Volatile Recycling at Tonga-Lau: A Global End-Member

Carbon-bearing sediments on the seafloor subduct into the deep earth. However, it is unclear how much carbon re-emerges at the surface in volcanic gases and eruptions and how much continues into the deep mantle. Many fundamental earth processes hang in the balance of this deep carbon cycle, such as the earth’s atmospheric evolution and the formation of diamonds, and yet the efficiency of this cycle is highly uncertain. This project will study the Tonga Islands, a location that is thought to output the least amount of carbon of any subduction zone. The Lau Basin, located behind the Tonga Islands is ideal for studying volatile recycling as it samples depths from 100 to 300 km. This is important, as carbon may be released at greater depths than water. Results will test recent predictions from models, calculations, and laboratory experiments and will apply to general carbon cycle models for other subduction zone systems. This project aims to measure the subducted carbon and water recycled to Tonga Arc and Lau Basin magmas using melt inclusions. Recent work has demonstrated how up to 90% of the CO2 in melt inclusions may be contained with their tiny shrinkage bubbles, which are rarely measured, and never have been for the Tonga arc. For the entire Lau Basin, there is a single published melt inclusion analysis with no CO2 measurement. Thus, the primary effort in this project is to experimentally rehomogenize melt inclusions and bubbles from the Tonga Arc and Lau back-arc using the piston cylinder apparatus and a technique developed at Lamont. All samples are available from existing collections. The Tonga islands are hypothesized to output the least amount of carbon of any subduction zone owing to the lack of carbon in subducting sediment input, a globally cold subducting slab that may not liberate substantial carbon beneath the volcanic arc, and a highly depleted mantle source. Preliminary data and calculations suggest that very little carbon recycles to the Tonga arc, but that some may emerge in the back-arc. This project will provide the first comprehensive estimates for CO2 (and H2O) for a paired arc-back-arc system. A major broader impact of this project is a Community Rehomogenization effort. The study of melt inclusions is undergoing a revolution, given the recognition that most of the 1000’s of published CO2 abundances are greatly under-estimated. Virtually every melt inclusion study now needs to be revisited in order to quantify the CO2 concentration in the shrinkage bubble. In order to meet this challenge, researchers will be invited to submit melt inclusions for experimental rehomogenization at Lamont, after which samples will be returned for analysis and publication. The goal is to generate 15-20 new datasets with restored CO2, prioritizing those that have coupled CO2/S volcanic gas records. This project will also support a PhD student.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.