Transcript Document

SUMMARY OF MANTLE
TEMPERATURES
DON L. ANDERSON
2006
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Bottom lines
• Geophysical global estimates of mantle
temperature are slightly higher and have a larger
range than petrological estimates from mature
spreading ridges (away from ‘plume influence’)
• The conduction geotherm may extend deeper
than the average depth of MORB extraction
(~280 vs ~100 km)
• The deeper geotherm is subadiabatic
• Middles of long-lived plates can be 30-50˚C
hotter than at mature ridges; “new” ridges can
give hotter MORB
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POTENTIAL TEMPERATURES (Tp)
• Global Geophysical Inversion [heatflow, spreading rates]
• 1410± 180°C [Kaula, 1983] (entire range)
• Ridges
– [melt petrology] 1370±70°C [Asimow, 2006] (2 sigma)
– [peridotites] ±100°C [Bonatti et al., 1993]
– [subsidence rates] ±100°C [Perrot et al., 1998]
• Kolbeinsey Ridge
– 1270-1360°C [Korenaga, Kodaira]
• Lower mantle
– 1500-1730°C [Zhao, Anderson, Stacey, Stixrude]
• Compare with McKenzie and Bickle [1988]
– Tp= 1280°C ± 20°C for normal mantle (thereby implying
plumes for Tp>1300 °C)
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TEMPERATURE BUMP IN UPPER
MANTLE
• Internal heating and secular cooling are expected to
decrease the radial geothermal gradient away from an
adiabat. Modeling shows that the average thermal
gradient is expected to be significantly subadiabatic
through much of the interior of the mantle
• There may be a maximum in T near 100-200 km below
the plate and below the depth of MORB extraction
• The geotherm is unlikely to be a TBL joining with an
adiabat at the ‘lithosphere’-asthenosphere boundary
(ala McKenzie, who uses the term ‘lithosphere’ for the
TBL )
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*
*more realistically, 30-50 C
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McKenzie and colleagues assume the upper
mantle to be homogeneous and isothermal. They
adopt a cold subsolidus potential temperature of
1280°C ± 20°C for normal mantle. Sleep
adopted T’s of >200 C to represent plumes
Most other ‘hotspots’ and LIPs have no thermal or heat
flow anomaly (see www.mantleplumes.org)
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Temperature: Iceland
Foulger et al. (2005)
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The total range sampled by ‘normal’ ridges
inferred from petrology is 1250-1450˚C
(Asimow, 2006) or 1475˚C if Iceland is a ridge
and is not built on a continental fragment (this
includes crustal thicknesses of 3-11 km and
includes 'ridge-like' ridges, away from
complexities that are likely to confound simple
relationships between potential temperature,
crustal thickness, and melt composition such as
active flow, fertile sources, non-steady flow,
focusing, EDGE effects).
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These petrological estimates are now
consistent with long-standing geophysical
estimates. Kaula (1983) estimated the minimal
upper mantle temperature variations that are
consistent with observed heat flow and plate
velocities. At the fully convective level, about
280 km depth, temperature variations are at
least ± 180˚C, averaged over 500 km spatial
dimensions. Tp under ridges was estimated at
1410˚C. There is some indication that MORB at
the onset of spreading are ~50˚ hotter
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Melting Temperatures
(solidi extrapolated to P=0)
Eclogite 1100˚C (extrapolated
from 1 MPa)
Peridotite 1300˚C (…from 1 MPa)
Melting anomalies may be due to
fertility streaks
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• The potential temperature of the present upper mantle is
1400±200˚C based on bathymetry, subsidence, heat flow,
tomography, plate motions, discontinuity depths
(Anderson, 2000). Temporal variations of ~200˚C over
~200 Myr are expected. Secular cooling of 100˚C in 1 Ga
is plausible.
• Temperatures at onset of spreading may be ~50˚C
warmer
• McKenzie and Bickle (1988) assumed the upper mantle to
be homogeneous and isothermal. They adopted a cold
subsolidus potential temperature of 1280 ± 20˚C for
normal mantle and thus require hot plumes elsewhere.
• If normal mantle temperatures are 1400 ± 200°C, or
even 1350 ± 150°C, there is no thermal requirement for
hot mantle plumes.
• Convection simulations without plumes give the above
ranges in temperature
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RIDGES
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Peridotite
liquidus
Eclogite
solidus
Eclogite
liquidus
HISTOGRAM FROM ASIMOW (2006)
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Eclogite 70% molten before
peridotite starts to melt
¥ eclogite 70%
molten at
peridotite solidus
¥ eclogite sinkers
w armed by
conduction
¥ rise before T has
risen to that of
ambient mantle
Cold eclogite can be negatively buoyant but it can have low shear wave
velocities & low melting point. Fertile eclogite blobs can be brought into
shallow mantle by entrainment or displacement or by melting
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Dry peridotite can only melt in shallow mantle
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COLD ECLOGITE CAN MIMIC HOT
UPWELLING
ridges
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Presnell, Gudfinnsson, Herzberg
Dense cold eclogite can have low seismic
velocities
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ARCS; the hot mantle wedge paradox
Kelemen et al, 2002
Extreme case of subadiabatic gradient
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• Figure 1. Predicted geotherms beneath arcs from thermal
modeling (small symbols and fine lines), compared to
petrological estimates of PT conditions in the uppermost
mantle and lowermost crust in arcs (large symbols and
thick lines).
• Most petrological estimates are several 100˚ hotter than
the highest temperature thermal models at a given depth.
• Wide grey lines illustrate a plausible thermal structure
consistent with the petrological estimates.
• Such a thermal structure requires adiabatic mantle
convection beneath the arc to a depth of ~ 50 km, instead
of minimum depths of ~ 80 km or more in most thermal
models.
• Deeper mantle may be hotter than usually modeled.
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Mantle Temperature Variations Beneath Back-arc Spreading
Centers Inferred from Seismology, Petrology, and Bathymetry
Douglas A. Wiens*, Katherine Kelley. Terry Plank
Earth and Planetary Science Letters
Compare
max T
with
Hawaii
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The currently high flux at Hawaii is
unusual
Van Ark & Lin, 2004
The quasi-periodic variations in the flux along the Hawaiian
ridge may be due to fertile streaks or stress variations rather
than pulsation of a plume. The highest flux is on the young
lithosphere between the Murray and Molokai FZ
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A fertility streak can be due to
subduction of an aseismic ridge or
seamount chain (about 20 are currently
entering subduction zones)
Hawaiian swell can be due to a
buried buoyant load at 120 km
depth (Van Ark & Lin, 2004).
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In an internally heated mantle or in a
mantle that is cooled by cold slabs, the
geotherm becomes subadiabatic.
This means that shallow mantle
temperatures can be hotter than at
~600 km.
Actual mantle temperatures and their
variations are greater, and the melting
temperatures can be less, than
assumed in plume modelling.
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