to the low fo2, low H2O conditions) that explains this?
Great question, Emily. I have some interesting figures that would answer your question. I will show those during the Q&A. If you aren't able to make it for the Q&A, please shoot me an email at: bradwpitcher@gmail.com. I would love to discuss further! I think this work has interesting implications for the Cascades!!!
Submitted by Marco Viccaro on Friday 26th June 11:50
Hi Brad, nice to meet here :-) Thank you for your interesting talk! Looking at radiogenic isotopes for some CA and TH magmas from the same settings, they suggest generation from compositionally distinct sources together with totally not comparable degrees of hydration in the source. How can you reconcile the striking different isotopic signatures with the distinct oxidative conditions experienced during differentiation?
Great question, Marco! You are totally right that in many cases the "CA-like" basalts and "TH-like" primitive basalts of a given arc or single volcano may have vastly different isotopic signatures indicating different sources. I am choosing to not even discuss the processes that lead to the formation of the different primitive basalt endmembers. However, I am saying that a High FeO/MgO (and high ASI) parent can evolve along a more strongly CA LLD than a low Fe parent if placed in slightly oxidizing conditions and with a minor amount of added H2O. (Higher Fe in the "TH-like" parent leads to earlier crystallization of spinel and thus a more Fe-depleted CA trend). In addition, given a CA-like (low Fe, low ASI) basalt parent, you will never evolve along a strongly TH LLD regardless of H2O or fO2 conditions. Granted, it may be difficult to even form a TH-like High Fe, high Al basaltic parent with >1% H2O and oxidizing conditions.
Hi Brad, nice to meet here :-) Thank you for your interesting talk! Looking at radiogenic isotopes for some CA and TH magmas from the same settings, they suggest generation from compositionally distinct sources together with totally not comparable degrees of hydration in the source. How can you reconcile the striking different isotopic signatures with the distinct oxidative conditions experienced during differentiation?
Great question, Marco! You are totally right that in many cases the "CA-like" basalts and "TH-like" primitive basalts of a given arc or single volcano may have vastly different isotopic signatures indicating different sources. I am choosing to not even discuss the processes that lead to the formation of the different primitive basalt endmembers. However, I am saying that a High FeO/MgO (and high ASI) parent can evolve along a more strongly CA LLD than a low Fe parent if placed in slightly oxidizing conditions and with a minor amount of added H2O. (Higher Fe in the "TH-like" parent leads to earlier crystallization of spinel and thus a more Fe-depleted CA trend). In addition, given a CA-like (low Fe, low ASI) basalt parent, you will never evolve along a strongly TH LLD regardless of H2O or fO2 conditions. Granted, it may be difficult to even form a TH-like High Fe, high Al basaltic parent with >1% H2O and oxidizing conditions.
Submitted by LeeAnn Srogi on Friday 26th June 19:59
Hi, Brad - great presentation and really interesting work! i love the approach of mapping out the effects of variables using MELTS. For your Rhyolite-MELTS runs, what was your Temperature step and did you model fractional crystallization or equilibrium crystallization? And am i correct that you constrained f(O2) to a single buffer (e.g., QFM) throughout? I've also been starting a MELTS run at a specified buffer at the Liquidus, then removing the buffer constraint and allowing f(O2) to vary according to the phase equilibria during the run (internal buffering?). What do you think of the variable f(O2) approach, and would it be worth trying to see its effect on your dataset?
Hello LeeAnn, Thank you for the questions. (1) I used 1 degree C temperature steps, and everything that I showed in the talk were equilibrium fractionation MELTS runs. The next step for this project will be to model using FC (I will reluctantly need to use a Mac, unfortunately). (2) You are correct: each MELTS run was at a single constrained fO2 buffer (which ranged from FMQ-1 to FMQ+3, at 0.5 log unit increments). Very early on in this project, I did a test of the effect of running "unconstrained fO2". At first glance, overall LLDs didn't change significantly enough for me to make that a priority at the time. Furthermore, at the time I was only looking to see if the unbuffered runs deviated significantly from observed data and never looked at the effect on THI of the LLDs. However, you have inspired me to take a closer look and really tease out differences between buffered and unbuffered. Thank you! I would be happy to talk with you further, and I would love to hear about your project. My email is: bradwpitcher@gmail.com
Hi, Brad - great presentation and really interesting work! i love the approach of mapping out the effects of variables using MELTS. For your Rhyolite-MELTS runs, what was your Temperature step and did you model fractional crystallization or equilibrium crystallization? And am i correct that you constrained f(O2) to a single buffer (e.g., QFM) throughout? I've also been starting a MELTS run at a specified buffer at the Liquidus, then removing the buffer constraint and allowing f(O2) to vary according to the phase equilibria during the run (internal buffering?). What do you think of the variable f(O2) approach, and would it be worth trying to see its effect on your dataset?
Hello LeeAnn, Thank you for the questions. (1) I used 1 degree C temperature steps, and everything that I showed in the talk were equilibrium fractionation MELTS runs. The next step for this project will be to model using FC (I will reluctantly need to use a Mac, unfortunately). (2) You are correct: each MELTS run was at a single constrained fO2 buffer (which ranged from FMQ-1 to FMQ+3, at 0.5 log unit increments). Very early on in this project, I did a test of the effect of running "unconstrained fO2". At first glance, overall LLDs didn't change significantly enough for me to make that a priority at the time. Furthermore, at the time I was only looking to see if the unbuffered runs deviated significantly from observed data and never looked at the effect on THI of the LLDs. However, you have inspired me to take a closer look and really tease out differences between buffered and unbuffered. Thank you! I would be happy to talk with you further, and I would love to hear about your project. My email is: bradwpitcher@gmail.com
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