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Is intra-plate volcanism dominated by deep or shallow processes?
Intra-plate ‘hotspots’ have for a long time been attributed to cylindrical plumes made of hot, buoyant mantle that rises from great depths as deep as the core-mantle boundary. However, there have been a lot of speculations and controversies surrounding the arguments on the true source of these volcanic ‘hot spots’ with numerous scientists siding to defend the shallow volcanicity process as a more tangible explanation. Theories and models of deep volcanicity processes nevertheless dominate the scientific field as the mantle plume-derived sources for hotspot tracks that occur along the oceanic lithosphere are a more straightforward standpoint of mantle dynamics. The model has clears explanation of convective instabilities that arise from boundary layers of the core and mantle, the thinning of the oceanic lithosphere as compared to the continental lithosphere as well as explanations of the upper mantle flow occurring beneath the interior of the large tectonic plates. It furthermore explains the intra-palate volcanic process in relation to plate tectonics with the aid of mechanisms like sea floor spreading, transform faults and subduction mechanisms. The mantle plume model therefore works well to represent the presence of oceanic hotspot tracks like the Hawaiian-Emperor, Cape Verde systems and Marquesas.
IS INTRA-PLATE VOLCANISM DOMINATED BY DEEP OR SHALLOW PROCESSES?
Volcanoes are among one of the most extraordinary natural features on Earth. They are not only powerful shapers of the Earth’s surface, but also influence the make-up of oceans, the lithosphere and the atmosphere and ultimately life in itself. Volcanoes have therefore inspired a lot of intense scientific speculation, sheer fascination as well as major controversies over the centuries (Gautam 2013, p.174). It is therefore astonishingthat the origins of some of the most powerful and spectacular volcanoes like the Hawaii, Yellowstone and Iceland are yet to be fully understood. Although the discovery of plate tectonics helped in the initial understanding of the causes and effect of volcanoes, there have been exceptions emanating from activities deep within the mantle (Gillian 2010, p.9). The fundamental question therefore is the link between the dynamic processes that occur deep in the mantle and volcanism. This paper aims at supporting and keenly analyzing the plume hypothesis as a more cohesive and palatable concept compared to the shallow mantle argument. Given its central significance to a wide range of intra-palate volcanoes including the Hawaiian- Emperor Chain, Cape Verde, Snake River Plain, Réunion and Marques systems, the mantle plume model hypothesis is more exploratory of the oceanic hotspot tracks.
Although over the past years there have been a group of scientists who have been working together to disqualify the plume hypothesis rejecting its concept by Wilson and Morgan of hot, deep lower mantle plumes, the plume hypothesis still became the ruling model flawed interpretation of helium isotope data during the mid-1980s (Smith 2013, p.66). Plumes were therefore imposed on models into the depleted mantle for crustal recycling. These scientists argued that such anomalous magmatism was the result of the melting of fertile bodies from temperature variations in the shallow mantle rather than the thermal plumes from the deep mantle. These investigators attributed intra-plate volcanism to be the result of shallow upper mantle processes like lithospheric extension, edge-driven convection at the boundaries between the thick and thin lithosphere and small-scale sub-lithospheric convection among others(Smith 2013, p.66).
Early Classic Models
Early models by Wegner after the discovery of the dynamism of the Earth proposed and supported the plate tectonic theory. Due to his work majorly recognized in 1924, numerous other scientists including Hess during the 1960’s delved deeper into the field birthing plate tectonics. The model generally explains the volcanic activity that is present at both the sub-ducting plate boundaries and spreading centers with support from geochemical and seismic evidence (Caroline 2003, p.1). However, because spreading and sub-duction has not been able to account for all other forms of magmatic activity on earth, scientist began exploring other plausible models and theories that explain the presence of volcanic activity at oceanislands, oceanic plateaux, centers on some mid ocean ridges, in large igneous provinces (LIPs), at sea mounts and at intra plate super volcanoes (Caroline 2003, p.1). The figure 1 gives a clear representation of the differences between the plume and the plate tectonics model.
It is this interest on the origin of basaltic volcanism occurring in the middle of the plate, also known as intra-palate volcanism, “hot-spot” or “plume volcanicity” that has been of keen concern over the past recent years. The Hawaiian Volcanic Islands within the Emperor Seamount Chain in the middle of the Pacific Plate are one of the most commonly cited examples of resultant features of this ingenious activity (Gautam 2013, p.174). There are many other oceanic islands that have been formed as a result of this type of volcanicity and the rocks that formislands widely referred to as Ocean Island Basalts. Wilson in 1963 was the first to propose that the Hawaiian Chain was formed as a result of ‘hot-spot’ volcanicity that was taking place on a migrating lithosphere. Later on in 1971, Morgan supported that the Hawaiian Volcanoes as well as other intra-palate volcanoes were the result of hot, buoyant mantle plumes that rose significantly from the mantle (Gillian 2010, p.4). He suggested a model with 20 deep mantle plumes that were rising in ‘unique’ positions flowing horizontally away from theircenters through the asthenosphere and that it is this movement which contributes to plate motion. As a result of the speculation in the field therefore, numerous researchers have since then attempted to gain a clear understanding of these plumes using numerical and laboratory simulation models as well as seismic tomography (Gautam 2013, p.174).
Figure 1: A Schematic cross - section of the Earth showing and contrasting the Plume and the Plate Model (Courtillotet al., 2003).According to the diagram, the left side shows the two different kinds of plumes which are the giant upwellings and the narrow tubes. The deep mantle is responsible for the provision of the chemical reservoirs and the heat. In the Plate model, the recycling depths are variable with volcanism concentrated in the extensional regions.
Existence of Deep Mantle Plumes
Plumes are basically proposed to be hot narrow regions of upwelling mantle having an increase intemperature with the surrounding mantle in the order of50-100 degrees Celsius. Their origin is still unclear but there is a high probability that they are originated from the core mantle probably from the core mantle boundary with topography of a few hundred meters in the location where the convecting liquid core lose heat through conduction. Proposals explain that this source of heat is responsible for the partial melt creating the plumes. Figure 2 and 3 give a clear representation of the mantle plumes explaining their formation processes.
Figure 2: This photograph has an illustration of a thermal plume that was formed in a laboratory as an experiment by injection of warm syrup to the bottom of a tank that was full of cooler syrup. Some of the features within the picture include the long hot tail, the heat source which is at the top leading edge, the source material of the original plum head which is within the head, the cooled source material at the outer edges as well as the heated entrained materials just below the outer edges (Campbell et al., 1989).
Figure 3: This diagram has a map representation of The Northwest Pacific showing the Hawaiian Emperor Hotspot Trail that explains the formation of the intra-plate sea mounts (Sandwell, D., & Smith, W. M. Gravity Anomaly Map based on Satellite Altimetry, Version 15.2). At the inset is another diagram providing a strong evidence for the existence of a mantle plume that originates from the deep as a result of thermal anomaly.
Plumes are believed to be able to survive for up to 10’s of millions of years and are able to cross the endothermic 670 km discontinuity during their ascent. There have however been recent suggestions that the plumes are the resultant features of at least three different sources within the mantle (Caroline 2003, p.1). One type of the plumes according to the explanations originates from the lowermantle from a chemical heterogeneity within the D" layer while the second type originates atthe base of transition zones corresponding to ‘superswells’. The third and last type originates from the upper mantle. Such models explain the diversity of the morphologies that are exhibited as a result of plume related volcanism (Caroline 2003, p.1).
Since at lithosphere base plume can no longer ascend their transfer of heat is restricted and hence causing thermal erosion of the lithosphere, consequently thinning it above a hot spot. This excess heat causes an uplift of 1 to 2 kilometers before volcanism as is evident in the Marques Volcanic System. Plume heads are therefore formed at the bases of the lithosphere during the initial phases of contact. The ‘stopping’ effect at the lithosphere base sufficiently raises the temperature producing partial melting that originates in in the primitive mantle before rising through the lithosphere and undergoing variable interactions with surroundingrocks and finally erupting as a plume induced volcanism (Caroline 2003, p.1). This is all supported by geochemistry that is suggestiveof a deep mantle source of the plumes. Plumes are therefore thought to be the initiating mechanisms for early stages of continental drift and break up. Very large volumes of magma erupted at the beginning of the development of LIPs from the excess heat of plume heads (Caroline 2003, p.1).
Evidence from distinct mineral composition
According to studies on the petrography and the mineral chemistry of basalts from the Snake River Plain of southern Idaho which is one of the few rare ‘hot-spots’, distinct crystallization conditions attributed to deep volcanic plumes have been established (Richard 2012, p.1). This is from a research carried out by the Snake River Scientific Drilling Project that has been collecting hydrologic, petrologic and geophysical data from three distinct deep holes namely the Kimberly, Kimama and Mountain Home in an attempt to examine the temporal evolution of theupper mantle and crust in the area. One of the main objectives of the project was to investigatethe geochemistry composition of the basalts to aid in understanding their sources. In the research, twenty-three samples of basalt from 20 separate lava flows were sampled from theupper 1000 m that was drilled on the axis of the Snake River Plain representing approximately 3 m.y. of the volcanism (Richard 2012, p.1). It was important to use the rocks from the upper 1000 m since they were typically fresh as compared to those in the lower core that are significantly altered and less likely to preserve the fresh phenocrysts needed for analysis.
The results established that the trace element and olivine chemistry has a basalt source of a spinel peridotite and not a pyroxenite. The average mantle potential temperature that were obtained for the samples was 1577°C,177°C which was generally hotter than the ambient mantle and hence suggested that the basaltic liquids had been derived from a deep thermal plume (Richard 2012, p.5). Furthermore, the silica activity barometry indicated that the melt segregation occurred between 80 and 110 km depth, which was within and very close to the spinel stability field, suggestive of the fact that the lithosphere had been eroded by the plume to a depth of 80 km and that the recent mantle tomography show that it may even be thinner.
The basalts of the Snake River Plain also have major element that have high proportional of Iron (Fe) and Titanium (Ti) which are generally enriched trace element patterns with high helium isotopic compositions suggesting a deep mantle source. This is in agreement with the high lead (Pb)isotopic ratio that similarly indicates a lithospheric source of the ‘hot-spot’ plume (Richard 2012, p.6).
In addition, there have also been several investigations of the ocean island basalt that have identified the possible role for pyroxenite as compared to peridotite within the mantle plumes.
This has been believed to occur as a result of the entraining of sub-ducted oceanic crust in plumes found in the lower mantle before later on reacting with the surrounding peridotite and hence creating pyroxenite lithology which consequently imposes the distinct chemical signature on the derived melts (Richard 2012, p.5).
Stability of ‘hot-spot’ Volcanoes
Heavy rare earth elements in the basalts of the Snake River Plain have over time been observed to remain resistant to depletion, a phenomena that has been attributed to it significant erosion resistant garnet source (Richard 2012, p.5). This therefore meant that the depth of last equilibration with the mantle was shallower than the spinel-garnet transition which was at 85-100 km. The estimates of the lithospheric thickness before the onset of hotspot volcanism were between 100 and 200 km, a level below the spinelgarnettransition that would place the segregation depth of the basalts to be within the lithosphere. The explanation has however been discredited by claims that this phenomena is the result of lithosphere erosion by the Yellowstone plume to much shallower depths. Mantle tomographic and magneto-telluric studies have indeed indicated that the lithosphere is very thin under the Snake River Plain and much thicker on its flanks to the northeast and south beyond the Yellowstone (Matthew 2012, p. 479).
As initially mentioned, according to the classical plume theory, it is the purely thermal up-welling that generate ‘hot spot’ volcanicity from layers beneath the lithosphere. However, there has been a general failure to recognize the dynamical effects of the heterogeneous composition that is carried by mantle plumes leading to the formation of ‘hot spot’ volcanoes. From numerical models, there have been suggestions that a compositionally heterogeneous hot mantle that contains dense eclogite components usually stop to pool at 300-410 km deep before they rise to feed the shallower sub-lithospheric layer. It is this double-layered structure of thermo-chemical plume that indicate further the consistency of the deep volcanic plumes theory with evidence from seismic-tomographic images of the Hawaii Islands (Maxim, Garrett, Cecily., & Sean 2013, p.160). Furthermore, there have also been observations that the thermochemical structure together with the dependence of the plume material that rises from the deep to the shallower layer, account for the long-term variation in volcanic asymmetry and activity in bathymetry, magnetic chemistry and seismic structure acrossthehotspot track (Maxim et al., 2013, p.160).
Volcanic islands like the Hawai`i, Réunion and the Cape Verde as well as sea- mounts also called swells are generally observed to be far from the plate-tectonic which have been linked to over 95% of Earth’s volcanic activity. Seamount trails have therefore been described as the surface expressions of the hot and buoyant mantle that rises from deep plumes originating in the mantle (Anthony & Anthony 2010, p. 44). The figure 4 has a vivid representation of the sea-mount formation.
Figure 4: Based on a combination of geophysical and geochemical observations of the different mantle convection as well as the Earth Structure Models, this diagram explains the mantle plumes concept as originating from the lower mantle. It shows the two-layeredmantle model that combines both the geophysical and geochemical evidence for the penetration of the subduction slabs (Van der Hilst et al., 1997).
The ‘hot spot’ model, consequently explains that these mush roomed shaped plume heads are responsible for the formation of these voluminous LIPs that include oceanic plateaus and flood basalts which override the tectonic plates as they impinge at the lithospheric base while flattening out. These LIPs that mark the beginnings of the narrow seamount trails as those of the Hawaiian Emperor Seamount Trail are believed to have a direct link with the Pacific Plate that overlies the buoyant plume. Satellite derived geoid and gravity data have enabled scientists to equate the sizes of the swells to standard vertical plume fluxes. Although volumes of active intra-palate volcanism are small compared to those of oceanic crusts at mid-ocean ridges and island arc volcanism, plume fluxes are still significant when they are integrated geologically with time with known ‘hot spot’ systems (Anthony & Anthony 2010, p. 44).
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