Reheated slabs II

Gasparik (1997), Balyshev & Ivanov (2001) and Ivanov (2003) suggest that recycled slabs may convert to low-velocity anomalies as they heat up. The latter authors propose that radioactivity in the slab is the source of heating and calculated that subducted slabs can be heated on a time-scale of 1-2 billion years and can be converted from high-velocity to low-velocity structures. A much faster conversion from low temperature slabs to low-velocity anomalies can be accomplished simply by conducting heat into the slab from the ambient mantle into which it settles. Slabs are composed in part of oceanic crust and in part of serpentinized peridotite. Both CO₂ and water occur in the upper part of the slab. All these effects serve to lower the melting point and seismic velocities of slabs compared to dry refractory peridotite. After the conversion of basalt to eclogite the melting point is still low. Even small amounts of fluid or melt can drastically lower the seismic velocity, even if the density remains high.

The time scale for heating a slab to above the dehydration and melting points is a small fraction of the age of the plate upon subduction. The low seismic velocities at the top of and above descending slabs are the clearest manifestation of this effect. Recycled slabs can indeed be converted to features that are not only low-velocity but also fertile, thus having the characteristics normally associated with plumes (Meibom & Anderson, 2003). However, the time scale of this conversion can be much less than the time scale of radioactive heating, and probably less than 20 million years. As far as the slab is concerned the surrounding mantle is an infinite heat source.

The idea that low-velocity material in the mantle can be low-temperature also is typically overlooked in most interpretations of seismic tomography images. Seismic velocity is controlled by composition, mineralogy, and volatile content as well as by temperature. If the melts and volatiles completely leave a reheated slab then it can become a high-seismic-velocity anomaly again, at least between depths of ~ 60 to 600 km (Anderson, 1989). Deeper than that the eclogite in a slab becomes low-density and low-velocity compared to normal mantle.

The abundances of K, U and Th decrease with time because of radioactive decay. This decay is the major heat source in the mantle. Higher mantle temperatures are expected for Archean mantle (Korenaga, 2003; Ernst et al., 2001). Since the shallow mantle is close to or above the solidus today this higher temperature means that more extensive and deeper melting should have occurred in the past. In fact, in the Archean mantle, radioactive heating was twice as high as today and the mantle had experienced less secular cooling. Although the need for localized high temperatures today may be debated, the evidence for higher temperatures and widespread volcanism in the Archean should not come as a surprise. If the surface of the Archaean Earth was blanketed with buoyant basalt then the recycling mechanism was depression of the buoyant lid into the upper mantle melting zone as new basalt overplated the old. The Archean Earth may have resembled Mars, Venus and some of the Jovian satellites in having non-plate-tectonic styles of heat removal. Emplacement of giant dikes and planetary resurfacing are styles of heat removal in such bodies. Even in a planet with plate tectonics, the plates may be permeable. Dikes can deliver heat to the near surface even if they do not erupt.

Simple heat conduction, rather than radioactivity, may thus explain the observations described by Ivanov (2003). Lord Kelvin neglected radioactivity in his calculations of the age of the Earth, and most subsequent workers have neglected terrestrial secular cooling (the Kelvin effect). Demonstrations of the Second Law of thermodynamics often invoke an infinite heat bath, as used above. Neglect of secular cooling, and the assumption of steady-state balance between heat production and heat loss, also violate the Second Law. The outer shells of Earth, at least, have certainly cooled and radioactive heating has certainly declined. Geological evidence against substantial cooling (survival of cratonic roots, komatiitic temperatures, and the Archaean catastrophe) have been addressed already (Korenaga, 2003; Lenardic, 1998).

In summary, both high-velocity and low-velocity tomographic anomalies can be generated by recycled slabs. Volatile and basalt-containing slab fragments are likely to be low-velocity anomalies, at least in the upper part of the mantle.