ibm crust/mantle structure
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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 - PowerPoint PPT PresentationTRANSCRIPT
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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
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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
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Seismic Structure of Crust/MantleSeismic Structure of Crust/Mantle
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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
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Sub-IBM Moho IdentificationSub-IBM Moho Identification
Sub-arc Moho: continuity from sub-BB ‘normal’ Moho seismic reflectors
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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
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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
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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
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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
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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
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Intermediate Middle Crust Intermediate Middle Crust CompositionComposition
Tanzawa Pluton: Obducted IBM Crust?Tanzawa Pluton: Obducted IBM Crust?
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IBM Magma CompositionsIBM Magma Compositions
Primary/differentiated Primary/differentiated basalt and felsic magmasbasalt and felsic magmas
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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
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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
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Uncertainty in degree of meltingUncertainty in degree of melting
Andesitic melt: F~0.3 0.15<F<0.5
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Restite+Cumulate VolumeRestite+Cumulate Volume
Andesitic melt: 0.15<F<0.5
calculated
observedtransformed
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◆ 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
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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
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Vp & Vp & Estimation Estimation
• Consistent with the observed seismic structure• Density inversion at the base of low-V upper mantle
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Phase Assemblages: boninite modelPhase Assemblages: boninite model
Uppermost mantle: peridotite not restite Middle crust: abundant hornbrende Uppermost lower crust: abundant pyroxenes
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Vp & Vp & Estimation (boninite model) Estimation (boninite model)
• Inconsistent with observed seismic structure• Extremely high-T is needed
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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
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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
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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…..