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The Role of Undercooling on Trace Element Partitioning in Sector-Zoned Clinopyroxene

MacDonald A, Ubide T, Masotta M, Mollo S & Pontesilli A

MacDonald A, Ubide T, Masotta M, Mollo S & Pontesilli A (2020) Goldschmidt Abstracts, 2020 1693

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05d: Room 2, View in program

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 Georg F. Zellmer on
Hi Alice, Very interesting talk, and the results of your work will become very important in the future when looking at volcanic eruptions, particularly the geochemical features in microphenocrysts and microlites. I am particularly interested in what you call "Al-rich skeletons", referring to the stripe in one of your crystals. I am wondering how that stripe comes about, i.e. how this stripe can be considered a skeleton. Is this simply an Al-rich acicular crystal that formed rapidly and than overgrew on the two long sides with more Si-rich material? If so, why did it not overgrow also on the short side, the tip of the crystal? And how come this "skeleton" was not preserved but ended up as a euhedral/subhedral crystal upon eruption? Did the degree of undercooling change during its growth? Was its growth initiated under high degrees of undercooling that then decreased during melt ascent, e.g. due to the release of latent heat of crystallization? Thanks! Georg
Hi Georg! Thanks for the question. The Al-enriched ‘stripe’ or skeleton we interpret to have formed first, with relatively high amounts of highly charged cations such as Ta (as seen in the maps.) This would have been followed by later infilling of an Al-depleted, Si-enriched overgrowth that is relatively depleted in trace elements. This mechanism is described in detail in my collaborators paper, Alessio Pontesilli, where he observed this phenomenon in time-series experiments on the same experimental composition. We don’t think the stripe is a microlite, as we analysed more crystals within the same sample and found that the stripe feature was more likely to be due to the cut of the crystal presented, as most of the other crystals have stripes that make up hopper-style skeletons. These features are not observed in naturally occurring Etnean clinopyroxenes as the degree of undercooling experienced by the magmas at Etna is considerably lower, with Etnean clinopyroxenes often exhibiting sector zoning which in our experiments is only observed at relatively low degrees of undercooling. Thank you again for your question and please don’t hesitate to contact me if you have any more questions or queries!

Submitted by Maurizio Petrelli on
Hi Alice, thanks for sharing your results. Do you think that integrating your results with more experimental data, you could develop a general relation between the parameters of the lattice strain model and the degree of undercooling?
Hi Maurizio, thanks for the question. Yes, it's interesting that Do and partition coefficients have a clear correlation with undercooling, yet the Youngs modulus and r0 didn't exhibit any systematic change with undercooling in these experiments. However, my collaborators and I have discussed doing more experiments and collecting more data to observe whether this behaviour changes at different conditions. Feel free to contact me if you have any other questions.

Submitted by Alice MacDonald on
Hi Maurizio, thanks for the question. It's definitely a possibility, although the Youngs modulus and r0 didn't exhibit any systematic change with undercooling in these experiments, my collaborators and I have discussed doing more experiments and collecting more data to observe whether this behaviour changes at different conditions. Feel free to contact me if you have any other questions.


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