Is There a Mantle Plume Below Italy?

Transcription

1 and the Quasi-Biennial Oscillation were favorable for the continued development and intensification of those tropical cyclones that formed. Finally the phase of the North Atlantic Oscillation and the position of the Bermuda High likely influenced the tracks of straightmoving hurricanes to make landfall along the southern U.S. coast, without the straight moving hurricanes experiencing the recurvature that has been typical during the last several North Atlantic hurricane seasons. Stochastic U.S. hurricane prediction methodologies based on previously developed regression models [Eisner and Bossak, 2001; 2004] will be utilized with final hurricane season climate data to verify these findings. A more statistically rigorous examination of climate influences relating to the 2004 U.S. hurricane events will be forthcoming. Acknowledgments I thank Kate Ciembronowicz for assistance with the figures, and James B. Eisner for helpful comments and suggestions. Eos, Vol. 85, No. 50,14 December 2004 References Bove, M.C., J. B. Eisner, C. W Landsea, X. Niu, and J. J. O'Brien (1998), Effect of El Nino on U.S. landfalling hurricanes, revisited, Bull. Am,. Meteorol. Soc, 79, Eisner, J. B.,K.-B.Liu,and B.Kocher (2000a),Spatial variations in major U.S. hurricane activity: Statistics and a physical mechanism, J. Clim., 73, Eisner, J. B.,T. Jagger,and X.-ENiu (2000b),Changes in the rates of North Atlantic major hurricane activity during the 20th century Geophy. Res. Lett, 27,\743-\746. Eisner, J. B., and B. H. Bossak (2001), Bayesian analysis of U.S. hurricane climate,./ Clim., 14, Elsner,J.B.,B.H. Bossak, and X.-FNiu (2001),Secular changes to the ENSO-U.S. hurricane relationship, Geophys. Res. Lett., 25, Eisner, J. B. (2003),Tracking hurricanes,bull. Am., Meteorol. Soc, 84, Eisner, J. B., and B. H. Bossak (2004), Hurricane landfall probability and climate, In Hurricanes and Typhoons: Past, Present, and Future, edited by R. J. Murnane and K.-B. Liu,464 pp., Columbia Univ., Press, New York. Goldenberg,S. B., C.W Landsea, A. M. Mestas-Nuhez, and WM.Gray (2001),The recent increase in Atlantic hurricane activity: Causes and implications, Science, 293, Landsea, C. W,W M. Gray, RW Mielke Jr., and K.J. Berry (1992),Long-term variations of western Sahelian monsoon rainfall and intense U.S. landfalling hurricanes, J. Clim., 5, Landsea, C.W, N. Nicholls,W M. Gray, and L.A.Avila (1996), Downward trends in the frequency of intense Atlantic hurricanes during the past five decades, Geophys. Res. Lett., 23, Maloney,E.D.,and D.L. Hartmann (2000), Modulation of hurricane activity in the Gulf of Mexico by the Madden-Julian Oscillation,Science, 287, Author Information Brian H. Bossak, U.S. Geological Survey, Florida Integrated Science Center, St. Petersburg For additional information, contact B. H. Bossak; bbossak@usgs.gov. Is There a Mantle Plume Below Italy? PAGES 541, Some of the most diverse igneous rocks found on Earth occur along the length of Italy and in many of the islands in the southeastern Tyrrhenian Sea, all the result of Cenozoic magmatism. Magmas extremely rich in alkalis, particularly potassium, and many undersaturated with respect to silica, were erupted, as well as others of calc-alkalic affinity (see legend in Figure 1). Their origin has been the subject of heated debate, and there is still no general consensus about how they formed. Most attribute them to subduction-related processes (see Beccaluva et al. [2004] for a review); others consider them to be the result of within-plate magmatism [e.g., Vollmer, 197'6; Lauecchia andstoppa, 1996]. Still others consider magmatism the result of a deep, mantle upwelling within a slab window coupled with mixing between isotopically different reservoirs [Gasperini et al, 2002]. The authors have re-evaluated some chemical (mainly isotopic) data from Italian rocks, and along with an assessment of some geodynamic numerical modeling, propose that magmatism is related to a large-scale, asymmetric plume beneath the Mediterranean region that has been in existence for at least Myr and continues to this day. In the opinion of the authors, there is just as much supporting evidence for a plume model as one involving subduction. Although the interpretation of Italian magmatism in terms of classic subduction has the comfort of tradition, it also has the discomfort of BY K. BELL, FCASTORINA, G. LAVECCHIA, G. ROSATELLI, AND F STOPPA being inconsistent with some of the available geochemical and structural data. The Isotopic Evidence A pronounced isotopic polarity along the length of Italy (northerly increase in 87 Sr/ 86 Sr, (TO, and (5 13 C, and southerly increase in 143 Nd/ 144 Nd, 206 Pb/ 204 Pb, and 3 He/ 4 He ratios) known for some time, and interpreted as mixing between two end-members, one depleted and the other enriched. The accumulation of data over the last decade and the authors' own unpublished analyses indicate a more complex story. Assessment of the isotopic data, particularly in terms of the mantle taxonomy established for oceanic basalts, shows that three quite distinct endmembers [Bell et a/., 2003] can adequately explain the broad trends of the isotopic ratio patterns shown in Figure 1. Two end-members are similar to the mantle components EMI (Enriched Mantle 1) and FOZO (Focus Zone) recognized in oceanic basalts. The third end-member, named ITEM (ITalian Enriched Mantle,Bell et al,2003), reflects a very enriched component yet to be recognized in oceanic basalts, with time-integrated moderately high U/Pb and Th/Pb, very high Rb/Sr, and low Sm/Nd ratios. The EM Hike component is recognized mainly in sodic-rich Tertiary basalts from Sardinia and is attributed by some to the melting of recycled plume heads stored within the deep mantle.the FOZO-like component is found in Quaternary lavas from Etna, Pantelleria, and the Sicily Channel, in Tertiary lavas from the Hyblean plateau (Sicily), and in late Paleocene lavas from Pietre Nere (Apulian foreland), and forms the common end-member to the overall data set. Although the origin of FOZO is still undergoing debate, it probably reflects a globally widespread, deep-seated, and perhaps more primitive component than any of the others [e.g.,hauri et al., 1994]. Gasperini et al. [2002] chose not to accept FOZO as one of their two end-members but instead considered it to be a mixture of Depleted MORB mantle (DMM) and high u mantle (HIMU), neither of which has been found as a pure end-member in many Italian lavas. ITEM is isotopically similar to upper continental crust and Atlantic pelagic sediments, and perhaps represents metasomatized mantle. An alternative might be a much deeper mantle reservoir sited at the D" core-mantle boundary layer and isolated from mantle convection. A noteworthy feature of the Italian data is the absence of any of these distinct, isotopic end-members in present-day subduction-related magmas.the contribution from DMM is minimal in most of the Italian lavas.the EMI- and FOZOlike signatures are usually associated with plume-related magmatism. ITEM has not been found in lavas from present-day subduction zones, and has not been recognized as a pure end-member in any of the rocks from the Aeolian Islands and the Tyrrhenian seafloor, areas considered to be typical of arc and back-arc basins. Rather, some of the data from the Aeolian Islands lie close to FOZO, and the same is true of samples from the Sicily Channel. Hence, the isotopic evidence seems inconsistent with subduction-related magmatism. Tectonic Context The Mediterranean region underwent a major change at the end of the Eocene, when the dominant compressional regime that had led to the development of the Alpine-Betic chain was replaced by dominant extension. This change coincided with complete subduction of the Tethyan oceanic lithosphere involved in the Alpine orogeny and strong decrease in

2 Eos, Vol. 85, No. 50,14 December 2004 the velocity of the relative northward motion of Africa relative to Eurasia.The opening and lithospheric stretching of the Mediterranean region started along the southwestward con tinuation of the Rhine-Rhone rift system and propagated eastward, accompanied by the subordinate development of the ApennineMagrhebian fold-and-thrust belt system at the outer margin of the extensional system. In spite of the general acceptance of subduction in controlling the evolution of the central Mediterranean and the Apennines [Faccenna et al, 2003, and references therein), several problems may be cited that are difficult to reconcile with arc-related models [Lavecchia and Creati, 2004].These include: (1) an incon sistency between the ~ 200-km along-strike length of the Benioff plane (offshore of Calabria) and the length of Apennine-Magrhebian system (about 3500 km from northern Liguria to the Gibraltar arc), (2) a lack of typical accretionary prism geometries, (3) rheological difficulties involved in subducting the Adriatic continental lithosphere, (4) poor definition of tomographically imaged slabs along peninsular Italy (5) a scarcity in peninsular Italy of c^dc-alkalinerocks, (6) the presence of rocks similar to those in rift-related settings (leucitites,carbonatites,and kamafugites),and (7) distinct isotopic endmembers never found in subduction-related magmas. The Plume ITEM m EM EMI %%*FOZQ2fi» & A " V + a DMM FOZOl Model During upward migration, large plumes can thermally entrain parts of the mantle other than their source, resulting in a plume head that is isotopically heterogeneous and chemi cally zoned.the isotopic signatures (similar to EMI and FOZO) of the Italian lavas, the peculiar and rare ultra-alkaline composition of some of the occurrences, and the intensive thinning of the lithosphere and mantle upwelling are features consistent with plume activity The absence in Italy of extensive volumes of flood basalt suggests that the plume head has not yet reached a sufficiently high level to produce basaltic liquids, a model previously proposed to explain the absence of flood basalts in East Africa [Griffiths and Campbell, 1991].Many Italian tectonic features (extensive stretching of the lithosphere, eastward tectonic and magmatic polarity, and metasomatism of the mantle) are consistent with an eastward growing, asymmetric plume head trapped within the transition zone between the 670-km and 410-km discontinuities (see Figure 2), perhaps from the deep mantle [Lavecchia and Creati, 2004], and similar to one of the numerical models proposed by Brunet and Yuen [2000].A volume excess within the asthenosphere, brought about by growth of the plume head within the transition zone, leads to stretching and thinning of the overlying lithosphere. In turn, rift-push forces developed at the outer border of the extended lithosphere would be responsible for the nucleation of the Apennine-Magrhebian compressional structures. Injection of deep mantle material into the plume head can theoretically repeat itself OZOl DMMQ EM V Western Alps (lamproite) A Eastern Alps (lamprophyre) Tuscany (lamproite/sub-alkaline) Roman Province (HK series) G IUP (kamafugite-carbonatite) m Vesuvius & Phlegrean Fields (HK series) Ischia & Procida, Ventotene (sub-alkaline) Vulture (Na-K-alkaline/carbonatite) A Punta Pietre Nere, La Queglia (lamprophyre) ^ Aeolian Islands (sub-alkaline/calcalkaline) * Etna (Na-alkaline) if Iblei (Na-alkaline) Sicily Channel and Pantelleria (Na-alkaline) -^Sardinia (lamprophyre/na-alkaline) -&#Tyrrhenian Sea (Na-alkaline/sub-alkaline) W Pb Fig. 1. (a) Italian mafic magmatism with spatial distribution of main rock types (IUP = Intramontane Ultra-alkaline Province), (b) Isotope ratio diagrams: DMM = Depleted MORB mantle, HIMU = high ju mantle, FOZO = Focus Zone (FOZOl and FOZ02 are different estimates of FOZO; see discussion in Hauri et al., 1994), EMI and EM2 = Enriched mantle 1 and 2, ITEM = Italian enriched mantle. Note the absence of DMM, HIMU, and EM2 in most of the Italian rocks, and convergence of data toward a FOZO-like end-member. Our best estimate for this common end-member is Sr/ Sr = , Nd/' Nd = (e = +l. 1), and Pb/ Pb = References for the data used in compiling this figure are available from the authors. Original color image appears at back of this volume. 87,43 44 m within a period of some tens of million years [Brunet and Yuen, 2000], generating a pulsating mechanism which may be responsible for episodic, intraplate volcanism.this mechanism might have been responsible not only for the magmatic activity coinciding with the opening of the Mediterranean basins, but also for older pulses of ultra-alkaline activity in Italy, such as indicated by the isotopic signatures of the late Cretaceous-early Paleocene lamprophyres from the Eastern Alps and Pietre Nere (Figure 1). Therefore, the plume activity might have already begun Myr ago One problem our model encounters is the tomographic velocity patterns detected in the Mediterranean region that show a large-scale high-velocity anomaly placed within the tran sition zone of the upper mantle, coupled with a large-scale, low-velocity anomaly in the overlying asthenosphere [e.g., Faccenna et al, 2003, and references therein]. In order to rec oncile the tomographic anomalies with our trapped plume model, it is hypothesized that perhaps some of the velocity patterns reflect chemical rather than thermal variations. Faster zones within the transition zone would represent

3 Eos,Vol. 85, No. 50,14 December 2004 a highly depleted plume head which has lost volatiles and fluids during upward migration. In turn, the overlying low velocity zone might represent a plume-modified asthenospheric region, metasomatised and enriched by H 0C0 rich fluids and volatiles released from the plume. 2 2 Concluding Remarks It is realized, of course, that isotopic data pri marily reflect sources rather than processes. But the presence of a common end-member in most of the Cenozoic rocks from Italy similar to FOZO, implies the involvement of a common, deep-seated source and perhaps, in turn, a sin gle geodynamic system. Although our plume model is by no means definitive, it does pro vide another way of looking at Italian magma tism and its geodynamic context. Acknowledgments This work was partly supported by the Natural Sciences and Engineering Research Council of Canada (K.B.) and funds from G.d'Annunzio University (G.L. and ES.). We thank A. Rukhlov for insightful comments, and help with the isotopic compilation. Useful discussions with I.H.Campbell, S. Conticelli, R. E. Ernst, M. Lustrino, H. Mirnejad, A. Peccerillo, and G. R.Tilton honed our ideas and helped toward a significantly improved manuscript, even though not all were in total agreement with our ideas. References Beccaluva,L.,G.Bianchini and ESiena (2004), Tertiary-Quaternary volcanism and tectono-magmatic evolution, in Geology of Italy, spec, vol., pp , Ital. Geol. Soc. Rome. Bell,K.,ECastorina,G.Rosatelli,and FStoppa (2003), Large scale, mantle plume activity below Italy: Isotopic evidence and volcanic consequences, Geophys. Res.Abstr., 5, Abstract Brunet, D.,and D.Yuen (2000), Mantle plumes pinched in the transition zone,earth Planet. Sci. Lett., 178, Faccenna, C, L. Jolivet, C. Piromallo, and A. Morelli (2003),Subduction and the depth of convection in the Mediterranean mantle, J. Geophys. Res., 707(B2),2O99,doi:lO.lO29/2OOUBOO169O. Gasperini, D., J. Blichert-Toft, D. Bosch, A. Del Moro, PMacera and EAlbarede (2002),Upwelling of deep mantle material through a plate window: evidence from the geochemistry of Italian basaltic volcanics, J. Geophys. Res., 76>7(B12),2367, doi: /2001jb Griffiths,R.W,and I.H.Campbell (1991),Interaction of mantle plume heads with the Earth's surface and onset of small-scale convection, J. Geophys. Res. 56,18,295-18,310. Hauri,E.H.,J.A.Whitehead,and S.R.Hart (1994), Fluid dynamic and geochemical aspects of entrainment in mantle plumes, J. Geophys. Res., 5/9,24,275-24,300. Lavecchia,G.,and N.Creati (2004),A mantle plume pinched in the transition zone beneath the Mediterranean: A preliminary idea,ann. Geophys., in press. Lavecchia, G., and EStoppa (1996),The tectonic significance of Italian magmatism: An alternative Fig. 2. (a) Lithospheric thickness in the Mediterranean (deduced from the regional dispersion of seismic surface waves) and major extensional and strike-slip tectonic lineaments at the outer boundaries of the extended region, (b) Interpretative transect showing a degassed mantle plume trapped in the Mediterranean transition zone; the trace of the transect is in Figure 2a. The geo metry of the plume is largely hypothetical and derived from Figure 6 of Brunet and Yuen [2000]; the size of the plume is predicted by Lavecchia and Creati [2004], based on a simple, areal-balance technique. Key: (1) lithosphere; (2) asthenosphere invaded by metasomatic fluids; (3) mesosphere; (4) degassed heterogeneous plume (the colors mark the decrease in density moving from the outer to the innermost parts of the plume); and (5) remnants of the Tethyan lithosphere which during the Cretaceous-Paleocene Alpine tectonic phase was subducted southeast, beneath the Adriatic fore land. The geometry of the overturned slab in Figure 2b is constrained by the distribution of the deep seismicity offshore of Calabria. Original color image appears at back of this volume. view to the popular interpretation, Terra Nova, 8, Vollmer, R. (1976), Rb-Sr and U-Th-Pb systematics of alkaline rocks: The alkaline rocks from Italy, Geochim. Cosmochim.Acta, 74, Author Information Keith Bell, Carleton University Ottawa, Ontario, Canada; Francesca Castorina,Universita"La Sapienza," Rome, Italy; and Giusy Lavecchia, Gianluigi Rosatelli, and Francesco Stoppa, Universita "G. d'annunzio," Chieti Scalo, Italy For additional information, contact K. Bell; kib@magma.ca.

4 Eos,Vol. 85, No. 50,14 December CZ3 ITEM ,EM ' EMI f$&$> 0 Sto»»FOZ02^ ' A DMMO DMM + OZOl ' ' ' ' EM V Western Alps (lamproite) Eastern Alps (lamprophyre) T Tuscany (lamproite/sub-alkaline) Roman Province (HK series) IUP (kamafugite-carbonatite) Vesuvius & Phlegrean Fields (HK series) Ischia & Procida, Ventotene (sub-alkaline) Vulture (Na-K-alkaline/carbonatite) Punta Pietre Nere, La Queglia (lamprophyre) Aeolian Islands (sub-alkaline/calcalkaline) Etna (Na-alkaline) Iblei (Na-alkaline) Sicily Channel and Pantelleria (Na-alkaline) A ^Sardinia (lamprophyre/na-alkaline) -&0Tyrrhenian Sea (Na-alkaline/sub-alkaline) I W 4 Pb Fig. 1. (a) Italian mafic magmatism with spatial distribution of main rock types (IUP = Intramontane Ultra-alkaline Province), (b) Isotope ratio diagrams: DMM - Depleted MORE mantle, HIMU = high ju mantle, FOZO = Focus Zone (FOZOI and FOZ02 are different estimates offozo; see discussion in Hauri et al., 1994), EMI and EM2 = Enriched mantle I and 2, ITEM = Italian enriched mantle. Note the absence of DMM, HIMU, and EM2 in most of the Italian rocks, and convergence of data toward a FOZO-like end-member. Our best estimate for this common end-member is 87 Sr/ 86 Sr = , M3 Nd/' 44 Nd = (e = +1.1), and 206 Pb/ m Pb = References for the data used in compiling this figure are available from the authors. Page 541

5 Eos,Vol. 85, No. 50,14 December 2004 Page 546 Fig. 2. (a) Lithospheric thickness in the Mediterranean (deduced from the regional dispersion of seismic surface waves) and major extensional and strike-slip tectonic lineaments at the outer boundaries of the extended region, (b) Interpretative transect showing a degassed mantle plume trapped in the Mediterranean transition zone; the trace of the transect is in Figure 2a. The geometry of the plume is largely hypothetical and derived from Figure 6 of Brunet and Yuen [2000]; the size of the plume is predicted by Lavecchia and Creati [2004], based on a simple, areal-balance technique. Key: (1) lithosphere; (2) asthenosphere invaded by metasomatic fluids; (3) mesosphere; (4) degassed heterogeneous plume (the colors mark the decrease in density moving from the outer to the innermost parts of the plume); and (5) remnants of the Tethyan lithosphere which during the Cretaceous-Paleocene Alpine tectonic phase was subducted southeast, beneath the Adriatic foreland. The geometry of the overturned slab in Figure 2b is constrained by the distribution of the deep seismicity offshore of Calabria.