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ISSN 0016-8521, Geotectonics, 2009, Vol. 43, No. 1, pp. 1–17. © Pleiades Publishing, Inc., 2009. Original Russian Text © V.N. Puchkov, 2009, published in Geotektonika, 2009, No. 1, pp. 3–22.

The Controversy over Plumes: Who Is Actually Right? V. N. Puchkov Institute of , Ufa Scientific Center, Russian Academy of Sciences, ul. K. Marksa 16/2, Ufa, 450000, Russia e-mail: [email protected] Received February 5, 2008

Abstract—The current state of the theory of plumes and its relation to classic show that the “plume” line of geodynamic research is in a period of serious crisis. The number of publications criticizing this concept is steadily increasing. The initial suggestions of plumes' advocates are disputed, and not without grounds. Questions have been raised as to whether all plumes are derived from the mantle–core interface; whether they all have a wide head and a narrow tail; whether they are always accompanied by uplifting of the ’s surface; and whether they can be reliably identified by geochemical signatures, e.g., by the helium-iso- tope ratio. Rather convincing evidence indicates that plumes cannot be regarded as a strictly fixed reference frame for moving lithospheric plates. More generally, the very existence of plumes has become the subject of debate. Alternative ideas contend that all plumes, or hot spots, are directly related to plate-tectonic mechanisms and appear as a result of shallow tectonic stress, subsequent decompression, and melting of the mantle enriched in basaltic material. Attempts have been made to explain the regular variation in age of volcanoes in ocean ridges by the crack propagation mechanism or by drift of melted segregations of enriched mantle in a nearly horizontal asthenospheric flow. In the author’s opinion, the crisis may be overcome by returning to the begin- nings of the plume concept and by providing an adequate specification of plume attributes. Only mantle flows with sources situated below the should be referred to as plumes. These flows are not directly related to such plate-tectonic mechanisms as passive rifting and decompression melting in the upper asthenos- phere and are marked by time-progressive volcanic chains; their subasthenospheric roots are detected in seismic tomographic images. Such plumes are mostly located at the margins of superswells, regions of attenuation of seismic waves at the mantle–core interface. DOI: 10.1134/S0016852109010014

INTRODUCTION my own estimate of the current state of the art, avoid- The history of Earth sciences has frequently ing, where possible, the emotions and biases inherent to involved crises accompanied by vigorous debate. The many participants in the debate. well-known controversies of neptunists and plutonists, catastrophists and evolutionists, and mobilists and fix- HOT SPOTS, HOT FIELDS, PLUMES, ists are examples. These great controversies, initiated SUPERPLUMES, AND LIPS by irreconcilable opponents with polarized views, led to fruitful search for new arguments and facts pro and The idea that chains of volcanoes on the oceanic contra, to disproofs and compromises, and finally, to a floor are hot spots, i.e., surface manifestations of local clear understanding of the frames of reference under anomalous zones of melting in the mantle, was set forth which the arguments of both sides may carry weight. by Wilson [71] more than 40 ago. Subsequently, A new debate in Earth sciences, which has flared up this idea was developed into the concept of mantle over the last decade, concerns the existence or nonex- plumes (plumages) [51], spatially fixed vertical convec- istence of so-called plumes. Ranking this discussion tive flows (upwelling) of hot light material ascending among the great controversies in the field, I would call from the mantle–core boundary. The volcanic trail of readers' attention not only to its passionate and at times these flows on the Earth’s surface depends on the biased character, but also to the great implications of its motion of lithospheric plates, allowing use of the pro- subject, which concerns processes that develop jection of any such flow on the Earth’s surface as a rel- throughout the mantle, especially its lower part, and are atively stable reference point. The Hawaiian–Emperor reflected in the Earth’s . The resolution of this con- Chain (Fig. 1) is a classic example. troversy will influence our basic concepts of global The idea of plumes has something in common, but and our knowledge of the tectonics and does not coincide completely, with the idea of astheno- metallogeny of the Earth’s crust and of the tectono- liths—columns of heated light material emerging in the sphere as a whole. mantle—stated by Belousov [2]. In contrast to This paper is an attempt to investigate the logic of Belousov, the advocates of the plume concept do not the arguments adduced by confronting sides and to give strive to deny horizontal displacements of lithospheric

1

2 PUCHKOV

Plate motion

Active

The Earth's surface

Bottom of the

Mantle plume Distance from Kilauea, km 6000 5000 4000 3000 2000 1000 0

80 80° 50°

60° 65 43 Ma, a moment of changed direction of plate motion

40° 40° 66 66 59 K–Ar age, Ma 48 20° 40 42 39 30° 29 20 28 5 Kilauea 20° C 170° E 180° 170° W 160°

Fig. 1. (Above) The classic model of hot-spot formation in terms of the plume hypothesis. The penetrates into the lithosphere and provokes a volcanic eruption. The lithosphere moves relative to the plume with the formation of a volcanic chain. (Below) Hawaiian–Emperor time-progressive volcanic chain with notation of age of igneous rocks, Ma. (Side) The relationship of age vs. distance from the active Kilauea volcano [53]. plates, and regard plume tectonics as a substantial sup- and maximum lifespans of ~50 Ma that have intraplate plement to the classic tectonics of lithospheric plates. tectonic settings or geochemical affinities and are charac- Over the last 35 years, plume tectonics has devel- terized by igneous pulse(s) of short duration (~1–5 Ma), oped actively, involving , petrol- during which a large proportion (>75%) of the total ogy, , and isotope geology in the sphere of igneous volume was emplaced” [19]. consideration. The concepts of hot fields [5], or large The development of LIPs has been attributed to the igneous provinces (LIPs) [26], have been developed. breakdown of , first Pangea [4, 64] and The recently specified definition of LIP is as follows: then Rodinia [48, 49]. The formation of LIPs is equated “Large igneous provinces are magmatic provinces with with superplumes, i.e., vast plumelike upwellings of areal extents >0.1 Mkm2, igneous volumes >0.1 Mkm3, mantle material resulting from relatively short-term

GEOTECTONICS Vol. 43 No. 1 2009

THE CONTROVERSY OVER PLUMES: WHO IS ACTUALLY RIGHT? 3 melting and eruption of enormous volumes of volcanic tinental lithosphere and on some oceanic is also rocks. The relationships of plumes to superplumes and attributed to plumes and related alkaline . of both to superswells—low-velocity regions in the D'' Presumably plume-related are classified as layer at the mantle–core boundary (see below)—have ocean- basalts (OIB), enriched in many lithophile trace been established. Comprehensive information on LIPs elements not only in oceans but also on continents [36]. is accessible at the website http://www.largeigneous- provinces.org. (6) LIPs, especially the trap provinces of the Siberia, Deccan, Parana, and Etendaka plateaus, and , are related to superplumes. The same is true of ATTRIBUTES AND PROPERTIES OF PLUMES giant swarms of Precambrian dolerite dikes in ancient AND SUPERPLUMES shields. Such swarms attain hundreds and even thou- sands of kilometers in extent and are radiate-arranged As the theory was developing, a set of attributes and (Fig. 5). The dikes are considered (not without properties ascribed to plumes and superplumes was grounds) to be plumbing systems of older, mainly Pre- proposed. cambrian, plateau basalts now eroded [32, 34]. The (1) The aforementioned time-progressive volcanic indications of lateral movement of along these chains suggested that hot spots and plumes actually channels allow us to avoid the suggestion of a sea of exist. As was inferred initially, plumes are strictly fixed magma beneath the trap field. and may serve as reference points for plate motions. To Despite certain problems, it seemed that plate and date, isotopic timing has confirmed the existence of at plume tectonics, initially existing as parallel para- least twenty such chains (Fig. 2). digms, the former responsible largely for the upper tec- (2) The vertical columns or inclined zones of low- tonosphere and the latter for the lower tectonosphere, density mantle material detected by seismic tomogra- would be able to develop without conflicts and merge phy beneath active plumes extend to a great depth, into a single consistent paradigm of global geodynam- often exceeding a few hundred kilometers and occa- ics. Attempts to present such a paradigm in the form of sionally reaching the upper– interface and a simplified model [28] were rather successful (Fig. 6). even the mantle–core boundary [75] (Fig. 3). Tectono- However, recently (approximately since the beginning physical modeling [24] suggests that these are nearly of the 21st century), the existence of plumes (at least in vertical cylindrical bodies with a thin feeder and a wide their classic interpretation) has become somewhat frontal portion (head–tail structure). Setting against the problematic and doubtful. Heated debates have flared lithosphere, the head spreads over a large territory and up in scientific journals and on websites, e.g., produces voluminous melts, whereas the volume of www.geolsoc.org and www.mantleplumes.org. melts related to the feeding conduit diminishes. (3) Individual plumes and superplumes are related OBJECTIONS OF THE “ANTIPLUME LOBBY” to low-density domains in the lower mantle (Fig. 4). Two large low--velocity provinces (LLSVPs) The “antiplume lobby” (the name given neatly by have been revealed beneath the southern Pacific Ocean Saunders [63]) has attacked the plume theory on several and Africa with seismic tomography [21]. The LLSVPs fronts. At times, the arguments of various parties, par- reach a thousand kilometers in plan view and extend ticularly of plume opponents, may seem to be passion- upward, gradually disintegrating throughout the mantle ate and even biased. This is reflected in the colorful rheto- and concentrating mainly above two superswells of the ric of the debate, often departing from academic style: “the D'' surface. The location of the Pacific and African plume myth,” “a nail into the coffin of plumes,” “the love superswells, in turn, is correlated with the geoid shape. affair with plumes,” “hell of a plume,” etc. It is pointed out that superswells are partly limited (1) It is stated (and it seems quite correctly) that by sharp boundaries and thus mark compositional het- geochemical indicators, including even the He-isotope erogeneities rather than purely thermal phenomena ratio, are not mandatory evidence for the origin of a [1, 21, 38, 70]. plume at the mantle–core interface [33, 50]. More and (4) Plumes are related to doming of the Earth’s sur- more evidence suggests that large-scale stirring (recy- face. cling) of the lithosphere and the mantle in the course of , and more precisely in the process of overall (5) Basalts of separate plumes are distinguished mantle with plumes as its ascending from MORB by higher LILE and LREE contents and streams, results in chemical heterogeneity of the man- elevated 87Sr/86Sr, 143Nd/144Nd, 206Pd/204Pd, and espe- tle, expressed in the fine geochemical features of volca- cially 3He/4He isotope ratios. The latter value is the nic rocks. Numerous enriched and depleted mantle res- ratio of primordial solar He to radiogenic He and is ervoirs with different He, Ar, Sr, Nd, Hf, Os, and Pb iso- regarded as an indicator of deep source [5]. It has been topic compositions are recognized and abbreviated as suggested that these characteristics suggest the derivation PM, BSE, PREMA, UM (MORB), DM, EM1, EM2, of volcanic rocks from the . The formation of SOPITA, HIMU, SCLM, FOZO, etc. [3, 76]. The com- carbonatites and kimberlites in the regions with thick con- plex mantle structure with neighboring undepleted,

GEOTECTONICS Vol. 43 No. 1 2009

4 PUCHKOV ° ° ° ° ° ° ° 90 W ° 90 60 30 0 30 60 ° 90 30 90 , Canary; Ea 2 , Marquises; Y 6 Can ° °

F , Yellowstone. , Mar Y 120 120 3 60 Pp Pt 21 1 the Pacific, Indian, and

Mar , ; K–B 11

27 , Society; 5 M ° ° S CÓ S , Galapagos; 24 150 4 Lo 150 Gal 65 Mu mid- on located is volcano frontal ) 2 29 95 46

, Louisville; °

° 77 he Columbia River Province in Province the he west Columbia of River Emp Lo inct volcanoes of volcanic time-progressive Sa 180 180 43 , Foundation; , Foundation; ) line connecting the volcanoes formerly belong- formerly volcanoes the connecting line ) significant displacement of the plume itself. The itself. plume the of displacement significant 1 4 F 100? Haw- 23 ( arbonatites); 80 , ; T , New England–Meteor; 7 ° ° Sa 10

Car 24 , Easter; ) approximate contour African of and the Pacific large low- 150 150 NE–M 5 Ea , Caroline; ° ° , Azores; Car , Pitcairn; Az Pt 120 120 ) Volcanic chains with active or recently extinct (<1 Ma) volcano at one end. The end. one at volcano Ma) (<1 extinct recently or active with chains Volcanic ) 1 , ; ° ° , Tasmanian; , Tasmanian; I T , Pukapuka; 90 90 118 38 Pp 38 65 34 K , Cobbs; ° ° , Kerguelen; , Kerguelen; 33 K 60 60 Co R , Reunion; ° ° R 30 30 , Kodiak–Bowie; , Kodiak–Bowie; 132 K–B

81 12 3 4 5 ° °

, Rio Grande;

0 0

72 TdC SH

63 R-G

3

Can M , Musicians; I Mu ° 60 °

30 30 Az 40 10-20

R-G 24-58? Tr 75 85 , da Tristan Cunha; ° °

127- ( 59]. 52–56, 46, 41, 39, 37, 27, 25, 24, [17, margins continental their and TdC 60 137 60 ) volcanic chains where the youngest extinct volcano is older than 1 Ma (numeral is the volcano’s age, Ma); ( Ma); age, volcano’s the is (numeral Ma 1 than older is volcano extinct youngest the where chains volcanic ) 81 3 NE–M , Hawaiian–Emperor; , Hawaiian–Emperor; Location of Mesozoic and time-progressive volcanic chains. The map is compiled on the basis of the data published for

124 20 ° W Gal , St. Helena; ° 90 ° ° ° ° ° N S 0 90 Haw–Emp ocean ridge; ( ridge; ocean Atlantic oceans Atlantic Fig. Fig. 2. ing to a single chain, which had crossed spreading an zone active and subsequently was by divided this zone; ( SH shear-velocity provinces shear-velocity (LLSVPs) at the mantle–core boundary projected on All surface or the active [21]. recently Earth’s ext , which chains are are related likely located to near within t volcanoes these contours, the except Cobb and Kodiak–Bowie America, controlled by a minor superswell [21]. North Notations of plumes (counterclockwise from the northwestern corner of the map): arrow arrow indicates the direction of increasing age, i.e., the direction of the lithospheric plate without correction for much less c and intrusions, ring traps, are these continents, (on event volcanic oldest the of age the indicates end other the at numeral ° ° 60 30 30 60 90 90

GEOTECTONICS Vol. 43 No. 1 2009 THE CONTROVERSY OVER PLUMES: WHO IS ACTUALLY RIGHT? 5

Tahiti

Ä Ç

0

500

1000 1500 2000 2500 –1% 0% +1%

Ç

Ä

Fig. 3. Vertical section of anomalies of P-wave velocities along the Hawaii–South Pacific Superswell line. The scale of deviation from the average value and the location of the section are shown below. The position of the Hawaii and hot spots [75] is noted by triangles.

–1% 0% +1%

Fig. 4. Anomalies of P-wave velocities near the mantle–core interface at a depth of 2730 km. The scale of velocities is given below. Hot spots on the Earth’s surface are noted by triangles [75]. depleted, secondary enriched (refertilized), and recy- (2) Doubts appear in real images of plumes based on cled domains are described in such figurative terms as seismic tomographic results, up to the assertion that the marble cake, spaghetti, plum cake, etc. Some authors detected seismic anomalies correspond to the noise assume that mantle domains of different types can level [44]. Indeed, by no means all plumes are traceable undergo He-isotope exchange over long intervals of down to the core, although the plume tail may be thin- geological time [14]. ner than the wavelength used, and thus undetectable.

GEOTECTONICS Vol. 43 No. 1 2009 6 PUCHKOV

Ural–Siberian superplume, which arose in the Early Triassic, when the Urals and western Siberia underwent substantial uplifting. As concerns the Ontong Java LIP, this volcanic plateau initially arose on the deep oceanic bottom, so that the uplift may have proceeded unno- ticed. In addition, it was shown in [23] that the effect of a plume on complexly built layered lithosphere may result in equivocal surface manifestation, e.g., as spatial alternation of elevated and subsided blocks. (4) The suggestion that plumes are an absolute ref- erence frame for permanently moving lithospheric plates has also proven incorrect. Displacements of the upper portions of plumes relative to their lower parts have been described [40, 73, 75, etc.]. Paleomagnetic studies have also revealed displacements of plume trails, which should most likely be regarded as floating anchors rather than fixed reference points [31]. The substantial shift of mantle sources for some volcanic chains is supported by paleomagnetic evidence as well [21, 55]. Nevertheless, these data do not impel us to dis- card the idea of plumes because the scale of such offsets is by an order of magnitude less than the displacement of the volcanic trail on the plate surface. Comparison of Fig. 5. The system of radiate dikes, sills, and layered intru- the paths of apparent polar wander established from sions 1270 Ma in age in the Mackenzie Province; an exam- paleomagnetic data and the motion of hot spots shows ple of a deeply eroded Proterozoic LIP with exposed plumb- their good consistency over the last 49 Ma and increas- ing system [32, 34]. ing discrepancy in older epochs [62]. Moreover, com- parison of the position of hot spots in the Pacific Ocean relative to the plumes in the Atlantic and Indian oceans The mushroomlike morphology of plumes is also not has confirmed their mutual deviations and correlation supported. Nevertheless, despite all the imperfections, within these groups [53, 59]. Three hot-spot families— seismic tomography reveals many important things. Pacific, Indo-Atlantic, and Icelandic—have been sug- The section along the Mid-Atlantic Ridge (Fig. 7) dem- gested [53]. As has been shown, the hot spots of the onstrates hot and cold zones, and even the zone of dry former two families have remained relatively fixed dur- spreading in the Central Atlantic. The root of Iceland ing the last 80 Ma. pierces the entire upper mantle. It should be noted that seismic tomography is a (5) The existence of anomalously high temperatures rather specialized kind of research, and the final word (up to 300–400°C above the background values) in the must belong to the experts. It is from the experts them- mantle, which is a necessary condition for melting selves, however, that we continue to obtain new and more under dry conditions [11], has been cast into doubt. detailed information each [47, 60, 66, 72, etc.], with Such temperatures should be expressed in a corre- the effect of multiplying, rather than reducing, the num- sponding increase in heat flow above the active plume, ber of known seismotomographic interpretations (and but actually this is not observed (see below). this pertains not only to plumes but also to subduction (6) Finally, attempts (in my opinion, clearly unsuc- zones). Before seismotomographic evidence, subduc- cessful) have been made to find an alternative explana- tion zones were traced down to 700 km on the basis of tion for the regular variation in age of volcanoes in the deep-sourced earthquakes. Seismic tomography traces submarine and on-land ridges pertaining to cold lithospheric slabs deeply in the lower mantle plume activity. Now it is impossible to dispute the (Fig. 8). One must have great courage to contest these existence of a great number of such ridges; up-to-date data, as Foulger has done [35], without being a seis- information about them was used in the compilation of mologist. Fig. 2. The necessity to explain this fact, which is (3) The statement that all plumes are predated by inconvenient for opponents of plumes, has sparked uplift of the Earth’s surface has been shown to be not renewed interest in the hypothesis of crack propagation fully valid. For example, the Siberian flood prov- as proposed by J. Dana [35, 69]. Seemingly, the most ince, the submarine , and the Dec- adequate approach to the verification of this hypothesis can traps are exceptions [35–65]. However, the Sibe- is consideration of the forces that drive lithospheric rian example, the most familiar one for me, does not plates and deform them. It would have to be shown that give an unequivocal answer. As has been emphasized a combination of forces or their transformation gives many times, in this case, we deal with the much greater rise to extension across the inferred fracture and the

GEOTECTONICS Vol. 43 No. 1 2009 THE CONTROVERSY OVER PLUMES: WHO IS ACTUALLY RIGHT? 7

Fig. 6. A model of the relationships between plate and plume tectonics [29]. (1) Superplume, (2) plunging lithospheric slabs. volcanic chain. However, in the most obvious case of the rearrangement of plate boundaries accompanied by the Hawaiian–Emperor and similar ridges, the subduc- extension. However, this concept is also based on the tion pull and rollback, mantle drag, and ridge push idea of crack propagation. do not correspond to the tensile stress. As was shown in [77], the maximum horizontal stress is commonly ori- It should be added that, in contrast to the plume ented nearly parallel to the absolute direction of plate model, this conjecture does not give satisfactory expla- motion, indicating that the driving force controls the nations of a number of phenomena. First, the strong stress distribution within the plate as well. Unfortu- anisotropy of oceanic, transitional, and marginal conti- nately, in the recently published substantiation of the nental crust crossed by volcanic chains does not affect crack propagation mechanism [69], this reasoning was their morphology. Second, the orientation of time-pro- not taken into account. The thermoelastic stress was gressive volcanic chains strikingly corresponds to the estimated for cooling of the Pacific Plate, considered as vectors of motion of lithospheric plates (Figs. 2, 9). For a spherical shell. Having omitted the powerful forces of example, the volcanic chains located on different slopes the plate-moving mechanism, the authors obtained the of the East Pacific Rise are distinguished by opposite desired tensile stress providing fracture formation. It is trends of age progression, as can be seen in Fig. 9. The common in physics to simplify a problem by omitting volcanic edifices located on the mid-ocean ridges (Ice- secondary factors; in this case, however, it is a major land, Azores, Galapagos) are split. The Reunion and force that has been neglected. Kerguelen volcanic chains cross the active Carlsberg mid-ocean ridge and extend further over the next plate A somewhat different approach to explanation of because the ridge itself drifts in the northeastern direc- time-progressive volcanic chains was demonstrated in tion. The Kodiak–Bowie and Cobb volcanic chains, on [67], where the formation of such chains is referred to the one side, and Yellowstone, on the other side of the

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14 17 2 6

2 6 14 17 0 100 200 300 400 500 600 Depth, km 700 800 –3% δV +3% 900 s 1000 0 20 40 60 80 100 120 140 160 180 200 10 30 50 70 90 110 130 150 170 190 Distance along ridge, degree

Fig. 7. Tomographic section along the Mid-Atlantic Ridge, modified after [61]. The geographic position of the section is shown in the upper panel. The solid line is the ridge axis; the gray circles are stakes spaced at 10° by latitude. The triangles are hot spots (stakes 2, 6, 14, and 17: Tristan da Cunha, Asuncion, Azores, and Iceland, respectively). The cross, added by the author, indicates a region of dry spreading with insignificant melting. The tomogram shown below demonstrates anomalies of S-wave velocity. Among hot spots, only Iceland is underlain by a deep (down to the transitional layer) column of low-density material, which does not reveal visible relations to the lower mantle but nevertheless extends deep into the mesosphere, indicating that Iceland may be regarded as a plume. The tomogram does not bear information on the of other hot spots. plate boundary are oriented strictly along the motion velocity regions that occupy about 2 wt % of the total vectors of host plates. Finally, the oblique northeastern mantle probably differ from the remainder in composi- orientation of the St. Helena and Tristan da Cunha tion [1, 38]. Because LIPs arose on the Earth’s surface chains exactly fits the direction of motion of the corre- during the last 2.5 Ga, it may be suggested that super- sponding plate. The active or recently extinct volcanoes swells with surrounding zones of plume generation of almost all chains are localized above superswells have existed as long as LIPs. close to their margins (Fig. 2). In contrast to the plume concept, neither the hypoth- The latter fact is also supported by paleomagnetic esis of crack propagation nor other alternative conjec- data [21, 22]. It has been shown that LIPs arising over tures (see below) explain the aforementioned phenom- the last 300 Ma, i.e., since the formation and break- ena. Given this fact, we should remember the principle down of Pangea, are situated exactly above the 1% con- “Leave well alone,” or as Ockham’s razor states entia tour line of S-wave attenuation surrounding the African non sunt multiplicanda praeter necessitatem (“Entities and Pacific provinces of low S-wave velocities at the must not be multiplied beyond necessity”)! mantle–core interface. The Siberian and Columbian LIPs surround swells of smaller dimensions. The loca- However, other alternatives should be mentioned for tion of the 24 most active volcanic hot spots coincides the sake of completeness. In particular, attempts have with the same boundaries, thus corresponding to zones been and continue to be made to explain the origin of of plume generation. The vertical projection of these regular volcanic chains by specially organized sublitho- zones fits the +10-m contour of the geoid. The low- spheric convection, e.g., related to the cooling effect of

GEOTECTONICS Vol. 43 No. 1 2009 THE CONTROVERSY OVER PLUMES: WHO IS ACTUALLY RIGHT? 9

(a) WE km 0 NF CL LSF T 410 660

1100 1200 –1,5% +1,5% 67 12 345 (b) NW SE km 0 Sl H SC S 410 K 660

1100

1800 –1% +1% 12345 Sea of Okhotsk Asian coast

(c) NW Kuril SE km 0 1

410

2

660

1000 –1.5% +1.5% 67 12345

Fig. 8. Structure of the mantle beneath the active zone of continent–ocean transition, after the data of deep seismic tomography (P-waves), modified after [20] and adapted after [8]: (a, b) sections of the West Pacific island-arc system: (a) Tonga island arc, (b) Sangihe and Halmahera. (1–5) Mantle domain with varying seismic velocities: (1, 2) slow; (3) medium; (4, 5) fast; (6) earth- quake source; (7) inferred deep detachment. Subduction zones (letters in circles): T, Tonga; S, Sangihe; H, Halmahera; fragments of abandoned subduction zones: CL, Colville–Lau (?) of Miocene age; K, Kalimantan of age; backarc basins: LSF, Lau and South Fiji; NF, North Fiji; Sl, Sulu; SC, South China; (c) deep structure of (1) ancient and (2) present-day subduction zones in the Kuril–Okhotsk region.

GEOTECTONICS Vol. 43 No. 1 2009 10 PUCHKOV 1 cm/yr

Fig. 9. Vectors of lithospheric plate motion in ITRF 2005 (http://itrf.ensg.ign.fr). Plate boundaries are shown by dotted lines.

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Observed VO = 10 cm/yr Observed Observed VO = 10 cm/yr Observed Hidden shear V O Total shear VO + VX = 20 cm/yr Lithosphere VL Lithosphere VL V A VX Asthenosphere Asthenosphere

VM VM Mantle Mantle

(a) (b)

Fig. 10. An attempt to explain the time progression of volcanoes along the Hawaiian–Emperor Ridge. The Hawaiian volcanic trail indicates that the connection between the magma source and the lithosphere is destroyed. The lithosphere moves to the west-north- west. Two alternatives are possible: (a) source is located beneath the asthenosphere; i.e, in the subasthenospheric mantle the total shear between the lithosphere and the mantle is recorded in the volcanic trail; (b) the source of the Hawaiian hot spot is situated in the asthenosphere; i.e., the total shear between the lithosphere and subasthenospheric mantle is not recorded in the volcanic trail because a part of this shear occurs beneath the source (deep, hidden shear) [Doglioni et al., 2005] (cited after [30]). the continental margin [45]. A hypothesis of small- tized attributes of plumes and the infinite expanding of scale roll convection with elongated cells parallel to the the term plume. This term is now used for any anoma- plate motion was proposed in [18]. The authors of this lous or seemingly anomalous (relative to classic plate model acknowledge that it fits for specific chains only tectonics) manifestation of igneous activity, whether and, as the preceding one, does not pretend to univer- deep- or shallow-seated. A certain portion of criticism sality. of the plume concept is added by advocates of passive Don Anderson [15, 16], another opponent of the rifting. This mechanism may be regarded as a partial plume concept, put forward the hypothesis of drifting alternative to plume tectonics, which explains shallow- melting source (easily melting blob) in the seated related to decompression. The asthenosphere, whose velocity is higher than the veloc- of East Africa or the Baikal are examples. ity of crust motion but lower than the velocity of the underlying mantle. A similar model was presented in [31] in graphic form (Fig. 10). ALTERNATIVES TO PLUME TECTONICS PROPOSED BY ITS OPPONENTS This interpretation turned out even worse than the idea of crack propagation. The hypothesis was worked Returning to the basic paper by Foulger [35], we out ad hoc for the Hawaiian–Emperor Ridge and is suit- note that this publication declares the passive extension able for explanation of other chains, where of the lithosphere, decompression, and local enriched become older westward. However, all of the hot spots domains of the mantle and crust as universal conditions in the and the eastern Atlantic become of all anomalies of melting. In other words, it proposes older in quite the opposite direction: to the northeast a model of passive rifting caused by plate-tectonic driv- and east rather than to the southwest (Figs. 11, 12; ing forces within intraplate space. This antiplume con- cf. Fig. 2). This hypothesis was considered in more cept is called the “plate” model for the genesis of melt- detail in [10], in my presentation at the EGU General ing anomalies [35], and is regarded as a universal Assembly 2008 [57, 58], and in discussion on the web- mechanism. Only the all-embracing approach provokes site http:www.mantleplumes.org. This discussion objection; the remainder is quite sensible. The model of turned out somewhat strange: my opponents said noth- passive rifting has been repeatedly and fruitfully dis- ing about the striking consistency in the vectors of plate cussed in Russian literature [6]. At the same time, its motion and orientation of time-progressive volcanic positioning as an absolute alternative to the model of chains and the coincident position of young volcanoes active intraplate is hardly reasonable. and superswell boundaries (see above), although this is The papers by Foulger (both cited and preceding) the main point. The probability of the random character are consonant with the aforementioned publications by of such coincidences is vanishingly small. Anderson [15, 16]. According to his statements, the (7) These “disclosures,” although readily refuted in domains of enriched mantle, which do not require many respects, manage to highlight the weak points in extremely high temperature for melting (see the section the model of plume tectonics, specifically the absolu- “Antiplume lobby,” paragraph 5), are composed largely

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60° N

30° N

0°

30° S

60° S

10 cm/yr

30° W 0° 30° E 60° E 90° E 120° E 150° E 180° 150° W 120° W 90° W 60° W 30° W

Fig. 11. Present-day plate velocities recalculated from the NUVELIA model of relative plate motions [De-Mets et al., 1990, 1994] under conditions that shallow-seated hot spots are taken as a reference frame. Note that in this system all plates move to the east. Rotation poles are denoted by circles (cited after [30]). of eclogite formed as a result of of thick- single but very important difference from the Ander- ened . The temperature of the mantle son’s model consists in its operation in the ascending below 100 km is lower than the melting temperature of flow of convecting mantle, i.e., in a plume. Moreover, dry peridotite but higher than the melting temperature without such a flow, eclogite cannot hang in the of basaltic or eclogitic components as products of ear- asthenosphere for tens of million years, as is accepted lier recycling (Fig. 13). in the model of time-progressive volcanic chains [30] (see above). The densities of , peridotite, and The advantage of such an approach for explanation eclogite in the crust and the upper mantle are 2.87, 3.30, of the geochemical features of the Hawaiian volcanics and 3.45 g/cm3, respectively. At a depth of ~150 km was recently demonstrated in [68]. A model of progres- eclogite must sink in the asthenosphere, whereas at a sive melting in the mantle has been proposed. At a depth of less than 50 km it is transformed into gabbro depth of ~150 km, in the low-velocity zone (asthenos- and floats to the roof of the athenosphere. phere?) lying within a depth interval of 130–170 km, eclogite as a product of lithosphere recycling starts to Subjecting the opponents of plumes to criticism, we melt in the center of plume. The percolating liquid must keep in mind that the idea of recycled mafic igne- reacts with overlying peridotite to form secondary ous and probably sedimentary rocks as possible enriched pyroxenite. In the course of further ascent of enriched sources of mantle melting are very advanta- plume, pyroxenite and then peridotite are involved in geous. As has been shown, they not only open the way melting. The incomplete magma mixing is expressed in to recognition of magma sources of volcanic rocks per- the geochemical variations of erupting volcanics. The taining to plumes and superplumes [68, 74], but in

GEOTECTONICS Vol. 43 No. 1 2009 THE CONTROVERSY OVER PLUMES: WHO IS ACTUALLY RIGHT? 13

60° N

Direction of mantle flow 0°

60° S

0° 60° E 120° 180° 120° W

Fig. 12. Correlating the directions of absolute plate motions that issue from the position of large rift zones 40 Ma ago, we obtain the field of coherent sinusoidal flow, along which the plates move with different relative velocities in the system of geographic coordinates [Doglioni, 1993] (cited after [30]). some cases allow us to manage without plumes at all, in the further development of global geodynamics, replacing them with quite plausible plate-tectonic mod- absolute rejection of plume tectonics must be consid- els, e.g., in the case of passive rifting. However, it ered unacceptable. Opponents of plume tectonics have should be emphasized that passive rifting is unable to not managed to adduce convincing arguments against explain melting in the midst of the asthenosphere or in the subasthenospheric layer, because decompression the two main pieces of evidence for the reality of and melting begin as soon as a fracture in the lithos- plumes. phere attains the asthenosphere, and this fracture can- not penetrate deeper, whether 30 or 300 km. Passive rifting is not the only non-plume mechanism 2000 Liquid of the formation of volcanic provinces. The vast prov- liquidus ince of the Baikal region, Mongolia, Northeast China, Lherzolite and Central Japan is an example of an alternative to the plume mechanism. A nearly horizontal lithospheric ° slab is currently detected beneath this province with C liquidus Eclogite seismic tomography [75, 78]. Due to its low density (?), 1500 this slab cannot sink below the phase boundary 70% melt between the upper and lower mantle and slips along this solidus Adiabat interface, inducing melting of the overlying mantle like Eclogite a common suprasubduction wedge (Fig. 14). This Temperature, Lherzolite Gabbro solidus mechanism is posited by some authors as an explana- eclogite tion of ancient LIPs, including the Siberian LIP [42]. Gabbro- Solid 1000 0 123 DISCUSSION AND CONCLUSIONS Pressure, MPa Although criticisms of the plume concept contain Fig. 13. Melting conditions of lherzolite, eclogite and gab- many rational, ideas which must be taken into account bro in the mantle [16].

GEOTECTONICS Vol. 43 No. 1 2009 14 PUCHKOV

Japanese Changbai Japanese Arc Trench A Ç

0

500

1000

1500

2000

2500

–1% 0% +1%

Ä Ç

Fig. 14. Vertical section of P-wave velocities across Northeast China and Central Japan. The scale of deviations from average velocity and location of section are shown below [75].

(1) Alternative interpretations of time-progressive limited by the lithosphere and the upper asthenosphere. volcanic chains, e.g., crack propagation or specially By definition, the explanation of the LIP origin by organized sublithospheric convection with horizontal anomalous behavior of subducted slabs, as applied to counter-flow in the asthenosphere and eclogitic body southeastern Asia (Fig. 14), cannot be universal, hanging for many tens of million years therein, cannot because the tomography of most slabs testifies to their compete with the plume model, which explains not extension into the lower mantle (Fig. 8) [43, 60, 75]. only the general behavior of volcanic chains but also many details related to plate motion in particular Thus, the results of seismic tomography, informa- regions (see above). tion on LIPs and time-progressive volcanic chains, as (2) Seismic tomographic data indicate that the anoma- well as comparative analysis of the orientation of these lies related to slab and hot spots extend into the mesos- chains and the direction of motion of lithospheric phere, i.e., beneath the asthenosphere (Figs. 3, 7, 8). This plates, not only support the existence of particular evidence contradicts the universal passive plate-tec- plumes but also show that they are tied up with one tonic mechanism (extension–decompression–melting another and correlated with other global events. The of enriched mantle source), because this mechanism is relationships between epochs of LIP formation, mag-

GEOTECTONICS Vol. 43 No. 1 2009 THE CONTROVERSY OVER PLUMES: WHO IS ACTUALLY RIGHT? 15 netic superchrones, and global climatic changes are Their Isotopic Geochemical Characteristics,” Petrologiya outlined in [30]. 11 (6), 618–654 (2003) [ 11 (6), 562–596 In fact, the results of seismic tomography do not (2003)]. always testify to the origin of plumes at the core–man- 4. N. L. Dobretsov, A. G. Kirdyashkin, and A. A. Kirdyash- tle interface; however, the resolution of this method is kin, Deep Geodynamics (Geo, Novosibirsk, 2001) [in yet not sufficient. Russian]. As suggested by many researchers, mantle plumes 5. L. P. Zonenshain and M. I. Kuz’min, Paleogeodynamics may originate at different levels within the Earth and at (Nauka, Moscow, 1993) [in Russian]. least three plume varieties may be recognized: primary 6. Yu. G. Leonov, “Continental Rifting: Modern Concepts,” Morgan plumes ascending from the lowermost mantle, in Fundamental Problems of General Tectonics (Nauchnyi intermediate plumes that originate at the base of the Mir, Moscow, 2001), pp. 154–173 [in Russian]. transitional zone, and Anderson upper-mantle plumes, 7. L. I. Lobkovsky, A. M. Nikishin, and V. E. Khain, probably arising in the upper asthenosphere as a Present-day Problems of Geotectonics and Geodynam- response to plate motions [28] (Fig. 6). The problem ics (Nauchnyi Mir, Moscow, 2004) [in Russian]. may be resolved in this way. However, the use of a sin- 8. E. N. Melankholina, “Ocean–Continent Active Transi- gle term for outwardly similar, but genetically different tion Zones,” in The Planet Earth, Encyclopedia: Tecton- types of volcanism, is not an ideal solution and brings ics and Geodynamics (VSEGEI, St. Petersburg, 2004), pp. 274–283 [in Russian]. about additional criticism, sometimes causing people to “throw the baby out with the bathwater.” 9. E. N. Melankholina, S. V. Ruzhentsev, and A. A. Mossa- kovsky, “Evolution of Deep Up- and Downwelling and The aforesaid to the conclusion that the tec- Geodynamics of the Earth,” in Fundamental Problems of tonic paradigm providing a consistent synthesis of plate General Tectonics (Nauchnyi Mir, Moscow, 2001), and plume tectonics is still a work in progress. Modern pp. 318–343 [in Russian]. geodynamics remains a rather controversial, continu- 10. V. N. Puchkov, “Relationships of Plume and Plate Tec- ously developing system of ideas, where researchers tonics in Perspectives of the Evolution of Global Geody- engage in active, and generally productive debate on namic Theory,” in Geodynamics, Magmatism, Metamor- the basis of new factual material and techniques. Irre- phism, and Mineralization (Inst. Geol. Geochem., Ural spective of who is victorious in a particular debate, sci- Division, Russian Acad. Sci., Yekaterinburg, 2007), ence will be the winner. pp. 23–51 [in Russian]. Despite all the controversies, the concept of convec- 11. N. D. Ryabchikov, “Mechanisms and Magma Formation tion in the asthenosphere and deeper levels of the man- Conditions in Mantle Plumes,” Petrologiya 11 (6), 548– tle will remain the foundation of global geodynamic 555 (2003) [Petrology 11 (6), 496–504 (2003)]. theory [4, 7, 9, 12, 13]. This statement follows from the 12. V. P. Trubitsyn, “Principles of the Tectonics of Floating results of seismic tomography and, in particular, from Continents,” Fiz. Zemli 36 (9), 3–40 (2000) [Izv. Phys. observation of the extending of slabs of the oceanic Solid Earth 36 (9), 708–741 (2000)]. lithosphere into the lower mantle. 13. V. P. Trubitsyn, “Geodynamic Model of the Evolution of The convection models elaborated in modern geo- the Pacific Ocean,” Fiz. Zemli 42 (2), 3–23 (2006) [Izv. physics reflect separate aspects of this process but do Phys. Solid Earth 42 (2), 93–113 (2006)]. not embrace it in full, and therefore are distinct from 14. F. Albarede and I. Kaneoka, “Ghost Primordial He and one another. The future will show whether a compre- Ne,” in Goldschmidt Conference Abstracts (2007), p. A9. hensive model can be created. 15. D. L. Anderson, “Large Igneous Provinces, Delamina- tion, and Fertile Mantle,” Elements, 271–275 (2005). 16. D. L. 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