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IBM crust/mantle structure

Crustal Growth Model for IBM: Arc Crust Evolution, Continental Crust Formation, and Crust-Mantle Transformation across The Transparent Moho. IBM crust/mantle structure. Petrologic modeling. New insights into Moho formation

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IBM crust/mantle structure

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  1. Crustal Growth Model for IBM:Arc 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

  2. IBM Arc-Trench System - an intra-oceanic arc - with backarc basins colliding with Japan arc C A A’ C’ B B’ • best surveyed arc • esp. seismic structure

  3. Seismic Structure of Crust/Mantle

  4. The Moho Discontinuity • A sharp seismological boundary • exhibiting Vp jump from 6-7 to >8 km/s • defining the crust/mantle boundary • Generally accepted as gabbro/peridotite boundary

  5. Sub-IBM Moho Identification Sub-arc Moho: continuity from sub-BB ‘normal’ Moho seismic reflectors

  6. Characteristic Seismic Structure • 6.0 - 6.5 km/s middle crust similar to intermediate average continental crust • 6.5-6.8 & 6.8-7.2 km/s lower crust layers • 7.4 - 7.7 km/s low-V uppermost mantle • reflectors near Moho and within upper mantle

  7. 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

  8. Two Possible Models Mantle-derived andesite model Mantle-derived basalt model Upper crust boninite Crustal melt or mixed magma Middle crust restite of m-crust melting Low-V restite of m-crust melting Basaltic Crust Lower crust Remaining initial basaltic crust Remaining initial basaltic crust High-V Moho low-V mantle restites of basaltic crust melting high-T peridotite normal mantle

  9. Arc Crust Evolution (basalt model): 1st Stage upper middle Partial Melt Basaltic Crust lower Crust Component Moho Basaltic Magma low-V Restite Melting front Mantle Initial Arc Crust Basaltic Underplating Crustal Anatexis Melt Migration Sub-arc Moho = Fossil melting front

  10. Arc Crust Evolution (basalt model): 2nd Stage Upper crust Middle crust Crustul Component Low-V Basaltic magma Basaltic Crust Lower crust High-V Moho Restite Restite low-V Mantle mormal Remelting of middle crust to create a restetic low-V lower crust layer

  11. Intermediate Middle Crust Composition Tanzawa Pluton: Obducted IBM Crust?

  12. IBM Magma Compositions Primary/differentiated basalt and felsic magmas

  13. Volume of Restite/Cumulate Felsic UC 246 Basaltic UC 486 N. Izu Andesitic MC 963 6.8 km/s LC 578 (restite for felsic UC) Anatexis (Unit volume: km3/km) Mixing 4157 4038 Observed Restite for andesitic MC 1812 1477 1882 Cumulate required for creating middle and low-V lower crust layers Based on experimental constraints on melting regime Crustal Component >> Seismic L-Crust

  14. Volume of Restite/Cumulate Upper Middle Lower required for creating middle and low-V lower crust layers ‘Excess’ ‘Excess’ crustal component ↓ Transformed into mantle across the transparent Moho

  15. Uncertainty in degree of melting Andesitic melt: F~0.3 0.15<F<0.5

  16. Restite+Cumulate Volume calculated transformed observed Andesitic melt: 0.15<F<0.5

  17. Vp &  Estimation for Petrologically Inferred Crust • Phase equilibria along inferred geotherm ← Perple_X (Connolly, 2005) ・low GT, 800 and 0°C at Moho and surface; 1400°C at 70km ・high GT, 200° higher T; 1400°C 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

  18. Phase Assemblages: basalt model Low-T: Garnet within the lower crust High-T: Melting at the base of lower crust Medium-T: Most plausible

  19. Vp &  Estimation • Consistent with the observed seismic structure • Density inversion at the base of low-V upper mantle

  20. Phase Assemblages: boninite model Uppermost mantle: peridotite not restite Middle crust: abundant hornbrende Uppermost lower crust: abundant pyroxenes

  21. Vp &  Estimation (boninite model) • Inconsistent with observed seismic structure • Extremely high-T is needed

  22. Arc Evolution & Transparent Moho Upper Crust Middle Crust Oceanic Crust Initial Arc Crust Moho Creation of the initial arc crust with a mafic composition Formation of the mature arc crust with an intermediate composition Transformed Crustal Component Crust-mantle transformation across the transparent Moho → Arc crust evolution from mafic to intermediate compositions

  23. Conclusion Seismic Petrologic upper Partial Melt middle Crustal Component CRUST lower Remaining Initial Crust Moho low-V Restite MANTLE Peridotites normal • 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’

  24. ThanksHow to test this model? Direct sampling of deep crust with Chikyu…..

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