Mantle hotspot volcanism

The energy that powers terrestrial processes is derived primarily from the sun and from the Earth s internal heat production (mostly radioactive decay). Solar energy drives atmospheric motions, ocean circulation (tidal energy is minor), the hydrologic cycle, and photosynthesis. The Earth s internal heat drives convection that is largely manifested at the Earth s surface by the characteristic deformation and volcanism associated with plate tectonics, and by the hotspot volcanism associated with rising plumes of especially hot mantle material. 

Various types of mantle heterogeneities have been identified that differ from one another in their major element, trace element, and isotope sys-tematics (see Chapter 2.03). A number of these distinct types of heterogeneities, particularly in isotopic composition, are most strongly manifest by hotspot volcanism (e.g.. Hart and Zindler, 1989). In addition, however, it is clear that there are dispersed heterogeneities in the upper mantle that are sampled by melting at mid-ocean ridges far removed from hotspot influences. 

Figure 1 Helium isotope data from various mantle-derived volcanics. The upper axis is the He/ He ratio R) normalized to the atmospheric ratio Rj f. As indicated by the data for selected segments of the MORB away from ocean islands falls almost entirely within the range of (7-9)Ra- While there are hotspot basalts that are characterized by high U/Pb ratios and low He/ He ratios (HIMU), many major oceanic hotspots, as well as continental hotspots, have high He/" He ratios (source Porcelli and Ballentine, 2002).

MORB mantle. The convecting upper mantle sampled by MORB has He/" He = (8 1) Ra away from hotspots (see Graham 2002, this volume). In some areas, such as southeast Australia, xenolith He appears to have MORB He isotope compositions. This is not surprising, considering that much of the mantle underlying the continents must have this composition. It is also not incompatible with the involvement of diapiric mantle hotspots in the local volcanism. The He isotope composition of many hotspots have not been clearly characterized, and while hotspots are often assumed to have very high He/" He ratios as seen in Iceland and Hawaii, it has not been established that other hotspots, with very different trace element characteristics, all have such ratios. Further, the presence of such material does not preclude the involvement of MORB mantle as well within a particular lithospheric region. 

Figure 4. Compilation of He isotope data from selected mid ocean ridges (MORE), continental hotspots, ocean island volcanism (OIB), and oceanic volcanism with sources having high U/Pb ratios (HD ) (after Barfod et al. 1999). The MORB source mantle is isotopically homogeneous compared with that supplying OIBs which, apart from HIMU, has an excess of He relative to " He. The creation and preservation of mantle supplying high He/" He and its interaction with the MORB-source mantle remains the focus of both experimental work and modeling.

Some volcanic centers, however, are better explained by either local melting anomalies (Hofmann 1997), comparatively broad upwellings associated with plate-scale flow (e.g., Darwin and African Rises Sleep 1990), or a plume of shallow origin (e.g., Tahiti Steinberger 2000). In order to determine the plume flux from the deep mantle, the proportion of the ocean island hotspots that are derived from the CMB, as well as the number of plumes beneath continental regions (e.g., Ritter et al. 2001), must be better constrained. 

The Hawaiian islands are volcanic in origin and each island is believed to have formed separately by the emergence above sea level of lava flows emanating from a single fixed melting source in the Earth s mantle. This is located under the central area of the Pacific Ocean. The northwestward movement of the Pacific tectonic plate over this "hotspot is... 

---