Phantom plumes in Europe and neighbouring areas Michele Lustrino and Eugenio “break-off” Carminati

1. Phantom plumes in Europe and neighbouring areas Michele Lustrino and Eugenio “break-off” Carminati Dipartimento di Scienze della Terra, Università degli Studi di Roma La Sapienza, P.le A. Moro, 5, 00185 Roma

2. The “anorogenic” magmatism of the circum-Mediterranean area (Tyrrhenian Sea, Sardinia, Sicily Channel and Middle East) and of continental Europe (French Massif Central, Eifel, Bohemian Massif and Pannonian Basin) has been proposed to be related to the presence of one or more mantle plumes.

3. We emphasize that such conclusions based on geochemical data and tomographic results are not fully justified because: • a given chemical and isotopic composition of a magma can be explained by different petrogenetic models; • a given petrogenetic process can produce magmas with different chemical and isotopic composition; • tomographic studies do not furnish unique results (i.e., different models can give contrasting conclusions); • seismic wave velocity anomalies interpreted exclusively in terms of temperature anomalies is not granted, since velocities are dependent also on other parameters (pressure, rock composition, melting, anisotropy and anelasticity).

4. Tomography and geochemistry are powerful tools but must be used in an interdisciplinary approach, in combination with geodynamics and structural geology. Alone they cannot provide compelling evidence for or against the existence of mantle plumes.

5. Why plumes? Geochemistry says: composition similar to oceanic intraplate basalts emplaced far away from subduction margins (i.e., OIB, Ocean Islands Basalts: Hawaiian-Emperor Chain, St. Helena, French Polynesia, and so on). Geochemists propose a contrasting model: from one side, they invoke isolated sources (considered to be primordial, never tapped by partial melts, undegassed with high 3He/4He ratios) but, at the same time, these must be open sources because they must allow entrance of subducted oceanic crust stored for at least 2 Ga (necessary to explain the high 206Pb/204Pb >21).

6. Upwelling of hot mantle (in solid state) is commonly called mantle plume The difference between the potential temperature of “normal” asthenosphere (with Tp ~1280 °C) and mantle plume material can range between 100 and 300 °C. Why invoke such a temperature excess? To explain huge volumes (millions of km3) of CFB and LIP in a short time (generally 1-2 Myr).

7. The “plume” models are based on the assumption that the source regions of large igneous provinces are entirely peridotitic. However, during the last decade, new models have suggested the presence of lithologies (eclogites, pyroxenites, garnet granulites and so on) with solidus temperature several hundred degrees lower than peridotitic mantle. At least in some cases, enhanced melt productivity can be consequence of chemical anomalies (e.g., presence of low temperature melting point assemblages) rather than thermal anomalies (as requested in the original mantle plume models).

8. Widespread volcanic activity accompanied the CiMACI (Circum Mediterranean Anorogenic Cenozoic Igneous) Province. • sodic mildly alkaline and tholeiitic rocks OIB-like; • oceanic floor rocks (from N- to E-MORB and low-K calcalkaline basalts and andesites); • calcalkaline rocks (resembling magmas emplaced in subduction-related settings); • potassic to ultrapotassic alkaline rocks with mildly to strongly SiO2-undersaturated compositions; • rare exotic compositions such as lamproites, lamprophyres and carbonatites.

9. The first problem is to try to define what an “anorogenic” magma is from a geochemical and geotectonic point of view. At the moment there is no consensus on “anorogenic” (or intra-plate) and “orogenic” (or subduction-related) terms.

10. What is important to stress is: Virtually all the igneous rocksreflect in their chemistry the effects of interactionbetweenmantle (i.e., peridotitic) and recycled crustal (i.e., pyroxenitic/eclogitic) lithologies.

11. An example?Hawaiian rocks are really “anorogenic”? • Yaxley and Sobolev (2007) High pressure experimental investigation of interactions between partial melts of gabbro and peridotitic mantle. Contrib. Mineral. Petrol. • 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. • Lassiter et al. (2000) Generation of Hawaiian post-erosional lavas by melting of a mixed lherzolite/pyroxenite source. Earth Planet. Sci. Lett.

12. In practice the mantle beneath Hawaii looks like this:

13. Notwithstanding this, the plume lovers are numerous. Several ad-hoc concepts like: fossil plume (Stein and Hofmann, 1992; Rotolo et al., 2006) dying plume (Davaille and Vatteville, 2005) recycled plume head (Gasperini et al., 2000) tabular plume (Hoernle et al., 1995) finger-like plume (e.g., Granet et al., 1995; Cadoux et al., 2007) baby plume (Ritter, 2006) channelled plume (Camp and Roobol, 1992; Oyarzun et al, 1997) thoroidal plume (Mahoney et al., 1992) head-free plume (e.g., Ritter, 2006) cold plume (Garfunkel, 1989; Hanguita and Hernan, 2000) depleted residual plume (e.g., Danyushevsky et al., 1995) pulsating plume (Krienitz et al., 2007) subduction fluid-fluxed refractory plume (Falloon et al., 2007)

14. CASE STUDIES TYRRHENIAN SEA Favouring Plume Bell et al., 2004 (Deep mantle plume. Opening of the Mediterranean region along the SW-ward continuation of the Rhine-Rhone rift system). On what grounds? Sr-Nd-Pb-O-C isotopic ratios. Locardi and Nicolich, 2005 (E-ward migrating deep-seated thermal plume). Seismic active belt in southern Italy? The effect of a convective cell associated with hot asthenolith inducing stress and seismic activity at the interface with the neighbouring cooler mantle.

15. CASE STUDIES TYRRHENIAN SEA Contrasting Plume 1) Oligocene-Recent volcanic activity with subduction-like signature from NW (Sardinia) to SE (Aeolian Archipelago); 2) middle Miocene-Quaternary igneous rocks along the W and E branch of the Tyrrhenian Sea completely different; 3) composition of Italian volcanic rocks (mostly potassic to ultrapotassic) never found among OIB; 4) depth of the Tyrrhenian Sea crust very deep compared to the depth of oceanic crust of a similar age; 5) calculated Tp of the Tyrrhenian Sea (~1320 °C vs. 1280 °C); 6) numerical modelling requires tectonic forces like those in subduction settings (subduction of lithosphere for >200 km in N. Apennines, >500 km in S. Apennines, >800 km in Calabria); 7) sub-crustal earthquakes indicate a slab below the Calabrian Arc up to a depth of 500 km;

16. CASE STUDIES SICILY CHANNEL SICILY SARDINIA MEDITERRANEAN SEA AND CENTRAL-WESTERN EUROPE FRENCH MASSIF CENTRAL EIFEL AND NEIGBOURING AREAS PANNONIAN BASIN MIDDLE EAST Blah blah blah… (See Lustrino and Carminati, 2007)

17. Many are the models proposed to explain the origin of CiMACI rocks. These can grouped in: • Models that require active upraise of asthenospheric mantle (or even deeper sources) (mantle plumes); • 2) Models that requires lithospheric extension (or detachment and delamination processes) to induce decompression melting and passive upraise of asthenospheric and lithospheric melts.

18. E C R I S According to Plume-lovers, the absence of igneous activity along most of the ECRIS (European Cenozoic Rift System) is evidence that continental rifting ALONE cannot promote partial melting of the mantle.

19. We suggest that if such a crustal thinning is associated to areas where lithosphere thickness is reduced (e.g., French Massif Central and Rhenish Massif) igneous activity may develop without requiring any thermal excess of the mantle.

20. CONCLUSIONS Evidence supporting Plumes: Overall geochemical similarities with OIB Geochemical homogeneity of the volcanic rocks Tomography sees “hot” areas beneath French Massif Central and Eifel Geothermometry of mantle minerals indicates excess temperature Evidence against Plumes: Small volume of volcanic products (with few exceptions); small f Very long magmatic activity No plume-tracks; No associated CFB No strong doming before magmatism No definitive and absolute message from geochemistry Tomography gives contrasting results Evidence of subduction and back-arc basin formation All major volcanic areas on thinned lithosphere and/or along plate margins

21. For further information:A. Peccerillo, M. Lustrino: Compositional variations of Plio-Quaternary magmatism in the circum-Tyrrhenian area: deep- versus shallow-mantle processes (2005) In: Foulger et al. (Eds). Plates, Plumes and Paradigms. Geol. Soc. Am. Spec. Paper, 418, 422-434M. Lustrino, M. Wilson: The Circum-Mediterranean Cenozoic Igneous Province (2007) Earth-Sci. Rev., 81, 1-65M. Lustrino, E. Carminati: Phantom plumes in Europe and neighbouring areas (2007) In: G. Foulger and D. Jurdy (Eds.) Plumes, Plates and Planetary Prospetives. Geol. Soc. Am. Spec. Paper (in press).