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. 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 progession of ages of volcanism along it were taken as important evidence in support of the mantle plume theory (Morgan, 1972 and Willson, 1963).
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[edit] 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.
[edit] Model of plume formation
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.
[edit] Role of the core
The most prominent compositional contrast known to exist in the deep (> ~400km) mantle is at the core-mantle boundary, and thus Morgan-type plumes are generally assumed to rise from this layer. Due to the depth of the boundary, proving that plumes are generated near or at the core is difficult and has led to controversy.
Also, a "superplume" is the term for a larger-scale plume. It is usually defined as a plume that has a diameter of at least 1500-3000 km by the time the plume head spreads at the base of the lithosphere. A "superplume event" is "a short-lived mantle event (100 Ma) during which many superplumes as well as smaller plumes bombard the base of the lithosphere" (Condie et al. (2001)). It is believed that such an event may have occurred in the mid-Cretaceous.
[edit] 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.
[edit] 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 younging 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 age, far to the southeast.
It is notable that these volcanic features young in the same vector as the motion of the Indo-Australian Plate, and match the younging of the intraplate ocean island basalts in the Indian Ocean.
[edit] 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). (However: alternate explanations have been proposed for this anomalous geochemical signature (Anderson, 1998).)
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.
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).
[edit] Geophysical anomalies
Geophysical 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).
More generally, by deploying a dense network of seismometers and a technique known as tomography, scientists can construct 3-d images of seismic velocities to try and identify vertical plume like structures (Yuan and Dueker, 2005).
Other indicators of plumes would be from the dynamic uplift of the surface (Burov, 2005) and an elevated heat flow.
Density differences between a mantle plume and cooler material that surrounds it enable researchers to distinguish between the two. Seismic waves generated by large earthquakes are used to determine structure below the Earth’s surface. The waves slow down when they travel through low-density material.
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)
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).
[edit] 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.
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.
[edit] Ore deposit association with mantle plume activity
- 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 probably plume-head eruptive event.
- Gold deposits (to a lesser extent)
[edit] Clarification
In a 2004 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."
[edit] References
- Anderson, Don L. & Natland, James H. (June 23, 2004). A brief history of the plume hypothesis and its competitors: Concept and controversy. Retrieved Sep. 15, 2004.
- Anderson, Don L., 1998. The helium paradoxes, Proc. Nat. Acad. Sci., 95, 4822-4827.
- Cohen, B., Vasconcelos, P.M.D., Knesel, K. M., 2004 Tertiary magmatism in Southeast Queensland in, Dynamic Earth: Past, Present and Future, pp. 256 - 256, Geological Society of Australia
- Courtillot, V., Davaille, A., Besse, J., Stock, J., 2003. Three distinct types of hotspots in the Earth's mantle. Earth and Planetary Science Letters 206, 295-308.
- Montelli R, Nolet G, Dahlen FA, Masters G, Engdahl ER, Hung SH (2004). "Finite-frequency tomography reveals a variety of plumes in the mantle". Science 303 (5656): 338-43. PMID 14657505.
- Farnetani, C.G., and H. Samuel. 2005. Beyond the thermal plume paradigm. Geophysical Research Letters 32(April 16):L07311. Abstract.
- Labrosse, S., Hotspots, mantle plumes and core heat loss, Earth Planet. Sci. Lett., 199, 147-156,2002.
- Lassiter, J. C., Constraints on the coupled thermal evoluution of the Earth's core and mantle, the age of the inner core, and the origin of the 186Os/188Os "core signal" in plume-derived lavas. Earth and Planetary Science Letters, v. 250, p. 306-317 (2006).
- Ratajeski, K. (November 25, 2005). The Cretaceous Superplume
- Ritsema, J., H.J. van Heijst, and J.H. Woodhouse, Complex shear wave velocity structure imaged beneath Africa and Iceland, Science, 286, 1925-1928, 1999.
