Mantle source variations beneath the Eastern Lau Spreading Center and the nature of subduction components in the Lau basin-Tonga arc system

International audienc

Trace element constraints on the origin of subduction component in the Eastern Lau Back-arc Spreading center

International audienc

Characterizing the effect of mantle source, subduction input and melting in the Fonualei Spreading Center, Lau Basin: Constraints on the origin of the boninitic signature of the back-arc lavas

New major element, trace element and Pb-Sr-Nd isotope data for glasses from the Fonualei Spreading Center (FSC) constrain the genesis of back-arc basin basalts and the origins of boninites. The FSC is an end-member for global back-arc lavas in terms of low Ti 8.0 and Na 8.0, and contains lavas with a boninitic signature. Latitudinal variations reveal a correspondence in location between back-arc and adjacent arc volcanism. The locations of spikes in subduction input and positive bathymetric anomalies along the FSC correspond to the projected location of the arc volcanoes, likely reflecting 3-D convective structure of the mantle wedge. Non-mobile trace elements in arc and back-arc lavas reveal an increasing proportion northward of a re-enriched refractory mantle source, which is supported by isotope data. Quantitative modeling constrains the extents of melting, fraction of enriched mantle and subduction input. For the FSC, extents of melting are exceptionally large. We show a general relationship between extent of melting, subduction input and distance from the arc that applies to both the Eastern Lau Spreading Center and the FSC segment closer to the arc. In the center of the FSC where the arc volcanism is captured by the back-arc, exceptionally high subduction input and even greater extents of melting are observed, producing melts with boninitic signature. Boninitic samples require the juxtaposition of high subduction input and refractory mantle, leading to large integrated extents of melting, an occurrence that can be produced by multiple causes

Assimilation of the plutonic roots of the Andean arc controls variations in U-series disequilibria at Volcan Llaima, Chile

U-series disequilibria provide important constraints on the processes and time scales of melt production, differentiation, and transport in subduction settings. Such constraints, which are essential for understanding the chemical evolution of the continental crust, are conventionally based on the assumption that the U-series disequilibria measured in mafic lavas are produced during mantle metasomatism and melting, and that intracrustal differentiation and assimilation have limited impacts. Here we show that mantle-derived U-series disequilibria in mafic lavas erupted at Volcán Llaima, Chile are significantly diminished by assimilation of plutonic rocks forming Llaima's subvolcanic basement. This contamination process is extremely subtle in terms of “classical” indicators of crustal assimilation like Sr, Nd or Pb isotopes because it is a manifestation of assimilative recycling of the plutonic roots of the arc. This process results in variations in U-series disequilibria and incompatible trace element ratios that are significant compared to regional and global variability in arc magmas. Furthermore, it yields linear correlations between U-series excesses and incompatible trace element ratios that are generally interpreted as slab-fluid indicators and chronometers, or tracers of sediment recycling in subduction zone. Cannibalization of ancestral magmas by ascending melts warrants careful evaluation when considering the components and chemical fluxes in subduction zones. Linear arrays defined by activity ratios of U-series nuclides with different half-lives may be the most reliable indicators of assimilative recycling of ancestral intrusive magmas