THE THERMAL SIGNATURE OF SUBDUCTED LITHOSPHERIC SLABS AT THE CORE-MANTLE BOUNDARY

We study the thermal structure around a cold deformable lithospheric s lab as it sinks to the core-mantle boundary and migrates along it. We present analytical results for the steady thermal structure establishe d by a steady but spatially varying motion. The analysis gives a time- like criterion for the thermal signature of a cold slab to persist by the time that the slab moves along the core-mantle boundary. The model is used to assess the feasibility of a purely thermal origin for some of the observed seismic reflectors near the core-mantle boundary. Cal culations of the time-like criterion show that the dynamical condition s in our model, namely the velocity and the thickness of the descendin g slab, are hard to reconcile with observations of subduction and seis mic features. Seismic reflections and refractions from anomalously fas t regions above the core-mantle boundary could be explained as thermal slabs if the thickness of the slab at subduction was larger than 200 km or somewhat less if the slab did not split at the core-mantle bound ary. A simple thermal model also predicts from mineral physics a certa in correlation between S- and P-wave velocity anomalies, which is not observed. However, a purely thermal origin cannot be ruled out if the slab is buckling. This process could be in agreement with the observat ions: the amplitude of the seismic anomalies, the vertical extent of h igh-gradient zones and the P versus S comparisons. Chemical heterogene ities and phase transformations remain alternative or complementary ex planations. (C) 1998 Elsevier Science B.V. All rights reserved.