A model for the evolution of strontium and lead isotopes in a dynamic Earth

Abstract

Contrasting interpretations of existing models of Sr and Pb isotope evolution can be eliminated with a model in which crustal material is recycled through the mantle. In this model the earth's crust and upper mantle (above approximately 500 km depth) are in a steady‐state system, and the volumes and bulk compositions of ocean, continent, and mantle have been nearly constant for at least the last 2.5 b.y. and probably for most of the earth's history. Sialic material is continuously eroded from continents into ocean basins and, as a consequence of this process, is isotopically homogenized. In continental‐margin orogenic belts and island arcs, the ocean basin, rise, and trench sediments are dragged into the mantle. Isotopic equilibration between sialic and simatic material takes place within the mantle, and the sialic material is returned to the continents or island arcs as juvenile‐appearing volcanics, thus completing the geochemical cycle. Most of the radioactive parent isotopes reside within the continental sial, whereas the mantle remains depleted and unable to sustain its observed isotope evolution. With this model it is possible to explain Pb isotope evidence of widespread ancient continents and common Pb evolution in a system which appears to have a very uniform U/Pb and Th/U ratio, even though most of the U and Th are highly enriched in the heterogeneous sialic crust. At the same time the model provides an explanation for Sr isotope evidence of continual addition of material to continents. Sr isotope evolution is dominated by the reservoir of Sr in the mantle; in contrast, Pb isotope evolution is dominated by isotopic mixing during erosion and sedimentation. The apparent differences in the evolutions of Sr and Pb isotopes are due to differing responses to various parts of the steady‐state cycle as a consequence of the differences in parent to daughter ratios in the sialic crust as compared with the upper mantle and in the degree of enrichment of parent and daughter products in the crust. Identical mathematical models may be used to describe the evolution of both isotope systems.