Skip to content

Alkaline magma generation and differentiation between proximal ocean island shield volcanoes: Insights from the Karthala and La Grille plumbing system(s), Grande Comore

^^ François J.P. Lötter^^ 1,2, Andrea Di Muro3, Nicolas Villeneuve1,2,4, Nils Lenhardt^5 ^ and Pascale Besson2

  • Affiliations: 1Laboratoire de Géosciences, Université de La Réunion, Saint Denis, France; 2Institut de Physique du Globe de Paris, Université de Paris, Paris , France; 3Laboratoire de géologie de Lyon (LGL-TPE), Université Claude Bernard Lyon1, Lyon, France; 4Observatoire Volcanologique du Piton de la Fournaise (OVPF), Institut de Physique du Globe de Paris (IPGP), Bourg Mourat ,France; 5Department of Geology, University of Pretoria, Pretoria, South Africa 

  • Presentation type: Poster

  • Presentation time: Friday 16:30 - 18:00, Room Poster Hall

  • Poster Board Number: 20

  • Programme No: 1.5.27

  • Theme 1 > Session 5

Abstract

Alkaline volcanism within ocean islands has long been associated with melts derived from a metasomatised mantle. Trace element ratios record compositional attributes of the mantle and preferentially melted metasomatised veins. The island of Grande Comore hosts La Grille and Karthala, two proximal, active alkaline volcanoes with differences in source characteristics, whole rock composition, and eruption styles. Metasomatic veins have been reported within xenoliths of La Grille lavas in addition to modal metasomatism of the mantle source inferred from trace elements. In contrast, Karthala's primitive lavas show only minor input from a modally metasomatised mantle. This study employs major and trace elements, Sr-Nd isotopes, and single mineral chemistry to elucidate the plumbing system by characterising the formation and differentiation of the alkaline products from the two volcanoes. Initial findings identify a new, third primitive magma series within Grande Comore, distinguishable by SiO2, HFSEs, and LREEs. Preliminary trace element modelling of La, Dy, and Yb in mantle residues, negative anomalies of K, and normalised incompatible element patterns characterised by HREE depletion indicate the influence of a metasomatised, garnet-bearing lherzolite-derived melt as at least one component for each primitive magma series. XRD analyses of aphanitic tephras trace the unique mineral assemblages that are associated with these distinct magma series, further defining the evolution of these compositional suites. These preliminary findings, in conjunction with single mineral chemistry of clinopyroxenes and clinopyroxene-based barometry, will detail the plumbing system beneath Grande Comore and provide new insights into small-scale mantle heterogeneities beneath ocean islands.