Contributions to Mineralogy and Petrology

, Volume 165, Issue 3, pp 601–622

Formation of U-depleted rhyolite from a basanite at El Hierro, Canary Islands

Authors

    • Laboratoire Magmas et VolcansCNRS-Université Blaise Pascal-IRD
    • NordVulk, Institute of Earth SciencesUniversity of Iceland
  • Didier Laporte
    • Laboratoire Magmas et VolcansCNRS-Université Blaise Pascal-IRD
  • Marion Carpentier
    • Laboratoire Magmas et VolcansCNRS-Université Blaise Pascal-IRD
  • Bertrand Devouard
    • Laboratoire Magmas et VolcansCNRS-Université Blaise Pascal-IRD
  • Jean-Luc Devidal
    • Laboratoire Magmas et VolcansCNRS-Université Blaise Pascal-IRD
  • Joan Marti
    • Institute of Earth Sciences ‘Jaume Almera’ CSIC
Original Paper

DOI: 10.1007/s00410-012-0826-5

Cite this article as:
Sigmarsson, O., Laporte, D., Carpentier, M. et al. Contrib Mineral Petrol (2013) 165: 601. doi:10.1007/s00410-012-0826-5

Abstract

Phonolite and trachyte are the felsic magmas of the alkaline magma suites, which characterize the Canary Islands. The October 2011 submarine eruption off El Hierro, the westernmost island, nevertheless, produced a small volume of rhyolitic magma. The rhyolite occurred as highly vesicular, white coloured pumices enveloped in and mingled with darker coloured basanitic pumice. The basanitic pumice is relatively crystal poor with a few euhedral olivines (mostly Fo77–79), clinopyroxenes and Fe-rich spinels, whereas very rare olivine of same composition is found together with equally rare Fe-sulphide and FeTi-rich oxides in the rhyolite. The Fe–Mg exchange equilibrium in the oxides permits to calculate an equilibrium temperature of 970–890 °C for the rhyolite, in agreement with quartz-melt equilibrium at ca. 930 °C. A striking mineralogical feature of the rhyolite is the presence of rounded to contorted grains of milky quartz, which are xenocrysts incorporated and partly dissolved into the magma. Analyses of residual volatile concentrations in the glasses show that the rhyolite melt was highly degassed, whereas the basanitic glass still has important halogen concentrations. Trace element patterns of the mafic glasses and their elevated incompatible element concentrations are typical of the western Canary Island basanites. In contrast, the trace element composition of the rhyolite shows surprisingly low concentrations for all elements except the most incompatible ones (e.g. Rb, Ba, K and Th). All other measured LILE, HFSE and REE have significantly lower concentration than the basanitic counterpart that can be explained by fractionation of accessory phases (1 % apatite, 1 % sphene and 0.1 % zircon). Surprisingly, low U concentration is presumably related to elevated oxygen fugacity in the rhyolite, causing U to be in a hexavalent state, and fluxing of F-rich gas leading to volatilization of UF6, known to emanate at low temperature. The results suggest that a gas-rich basanitic melt remobilized a small volume of stagnant rhyolitic melt formed by incorporation of approximately 10 % quartz-rich sediment into a late differentiate of trachytic composition. Sediments at the interface of an old oceanic crust adjacent to a continental shield and younger volcanic island are likely to act as magma traps were sediment assimilation may alter the mantle-derived magma composition. Quartz assimilation thus explains the production of rhyolite magma in a volcanic island characterized by an alkaline magma series from primitive basanites to trachytes.

Keywords

El Hierro Quartz Magma series Oxygen fugacity Accessory phases U volatilization

Supplementary material

410_2012_826_MOESM1_ESM.pdf (1.6 mb)
Supplementary material 1 (PDF 1591 kb)
410_2012_826_MOESM2_ESM.xls (54 kb)
Supplementary material 2 (XLS 53 kb)

Copyright information

© Springer-Verlag Berlin Heidelberg 2012