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(2020) Equilibrium, Disequilibrium, and Crystal Growth in Basaltic Magmas

Hartley M, Arzilli F, Polacci M, La Spina G, Burton M, Le Gall N & Maclennan J


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05c: Plenary Hall, Friday 26th June 00:39 - 00:42

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Submitted by Margaret Hartley on Monday 22nd June 14:33
That's a great question. Yes, the textures are remarkably symmetrical and I suspect that we are observing a lot of heterogeneous nucleation on the alumina crucible walls. We could perhaps partially suppress this by doing the experiment in a Pt crucible, but then we would lose X-ray transparency. It is a tricky balance! I had been hoping to investigate heterogeneous nucleation effects by doing some statistical nearest-neighbour analysis on QEMSCAN images. I'd like to know if there is a difference in the nucleation delay and/or the crystal shape in heterogeneously vs homogeneously nucleated crystals. Unfortunately the QEMSCAN work has been postponed during the lockdown so I don't have any answers yet! Thanks for the pointer to the lunar experiments - I will take a closer look at those.

Submitted by LeeAnn Srogi on Friday 26th June 00:17
Hi, Margaret - fascinating work and a great talk! I looked up the experimental work in the Corsaro reference, and they ran experiments at QFM and olivine crystallized as well as plag and cpx. Would you say you observed suppression of olivine crystallization? And how much effect do you think the oxygen fugacity has on your experimental results? Thanks! LeeAnn
Hi LeeAnn, thanks for your question! We haven't managed to crystallize olivine in any of the experiments. So yes, I do think that crystallization of olivine is suppressed. The experiments I showed in the talk are run in air, so extremely oxidising conditions, whereas Etna magmas have typical fO2 around NNO+1. I think this has a huge effect on the results. We get far more oxides crystallizing than the natural samples. In some more recent experiments we have tried to control the fO2 by adding a nickel disc, and this seems to help, although the experiment charges have too little glass for us to be able to confirm the fO2 conditions by EPMA or XANES. But even in these experiments we didn't grow olivine.

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