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

Abstract

The residence time of highly incompatible lithophile elements in the upper mantle such as Th, U and Pb has previously been estimated at ⩽ 1 Ga. The concept of steady-state residence time is extended to include He and other rare gases. For a layered model of the silicate earth long-term storage of the non-radiogenic isotopes of He, Ne and Ar occurs in the lower mantle. The upper mantle is considered to be an extremely degassed reservoir but a source of radiogenic rare gas isotopes. He and other incompatible elements are introduced into the upper mantle by plumes, which bear lower mantle components, and in the case of lithophile elements from the lithospheric boundary layer also. 3He/22Ne and 3He/36Ar ratios are shown to be similar for the upper mantle and the plume sources, implying that He, Ne and Ar may have similar residence times in the upper mantle. 238U/3He ratios are used to show that ca. 90% of the steady-state U and other highly incompatible elements present in the upper mantle are recycled from the lithosphere and only ca. 10% can be introduced directly with 3He 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-day3He loss rate from ridges.