HR: 08:45h
AN: V51A-04    [PDF]
TI: Boron isotope fractionation during slab dehydration and across-arc $\delta$$^{11}$B variations
AU: * Rosner, M
EM: rosner@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Trumbull, R
EM: bobby@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Erzinger, J
EM: erz@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Franz, G
EM: Gerhard.Franz@berlin.de
AF: Technische Universitaet Berlin, Institut fuer Angewandte Geowissenschaften, Ernst-Reuter-Platz 1, Berlin, 10587 Germany
AB: Boron is one of the most suitable elements to study the fluid driven mass transfer from the subducted slab into arc magmas due to its mobility in aqueous fluids, incompatibility during most magmatic processes and large compositional differences among the main geological reservoirs. Across-arc variations in B-isotopes from western Pacific arcs can be interpreted as mainly the result of a decreasing amount of slab-derived B with a constant AOC-like composition over the arc profiles relative to a constant mantle contribution. However, we present a new model for fluid-mineral B-isotope fractionation that explains $\delta$$^{11}$B across-arc variations by B-isotope fractionation during progressive slab dehydration alone. A combination of the fractionation model with a temperature model for the Central Andean subduction zone fits the observed across-arc variations of $\delta$$^{11}$B in volcanic rocks from the Central Andes as well as literature data for slab restites and fore arc fluids from other settings. We conclude that the B-isotope composition of arc magmas in general is dominated by changing $\delta$$^{11}$B composition of B-rich slab fluids as a result of progressive dehydration during subduction. In contrast to most previous studies, we deny a major role of mantle-derived B, because concentrations in the mantle are too low to affect the B budget of arc volcanics significantly. Our modeled slab restite compositions also offer an explanation for the negative $\delta$$^{11}$B values of OIB, due to recycling of $^{11}$B-depleted subducted slabs into the OIB magma source. A major uncertainty of all B-models is the composition of the input to the subduction zone. We assumed that the AOC chiefly contributes B to the slab-fluid, but as shown by numerous studies, several other major lithologies contribute to subduction zone fluids. In particular, the importance of subducted sediments and serpentinized peridotite within or above the slab is key to refining models of B-isotope fractionation and mass transfer in subduction zones.
DE: 1010 Chemical evolution
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting