Lithium Isotope Cycling in Subduction Zones: The Tonga Island arc/back-arc System

We carried out a Li isotope study for the Tonga arc system to understand how Li is cycled in subduction zones. We characterized the INPUT of the subducting plate as well as the magmatic OUTPUT in terms of Li isotope composition (reported in ‰ relative NIST 8545). INPUT: Altered basalts from ODP sites 595 have Li contents between 12 and 18 ppm and range in δ7Li from +3.4 to +5.9‰. Sediments (DSDP/ODP sites 204 and 596) show a huge range in Li contents (12 to 152 ppm) and isotope compositions (+1.4 to +10.1‰). Most sediments have more than 40 ppm Li and the volcanoclastics from the Louisville seamount chain are isotopically heavy (+3.9 to +10.1‰). Fluid phases coming off the subducting slab, regardless if aqueous fluids or silicate melts, should be destinctly higher in δ7Li than a MORB like mantle. OUTPUT: Basalts and basaltic andesites from the main Tonga arc have Li contents between 3.7 and 9.3 ppm and δ7Li values ranging from +3.5 to +8.4‰. Samples from the southern Tonga arc have MORB-like Li isotope compositions, whereas samples from the northernmost islands have higher δ7Li values. Submarine basalts and basaltic andesites from the Tonga back-arc (Lau basin and Valu Fa Ridge) are MORB-like and extremely uniform in δ7Li (+2.8 to +3.6‰). The δ7Li values of magmas generated by back-arc volcanism behind the Tonga arc do not vary along strike and therefore we conclude that their Li isotope composition is not effected by the slab-fluid input, which increase to the south (high B/Be and Ba/Nb at the Valu Fa Ridge). If 7Li-rich aqueous slab fluids would be involved in triggering melting beneath the Valu Fa Ridge this lavas should have δ7Li values significantly higher than fresh MORB, but this is not the case. The uniformal MORB like δ7Li values of the Tonga back-arc lavas call for fluid-mantle interaction with selective Li-uptake in the mantle during ascent of the slab-fluid. Our data support the interpretation of Tomascak et al. (2002, EPSL) that initially 7Li-rich slab-fluids lose their Li slab-signature before triggering mantle melting beneath the arc/back-arc, due to Li-substitution in Mg-silicates. Li isotope data of arc lavas are therefore not suitable to trace mass transfer by aqueous slab-fluids or to prove if Li isotope fractionation is important at sub-arc depths. Most of the Tonga arc δ7Li values overlap the range of published values for fresh MORB, except the samples from the northernmost Tonga arc islands. Along arc δ7Li values are lowest in the south and highest in the north and show an apparent negative correlation with Nd isotope ratios. As Nd is thought to be relatively immobile during dehydration low 143Nd144Nd ratios coupled with high δ7Li values are explained by involvement of slab derived melt, most likely of the sedimentary cover of the subducted plate. However, the Li-Nd relationship could also reflect a systematic change in the composition and/or amount of the subducted sediments (Louisville component) or a south north change in the sub-arc mantle source. δ7Li values of Tonga arc and back-arc lavas imply that the mantle source and slab derived melts are the controlling reservoires and processes for Li isotope compositions of arc magmas in general. Direct mass transfer of Li from the slab into the suprasubduction zone crust occurs solely via melts and not via aqueous slab-fluids.