THERMAL STATE AND GEOCHEMICAL COMPOSITION OF THE MANTLE. WHAT CAN WE INFER FROM IGNEOUS ROCKS? Michele Lustrino Dipartimento di Scienze della Terra, Univ

THERMAL STATE AND THERMAL STATE AND GEOCHEMICAL COMPOSITION OF GEOCHEMICAL COMPOSITION OF

THE MANTLE.THE MANTLE.WHAT CAN WE INFER FROM WHAT CAN WE INFER FROM

IGNEOUS ROCKS?IGNEOUS ROCKS?Michele LustrinoMichele Lustrino

Dipartimento di Scienze della Terra, Univ. La Sapienza, Dipartimento di Scienze della Terra, Univ. La Sapienza, RomaRoma

[email protected]@uniroma1.itThanks to:Thanks to:Don L. Anderson, Carlo Doglioni, Gill Don L. Anderson, Carlo Doglioni, Gill Foulger, Jim Natland, Giuliano Panza, Foulger, Jim Natland, Giuliano Panza, Dean Presnall, Bob Stern and all the Dean Presnall, Bob Stern and all the www.mantleplumes group.www.mantleplumes group.

!Attention Plumers:Attention Plumers:

This is an This is an Anti-Plume Talk!Anti-Plume Talk!

Chemical Geodynamics (Zindler and Chemical Geodynamics (Zindler and Hart, 1986) has proposed a strict link Hart, 1986) has proposed a strict link between geochemistry and between geochemistry and geophysics.geophysics.Geochemical models/hypotheses have Geochemical models/hypotheses have been commonly, been commonly, AND DANGEROUSLYAND DANGEROUSLY, ,

translated into physical concepts.translated into physical concepts.

A mental loop or circular reasoning is A mental loop or circular reasoning is created when geochemical concepts created when geochemical concepts are used to reinforce geophysical are used to reinforce geophysical evidence (and vice-versa).evidence (and vice-versa).

No clear and unequivocal thermal and No clear and unequivocal thermal and chemical models for the upper/whole chemical models for the upper/whole mantle are yet available.mantle are yet available.

Important and fundamental concepts Important and fundamental concepts are not yet fully understood and are not yet fully understood and

accepted worldwide with the same accepted worldwide with the same significance:significance:Upper Mantle?Upper Mantle?

Transition Zone Mantle?Transition Zone Mantle?Lithosphere?Lithosphere?Asthenosphere?Asthenosphere?Boundary Layers?Boundary Layers?Depleted/Enriched/Primitive/Fertile Mantle?Depleted/Enriched/Primitive/Fertile Mantle?

Unsupported Unsupported assumptions:assumptions:

Upper mantle is:Upper mantle is:- HomogeneousHomogeneous- Chemically depletedChemically depleted

This is a BASIC ASSUMPTION This is a BASIC ASSUMPTION (Green (Green and Ringwood, 1967; De Paolo and and Ringwood, 1967; De Paolo and Wasserburg, 1976)Wasserburg, 1976)..

PlumersPlumers adopted this approach to adopted this approach to propose deeper sources for OIB-like propose deeper sources for OIB-like

magmas.magmas.

11

The mantle sources of several OIB types are The mantle sources of several OIB types are not much different from MORB source from a not much different from MORB source from a

Sr-Nd-Pb isotopic point of view.Sr-Nd-Pb isotopic point of view.

MORBsMORBs MORBsMORBs

From: White, 2010 (Ann. Rev. Earth Planet. Sci.)


Sr-Nd-Pb isotopic point of view.Sr-Nd-Pb isotopic point of view.

From: Hofmann, 2004 (Encyclopedia of Geochemistry)

Depleted Isotopic Field

1)1) MORB sources are isotopically MORB sources are isotopically depleted.depleted.

2)2) The depletion is ancient The depletion is ancient (evolution with low Rb/Sr and (evolution with low Rb/Sr and low Nd/Sm for several Ga).low Nd/Sm for several Ga).


Sr-Nd-Pb isotopic point of view.Sr-Nd-Pb isotopic point of view.From: Hofmann, 2004 From: Hofmann, 2004 (Encyclopedia of Geochemistry)(Encyclopedia of Geochemistry)

Depleted Isotopic Field

OIBOIB

From: Hofmann, 1997 (Nature, 385, 219-229)From: Hofmann, 1997 (Nature, 385, 219-229)

Vague Vague definitiondefinition

OKOK

Vague Vague definition based definition based on assumptionson assumptions

Again vague Again vague definitiondefinition

Definition Definition based on based on

assumptions, assumptions, not evidencenot evidenceUnrelated to Unrelated to subduction?subduction?

Are Hawaiian magmas geochemically unrelated to subduction?

Are Hawaiian magmas geochemically unrelated to subduction?

Nielsen et al. (2006) Thallium isotopic Nielsen et al. (2006) Thallium isotopic evidence for ferromanganese evidence for ferromanganese sedimentssediments in the mantle source of Hawaiian basalts. Nature in the mantle source of Hawaiian basalts. Nature

Nielsen et al. (2006) Thallium isotopic Nielsen et al. (2006) Thallium isotopic evidence for ferromanganese evidence for ferromanganese sedimentssediments in the mantle source of Hawaiian basalts. Nature in the mantle source of Hawaiian basalts. Nature

Huang e Frey (2005) Huang e Frey (2005) Recycled oceanic crustRecycled oceanic crust in the Hawaiian plume: evidence in the Hawaiian plume: evidence from temporal geochemical variations within the Koolau Shield. Contrib. from temporal geochemical variations within the Koolau Shield. Contrib. Mineral. Petrol.Mineral. Petrol.

Huang e Frey (2005) Huang e Frey (2005) Recycled oceanic crustRecycled oceanic crust in the Hawaiian plume: evidence in the Hawaiian plume: evidence from temporal geochemical variations within the Koolau Shield. Contrib. from temporal geochemical variations within the Koolau Shield. Contrib. Mineral. Petrol.Mineral. Petrol.Gaffney et al. (2005) Melting in the Hawaiian Plume at 1-2 Ma as recorded at Gaffney et al. (2005) Melting in the Hawaiian Plume at 1-2 Ma as recorded at Maui Nui: Maui Nui: the role of eclogitethe role of eclogite, peridotite and source mixing. Geochem. , peridotite and source mixing. Geochem. Geophys. Geosyst.Geophys. Geosyst.

Gaffney et al. (2005) Melting in the Hawaiian Plume at 1-2 Ma as recorded at Gaffney et al. (2005) Melting in the Hawaiian Plume at 1-2 Ma as recorded at Maui Nui: Maui Nui: the role of eclogitethe role of eclogite, peridotite and source mixing. Geochem. , peridotite and source mixing. Geochem. Geophys. Geosyst.Geophys. Geosyst.

Herzberg (2006) Petrology and thermal structure of the Hawaiian plume from Herzberg (2006) Petrology and thermal structure of the Hawaiian plume from Mauna Kea volcano. NatureMauna Kea volcano. Nature

Herzberg (2006) Petrology and thermal structure of the Hawaiian plume from Herzberg (2006) Petrology and thermal structure of the Hawaiian plume from Mauna Kea volcano. NatureMauna Kea volcano. Nature

Sobolev et al. (2007) The amount of Sobolev et al. (2007) The amount of recycled crustrecycled crust in sources of mantle- in sources of mantle-derived melts. Science.derived melts. Science.

Sobolev et al. (2007) The amount of Sobolev et al. (2007) The amount of recycled crustrecycled crust in sources of mantle- in sources of mantle-derived melts. Science.derived melts. Science.

Blichert-Toft and Albarede (2009) Mixing of Blichert-Toft and Albarede (2009) Mixing of isotopic heterogeneitiesisotopic heterogeneities in the in the Mauna Kea plume conduit. Earth Planet. Sci. Lett.Mauna Kea plume conduit. Earth Planet. Sci. Lett.

Blichert-Toft and Albarede (2009) Mixing of Blichert-Toft and Albarede (2009) Mixing of isotopic heterogeneitiesisotopic heterogeneities in the in the Mauna Kea plume conduit. Earth Planet. Sci. Lett.Mauna Kea plume conduit. Earth Planet. Sci. Lett.

Huang et al (2009) Huang et al (2009) Ancient carbonate sedimentary signatureAncient carbonate sedimentary signature in the Hawaiian in the Hawaiian plume: evidence from Mahukona volcano, Hawaii. Geochem. Geophys. plume: evidence from Mahukona volcano, Hawaii. Geochem. Geophys. Geosyst.Geosyst.

Huang et al (2009) Huang et al (2009) Ancient carbonate sedimentary signatureAncient carbonate sedimentary signature in the Hawaiian in the Hawaiian plume: evidence from Mahukona volcano, Hawaii. Geochem. Geophys. plume: evidence from Mahukona volcano, Hawaii. Geochem. Geophys. Geosyst.Geosyst.

Fodor and Bauer (2010) Kahoolawe Island, Hawaii: The role of an Fodor and Bauer (2010) Kahoolawe Island, Hawaii: The role of an ‘inaccessible’ shieldv olcano in the petrologyof the Hawaiian islands and ‘inaccessible’ shieldv olcano in the petrologyof the Hawaiian islands and plume. Chem Erdieplume. Chem Erdie

Fodor and Bauer (2010) Kahoolawe Island, Hawaii: The role of an Fodor and Bauer (2010) Kahoolawe Island, Hawaii: The role of an ‘inaccessible’ shieldv olcano in the petrologyof the Hawaiian islands and ‘inaccessible’ shieldv olcano in the petrologyof the Hawaiian islands and plume. Chem Erdieplume. Chem Erdie

Sobolev et al. (2005) An Sobolev et al. (2005) An olivine-free mantleolivine-free mantle source for Hawaiian shield lavas. source for Hawaiian shield lavas. Nature.Nature.

Sobolev et al. (2005) An Sobolev et al. (2005) An olivine-free mantleolivine-free mantle source for Hawaiian shield lavas. source for Hawaiian shield lavas. Nature.Nature.

Ren et al (2009) Geochemical differences of the Hawaiian shield lavas: Ren et al (2009) Geochemical differences of the Hawaiian shield lavas: implications for melting process in the heterogeneous Hawaiian plume. J. implications for melting process in the heterogeneous Hawaiian plume. J. Petrol.Petrol.

Ren et al (2009) Geochemical differences of the Hawaiian shield lavas: Ren et al (2009) Geochemical differences of the Hawaiian shield lavas: implications for melting process in the heterogeneous Hawaiian plume. J. implications for melting process in the heterogeneous Hawaiian plume. J. Petrol.Petrol.

Sobolev et al (2011) A young source for the HawaiianPlume. NatureSobolev et al (2011) A young source for the HawaiianPlume. NatureSobolev et al (2011) A young source for the HawaiianPlume. NatureSobolev et al (2011) A young source for the HawaiianPlume. Nature

Huang et al (2011) Stable calcium isotopic compositions of Hawaiian lavas: Huang et al (2011) Stable calcium isotopic compositions of Hawaiian lavas: Evidence for Evidence for recycling of ancient marine carbonates recycling of ancient marine carbonates into the mantle. into the mantle. Geochim. Cosmochim. ActaGeochim. Cosmochim. Acta

Huang et al (2011) Stable calcium isotopic compositions of Hawaiian lavas: Huang et al (2011) Stable calcium isotopic compositions of Hawaiian lavas: Evidence for Evidence for recycling of ancient marine carbonates recycling of ancient marine carbonates into the mantle. into the mantle. Geochim. Cosmochim. ActaGeochim. Cosmochim. Acta


This is a BASIC ASSUMPTION This is a BASIC ASSUMPTION (Green (Green and Ringwood, 1967; De Paolo and and Ringwood, 1967; De Paolo and Wasserburg, 1976)Wasserburg, 1976)..

The concept itself of The concept itself of “Normal-MORB” is an invention.“Normal-MORB” is an invention.

……Normal with respect to what?Normal with respect to what?

11


Was Igor Was Igor Normal?Normal?

……Probably yes, Probably yes, but only in the but only in the Frankenstein Frankenstein Castle…Castle…


11


……..DD-MORB-MORB, , NN-MORB-MORB, , TT-MORB-MORB, , EE-MORB-MORB, , PP--MORBMORB

Anomalies along the ridge system–Anomalies along the ridge system–elevation, chemistry, physical properties elevation, chemistry, physical properties are part of a continuum and are part of a continuum and the distinction the distinction between ‘normal’ and ‘anomalous’ ridge between ‘normal’ and ‘anomalous’ ridge segments is arbitrary and model-segments is arbitrary and model-dependent. dependent. (NewTOE; Anderson, 2007)(NewTOE; Anderson, 2007)


11



11

Selected trace Selected trace element element variability of variability of a a SMALL set of SMALL set of MORBsMORBs (40-55° S. (40-55° S. Atlantic Ocean)Atlantic Ocean) From: Hofmann, 2004 From: Hofmann, 2004

(Encyclopedia of Geochemistry)(Encyclopedia of Geochemistry)



11

““Heterogeneities from Heterogeneities from plumes plumes may comprise a may comprise a substantial fraction of substantial fraction of all heterogeneities all heterogeneities in in the MORB source.the MORB source.”” (Davies, 2009, G3)(Davies, 2009, G3)

…Mmmhhh… Heterogeneous MORB sources?


Pay attention in distinguishing “Pay attention in distinguishing “FertileFertile” ” from “from “EnrichedEnriched”.”.

MORB sources are not enrichedMORB sources are not enriched (low (low incompatible trace element content) incompatible trace element content) but but

are not necessarily sterile or refractoryare not necessarily sterile or refractory (they are essentially four-phase lherzolite, (they are essentially four-phase lherzolite,

producing lithophile-element-rich melts).producing lithophile-element-rich melts).


11


Pay attention in distinguishing “Pay attention in distinguishing “FertileFertile” ” from “from “EnrichedEnriched”.”.

Fertile vs. SterileFertile vs. Sterile(capacity or lack there of to generate basaltic (capacity or lack there of to generate basaltic

melts)melts)

Enriched vs. DepletedEnriched vs. Depleted(incompatible trace element content).(incompatible trace element content).


11


Basalts (and therefore MORBs too) are Basalts (and therefore MORBs too) are generated when all four of the lherzolite generated when all four of the lherzolite phases are present in some proportion.phases are present in some proportion.

The relative amounts of these minerals is The relative amounts of these minerals is not important, so that some of these not important, so that some of these

basalt-yielding source regions would be basalt-yielding source regions would be called pyroxenites, harzburgites or called pyroxenites, harzburgites or

lherzolites.lherzolites.


11


Mantle rocks can be, at the same Mantle rocks can be, at the same time:time:

Fertile and Enriched Fertile and Enriched (e.g., OIB sources)(e.g., OIB sources)

Fertile and Depleted Fertile and Depleted (e.g., MORB sources)(e.g., MORB sources)

Sterile and Enriched Sterile and Enriched ((e.g.e.g. Harzburgite Harzburgite xenolithsxenoliths))

Sterile and Depleted Sterile and Depleted ((unable to produce unable to produce basaltsbasalts))


11


Asthenosphere is:Asthenosphere is:- Fully convectingFully convecting-Chemically homogeneous and Chemically homogeneous and depleteddepletedThe Asthenosphere is a layer that is able to The Asthenosphere is a layer that is able to

flowflow or or creepcreep. . It is not elastic and not rigidIt is not elastic and not rigid. . It deforms under a load because it has It deforms under a load because it has relatively low viscosity. This deformation relatively low viscosity. This deformation can be simple laminar flow, as when a can be simple laminar flow, as when a plate moves over a viscous fluid. plate moves over a viscous fluid. This does This does not mean that not mean that it is homogeneous or fully it is homogeneous or fully convectingconvecting..

22


The axiom: Asthenosphere = Convecting The axiom: Asthenosphere = Convecting mantle = Vigorously stirred mantle = mantle = Vigorously stirred mantle = Homogeneous mantle Homogeneous mantle is simply not correctis simply not correct..

This is based on the assumption that This is based on the assumption that MORBs = Homogeneous magmas = MORBs = Homogeneous magmas =

Homogeneous mantle sources = Homogeneous mantle sources = Convecting = Well mixed source.Convecting = Well mixed source.

Asthenosphere is:Asthenosphere is:- Fully convectingFully convecting-Chemically homogeneous and Chemically homogeneous and depleteddepleted

22


[…] “[…] “The presence in oceanic basalts of a common The presence in oceanic basalts of a common mantle component that is not the ubiquitous depleted mantle component that is not the ubiquitous depleted upper mantle (asthenosphere) of mid-ocean ridge upper mantle (asthenosphere) of mid-ocean ridge basalts (MORB) is probably one of the major findings of basalts (MORB) is probably one of the major findings of igneous isotope geochemistryigneous isotope geochemistry”” (Cadoux et al., 2007 EPSL) (Cadoux et al., 2007 EPSL)..

[…] “[…] “Geophysical evidence and numerical models of Geophysical evidence and numerical models of mantle stirring imply the source of mid-ocean ridge mantle stirring imply the source of mid-ocean ridge basalts (MORBs) comprises most of the mantle, basalts (MORBs) comprises most of the mantle, excepting only the D” region and the ‘‘superpile’’ excepting only the D” region and the ‘‘superpile’’ anomalies deep under Africa and the Pacific.anomalies deep under Africa and the Pacific.”” (Davies, (Davies,

2009 G3)2009 G3)..

[...] “[...] “We modelWe modeltwo mantle reservoirs corresponding in two mantle reservoirs corresponding in mass to the Earth’s upper mantle (MORB source) and mass to the Earth’s upper mantle (MORB source) and lower mantle (OIB source), respectivelylower mantle (OIB source), respectively.” .” (Gonnermann (Gonnermann and Mukhopadhyay, 2010, Nature)and Mukhopadhyay, 2010, Nature)


The asthenosphere is a relatively low-The asthenosphere is a relatively low-viscosity layer, not a vigorously stirred viscosity layer, not a vigorously stirred and convecting layer.and convecting layer.

Plate tectonics and post-glacial Plate tectonics and post-glacial rebound (isostasy) require a low rebound (isostasy) require a low

viscosity, not vigorous convection.viscosity, not vigorous convection.

Asthenosphere is:Asthenosphere is:- Fully convectingFully convecting-Chemically homogeneous and Chemically homogeneous and depleteddepleted

22


“Fixed (convecting)”

MantleMelt Fraction

Upper Upper Boundary Boundary

LayerLayerB Region B Region of of Gutenberg (1959)Gutenberg (1959)

Laterally Laterally Advecting and Advecting and

Anisotropic Anisotropic MantleMantle

Classically defined

“Asthenosphere”

50-120 km

From: Anderson, 2011 (J. Petrol.)From: Anderson, 2011 (J. Petrol.)

Conductive

Laye

r


50-120 km

Melt InMelt In(or fluid-rich)(or fluid-rich)

Melt Out Melt Out (or fluid-poor)(or fluid-poor)

G G DiscontinuityDiscontinuity

L L DiscontinuityDiscontinuity



Mantle

Homogeneous (convecting)

“Asthenosphere”Melt Fraction

Conductive

Laye

r


Classically defined

“Asthenosphere”

50-120 km

By definition By definition (McKenzie and (McKenzie and Bickle, 1988) the Bickle, 1988) the LITHOSPHERELITHOSPHERE is is the non-the non-convecting part of convecting part of the mantle the mantle characterized by characterized by conductive conductive geothermgeotherm



Mantle

Homogeneous (convecting)

“Asthenosphere”

LIT

HO

SP

HER

E Seis

mic

Lid


Classically defined

“Asthenosphere”

Tomography can be used to Tomography can be used to measure the temperature of the measure the temperature of the mantlemantlePositive Vs and Vp anomalies can be Positive Vs and Vp anomalies can be

related to the presence of less dense related to the presence of less dense material (e.g., depleted harzburgite, material (e.g., depleted harzburgite, seismic lid) and low velocity anomalies can seismic lid) and low velocity anomalies can be dense eclogite.be dense eclogite.

Tomographic images are perturbations of Tomographic images are perturbations of an an initial reference modelinitial reference model, and the , and the

assumed model may greatly influence the assumed model may greatly influence the final results.final results.

33


a = Grand et al. a = Grand et al. 20072007b = Mègnin and b = Mègnin and Romanowicz, 2000Romanowicz, 2000c = Ritsema et al., c = Ritsema et al.,

19991999d = Montelli et al., d = Montelli et al.,

20062006

From: Kumagai et al., 2008 From: Kumagai et al., 2008 (GRL)(GRL)

Plume or not under Plume or not under Iceland?Iceland?a b

c d


Pacific Plate Pacific Plate TomographyTomography

100 km50 km

150 km 200 km250 km 300 km

Maggi et al., 2006 (EPSL)Maggi et al., 2006 (EPSL)

Where is the Where is the Hawaiian thermal Hawaiian thermal

plume?plume?


Kustwosky et al., 2008 (J Geophys Kustwosky et al., 2008 (J Geophys Res)Res)

Pacific Plate Pacific Plate TomographyTomography

Where is the Where is the Hawaiian thermal Hawaiian thermal plume?plume?


Mantle Plume Mantle Plume trace?trace? This too?This too?

Schmerr et al. 2010 (EPSL)Schmerr et al. 2010 (EPSL)


““Red" patches in tomographic images can Red" patches in tomographic images can be fine grained peridotite, eclogite, Hbe fine grained peridotite, eclogite, H22O, O, COCO22 or melt. or melt.

With volatiles or eclogite components it is With volatiles or eclogite components it is not necessary to dream up mechanisms to not necessary to dream up mechanisms to

cause melting or raise the temperature.cause melting or raise the temperature.

Tomography can be used to Tomography can be used to measure the temperature of the measure the temperature of the mantlemantle

33


Seismic wave velocity is also dependent Seismic wave velocity is also dependent on the direction in which the waves on the direction in which the waves travel.travel.

The velocities of surface waves (large The velocities of surface waves (large horizontal component of motion) are horizontal component of motion) are

different from steeply up-coming S different from steeply up-coming S waves (large vertical component). waves (large vertical component).


33


““Improved seismology is likely to Improved seismology is likely to become definitive on the question of become definitive on the question of existence of plumes in the mid-existence of plumes in the mid-mantle. mantle. We really do not know how We really do not know how the deep Earth works. We need much the deep Earth works. We need much more seismic datamore seismic data.” .” (Sleep, 2006, Earth (Sleep, 2006, Earth Sci. Rev.)Sci. Rev.)


33


Seismology has simply no Seismology has simply no more power to map hot more power to map hot

plumes than geochemistryplumes than geochemistry(and geochemistry can say (and geochemistry can say

NOTHING about T)NOTHING about T)


33


““Between the depths of 100 and 250 km, the velocity Between the depths of 100 and 250 km, the velocity anomalies detected below the present study region anomalies detected below the present study region are approximately 2–2.5% slower than average, are approximately 2–2.5% slower than average, implying a temperature excess of about 220–280 Kimplying a temperature excess of about 220–280 K, , which is consistent with estimates for other mantle which is consistent with estimates for other mantle plumesplumes.” .” (Macera et al., 2003 J. Geodyn.)(Macera et al., 2003 J. Geodyn.)

““[…] […] we compute instantaneous, three-dimensional we compute instantaneous, three-dimensional spherical-mantle flow driven by temperature spherical-mantle flow driven by temperature (density) anomalies as inferred from seismic (density) anomalies as inferred from seismic tomography, tomography, assuming thatassuming that velocityvelocity anomaliesanomalies areare

simply simply related to temperaturerelated to temperature.” .” (Faccenna and Becker, (Faccenna and Becker, 2010, Nature)2010, Nature)


33


The potential temperature is the The potential temperature is the temperature the mass would have (hence temperature the mass would have (hence the term “potential”) if it were compressed the term “potential”) if it were compressed or expanded to some constant reference or expanded to some constant reference pressure (1 atm).pressure (1 atm).

This concept is based on the assumption This concept is based on the assumption of a homogeneous and isothermal of a homogeneous and isothermal

upper mantle at a given depth.upper mantle at a given depth.

44The Potential Temperature (Tp) of the The Potential Temperature (Tp) of the mantle at the base of the Plate (~100 mantle at the base of the Plate (~100 km) and for the whole upper mantle is km) and for the whole upper mantle is ~~1280°C.1280°C.


The concept itself of Tp should be The concept itself of Tp should be considered in relation to the depth of considered in relation to the depth of magma formation.magma formation.

Magmas formed at high P show high Tp; Magmas formed at high P show high Tp; magmas formed at shallower P show lower magmas formed at shallower P show lower

Tp.Tp.

This does not imply any kind of thermal This does not imply any kind of thermal anomaly, but it indicates a temperature anomaly, but it indicates a temperature

gradient in the mantle.gradient in the mantle.

44The Potential Temperature (Tp) of the The Potential Temperature (Tp) of the mantle at the base of the Plate (~100 mantle at the base of the Plate (~100 km) and for the whole upper mantle is km) and for the whole upper mantle is ~~1280°C.1280°C.


Two main models concerning Mantle Potential Two main models concerning Mantle Potential Temperature:Temperature:

Potential TemperaturePotential Temperature

The difference between the two models described above The difference between the two models described above depends on the assumptions:depends on the assumptions:

Assuming a “normal” mantle potential Assuming a “normal” mantle potential temperature oftemperature of~~1280 °C, magmas formed at 1280 °C, magmas formed at higher temperatures (e.g., in mid-plate area higher temperatures (e.g., in mid-plate area ssuchas Hawaii) comes from hotter sources.ssuchas Hawaii) comes from hotter sources.

““……Geochemistry provides Geochemistry provides convincing convincing evidence that mantle plumes are 100–300 evidence that mantle plumes are 100–300 °C hotter than normal °C hotter than normal upper mantleupper mantle” ” W. M. W. M. White, 2010 White, 2010 (Oceanic Island Basalts and Mantle (Oceanic Island Basalts and Mantle Plumes: The Geochemical Perspective. Ann. Rev. Earth Plumes: The Geochemical Perspective. Ann. Rev. Earth Planet. Sci. 38, 133-160)Planet. Sci. 38, 133-160)




Assuming a “normal” mantle potential Assuming a “normal” mantle potential temperature oftemperature of~~1280 °C, magmas formed at 1280 °C, magmas formed at higher temperatures (e.g., in mid-plate areas higher temperatures (e.g., in mid-plate areas suchas Hawaii) comes from hotter sources.suchas Hawaii) comes from hotter sources.

Alternatively:Alternatively:

The MORB source is colder than elsewhere The MORB source is colder than elsewhere (because extensive melting cools the (because extensive melting cools the upper mantle). In this case, the anomaly upper mantle). In this case, the anomaly is not the mid-plate mantle, but the MORB is not the mid-plate mantle, but the MORB sources.sources.




““Long wavelength temperature variation sof the Long wavelength temperature variation sof the asthenosphere (LAM) depart from the mean by asthenosphere (LAM) depart from the mean by ±200 °C, not the ±20 °C adopted by plume ±200 °C, not the ±20 °C adopted by plume theoricians.theoricians.

The ‘normal’ variation, caused by plate tectonic The ‘normal’ variation, caused by plate tectonic processes (subduction cooling, continental processes (subduction cooling, continental insulation, small scale convection) encompasses insulation, small scale convection) encompasses the temperature excesses that have been the temperature excesses that have been attributed to hot jets and thermal plumes.” attributed to hot jets and thermal plumes.” (Anderson, 2000, Geophys. Res. Lett.).(Anderson, 2000, Geophys. Res. Lett.).

ll

900900 11001100 13001300 15001500 17001700

Temperature (°C)Temperature (°C)

00

22

44

66

88

Pre

ssure

(G

Pa)

Pre

ssure

(G

Pa)

Depth

(km)

Depth

(km)

100100

200200

A magma A magma forms where forms where the mantle the mantle temperature temperature crosses the crosses the Solidus.Solidus.

Potential TemperaturePotential TemperatureC

on

du

ctive La

yerGeother

m

or T

herm

al B

oundary

Layer

Anhydrous

Solidus

(one of the possible)

AB

C

D

E155

01480

1350

1220

1110

An An upwellingupwelling mantle volume mantle volume may start may start melting at A, B, melting at A, B, C, D, E, …, C, D, E, …, when it crosses when it crosses the local the local solidus.solidus.

A, B, A, B, C, D, E, …C, D, E, …

What is the Mantle Potential Temperature at these points?

Hawaii may have ambient Tp up to 1600 Hawaii may have ambient Tp up to 1600 °C, but °C, but so does most of the mantle away so does most of the mantle away from ridgesfrom ridges..

The Pacific asthenosphere away from The Pacific asthenosphere away from hot-spots is as hot as Hawaii hot-spots is as hot as Hawaii

asthenosphere (e.g., heatflow).asthenosphere (e.g., heatflow).

44The Potential Temperature (Tp) of the The Potential Temperature (Tp) of the mantle at the base of the Plate (~100 mantle at the base of the Plate (~100 km) and for the whole upper km) and for the whole upper mantleismantleis~~1280°C.1280°C.


Much is based on the original model of Much is based on the original model of crustal recycling by A.W. Hofmann.crustal recycling by A.W. Hofmann.

Very radiogenic Very radiogenic 206206Pb/Pb/204204Pb isotopic ratios Pb isotopic ratios (>21) of (>21) of an extremely rare group of OIBs an extremely rare group of OIBs

(~1-2%) (HIMU-like) is compatible with the (~1-2%) (HIMU-like) is compatible with the recycling of high recycling of high 238238U/U/204204Pb altered oceanic Pb altered oceanic

crust and very long storage and isotopic crust and very long storage and isotopic growth of growth of 206206Pb from the parent Pb from the parent 238238U (>2 U (>2

Ga).Ga).

55Geochemistry Geochemistry clearly indicates clearly indicates provenance of OIB from deep provenance of OIB from deep mantle.mantle.


Storage in the deepest lower mantle was Storage in the deepest lower mantle was considered necessary to allow the isotopic considered necessary to allow the isotopic growth in the recycled slab to be not involved growth in the recycled slab to be not involved in the supposedly vigorous stirring of the rest in the supposedly vigorous stirring of the rest of the mantle.of the mantle.

Recently this long isolation (>2 Ga) has been Recently this long isolation (>2 Ga) has been considered not necessary. Sr isotopes on considered not necessary. Sr isotopes on Hawaiian melt inclusions require younger Hawaiian melt inclusions require younger recycling ages (0.2-0.6 Ga)recycling ages (0.2-0.6 Ga)(Sobolev et al., (Sobolev et al.,

2011, Nature)2011, Nature)

55Geochemistry Geochemistry clearly indicates clearly indicates provenance of OIB from deep provenance of OIB from deep mantle.mantle.


Ringw.Pvsk + MgWust.

Lower Mantle

Liquid Core

From: Stern (2002) Rev. From: Stern (2002) Rev. Geophys., 40, Geophys., 40, doi:10.1029/2001RG000108doi:10.1029/2001RG000108

Storage of high Storage of high 238238U/U/204204Pb (High U/Pb) Pb (High U/Pb)

recycled oceanic recycled oceanic crust for >2 Ga, crust for >2 Ga, allowing isotopic allowing isotopic growth of growth of 206206PbPb

11stst possibility: possibility: Recycling and folding Recycling and folding at 670 kmat 670 km

22ndnd possibility: Recycling possibility: Recycling and folding at D” (2900 and folding at D” (2900 km)km)

Only after substantial isotopic growth would the 206Pb/204Pb have reached very radiogenic values

(up to 21-22)

??

XX

This concept is based on the assumption of This concept is based on the assumption of a distribution coefficient of Fe and Mg a distribution coefficient of Fe and Mg between an Mg-rich solid source (Mg# between an Mg-rich solid source (Mg# ~~90) and a partial melt.90) and a partial melt.

Upper mantle is characterized also by the Upper mantle is characterized also by the presence of Mg#-poorer lithologies (e.g., presence of Mg#-poorer lithologies (e.g.,

eclogites or pyroxenites s.l.).eclogites or pyroxenites s.l.).

Magmas in equilibrium with mantle Magmas in equilibrium with mantle sources (primitive melts) must sources (primitive melts) must have Mg# [Mg/(Mg+Fe)] have Mg# [Mg/(Mg+Fe)] ~~0.70.7

66


This may have strong effects when This may have strong effects when recalculating the “original” melt recalculating the “original” melt composition of basaltic rocks assuming composition of basaltic rocks assuming melts with MgO up to 15 wt% in equilibrium melts with MgO up to 15 wt% in equilibrium with mantle residua.with mantle residua.

This would mean that some (or all) the This would mean that some (or all) the olivine-melt thermometric estimates are olivine-melt thermometric estimates are

overestimated.overestimated.

Magmas in equilibrium with mantle Magmas in equilibrium with mantle sources (primitive melts) must sources (primitive melts) must have Mg# [Mg/(Mg+Fe)] have Mg# [Mg/(Mg+Fe)] ~~0.70.7

66


This definition is not correct, because This definition is not correct, because high melt productivity can be related to high melt productivity can be related to High Homologous Temperature.High Homologous Temperature.

The Homologous TemperatureThe Homologous Temperature is the is the ratio of the temperature of a substance ratio of the temperature of a substance to the melting temperature (to the melting temperature (solidussolidus for for

natural systems) of the same substance.natural systems) of the same substance.

High magma production is related High magma production is related to High Absolute Temperature.to High Absolute Temperature.77



In a lherzolitic mantle at a given depth, the H.T. may In a lherzolitic mantle at a given depth, the H.T. may be: 1000°C/1300°C = 0.76.be: 1000°C/1300°C = 0.76.

At the same depth, in an eclogite-bearing mantle the At the same depth, in an eclogite-bearing mantle the H.T. may be: 1000°C/1000°C. = 1.00.H.T. may be: 1000°C/1000°C. = 1.00.

An eclogite-bearing mantle has higher An eclogite-bearing mantle has higher H.T.H.T.

High magma production is related High magma production is related with High Absolute Temperature.with High Absolute Temperature.77



Huge amounts of melts can, thus, be Huge amounts of melts can, thus, be produced at “normal/average” mantle produced at “normal/average” mantle

temperatures from low temperature-temperatures from low temperature-melting mantle assemblages (e.g., melting mantle assemblages (e.g.,

eclogite-bearing peridotites).eclogite-bearing peridotites).



Many geochemists assert that the whole Many geochemists assert that the whole upper mantle is MORB, cold and upper mantle is MORB, cold and homogeneous and that MORB comes from homogeneous and that MORB comes from ambient convecting mantle.ambient convecting mantle.

There is plenty of room and magma in the 220 There is plenty of room and magma in the 220 km-thick Boundary Layer to provide Hawaii, km-thick Boundary Layer to provide Hawaii,

Ethiopia, Siberia, Deccan, Kerguelen and Ontong Ethiopia, Siberia, Deccan, Kerguelen and Ontong Java LIPs.Java LIPs.



True INTRA-PLATE magmatism does not True INTRA-PLATE magmatism does not exist. Igneous activity always develops exist. Igneous activity always develops at plate margins (i.e., along lithospheric at plate margins (i.e., along lithospheric discontinuities).discontinuities).

Edge-driven effects, lithosphere cracking, Edge-driven effects, lithosphere cracking, small-scale convection beneath the seismic small-scale convection beneath the seismic

lid and/or shear heating at the base of the lid and/or shear heating at the base of the lithosphere can contribute to magma lithosphere can contribute to magma

formation.formation.

Intra-plate magmatism is relatedto the Intra-plate magmatism is relatedto the presence of mantle plumes.presence of mantle plumes.88


88Intra-plate magmatism is relatedto the Intra-plate magmatism is relatedto the presence of mantle plumes.presence of mantle plumes.


88

From: Babuska et al. (2002) Tectonics, 21, 10.1029/2001TC901035

Intra-plate magmatism is relatedto the Intra-plate magmatism is relatedto the presence of mantle plumes.presence of mantle plumes.


PLUMEPLUME

88

From: Sleep (2006) Earth Sci. From: Sleep (2006) Earth Sci. Rev.Rev.

Why invoke the Why invoke the presence of a presence of a

PLUME?PLUME?

Igneous activity in Igneous activity in a cratonic area? a cratonic area?

An oxymoron. An oxymoron.This model works also without a plume. It is a sort of edge-driven effect



88[…] “[…] “for certain geometries for certain geometries and viscosity ratios, and viscosity ratios, circulatory flow circulatory flow develops within a “cavity” or “step” develops within a “cavity” or “step” embedded into the lithospheric base, or within a embedded into the lithospheric base, or within a low-viscosity “pocket” embedded within the low-viscosity “pocket” embedded within the asthenospheric layer.”asthenospheric layer.”

Calculated Calculated Shear-Driven UpwellingShear-Driven Upwelling rates f rates for or asthenosphere asthenosphere shearing at 5 cm/yr: shearing at 5 cm/yr: 0.2 cm/yr 0.2 cm/yr (continental rift), (continental rift), 0.5 cm/yr 0.5 cm/yr (craton edge), (craton edge), 1.0 1.0

cm/yr cm/yr (within a “pocket” of low-viscosity (within a “pocket” of low-viscosity asthenospherere) asthenospherere) (Conrad et al., 2009, Phys. Earth (Conrad et al., 2009, Phys. Earth

Planet. Int.)Planet. Int.)



88““Such asthenosphere viscosity heterogeneity Such asthenosphere viscosity heterogeneity may be associated with thermal, chemical, may be associated with thermal, chemical, melting, volatile, or grain-size anomalies, and is melting, volatile, or grain-size anomalies, and is consistent with tomographic constraints on consistent with tomographic constraints on asthenospheric variability. We estimate that asthenospheric variability. We estimate that shear-driven upwelling may generate up to 2.5 shear-driven upwelling may generate up to 2.5 km/Myr of melt that is potentially eruptible as km/Myr of melt that is potentially eruptible as surface volcanismsurface volcanism” ” (Conrad et al., 2009, Phys. Earth (Conrad et al., 2009, Phys. Earth Planet. Int.)Planet. Int.)




Ac = Wc/Hc(width of the cavity)

Tc = Hasth/(Hasth+Hc)(asthenosphere thickness with and without the cavity)



Flow velocity in the cavity as a fraction of the assumed original velocity of the asthenosphere below the cavity (e.g., 5 cm/yr)



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.



Upwelling velocity in the cavity. Tc = height of the cavity; Ac = width of the cavity.

The same as before, but assuming a low-viscosity asthenospheric layer in the cavity.



In this case no lid cavity is present.

A low-viscosity volume is assumed within the asthenosphere.



AALVLV = W = WLVLV/H/HLVLV

H’H’LVLV = H = HLVLV/H/Hasthasth

D’D’LVLV = D = DLVLV/H/Hasthasth

''LVLV = = LVLV//asthasth



Assuming:Assuming:''LVLV = = LVLV//asthasth = 0.01= 0.01(i.e., a low-(i.e., a low-viscosity layer viscosity layer 100 times less 100 times less viscous than viscous than ambient ambient asthenosphere)asthenosphere)



ALV

H’ L

V

In this case, assuming In this case, assuming an original an original asthenospheric flow asthenospheric flow velocity of 5 cm/yr, with velocity of 5 cm/yr, with ’’LVLV = 0.01, it is possible = 0.01, it is possible to develop upwelling to develop upwelling flows with velocities ~1 flows with velocities ~1 cm/yrcm/yr


33He is the stable Helium isotope. He is the stable Helium isotope. 44He is He is the Helium isotope produced by decay the Helium isotope produced by decay of U and Th.of U and Th.

MORBs have typically MORBs have typically lower lower 33He/He/44He He (but much higher (but much higher 33He and He and 44He) than He) than

OIBsOIBs..

High High 33He/He/44He ratios in basaltic He ratios in basaltic melts indicate undegassed melts indicate undegassed (primitive) mantle sources (= deep (primitive) mantle sources (= deep mantle origin).mantle origin).

99


33He is the stable Helium isotope. He is the stable Helium isotope. 44He is He is the Helium isotope produced by decay the Helium isotope produced by decay of U and Th.of U and Th.

Refractory peridotites with almost no Refractory peridotites with almost no 33He have high He have high 33He/He/238238U, just as U, just as

"undegassed or primordial" mantle."undegassed or primordial" mantle.


99


High concentrations of […] High concentrations of […] 33He/He/44He ratios of He ratios of about 50 Ra, noble gas characteristics that about 50 Ra, noble gas characteristics that are normally attributed to a primitive are normally attributed to a primitive mantle or hidden reservoirs, can be mantle or hidden reservoirs, can be preserved in a convecting and preserved in a convecting and processed processed lower mantlelower mantle. (From: Gonnermann and . (From: Gonnermann and Mukhopadhyay, 2009, Nature)Mukhopadhyay, 2009, Nature)


99


Early workers assumed high Early workers assumed high 33He for high He for high 33He/He/44He (therefore undegassed and, He (therefore undegassed and, consequentially, primitive mantle), rather than consequentially, primitive mantle), rather than low low 44He.He.

HighHigh 33He/He/44He (up to 50 Ra) has been found in He (up to 50 Ra) has been found in UHP crustal terranes,in Baffin Bay depleted UHP crustal terranes,in Baffin Bay depleted

picrites, Lau back-arc basalts, South Arc picrites, Lau back-arc basalts, South Arc basalts, dunite cumulates,basalts, dunite cumulates, andand otherother “not-Hot-“not-Hot-

Spot”Spot” lowlow 44He cases.He cases.


99


Helium and Carbon are absolutely Helium and Carbon are absolutely incompatible in silicate mantle mineral incompatible in silicate mantle mineral structure.structure.

33He/COHe/CO22 is the same for OIB and is the same for OIB and MORB. MORB. 44He/COHe/CO2 2 is higher for MORB.is higher for MORB.

This supports the idea that This supports the idea that 44He is He is responsible for MORB-OIB differences, not responsible for MORB-OIB differences, not

33He!He!


99


What should be clear is that:What should be clear is that:

Low Low 33He/He/44He does not imply He does not imply "degassed" nor does High "degassed" nor does High 33He/He/44He imply “undegassed”.He imply “undegassed”.


99


This definition/model/assumption This definition/model/assumption does not stand up.does not stand up.

The trace element and isotopic The trace element and isotopic overlap of different igneous rocks is overlap of different igneous rocks is evidence for the derivation from the evidence for the derivation from the same physical sources.same physical sources.

1010


CsCsRbRb

BaBaThTh

UUNbNb

TaTaKK

LaLaCeCe

PbPbPrPr

SrSrPP

NdNdSmSm

ZrZrHfHf

EuEuTiTi

GdGdTbTb

DyDyYY

HoHoErEr

TmTmYbYb

LuLu

11

1010

100100

10001000S

am

ple

/Pri

mit

ive

Sam

ple

/Pri

mit

ive

Man

tle

Man

tle

Canary IslandsCanary IslandsBohemian Bohemian MassifMassifPannonian Pannonian BasinBasin FrancFranc

eeGermaGermanyny SpaiSpai

nn

Area covered:Area covered:More than 3000 km-longMore than 3000 km-long

Ba/Nb

CsCsRbRb

BaBaThTh

UUNbNb

TaTaKK

LaLaCeCe

PbPbPrPr

SrSrPP

NdNdSmSm

ZrZrHfHf

EuEuTiTi

GdGdTbTb

DyDyYY

HoHoErEr

TmTmYbYb

LuLu

11

1010

100100

10001000S

am

ple

/Pri

mit

ive

Sam

ple

/Pri

mit

ive

Man

tle

Man

tle

FrancFranceeGermaGerma

nyny SpaiSpainn

St. HelenaSt. Helena

St. Helena basalts: Typical

HIMU-OIBs

St. Helena Island

Does anybody believe in a single mantle plume Does anybody believe in a single mantle plume origin for St. Helena basalts and those of origin for St. Helena basalts and those of

Germany or Bohemian massif on the basis of Germany or Bohemian massif on the basis of geochemical similarities?geochemical similarities?

Around the Mediterranean many “intraplate” Around the Mediterranean many “intraplate” igneous rocks occur.igneous rocks occur.

“Intraplate” (or “anorogenic”) Cenozoic igneous rocks

Essentially: low-volume, low-degree partial melts Essentially: low-volume, low-degree partial melts with alkaline sodic to tholeiitic compositions.with alkaline sodic to tholeiitic compositions.

Just very few geochemical comments on the

“anorogenic” igneous rock of the Circum-

Mediterranean area:

Canary IslandsMadeiraPortugalSpainMaghrebFranceSardinia UPVSardinia RPVGermanyUstica and Sicily Channel)

Etna-Hyblean Mts.Veneto DistrictItaly (Pietre Nere)Bohemian MassifLibyaPannonian BasinEast EuropeTurkeyMashrek

0.51200.51220.51240.51260.51280.51300.51320.5134

0.7020.704 0.706 0.708 0.710 0.712 0.714 0.716

143144Nd/Nd

8786 Sr/Sr

From: Lustrino From: Lustrino and Wilson and Wilson

(2007) Earth-(2007) Earth-Sci. Rev.Sci. Rev.

Database from: Lustrino (2011) Geol. Database from: Lustrino (2011) Geol. Mag.Mag.

0

50

100

150

0.700 0.705 0.710 0.715

0.5122

0.5124

0.5126

0.5128

0.5130

0.5132

87

Sr 86

Sr87Sr/86Sr

Surprisingly small spread of data of

the bulk of the samples

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.

Database from: Lustrino (2011) Geol. Database from: Lustrino (2011) Geol. Mag.Mag.

As concerns the depth of magma As concerns the depth of magma formation, …formation, …

It is one thing to deal with the depth of It is one thing to deal with the depth of magma extraction from the solid magma extraction from the solid residue.residue.

It is another to deal with the depth of It is another to deal with the depth of melting (magma formation).melting (magma formation).

And yet another to deal with the depth And yet another to deal with the depth of provenance of the solid source.of provenance of the solid source.


Thermal Thermal Plume (Plume (11); ); Fossil Fossil Plume (Plume (22); ); ChannelledChannelled Plume Plume ((33); ); ToroidalToroidal Plume ( Plume (44); ); TabularTabular Plume ( Plume (55); ); Depleted Depleted residualresidual Plume ( Plume (66); ); Finger-likeFinger-like Plume ( Plume (77); ); RecycledRecycled Plume head (Plume head (88); ); EdgeEdge Plume ( Plume (99); ); ColdCold Plume ( Plume (1010); ); CactoCactoplume (plume (1111); ); Super Super Plume (Plume (1212); ); Asthenospheric Asthenospheric Plume (Plume (1313) ) DyingDying Plume ( Plume (1414); ); Not very energetic Not very energetic Plume Plume ((1515); ); Spaghetti Spaghetti Plume (Plume (1616); ); BabyBaby Plume ( Plume (1717); ); Head-freeHead-free Plume (Plume (1818); ); Splash Splash Plume (Plume (1919); ); Pulsating Pulsating Plume (Plume (2020); ); Subduction fluid-fluxed refractory Subduction fluid-fluxed refractory Plume(Plume(2121); ); Hydrogen Hydrogen Plume (Plume (2222); ); HeterogeneousHeterogeneous Plume ( Plume (2323); ); Flattened Onion Flattened Onion Plume (Plume (2424); ); Subduction-driving Subduction-driving Plume (Plume (2525); ); Subduction-Subduction-triggered triggered Plume (Plume (2626); ); Washboard Washboard Plume (Plume (2727))11 ( (Griffiths and Campbell, 1990Griffiths and Campbell, 1990); ); 22 ( (Stein and Hofmann, 1992Stein and Hofmann, 1992); ); 33 ( (Camp and Camp and Roobol, 1992Roobol, 1992););44 ( (Mahoney et al., 1992Mahoney et al., 1992); ); 55 ( (Hoernle et al., 1995Hoernle et al., 1995), ), 6 6 ((Danyushevsky Danyushevsky et al., 1995et al., 1995); ); 77 ( (Granet et al., 1995Granet et al., 1995); ); 88 ( (Gasperini et al., 2000Gasperini et al., 2000); ); 99 ( (King and King and Ritsema, 2000Ritsema, 2000); ); 1010((Hanguita and Hernan, 2000Hanguita and Hernan, 2000); ); 1111 ( (Lundin, 2003Lundin, 2003); ); 1212 ( (Condie, Condie, 20042004); ); 1313 ( (Seghedi et al., 2004Seghedi et al., 2004); ); 1414 ( (Davaille and Vatteville, 2005Davaille and Vatteville, 2005); ); 1515 ( (Michon Michon and Merle, 2005and Merle, 2005); ); 1616 ( (Abouchami et al., 2005Abouchami et al., 2005); ); 1717 ( (Ritter, 2006Ritter, 2006); ); 1818 ( (e.g., Ritter, e.g., Ritter, 20062006); ); 1919 ( (Davies and Bunge, 2006Davies and Bunge, 2006); ); 20 20 ((Krienitz et al., 2007Krienitz et al., 2007); ); 21 21 ((Falloon et al., Falloon et al., 20072007); ); 2222 ( (Dobretsov, 2008Dobretsov, 2008); ); 2323 ( (Ren et al., 2009Ren et al., 2009); ); 2424 ( (Beccaluva et al., 2010Beccaluva et al., 2010); ); 2525 ((Burov and Cloetingh 2010Burov and Cloetingh 2010); ); 2626 ( (Faccenna et al., 2010Faccenna et al., 2010); ); 2727 ( (Ballmer et al, 2011Ballmer et al, 2011).).

Comparison with Comparison with geosynclinesgeosynclines

• Mio-geosynclineMio-geosyncline• Eu-geosynclineEu-geosyncline• Ortho-geosynclineOrtho-geosyncline• Primary geosynclinePrimary geosyncline• Zeugo-geosynclineZeugo-geosyncline• Para-geosynclinePara-geosyncline• Exo-geosynclineExo-geosyncline• Taphro-geosynclineTaphro-geosyncline• Paralia-geosynclineParalia-geosyncline

Thanks to Gill FoulgerThanks to Gill Foulger

Why are deep mantle plumes needed?Why are deep mantle plumes needed?-High melt productivity of LIPs?High melt productivity of LIPs? No.No. High Homologous High Homologous Temperature (chemical rather temperature anomalies).Temperature (chemical rather temperature anomalies).

-Peculiar Sr-Nd-Pb isotopic composition of OIBs?Peculiar Sr-Nd-Pb isotopic composition of OIBs? No.No. ALL the most peculiar geochemical characteristics of ALL the most peculiar geochemical characteristics of OIBs require recycled crustal lithologies, not deep OIBs require recycled crustal lithologies, not deep sources.sources.

- High High 33He/He/44He means undegassed - therefore never-He means undegassed - therefore never-tapped by basaltic magmatism - mantle sources?tapped by basaltic magmatism - mantle sources? No.No. Helium isotopes do not support this.Helium isotopes do not support this.

-Doming in some CFB?Doming in some CFB? No.No. Presence of abundant Presence of abundant (buoyant) basaltic melt, not hot mantle sources. (buoyant) basaltic melt, not hot mantle sources. Doming not ubiquitousDoming not ubiquitous

-Vs and Vp anomalies in tomographic images?Vs and Vp anomalies in tomographic images? No.No. Seismic anomalies rather reflect chemical Seismic anomalies rather reflect chemical heterogeneity. Mantle plumes have been proven heterogeneity. Mantle plumes have been proven difficult to image using seismology.difficult to image using seismology.

Why are deep mantle plumes needed?Why are deep mantle plumes needed?- High potential temperatures of some OIBs compared High potential temperatures of some OIBs compared with MORBs?with MORBs? No.No. OIB Tp is ambient mantle OIB Tp is ambient mantle temperature. MORBs are colder than “average”.temperature. MORBs are colder than “average”.

-Age progression in some Island Chains?Age progression in some Island Chains? No.No. Can be Can be explained by progressive cracks in the lithosphere.explained by progressive cracks in the lithosphere.

- Long isolation time to allow isotopic growth of Long isolation time to allow isotopic growth of 206206Pb/Pb/204204Pb ratios?Pb ratios? No.No. Isolation may happen also in the Isolation may happen also in the shallow, non convecting mantle (B Layer of Gutenberg) shallow, non convecting mantle (B Layer of Gutenberg) or at 670 km.or at 670 km.

-Peculiar trace element composition of OIBs?Peculiar trace element composition of OIBs? No.No. OIBs OIBs are very heterogeneous from an incompatible trace are very heterogeneous from an incompatible trace element point of view. All of them require the element point of view. All of them require the involvement of crustal lithologies in their sources. involvement of crustal lithologies in their sources.

Why are deep mantle plumes needed?Why are deep mantle plumes needed?- Geochemical similarity of areally dispersed OIB-like Geochemical similarity of areally dispersed OIB-like igneous rocks?igneous rocks? No.No. Similar mantle processes, not the Similar mantle processes, not the same physical sources.same physical sources.

-The only model to explain the fixity of hot-spot tracks? The only model to explain the fixity of hot-spot tracks? No.No. Also in the Hawaii-Emperor Chain case such a fixity Also in the Hawaii-Emperor Chain case such a fixity is not demonstrated (i.e. the geographic coordinates of is not demonstrated (i.e. the geographic coordinates of the source move).the source move).

- Do you suggest other questioned features for mantle Do you suggest other questioned features for mantle plumes?plumes? ………. ……….

Mantle

Plumes

Message to take Message to take away:away:

Thanks for your attentionThanks for your attentionVisit: www.mantleplumes.orgVisit: www.mantleplumes.org

Documents

THERMAL STATE AND GEOCHEMICAL COMPOSITION OF THE MANTLE. WHAT CAN WE INFER FROM IGNEOUS ROCKS? Michele Lustrino Dipartimento di Scienze della Terra, Univ