Arc–Backarc Exchange Along the Tonga–Lau System: Constraints From Volatile Elements

Abstract

Abstract Analyses of olivine-, pyroxene- and plagioclase-hosted melt (now preserved as glass) inclusions (MI) in submarine glasses and subaerial scoria from seven volcanoes along the volcanic front of the Tonga arc provide the first comprehensive dataset including volatile contents (H2O, CO2, S, Cl and F) for this arc. Maximum water contents at each volcano are typical (3–5 wt %) of other arc volcanoes worldwide, and within each volcano, water generally correlates with sulfur, consistent with degassing but not diffusive re-equilibration. The Tonga arc is notable for the eruption of magmas sourced from strongly depleted upper mantle, including boninites in the northern half of the active arc. A key question has been whether such boninites are derived from high degrees of melting actively occurring under the arc, driven by high mantle temperatures and water contents, or from high integrated degrees of melting including melting of depleted mantle in the backarc. This study supports the latter view, based on sub-arc mantle melt fractions of 17–23%, calculated from primary melt water contents and mantle-melt thermobarometry. Such fractions are not high enough to generate boninites from a typical, fertile mantle source in a single melting stage, and melt fractions do not increase north of 22°S, where the boninitic magmas appear. Instead, the northern Tonga arc boninites reflect high cumulative degrees of melting (>30%, with respect to fertile mantle) and occur along the volcanic front where adjacent backarc spreading is fast, proximal and mature. Slab tracers such as the Ba/La ratio peak at around 20–21°S, in the central part of the Tonga arc. A Ba/La peak also appears at about the same latitude in the Lau backarc basin. This peak is consistent with the coolest slab temperatures beneath the arc and backarc at this latitude, as the slab warms to the north due to an increase in mantle potential temperature, and to the south due to less heat extraction from melting. A new concept developed here is cooling of the mantle wedge by melting, due to the supply of the heat of fusion and melt removal. Such a process can cool the mantle by more than 100 degrees, and thus affect slab heating as well. We find a critical distance of slab influence in the Lau spreading centers; back-arc magmas erupted >70 km from the arc are >210 km above the slab, beyond the point of slab dehydration, and show no enrichment in Ba/La. The coupled dynamics of the Tonga arc-Lau basin are rich in their connections, and demonstrate how melting processes beneath the back-arc and arc can affect slab processes, which in turn can affect the composition of fluids that feed the arc and backarc.