THERMAL STATE AND
GEOCHEMICAL COMPOSITION OF
THE MANTLE.
WHAT CAN WE INFER FROM
IGNEOUS ROCKS?
Michele Lustrino
Dipartimento di Scienze della Terra, Univ. La Sapienza, Roma
michele.lustrino@uniroma1.it
Thanks to:
Don L. Anderson, Carlo Doglioni, Gill
Foulger, Jim Natland, Giuliano Panza,
Dean Presnall, Bob Stern and all the
www.mantleplumes group.
Attention Plumers:
This is an
Anti-Plume Talk!
Chemical Geodynamics (Zindler and Hart,
1986) has proposed a strict link between
geochemistry and geophysics.
Geochemical models/hypotheses have
been commonly, AND DANGEROUSLY,
translated into physical concepts.
A mental loop or circular reasoning is
created when geochemical concepts are
used to reinforce geophysical evidence
(and vice-versa).
No clear and unequivocal thermal and
chemical models for the upper/whole
mantle are yet available.
Important and fundamental concepts are
not yet fully understood and accepted
worldwide with the same significance:
Upper Mantle?
Transition Zone Mantle?
Lithosphere?
Asthenosphere?
Boundary Layers?
Depleted/Enriched/Primitive/Fertile Mantle?
Unsupported assumptions:
1
Upper mantle is:
- Homogeneous
- Chemically depleted
This is a BASIC ASSUMPTION (Green and
Ringwood, 1967; De Paolo and Wasserburg,
1976).
Plumers adopted this approach to propose
deeper sources for OIB-like magmas.
The mantle sources of several OIB types are
not much different from MORB source from a
Sr-Nd-Pb isotopic point of view.
MORBs
From: White, 2010
(Ann. Rev. Earth
Planet. Sci.)
MORBs
The mantle sources of several OIB types are
not much different from MORB source from a
Sr-Nd-Pb isotopic point of view.
Depleted
Isotopic Field
1) MORB sources are isotopically
depleted.
2) The depletion is ancient
(evolution with low Rb/Sr and low
Nd/Sm for several Ga).
From: Hofmann, 2004
(Encyclopedia of Geochemistry)
The mantle sources of several OIB types are
not much different from MORB source from a
Sr-Nd-Pb isotopic point of view.
Depleted
Isotopic Field
From: Hofmann, 2004
(Encyclopedia of Geochemistry)
OIB
Vague
definition
OK
Vague definition
based on
assumptions
Again vague
definition
Definition based
on assumptions,
not evidence
Unrelated to
subduction?
From: Hofmann, 1997 (Nature, 385, 219-229)
Are Hawaiian magmas geochemically unrelated
to subduction?
Sobolev et al (2011) A young source for the HawaiianPlume. Nature
Huang et al (2011) Stable calcium isotopic compositions of Hawaiian lavas: Evidence
for recycling of ancient marine carbonates into the mantle. Geochim. Cosmochim.
Acta
Fodor and Bauer (2010) Kahoolawe Island, Hawaii: The role of an ‘inaccessible’
shieldv olcano in the petrologyof the Hawaiian islands and plume. Chem Erdie
Ren et al (2009) Geochemical differences of the Hawaiian shield lavas: implications
for melting process in the heterogeneous Hawaiian plume. J. Petrol.
Huang et al (2009) Ancient carbonate sedimentary signature in the Hawaiian plume:
evidence from Mahukona volcano, Hawaii. Geochem. Geophys. Geosyst.
Blichert-Toft and Albarede (2009) Mixing of isotopic heterogeneities in the Mauna
Kea plume conduit. Earth Planet. Sci. Lett.
Sobolev et al. (2007) The amount of recycled crust in sources of mantle-derived
melts. Science.
Nielsen et al. (2006) Thallium isotopic evidence for ferromanganese sediments in
the mantle source of Hawaiian basalts. Nature
Herzberg (2006) Petrology and thermal structure of the Hawaiian plume from
Mauna Kea volcano. Nature
Huang e Frey (2005) Recycled oceanic crust in the Hawaiian plume: evidence from
temporal geochemical variations within the Koolau Shield. Contrib. Mineral. Petrol.
Gaffney et al. (2005) Melting in the Hawaiian Plume at 1-2 Ma as recorded at Maui
Nui: the role of eclogite, peridotite and source mixing. Geochem. Geophys.
Geosyst.
Sobolev et al. (2005) An olivine-free mantle source for Hawaiian shield lavas.
Nature.
Unsupported assumptions:
1
Upper mantle is:
- Homogeneous
- Chemically depleted
This is a BASIC ASSUMPTION (Green and
Ringwood, 1967; De Paolo and Wasserburg,
1976).
The concept itself of
“Normal-MORB” is an invention.
…Normal with respect to what?
Unsupported assumptions:
1
Upper mantle is:
- Homogeneous
- Chemically depleted
Was Igor Normal?
…Probably yes,
but only in the
Frankenstein
Castle…
Unsupported assumptions:
1
Upper mantle is:
- Homogeneous
- Chemically depleted
….D-MORB, N-MORB, T-MORB, E-MORB, P-MORB
Anomalies along the ridge system–elevation,
chemistry, physical properties are part of a
continuum and the distinction between ‘normal’
and ‘anomalous’ ridge segments is arbitrary and
model-dependent. (NewTOE; Anderson, 2007)
Unsupported assumptions:
1
Upper mantle is:
- Homogeneous
- Chemically depleted
Selected trace
element variability
of a SMALL set of
MORBs (40-55°
S. Atlantic Ocean)
From: Hofmann, 2004
(Encyclopedia of Geochemistry)
Unsupported assumptions:
1
Upper mantle is:
- Homogeneous
- Chemically depleted
…Mmmhhh…
Heterogeneous
MORB sources?
“Heterogeneities from
plumes may comprise a
substantial fraction of all
heterogeneities in the
MORB source.” (Davies, 2009,
G3)
Unsupported assumptions:
1
Upper mantle is:
- Homogeneous
- Chemically depleted
Pay attention in distinguishing “Fertile” from
“Enriched”.
MORB sources are not enriched (low
incompatible trace element content) but are
not necessarily sterile or refractory (they
are essentially four-phase lherzolite,
producing lithophile-element-rich melts).
Unsupported assumptions:
1
Upper mantle is:
- Homogeneous
- Chemically depleted
Pay attention in distinguishing “Fertile” from
“Enriched”.
Fertile vs. Sterile
(capacity or lack there of to generate basaltic melts)
Enriched vs. Depleted
(incompatible trace element content).
Unsupported assumptions:
1
Upper mantle is:
- Homogeneous
- Chemically depleted
Basalts (and therefore MORBs too) are
generated when all four of the lherzolite
phases are present in some proportion.
The relative amounts of these minerals is not
important, so that some of these basaltyielding source regions would be called
pyroxenites, harzburgites or lherzolites.
Unsupported assumptions:
1
Upper mantle is:
- Homogeneous
- Chemically depleted
Mantle rocks can be, at the same time:
Fertile and Enriched (e.g., OIB sources)
Fertile and Depleted (e.g., MORB sources)
Sterile and Enriched (e.g. Harzburgite xenoliths)
Sterile and Depleted (unable to produce basalts)
Unsupported assumptions:
2
Asthenosphere is:
- Fully convecting
-Chemically homogeneous and depleted
The Asthenosphere is a layer that is able to
flow or creep. It is not elastic and not rigid. It
deforms under a load because it has relatively
low viscosity. This deformation can be simple
laminar flow, as when a plate moves over a
viscous fluid. This does not mean that it is
homogeneous or fully convecting.
Unsupported assumptions:
2
Asthenosphere is:
- Fully convecting
-Chemically homogeneous and depleted
The axiom: Asthenosphere = Convecting mantle
= Vigorously stirred mantle = Homogeneous
mantle is simply not correct.
This is based on the assumption that MORBs =
Homogeneous magmas = Homogeneous mantle
sources = Convecting = Well mixed source.
Unsupported assumptions:
[…] “The presence in oceanic basalts of a common mantle
component that is not the ubiquitous depleted upper mantle
(asthenosphere) of mid-ocean ridge basalts (MORB) is
probably one of the major findings of igneous isotope
geochemistry” (Cadoux et al., 2007 EPSL).
[…] “Geophysical evidence and numerical models of mantle
stirring imply the source of mid-ocean ridge basalts
(MORBs) comprises most of the mantle, excepting only the
D” region and the ‘‘superpile’’ anomalies deep under Africa
and the Pacific.” (Davies, 2009 G3).
[...] “We modeltwo mantle reservoirs corresponding in mass
to the Earth’s upper mantle (MORB source) and lower
mantle (OIB source), respectively.” (Gonnermann and
Mukhopadhyay, 2010, Nature)
Unsupported assumptions:
2
Asthenosphere is:
- Fully convecting
-Chemically homogeneous and depleted
The asthenosphere is a relatively lowviscosity layer, not a vigorously stirred
and convecting layer.
Plate tectonics and post-glacial rebound
(isostasy) require a low viscosity, not
vigorous convection.
Unsupported assumptions:
From: Anderson, 2011 (J. Petrol.)
Upper
Boundary
Layer
B Region of
Gutenberg (1959)
Laterally Advecting
Melt
and Anisotropic
Fraction
Mantle
50-120
km
Classically
defined
“Asthenosphere”
“Fixed (convecting)”
Mantle
Unsupported assumptions:
From: Anderson, 2011 (J. Petrol.)
50-120
km
G Discontinuity
Melt In
(or fluid-rich)
Classically
defined
“Asthenosphere”
L Discontinuity
Melt Out
(or fluid-poor)
Melt
Fraction
“Fixed (convecting)”
Homogeneous
(convecting)
“Asthenosphere”
Mantle
Unsupported assumptions:
Seismic Lid
LITHOSPHERE
By definition
(McKenzie and
Bickle, 1988) the
LITHOSPHERE is
the non-convecting
part of the mantle
characterized by
conductive
geotherm
From: Anderson, 2011 (J. Petrol.)
50-120
km
Classically
defined
“Asthenosphere”
“Fixed (convecting)”
Homogeneous
(convecting)
“Asthenosphere”
Mantle
Unsupported assumptions:
3
Tomography can be used to measure
the temperature of the mantle
Positive Vs and Vp anomalies can be related to
the presence of less dense material (e.g.,
depleted harzburgite, seismic lid) and low
velocity anomalies can be dense eclogite.
Tomographic images are perturbations of an
initial reference model, and the assumed model
may greatly influence the final results.
Unsupported assumptions:
From: Kumagai et al., 2008 (GRL)
a = Grand et al. 2007
b = Mègnin and
Romanowicz, 2000
c = Ritsema et al.,
1999
d = Montelli et al.,
2006
Plume or not under Iceland?
a
b
c
d
Unsupported assumptions:
50 km
100 km
Pacific Plate
Tomography
Where is the Hawaiian
thermal plume?
Maggi et al., 2006 (EPSL)
150 km
200 km
250 km
300 km
Unsupported assumptions:
Where is the Hawaiian
thermal plume?
Kustwosky et al., 2008 (J Geophys Res)
Pacific Plate
Tomography
Unsupported assumptions:
Mantle Plume
trace?
This too?
Schmerr et al. 2010 (EPSL)
Unsupported assumptions:
3
Tomography can be used to measure
the temperature of the mantle
“Red" patches in tomographic images can be
fine grained peridotite, eclogite, H2O, CO2 or
melt.
With volatiles or eclogite components it is not
necessary to dream up mechanisms to cause
melting or raise the temperature.
Unsupported assumptions:
3
Tomography can be used to measure
the temperature of the mantle
Seismic wave velocity is also dependent on
the direction in which the waves travel.
The velocities of surface waves (large
horizontal component of motion) are
different from steeply up-coming S waves
(large vertical component).
Unsupported assumptions:
3
Tomography can be used to measure
the temperature of the mantle
“Improved seismology is likely to become
definitive on the question of existence
of plumes in the mid-mantle. We really
do not know how the deep Earth works.
We need much more seismic data.”
(Sleep, 2006, Earth Sci. Rev.)
Unsupported assumptions:
3
Tomography can be used to measure
the temperature of the mantle
Seismology has simply no more
power to map hot plumes than
geochemistry
(and geochemistry can say NOTHING
about T)
Unsupported assumptions:
3
Tomography can be used to measure
the temperature of the mantle
“Between the depths of 100 and 250 km, the velocity
anomalies detected below the present study region are
approximately 2–2.5% slower than average, implying a
temperature excess of about 220–280 K, which is
consistent with estimates for other mantle plumes.”
(Macera et al., 2003 J. Geodyn.)
“[…] we compute instantaneous, three-dimensional
spherical-mantle flow driven by temperature (density)
anomalies as inferred from seismic tomography, assuming
that velocity anomalies are simply related to temperature.”
(Faccenna and Becker, 2010, Nature)
Unsupported assumptions:
4
The Potential Temperature (Tp) of the
mantle at the base of the Plate (~100 km)
and for the whole upper mantle is
~1280°C.
The potential temperature is the temperature
the mass would have (hence the term
“potential”) if it were compressed or expanded
to some constant reference pressure (1 atm).
This concept is based on the assumption
of a homogeneous and isothermal
upper mantle at a given depth.
Unsupported assumptions:
4
The Potential Temperature (Tp) of the
mantle at the base of the Plate (~100 km)
and for the whole upper mantle is
~1280°C.
The concept itself of Tp should be considered
in relation to the depth of magma formation.
Magmas formed at high P show high Tp;
magmas formed at shallower P show lower Tp.
This does not imply any kind of thermal
anomaly, but it indicates a temperature
gradient in the mantle.
Potential Temperature
Two main models concerning Mantle Potential
Temperature:
The difference between the two models described above
depends on the assumptions:
Assuming a “normal” mantle potential temperature
of~1280 °C, magmas formed at higher
temperatures (e.g., in mid-plate area ssuchas
Hawaii) comes from hotter sources.
“…Geochemistry provides convincing evidence
that mantle plumes are 100–300 °C hotter
than normal upper mantle” W. M. White, 2010
(Oceanic Island Basalts and Mantle Plumes: The Geochemical
Perspective. Ann. Rev. Earth Planet. Sci. 38, 133-160)
Potential Temperature
Two main models concerning Mantle Potential
Temperature:
The difference between the two models described above
depends on the assumptions:
Assuming a “normal” mantle potential temperature
of~1280 °C, magmas formed at higher
temperatures (e.g., in mid-plate areas suchas
Hawaii) comes from hotter sources.
Alternatively:
The MORB source is colder than elsewhere
(because extensive melting cools the upper
mantle). In this case, the anomaly is not the
mid-plate mantle, but the MORB sources.
Potential Temperature
Two main models concerning Mantle Potential
Temperature:
The difference between the two models described above
depends on the assumptions:
“Long wavelength temperature variation sof the
asthenosphere (LAM) depart from the mean by ±200
°C, not the ±20 °C adopted by plume theoricians.
The ‘normal’ variation, caused by plate tectonic
processes (subduction cooling, continental insulation,
small scale convection) encompasses the temperature
excesses that have been attributed to hot jets and
thermal plumes.” (Anderson, 2000, Geophys. Res. Lett.).
Potential Temperature
900
0
4
6
8
1110
E
1300
1220
1350
1500
1480 1550
D
C
B
(one of the possible)
Geotherm
A
What is the
Mantle Potential
Temperature at
these points?
1700
100l
Depth (km)
Pressure (GPa)
2
1100
Conductive Layer
An upwelling
mantle volume
may start
melting at A, B,
C, D, E, …
…, when
it crosses the
local solidus.
Temperature (°C)
or Thermal Boundary Layer
A magma
forms where
the mantle
temperature
crosses the
Solidus.
200
Unsupported assumptions:
4
The Potential Temperature (Tp) of the
mantle at the base of the Plate (~100 km)
and for the whole upper
mantleis~1280°C.
Hawaii may have ambient Tp up to 1600 °C,
but so does most of the mantle away from
ridges.
The Pacific asthenosphere away from hotspots is as hot as Hawaii asthenosphere
(e.g., heatflow).
Unsupported assumptions:
5
Geochemistry clearly indicates
provenance of OIB from deep mantle.
Much is based on the original model of crustal
recycling by A.W. Hofmann.
Very radiogenic 206Pb/204Pb isotopic ratios
(>21) of an extremely rare group of OIBs (~12%) (HIMU-like) is compatible with the
recycling of high 238U/204Pb altered oceanic
crust and very long storage and isotopic
growth of 206Pb from the parent 238U (>2 Ga).
Unsupported assumptions:
5
Geochemistry clearly indicates
provenance of OIB from deep mantle.
Storage in the deepest lower mantle was
considered necessary to allow the isotopic growth
in the recycled slab to be not involved in the
supposedly vigorous stirring of the rest of the
mantle.
Recently this long isolation (>2 Ga) has been
considered not necessary. Sr isotopes on Hawaiian
melt inclusions require younger recycling ages
(0.2-0.6 Ga)(Sobolev et al., 2011, Nature)
1st possibility: Recycling
and folding at 670 km
2nd possibility: Recycling
and folding at D” (2900 km)
?
X
Only after substantial
isotopic growth would the
206Pb/204Pb have reached
very radiogenic values
(up to 21-22)
Storage of high
(High
U/Pb) recycled
oceanic crust for >2
Ga, allowing isotopic
growth of 206Pb
238U/204Pb
From: Stern (2002) Rev. Geophys.,
40, doi:10.1029/2001RG000108
Unsupported assumptions:
6
Magmas in equilibrium with mantle
sources (primitive melts) must have
Mg# [Mg/(Mg+Fe)] ~0.7
This concept is based on the assumption of a
distribution coefficient of Fe and Mg between
an Mg-rich solid source (Mg# ~90) and a
partial melt.
Upper mantle is characterized also by the
presence of Mg#-poorer lithologies (e.g.,
eclogites or pyroxenites s.l.).
Unsupported assumptions:
6
Magmas in equilibrium with mantle
sources (primitive melts) must have
Mg# [Mg/(Mg+Fe)] ~0.7
This may have strong effects when
recalculating the “original” melt composition of
basaltic rocks assuming melts with MgO up to
15 wt% in equilibrium with mantle residua.
This would mean that some (or all) the olivinemelt thermometric estimates are
overestimated.
Unsupported assumptions:
7
High magma production is related to
High Absolute Temperature.
This definition is not correct, because high
melt productivity can be related to High
Homologous Temperature.
The Homologous Temperature is the ratio
of the temperature of a substance to the
melting temperature (solidus for natural
systems) of the same substance.
Unsupported assumptions:
7
High magma production is related
with High Absolute Temperature.
This definition is not correct, because high
melt productivity can be related to High
Homologous Temperature.
In a lherzolitic mantle at a given depth, the H.T. may be:
1000°C/1300°C = 0.76.
At the same depth, in an eclogite-bearing mantle the H.T.
may be: 1000°C/1000°C. = 1.00.
An eclogite-bearing mantle has higher H.T.
Unsupported assumptions:
7
High magma production is related to
High Absolute Temperature.
This definition is not correct, because high
melt productivity can be related to High
Homologous Temperature.
Huge amounts of melts can, thus, be
produced at “normal/average” mantle
temperatures from low temperature-melting
mantle assemblages (e.g., eclogite-bearing
peridotites).
Unsupported assumptions:
7
High magma production is related to
High Absolute Temperature.
Many geochemists assert that the whole upper
mantle is MORB, cold and homogeneous and
that MORB comes from ambient convecting
mantle.
There is plenty of room and magma in the 220 kmthick Boundary Layer to provide Hawaii, Ethiopia,
Siberia, Deccan, Kerguelen and Ontong Java LIPs.
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
True INTRA-PLATE magmatism does not
exist. Igneous activity always develops at
plate margins (i.e., along lithospheric
discontinuities).
Edge-driven effects, lithosphere cracking,
small-scale convection beneath the seismic lid
and/or shear heating at the base of the
lithosphere can contribute to magma formation.
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
From: Babuska et al. (2002) Tectonics,
21, 10.1029/2001TC901035
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
Igneous activity in a
cratonic area?
An oxymoron.
This model works also without
a plume. It is a sort of edgedriven effect
Why invoke the
presence of a PLUME?
From: Sleep (2006) Earth Sci. Rev.
PLUME
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
[…] “for certain geometries and viscosity ratios,
circulatory flow develops within a “cavity” or “step”
embedded into the lithospheric base, or within a lowviscosity “pocket” embedded within the asthenospheric
layer.”
Calculated Shear-Driven Upwelling rates for
asthenosphere shearing at 5 cm/yr: 0.2 cm/yr
(continental rift), 0.5 cm/yr (craton edge), 1.0 cm/yr
(within a “pocket” of low-viscosity asthenospherere)
(Conrad et al., 2009, Phys. Earth Planet. Int.)
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
“Such asthenosphere viscosity heterogeneity may be
associated with thermal, chemical, melting, volatile,
or grain-size anomalies, and is consistent with
tomographic constraints on asthenospheric
variability. We estimate that shear-driven upwelling
may generate up to 2.5 km/Myr of melt that is
potentially eruptible as surface volcanism” (Conrad et
al., 2009, Phys. Earth Planet. Int.)
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
Ac = Wc/Hc
(width of the cavity)
Tc = Hasth/(Hasth+Hc)
(asthenosphere thickness with
and without the cavity)
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
Flow velocity in the cavity as a fraction of the assumed original velocity of the
asthenosphere below the cavity (e.g., 5 cm/yr)
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
Assuming an
original
asthenospheric
flow velocity of
5 cm/yr, it is
possible to
develop
upwelling flows
with velocities
>0.5 cm/yr
Upwelling velocity in the cavity. Tc = height
of the cavity; Ac = width of the cavity.
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
The same as
before, but
assuming a lowviscosity
asthenospheric
layer in the
cavity.
Upwelling velocity in the cavity. Tc = height
of the cavity; Ac = width of the cavity.
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
In this case no lid
cavity is present.
A low-viscosity
volume is assumed
within the
asthenosphere.
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
ALV = WLV/HLV
H’LV = HLV/Hasth
D’LV = DLV/Hasth
h'LV = hLV/hasth
Unsupported assumptions:
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
Assuming:
h'LV = hLV/hasth
= 0.01
(i.e., a lowviscosity layer 100
times less viscous
than ambient
asthenosphere)
Unsupported assumptions:
In this case, assuming an
original asthenospheric
flow velocity of 5 cm/yr,
with h’LV = 0.01, it is
possible to develop
upwelling flows with
velocities ~1 cm/yr
H’LV
8
Intra-plate magmatism is relatedto the
presence of mantle plumes.
ALV
Unsupported assumptions:
9
High 3He/4He ratios in basaltic melts
indicate undegassed (primitive)
mantle sources (= deep mantle origin).
3He is the stable Helium isotope. 4He is
the Helium isotope produced by decay of
U and Th.
MORBs have typically lower 3He/4He (but
much higher 3He and 4He) than OIBs.
Unsupported assumptions:
9
High 3He/4He ratios in basaltic melts
indicate undegassed (primitive)
mantle sources (= deep mantle origin).
3He is the stable Helium isotope. 4He is
the Helium isotope produced by decay of
U and Th.
Refractory peridotites with almost no
3He have high 3He/238U, just as
"undegassed or primordial" mantle.
Unsupported assumptions:
9
High 3He/4He ratios in basaltic melts
indicate undegassed (primitive)
mantle sources (= deep mantle origin).
High concentrations of […] 3He/4He ratios of
about 50 Ra, noble gas characteristics that
are normally attributed to a primitive mantle
or hidden reservoirs, can be preserved in a
convecting and processed lower mantle. (From:
Gonnermann and Mukhopadhyay, 2009, Nature)
Unsupported assumptions:
9
High 3He/4He ratios in basaltic melts
indicate undegassed (primitive)
mantle sources (= deep mantle origin).
Early workers assumed high 3He for high 3He/4He
(therefore undegassed and, consequentially,
primitive mantle), rather than low 4He.
High 3He/4He (up to 50 Ra) has been found in UHP
crustal terranes,in Baffin Bay depleted picrites,
Lau back-arc basalts, South Arc basalts, dunite
cumulates, and other “not-Hot-Spot” low 4He cases.
Unsupported assumptions:
9
High 3He/4He ratios in basaltic melts
indicate undegassed (primitive)
mantle sources (= deep mantle origin).
Helium and Carbon are absolutely incompatible
in silicate mantle mineral structure.
3He/CO
is the same for OIB and MORB.
4He/CO is higher for MORB.
2
2
This supports the idea that 4He is responsible
for MORB-OIB differences, not 3He!
Unsupported assumptions:
9
High 3He/4He ratios in basaltic melts
indicate undegassed (primitive)
mantle sources (= deep mantle origin).
What should be clear is that:
3He/4He
Low
does not imply
"degassed" nor does High
3He/4He imply “undegassed”.
Unsupported assumptions:
10
The trace element and isotopic overlap
of different igneous rocks is evidence
for the derivation from the same
physical sources.
This definition/model/assumption
does not stand up.
1000
Sample/Primitive Mantle
Area covered:
More than 3000 km-long
100
Canary Islands
Bohemian Massif
Pannonian Basin
France
Germany
Spain
10
1
Rb Th Nb
K
P Sm Hf Ti
Er
Ce Pr
Y
Yb
Tb
Cs Ba
U
Ta
La Pb
Sr Nd Zr
Eu Gd
Dy Ho Tm Lu
Sample/Primitive Mantle
1000
St. Helena
France
basalts: Typical
Germany
Spain
HIMU-OIBs
100
Ba/Nb
St. Helena
10
1
Rb Th Nb
K
P Sm Hf Ti
Er
Ce Pr
Y
Yb
Tb
Cs Ba
U
Ta
La Pb
Sr Nd Zr
Eu Gd
Dy Ho Tm Lu
Does anybody believe in a single mantle plume origin
for St. Helena basalts and those of Germany or
Bohemian massif on the basis of geochemical
similarities?
St. Helena
Island
Around the Mediterranean many “intraplate” igneous
rocks occur.
Essentially: low-volume, low-degree partial melts with
alkaline sodic to tholeiitic compositions.
“Intraplate” (or
“anorogenic”) Cenozoic
igneous rocks
0.5134
Just
very
few
geochemical
0.5132
comments on the
0.5130
“anorogenic” igneous rock of
0.5128
the Circum-Mediterranean
0.5126
area:
1 43
144
Nd/ Nd
Canary Islands
Etna-Hyblean Mts.
Madeira
Veneto District
Portugal
Italy (Pietre Nere)
Spain
Bohemian Massif
Maghreb
Libya
France
Pannonian Basin
Sardinia UPV
East Europe
Sardinia RPV
Turkey
Germany
Mashrek
Ustica and Sicily Channel)
From: Lustrino and
Wilson (2007)
Earth-Sci. Rev.
0.5124
0.5122
0.5120
0.702
0.704
0.706
0.708
87
0.710
0.712
0.714
0.716
86
Sr/
Sr
Database from: Lustrino (2011) Geol. Mag.
The low volume, low
degree partial melting,
geographic position, age,
temperature, heat-flow
measurements, absence
of tracks and geological
setting are incompatible
with a thermal mantle
plume origin.
0.5132
Nd
0.5130
143
Nd
144
0.5128
0.5126
Surprisingly small
spread of data of the
bulk of the samples
0.5124
0.5122
0.700
0.705
0.710
150
100
50
0
0.715
87Sr/86Sr
87
86
Sr
SrDatabase from: Lustrino (2011) Geol. Mag.
Unsupported assumptions:
As concerns the depth of magma
formation, …
It is one thing to deal with the depth of
magma extraction from the solid residue.
It is another to deal with the depth of
melting (magma formation).
And yet another to deal with the depth
of provenance of the solid source.
Thermal Plume (1); Fossil Plume (2); Channelled Plume (3);
Toroidal Plume (4); Tabular Plume (5); Depleted residual
Plume (6); Finger-like Plume (7); Recycled Plume head (8);
Edge Plume (9); Cold Plume (10); Cactoplume (11); Super
Plume (12); Asthenospheric Plume (13) Dying Plume (14); Not
very energetic Plume (15); Spaghetti Plume (16); Baby Plume
(17); Head-free Plume (18); Splash Plume (19); Pulsating
Plume (20); Subduction fluid-fluxed refractory Plume(21);
Hydrogen Plume (22); Heterogeneous Plume (23); Flattened
Onion Plume (24); Subduction-driving Plume (25);
Subduction-triggered Plume (26); Washboard Plume (27)
1 (Griffiths and Campbell, 1990); 2 (Stein and Hofmann, 1992); 3 (Camp and Roobol,
1992);4 (Mahoney et al., 1992); 5 (Hoernle et al., 1995), 6 (Danyushevsky et al., 1995); 7
(Granet et al., 1995); 8 (Gasperini et al., 2000); 9 (King and Ritsema, 2000); 10(Hanguita
and Hernan, 2000); 11 (Lundin, 2003); 12 (Condie, 2004); 13 (Seghedi et al., 2004); 14
(Davaille and Vatteville, 2005); 15 (Michon and Merle, 2005); 16 (Abouchami et al.,
2005); 17 (Ritter, 2006); 18 (e.g., Ritter, 2006); 19 (Davies and Bunge, 2006); 20
(Krienitz et al., 2007); 21 (Falloon et al., 2007); 22 (Dobretsov, 2008); 23 (Ren et al.,
2009); 24 (Beccaluva et al., 2010); 25 (Burov and Cloetingh 2010); 26 (Faccenna et al.,
2010); 27 (Ballmer et al, 2011).
Comparison with geosynclines
•
•
•
•
•
•
•
•
•
Mio-geosyncline
Eu-geosyncline
Ortho-geosyncline
Primary geosyncline
Zeugo-geosyncline
Para-geosyncline
Exo-geosyncline
Taphro-geosyncline
Paralia-geosyncline
Thanks to Gill Foulger
Why are deep mantle plumes needed?
-High melt productivity of LIPs? No. High Homologous
Temperature (chemical rather temperature anomalies).
-Peculiar Sr-Nd-Pb isotopic composition of OIBs? No. ALL
the most peculiar geochemical characteristics of OIBs
require recycled crustal lithologies, not deep sources.
- High 3He/4He means undegassed - therefore never-tapped
by basaltic magmatism - mantle sources? No. Helium
isotopes do not support this.
-Doming in some CFB? No. Presence of abundant (buoyant)
basaltic melt, not hot mantle sources. Doming not ubiquitous
-Vs and Vp anomalies in tomographic images? No. Seismic
anomalies rather reflect chemical heterogeneity. Mantle
plumes have been proven difficult to image using seismology.
Why are deep mantle plumes needed?
- High potential temperatures of some OIBs compared with
MORBs? No. OIB Tp is ambient mantle temperature.
MORBs are colder than “average”.
-Age progression in some Island Chains? No. Can be
explained by progressive cracks in the lithosphere.
- Long isolation time to allow isotopic growth of 206Pb/204Pb
ratios? No. Isolation may happen also in the shallow, non
convecting mantle (B Layer of Gutenberg) or at 670 km.
-Peculiar trace element composition of OIBs? No. OIBs are
very heterogeneous from an incompatible trace element
point of view. All of them require the involvement of crustal
lithologies in their sources.
Why are deep mantle plumes needed?
- Geochemical similarity of areally dispersed OIB-like
igneous rocks? No. Similar mantle processes, not the same
physical sources.
-The only model to explain the fixity of hot-spot tracks?
No. Also in the Hawaii-Emperor Chain case such a fixity is
not demonstrated (i.e. the geographic coordinates of the
source move).
- Do you suggest other questioned features for mantle
plumes? ……….
Message to take away:
Mantle
Plumes
Thanks for your attention
Visit: www.mantleplumes.org