Transcript Document

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
The “anorogenic” magmatism of the circumMediterranean 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.
We emphasize that such conclusions based on
geochemical data and tomographic results are not
fully justified because:
1) a given chemical and isotopic composition of a magma
can be explained by different petrogenetic models;
2) a given petrogenetic process can produce magmas with
different chemical and isotopic composition;
3) tomographic studies do not furnish unique results (i.e.,
different models can give contrasting conclusions);
4) 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).
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.
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).
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).
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).
Widespread volcanic activity accompanied the CiMACI
(Circum Mediterranean Anorogenic Cenozoic Igneous)
Province.
1) sodic mildly alkaline and tholeiitic rocks OIB-like;
2) oceanic floor rocks (from N- to E-MORB and low-K
calcalkaline basalts and andesites);
3) calcalkaline rocks (resembling magmas emplaced in
subduction-related settings);
4) potassic to ultrapotassic alkaline rocks with mildly to
strongly SiO2-undersaturated compositions;
5) rare exotic compositions such as lamproites,
lamprophyres and carbonatites.
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.
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.
An example?
Hawaiian rocks are really “anorogenic”?
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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.
In practice the mantle beneath Hawaii
looks like this:
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)
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-NdPb-O-C isotopic ratios.
Locardi and Nicolich, 2005 (E-ward migrating deepseated 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.
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;
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)
Many are the models proposed to explain the origin
of CiMACI rocks. These can grouped in:
1) 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.
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.
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.
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
For further information:
A. Peccerillo, M. Lustrino: Compositional variations of Plio-Quaternary magmatism in the circumTyrrhenian area: deep- versus shallow-mantle processes (2005) In: Foulger et al. (Eds). Plates,
Plumes and Paradigms. Geol. Soc. Am. Spec. Paper, 418, 422-434
M. Lustrino, M. Wilson: The Circum-Mediterranean Cenozoic Igneous Province (2007) Earth-Sci.
Rev., 81, 1-65
M. 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).