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

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Presentation on theme: "THERMAL STATE AND GEOCHEMICAL COMPOSITION OF THE MANTLE. WHAT CAN WE INFER FROM IGNEOUS ROCKS? Michele Lustrino Dipartimento di Scienze della Terra, Univ."— Presentation transcript:

1 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 Thanks to: Don L. Anderson, Carlo Doglioni, Gill Foulger, Jim Natland, Giuliano Panza, Dean Presnall, Bob Stern and all the group.

2 ! Attention Plumers: This is an Anti-Plume Talk!

3 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).

4 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?

5 Unsupported assumptions: 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. 1

6 The mantle sources of several OIB types are not much different from MORB source from a Sr-Nd-Pb isotopic point of view. MORBsMORBs From: White, 2010 (Ann. Rev. Earth Planet. Sci.)

7 The mantle sources of several OIB types are not much different from MORB source from a Sr-Nd-Pb isotopic point of view. From: Hofmann, 2004 (Encyclopedia of Geochemistry) 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).

8 The mantle sources of several OIB types are not much different from MORB source from a Sr-Nd-Pb isotopic point of view. From: Hofmann, 2004 (Encyclopedia of Geochemistry) Depleted Isotopic Field OIB

9 From: Hofmann, 1997 (Nature, 385, ) Vague definition OK Vague definition based on assumptions Again vague definition Definition based on assumptions, not evidence Unrelated to subduction?

10 Are Hawaiian magmas geochemically unrelated to subduction? Nielsen et al. (2006) Thallium isotopic evidence for ferromanganese sediments in the mantle source of Hawaiian basalts. 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. Herzberg (2006) Petrology and thermal structure of the Hawaiian plume from Mauna Kea volcano. Nature Sobolev et al. (2007) The amount of recycled crust in sources of mantle-derived melts. Science. Blichert-Toft and Albarede (2009) Mixing of isotopic heterogeneities in the Mauna Kea plume conduit. Earth Planet. Sci. Lett. Huang et al (2009) Ancient carbonate sedimentary signature in the Hawaiian plume: evidence from Mahukona volcano, Hawaii. Geochem. Geophys. Geosyst. Fodor and Bauer (2010) Kahoolawe Island, Hawaii: The role of an ‘inaccessible’ shieldv olcano in the petrologyof the Hawaiian islands and plume. Chem Erdie Sobolev et al. (2005) An olivine-free mantle source for Hawaiian shield lavas. Nature. Ren et al (2009) Geochemical differences of the Hawaiian shield lavas: implications for melting process in the heterogeneous Hawaiian plume. J. Petrol. 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

11 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. The concept itself of “Normal-MORB” is an invention. …Normal with respect to what? 1 Unsupported assumptions:

12 Was Igor Normal? …Probably yes, but only in the Frankenstein Castle… Upper mantle is: - Homogeneous - Chemically depleted 1 Unsupported assumptions:

13 ….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) Upper mantle is: - Homogeneous - Chemically depleted 1 Unsupported assumptions:

14 Upper mantle is: - Homogeneous - Chemically depleted 1 Selected trace element variability of a SMALL set of MORBs (40-55° S. Atlantic Ocean) From: Hofmann, 2004 (Encyclopedia of Geochemistry) Unsupported assumptions:

15 Upper mantle is: - Homogeneous - Chemically depleted 1 “Heterogeneities from plumes may comprise a substantial fraction of all heterogeneities in the MORB source.” (Davies, 2009, G3) …Mmmhhh… Heterogeneous MORB sources? Unsupported assumptions:

16 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). Upper mantle is: - Homogeneous - Chemically depleted 1 Unsupported assumptions:

17 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). Upper mantle is: - Homogeneous - Chemically depleted 1 Unsupported assumptions:

18 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 basalt- yielding source regions would be called pyroxenites, harzburgites or lherzolites. Upper mantle is: - Homogeneous - Chemically depleted 1 Unsupported assumptions:

19 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 ) Upper mantle is: - Homogeneous - Chemically depleted 1 Unsupported assumptions:

20 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. 2 Unsupported assumptions:

21 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. Asthenosphere is: - Fully convecting -Chemically homogeneous and depleted 2 Unsupported assumptions:

22 […] “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:

23 The asthenosphere is a relatively low- viscosity layer, not a vigorously stirred and convecting layer. Plate tectonics and post-glacial rebound (isostasy) require a low viscosity, not vigorous convection. Asthenosphere is: - Fully convecting -Chemically homogeneous and depleted 2 Unsupported assumptions:

24 “Fixed (convecting)” Mantle Melt Fraction Upper Boundary Layer B Region of Gutenberg (1959) Laterally Advecting and Anisotropic Mantle Classically defined “Asthenosphere” km From: Anderson, 2011 (J. Petrol.) Conductive Layer Unsupported assumptions:

25 km Melt In (or fluid-rich) Melt Out (or fluid-poor) G Discontinuity L Discontinuity From: Anderson, 2011 (J. Petrol.) “Fixed (convecting)” Mantle Homogeneous (convecting) “Asthenosphere” Melt Fraction ConductiveLayer Unsupported assumptions: Classically defined “Asthenosphere”

26 km 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.) “Fixed (convecting)” Mantle Homogeneous (convecting) “Asthenosphere” LITHOSPHERE Seismic Lid Unsupported assumptions: Classically defined “Asthenosphere”

27 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. 3 Unsupported assumptions:

28 a = Grand et al b = Mègnin and Romanowicz, 2000 c = Ritsema et al., 1999 d = Montelli et al., 2006 From: Kumagai et al., 2008 (GRL) Plume or not under Iceland? ab cd Unsupported assumptions:

29 Pacific Plate Tomography 100 km50 km 150 km200 km 250 km 300 km Maggi et al., 2006 (EPSL) Where is the Hawaiian thermal plume? Unsupported assumptions:

30 Kustwosky et al., 2008 (J Geophys Res) Pacific Plate Tomography Where is the Hawaiian thermal plume? Unsupported assumptions:

31 Mantle Plume trace? This too? Schmerr et al (EPSL) Unsupported assumptions:

32 “Red" patches in tomographic images can be fine grained peridotite, eclogite, H 2 O, CO 2 or melt. With volatiles or eclogite components it is not necessary to dream up mechanisms to cause melting or raise the temperature. Tomography can be used to measure the temperature of the mantle 3 Unsupported assumptions:

33 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). The velocities of surface waves (large horizontal component of motion) are different from steeply up-coming S waves (large vertical component). Tomography can be used to measure the temperature of the mantle 3 Unsupported assumptions:

34 “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.) Tomography can be used to measure the temperature of the mantle 3 Unsupported assumptions:

35 Seismology has simply no more power to map hot plumes than geochemistry (and geochemistry can say NOTHING about T) Tomography can be used to measure the temperature of the mantle 3 Unsupported assumptions:

36 “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) Tomography can be used to measure the temperature of the mantle 3 Unsupported assumptions:

37 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. 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. Unsupported assumptions:

38 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. 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. Unsupported assumptions:

39 Two main models concerning Mantle Potential Temperature: 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, )

40 Two main models concerning Mantle Potential Temperature: 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.

41 Two main models concerning Mantle Potential Temperature: 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.).

42 l Temperature (°C) Pressure (GPa) Depth (km) A magma forms where the mantle temperature crosses the Solidus. Potential Temperature Conductive Layer Geotherm or Thermal Boundary Layer Anhydrous Solidus (one of the possible) A B C D E An upwelling mantle volume may start melting at A, B, C, D, E, …, when it crosses the local solidus. A, B, C, D, E, … A, B, C, D, E, … What is the Mantle Potential Temperature at these points?

43 Hawaii may have ambient Tp up to 1600 °C, but so does most of the mantle away from ridges. The Pacific asthenosphere away from hot- spots is as hot as Hawaii asthenosphere (e.g., heatflow). 4 The Potential Temperature (Tp) of the mantle at the base of the Plate (~100 km) and for the whole upper mantleis~1280°C. Unsupported assumptions:

44 Much is based on the original model of crustal recycling by A.W. Hofmann. Very radiogenic 206 Pb/ 204 Pb isotopic ratios (>21) of an extremely rare group of OIBs (~1- 2%) (HIMU-like) is compatible with the recycling of high 238 U/ 204 Pb altered oceanic crust and very long storage and isotopic growth of 206 Pb from the parent 238 U (>2 Ga). 5 Geochemistry clearly indicates provenance of OIB from deep mantle. Unsupported assumptions:

45 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 ( Ga) (Sobolev et al., 2011, Nature) 5 Geochemistry clearly indicates provenance of OIB from deep mantle. Unsupported assumptions:

46 Ringw. Pvsk + MgWust. Lower Mantle Liquid Core From: Stern (2002) Rev. Geophys., 40, doi: /2001RG Storage of high 238 U/ 204 Pb (High U/Pb) recycled oceanic crust for >2 Ga, allowing isotopic growth of 206 Pb 1 st possibility: Recycling and folding at 670 km 2 nd possibility: Recycling and folding at D” (2900 km) Only after substantial isotopic growth would the 206 Pb/ 204 Pb have reached very radiogenic values (up to 21-22) ? X

47 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.). Magmas in equilibrium with mantle sources (primitive melts) must have Mg# [Mg/(Mg+Fe)] ~0.7 6 Unsupported assumptions:

48 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 olivine- melt thermometric estimates are overestimated. Magmas in equilibrium with mantle sources (primitive melts) must have Mg# [Mg/(Mg+Fe)] ~0.7 6 Unsupported assumptions:

49 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. High magma production is related to High Absolute Temperature. 7 Unsupported assumptions:

50 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 = At the same depth, in an eclogite-bearing mantle the H.T. may be: 1000°C/1000°C. = An eclogite-bearing mantle has higher H.T. High magma production is related with High Absolute Temperature. 7 Unsupported assumptions:

51 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). High magma production is related to High Absolute Temperature. 7 Unsupported assumptions:

52 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 km- thick Boundary Layer to provide Hawaii, Ethiopia, Siberia, Deccan, Kerguelen and Ontong Java LIPs. High magma production is related to High Absolute Temperature. 7 Unsupported assumptions:

53 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. Intra-plate magmatism is relatedto the presence of mantle plumes. 8 Unsupported assumptions:

54 8 Intra-plate magmatism is relatedto the presence of mantle plumes. Unsupported assumptions:

55 8 From: Babuska et al. (2002) Tectonics, 21, /2001TC Intra-plate magmatism is relatedto the presence of mantle plumes. Unsupported assumptions:

56 PLUME 8 From: Sleep (2006) Earth Sci. Rev. Why invoke the presence of a PLUME? Igneous activity in a cratonic area? An oxymoron. This model works also without a plume. It is a sort of edge- driven effect Intra-plate magmatism is relatedto the presence of mantle plumes. Unsupported assumptions:

57 8 […] “for certain geometries and viscosity ratios, circulatory flow develops within a “cavity” or “step” embedded into the lithospheric base, or within a low- viscosity “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.) Intra-plate magmatism is relatedto the presence of mantle plumes. Unsupported assumptions:

58 8 “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.) Intra-plate magmatism is relatedto the presence of mantle plumes. Unsupported assumptions:

59 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:

60 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:

61 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:

62 8 Intra-plate magmatism is relatedto the presence of mantle plumes. 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. Unsupported assumptions:

63 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:

64 8 Intra-plate magmatism is relatedto the presence of mantle plumes. A LV = W LV /H LV H’ LV = H LV /H asth D’ LV = D LV /H asth  ' LV =  LV /  asth Unsupported assumptions:

65 8 Intra-plate magmatism is relatedto the presence of mantle plumes. Assuming:  ' LV =  LV /  asth = 0.01 (i.e., a low- viscosity layer 100 times less viscous than ambient asthenosphere) Unsupported assumptions:

66 8 Intra-plate magmatism is relatedto the presence of mantle plumes. A LV H’ LV In this case, assuming an original asthenospheric flow velocity of 5 cm/yr, with  ’ LV = 0.01, it is possible to develop upwelling flows with velocities ~1 cm/yr Unsupported assumptions:

67 3 He is the stable Helium isotope. 4 He is the Helium isotope produced by decay of U and Th. MORBs have typically lower 3 He/ 4 He (but much higher 3 He and 4 He) than OIBs. High 3 He/ 4 He ratios in basaltic melts indicate undegassed (primitive) mantle sources (= deep mantle origin). 9 Unsupported assumptions:

68 3 He is the stable Helium isotope. 4 He is the Helium isotope produced by decay of U and Th. Refractory peridotites with almost no 3 He have high 3 He/ 238 U, just as "undegassed or primordial" mantle. High 3 He/ 4 He ratios in basaltic melts indicate undegassed (primitive) mantle sources (= deep mantle origin). 9 Unsupported assumptions:

69 High concentrations of […] 3 He/ 4 He 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) High 3 He/ 4 He ratios in basaltic melts indicate undegassed (primitive) mantle sources (= deep mantle origin). 9 Unsupported assumptions:

70 Early workers assumed high 3 He for high 3 He/ 4 He (therefore undegassed and, consequentially, primitive mantle), rather than low 4 He. High 3 He/ 4 He (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 4 He cases. High 3 He/ 4 He ratios in basaltic melts indicate undegassed (primitive) mantle sources (= deep mantle origin). 9 Unsupported assumptions:

71 Helium and Carbon are absolutely incompatible in silicate mantle mineral structure. 3 He/CO 2 is the same for OIB and MORB. 4 He/CO 2 is higher for MORB. This supports the idea that 4 He is responsible for MORB-OIB differences, not 3 He! High 3 He/ 4 He ratios in basaltic melts indicate undegassed (primitive) mantle sources (= deep mantle origin). 9 Unsupported assumptions:

72 What should be clear is that: Low 3 He/ 4 He does not imply "degassed" nor does High 3 He/ 4 He imply “undegassed”. High 3 He/ 4 He ratios in basaltic melts indicate undegassed (primitive) mantle sources (= deep mantle origin). 9 Unsupported assumptions:

73 This definition/model/assumption does not stand up. The trace element and isotopic overlap of different igneous rocks is evidence for the derivation from the same physical sources. 10 Unsupported assumptions:

74 Cs Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Sm Zr Hf Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Sample/Primitive Mantle Canary Islands Bohemian Massif Pannonian Basin France Germany Spain Area covered: More than 3000 km-long

75 Ba/Nb Cs Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Sm Zr Hf Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Sample/Primitive Mantle France Germany Spain St. Helena St. Helena basalts: Typical HIMU-OIBs

76 St. Helena Island 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?

77 Around the Mediterranean many “intraplate” igneous rocks occur. “Intraplate” (or “anorogenic”) Cenozoic igneous rocks Essentially: low-volume, low-degree partial melts with alkaline sodic to tholeiitic compositions.

78 Just very few geochemical comments on the “anorogenic” igneous rock of the Circum-Mediterranean area: From: Lustrino and Wilson (2007) Earth-Sci. Rev. Database from: Lustrino (2011) Geol. Mag.

79 87 Sr/ 86 Sr 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. Mag.

80 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. Unsupported assumptions:

81 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).

82 Comparison with geosynclines 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 Foulger

83 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 3 He/ 4 He 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.

84 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 206 Pb/ 204 Pb 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.

85 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? ……….

86 Mantle Plumes Message to take away: Thanks for your attention Visit:


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