Number 155 (Story #4), December 13, 1993 by Phillip F. Schewe and Ben Stein MANTLE PLUMES CONTAIN RECYCLED OCEAN CRUST AND SEDIMENT, William M. White of Cornell University reported at the AGU meeting. Mantle plumes are the upwellings of hot rock that originate either at the boundary between the mantle and the core 2900 km below the surface, or at a convective boundary (if one exists) between the upper and lower mantle, which seismic measurements suggest may be located 660 km below the surface. Measurements of the lead-to-cerium ratio in volcanic rock originating from plumes in the Society Islands, which include Tahiti, led White's team to conclude that some of the material must have been part of the ocean floor in the past. These findings suggest a picture whereby ocean slabs sink very deeply into the mantle, only to re-emerge on the surface as material in mantle plumes. Earlier this year, Jon D. Woodhead of Australian National University and his colleagues made a similar conclusion through analysis of oxygen isotope ratios from igneous rocks studied in a volcanic region near Pitcairn Island.