Collaborative Research: Halogen Behavior In the Pluton-to-Volcanic Arc System

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Halogen elements (fluorine, chlorine, bromine, and iodine) are abundant, highly reactive elements in the atmosphere, oceans, and lithosphere. Halogens within Earth’s crust can affect magmatic properties and carry elements to form economic mineral deposits. Within volcanic arc systems, halogen elements are carried to depth by subducting crust and can travel within melts through the overriding crust and out at volcanoes. Estimates of halogen inputs at subduction zones and outputs through volcanic gases show a deficit that suggest a portion of halogens are trapped deep within the Earth. There are multiple possible reservoirs to host halogens that are not well characterized because they are too difficult to access and sample. One example is the magmatic system under volcanoes where concentrations of halogens in subsurface magma chambers and erupted volcanic rocks are not well known. This team will measure the full halogen suite in intrusive plutonic rocks and extrusive volcanic rocks from the same ancient arc system now exposed in the Sierra Nevada, California. This will allow them to address key open questions including: whether halogens are enriched in the plutonic magma chamber or erupted volcanic rock; where halogens are hosted within the rocks; and what impact halogens have on magmatic processes and mineral deposits at volcanic arcs. Through this research, the principal investigators and graduate students will design an undergraduate mineralogy and petrology class with accessibility, equity, and inclusion principles and engage in undergraduate students in research and safe, inclusive field experiences. A subset of the samples used for this study come from an historic collection. These samples will be digitized using 3D technology and created into an educational resource hosted on the Virtual Microscope public website.Halogen movement and reservoirs within the deep Earth are highly uncertain due to the lack of data from key reservoirs, namely, the fore-arc, deeply subducted slab residues, subcontinental mantle, and – the focus of this study – continental crust. Given halogen incompatibility in the mantle and affinity towards more evolved melts, continental crust is a potentially important reservoir; however, bulk rock compositions of plutonic and volcanic arc rocks are relegated to a handful of studies. This research will characterize the full halogen (Fl, Cl, Br, I) composition and behavior within an arc magmatic system by analyzing a suite of spatially-related plutonic and volcanic rocks that share similar chemical composition and age from the Sierra Nevada Arc. Specifically, this team will measure bulk rock halogen (Cl, F, Br, I) concentrations, in-situ mineral concentrations (F, Cl), and O and H-isotopes on a suite of geochemically well-characterized samples. This sample suite will be used to 1) determine whether halogens are concentrated in the plutonic or volcanic section within a single arc system, 2) identify where halogens are hosted (i.e., in minerals or other sites), and 3) assess the role of halogens in magmatic processes during the formation of continental crust. In order to interpret the role of processes, such as assimilation and hydrothermal alteration, on halogen behavior, they also propose to analyze a complimentary suite that includes lower crustal xenoliths, host wall rocks (e.g., schists, marbles), hydrothermally altered rocks and associated veins, and more mafic lithologies (e.g., gabbros) to strengthen our interpretations. The dataset will significantly contribute to what is known of bulk rock halogen compositions for continental arc crust, and be used with major element, trace element, and isotopic compositions to evaluate the impact of primary magmatic processes (e.g., crustal assimilation, fractional crystallization) on the halogen record and interpret overall halogen systematics through the arc magmatic system.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.