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LARGE IGNEOUS PROVINCES: Results of Delamination? . Don L. Anderson Caltech. A new GSA book. Delamination: The Eclogite Engine. Kay, R.W. & Kay, S.M., Delamination and delamination magmatism, Tectonophysics, 219, 177-189, 1993.

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delamination the eclogite engine
Delamination:The Eclogite Engine
  • Kay, R.W. & Kay, S.M., Delamination and delamination magmatism, Tectonophysics, 219, 177-189, 1993.
  • mechanism can explain some long-standing geophysical problems, e.g.
    • subsidence prior to LIP emplacement
    • short duration
    • bottoming of seismic tomography anomalies beneath “hot spots”
  • but what happens to this lower crust?
summary of model
Summary of model
  • When crust thickens to > 50 km:
    • converts to dense eclogite
    • delaminates
    • sinks
    • heats up
    • rises
  • eclogites have low Vs for their density - may be confused with high T
rocks and minerals arranged by density crust upper mantle
Rocks and minerals arranged by density: crust & upper mantle

eclogite: here used as a general term for garnet & pyroxene-rich rock

rocks and minerals arranged by density crust upper mantle1
Rocks and minerals arranged by density: crust & upper mantle
  • delaminates when crust > 50 km thick
  • warmer than MORB
rocks and minerals arranged by density upper mantle
Rocks and minerals arranged by density: upper mantle

Where does delaminate reach neutral buoyancy?

r

Vs

delaminated roots warm quickly
Delaminated roots warm quickly
  • will start to melt before reaching same T as surrounding mantle
  • already in TBL, so starts off warm
  • when 30% melt, garnet mostly gone & will start to rise
slide9

pink eclogite is only temporarily stable at these depths

  • “arclogites” less SiO2 than MORB eclogite – do not sink so far
  • Vs of eclogite low at depth
  • low melting point
  • as it warms, it rises
mantle stratification
Mantle stratification
  • irregular chemical discontinuities expected
  • difficult to see in tomography
  • can be seen in receiver functions
underside reflections 0 1 000 km depth
Underside reflections 0 – 1,000 km depth
  • 410 & 660-km discontinuities clear
  • ~ 10 others
  • may be chemical
slide12

Delamination cycle

  • dense roots
    • fall off
    • warm up in ambient mantle
    • rise
  • possible mechanism for Atlantic & Indian ocean plateaus & DUPAL anomaly
many ways for eclogite to get into the mantle
Many ways for eclogite to get into the mantle
  • collision belts, arcs
  • can fuel melting anomalies at normal T
lips are associated with continental breakup
LIPs are associated with continental breakup
  • reconstruction at ~ 30 Ma
  • dual volcanism
    • on breakup
    • ~ 30-40 Myr later
  • oceanic plateaus form ~ 1,000 km offshore
  • = rising of delaminated root?
eclogite 70 molten before peridotite starts to melt
Eclogite 70% molten before peridotite starts to melt
  • eclogite sinkers warmed by conduction
  • rise before T has risen to that of ambient mantle
  • eclogite 70% molten at peridotite solidus
slide16

delamination controls crustal thickness

  • very sharp cut-off at 50 km
  • interpreted as eclogite phase change

from Mooney et al., 1998

example 2 sierra nevada

garnet peridotite

garnet pyroxenite

Example 2: Sierra Nevada

attenuation

P-wave slowness

Vp/Vs

anisotropy

from Boyd et al., 2004

example 3 iceland
Example 3: Iceland
  • Restricted LVZ
  • possibly Caledonian arc roots delaminated on breakup
  • Cold, dense, sinking eclogite can be LVZ
  • warmed, melted, rising eclogite can also be buoyant if ~ 1/2 garnet eliminated

Ritsema et al., 1999

summary
Summary
  • Dense, mafic cumulates may be twice the thickness of arc crust
  • Delamination accompanied by upwelling & adiabatic decompression of the asthenosphere; a whole cycle may take 30-40 Myr
  • The global recycling flux of arcologite is ~ 10% that of oceanic crust, i.e. ~ hotspot volume rate
  • It starts out hotter & by-passes normal subduction zone processing
  • Delaminated arclogites preferentially melt & form a unique component of hotspot & ridge magmas (e.g. suggested DUPAL = Gondwana crust).
resources
Resources

Please visit:

www.mantleplumes.org/Eclogite.html

www.mantleplumes.org/LowerCrust.html

End