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 structureIBM crust/mantle structure

• Petrologic modelingPetrologic modeling

• New insights into New insights into Moho formation Moho formation and arc evolution and arc evolution

Y. TATSUMIIFREE/JAMSTEC

IBM Arc-Trench SystemIBM Arc-Trench System

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

A A’

B B’

C

C’

- best surveyed arc esp. seismic structure

Seismic Structure of Crust/MantleSeismic Structure of Crust/Mantle

The Moho DiscontinuityThe 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

Sub-IBM Moho IdentificationSub-IBM Moho Identification

Sub-arc Moho: continuity from sub-BB ‘normal’ Moho seismic reflectors

Characteristic Seismic StructureCharacteristic Seismic Structure

6.0 - 6.5 km/s middle crust similar to intermediate average continental crustsimilar to intermediate average continental crust 6.5-6.8 & 6.8-7.2 km/s lower crust layers6.5-6.8 & 6.8-7.2 km/s lower crust layers 7.4 - 7.7 km/s low-V uppermost mantle7.4 - 7.7 km/s low-V uppermost mantle reflectors near Moho and within upper mantlereflectors near Moho and within upper mantle

Origin of Characteristics Middle Origin of Characteristics Middle CrustCrust

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

Two Possible ModelsTwo Possible Models

Mantle-derivedbasalt model

Crustal melt or mixed magmarestite of m-crust melting

Remaininginitial basaltic crust

restites of basaltic crust melting

Mantle-derivedandesite model

boniniterestite of m-crust melting

Remaininginitial basaltic crust

high-T peridotite

BasalticCrustHigh-V

Low

er

crust

normalmantle

Upper crust

Middle crust

Low-V

low-Vmantle

Moho

Arc Crust Evolution (basalt model): 1st StageArc Crust Evolution (basalt model): 1st Stage

CrustalAnatexis

Initial ArcCrust

Mantle

BasalticCrust

Moho

BasalticUnderplating

BasalticMagma

MeltMigration

Restite

Partial Meltuppermiddle

lower

low-VM

elting front

Sub-arc Moho = Fossil melting front

Cru

st C

ompo

nent

Arc Crust Evolution (basalt model): 2nd StageArc Crust Evolution (basalt model): 2nd Stage

Remelting of middle crust to create a restetic low-V lower crust layer

Restite

Basaltic magma

Mantle

BasalticCrust

Upper crustMiddle crustLow-V

low-VRestite

High-V Lower crust

Moho

mormal

Crus

tul

Com

pone

nt

Intermediate Middle Crust Intermediate Middle Crust CompositionComposition

Tanzawa Pluton: Obducted IBM Crust?Tanzawa Pluton: Obducted IBM Crust?

IBM Magma CompositionsIBM Magma Compositions

Primary/differentiated Primary/differentiated basalt and felsic magmasbasalt and felsic magmas

Volume of Restite/CumulateVolume of Restite/Cumulaterequired for creating middle and low-V lower crust required for creating middle and low-V lower crust

layerslayers

N. IzuN. Izu

Cumulate

Restite for andesitic MC

Felsic UC 246Basaltic UC 486

Andesitic MC 963

6.8 km/s LC 578(restite for felsic UC)

(Unit volume: km3/km)

Observed

1882

Anatexis

4157

1812

Mixing

4038

1477

Crustal Component >> Seismic L-Crust

Based on experimental constraints on melting regime

Volume of Restite/CumulateVolume of Restite/Cumulaterequired for creating middle and low-V lower crust required for creating middle and low-V lower crust

layerslayers

Upper

MiddleLo

wer

‘Exc

ess’

‘Excess’ crustal component↓

Transformed into mantleacross

the transparent Moho

Uncertainty in degree of meltingUncertainty in degree of melting

Andesitic melt: F~0.3 0.15<F<0.5

Restite+Cumulate VolumeRestite+Cumulate Volume

Andesitic melt: 0.15<F<0.5

calculated

observedtransformed

◆ 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

Vp Vp & & Estimation Estimation for Petrologically Inferred Crustfor Petrologically Inferred Crust

Phase Assemblages: basalt modelPhase Assemblages: basalt model

Low-T: Garnet within the lower crust High-T: Melting at the base of lower crustMedium-T: Most plausible

Vp & Vp & Estimation Estimation

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

Phase Assemblages: boninite modelPhase Assemblages: boninite model

Uppermost mantle: peridotite not restite Middle crust: abundant hornbrende Uppermost lower crust: abundant pyroxenes

Vp & Vp & Estimation (boninite model) Estimation (boninite model)

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

Oceanic Crust

Moho

Initial ArcCrust

Creation of the initial arc crust with a mafic composition

MiddleCrust

UpperCrust

TransformedCrustalComponent

Arc Evolution & Transparent MohoArc Evolution & Transparent Moho

Formation of the mature arc crust with an intermediate composition

Crust-mantle transformation across the transparent Moho

→ Arc crust evolution from mafic to intermediate compositions

ConclusionConclusion • 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 fo

ssil melting front• Delamination of mafic restite wil

l take place at its base• Mafic initial arc crust will differe

ntiates into intermediate ‘continental crust’

Restite

Partial Melt

Petrologic

RemainingInitial Crust

Peridotites

CR

UST

MA

NTL

E

upper

middle

lower

low-V

Seismic

normal

Moho Cru

stal

Com

pone

nt

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