Abstract Details
(2020) Fe-Mg Zoning in Olivine during Rapid Growth: Kinetic Effects are Small, Zoning Nearly Flatlines
Shea T & Mourey A
https://doi.org/10.46427/gold2020.2354
The author has not provided any additional details.
05c: Plenary Hall, Friday 26th June 00:45 - 00:48
Thomas Shea
View all 7 abstracts at Goldschmidt2020
View abstracts at 6 conferences in series
Adrien Mourey View all 3 abstracts at Goldschmidt2020 View abstracts at 6 conferences in series
Adrien Mourey View all 3 abstracts at Goldschmidt2020 View abstracts at 6 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 Marc Norman on Monday 22nd June 02:46
Nice talk, well presented. Just curious as to where the concept of tree-ring zoning came from? It's been clear for a long time (e.g., Donaldson 76 CMP) that olivine morphology and composition is a complex function of crystal growth. One of the challenges is relating olivine morphologies and compositions to volcanic setting - would you have any thoughts on this, e.g., for evolution of Hawaiian volcanoes?
Many thanks for the questions Marc. The concept of tree-ring zoning for olivine is still ingrained in the literature. Authors studying P zoning that looks concentric in nature for instance, interpret growth as a sequence of added layers to a core (e.g Milman-Barris et al. 2008). For concentric growth-related Fe-Mg zoning, see for instance the excellent paper on sectioning effects by Pearce (1984). Donaldson's seminal work on olivine growth did not actually look at compositional effects in the crystals. Textural development and the development of olivine morphotypes as a function of undercooling/cooling rate, superheating was the focus of these remarkable studies (Donaldson 1975, 76; more recently the works of Francois Faure and Pierre Schiano). He even looked at the formation of boundary layers in the melt, truly before his time (Donaldson 1979). The way I think about it these days: if olivine grows in a skeletal fashion, than growth zoning would also have a skeletal-looking element distribution between rapidly grown cores and slowly matured rims. As for your second point - if I understood correctly - it's another good question that is difficult question to answer. Morphology isn't necessarily indicative since it evolves so much through cycles of olivine growth, maturation, and potential resorption-regrowth. From my own experience, I can say that skeletal crystals are more abundant in OIB and MORB for some reason than they are in Arc basalt. We speculated in Mourey and Shea (2019) that this may have to do with water content and the faster diffusivity of elements in the melt, essentially requiring larger undercooling to produce the same boundary layers in Mg (and thus initiating skeletal growth).
Nice talk, well presented. Just curious as to where the concept of tree-ring zoning came from? It's been clear for a long time (e.g., Donaldson 76 CMP) that olivine morphology and composition is a complex function of crystal growth. One of the challenges is relating olivine morphologies and compositions to volcanic setting - would you have any thoughts on this, e.g., for evolution of Hawaiian volcanoes?
Many thanks for the questions Marc. The concept of tree-ring zoning for olivine is still ingrained in the literature. Authors studying P zoning that looks concentric in nature for instance, interpret growth as a sequence of added layers to a core (e.g Milman-Barris et al. 2008). For concentric growth-related Fe-Mg zoning, see for instance the excellent paper on sectioning effects by Pearce (1984). Donaldson's seminal work on olivine growth did not actually look at compositional effects in the crystals. Textural development and the development of olivine morphotypes as a function of undercooling/cooling rate, superheating was the focus of these remarkable studies (Donaldson 1975, 76; more recently the works of Francois Faure and Pierre Schiano). He even looked at the formation of boundary layers in the melt, truly before his time (Donaldson 1979). The way I think about it these days: if olivine grows in a skeletal fashion, than growth zoning would also have a skeletal-looking element distribution between rapidly grown cores and slowly matured rims. As for your second point - if I understood correctly - it's another good question that is difficult question to answer. Morphology isn't necessarily indicative since it evolves so much through cycles of olivine growth, maturation, and potential resorption-regrowth. From my own experience, I can say that skeletal crystals are more abundant in OIB and MORB for some reason than they are in Arc basalt. We speculated in Mourey and Shea (2019) that this may have to do with water content and the faster diffusivity of elements in the melt, essentially requiring larger undercooling to produce the same boundary layers in Mg (and thus initiating skeletal growth).
Submitted by Timothy Hampel on Monday 22nd June 15:44
Great presentation Tom, really intriguing. I have a few questions 1) In your presentation, you mentioned that the kinetic effects to be considered were boundary layers enriched and depleted in elements. Are their other kinetic effects to be considered in this research and incorporated into the model? 2) Have you noted these same kinetic effects (esp. boundary layer enrichment or depletion) in other minerals, not just olivine, in these rocks? 3) With respect to the olivines in this research, I understand that Fe and Mg would be the primary focus, but are trace elements also being considered in these boundary layers, especially since you noted that there is still disequilibrium occurring even if the olivine is unzoned?
Hi Timothy, thanks for asking those questions. (1) Other kinetic effects: not that I can think of. It has been proposed that anomalous (i.e. disequilibrium) compositions in the crystal itself can arise not just from depleted/enriched boundary layers but that there is a growth rate above which compositionally anomalous layers can get trapped in the growing lattice on the atomic scale Growth entrapment model, e.g. Watson and Liang 1995). It's a competition between diffusion of certain element in these growing layers crystal and growth. So it's possible that it's not just the boundary layer on the macroscopic scale that influences the composition of the growing olivine. But much harder to demonstrate/observe processes at the interface scale than the macroscale, particularly compositional changes. (2) Kinetic effects in other minerals: In these charge, there is only olivine growing. I haven't notice boundary layers preserved very often around plag or cpx phenocrysts in natural samples (around microcrysts, yes). (3) Yes, in fact we recently published as study of two trace elements (P and Al) with quite different behaviors during rapid olivine growth (Shea et al. 2019 CMP). Have not yet looked at other trace elements than those two though.
Great presentation Tom, really intriguing. I have a few questions 1) In your presentation, you mentioned that the kinetic effects to be considered were boundary layers enriched and depleted in elements. Are their other kinetic effects to be considered in this research and incorporated into the model? 2) Have you noted these same kinetic effects (esp. boundary layer enrichment or depletion) in other minerals, not just olivine, in these rocks? 3) With respect to the olivines in this research, I understand that Fe and Mg would be the primary focus, but are trace elements also being considered in these boundary layers, especially since you noted that there is still disequilibrium occurring even if the olivine is unzoned?
Hi Timothy, thanks for asking those questions. (1) Other kinetic effects: not that I can think of. It has been proposed that anomalous (i.e. disequilibrium) compositions in the crystal itself can arise not just from depleted/enriched boundary layers but that there is a growth rate above which compositionally anomalous layers can get trapped in the growing lattice on the atomic scale Growth entrapment model, e.g. Watson and Liang 1995). It's a competition between diffusion of certain element in these growing layers crystal and growth. So it's possible that it's not just the boundary layer on the macroscopic scale that influences the composition of the growing olivine. But much harder to demonstrate/observe processes at the interface scale than the macroscale, particularly compositional changes. (2) Kinetic effects in other minerals: In these charge, there is only olivine growing. I haven't notice boundary layers preserved very often around plag or cpx phenocrysts in natural samples (around microcrysts, yes). (3) Yes, in fact we recently published as study of two trace elements (P and Al) with quite different behaviors during rapid olivine growth (Shea et al. 2019 CMP). Have not yet looked at other trace elements than those two though.
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