1994 atlantic sediment data from white et al 1985

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Unformatted text preview: ta from White et al. (1985). Figure 16-13. Variation in 207Pb/204Pb vs. 206Pb/204Pb for oceanic island arc volcanics. Included are the isotopic reservoirs and the Northern Hemisphere Reference Line (NHRL) proposed in Chapter 14. The geochron represents the mutual evolution of 207 Pb/204Pb and 206Pb/204Pb in a single-stage homogeneous reservoir. Data sources listed in Wilson (1989). Be created by cosmic rays + oxygen and nitrogen in upper atmos. 3 → Earth by precipitation & readily → clay-rich oceanic seds 10 10 3 Half-life of only 1.5 Ma (long enough to be subducted, but Half-life 10 quickly lost to mantle systems). After about 10 Ma 10Be is no Be longer detectable longer 11 Be/9Be averages about 5000 x 10--11 in the uppermost oceanic sediments oceanic 3 10 3 In mantle-derived MORB and OIB magmas, & continental In 10 crust, 10Be is below detection limits (<1 x 106 atom/g) and 10 10Be/9Be is <5 x 10-14 B is a stable element 3 Very brief residence time deep in subduction zones 3 B in recent sediments is high (50-150 ppm), but has a greater in affinity for altered oceanic crust (10-300 ppm) affinity 3 In MORB and OIB it rarely exceeds 2-3 ppm 10 Be/Betotal vs. B/Betotal diagram (Betotal ≈ 9Be since 10Be is so Be Be rare) rare) 10 10 Figure 16-14. 10Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88, 111-123. Petrogenesis of Island Arc Magmas q Why is subduction zone magmatism a paradox? Why Of the many variables that can affect the isotherms in subduction zone systems, the main ones are: 1) the rate of subduction 1) rate 2) the age of the subduction zone age zone 3) the age of the subducting slab age slab 4) the extent to which the subducting slab induces flow in the mantle wedge flow Other factors, such as: 3 dip of the slab 3 frictional heating 3 endothermic metamorphic reactions 3 metamorphic fluid flow metamorphic are now thought to play only a minor role are q q Typical thermal model for a subduction zone Isotherms will be higher (i.e. the system will be hot...
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