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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
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
quickly lost to mantle systems). After about 10 Ma 10Be is no
Be/9Be averages about 5000 x 10--11 in the uppermost
oceanic 3 10 3 In mantle-derived MORB and OIB magmas, & continental
crust, 10Be is below detection limits (<1 x 106 atom/g) and
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
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
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
2) the age of the subduction zone
3) the age of the subducting slab
4) the extent to which the subducting slab induces
flow in the mantle wedge
Other factors, such as:
3 dip of the slab
3 frictional heating
3 endothermic metamorphic reactions
3 metamorphic fluid flow
are now thought to play only a minor role
q Typical thermal model for a subduction zone
Isotherms will be higher (i.e. the system will be hot...
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- Spring '13