Project Title: Volatiles in Tonga Arc Magmas
PI: T.Plank, BU
Project Summary (NSF Funded Project)
Intellectual Merit. Volatile species (e.g., H2O, CO2, SO2, Cl, F) are central to the origin of magmas associated with subduction zones. Water affects not only the physics of subduction and convection, but also drives mantle melting beneath arc volcanoes and material recycling from the downgoing plate. This proposal seeks to develop the first comprehensive study of the volatile content of Tonga arc magmas. The Tonga arc provides a unique opportunity to test hypotheses regarding the 1) composition of slab fluids, 2) their distribution in the mantle, and 3) their effect on melting. The convergence rate increases dramatically northward along the arc, where it reaches a global maximum. At the same time, sediment flux to the trench is near the global minimum, so the Tonga arc is perhaps the best place to test current models for the composition of fluid derived from subducted oceanic crust, without the complications of sedimentary components. The increase in convergence rate along-strike leads to an increase in fluid flux from the slab, which should manifest in variations in composition and/or volume of volcanism along-strike. The abundant magmatism in the backarc, at varying distances from the trench, is one of the major reasons for the selection of the adjacent Lau Basin as a RIDGE2000 (R2K) integrated study site. Back-arc data already show across-strike gradients in H2O and other slab tracers that have yet to be measured at their (presumed) maximum expression in the arc. Finally, the higher-than average temperature of the Lau basin mantle, and the high predicted water inputs should lead to very high integrated mantle melt fractions beneath the Tonga arc. While certain trace element and isotopic systematics are consistent with this view, these ideas have yet to be tested with actual water measurements linked to measures of mantle melt fraction. This project will attack these problems by measuring volatile concentrations in melt inclusions (by ion and electron probe), combined with major and trace element compositions (by electron probe and laser-ICPMS) in order to develop quantitative relationships between slab fluid compositions and mantle melting progress. This project follows from the PI's participation (through her Ph.D. student, Lauren Cooper) on a research cruise to the Northern Tonga arc (NoToVE), led by Richard Arculus of the Australian National University. The cruise sampled submarine volcanoes, which, by volume, are the dominant form of volcanism at the Tonga arc. The submarine volcanics contain abundant olivine, which is otherwise rare in subaerial samples, but critical to preserving are currently in-hand from the NoToVE cruise, as well as a prior cruise to the south (TELVE), providing continuous coverage of the 1000 km long Tonga arc. This project is a thematic extension of the PI's prior work studying volatiles and mantle melting at the Mariana arc, and complements on-going work in Central American and the Aleutians.
Broader Impact: Volatile cycling is a major theme in earth science, and results from this work will impact science problems in tectonics, petrology, and the deep mantle. While not precisely within one of the R2K or MARGINS focus areas, study of the Tonga arc would clearly add value to on-going R2K activities in the Lau Basin, as well as contribute substantially to the volatile-flux theme in the MARGINS Subduction Factory Science Plan. This project also includes continuing efforts to develop a precise hygrometer, which is essential to obtaining widespread magmatic water estimates, in the absence of olivine-hosted melt inclusions. Another outcome will be a comprehensive phenocryst dataset, which is poorly represented in current databases. This project supports the Ph.D. thesis of a female graduate student, and her training her in state-of-the-art microbeam techniques. This project also leverages infrastructure and shiptime costs contributed from Australian sources.