Mantle hot-spots

About 80% of magmatic activity in the ocean occurs at the MOR, with another 10% each at subduction zones and mantle hot spots. Hydrothermal interactions take place at convergent plate boundaries where oceanic crust is being subducted back into the... 

Langmuir C., Humphris S., Fomari D., Van Dover C., Von Damm K., Tivey M. K., Colodner D., Charlou J.-L., Desonie D., Wilson C., Fouquet Y., Klinkhammer G., and Bougault H. (1997) Description and significance of hydrothermal vents near a mantle hot spot the lucky strike vent field at 37°N on the Mid-Atlantic Ridge. Earth Planet. Sci. Lett. 148, 69-92. 

Exley R.A., Matt DJP., Qague DJV. and Pillinger C.T., 1986, Carbon isotope systematics of a mantle hot-spot a o>mparison of Loihi seamount and MORfi glasses. Earth Planet. Set. 78, 189-199. -. . ... 

Some ridge sections are underlain by mantle melt anomalies, or hot spots, such as at the Azores and Galapagos Islands. These are marked by the stars in Figure 19.1 and data points 12 and 13 in Figure 19.5. Mantle upwelhng beneath both these ridge sections has abnormally thickened the oceanic crust to at least about 10 km. Most of the 47 known hot spots lie more than 500 km from a ridge axis. The Hawaiian islands are a notable example. 

In the modern tectonic environment, melt extraction from the uppermost mantle occurs in numerous settings that include oceanic and continental spreading centers, hot-spots, and sub-duction zones, with the vast majority of melt... 

Siena F., Beccaluva L., Coltorti M., Marchesi S., and Morra V. (1991) Ridge to hot-spot evolution of the Atlantic lithospheric mantle evidence from Lanzarote peridotite xenoliths (Canary Islands). J. Petrol. (Special LherzoUtes Issue) 271-290. 

The formation of basalts by partial melting of the upper mantle at mid-oceanic ridges and hot spots provides the opportunity to determine mantle composition. Early studies of radiogenic isotopes in oceanic basalts (e.g., Eaure and Hurley, 1963 Hart et al, 1973 Schilling, 1973) showed fundamental chemical differences between OIBs and MORBs (see Chapter 2.03). This led to the development of the layered mantle model, which consists essentially of three different reservoirs the lower mantle, upper mantle, and continental cmst. The lower mantle is assumed primitive and identical to the bulk silicate earth (BSE), which is the bulk earth composition minus the core (see also Chapters 2.01 and 2.03). The continental cmst is formed by extraction of melt from the primitive upper mantle, which leaves the depleted upper mantle as third reservoir. In this model, MORB is derived from the depleted upper mantle, whereas OIB is formed from reservoirs derived by mixing of the MORB source with primitive mantle (e.g., DePaolo and Wasserburg, 1976 O Nions et al., 1979 Allegre et al., 1979). 

In particular, the study of noble gas isotopes has provided a compelling case for mantle layering and the preservation of primitive mantle (see Chapter 2.06). For example, He/" He values for MORBs are nearly uniform, but large departures are seen for OIBs. Most oceanic hot spots have elevated He/" He values compared to MORBs. Helium-4 is generated by the decay of uranium... 

Several lines of evidence indicate problems with strict application of the layered model. It is particularly difficult to satisfy the requirement for primitive composition of the lower mantle. For example, Zindler and Hart (1986) showed that the isotopic database for the oceanic mantle could be described by mixing between depleted MORE mantle and enriched components that have compositions quite unlike primitive mantle. Similarly, the constancy of Nb/U and Ce/Pb (Hofmann, 1986 Newsom et al., 1986) suggests that there is no current reservoir with primitive mantle ratios. Interestingly, there is a strong correlation between a depleted component, termed FOZO by Hart et al. (1992), and the high He source observed in many hot spots (Hart et al., 1992 Farley et al., 1992 Hanan and Graham, 1996). 

In recent years, we have seen the use of new isotopic systems, including those of osmium and oxygen, in the description of oceanic basalts. Osmium isotopes can trace the addition of mafic crust or melt to the mantle source (Hauri and Hart, 1993) and have been used to highlight the presence of recycled mafic crust in sources of hot spots (Shirey and Walker, 1998). In a complimentary fashion, oxygen isotope variations trace directly... 

The cause and source regions of hot spots. Since plumes are considered to deliver the signals from the deep mantle, it is essential to understand how plumes influence mantle mixing. Of particular concern are the low internal viscosity of mantle plumes and the interaction with the transition zone and lithosphere (e.g., Hauri et al., 1994 Marquart and Schmeling, 2000 Fametani et al., 2002 Kumagai, 2002). 

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