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7.09 Hot Spots and Melting Anomalies G 7.09 Hot Spots and Melting Anomalies G. Ito, University of Hawaii, Honolulu, HI, USA P. E. van Keken, University of Michigan, Ann Arbor, MI, USA ª 2007 Elsevier B.V. All rights reserved. 7.09.1 Introduction 372 7.09.2 Characteristics 373 7.09.2.1 Volcano Chains and Age Progression 373 7.09.2.1.1 Long-lived age-progressive volcanism 373 7.09.2.1.2 Short-lived age-progressive volcanism 381 7.09.2.1.3 No age-progressive volcanism 382 7.09.2.1.4 Continental hot spots 383 7.09.2.1.5 The hot-spot reference frame 386 7.09.2.2 Topographic Swells 387 7.09.2.3 Flood Basalt Volcanism 388 7.09.2.3.1 Continental LIPs 388 7.09.2.3.2 LIPs near or on continental margins 389 7.09.2.3.3 Oceanic LIPs 391 7.09.2.3.4 Connections to hot spots 392 7.09.2.4 Geochemical Heterogeneity and Distinctions from MORB 393 7.09.2.5 Mantle Seismic Anomalies 393 7.09.2.5.1 Global seismic studies 393 7.09.2.5.2 Local seismic studies of major hot spots 395 7.09.2.6 Summary of Observations 399 7.09.3 Dynamical Mechanisms 400 7.09.3.1 Methods 400 7.09.3.2 Generating the Melt 401 7.09.3.2.1 Temperature 402 7.09.3.2.2 Composition 402 7.09.3.2.3 Mantle flow 404 7.09.3.3 Swells 405 7.09.3.3.1 Generating swells: Lubrication theory 405 7.09.3.3.2 Generating swells: Thermal upwellings and intraplate hot spots 407 7.09.3.3.3 Generating swells: Thermal upwellings and hot-spot–ridge interaction 408 7.09.3.4 Dynamics of Buoyant Upwellings 410 7.09.3.4.1 TBL instabilities 410 7.09.3.4.2 Thermochemical instabilities 411 7.09.3.4.3 Effects of variable mantle properties 412 7.09.3.4.4 Plume buoyancy flux and excess temperature 412 7.09.3.5 Chains, Age Progressions, and the Hot-spot Reference Frame 413 7.09.3.6 Large Igneous Provinces 414 7.09.3.7 Hot Spots: Modifications and Alternatives 417 7.09.3.7.1 Variable hot-spot durations from transient thermal plumes 417 7.09.3.7.2 Forming melting anomalies by upper-mantle processes 418 7.09.3.8 Geochemistry of Hotspots and Melting Anomalies Vs MORB 420 7.09.4 Conclusions and Outlook 421 References 422 371 372 Hot Spots and Melting Anomalies Nomenclature Ra thermal Rayleigh number À2 g acceleration of gravity (m s ) Rac critical Rayleigh number h average swell height (m) T temperature (K) qp plume heat flux associated with swell Up, U plate speed, seafloor spreading rate buoyancy flux (m sÀ1) s hot-spot swell volume flux V volume (m3) t time (s) W average intraplate swell width (m), or steady-state ridge-axis swell depth (m) x horizontal dimension (m) W swell width (m) xr distance between plume source and W0 characteristic width scale (m) ridge axis (m) thermal expansivity (KÀ1) B buoyancy flux (kg sÀ1) thickness of boundary layer (m) C composition thermal diffusivity (m2 sÀ1) C1, C2, constants used in scaling of swell width characteristic growth time (s) C3 , viscosity (Pa s) E equation of an ellipse density (kg mÀ3) À3 F fraction partial melting c crustal density (kg m ) À3 H thickness of fluid m mantle density (kg m ) À3 L0 characteristic length scale (m) w density of sea water (kg m ) M volumetric rate of melt generation ÁT Temperature (contrast (K)) (m3 sÀ1) Á density difference between buoyant and P pressure (Pa) normal mantle (kg mÀ3) Q volume flux of buoyant material (m3 sÀ1) 7.09.1 Introduction that hot spots are generated by convective upwellings, or plumes of unusually hot, buoyant mantle, which rise The original work by Wilson (1963, 1973), Morgan from the lower mantle (Wilson, 1963, 1973; Morgan, (1971, 1972), and Crough (1978) established the concept 1971, 1972; Whitehead and Luther, 1975)possibly of ‘hot spot’ as a broad swelling of topography capped through a chemically stratified mantle (e.g., Richter by volcanism, which, combined with plate motion, gen- and McKenzie, 1981). The large mushroom-shaped erates volcanoes aligned in a chain and with ages that head of an initiating mantle plume and the trailing, progress monotonically. In some cases, these chains more narrow plume stem has become a popular expla- project back to massive volcanic plateaus, or large nationfortheformationofaLIPfollowedbyahot-spot igneous provinces (LIPs), suggesting that hot-spot track (e.g., Richards et al., 1989; Campbell and Griffiths, activity began with some of the largest magmatic out- 1990). bursts evident in the geologic record (Morgan, 1972; Studies of hot spots have flourished over the past Richards et al., 1989; Duncan and Richards, 1991). Hot- few decades. Recent articles and textbooks have spot volcanism is dominantly basaltic and therefore reviewed some of the classic connections between largely involves melting of mantle peridotite, a process hot spots and mantle plumes (e.g., Jackson, 1998; that also produces mid-oceanic ridge volcanism. Yet Davies, 1999; Condie, 2001; Schubert et al., 2001), the mid-ocean ridge basalts (MORBs) and hot-spot basalts role of mantle plumes in deep-mantle convection and typically have distinct radiogenic isotope characteristics chemical transport (Jellinek and Manga, 2004), and (Hart et al., 1973; Schilling, 1973). These differences oceanic hot spots (e.g., Ito et al., 2003; Hekinian et al., indicate that the two forms of magmatism come from 2004). Alternative mechanisms, which emphasize pro- mantle materials that have preserved distinct chemical cesses in the asthenosphere and lithosphere, are being identities for hundreds of millions of years. re-evaluated and some new ones proposed (Foulger The above characteristics suggest that hot-spot vol- et al., 2005). It has become clear that few hot spots canism has an origin that is at least partly decoupled confidently show all of the above characteristics of the from plate processes. A straightforward explanation is classicdescription.Thetermhotspotitselfimpliesa Hot Spots and Melting Anomalies 373 localized region of anomalously high mantle tempera- concept. As both chains terminate at subduction ture, but some features that were originally called hot zones, the existing volcanoes likely record only part of spots may involve mantle with little or no excess heat, the activities of these hot spots. The Gala´pagosisthe volcanoes spanning large distances of a chain with other Pacific hot spot with a similar duration. Its inter- similar ages, or both. Thus, the terms ‘magmatic action with the Gala´pagos Spreading Center has anomaly’ or ‘melting anomaly’ may be more general produced two chains: the Gala´pagos Archipelago– and appropriate to describe the topic of this chapter. Carnegie Ridge on the Nazca Plate (Sinton et al., Progress made in the last decade on studies of hot 1996) and the Cocos Ridge on the Cocos Plate. The spots and melting anomalies is emphasized here. We Cocos Ridge records oceanic volcanism for 14.5 My summarize the recent observations and discuss the (Werner et al., 1999) and projects toward the Caribbean major dynamical processes that have been explored LIP (Duncan and Hargraves, 1984), which has and evaluate their ability to explain the main char- 40Ar/39Ar dates of 69–139 Ma (e.g., Sinton et al., 1997; acteristics. Mechanisms involving hot mantle plumes Hoernle et al., 2004). The geochemical similarity of have seen the most extensive quantitative testing, but these lavas with the Gala´pagos Archipelago is compel- the recent observations compel the exploration and ling evidence for a 139MylifespanfortheGala´pagos rigorous testing of other mechanisms. We summarize hotspot(Hoernle et al., 2002, 2004). the main observations, outline mechanisms that have In the Indian Ocean, Mu¨ller et al.’s (1993b) com- been proposed, and pose questions that need quanti- pilation of ages associates the Re´union hot spot with tative answers. volcanism on the Mascarene Plateau at 45 Ma (Duncan et al., 1990), the Cocos–Laccadive Plateau 60 Ma (Duncan, 1978, 1991), and finally the Deccan 7.09.2 Characteristics flood basalts in India, which are dated at 65–66 Ma (see also Sheth (2005)). The Comoros hot spot can be Guided by the classical description of hot spots, we linked to volcanism around the Seychelles islands examine four main characteristics: (1) geographic dated at 63 Ma (Emerick and Duncan, 1982; Mu¨ller age progression along volcano chains, (2) initiation et al., 1993b). Volcanism associated with the Marion by massive flood basalt volcanism, (3) anomalously hot-spot projects from Marion island (<0.5 Ma shallow topography surrounding volcanoes (i.e., a (McDougall et al., 2001)) along a volcanic ridge to hot-spot swell), and (4) basaltic volcanism with geo- Madagascar. While geologic dating is sparse, Storey chemical distinction from MORBs. Given the et al. (1997) infer an age progression along this track marked progress in seismic methods over the past back to 88 Ma. The Kerguelen hot spot is linked to decade, we also summarize the findings of mantle Broken Ridge and Ninetyeast Ridge on the seismic structure beneath hot spots and surface melt Australian Plate, as well as multiple stages of volcan- anomalies. Table 1 summarizes what we have com- ism on the Kerguelen Plateau dating to 114 Ma (Frey piled about the above characteristics for 69 hot spots et al., 2000; Nicolaysen et al., 2000)(Figure 2). and melting anomalies. Figure 1 shows a global map In the Atlantic Ocean, the Tristan–Gough and of their locations with abbreviations and the main St. Helena chains record volcanism on the African large igneous provinces that we will discuss.
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