Experimental determination of boron isotope fractionation between silicate melts and aqueous fluids, with application to understanding magmatic-hydrothermal ore genesis

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This proposal will address a critical gap of understanding the formation of ore deposits related to granites and pegmatites: what is the role of the magmatic-hydrothermal transition? Theory predicts that boron isotopes should fractionate between silicate melt and fluid at this transition, and empirical studies suggest that tourmaline can record this shift in natural rocks. However, the information needed to validate this is lacking because there is no current agreement on the extent of B-isotope fractionation between granitic melts and aqueous fluids derived from them. The research proposed here aims to resolve this question by conducting experimental studies of B-isotope exchange between fluids and synthetic granitic melts with variable compositions and controlled pressure-temperature conditions that are realistic for natural systems. The investigations will include: a) time-series experiments tested with SIMS B-isotope profiling to establish attainment of equilibrium exchange, b) B-isotopic analysis of quenched melt and fluid by TIMS, c) determining the coordination number of boron in compositionally variable synthetic melts using NMR spectroscopy and d) complementing the experimental work with molecular dynamic simulations of boron speciation in melt and fluid phases. Application of the results to natural ore-forming systems will be done by re-interpreting the abundant existing data from studies of B-isotope variations in granite- and pegmatite-related ore deposits from the Potsdam SIMS laboratory and other published sources, where tourmaline was suspected to have crystallized across the magmatic-hydrothermal transition.