Phantom plumes in Europe and neighbouring areas

1
Phantom plumes in Europe and neighbouring areas
Michele Lustrino and Eugenio break-off Carmi
nati Dipartimento di Scienze della Terra, Uni
versità degli Studi di Roma La Sapienza, P.le A.
Moro, 5, 00185 Roma
2
The anorogenic magmatism of the
circum-Mediterranean area (Tyrrhenian Sea, Sardin
ia, 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 on
e 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 betwe
en 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 explai
n 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 rocks reflect in their
chemistry the effects of interaction between
mantle (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) ta
bular plume (Hoernle et al., 1995)
finger-like plume (e.g., Granet et al., 1995
Cadoux et al., 2007) baby plume (Ritter, 2006) c
hannelled plume (Camp and Roobol, 1992 Oyarzun
et al, 1997) thoroidal plume (Mahoney et al., 199
2) head-free plume (e.g., Ritter, 2006) cold plu
me (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 (Fallo
on 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-Re
cent volcanic activity with subduction-like
signature from NW (Sardinia) to SE (Aeolian
Archipelago) 2) middle Miocene-Quaternary igneou
s 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 de
ep compared to the depth of oceanic crust of a
similar age 5) calculated Tp of the Tyrrhenian S
ea (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
MIDDLE EAST AND SICILY CHANNEL
Favouring Plume The presence of Fossil Plume He
ads. Why? Relative constancy of 87Sr/86Sr and 1
43Nd/144Nd isotopic ratios of Israeli basalts
erupted during the Meso-Cenozoic (Stein and
Hofmann, 1992). Existence of a common HIMU/FOZO
component in the volcanic rocks of Pantelleria
and Linosa islands in the Sicily Channel, Mt.
Etna and Hyblean Mts. in Sicily, Ustica island
(NW Sicily) and Alicudi (Rotolo et al., 2006).
17
CASE STUDIES
MIDDLE EAST AND SICILY CHANNEL Contrasting
Plume Strong isotopic difference between Miocen
e-Pliocene Israeli lavas and coeval igneous rocks
from neighbouring areas (e.g., Lebanon, Syria and
Jordan ). Not so close geochemical similarities
between southern Italy volcanoes. Moreover
GEOCHEMICAL SIMILARITIES DO NOT IMPLY THE
PRESENCE OF COMMON PHYSICAL MANTLE SOURCES BUT,
RATHER, MAY IMPLY THE PRESENCE OF SIMILAR
PETROGENETIC PROCESSES.
18
CASE STUDIES
SICILY CHANNEL SICILY SARDINIA MEDITERRANEAN SE
A AND CENTRAL-WESTERN EUROPE FRENCH MASSIF CENTRA
L EIFEL AND NEIGBOURING AREAS PANNONIAN BASIN M
IDDLE EAST Blah blah blah (See Lustrino
and Carminati, 2007)
19

• 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.

20
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.
21
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.
22
CONCLUSIONS
Evidence supporting Plumes Overall geochemical s
imilarities with OIB Geochemical homogeneity of t
he volcanic rocks Tomography sees hot areas be
neath French Massif Central and Eifel
Geothermometry of mantle minerals indicates
excess temperature Evidence against Plumes Sm
all 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 absolut
e message from geochemistry Tomography gives cont
rasting results Evidence of subduction and back-a
rc basin formation All major volcanic areas on th
inned lithosphere and/or along plate margins
23
For further informationA. 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).