Project Title: Magmatic Water along the Central American Arc
PI: T.Plank, BU
Project Summary (NSF-MARGINS Funded Project)
Intellectual Merit. The water content of arc magmas affects every aspect of their evolution, from their origin as wet mantle melts, to their fractionation in the crust, and final eruption. Arc magmas also serve as primary constraints on physical models of water fluxes through the subduction zone. A mature suite of geochemical and geophysical models for the Central American margin now bear testing against water measurements in arc magmas. This proposal aims to provide that primary data set for magmatic water contents for Costa Rican volcanoes. Existing models for the Central American arc call for greater extents of water recycled to Nicaraguan than Costa Rican volcanoes, based on 1) an abundance of outer rise faults seaward of Nicaragua that may hydrate and serpentinize the downgoing plate prior to subduction, 2) anomalously low seismic velocities in the Nicaragua slab that may reflect greater initial states of hydration, 3) near global maxima in slab tracers such as Ba/La and 10Be in Nicaragua volcanoes, 4) anomalously low d18O in Nicaragua olivines, and 5) petrological parameters in Nicaraguan magmas that may reflect higher degrees of water-fluxed mantle melting. Ongoing seismic and geochemical studies will no doubt refine these predictions, and lead to new ones, but a common theme is the requirement of water contents in volcanic output as quantitative tests. Preliminary work (some by the PI's group of students) shows variations along-strike in magmatic water contents from Nicaragua to Costa Rica. In this project, we will exploit what has become a successful technique for measuring volatile concentrations (H2O, CO2, S, Cl, F) in olivine-hosted melt inclusions, by ion and electron probe. Another major focus, partly driven by the lack of abundant olivine in Costa Rican tephra, will be the development of new techniques, one a hygrometer based on the rare earth element pattern recorded in clinopyroxenes, and the other based on the H2O concentration in clinopyroxenes. With these techniques we hope to contribute to a comprehensive dataset for water contents along the Costa Rica-Nicaragua arc sector, including approx. 20 arc volcanoes. This will enable rigorous testing against the existing geochemical gradients, as well as results of seismic imaging.
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, and most directly, primary themes within the MARGINS Subduction Factory Initiative (see below). This project also includes continuing efforts to develop a precise hygrometer, which may enable widespread magmatic water estimates in the absence of olivine-hosted melt inclusions. The impetus behind this project, and the major effort expended, is in support of the Ph.D. thesis of a female graduate student.
MARGINS Relevancy: This project squarely addresses one of the three primary science goals of the MARGINS Subduction Factory Initiative: The Volatile Cycle. The current lack of constraints on water contents in Costa Rica magmas is a large hole in the Central American focus area, one that this projects seeks to fill. The PI is actively collaborating with petrologists measuring other geochemical and volatile species, as well as geophysicists developing images of the slab, mantle and crust along the Central American arc. Finally, the PI's experience with the Mariana arc system will permit integration across the two Subduction Factory focus areas.