MASS TRANSFER OF XENON THROUGH A STEADY-STATE UPPER MANTLE

Abstract

We present a model of steady-state transport of Xe through the upper mantle with inputs from an undergassed lower mantle by mass transport without fractionation and subducted atmospheric gases. The model is explored using box model mass transport equations for each isotope of Xe coupled with the transport equations for He. The mantle is assumed to have had initially uniform Pu,I, and U concentrations and is divided into two reservoirs. The lower mantle is assumed to have evolved approximately as a closed-system containing initially trapped Xe along with xenon isotopes produced by ¹²⁹I, ²⁴⁴Pu, and ²³⁸U decay. The concentration and present isotopic composition of lower mantle Xe are unknowns. The upper mantle is considered to have inputs of Xe by mass transfer from the lower mantle at hotspots and from the atmosphere by subduction. In addition, the decay of U within the upper mantle contributes radiogenic Xe as well as ⁴He. The flows of Xe into the upper mantle are balanced by flows out of the upper mantle at mid-ocean ridges and hotspots, so that the upper mantle Xe concentration is assumed to be in a steady state. In this model, upper mantle Xe reflects the isotopic characteristics of the lower mantle altered by mixing with subducted atmospheric Xe and the present decay of U in the upper mantle. This is in contrast to previous models in which upper mantle Xe is what remains after degassing of the atmosphere, with a present isotopic composition that reflects the integrated history of continuous losses of Xe and grow-in of radiogenic Xe isotopes within the upper mantle. The observed correlation of and ratios for MORB found by other workers is interpreted here as reflecting different degrees of atmospheric contamination to a MORB Xe composition.