2000 Western Pacific Geophysics Abstract (Wood)

Bina, C. R., and B. J. Wood, Thermal and compositional implications of seismic velocity anomalies in the lower mantle, Eos, Transactions of the American Geophysical Union, Western Pacific Geophysics Meeting Supplement, T32B-03, 2000.


Seismic velocity heterogeneity, as revealed for example by seismic tomography, is often interpreted in terms of strictly thermal origins. However, such lateral variations in seismic velocity may in fact arise from perturbations to temperature, composition, and other variables. Furthermore, they should be accompanied associated density anomalies. Thus, lower mantle seismic velocity anomalies may reflect local heterogeneity in chemical composition, such as variations in iron-magnesium ratio and silica content, rather than simply variations in temperature. Together, thermal and chemical variations may jointly explain the manner in which the RMS amplitudes of seismic velocity anomalies appear to vary with depth. The distinction is important because velocity anomalies arising from compositional differences are not subject to the same temporal decay as those due to thermal perturbations.

For example, formation of oceanic lithosphere involves chemical differentiation of mantle lherzolite into overlying harzburgitic and gabbroic layers. Upon subduction into the lower mantle, both of these layers would be fast relative to the surrounding mantle due to their chemistry alone, the former by several tenths of a percent and the latter by up to several percent, because of the coupled effects of Si-enrichment and Mg-depletion. Also, the former will be intrinsically less dense and the latter more dense than the mantle. Furthermore, the amplitudes of these velocity anomalies of petrological origin should increase with increasing depth. In addition, the low temperatures of slab material would contribute fast velocity anomalies, of decreasing amplitude with increasing depth, accompanied by density increases, but such anomalies of thermal origin will decay with time as cold slab material is thermally assimilated. Thus, in the lower mantle, enhanced silica content or depressed temperatures may increase both density and seismic velocity, whereas a higher Mg/Fe ratio may increase seismic velocity while simultaneously decreasing density.

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