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Anatexis in Subduction Zones: Models and Evidence

Explore two models - one involving subducted sediment and oceanic crust, the other focusing on subducted oceanic crust and mantle - with supporting and conflicting evidence for magma genesis. Understand the processes through major elements, trace elements, and isotopic characteristics. Analyze the role of 10Be as a sediment tracer and evaluate the likelihood of andesitic magma formation in subduction zones.

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Anatexis in Subduction Zones: Models and Evidence

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  1. Group A: Model 1 & Model 3 Ricci Keller Symone Stinson Amber Colter

  2. Model 1: Anatexis of Subducted Sediment(C) ±Oceanic Crust (B) • Sediment is subducted with oceanic crust to produce arc-andesitic magma • Sediment from eroded continents has high Sr/Pb ratios • Evidence for some involvement of sediment: • Major Elements • Trace Elements • Isotopic Characteristics

  3. Support for Model 1 • 10Be • Formed in the atmosphere • Transferred to Earth by rain • Absorbed by sediment and soil • Has a half life of 1.5my • Is a tracer of sediment moving into a subduction zone and back to the surface via magma transport • Can only be recycled once • Samples from several arcs show elevated levels of 10Be • Evidence for participation of sediment in magma genesis http://www.gfz-potsdam.de/portal/gfz/Struktur/Departments/Department+3/sec34/Bilder/Meteoric+10Be?binary=true&status=300&language=en

  4. Evidence Against Model 1 • No compelling case of large scale sediment anatexis • Sediment melting is indistinguishable from crustal contamination • Sediment melting is most likely part of a more complex model involving andesite genesis • Be isotope data suggests a less than 4% sediment component

  5. Model 3:Anatexis of Subducted Oceanic Crust (B) ± Mantle (A) • MaficOceanic Crust converts to eclogite during subduction and is melted to yield a primary andesitic magma • The product of this process is Adakitemagma

  6. Model 3:Anatexis of Subducted Oceanic Crust (B) ± Mantle (A) Ecolgite • Eclogite • Formed via metamorphism during the subduction process and is the direct parent of adakite • Adakite • Silica Rich • Volcanic or plutonic rock derived from melting of the basaltic portion of oceanic crust subducted beneath volcanic arcs Adakite

  7. Evidence Against Model 3 Evidence For Model 3 • Experimental Melts • Theoretical Calculations • Major/Trace Element Data • Experimental data found does not match andesite data found in the field • ie: REE patterns • “…most thermal models of subduction zones do not predict temperatures high enough to melt subducted crustal rocks at sites near the top of the plate below the arc.” • Model 3 opposes fractional crystallization and magma mixing • Fractional Crystallization is a major component of andesite formation • The model does not explain why basalts and andesites commonly occur together in volcanic arc settings • Equillibrium crystallization models suggest that high alumina- andesites are not likely to be primary melts.

  8. Conclusion for Model 3 Except for Adakites, it is unlikely that anatexis of a mafic, oceanic crust during subduction, will produce magma with andesitic composition.

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