BEHAVIOR AND RESIDENCE TIMES OF LITHOPHILE AND RARE-GAS TRACERS IN THE UPPER-MANTLE

The residence time of highly incompatible lithophile elements in the u pper mantle such as Th, U and Pb has previously been estimated at less -than-or-equal-to 1 Ga. The concept of steady-state residence time is extended to include He and other rare gases. For a layered model of th e silicate earth long-term storage of the non-radiogenic isotopes of H e, Ne and Ar occurs in the lower mantle. The upper mantle is considere d to be an extremely degassed reservoir but a source of radiogenic rar e gas isotopes. He and other incompatible elements are introduced into the upper mantle by plumes, which bear lower mantle components, and i n the case of lithophile elements from the lithospheric boundary layer also. He-3/Ne-22 and He-3/Ar-36 ratios are shown to be similar for th e upper mantle and the plume sources, implying that He, Ne and Ar may have similar residence times in the upper mantle. U-238/He-3 ratios ar e used to show that ca. 90% of the steady-state U and other highly inc ompatible elements present in the upper mantle are recycled from the l ithosphere and only ca. 10% can be introduced directly with He-3 from a lower mantle region. Less than 1% of the lower mantle mass/ Ga needs to be entrained into the upper mantle to sustain the present-day He-3 loss rate from ridges.