Closure and Isolation of Mantle Reservoirs and
the Role of Plumes

R. K. O'Nions Dept of Earth Sciences , Univ.Oxford, Parks Rd.,Oxford OX1 3PR, UK

I. N. Tolstikhin C.P.R.G., Nancy, France

Rare gas geochemistry provides constraints on the time of closure of the earth's lower mantle and atmosphere to major gas loss. The closure time is that time at which transport out of the relevant reservoir becomes near- negligible with regard to that lost during accretion and differentiation . From Pu-U-I-Xe constraints the best estimate for closure time of the atmosphere is close to 4.4 Ga, and the lower mantle became closed(for Xe at least) prior to 4.4 Ga. It is assumed in making this estimate that 129Xe* is stored long-term in the lower mantle. The role of plumes in transporting material from the lower to the upper mantle,crust and atmosphere over >4.4 Ga should be viewed in this light.

The rare gases also provide three semi-independent estimates for the lower to upper mantle mass flux. In each case it is assumed that rare gases are uniformly distributed in the lower mantle and that the upper mantle is at steady state. The first is derived from the contemporary 3He flux from ridges which is equated with the lower to upper mantle flux of 3He. This suggests a corresponding mass flux of <3¥1013 Kg.a-1. A second estimate from He-Ar data includes the accumulated atmospheric inventories of 36Ar and 40Ar, and provides an averaged mass flux over the period 4.4 Ga to present. This mass flux is <2¥1013 Kg.a-1. Lastly Pu-U-I-Xe data together with the assumption that 129Xe* resides principally in the lower mantle gives an estimate of <1¥1013 Kg.a-1.

In each case these estimates both for a long-term average and contemporary values are 1 to 2 orders of magnitude less than the present day slab flux of 1015 Kg.a-1. If the slab flux into the lower mantle is of this magnitude then the distribution of rare gases in the lower mantle must be highly non-uniform. The plume mass flux derived from the plume bouyancy flux estimates is 2.6¥1014 Kg.a-1, an order of magnitude higher than the lower to upper mantle flux estimates. This limits the maximum amount of uniform lower mantle material entrained by plumes in the recent past to around 10% of total, and further shows that the lithophile elements must be sourced from the upper mantle and dominated by recycled lithosphere.

Over some 4.4 Ga the average lower mantle mass flux is <2¥1013 Kg.a-1 and this is the maximum that plume activity would have entrained over this time period. The role of convective avalanches and massive transport of lower mantle material by plumes must be consistent with these constraints.


O'Nions, R.K. & Tolstikhin, I.N., Earth Plan. Sci.Lett. 124 131-138 (1994).