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(2020) Forensic Geochronology of an Explosive-Effusive Transition: Ascension Island

Scarrow JH, Schmitt AK, Danisik M, Montero P, Preece KJ, Davies BV, Brown RJ, Mark D & Barclay J


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05d: Room 2, Friday 26th June 06:18 - 06:21

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Submitted by Georg F. Zellmer on Monday 22nd June 05:14
Hi Jane, this appears to constitute some very time-intensive zircon dating work, and it is beautiful to see remobilization of plutonic fragments in these eruptions. The last sentence of your abstract indicates rapid magma ascent/emplacement/cooling, too fast to completely reset the He systematics of the remobilized zircon "antecrysts". I was hoping for some more quantitative timing on your poster, but there is no additonal information on that. Rapid ascent through tectonism, and uptake of the crystal cargo from pre-existing intrusives, seems to be a recurring theme these days, and getting a handle on the ascent rate of the initially aphyric or sparsely phyric melts represented by the glass/groundmass is quite important. Does your data provide any insights to this aspect of the study? Thanks! Georg
Hello Georg, Thank you for your interest in our work. We are currently working to refine information about the ascent rate. What we see in the data we have at present is that He systematics of older, pre-existing, zircon crystals were not reset in the lower explosive pumice, but were apparently partially reset in deposits of the upper part of the sequence. Unfortunately the lava breccia near the top of the sequence gave a very low zircon yield. So, we are going to process more sample to obtain zircon from this, and the overlying baked pumice capping the sequence, for further analysis: U-Th disequilibrium (crystallisation age) and (U-Th)/He (cooling below ~200ºC). Once we have the new data we should be able to model how long zircon would have to be held at given temperatures to produce the measured range of (U-Th)/He ages. Clearly the constraint is the time-temperature relationship needed to reset the He in the zircon. At temperatures estimated for the felsic magma, ~770ºC-1000ºC (Chamberlain et al. 2019, J. Pet., 60, 1489–1522. doi: 10.1093/ petrology/egz037) this may be a matter of days (Martin Danisik, pers. comm.). Other work by our research group, (Chamberlain et al. 2020, Volcanica, 3, 139–153. doi: 10.30909/vol.03.01.139153) also suggest very fast ascent rates, ~24 hours, from preservation of compositional bimodality in the glass matrix of mingled magmas for which injection of mafic magma was apparently the eruptive trigger. Best regards, Jane

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