| The H2O content of magmas is fundamental to their genesis, evolution, and eruption, but most models make predictions that have yet to be tested against actual H2O measurements. This is because few direct measurements of magmatic H2O exist, as rocks found at the surface have extensively degassed upon eruption. Olivine-hosted melt inclusions (MI), trapped at depth, provide an opportunity to measure volatiles in undegassed magma. Chapter One provides the first direct measurements of water in melts from Arenal volcano, Costa Rica. Analyses of olivine-hosted melt inclusions (MI) reveal higher water contents than expected (up to 4 wt.% H2O) given the relatively weak subduction signal (e.g. low 10Be and B) recorded geochemically.; To better predict magmatic water contents, a larger magmatic H2 O dataset is desired. However, many volcanic deposits do not contain MI large enough for analysis. Chapter Two describes a new method to calculate magmatic H2O from direct measurements of H2O in clinopyroxenes. In the volcanoes studied, magmatic H2O contents calculated from the clinopyroxene agree well with MI data from the same samples, thus demonstrating the viability of this new technique.; Chapter Three reports new MI data, and combines it with the MI and clinopyroxene-derived data to model the slab fluids contributing to Central American magmas. The data support the use of Ba/La (in melts) and Sr/Nd (in clinopyroxene) as water proxies. At least two fluids are at work beneath SE Central America. The first, dominant in Costa Rica, is relatively water-poor and highly saline, with low Ba/La, Sr/Nd. It may be derived in part from sediments associated with the Cocos Ridge. The second potential subduction fluid, dominant in Nicaragua, is relatively water-rich, with low salinity and high Ba/La, Sr/Nd. It may be derived, in part, from the breakdown of serpentinite in the down-going plate, and overlying pelagic sediments. The potential of combining clinopyroxene-derived H2O and trace elements with an expanded MI dataset creates a new and powerful tool in igneous petrology and the modeling of the fluids which generate subduction zone magmas. |
