THE NATURE AND SOURCE OF MAJOR MAGMATIC VOLATILES: OPEN-VENT DEGASSING VOLCANOES IN THE CENTRAL AMERICAN VOLCANIC ARC

Major volatiles play an important role in subduction zone magmatism, from magma generation in the mantle, to crustal ascent and evolution, until its dramatic expression during a volcanic eruption. In the attempt to add a piece of information on source and behavior of major volatiles in arc volcanism, I here report on the geochemistry of H2O, CO2, S and Cl in two volcanic systems in NW Nicaragua (Central America), San Cristóbal and Telica. The observational approach is based upon combination of some of the most recent techniques in volcanic gas monitoring and petrology. I here explore quiescent and eruptive degassing processes by combining both volcanic gas and melt inclusion approaches. Field gas measurements were primarily obtained via the Multi-Component Gas Analyzer system (Multi- GAS) technique, complemented with additional remote sensing observations via open-path Fourier Transform IR (OP-FTIR) spectroscopy, ultraviolet differential optical absorption spectroscopy (UV-DOAS). These techniques together allowed characterizing compositions and fluxes of CO2, H2O, S and Cl in the central crater plumes of both quiescent degassing volcanoes. These measurements contribute to our understanding of the current structure and state of the shallow plumbing systems that feed the surface gas emissions. The targeted San Cristóbal and Telica volcanoes are also investigated by characterizing, for the first time, the volatiles contents (CO2, H2O, S and Cl, F) in their melt inclusions. Using a series of laboratory instrumental facilities, such as electron microprobe, micro FTIR spectroscopy, Raman spectroscopy and Nano secondary ions mass spectroscopy (NanoSIMS), I show that olivine- and pyroxene-hosted melt inclusions can reveal crucial information on preeruptive magma conditions at both San Cristóbal and Telica. I propose that polybaric degassing and crystallization models can suitably describe major volatile abundance and behavior during magmatic evolution. These results are synthesized in 3D illustrative sketches that represent the crustal structure of San Cristóbal and Telica volcano-magmatic systems. Water is the primary agent during magmatic fragmentation and strongly influences eruptive style. CO2, the second most abundant volatile in magmas, is generally more difficult to quantify in melt inclusion studies, but its role in volcanic processes is revealing more and more important as volcanological research advances. I therefore spend major attention in this study to charactering the carbon content in San Cristóbal and Telica primitive magmas. CO2, apart from being an excellent chemical tracer of magmatic degassing processes, is one of the most important triggers of explosive eruptions in basic to intermediate magmas. The volatile contents of Central American magmas show wide regional-scale variations. It is shown here that gas signature (carbon abundance) of Central American volcanic gases correlates with petrological tracers of magma compositions, and specifically with the trace element proxies of slab-fluid contribution to the mantle wedge. This brings novel constraints on carbon source in volcanic arc regions. I also show, using noble gas isotope compositions of fluid inclusions in olivine-pyroxene crystals, that a common mantle fluid signature persists along the entire Nicaragua volcanic segment. Controls on magma generation are, instead, better reconstructed using incompatible trace elements; these data reveal that the degree of mantle melting and sedimentary fluid incorporation are higher in Nicaragua than in other sectors of the arc. We find that local variations exist also at the scale of a single magma system. These variations, at different spatial and temporal timescales, prove that regional compositional heterogeneities exist underneath volcanic arcs.