Nakano, Toshio & Eizo Nakamura (2001) Boron isotope geochemistry of metasedimentary rocks and tourmalines in a subduction zone metamorphic suite, Physics of The Earth and Planetary Interiors, 127, 233-252, doi: 10.1016/S0031-9201(01)00230-8. < 20091022173059086.tota > pub

Map on Earth or stone. Places and analyses are shown.

  • abstract:

    In order to understand the behavior of boron (B) and its isotope fractionation during subduction zone metamorphism, B contents and isotopic compositions together with major element compositions were determined for metasedimentary rocks and tourmalines from the Sambagawa Metamorphic Belt, central Shikoku, Japan. No systematic changes in whole-rock B content and isotope composition of the metasediments were observed among the different metamorphic grades, indicating the lack of a bulk fluid-rock B isotope fractionation as a result of devolatilization.
    Both modal abundance and grain size of tourmaline increase with increasing metamorphic grade. In contrast, B contents in muscovite and chlorite decrease with increasing metamorphic grade. These observations combined with mass balance calculations of B suggest the formation of tourmaline during progressive metamorphism from metamorphic fluids containing B mainly derived from muscovite and subordinately from chlorite without allowing significant net removal of B from the metasedimentary rocks. Tourmalines in the higher-grade metasedimentary rocks have zonal structure of B isotope and major element composition with decreasing δ11B and increasing Mg/(Mg+Fe) from the inner rim (core) to the outer rim. The change of Mg/(Mg+Fe) in the tourmalines with increasing grade is paralleled by similar variation in chlorite. These observations suggest that the growing tourmalines record the progressive evolution of the B isotopic composition of the metamorphic fluid, in the outermost rims preserving the isotope signature of peak metamorphic P–T-fluid conditions.
    Based on the above observations, the δ11B of the tourmaline is thought to have been nearly identical to that of the metamorphic fluid resulting in the “apparent” B isotopic fractionation factor between metamorphic fluid and whole-rock (α = (11B/10B)fluid /(11B/10B)whole - rock) which decreases from 1.007±0.003 to 1.001±0.003 from chlorite to biotite zone metamorphism. Such results together with the formation of tourmaline from (and sequestering of) B in metamorphic fluids may lead to less B isotopic fractionation as a result of subduction zone devolatilization than noted in suites containing less tourmaline. This, therefore, makes it possible to transport B isotopic signatures, which ultimately reflect Earth’s surface materials, to the deep mantle, perhaps resulting in mantle B isotope anomalies near convergent margins.

  • doi: 10.1016/S0031-9201(01)00230-8
  • modified at 16 h ago


(a) B_procedures of Nakamura et al. (1992) on TIMS Thermo-Fisher MAT261
(b) XRF procedures of Takei (2002) on XRF PANalytical PW2400


(a) B_procedures of Nakamura et al. (1992) on TIMS Thermo-Fisher MAT261
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Tixhr973765077 thumb 91072603-S < 20180219112418-534345 >pub
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Tixhr1351542088 thumb 89052502S < 20180216111602-303679 >pub
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Img 4063 thumb 91072702P < 20110720094230-290-031 >pub
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caption ID assembly stone
Table 1. Whole-rock boron concentrations, boron isotope compositions, and major element compositions, and modal abundances of metamorphic minerals in metasedimentary rocks of the Sambagawa Metamorphic Belt pub 20190909142026-190790 major+B (Nakano, 2001) 16
Table 3. B concentrations of minerals in metasedimentary rocks in the Sambagawa Metamorphic Belt, as a function of metamorphic grade pub 20190909153127-951778 B (Nakano., 2001) 24
Table 2a. Mass discrimination factors for various tourmalines (TIMS) pub 20190909143950-368455 B_TIMS (Nakano, 2001) 5
Table 2b. Mass discrimination factors for various tourmalines (SIMS) pub 20190909150154-693119 B_SIMS (Nakano, 2001) 5
manual
  • name
  • Nakano, Toshio
  • Eizo Nakamura