1 / 24

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

richert
Download Presentation

IBM crust/mantle structure

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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…..

More Related