MARGINS: Central American Magmatic Volatile Histories as Recorded by Apatite Phenocrysts

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)." Abstract This project will undertake an experimental study to calibrate volatile (for H, Cl, and F) solubilities in the mineral apatite, and to apply the resulting 'barometer' to investigate volatile contents in magmas erupted from four volcanoes in the Central American volcanic arc (CAVA). The work will provide independent constraints on pre-eruptive magmatic volatile contents and their bearing on melt generation and eruptive processes. Intellectual Merit. The ability of apatite to record magmatic processes affecting volatile elements has been demonstrated for a wide range of magmatic compositions. Apatite is especially valuable where other monitors of volatile evolution (such as melt inclusions) are not available, or as a companion to melt inclusions, because it offers temporal information via core-to-rim relationships, diffusion profiling, and isotope geochronology. Measurements of H, F, and Cl in apatite phenocrysts have been used to infer complex pre-eruptive processes in the magma chamber that produced the ~1000 km3 Cerro Galan ignimbrite (Boyce and Hervig, 2008a) in NW Argentina. More recently, Boyce and Hervig (2008b) showed that apatite phenocrysts from the 1723 and 1963 eruptions of Volcan Irazú, Costa Rica record a volatile history compatible with that recorded in olivine- and clinopyroxene-hosted melt inclusions from the same samples. It is proposed to extend this work of by collecting and characterizing apatite-bearing volcanic rocks from four volcanoes in CAVA. Volatile histories have previously been characterized for three of these volcanoes (Irazú, Arenal, and Masaya), making it possible to directly test apatite volatile barometry. The fourth (Concepcion) has units with large, abundant apatite, making it an ideal target for this study. Because these volcanoes span much the southern half of the active portion of CAVA, it will be possible to compare volatile distributions in apatite to other regional-scale geochemical observations. A series of piston-cylinder experiments will be conducted to equilibrate volatile solubility in apatite and representative melts and to develop an empirical functional relationship describing the relation between apatite and melt compositions. Additional experiments will be designed to determine the sensitivity of apatite volatile barometry to pressure, temperature, and other factors. Finally, the volatile barometer will be applied to determine pre-eruptive H, F, and Cl contents in selected magmas. Measurements of H, Cl, and F in apatite will be made by secondary ion mass spectrometry (SIMS) with a spatial resolution at the micron-scale. Broader Impacts. This research will advance the utility of a developing technique for determining pre-eruptive magmatic H2O (as well as F, and Cl) contents. As volatiles play a major role in both local and global hazards stemming from volcanic eruptions, improving constraint on the pre-eruptive volatile histories of CAVA volcanoes will lead to better long-term hazards estimates. The project will support the PI, an early-career scientist, in developing a research program while seeking a permanent position in academia. The project is being undertaken with the full cooperation of local scientists in Costa Rica and Nicaragua, and will help to build a long term, mutually beneficial cooperative relationship between scientists in Central America and the USA. The project will also involve an undergraduate researcher (Kristina Walowski), and will provide mentoring experience for the PI. The results of this study will be disseminated at meetings and through peer-reviewed publications.