The historical basanite - alkali basalt - tholeiite suite at Lanzarote, Canary Islands: Carbonated melts of heterogeneous mantle source?

Abstract

For the last three decades recycled oceanic lithosphere has been proposed as the principal mantle source for mafic magma erupting on oceanic islands. However, the composition of this lithosphere, as well as how and what part of it melts, is a matter of debate. Several melting experiments show that a mafic pyroxenite/eclogite lithology melts at higher pressure than that of ultramafic peridotite/lherzolite. In addition, the role of carbonate recycling and a carbon-rich fluxing agent producing carbonated silicate melt is increasingly being discussed. The historical Timanfaya eruption (1730–1736) at Lanzarote, Canary Islands, produced a range of mafic magmas that evolved with time from basanite trough alkali basalt to silica-saturated tholeiite, whereas only basanite was erupted in the smaller 1824 eruption. Mantle nodules are common and most lavas and tephra are characterised by abundant olivine macrocrysts. Variable trace element ratios of whole-rock tephra (Rb/Th: 3.3–7.7; La/Yb: 12–44; Nb/Ta: 16–21), and more especially those of olivine-bearing melt inclusions (Rb/Th: 2.5–12; La/Yb: 9–77; Nb/Ta: 14–22) suggest rapid melt extraction from a heterogeneous mantle source beneath the island. Increasing (Nb,Ta)/Th, Zr/Hf and decreasing Nb/Ta from basanite through alkali basalt to tholeiite reflect decreasing carbon-rich fluxing on a time scale of less than a few years. Increasing K, Rb, Cs, and Ba over Th with time during the Timanfaya eruption most likely result from diminishing phlogopite control during the production of increasingly silica-saturated magma. Highly variable melt inclusion compositions are compatible with fractional melting of carbonated pyroxenite, with the whole-rock composition most likely representing the amalgamation of these fractional melts at the asthenosphere-lithosphere boundary beneath the eastern Canary Islands.