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Published 1987 | public
Book Section - Chapter

The depths of mantle reservoirs


Many petrological studies are concerned with the temperature and pressure of final equilibrium of erupted magmas and residual crystals. The average composition of the source region and its original depth are also of interest but these cannot be determined unambiguously from petrology. Seismic techniques can be used to infer the mineralogy of various regions of the mantle and the probable depth extent of the low-velocity zones (LVZ) associated with high-temperature buoyant upwellings. Oceanic ridges are characterized by broad deep LVZ's which extend locally to depths in excess of 400 km. Partial melting is implied to depths of at least 300 km. The same is true for some young continental regions such as northeast Africa and western North America, and some midplate regions such as the central Pacific. Hotspots occur on the edges of broad upper mantle low-velocity anomalies, often in regions of thick crust and/or old and thick lithosphere. Continental shields have thick (150 km), cold, refractory lithospheres which are unlikely sources of voluminous plateau basalt outpourings. The rapid decrease in velocity between 150 and 200 km beneath shields implies a high thermal gradient and a change in mineralogy. From 200 to 400 km the seismic velocities beneath shields fall on the 1400°C adiabat. This suggests that the stable continental plate is 150 km thick and that it is underlain by a thermal boundary layer which grades downward into a convective gradient. Continental and oceanic basalts probably share a common source region which is deeper than 350 km. When hot, this source region becomes buoyant, because of thermal expansion and the reduction or elimination of dense phases, such as garnet, rises into the shallow mantle, adiabatically decompresses, becomes an LVZ and a potential source of magma.

Additional Information

© 1987 Geochemical Society. Much of the research reported here was done in collaboration with Ichiro Nakanishi, Henri-Claude Nataf, Jay Bass, Toshiro Tanimoto, and Adam Dziewonski. The author would like to acknowledge many enjoyable hours of cooperation and discussion with these colleagues. This research was supported by NSF Grants EAR-8509350 and EAR-8317623 and support from DARPA. Contribution 4354, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.

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