238U-230Th-226Ra disequilibria constraints on the magmatic evolution of the cumbre vieja volcanics on la palma, Canary Islands

S. Turner, K. Hoernle, F. Hauff, T. S. Johansen, A. Klügel, T. Kokfelt, Craig Campbell Lundstrom

Research output: Contribution to journalArticle

Abstract

A suite of 48 samples, including both historical and prehistoric lavas and some plutonic rocks, have been analysed from the Cumbre Vieja rift, La Palma, Canary Islands. Additionally, mineral-melt partition coefficients have been measured for clinopyroxene, plagioclase, amphibole, titanite and apatite in selected rocks. The lavas range from basanite to phonolite (SiO2 = 41·2-57·5 wt % and MgO = 10-0·8 wt %) in composition and form coherent, curvilinear major and trace element arrays in variation diagrams, irrespective of eruption age. The mafic lavas have typical ocean island incompatible trace element patterns and Sr, Nd and Pb isotope compositions show little variation but have a HIMU-type character. Generation of the parental magmas is inferred to have involved ~4% dynamic melting of a garnet lherzolite source that may have previously been metasomatized by melts derived from a recycled mafic component containing residual phlogopite. The major process of differentiation to phonotephrite involved fractional crystallization of basanitic magmas that evolved along the same liquid line of descent under similar pressure-temperature conditions. Numerical simulations using the MELTS algorithm suggest that this occurred across a temperature interval from c. 1320 to 950°C at 400 MPa and an oxygen fugacity equivalent to quartz-fayalite-magnetite (QFM), with an initial H2O content of 0·3 wt %. The later stages of differentiation (<5 wt % MgO) were dominated by mixing with partial melts of young syenites formed from earlier magma batches. All of the lavas are characterized by 230Th and 226Ra excesses and (230Th/238U) decreases with decreasing Nb/U and increasing SiO2, with no accompanying change in (226Ra/230Th). To explain the observations, we propose a model in which there was a significant role for amphibole, and more importantly accessory titanite, in decre'asing Nb/U, Ce/Pb and Th/U ratios and increasing or buffering (226Ra/230Th) ratios during the later stages of differentiation and magma mixing. These processes all occurred over a few millennia in small magma batches that were repeatedly emplaced within the mid-crust of the Cumbre Vieja rift system prior to rapid transport to the surface.

Original languageEnglish (US)
Pages (from-to)1999-2024
Number of pages26
JournalJournal of Petrology
Volume56
Issue number10
DOIs
StatePublished - Dec 17 2014

Keywords

  • La Palma
  • magmatic timescales
  • ocean island basalt evolution
  • titanite fractionation

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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