Unsuccessful initial search for a midmantle chemical boundary with seismicarrays

Compositional layering of the midmantle has been proposed to account for se ismic and geochemical patterns [van der Hilst and Karason, 1999], and infer red radiogenic heat source concentrations [Kellogg et al., 1999]. Compositi onal layering would require thermal boundary layers both above and below an interface. We construct a minimal 1-D model of a mid-mantle boundary consi stent with the observed nearly adiabatic compressional velocity structure [ Dziewonski and Anderson, 1981] and the proposed high heat flow from the low er mantle [Albarede and van der Hilst, 1999; Kellogg et al., 1999]. Ray tra cing and reflectivity synthetic seismograms show that a distinct triplicati on is predicted for short-period P waves. Although topography on a boundary would cause uncertainty in the strength and the range of the triplication, many clear observations would be expected. We examine data from the US Wes t Coast regional networks in the most likely distance range of 60 degrees t o 70 degrees for a 1770-km-depth boundary, and find no evidence for P wave triplications.