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
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…..