Sequestration of Halogens in the Sub-continental Lithospheric Mantle: Implications for Global Element Cycling

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The halogen elements (chlorine, fluorine, iodine, and bromine) play a fundamental role in modifying the properties of fluids and minerals on Earth. For example, the addition of halogens to fluids can cause minerals to break down at different temperatures and pressures, as well as, increase the ability of the fluid to transport of metals. Halogens are enriched in surface reservoirs (e.g., sedimentary pore fluids, sediments) compared to the Earth's mantle. The subduction of these reservoirs brings halogens into the mantle. Halogens can then be returned to the Earth's surface at arc volcanoes. However, mass balance constraints suggest that more halogens are subducted to the mantle than are returned to the Earth's surface via arc volcanism. These observations lead most researchers to suggest that an excess of halogens are being continually delivered to the mantle. This imbalance as implications for the long-term chemical evolution of the Earth's surficial reservoirs and mantle. However, several halogen reservoirs remain unquantified and may account for this disconnect. Here it is proposed that halogens may be sequestered in the sub-continental lithospheric mantle (i.e., the upper mantle below the base of the continental crust) and thus not accounted for in calculations. By quantifying the concentrations of halogens in the sub-continental lithospheric mantle, the overall halogen budget can be constrained. In addition to the intellectual impacts of improved understanding of elemental transfer through subductions zones, the broader impacts involve training of graduate and undergraduate students and participation in GeoFORCE, a summer program that engages outstanding students from select low performing minority-serving high schools in STEM fields.This work will constrain the effects of subduction on the subcontinental lithospheric mantle halogen budget and the extent to which the subcontinental lithospheric mantle may act as a major halogen reservoir and control the overall halogen budget by "isolating" halogens from the subduction zone cycle for potentially long periods of time. This work will also provide insights as to how subduction processes modify the halogen concentrations and ratios in material returned to the convecting mantle. Bulk halogen concentrations, as well as, in situ halogen concentrations of hydrous (e.g., chlorite, amphibole, antigorite) and anhydrous minerals (e.g., pyroxene, olivine) of mantle xenoliths from the western United States that have been geochemically modified by the subducting Farallon slab will be measured. These data will be compared with other lithophile tracers of subduction input and melt depletion. The subduction-influenced xenoliths will be compared to non-subduction-modified mantle xenoliths and abyssal peridotites, which will serve as a baseline for halogen content in the convecting mantle.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.