Intellectual merit. The volatile components water, carbon, sulfur, chlorine, and fluorine affect magma rheology and differentiation, control processes of volcanic degassing and eruption, and influence the dissolution and transport of magmatic components in fluids that cause metasomatism and hydrothermal mineralization. The degassing of magmatic volatiles from volcanoes also fundamentally affects the geochemistry of the atmosphere and oceans - thus influencing Earth's climate. The efficacy of these processes varies directly with the abundances of volatiles. Despite extensive study of magmatic volatile components via analyses of trapped silicate melt inclusions and hydrous minerals in volcanic and plutonic rocks, and complementary experimental and theoretical research, current knowledge of the behavior and abundances of volatiles is insufficient for accurate modeling of fluid processes during magma evolution. In particular, we must better understand when volatile-rich magmatic fluids first exsolve, how their compositions change during magma evolution, and how these fluids accumulate in the apices of magma chambers to control magmatic processes and volcanic activities. The ubiquitous mineral apatite [Ca5(PO4)3(OH,F,Cl)] contains fluorine, chlorine, and hydroxyl ions as essential constituents and may also contain trace- to major-element levels of sulfur. If we have quantitative knowledge of element partitioning, apatite potentially can be used to monitor volatile contents in the magmas from which it forms. The proposed investigation aims to provide that essential calibration information via controlled experiments and detailed analysis of run products formed at known pressure, temperature, and melt composition. The ultimate goal is to develop apatite as a geochemical tool for: (1) estimating magmatic volatile contents at various stages of melt evolution (not always represented by presence of primary melt inclusions), (2) tracking the behavior of volatile components during progressive magma evolution, (3) elucidating the role of magmatic fluids in processes of degassing and volcanic eruption, and (4) improving thermodynamic models that can predict these exchange processes and apply to natural systems.
Broader Impacts. This project will likely result in development of powerful alternative methods for assessing magmatic volatile contents. The scientific results and their relevance to society will be conveyed to summer interns participating in an NSF-supported Research Experiences for Undergraduate students (REU) program at the American Museum of Natural History, and to teachers and the generall public via a lecture program (focused on volatiles and geologic processes) at the museum. The museum offers numerous other opportunities to share the results of scientific research with the public through electronic media in exhibition halls. The results will also be shared by training undergraduate students at the University of Maryland and by educating the public through an open-house event on Earth science at the University of Maryland.
Understanding the behavior of volatile compounds, such as H2O, CO2, SO2, HCl, and HF, in magmas – particularly for magmas that erupt explosively or are genetically involved in the formation of metallic mineral deposits – is fundamentally important in Earth science. The concentrations of these volatiles in magmas and their role in magmatic processes are difficult to determine because of the fugitive character of these volatile compounds when at high magmatic temperatures. Inclusions of silicate glass trapped in minerals have been analyzed and applied to better understand magmatic volatiles, because some of these glass inclusions are chemically representative of the melt in magmas. However, other glass inclusions are subject to problems that limit or preclude their use in this regard. The mineral apatite [Ca5(PO4)3(OH,Cl,F)] incorporates these same volatiles in its structure, and apatite is common to many magmatic rocks, so the chemical composition of apatite has been measured and used to better interpret processes of magma evolution, volcanic eruption, and formation of magmatic-hydrothermal ore deposits. To apply the composition of apatites present in natural magmatic rocks in order to understand the behavior of volatile compounds, however, one needs access to experimental or theoretical constraints that quantify how OH is distributed between apatite and melt and/or hydrothermal fluids at set conditions of pressure and temperature. The same constraints are necessary for F, Cl, S, and CO2. For this project, we have conducted laboratory experiments in which rhyolitic to phonolitic rocks were melted, pressurized, and allowed to react with hydrothermal fluids and microscopic grains of apatite for long periods of time. At the conclusion of these experiments, we analyzed the quenched glasses (which represent the melt), the apatite, and the fluids to determine how F, Cl, and OH are distributed between these phases as a function of varying pressure, temperature, and melt and fluid composition. The experimental results will provide a better understanding of magmatic systems, based on analyses of natural apatite grains and via interpretation of natural grains of apatite with our experimental data in future research. In the course of this project, we also determined how the solubilities of the volatiles H2O, CO2, S, Cl, and F in rhyolitic and phonolitic melts change as a result of strong variations in the composition of the coexisting fluid phases, in particular with two fluids (both a vapor phase and a saline brine phase) stable. These experiments were conducted at a single pressure of 2000 bars. The results provide important new constraints on the solubilities of these volatiles in melts and on how these volatiles are distributed between the melts and fluid phases. These data are useful for interpreting degassing processes in eruptive, volcanic magmas.
