Crustal Growth Model for IBM: Arc Crust Evolution, Continental Crust Formation, and Crust-Mantle Transformation across The Transparent Moho

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Crustal Growth Model for IBMArc Crust
Evolution, Continental Crust Formation,and
Crust-Mantle Transformation across
The Transparent Moho

• IBM crust/mantle structure

• Petrologic modeling

• New insights into
• Moho formation
• and arc evolution
  

Y. TATSUMI IFREE/JAMSTEC
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IBM Arc-Trench System

• - an intra-oceanic arc
• - with backarc basins
• colliding with Japan arc

• best surveyed arc
• esp. seismic structure

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Seismic Structure of Crust/Mantle
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The Moho Discontinuity

• A sharp seismological boundary
• exhibiting Vp jump from 6-7 to gt8 km/s
• defining the crust/mantle boundary

• Generally accepted as gabbro/peridotite boundary

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Sub-IBM Moho Identification
Sub-arc Moho continuity from sub-BB normal
Moho seismic
reflectors
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Characteristic Seismic Structure
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Origin of Characteristics Middle Crust

• Mafic plutonics
• ?too high-T
• Boninitic plutonics mantle-derived andesite
  model
• ? boninite magmatism in the initial IBM arc
• Intermediate plutonics mantle-derived basalt
  model
• anatexis of pre-existing basaltic crust
• mixing of mafic and felsic magmas

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Two Possible Models
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Arc Crust Evolution (basalt model) 1st Stage
Sub-arc Moho Fossil melting front
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Arc Crust Evolution (basalt model) 2nd Stage
Remelting of middle crust to create a restetic
low-V lower crust layer
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Intermediate Middle Crust Composition
Tanzawa Pluton Obducted IBM Crust?
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IBM Magma Compositions
Primary/differentiated basalt and felsic magmas
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Volume of Restite/Cumulate
required for creating middle and low-V lower
crust layers
Based on experimental constraints on melting
regime
Crustal Component gtgt Seismic L-Crust
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Volume of Restite/Cumulate
required for creating middle and low-V lower
crust layers
Excess
Excess crustal component ? Transformed into
mantle across the transparent Moho
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Uncertainty in degree of melting
Andesitic melt F0.3
0.15ltFlt0.5
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RestiteCumulate Volume
Andesitic melt 0.15ltFlt0.5
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Vp ? Estimation for Petrologically Inferred
Crust

• Phase equilibria along inferred geotherm
• ? Perple_X (Connolly, 2005)
• low GT, 800 and 0C at Moho and
  surface 1400C at 70km
• high GT, 200 higher T 1400C at 70km
• Vp and ? calculation
• ? Hacker et al. (2003)
• H2O basalt, 0.1 intermediate partial melt, 0.3
  wt
• ? IBM tonalite 10-20 vol hornblende

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Phase Assemblages basalt model
Low-T Garnet within the lower crust High-T
Melting at the base of lower crust Medium-T Most
plausible
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Vp ? Estimation

• Consistent with the observed seismic structure
• Density inversion at the base of low-V upper
  mantle

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Phase Assemblages boninite model
Uppermost mantle peridotite not restite Middle
crust abundant hornbrende Uppermost lower
crust abundant pyroxenes
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Vp ? Estimation (boninite model)

• Inconsistent with observed seismic structure
• Extremely high-T is needed

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Arc Evolution Transparent Moho
Crust-mantle transformation across the
transparent Moho ? Arc crust evolution from
mafic to intermediate compositions
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Conclusion

• Sub-IBM seismic crust/mantle structure can be
  reasonably explained by melting and
  differentiation regime of arc basalt magmas
• Mafic crustal component, i.e., restites, will be
  transformed to the mantle during arc evolution
• Sub-arc Moho is transparent
• Sub-arc Moho represents the fossil melting front
• Delamination of mafic restite will take place at
  its base
• Mafic initial arc crust will differentiates into
  intermediate continental crust

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Thanks How to test this model?
Direct sampling of deep crust with Chikyu..