Abstract Details
(2020) Comparative Zoning Studies in Titanite
Bonamici C
https://doi.org/10.46427/gold2020.219
The author has not provided any additional details.
04h: Room 1, Friday 26th June 22:18 - 22:21
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 Charles Magee on Monday 22nd June 11:55
Does the d18O value on the edges of the sphenes represent a move towards the value in high temperature equilibrium with the particular mineral adjacent to the sphene on that edge, towards the bulk rock d18O, or towards some cryptic value?
The grain boundary oxygen isotope values for sphene represent the equilibrium oxygen isotope fractionation at the closure temperature for oxygen diffusion in sphene. That closure occurred around 500-550 C in this particular rock. We can successfully model this with the Fast Grain Boundary model, which assumes that mass-balanced oxygen isotope equilibrium is maintained along the grain boundaries in a rock but not necessarily within grain interiors (which are subject to slower volume diffusion and trying to equilibriate with their grain boundaries).
Does the d18O value on the edges of the sphenes represent a move towards the value in high temperature equilibrium with the particular mineral adjacent to the sphene on that edge, towards the bulk rock d18O, or towards some cryptic value?
The grain boundary oxygen isotope values for sphene represent the equilibrium oxygen isotope fractionation at the closure temperature for oxygen diffusion in sphene. That closure occurred around 500-550 C in this particular rock. We can successfully model this with the Fast Grain Boundary model, which assumes that mass-balanced oxygen isotope equilibrium is maintained along the grain boundaries in a rock but not necessarily within grain interiors (which are subject to slower volume diffusion and trying to equilibriate with their grain boundaries).
Submitted by Matthias Konrad-Schmolke on Wednesday 24th June 15:36
Hi Chloe, I have a question regarding your statement that major elements were decoupled from the oxygen isotope zoning. If you expanded the compositional range in your diagrams, wouldn't the majors show compositional variations that spatially coincide with the areas where oxygen is zoned?
If I understanding your question, you are wondering if one expanded the y axis for some of the major element profiles whether you might not see a similar zoning profile shape in these elements to the oxygen. We've tried this but there is no correlation between most major elements in most grains and that does not change even if expanding the concentration axes to try to parse very small major element variations. The possible exception is Fe, which shows near-rim decreases in some grains that look like they *could* be diffusion profiles. We've not explored the diffusivity of Fe in our work. For reference, the graph symbol sizes for major element measurements on slide 5 are representative of their uncertainties.
Hi Chloe, I have a question regarding your statement that major elements were decoupled from the oxygen isotope zoning. If you expanded the compositional range in your diagrams, wouldn't the majors show compositional variations that spatially coincide with the areas where oxygen is zoned?
If I understanding your question, you are wondering if one expanded the y axis for some of the major element profiles whether you might not see a similar zoning profile shape in these elements to the oxygen. We've tried this but there is no correlation between most major elements in most grains and that does not change even if expanding the concentration axes to try to parse very small major element variations. The possible exception is Fe, which shows near-rim decreases in some grains that look like they *could* be diffusion profiles. We've not explored the diffusivity of Fe in our work. For reference, the graph symbol sizes for major element measurements on slide 5 are representative of their uncertainties.
Submitted by Andy Smye on Thursday 25th June 21:51
Hi Chloe, great talk. Some of your d18O profiles appear to have "steps" preserved in an otherwise error-function shape. Do you think these are the result of low-T deformation or heterogeneous diffusion? Thanks.
Some of the profiles have likely been modified by later deformation or recrystallization, which have either altered diffusivity within parts of the grain through a distributed network of dislocations or created fast pathways along nascent subgrain or twin boundaries. We have avoided using these profiles to model volume diffusion recognizing that they've been affected by other processes. That is why, on slide 6, I mention that the modeled profiles are a subset of the data and show the blue model curves for only one side of several profiles, rather than both.
Hi Chloe, great talk. Some of your d18O profiles appear to have "steps" preserved in an otherwise error-function shape. Do you think these are the result of low-T deformation or heterogeneous diffusion? Thanks.
Some of the profiles have likely been modified by later deformation or recrystallization, which have either altered diffusivity within parts of the grain through a distributed network of dislocations or created fast pathways along nascent subgrain or twin boundaries. We have avoided using these profiles to model volume diffusion recognizing that they've been affected by other processes. That is why, on slide 6, I mention that the modeled profiles are a subset of the data and show the blue model curves for only one side of several profiles, rather than both.
Submitted by Thomas Zack on Friday 26th June 17:51
Hi Chloe, I had a similar question to Dave regarding common Pb (in his case for apatite): could you potentially have variable common Pb compositions in your titanites? One might envision a scenario where titanite grew in a short time span, but with changing common Pb coming in via fluids/melts. This could create an apparent spread in ages as you observe. Not saying it should be this way- just worth considering.
Yes! We think that part of the dispersion in U-Pb data we see is due to diffusive Pb loss and part is due to a change in the composition of initial Pb incorporated into the titanite. The main effect of the initial Pb variation is to slightly change the slope of the U-Pb data array. We have a forthcoming paper in American Mineralogist that parses this out in detail (pre-print doi: 10.2138/am-2020-7274), if you care to read more.
Hi Chloe, I had a similar question to Dave regarding common Pb (in his case for apatite): could you potentially have variable common Pb compositions in your titanites? One might envision a scenario where titanite grew in a short time span, but with changing common Pb coming in via fluids/melts. This could create an apparent spread in ages as you observe. Not saying it should be this way- just worth considering.
Yes! We think that part of the dispersion in U-Pb data we see is due to diffusive Pb loss and part is due to a change in the composition of initial Pb incorporated into the titanite. The main effect of the initial Pb variation is to slightly change the slope of the U-Pb data array. We have a forthcoming paper in American Mineralogist that parses this out in detail (pre-print doi: 10.2138/am-2020-7274), if you care to read more.
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