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Mantle plume
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A mantle plume is an upwelling of abnormally hot rock within the Earth's mantle. As the heads of mantle plumes can partly melt when they reach shallow depths, they are thought to be the cause of volcanic centers known as hotspots and probably also to have caused flood basalts.
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A mantle plume is an upwelling of abnormally hot rock within the Earth's mantle. As the heads of mantle plumes can partly melt when they reach shallow depths, they are thought to be the cause of volcanic centers known as hotspots and probably also to have caused flood basalts. It is a secondary way that Earth loses heat, much less important in this regard than is heat loss at plate margins (see Plate tectonics). Some scientists think that plate tectonics cools the mantle, and mantle plumes cool the core.
The geometry of the Hawaiian-Emperor seamount chain and the regular progression of ages of volcanism along it were taken as important evidence in support of the mantle plume theory (Morgan, 1972 and Willson, 1963).
Concepts
In 1971, geophysicist W. Jason Morgan proposed the theory of mantle plumes. In this theory, convection in the mantle slowly transports heat from the core to the Earth's surface. It is now understood that two convective processes drive heat exchange within the earth: plate tectonics, which is driven primarily by the sinking of cold plates of lithosphere back into the mantle asthenosphere, and mantle plumes, which carry heat upward in rising columns of hot material, driven by heat exchange across the core-mantle boundary. The sinking of vast sheets of oceanic lithosphere back into the mantle is the primary driving force of plate tectonics, where the sinking of these slabs is balanced by the passive upwelling of asthenosphere along mid-oceanic ridges. In contrast, mantle plumes are narrow columns of material that rise more-or-less independently of plate motions.
Fluid dynamics experiments in the early 1970's (Whitehead and Luther, 1974) produced models of mantle plumes that consist of two parts: a long thin conduit that connects the top of the plume to its base, and a bulbous head that expands in size as the plume rises upward in the mantle; the result looks like a mushroom with a thin stalk and large top. The bulbous head forms because hot material moves upward through the plume conduit faster than the plume itself rises through the surrounding asthenosphere. In the late 1980's and early 1990's, experiments with thermal models shows that as the bulbous head expands it may entrain some of the adjacent asthenosphere into the rising head.
When the plume head encounters the base of the lithosphere, it flattens out against this surface barrier and undergoes widespread decompression melting to form enormous volumes of basalt magma. This basalt may erupt onto the surface over very short time scales (less than 1 million years) to form a continental flood basalt (if it erupts through continental crust) or an oceanic plateau (if it erupts through oceanic crust). Prominent continental flood basalt provinces include the Deccan traps and the Rajmahal traps in India, the Siberian traps of Asia, the Karmutsen Formation in British Columbia, Canada, the Karoo basalts in South Africa, the Ferrar dolerite of Antarctica (conjugate with the Karoo), the Parana basalts in South America and the Etendeka basalts in Africa (formerly a single province separated by opening of the South Atlantic ocean), and the Columbia River basalts of North America. Plume-related oceanic plateaux include the Ontong Java plateau of the southwest Pacific ocean and the Maniheken plateau of the Indian ocean.
The plume tail may continue to move material from the Earth's interior to the surface, providing a continuous supply of magma in a fixed location, often referred to as a hotspot. As the lithosphere moves over this fixed hotspot due to plate tectonics, the eruption of magma from the fixed hotspot onto the surface forms a chain of volcanoes that parallels plate motion (Skilbeck and Whitehead 1978. The classic example of this is the Hawaiian island chain in the Pacific ocean.
The eruption of continental flood basalts is often associated with continental rifting and breakup, leading to the hypothesis that mantle plumes play an important role in continental rifting and the formation of ocean basins. Where this association of flood basalts with continental rifting is observed, it is not uncommon to find linear chains of volcanic islands that parallel the motion of plates on either side of the spreading center (South Atlantic ocean).
Model of plume formation
The chemical and isotopic composition of basalts found in hotspots and inferred to form by partial melting of mantle plumes suggest that several components are involved, including primordial mantle with unfractionated noble gases, subducted oceanic crust and mantle lithosphere, and subducted sediments. The processing of oceanic crust, lithosphere, and sediment through a subduction zone decouples the water soluble trace elements (e.g., K, Rb, Th) from the immobile trace elements (e.g., Ti, Nb, Ta), concentrating the immobile elements in the oceanic slab (the water soluble elements are added to the crust in island arc volcanoes). Seismic tomography shows that subducted oceanic slabs may sink directly to the core-mantle boundary, or pause for long periods at the mantle transition zone (400-660 km depth) before sinking to the core-mantle boundary. The subducted slabs accumulate at the core-mantle boundary and form a seismically distinct layer called the D" (Dee-double prime). This appears to be the source of most deep mantle plumes, as shown by seismic tomography (Montelli et al, 2005).
Because there is little material transport across the core-mantle boundary, heat transfer must occur by conduction, with well-stirred adiabatic gradients above and below this boundary. As a result, the core-mantle boundary represents a significant thermal (temperature) discontinuity, with the core at temperatures several hundred degrees Celsius hotter than the overlying mantle. As heat is transferred across this boundary by conduction, material in the D" layer becomes hotter and thus more buoyant. When it becomes sufficiently buoyant, material begins to rise from the D" layer to form a mantle plume.
In concert with hypothesised slow-down in plate tectonic motion, which may be associated with prolonged periods of supercontinent formation, it is theorised that without an actively convecting asthenosphere, the lower mantle will begin to locally overheat. These overheated portions of the mantle near the core-mantle boundary become buoyant relative to their surroundings, and begin to rise via diapirism.
This plume of material rises through the mantle. Upon reaching shallower depths within the asthenosphere, decompression melting occurs in the plume head, creating large volumes of magma. The magma rises through the asthenosphere until it reaches the Earth's crust where it causes a hotspot.
Miniplumes
The term mini-plumes refers to smaller plumes that may originate in the upper mantle rather than the more common deep mantle plumes. No conclusive examples have yet been identified. One possible example, however, is the Anahim plume at the Anahim hotspot in central British Columbia, Canada.
Role of the core
The most prominent compositional contrast known to exist in the deep (> ~400km) mantle is at the core-mantle boundary. Morgan-type plumes are generally assumed to rise from this layer for two reasons. First, this boundary represents a major thermal discontinuity because the top of the core is much hotter than the base of the mantle. Secondly, the base of the mantle is characterized by the D" layer that is seismically distinct from the overlying mantle. The D" layer appears to be compositionally distinct from the overlying mantle, and seismic tomography of subducted lithosphere suggests that the D" layer may represent the accumulation of these subducted slabs at the base of the mantle.
Very large, broad plumes that spawn a series of smaller plumes in the upper mantle are sometimes referred to as "superplumes". These are usually defined as a plume that has a diameter of at least 1500-3000 km by the time the plume head reaches the upper mantle. A "superplume event" is a short-lived mantle event (100 million years) during which a superplume and the smaller plumes that form from it bombard the base of the lithosphere (Condie et al. (2001)). It is believed that such an event may have occurred in the mid-Cretaceous.
Evidence for the theory
Mantle plumes provide an explanation for intra-plate tectonic volcanism called 'hotspots'. There are several lines of evidence used to support the theory: linear volcanic centers, hotspot fixity, geochemical, noble gas isotopes, and geophysical anomalies.
Linear volcanic tracks
The apparent linear, age-progressive distribution of the Hawaiian-Emperor seamount chain is explained in this context as a result of a fixed, deep-mantle plume impinging into the upper mantle, partly melting, and causing a "track" as the plate moves with respect to the plume source (Morgan, 1972).
Smaller plumes, arguably called petitspots, are also common within intraplate areas. For instance, tracks of ocean island basalts are found within the Indian Plate, namely the Marshall Islands hotspot.
Continental flood basalt in Oregon and Washington and the Yellowstone caldera-forming event are also used as evidence for mantle plumes, with the voluminous flood basalt envisaged as a product of the vigorous mantle plume head, and the hot 'tail' to the plume driving a progressively younger series of caldera events as the North American continental mass tracks above it.
Smaller series of intracontinental volcanic rocks are also ascribed to small plumes or petitspots. These are notably the Glasshouse Mountains in Queensland (Cohen et al. 2004), which are the oldest Tertiary (25 Ma) members of a progressively younger trend of basaltic and intraplate volcanic cones and plugs culminating in the maars and small peridotitic basalts of the Newer Volcanics in Victoria of 40,000 years ago, far to the southeast.
It is notable that these volcanic features become younger in the same vector as the motion of the Indo-Australian Plate, and matching the trend of the intraplate ocean island basalts in the Indian Ocean.
Noble gas and other isotopes
The standard 3He is considered a primordial isotope as it was formed in the Big Bang and very little is produced or added to the Earth by other processes since then (Anderson, 1989). 4He includes a primordial component, but it is also produced by the natural radioactive decay of U and Th. 3He is 25% lighter than 4He, so over time He in the upper atmosphere becomes depleted in 3He as it is lost into space. All of these processes contribute to low ratios of 3He/4He in the atmosphere and in the Earth's crust and upper mantle. Thus, the only potential source of He with elevated 3He/4He ratios is the deep interior of the Earth, which must still remain a reservoir of primordial He and other gases. Thus, anomalous 3He/4He isotopic ratios with respect to mean mid-ocean ridge basalts (MORB) (see basalts), as found in Hawaiian volcanic rocks, are assumed to provide a signature of primordial, non-degassed mantle (however: alternate explanations have been proposed for this anomalous geochemical signature (Anderson, 1998).)
Relative abundances of osmium isotopes in Hawaiian basalts have also been taken as signatures of plume formation at the core-mantle boundary, with incorporation of some core-derived material. That explanation for the osmium isotope abundances remains controversial (Lassiter, 2006).
Geophysical anomalies
Geophysical anomalies associated with hotspots and plumes include thermal, seismic, and geodetic. Thermal anomalies are inherent in the term "hotspot." Thermal anomalies are reflected in high heat flow values at the Earth's surface and excess volcanism. Thermal anomalies also produce anomalies in the travel times of seismic waves.
Seismic anomalies are identified by measuring spatial variations in the time it takes seismic waves to travel through the earth. A fluid body with a lower density (e.g., a hot mantle plume or wetter mantle) exhibits lower seismic velocity compared to surrounding mantle. Observations of regions where seismic waves take longer to arrive are used as evidence for regions of anomalously hot mantle, as is observed underneath Hawaii (Ritsema et al., 1999). Other indicators of plumes would be from the dynamic uplift of the surface (Burov, 2005) and an elevated heat flow.
By deploying a dense network of seismometers and a technique known as seismic tomography, scientists can construct 3-d images of seismic velocities to try and identify vertical plume like structures (Yuan and Dueker, 2005). This is referred to as seismic tomography because it uses techniques similar to medical tomography.
Seismic waves generated by large earthquakes are used to determine structure below the Earth’s surface because they can be detected far from the earthquake epicenter. Far-travelled seismic waves (also called teleseismic waves) are especially useful for seismic tomography because they have steep travel paths that sample smaller longitudinal domains. Density differences between a mantle plume and cooler material that surrounds it enable researchers to distinguish between the two. Seismic waves slow down when they travel through low-density (hotter) material, and speed up when traveling through denser (cooler) material. Density differences may also arise from compositional differences between the plume material and the surrounding mantle.
By analyzing pressure pulses, or P-waves, a group of scientists at Princeton have identified 32 regions throughout the world where P-waves travel slower than average. They conclude that these areas are mantle plumes. The team used analysis of S-waves, another type of seismic wave generated by earthquakes, to determine that those plumes extend to the core-mantle boundary (Montelli et al. 2004).
Geodetic anomalies are reflected in topographic bulges above the plume location, and in positive geoid anomalies. The geoid is a potential surface that reflects the theoretical height to sealevel if mass was distributed uniformly within the Earth. Positive geoid anomalies reflect excess mass associated with uplift and doming over a thermal plume. The Yellowstone plume has a positive geoid anomaly of around +15 meters at its center, and over 1000 km in diameter (Smith & Braile, 1994).
Computer modeling of the mantle plume theory shows that changes of temperature and chemical composition of rising plumes can lead to plumes of varying contours as opposed to the early conceptualization that plumes developed as a homogeneous mushroom shape (Farnetani & Samuel, 2005).
Geochemistry
Basalts associated with hotspots or mantle plumes are geochemically distinct from mid-ocean ridge basalts and from lavas associated with island arc volcanoes. In major elements, hotspot basalts are typically higher in iron (Fe) and titanium (Ti) than mid-ocean ridge basalts at similar magnesium (Mg) contents, reflecting their higher temperatures of formation. In trace elements, hotspot basalts are typically more enriched in the light rare earth elements than mid-ocean ridge basalts. Compared to island arc basalts, hotspot basalts are lower in alumina (Al2O3) and much higher in the immobile trace elements (e.g., Ti, Nb, Ta).
The significance of these differences among ocean island basalts (hotspots), mid-ocean ridge basalts, and island arc basalts rests on processes that occur during subduction of oceanic crust and mantle lithosphere. Oceanic crust (and to a lesser extent, the underlying mantle) typically becomes hydrated to varying degrees on the seafloor, partly as the result of seafloor weathering, and partly in response to hydrothermal circulation near the ridge crest. As oceanic crust-lithosphere subduct, water is released by dehydration reactions, along with water-soluble chemical elements and trace elements. This enriched fluid rises to metasomatize the overlying mantle wedge and leads to the formation of island arc basalts. The subducting slab is depleted in these water-mobile elements (e.g., K, Rb, Th, Pb) and thus relatively enriched in elements that are not water-mobile (e.g., Ti, Nb, Ta) compared to both mid-ocean ridge and island arc basalts.
Ocean island basalts, which represent the volcanic product of mantle plumes, are also relatively enriched in the immobile elements relative to the water-mobile elements, leading to the conclusion that subducted oceanic crust plays a major role in their origin.
Suggested mantle plume locations
Two of the most well known locations that fit the mantle plume theory are Hawaii and Iceland as both have volcanic activity. Other island chains that parallel plate motion include the Society Islands (e.g., Tahiti), St Helena-Ascension-Gough, and the Ninety-east ridge (Indian ocean). One of the dormant in Asia that fits the mantle plume theory is Mount Halla(Hallasan) in Jeju island(Jeju-do).
The P-wave and S-wave images show other locations that fit the mantle plume model. Ascension Island and St. Helena appear to originate from the same plume. Similarly, volcanic activity in the Azores and Canary Islands branch from a single trunk.
South of Java and in the Coral Sea, the images show possible formation of future plumes that currently extend only halfway to the surface.
Ore deposit associations
- Nickel-Copper-PGE deposits. For instance the giant Norilsk nickel deposit in Russia is considered to be associated with the Permian Siberian Traps volcanism, a probable plume-head eruptive event.
- Gold deposits (to a lesser extent)
Alternative models of hotspot formation
It is important to distinguish between observation and interpretation or hypothesis. Hotspots are observed surface features characterized by volcanic effusions in excess of what is normally expected for their nominal setting; mantle plumes are interpreted to be the cause of many or most hotspots. In a 2005 paper, Don L. Anderson and James H. Natland wrote:
- "Unfortunately, the terms hotspot and plume have become confused. In recent literature the terms are used interchangeably. A plume is a hypothetical mantle feature. A hotspot is a region of magmatism or elevation that has been deemed to be anomalous in some respect because of its volume or location. In the plume hypothesis, a hotspot is the surface manifestation of a plume, but the concepts are different; one is the presumed effect, and the other is the cause."
Although mantle plumes are currently the dominant hypothesis for creating hotspots, flood basalts, and oceanic plateaux, collectively referred to as large igneous provinces (LIPS) (Saunders 2005; Campbell 2005), many geoscientists prefer models that are confined to the upper mantle and crust, and do not require deep thermal anomalies. These hypotheses include:
- Crustal delamination: the delamination and sinking of large portions of lower continental crust (assumed to have transformed into the dense rock eclogite), which allows the influx of asthenosphere from the low velocity zone and subsequently melts to form continental flood basalts (e.g., Anderson, 2005).
- Edge effects: thick continental lithosphere insulates the underlying asthenosphere, causing heat buildup that leads to buoyancy. The buoyant asthenosphere moves toward the edge of the cratonic lithosphere, where it can rise and melt (e.g., Anderson, 2005). This model is often paired with rifting of the continental crust to explain formation of ocean basins.
- Meteorite impacts: the impact of large meteorites into oceanic crust may cause large parts of the transient cavity to melt, forming melt sheets similar in volume to flood basalts (Jones, 2005). This model works less well in continental crust, which lacks a basaltic bulk composition.
The current debate has stimulated an increased interest in research to distinguish between these models. Recent advances in seismic tomography have enhanced its spatial resolution in both the upper and lower mantle. New seismic tomographs resolve anomalous features consistently within the upper mantle, and in places to the lower mantle (e.g., Montelli et al 2004). It is becoming difficult to explain these data by processes in the uppermost mantle.
See also
External links
- Campbell, Ian H. and Davies, Geoffrey F. (2006) , (pdf), Episodes, volume 29, number 3, pages 162-168. Retrieved on 24 October 2007
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