Effects of chemical fractionation of heat-producing elements on mantle evolution inferred from a numerical model of coupled magmatism-mantle convection system

Abstract

A numerical model of a coupled magmatism-mantle convection system has been developed to study how a strong chemical fractionation of heat-producing elements as a result of mantle magmatism influences the evolution of the uppermost mantle. Mantle convection is modeled by a convection of a binary eutectic material A ξ B 1− ξ with a Newtonian rheology in a two-dimensional rectangular box; A stands for olivine and B stands for a mixture of pyroxene and garnet. The viscosity depends strongly on temperature and the density increases with decreasing ξ and temperature. The material contains decaying heat-producing elements. Mantle magmatism is modeled by an extraction of melt generated at depth by a pressure-release partial melting and a deposit of the extracted melt in a thin crustal layer at the top of the box. The melt extraction and deposit give rise to heterogeneity in the distribution of heat-producing elements as well as in ξ distribution. A dense, low-ξ magmatic product is more enriched in heat-producing elements than is a less dense, high-ξ residual material. The positive thermal buoyancy that a low-ξ material gains owing to its enrichment with heat-producing elements causes a catastrophic change in the chemical structure of the uppermost mantle with time when the viscosity in the top thermal boundary layer (lithosphere) is sufficiently high. A chemically stratified structure with a high-ξ residual material in the upper part and a low-ξ material in the lower part of the box develops at the beginning, when active magmatism occurs owing to strong internal heating. As the heat-producing elements decay and the magmatism becomes weaker, a steep gradient or a discontinuity in ξ distribution develops between the high-ξ part and the low-ξ part of the box, and the convection occurs temporarily as a layered convection. The chemically layered structure is, however, catastrophically destroyed and the box suddenly becomes more chemically homogeneous at a subsequent time when the positive thermal buoyancy of the low-ξ material at depth becomes stronger than the chemically induced negative buoyancy of the low-ξ material. The catastrophic change in chemical structure induces a drop in temperature at depth and a sudden enhancement of mantle magmatism. When the viscosity in the lithosphere is sufficiently low, in contrast, the chemically layered structure is stable and the catastrophic change from a chemically layered structure to a more chemically homogeneous structure does not occur. A catastrophic destruction of a chemically layered structure in the upper mantle as a result of strong fractionation of heat-producing elements may have occurred in the early Earth.