Melt migration and mantle chromatography, 1: simplified theory and conditions for chemical and isotopic decoupling

Numerical modeling (1-D) of the geochemical effects of melt migration has been undertaken in order to determine the conditions necessary for disequilibrium transport of mantle derived magmas. This, numerical results demonstrate a first-order concordance between models incorporating all combinations of porous flow or channel flow with chemical exchange governed by either solid-phase diffusion or dissolution-precipitation mechanisms. The chemical changes imparted on the magma and solid mantle during melt transport are demonstrated to be functions only of the partition coefficients and the local Damkohler number, a dimensionless combination of melt velocity, effective grain size, porosity, exchange rate, and migration distance. Geochemical shifts during melt migration are not strongly dependent on the exact mechanisms of melt flow (porous versus channel) or chemical exchange (reaction versus diffusion) between the melt and mantle; the only difference among these mechanisms is the timescale of chemical exchange. A semi-analytic solution is presented which simplifies the calculation of the effects of melt migration on trace element concentrations and isotopic compositions, These simple equations can be easily incorporated into complex numerical models for the flow of mantle and melt in order to account for the effects of disequilibrium melt transport on magma and mantle chemistry. This formulation also permits a straightforward application to geochemical data sets, which allows an estimation of the local Damkohler number prevailing during melt transport. Analysis of trace element data from oceanic mantle xenoliths demonstrates the ability to resolve a 'chromatographic' trace element fractionation signal, The geochemistry of incompatible elements in some late-stage alkalic basalts from Haleakala, Hawaii, are consistent with chromatographic fractionation, however this signal is weak or absent in magmas erupted during the main phase of shield building (see companion paper). Osmium isotope correlations in oceanic island basalts require transport of plume magmas in transient open channels, reaching from within hundreds of meters of the melting zone to near-surface levels. The ability to identify a chromatographic chemical signature is greatly improved by acquisition of isotopic data for elements with a wide range of solid-melt compatibilities. (C) 1997 Elsevier Science B.V.