On the Time Scales of Magma Genesis, Melt Evolution, Crystal Growth Rates and Magma Degassing in the Erebus Volcano Magmatic System Using the 238U, 235U and 232Th Decay Series

• Published in: Journal of Petrology
• Main Authors: Sims, Kenneth W.W., Pichat, Sylvain, Reagan, Mark K., Kyle, Philip R., Dulaiova, Henrietta, Dunbar, Nelia W., Prytulak, J., Sawyer, Georgina, Layne, Graham D., Blichert-Toft, Janne, Gauthier, Pierre-Jean, Charette, Matthew A., Elliott, Timothy
• Other Authors: Wyoming High-Precision Isotope Laboratory, Department of Geology and Geophysics, University of Wyoming (UW), Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement Lyon (LGL-TPE), École normale supérieure - Lyon (ENS Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Department of Geosciences, University of Iowa Iowa City, Department of Earth and Environmental Science Socorro, New Mexico Institute of Mining and Technology New Mexico Tech (NMT), Department of Geology and Geophysics, University of Hawai‘i Mānoa (UHM), Department of Atmospheric, Oceanic and Planetary Physics Oxford (AOPP), University of Oxford Oxford, School of Earth Sciences Bristol, University of Bristol Bristol, Department of Geography Cambridge, UK, University of Cambridge UK (CAM), Department of Earth Sciences St. John's, Memorial University of Newfoundland = Université Memorial de Terre-Neuve St. John's, Canada (MUN), Laboratoire Magmas et Volcans (LMV), Observatoire de Physique du Globe de Clermont-Ferrand (OPGC), Université Blaise Pascal - Clermont-Ferrand 2 (UBP)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Blaise Pascal - Clermont-Ferrand 2 (UBP)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Jean Monnet Saint-Étienne (UJM)-Centre National de la Recherche Scientifique (CNRS), Department of Marine Chemistry and Geochemistry (WHOI), Woods Hole Oceanographic Institution (WHOI)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2013
• Subjects: Erebus volcano; U-series isotopes; HIMU; Antarctica; magma chamber residence time; open-system crystallization; magma degassing; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; [SDE.MCG]Environmental Sciences/Global Changes; Antarc*
• Online Access: https://hal.archives-ouvertes.fr/hal-01066877; https://doi.org/10.1093/petrology/egs068

International audience We investigate the time scales of magma genesis, melt evolution, crystal growth rates and magma degassing in the Erebus volcano magmatic system using measurements of 238U-230Th-226Ra-210Pb-210Po, 232Th-228Ra-228Th and 235U-231Pa-227Ac. These are the first measurements of 231Pa-227Ac in volcanic samples and represent the first set of data in a volcanic system to examine the entire suite of relevant 238U, 235U and 232Th decay series nuclides. Our sample suite consists of 22 phonolite volcanic bombs, erupted between 1972 and 2005, and five anorthoclase megacrysts separated from bombs erupted in 1984, 1989, 1993, 2004 and 2005. The 238U-230Th, 230Th-226Ra and 235U-231Pa systems are uniform over the 34 years examined. The anorthoclase megacrysts and phonolite glasses show complementary 226Ra/230Th disequilibria with (226Ra/230Th) ∼40 in the anorthoclase and ∼0*75 in the phonolite glass. In all samples, (210Pb/226Ra) is in radioactive equilibrium for both phases. In two phonolite glass samples (227Ac/231Pa) is unity. For the phonolite glasses (228Ra/232Th) is in equilibrium, whereas in the anorthoclase megacrysts it is significantly greater than unity. Instantaneous crystal fractionation, with magma residence times greater than 100 years and less than 10 kyr, can account for the measured 238U-230Th-226Ra-210Pb and 235U-231Pa-227Ac. However, the significant 228Ra/232Th disequilibria in the anorthoclase megacrysts preclude this simple interpretation. To account for this apparent discrepancy we therefore developed an open-system, continuous crystallization model that incorporates both nuclide ingrowth and decay during crystallization. This open-system model successfully reproduces all of the measured 238U and 232Th disequilibria and suggests that the shallow magma reservoir at Erebus is growing. The implication of this modeling is that when the time scale of crystallization is comparable with the half-life of the daughter nuclide of interest (e.g. 226Ra) the simple isochron techniques typically ...