HR: 08:30h
AN: V11C-03    [Abstracts]
TI: Metasomatic Mechanisms for Fractionation of Slab-Derived B Isotope Signatures Within the Mantle Wedge: Evidence From HP-Metasomatized Peridotites
AU: * King, R L
EM: robbie@lehigh.edu
AF: Earth and Env. Sciences, Lehigh Univ., 31 Williams Drive, Bethlehem, PA 18015 United States
AU: Savov, I P
AF: Dept. of Geology, Univ. of South Florida, 4204 E. Fowler Ave, SCA 528, Tample, FL 33620 United States
AU: Bebout, G E
AF: Earth and Env. Sciences, Lehigh Univ., 31 Williams Drive, Bethlehem, PA 18015 United States
AU: Tonarini, S
AF: Istituto di Geoscienze e Georisorse, Area di Ricerca di Pisa Via Moruzzi 1, Pisa, I-56127 Italy
AU: Ryan, J G
AF: Dept. of Geology, Univ. of South Florida, 4204 E. Fowler Ave, SCA 528, Tample, FL 33620 United States
AB: We present B and $\delta^{11}$B data for metasomatic zones within exotic peridotite blocks of the Franciscan Complex, CA, that further constrain the behavior of this valuable tracer in subduction zones. These blocks preserve a series of metasomatic zones (serpentinite/talc schist/tremolite schist) imposed by Si-enriched hydrous fluids and are thought to be analogous to those developed in the mantle wedge adjacent to the slab-mantle interface. $\delta^{18}$O data indicate that these zones equilibrated with a single fluid at temperatures of $450-500\deg$C; this metasomatism is akin to that producing serpentinite seamounts in the Mariana subduction zone, although at higher temperature and with greater extents of reaction progress. Incipiently serpentinized protoliths contain 10-50ppm B, which overwhelms the likely initial $\delta^{11}$B of these peridotites. $\delta^{11}$B for these least-altered peridotites ranges from +6 to +8$\permil$, which agrees well with estimates for slab-derived fluid $\delta^{11}$B for the temperature recorded by these samples. Serpentinites formed after peridotite contain ~10-80ppm B, and increasing B concentrations exhibit a marked positive correlation with serpentine $\delta^{18}$O and whole-rock SiO$_{2}$ indicating that progressive serpentinization correlates with increasing B concentrations. However, as B contents increase, $\delta^{11}$B becomes negative (0 to -10$\permil$). As increasing B supports progressive removal of B from fluids, negative $\delta^{11}$B indicates a fractionation process. The direction of this fractionation is the opposite of that expected for a Rayleigh distillation process, and indicates that a shift in the $\delta^{11}$B composition of the fluid phase during serpentinization is most likely. Formation of a talc-dominated rock from serpentinites resulted in drastically reduced B concentrations of 3-20ppm, but with similar $\delta^{11}$B. This reaction, although driven by an influx of fluid, is a dehydration reaction and reduced B concentrations imply remobilization of B by dehydration fluids, but no fractionation of $\delta^{11}$B due to dehydration is apparent. Tremolite-dominated samples formed after talc schists likewise have similar B concentrations and $\delta^{11}$B, indicating that this reaction has little effect on B systematics. Data in the literature strongly indicate that subduction zone fluids have basic pH. If metamorphic silicates faithfully record only the isotopic composition of B speciated tetrahedrally within fluids, our data suggest that the pH of serpentinizing fluids is lowered within the serpentinite metasomatic zone. The most likely agent for a shift in fluid chemistry during serpentinization is the formation of magnetite, in which silicate ferrous iron is oxidized; a by-product of this reaction is probably H$_{2}$ gas, which would act to lower fluid pH. The evolution of $\delta^{11}$B to negative values during serpentinization therefore suggests that B and its isotopes can be significantly fractionated by metasomatic processes in the mantle wedge, and that true "slab-derived" $\delta^{11}$B may not be sampled by arc volcanics.
DE: 3660 Metamorphic petrology
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting