HR: 08:30h
AN: V51A-03    [PDF]
TI: Boron Isotope Compositions of Selected Fresh MORB Glasses From the Northern EPR (8-10$\deg$N): Implications for MORB Magma Contamination
AU: * le Roux, P J
EM:
AF: Dept of Terrestrial Magnetism, Carnegie Inst. of Washington, Washington, DC 20015 United States
AU: Shirey, S B
EM:
AF: Dept of Terrestrial Magnetism, Carnegie Inst. of Washington, Washington, DC 20015 United States
AU: Hauri, E H
EM:
AF: Dept of Terrestrial Magnetism, Carnegie Inst. of Washington, Washington, DC 20015 United States
AU: Perfit, M R
EM:
AF: Dept of Geological Sciences, University of Florida, Gainesville, FL 3261 United States
AB: The petrogenetic role of seawater and seawater-equilibrated altered crust in the magmatic evolution of basalts formed at mid-ocean ridges is not well-constrained. Observed excess Cl in oceanic basaltic magmas led to established models of assimilation of a saline brine component, although the physical form of this component and whether any other components contaminate MORB magmas remain unresolved. Light stable isotopes such as B are valuable in further refining our understanding of these magmatic processes. The light element B has two stable isotopes (mass 11 and 10) and B isotopic ratio ranges significantly in oceanic settings: e.g. depleted upper mantle ($\delta$$^{11}$B -10$\permil$), fresh MORB magmas ($\delta$$^{11}$B -1.2 to -6.5$\permil$), altered oceanic crust ($\delta$$^{11}$B +2 to +9$\permil$), hydrothermal fluids ($\delta$$^{11}$B +30 to +36.5$\permil$), and seawater ($\delta$$^{11}$B +38.5$\permil$). We have developed an in situ laser ablation, multiple multiplier ICP-MS at DTM (see le Roux et al., in press) that has reliable uncertainties for B isotope analyses better than 1$\permil$ (2$\sigma$) over concentration ranges from 0.3 ppm to above 30 ppm. This technique makes nearly any size glass sample amenable to B isotope study. B isotopic compositions were obtained on 16 fresh MORB magmas from the northern East Pacific Rise (EPR) from 8 to 10$\deg$N. This region of the EPR has an extensive, existing MORB glass collection, with well-constrained general geochemistry and petrology, recovered from sites on-axis, off-axis (including young off-axis eruptions; abyssal hills), and the Siquerios fracture zone. Geophysical data from this region imaged the top of the dike section (layer 2A) and the sub-axial magma chambers (AMC). Data for these MORB glasses indicate the variable addition of H$_{2}$O, Cl, F, Li and B to these magmas prior to eruption. The excess Cl can be accounted for by variable ($<$0.5wt%) magma contamination with a saline brine (NaCl 15-50wt%; Kent et al, 1999). Variable magma degassing places the contamination of some of these magmas at depths within the oceanic crust close to the top of the AMC. These MORB samples have 0.56 to 2.61 ppm B, and B isotope compositions that are surprisingly restricted ranging from $\delta$$^{11}$B -5.50 to -8.96$\permil$. The low $\delta$$^{11}$B values are close to the depleted upper mantle value (-10$\permil$). The $\delta$$^{11}$B data do not correlation with B concentrations, Mg\#, Sr, Nd or Pb isotopes, or proxies for brine addition (e.g. Cl/Nb). The lowest $\delta$$^{11}$B samples are also the most-incompatible element depleted (high B/Nb ratios). The $\delta$$^{11}$B of the on-axis samples increases slightly with increased levels of magma degassing (i.e. lowest $\delta$$^{11}$B values in samples extracted undegassed from depths closest to AMC top).Therefore, although the Cl data indicate significant addition of probably a saline brine component to both on- and off-axis MORB magmas, their $\delta$$^{11}$B compositions were not significantly affected by this process and the observed variations in $\delta$$^{11}$B may have a different origin. Possibly, the low B/Cl ratio of seawater ($\sim$ 0.001) coupled with preferential partitioning of Cl relative to B into brines during supercritical phase separation (Berndt and Seyfried, 1990) of seawater in hydrothermal system, results in very saline brines with low boron concentrations. The coupled B-Cl data effectively eliminates simple magmatic assimilation of altered Cl-rich high-B isotope composition oceanic crust in this region.
DE: 1020 Composition of the crust
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting