Fluid-Mobile Element Constraints on Processes Within Subduction Zones: Applications of Boron Geochemistry to Arc Volcanic Rocks Curves show mixing between mantle (M) and fluids with high B/Nb but different B isotopic compositions as specified SSW d11B = +16 Tonga, Izu d11B = +8 Ryan et al. (1995) Leeman et al. (2004) C Aleut d11B = +3 d11B = -2
Fa Cascade arc crossection showing inferred segregation depths for primitive basalts. Group I: LKT (squares) & OIB-like (circles) Group II: Calcalkalic types (triangles) Fb Fluid type is related to slab history, whereas intrinsic magma composition may be partly inherited from a compositionally stratified mantle wedge Cascades: (a) d7Li–Li/Y and (b) d11B–B/Nb systematics. Fields are shown for frontal arc samples. Group I (squares); Group II (triangles).
Simulated SZ thermal models (Huang, 2001) Cascades 2 cm/yr 9 Ma E. Aleutians 5.8 cm/yr 50 Ma Kuril 9.2 cm/yr 89 Ma C. America N. 8.8 cm/yr 25 Ma Hokkaido 9.5 cm/yr 94 Ma C. America S. 8.5 cm/yr 15 Ma Marianas 10 cm/yr 134 Ma S. America N. 7.8 cm/yr 57 Ma S. America C. 8.3 cm/yr 112 Ma S. America S. 8.4 cm/yr 30 Ma
11Bsc determined from arc lava suites (cf. Leeman & Tonarini, 2002). Because primitive mantle values are low (less than -3 permil; Chaussidon & Marty, 1995), elevated 11Bsc values are attributed to fluids derived from altered oceanic crust (Ishikawa & Nakamura, 1992; Staudigel et al., 1996). Estimated slab-surface temperature (°C) at 100 km depth SSTs at 100 km depth, based on multilinear regression of numerical models (Huang, 2001), reflect relative differences between the SZs. Absolute values could be systematically higher, but probably no more than 100-200°C based on comparisons with other models (Conder et al., 2002; Rüpke et al., 2002; Kelemen et al., 2002; Currie et al., 2004).
Observed cross-arc variations in B enrichment Suggest progressive loss of FMEs with depth Ryan et al. (1995) Leeman et al. (2004)
SSW d11B = +16 Tonga, Izu d11B = +8 C Aleut d11B = +3 d11B = -2 Evaluating B isotope composition of subduction component Curves show mixing between mantle (M) and fluids with high B/Nb but different B isotopic compositions as specified